## Re: hull speed, hull shape, Bernoulli

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• Have a look at these URLs: http://lists.samurai.com/pipermail/trawlers-and-trawlering/1999- January/008309.html
Message 1 of 30 , Oct 1, 2004
Have a look at these URLs:
http://lists.samurai.com/pipermail/trawlers-and-trawlering/1999-
January/008309.html
http://www.irbs.com/lists/trawlerworld/9901/0388.html

Until you get "On the step" with a seaplane... before
it starts to plane... there is suction on the floats.
That's Bernoulli.

The "Lift" on the hull's surface is not tending to lift
the boat, but rather to suck it downward.

The Lift will be CL * 1/2 rho V^2.
rho is much higher for a dense fluid like water than it is
for a fluid like air. Which is why the airflow over the deck
is negligible (though it does suck the deck upward) than
the flow of water over the hull, which create low pressure
on the hull, which sucks the hull deeper into the water.

An airplane wing develops lift two ways.
It develops a low pressure on the upper surface
which is lower than the low pressure on the lower
surface because the flow velocity is greater on
the upper surface.
It also gains lift in reaction to the net downward
change in the velocity of the flow (behind the wing).

As speed of waterflow over a hull increases, the
pressure gets lower. It gets lower on the deck too,
but not by as much. Pressure is lower on the lower
surface... net effect (compared to static condition)
is downward. As speed increases, the boat sinks downward.

The hull of a boat is sucked downward because of the
Bernoulli principal, and because it is pushing water
upward (waves). Once the net effect becomes pushing water
downward, as it does in planing condition, the opposing
reaction changes to an upward one (relative to the boat
when it is not moving). The boat is in effect throwing
water downward (actually it's climbing on top of the water)

But overall...
The boat sinks down some before it starts to lift
out of the water.

A boat squats downward before it begins to lift...
I have done the experiment, and challeng you to do
the same. Observe the reduction in freeboard (the boat
sits deeper) as you gradually increase speed from 0 to
.5... then from .5 to 1 mph, then 1 to 2, and so forth.

My kids enjoy these types of experiments. Some more than
others... 4th of July is their favorite series of experiments,
though we've had a few exciting ones indoors too (much to
my wife's chagrin)

In the low speed regions Bernoulli is more of an influece
on freeboard than wave effect. After 3 or 4 mph, the transition
begins as the boat starts to climb on the wave (though the
stern settles, and the net effect on the waterline is
still a raisning of the waterline on the hull).

Here are some other interesting articles:
http://boatbuilding.com/content/Redwing.html
http://www.irbs.com/lists/trawlerworld/9901/0377.html

--- In SJ-24@yahoogroups.com, "Gil Lund" <gil@l...> wrote:
> The Bernoullie equation covers ALL fluid flow. It is esentially a
> statement of the "principle of energy conservation" applied to
fluid
> or gas flow.
>
> An airplane develops lift because the ABSOLUTE air pressures on the
> bottom of the wing is larger than the ABSOLUTE air pressures on the
> top. Both are lower (or sucks) relative to the ABSOLUTE pressure at
> the stagnation point on the leading edge.
>
> A surface vessel develops lift because the ABSOLUTE water pressures
> on the bottom of the hull is much larger than the ABSOLUTE air
> pressures on the deck. Both are lower (or sucks) relative to the
> ABSOLUTE pressure at the stagnation point on the submerged portion
of
> the hull.
>
> The lift and wave drag (but not friction drag) on airplanes,
surface
> vessels and submarines can be calculated using Bernoullies
equation.
> It is proportional to density of the fluids and speed2.
>
> It follows from this that lift is always present when a
conventional
> boat moves.
>
> When the lift has increased enough to equal the weight of the boat
it
> will try to climb out of the water. However, as soon as it lifts
> enough to clear the water the lift decreases and it fall back into
> the water. It then develops more lift and rises again etc,etc..
This
> semi unstable condition is called "Planing".
>
> If you have experienced flying seaplanes the physics of this will
be
> self evident.
>
> Gil
>
>
>
>
>
>
>
> -- In SJ-24@yahoogroups.com, m_kanzler@y... wrote:
> > But, as I tried to say, some component of the vectors always
> > points downward (or upward if pressure is above static pressure...
> > which is only where impingement occurs).
> >
> > All of the vectors point outward and downward or
> > outward and upward, except where the surfaces are
> > vertical.
> >
> > I do think the Bernoulli effect is samll compared with
> > wave effects, my point was that overall there is no
> > lift on the hull until it starts to plane.
> >
> > --- In SJ-24@yahoogroups.com, "Dave Brezina" <dbrezina@r...>
wrote:
> > > ... Larsson and Eliason graph it, calling it
> > > "suction". Also, it's not "pointing" to the bottom. That
would
> > only
> > > occur on the centerline of the boat. Boats are three
> dimensional,
> > so
> > > at the waterline, the vector points off toward the side
> > considerably.
> > >
• About this hull speed thing.............. I am following with great interest but I get lost once you get into the too theoretical. Can someone put the various
Message 2 of 30 , Oct 1, 2004
I am following with great interest but I get lost once you get into the too
theoretical. Can someone put the various theories and their proponents into
layman's language so I can tell whose ahead?
George #18 "Spaghetti"

[Non-text portions of this message have been removed]
• Couldnt have said it better myself. and to help prove this, I have to make sure my plugs are in or my cockpit are will fill up with water as the stern lowers
Message 3 of 30 , Oct 1, 2004
Couldnt have said it better myself. and to help prove this, I have to make
sure my plugs are in or my cockpit are will fill up with water as the stern
lowers into the water. I have also sent someone up front and they noticed
that at full speed on the 8hp outboard the whole boat sucks down into the
-----Original Message-----
From: m_kanzler@... [mailto:m_kanzler@...]
Sent: Friday, October 01, 2004 3:28 PM
To: SJ-24@yahoogroups.com
Subject: [SJ-24] Re: hull speed, hull shape, Bernoulli

Have a look at these URLs:
http://lists.samurai.com/pipermail/trawlers-and-trawlering/1999-
January/008309.html
http://www.irbs.com/lists/trawlerworld/9901/0388.html

Until you get "On the step" with a seaplane... before
it starts to plane... there is suction on the floats.
That's Bernoulli.

The "Lift" on the hull's surface is not tending to lift
the boat, but rather to suck it downward.

The Lift will be CL * 1/2 rho V^2.
rho is much higher for a dense fluid like water than it is
for a fluid like air. Which is why the airflow over the deck
is negligible (though it does suck the deck upward) than
the flow of water over the hull, which create low pressure
on the hull, which sucks the hull deeper into the water.

An airplane wing develops lift two ways.
It develops a low pressure on the upper surface
which is lower than the low pressure on the lower
surface because the flow velocity is greater on
the upper surface.
It also gains lift in reaction to the net downward
change in the velocity of the flow (behind the wing).

As speed of waterflow over a hull increases, the
pressure gets lower. It gets lower on the deck too,
but not by as much. Pressure is lower on the lower
surface... net effect (compared to static condition)
is downward. As speed increases, the boat sinks downward.

The hull of a boat is sucked downward because of the
Bernoulli principal, and because it is pushing water
upward (waves). Once the net effect becomes pushing water
downward, as it does in planing condition, the opposing
reaction changes to an upward one (relative to the boat
when it is not moving). The boat is in effect throwing
water downward (actually it's climbing on top of the water)

But overall...
The boat sinks down some before it starts to lift
out of the water.

A boat squats downward before it begins to lift...
I have done the experiment, and challeng you to do
the same. Observe the reduction in freeboard (the boat
sits deeper) as you gradually increase speed from 0 to
.5... then from .5 to 1 mph, then 1 to 2, and so forth.

My kids enjoy these types of experiments. Some more than
others... 4th of July is their favorite series of experiments,
though we've had a few exciting ones indoors too (much to
my wife's chagrin)

In the low speed regions Bernoulli is more of an influece
on freeboard than wave effect. After 3 or 4 mph, the transition
begins as the boat starts to climb on the wave (though the
stern settles, and the net effect on the waterline is
still a raisning of the waterline on the hull).

Here are some other interesting articles:
http://boatbuilding.com/content/Redwing.html
http://www.irbs.com/lists/trawlerworld/9901/0377.html

--- In SJ-24@yahoogroups.com, "Gil Lund" <gil@l...> wrote:
> The Bernoullie equation covers ALL fluid flow. It is esentially a
> statement of the "principle of energy conservation" applied to
fluid
> or gas flow.
>
> An airplane develops lift because the ABSOLUTE air pressures on the
> bottom of the wing is larger than the ABSOLUTE air pressures on the
> top. Both are lower (or sucks) relative to the ABSOLUTE pressure at
> the stagnation point on the leading edge.
>
> A surface vessel develops lift because the ABSOLUTE water pressures
> on the bottom of the hull is much larger than the ABSOLUTE air
> pressures on the deck. Both are lower (or sucks) relative to the
> ABSOLUTE pressure at the stagnation point on the submerged portion
of
> the hull.
>
> The lift and wave drag (but not friction drag) on airplanes,
surface
> vessels and submarines can be calculated using Bernoullies
equation.
> It is proportional to density of the fluids and speed2.
>
> It follows from this that lift is always present when a
conventional
> boat moves.
>
> When the lift has increased enough to equal the weight of the boat
it
> will try to climb out of the water. However, as soon as it lifts
> enough to clear the water the lift decreases and it fall back into
> the water. It then develops more lift and rises again etc,etc..
This
> semi unstable condition is called "Planing".
>
> If you have experienced flying seaplanes the physics of this will
be
> self evident.
>
> Gil
>
>
>
>
>
>
>
> -- In SJ-24@yahoogroups.com, m_kanzler@y... wrote:
> > But, as I tried to say, some component of the vectors always
> > points downward (or upward if pressure is above static pressure...
> > which is only where impingement occurs).
> >
> > All of the vectors point outward and downward or
> > outward and upward, except where the surfaces are
> > vertical.
> >
> > I do think the Bernoulli effect is samll compared with
> > wave effects, my point was that overall there is no
> > lift on the hull until it starts to plane.
> >
> > --- In SJ-24@yahoogroups.com, "Dave Brezina" <dbrezina@r...>
wrote:
> > > ... Larsson and Eliason graph it, calling it
> > > "suction". Also, it's not "pointing" to the bottom. That
would
> > only
> > > occur on the centerline of the boat. Boats are three
> dimensional,
> > so
> > > at the waterline, the vector points off toward the side
> > considerably.
> > >

----------------------------------------------------------------------------
--

a.. To visit your group on the web, go to:
http://groups.yahoo.com/group/SJ-24/

b.. To unsubscribe from this group, send an email to:
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[Non-text portions of this message have been removed]
• I think what you re seeing is what I said before: a big wave at the bow and one at the stern--that explains the backup in the cockpit drains. In other words,
Message 4 of 30 , Oct 1, 2004
I think what you're seeing is what I said before: a big wave at the bow and one at the
stern--that explains the backup in the cockpit drains. In other words, the bow and
stern appear "sucked down" while the midships area appears to be floating high.
Overall, the same amount of water is displaced as when the boat is at rest. Now I
know there's probably some small variation from this due to esoteric hydrodynamic
effects, but it seems to me basically descriptive of what's happening.

--- In SJ-24@yahoogroups.com, "pmontee" <pmontee@y...> wrote:
> Couldnt have said it better myself. and to help prove this, I have to make
> sure my plugs are in or my cockpit are will fill up with water as the stern
> lowers into the water. I have also sent someone up front and they noticed
> that at full speed on the 8hp outboard the whole boat sucks down into the
> -----Original Message-----
> From: m_kanzler@y... [mailto:m_kanzler@y...]
> Sent: Friday, October 01, 2004 3:28 PM
> To: SJ-24@yahoogroups.com
> Subject: [SJ-24] Re: hull speed, hull shape, Bernoulli
>
>
> Have a look at these URLs:
> http://lists.samurai.com/pipermail/trawlers-and-trawlering/1999-
> January/008309.html
> http://www.irbs.com/lists/trawlerworld/9901/0388.html
>
> Until you get "On the step" with a seaplane... before
> it starts to plane... there is suction on the floats.
> That's Bernoulli.
>
> The "Lift" on the hull's surface is not tending to lift
> the boat, but rather to suck it downward.
>
> The Lift will be CL * 1/2 rho V^2.
> rho is much higher for a dense fluid like water than it is
> for a fluid like air. Which is why the airflow over the deck
> is negligible (though it does suck the deck upward) than
> the flow of water over the hull, which create low pressure
> on the hull, which sucks the hull deeper into the water.
>
> An airplane wing develops lift two ways.
> It develops a low pressure on the upper surface
> which is lower than the low pressure on the lower
> surface because the flow velocity is greater on
> the upper surface.
> It also gains lift in reaction to the net downward
> change in the velocity of the flow (behind the wing).
>
> As speed of waterflow over a hull increases, the
> pressure gets lower. It gets lower on the deck too,
> but not by as much. Pressure is lower on the lower
> surface... net effect (compared to static condition)
> is downward. As speed increases, the boat sinks downward.
>
> The hull of a boat is sucked downward because of the
> Bernoulli principal, and because it is pushing water
> upward (waves). Once the net effect becomes pushing water
> downward, as it does in planing condition, the opposing
> reaction changes to an upward one (relative to the boat
> when it is not moving). The boat is in effect throwing
> water downward (actually it's climbing on top of the water)
>
> But overall...
> The boat sinks down some before it starts to lift
> out of the water.
>
> A boat squats downward before it begins to lift...
> I have done the experiment, and challeng you to do
> the same. Observe the reduction in freeboard (the boat
> sits deeper) as you gradually increase speed from 0 to
> .5... then from .5 to 1 mph, then 1 to 2, and so forth.
>
> My kids enjoy these types of experiments. Some more than
> others... 4th of July is their favorite series of experiments,
> though we've had a few exciting ones indoors too (much to
> my wife's chagrin)
>
> In the low speed regions Bernoulli is more of an influece
> on freeboard than wave effect. After 3 or 4 mph, the transition
> begins as the boat starts to climb on the wave (though the
> stern settles, and the net effect on the waterline is
> still a raisning of the waterline on the hull).
>
> Here are some other interesting articles:
> http://boatbuilding.com/content/Redwing.html
> http://www.irbs.com/lists/trawlerworld/9901/0377.html
>
> --- In SJ-24@yahoogroups.com, "Gil Lund" <gil@l...> wrote:
> > The Bernoullie equation covers ALL fluid flow. It is esentially a
> > statement of the "principle of energy conservation" applied to
> fluid
> > or gas flow.
> >
> > An airplane develops lift because the ABSOLUTE air pressures on the
> > bottom of the wing is larger than the ABSOLUTE air pressures on the
> > top. Both are lower (or sucks) relative to the ABSOLUTE pressure at
> > the stagnation point on the leading edge.
> >
> > A surface vessel develops lift because the ABSOLUTE water pressures
> > on the bottom of the hull is much larger than the ABSOLUTE air
> > pressures on the deck. Both are lower (or sucks) relative to the
> > ABSOLUTE pressure at the stagnation point on the submerged portion
> of
> > the hull.
> >
> > The lift and wave drag (but not friction drag) on airplanes,
> surface
> > vessels and submarines can be calculated using Bernoullies
> equation.
> > It is proportional to density of the fluids and speed2.
> >
> > It follows from this that lift is always present when a
> conventional
> > boat moves.
> >
> > When the lift has increased enough to equal the weight of the boat
> it
> > will try to climb out of the water. However, as soon as it lifts
> > enough to clear the water the lift decreases and it fall back into
> > the water. It then develops more lift and rises again etc,etc..
> This
> > semi unstable condition is called "Planing".
> >
> > If you have experienced flying seaplanes the physics of this will
> be
> > self evident.
> >
> > Gil
> >
> >
> >
> >
> >
> >
> >
> > -- In SJ-24@yahoogroups.com, m_kanzler@y... wrote:
> > > But, as I tried to say, some component of the vectors always
> > > points downward (or upward if pressure is above static pressure...
> > > which is only where impingement occurs).
> > >
> > > All of the vectors point outward and downward or
> > > outward and upward, except where the surfaces are
> > > vertical.
> > >
> > > I do think the Bernoulli effect is samll compared with
> > > wave effects, my point was that overall there is no
> > > lift on the hull until it starts to plane.
> > >
> > > --- In SJ-24@yahoogroups.com, "Dave Brezina" <dbrezina@r...>
> wrote:
> > > > ... Larsson and Eliason graph it, calling it
> > > > "suction". Also, it's not "pointing" to the bottom. That
> would
> > > only
> > > > occur on the centerline of the boat. Boats are three
> > dimensional,
> > > so
> > > > at the waterline, the vector points off toward the side
> > > considerably.
> > > >
>
>
>
>
>
>
>
>
------------------------------------------------------------------------
----
> --
>
> a.. To visit your group on the web, go to:
> http://groups.yahoo.com/group/SJ-24/
>
> b.. To unsubscribe from this group, send an email to:
> SJ-24-unsubscribe@yahoogroups.com
>
>
>
>
> [Non-text portions of this message have been removed]
• My Best hull speed was about 75 mph, but that was on I-5, coming off the back side of Siskiyou Summit in southern Oregon. Gotta watch that 4th gear overdrive
Message 5 of 30 , Oct 1, 2004
My Best hull speed was about 75 mph, but that was on I-5, coming off
the back side of Siskiyou Summit in southern Oregon.

Gotta watch that 4th gear overdrive ;)

Peter.

--- In SJ-24@yahoogroups.com, "Tim Stokes" <tmstokes@b...> wrote:
> I think what you're seeing is what I said before: a big wave at
the bow and one at the
> stern--that explains the backup in the cockpit drains. In other
words, the bow and
> stern appear "sucked down" while the midships area appears to be
floating high.
> Overall, the same amount of water is displaced as when the boat is
at rest. Now I
> know there's probably some small variation from this due to
esoteric hydrodynamic
> effects, but it seems to me basically descriptive of what's
happening.
>
>
>
> --- In SJ-24@yahoogroups.com, "pmontee" <pmontee@y...> wrote:
> > Couldnt have said it better myself. and to help prove this, I
have to make
> > sure my plugs are in or my cockpit are will fill up with water as
the stern
> > lowers into the water. I have also sent someone up front and they
noticed
> > that at full speed on the 8hp outboard the whole boat sucks down
into the
> > water about 6-8 inches.
> > -----Original Message-----
> > From: m_kanzler@y... [mailto:m_kanzler@y...]
> > Sent: Friday, October 01, 2004 3:28 PM
> > To: SJ-24@yahoogroups.com
> > Subject: [SJ-24] Re: hull speed, hull shape, Bernoulli
> >
> >
> > Have a look at these URLs:
> > http://lists.samurai.com/pipermail/trawlers-and-trawlering/1999-
> > January/008309.html
> > http://www.irbs.com/lists/trawlerworld/9901/0388.html
> >
> > Until you get "On the step" with a seaplane... before
> > it starts to plane... there is suction on the floats.
> > That's Bernoulli.
> >
> > The "Lift" on the hull's surface is not tending to lift
> > the boat, but rather to suck it downward.
> >
> > The Lift will be CL * 1/2 rho V^2.
> > rho is much higher for a dense fluid like water than it is
> > for a fluid like air. Which is why the airflow over the deck
> > is negligible (though it does suck the deck upward) than
> > the flow of water over the hull, which create low pressure
> > on the hull, which sucks the hull deeper into the water.
> >
> > An airplane wing develops lift two ways.
> > It develops a low pressure on the upper surface
> > which is lower than the low pressure on the lower
> > surface because the flow velocity is greater on
> > the upper surface.
> > It also gains lift in reaction to the net downward
> > change in the velocity of the flow (behind the wing).
> >
> > As speed of waterflow over a hull increases, the
> > pressure gets lower. It gets lower on the deck too,
> > but not by as much. Pressure is lower on the lower
> > surface... net effect (compared to static condition)
> > is downward. As speed increases, the boat sinks downward.
> >
> > The hull of a boat is sucked downward because of the
> > Bernoulli principal, and because it is pushing water
> > upward (waves). Once the net effect becomes pushing water
> > downward, as it does in planing condition, the opposing
> > reaction changes to an upward one (relative to the boat
> > when it is not moving). The boat is in effect throwing
> > water downward (actually it's climbing on top of the water)
> >
> > But overall...
> > The boat sinks down some before it starts to lift
> > out of the water.
> >
> > A boat squats downward before it begins to lift...
> > I have done the experiment, and challeng you to do
> > the same. Observe the reduction in freeboard (the boat
> > sits deeper) as you gradually increase speed from 0 to
> > .5... then from .5 to 1 mph, then 1 to 2, and so forth.
> >
> > My kids enjoy these types of experiments. Some more than
> > others... 4th of July is their favorite series of experiments,
> > though we've had a few exciting ones indoors too (much to
> > my wife's chagrin)
> >
> > In the low speed regions Bernoulli is more of an influece
> > on freeboard than wave effect. After 3 or 4 mph, the transition
> > begins as the boat starts to climb on the wave (though the
> > stern settles, and the net effect on the waterline is
> > still a raisning of the waterline on the hull).
> >
> > Here are some other interesting articles:
> > http://boatbuilding.com/content/Redwing.html
> > http://www.irbs.com/lists/trawlerworld/9901/0377.html
> >
> > --- In SJ-24@yahoogroups.com, "Gil Lund" <gil@l...> wrote:
> > > The Bernoullie equation covers ALL fluid flow. It is
esentially a
> > > statement of the "principle of energy conservation" applied to
> > fluid
> > > or gas flow.
> > >
> > > An airplane develops lift because the ABSOLUTE air pressures
on the
> > > bottom of the wing is larger than the ABSOLUTE air pressures
on the
> > > top. Both are lower (or sucks) relative to the ABSOLUTE
pressure at
> > > the stagnation point on the leading edge.
> > >
> > > A surface vessel develops lift because the ABSOLUTE water
pressures
> > > on the bottom of the hull is much larger than the ABSOLUTE air
> > > pressures on the deck. Both are lower (or sucks) relative to
the
> > > ABSOLUTE pressure at the stagnation point on the submerged
portion
> > of
> > > the hull.
> > >
> > > The lift and wave drag (but not friction drag) on airplanes,
> > surface
> > > vessels and submarines can be calculated using Bernoullies
> > equation.
> > > It is proportional to density of the fluids and speed2.
> > >
> > > It follows from this that lift is always present when a
> > conventional
> > > boat moves.
> > >
> > > When the lift has increased enough to equal the weight of the
boat
> > it
> > > will try to climb out of the water. However, as soon as it
lifts
> > > enough to clear the water the lift decreases and it fall back
into
> > > the water. It then develops more lift and rises again
etc,etc..
> > This
> > > semi unstable condition is called "Planing".
> > >
> > > If you have experienced flying seaplanes the physics of this
will
> > be
> > > self evident.
> > >
> > > Gil
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > > -- In SJ-24@yahoogroups.com, m_kanzler@y... wrote:
> > > > But, as I tried to say, some component of the vectors always
> > > > points downward (or upward if pressure is above static
pressure...
> > > > which is only where impingement occurs).
> > > >
> > > > All of the vectors point outward and downward or
> > > > outward and upward, except where the surfaces are
> > > > vertical.
> > > >
> > > > I do think the Bernoulli effect is samll compared with
> > > > wave effects, my point was that overall there is no
> > > > lift on the hull until it starts to plane.
> > > >
> > > > --- In SJ-24@yahoogroups.com, "Dave Brezina" <dbrezina@r...>
> > wrote:
> > > > > ... Larsson and Eliason graph it, calling it
> > > > > "suction". Also, it's not "pointing" to the bottom. That
> > would
> > > > only
> > > > > occur on the centerline of the boat. Boats are three
> > > dimensional,
> > > > so
> > > > > at the waterline, the vector points off toward the side
> > > > considerably.
> > > > >
> >
> >
> >
> >
> >
> >
> >
> >
> --------------------------------------------------------------------
----
> ----
> > --
> >
> > a.. To visit your group on the web, go to:
> > http://groups.yahoo.com/group/SJ-24/
> >
> > b.. To unsubscribe from this group, send an email to:
> > SJ-24-unsubscribe@yahoogroups.com
> >
> > c.. Your use of Yahoo! Groups is subject to the Yahoo! Terms
of Service.
> >
> >
> >
> > [Non-text portions of this message have been removed]
• A boat can only go so fast. Dan Night Train ... From: To: Sent: Friday, October 01, 2004 3:51 PM Subject: Re: [SJ-24]
Message 6 of 30 , Oct 1, 2004
A boat can only go so fast.
Dan "Night Train"

----- Original Message -----
From: <gc138@...>
To: <SJ-24@yahoogroups.com>
Sent: Friday, October 01, 2004 3:51 PM
Subject: Re: [SJ-24] Re: hull speed, hull shape, Bernoulli

>
> I am following with great interest but I get lost once you get into the
> too
> theoretical. Can someone put the various theories and their proponents
> into
> layman's language so I can tell whose ahead?
> George #18 "Spaghetti"
>
>
> [Non-text portions of this message have been removed]
>
>
>
>
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>
• ... There are stories of completely pulling a boat under when towing with too much horsepower (Though I honestly have to questiond the authenticity of such
Message 7 of 30 , Oct 2, 2004
--- In SJ-24@yahoogroups.com, "Tim Stokes" <tmstokes@b...> wrote:
> Overall, the same amount of water is displaced as when the boat
> is at rest. Now I know there's probably some small variation
> from this due to esoteric hydrodynamic effects, but it seems
> to me basically descriptive of what's happening.

There are stories of completely pulling a boat under when
towing with too much horsepower (Though I honestly have to
questiond the authenticity of such stories).

Here's one:

Someone on the group here (pmontee) said:
"I have also sent someone up front and they noticed
that at full speed on the 8hp outboard the whole boat
sucks down into the water about 6-8 inches."
http://groups.yahoo.com/group/SJ-24/message/9350

As I said before, the best way to remove all doubt is to
try it. If you have a few observers put them in different
places ob the boat and have them watch the waterline as

Maybe someday we'll have a get-together cruis here on
Puget Sound and I'll put hash marks on my boat above
the waterline with a grease pencil (China Marker) and
we can go out with a few people and some video cameras
and take data.

Or I'll build a flow tank and some models...
(In my spare time).
• Guys Fluid Dynamics 101!! It is not the boat that sucks down it is the water that rises up around it to get out of the way. If you had a sensitive altimeter
Message 8 of 30 , Oct 2, 2004
Guys

Fluid Dynamics 101!!

It is not the boat that "sucks" down it is the water that rises up
around it to get out of the way. If you had a sensitive altimeter you
woulds see that both the water level around the boat and the boat
itself would actually rise up as speed is increased. If you get to
fly a seaplanes this is easy to notice when you do a take off run.

It is not possible to tow a boat under unless the bow digs in and you
get water pressure on the deck.

gil

--- In SJ-24@yahoogroups.com, m_kanzler@y... wrote:
>
> --- In SJ-24@yahoogroups.com, "Tim Stokes" <tmstokes@b...> wrote:
> > Overall, the same amount of water is displaced as when the boat
> > is at rest. Now I know there's probably some small variation
> > from this due to esoteric hydrodynamic effects, but it seems
> > to me basically descriptive of what's happening.
>
> There are stories of completely pulling a boat under when
> towing with too much horsepower (Though I honestly have to
> questiond the authenticity of such stories).
>
> Here's one:
>
> Someone on the group here (pmontee) said:
> "I have also sent someone up front and they noticed
> that at full speed on the 8hp outboard the whole boat
> sucks down into the water about 6-8 inches."
> http://groups.yahoo.com/group/SJ-24/message/9350
>
> As I said before, the best way to remove all doubt is to
> try it. If you have a few observers put them in different
> places ob the boat and have them watch the waterline as
>
> Maybe someday we'll have a get-together cruis here on
> Puget Sound and I'll put hash marks on my boat above
> the waterline with a grease pencil (China Marker) and
> we can go out with a few people and some video cameras
> and take data.
>
> Or I'll build a flow tank and some models...
> (In my spare time).
• I guess that this explains the disappearance of some of the clipper ships in the 1800s during fair weather. I ve been told that as they were being challenged
Message 9 of 30 , Oct 13, 2004
I guess that this explains the disappearance of some of the clipper
ships in the 1800s during fair weather. I've been told that as they
were being challenged by steamships, they added more sail for more
speed. Unfortunately, the greater speed resulted in greater squat.
Add an overload of cargo and some of them literally sailed themselves
under.
The SJ-24 has the same (though less drastic) problem. If you want to
experiment, try motoring full blast without the cockpit drain holes
plugged. Your feet will be very wet, very quickly, as the stern squats.

Graeme Jannaway
Matta II, #308
--- In SJ-24@yahoogroups.com, m_kanzler@y... wrote:
> As speed increases, the flow velocity over the hull increases.
> The flow velocity over the deck increases too.
>
> I think we agree that the flow over the deck (air) can
> be considered negligible.
>
> As flow increases over a surface pressure drops.
>
> As pressure (locally) drops, the surface gets a
> suction effect on it because away from the surface
> (outside the transition layer) pressure is higher.
>
> This is why when you take a shopvac and use the blow
> (output) connection on the hose you can suspend a
> ball in the flow, and if you try to push it out
> of the flow it will restabilize in the flow (up
> to a point).
>
> The boat is sucked downward. It can't go too far down because
> the bouyancy is acting on the transition and boundary layer
> and transmitted to the hull.
>
> In otherwords, the boat is sucked downward into the layer
> where there is an increased velocity (transition layer)
> until it encounters the stagnat fluid outside the layer
> that is influenced by the hull's velocity.
>
> Watch a tugboat that is in a hurry and not towing sometime...
> they tend to squat down before they begin to lift back up.
> The squating is the hydodyanamic suction on the hull.
> The subsequent raisning is when they start to climb up the
> bow wave.
>
> You can see this on a sailboat too... get to about 5 mph
> (not knots) and see where the water is on your waterline
> near the middle (fore/aft) of the boat.
>
> Then, if you have enough horsepower, keep increasing throttle
> and sampling depth. You'll see that the overall freeboard
> decreases as the boat is sucked downward.
>
> --- In SJ-24@yahoogroups.com, "Gil Lund" <gil@l...> wrote:
> > Just like an airplane. You have to increase the angle of attack to
> > climb.
> >
> > Because of the 800/1 density difference between water and air the
> > absolute static pressure in the water flowing under the hull is
> lower
> > or "suck" relative to the stagnation pressure on the bow. However,
> it
> > is always much larger in magnitude than the absolute static
> pressure
> > in the air flowing over an adjacent point on the deck.
> >
> > It is the difference in absolute pressure between two adjacent
> points
> > on the hull and deck that lifts you up. Integrating the absolute
> > pressures over the deck and hull of a ship normally gives you a
> total
> > force vector pointing up not towards the bottom. That is unless
> you
> > signed up to crew on the Titanic.
> > gil
> >
> >
> > --- In SJ-24@yahoogroups.com, m_kanzler@y... wrote:
> > > Until the angle of attack of the boat with the surface
> > > begins to cause the boat to plane, bernoulli predicts that
> > > as speed increases, the boat gets sucked downward.
> > >
> > > Draw the lift vecotors on the surface of the hull...
> > > they all point (at least partially) to the ocean's bottom.
> > >
> > > The density doesn't come into the equation. You don't need
> > > to look at the lift caused by air over the cabin.
> > >
> > > There is low pressure all around the hull, except at the bow
> > > where the flow impinges at 90° to the surface. Any flow
> > > along the surface creates suction, just like on the upper
> > > surface of an airplane's wing.
> > >
> > > Try this experiment:
> > > Hold a piece of paper by the end and blow over the top of
> > > the paper. Unless you blow direcly downward the paper will
> > > lift, even if you blow at 45°
> > >
> > > Here's a picture of the experiment:
> > >
> > > http://www.nasm.si.edu/exhibitions/gal109/LESSONS/TEXT/TEASERS.HTM
> > >
> > > --- In SJ-24@yahoogroups.com, "Gil Lund" <gil@l...> wrote:
> > > >
> > > > That is correct, but only if you are a submarine.
> > > > For a surface vessels the combined hydro-aero dynamic upforce
> or
> > > lift
> > > > increases with speed. This all conforms to Bernoulli's
> principle
> > > > quite well if you remember that the fluid density is also a
> part
> > of
> > > > the equation and that the density of air is about 800 times
> > smaller
> > > > than the density of water.
> > > > gil
> > > >
> > > > --- In SJ-24@yahoogroups.com, m_kanzler@y... wrote:
> > > > > Actually, as the boat goes faster the hull doesn't create
> > > > > lift, it does the opposite. From Bernoulli's principle you
> > > > > can see that as the fluid velocity over the surface of the
> > > > > hull increases, a low pressure area is created which sucks
> > > > > the boat downward.
> > > > >
> > > > > As the boat tries to climb its bow wave, it is deterred
> > > > > by gravity. Some heavily powered boats, like tugs, can
> > > > > climb part way up that bow wave and can exceed their
> theoretical
> > > > > hull speed, but it takes tons of power.
> > > > >
> > > > > Some other boats which are lightweight with a flat
> > > > > enough foresection can succeed at climbing the bow
> > > > > wave, and that his how they get up on a plane.
> > > > >
> > > > > In between are the "Semi-displacement" hulls which begin
> > > > > to plane, but don't quite break into a full plane.
> > > > >
> > > > > Here is reputable (Antrim) a reference:
> > > > > http://www.antrimdesign.com/articles/hullspeed.html
> > > > >
> > > > > And a bit about hull shapes:
> > > > > http://www.oceannavigator.com/site/csrv/content.asp?id=1314
> > > > > http://cruisenews.net/cgi-
> > > > > bin/dockside/webbbs_config.pl/noframes/read/416
> > > > >
> > > > >
> > > > > --- In SJ-24@yahoogroups.com, "Gil Lund" <gil@l...> wrote:
> > > > > > Tim ,
> > > > > >
> > > > > > The wave pattern developed by the boat has little or no
> > > > > relationship
> > > > > > to its "hull speed".
> > > > > >
> > > > > > The following is the official fluid dynamics definition of
> > > > > the "hull
> > > > > > speed" concept.
> > > > > >
> > > > > > Any boat floating in water will displace an amount of water
> > > equal
> > > > > to
> > > > > > its weight.(about 62lb/ft3) Remember the guy in the bathtub.
> > > > > >
> > > > > > If the boat starts to move the hull normally develops some
> > > > uplift.
> > > > > > Because of this the water displaced by the hull then equals
> > the
> > > > > > weight minus this uplift. As the boat moves thru the water
> it
> > > > must
> > > > > be
> > > > > > pushed up and out of the boats way. This will lift the
> water
> > > > > surface
> > > > > > around the hull creating waves.
> > > > > >
> > > > > > The power needed to do the work lifting this water has to
> > come
> > > > from
> > > > > > the boats engine or sail. It incrases with the third power
> of
> > > the
> > > > > > boat speed. Obviously as you go faster the power needs
> > > increases
> > > > > > much, much, much faster. For a given boat hull you finally
> > > reach
> > > > a
> > > > > > speed at which it is not "practical" to to add more power
> to
> > go
> > > > > > faster. We call this speed the "hull speed". You can go
> > faster
> > > > than
> > > > > > this but the cost of fuel will kill you.
> > > > > >
> > > > > > The formula to calculate "hull speed" is purly developed
> > using
> > > > > > imperical and practical considerations. For a hull that
> > > develops
> > > > > > minimal lift it is approximatly given by 1.3*SquareRoot of
> > the
> > > > > > waterline length. The factor (1.3) is determined by the
> hull
> > > > shape.
> > > > > >
> > > > > > The only way to beat the water displacement drag problem is
> > to
> > > > > > develop more hull lift. If the lift is equal to the weight
> of
> > > the
> > > > > > boat there is no need to displace water and you can "plane"
> > on
> > > > top
> > > > > of
> > > > > > the water without displacing it.
> > > > > >
> > > > > > In practice all boat hulls have some lift reducing the
> > > > displacement
> > > > > > drag and no hulls have enough lift to totally elliminate
> the
> > > > > > displacement drag. That is unless they become airborne.
> > > > > >
> > > > > > SJ24 hulls will need a tremendous amount of power to reach
> > > > planing
> > > > > > speeds.
> > > > > >
> > > > > > gil
> > > > > >
> > > > > >
> > > > > >
> > > > > >
> > > > > >
> > > > > >
> > > > > > --- In SJ-24@yahoogroups.com, "Tim Stokes" <tmstokes@b...>
> > > wrote:
> > > > > > > Marc--I'll throw in my two cents. "Hull speed" means as
> > much
> > > as
> > > > > it
> > > > > > ever did for those
> > > > > > > boats to which it applies, including a San Juan 24.
> > > > > > hull shapes do not
> > > > > > > plane; i.e., each end of the hull has to be supported.
> > When
> > > > the
> > > > > > boat is moving slowly
> > > > > > > you'll notice a number of little ripples emanating from
> the
> > > > > hull.
> > > > > > As speed increases
> > > > > > > the number of crests decreases and the height and
> distance
> > > > > between
> > > > > > each crest
> > > > > > > increases until there is a big crest at the bow and a big
> > > crest
> > > > > at
> > > > > > the stern. This is
> > > > > > > maximum hull speed because any further increase in speed
> > > would
> > > > > > result in a
> > > > > > > lengthening distance between crests and the stern would
> > fall
> > > > into
> > > > > a
> > > > > > trough-- the boat
> > > > > > > would be trying to sail uphill. The reason for the stern
> > > > > overhang
> > > > > > was an attempt to
> > > > > > > cheat the rule. The static waterline is 19.5 feet, but
> > when
> > > > > > sailing the overhang
> > > > > > > provides some extra waterline as the stern wave builds
> up.
> > > The
> > > > > > true dynamic
> > > > > > > waterline is probably more like 21 feet.
> > > > > > >
> > > > > > > None of that applies, or applies in a reduced degree, to
> a
> > > > > planing
> > > > > > hull in planing
> > > > > > > conditions. I'm sure you've noticed that newer shapes
> > > feature
> > > > > > broad stern sections
> > > > > > > and relatively flat bottoms, compared to our round
> bottoms
> > > and
> > > > > > narrow sterns. I've
> > > > > > > never experienced a SJ24 planing and don't think anyone
> > has,
> > > > > > although they will surf
> > > > > > > briefly downwind off a big wave.
> > > > > > >
> > > > > > > Tell Janie hi.
> > > > > > >
> > > > > > > --- In SJ-24@yahoogroups.com, "Marc Marschark"
> > <memrtl@n...>
> > > > > wrote:
> > > > > > > > George (and Dave),
> > > > > > > >
> > > > > > > > I was eye-balling it at 20 ft and used the 1.4 I read
> in
> > > Sail
> > > > > > magazine.
> > > > > > > > Using, admittedly, a yankee calculator, that comes to
> > 6.26.
> > > > > > > >
> > > > > > > > But, no matter, the question is still "does hull speed
> > > > actually
> > > > > > mean
> > > > > > > > anything these days?"
> > > > > > > >
> > > > > > > > Marc
> > > > > > > >
> > > > > > > > -----Original Message-----
> > > > > > > > From: gc138@a... [mailto:gc138@a...]
> > > > > > > > Sent: Thursday, September 23, 2004 10:52 AM
> > > > > > > > To: SJ-24@yahoogroups.com
> > > > > > > > Subject: Re: [SJ-24] hull speed
> > > > > > > >
> > > > > > > >
> > > > > > > > Marc - How do you get 6.4 kts? By my calculations, that
> > > would
> > > > > be
> > > > > > for a
> > > > > > > > boat
> > > > > > > > with a waterline length of about 23 feet. The number I
> > get
> > > is
> > > > > > 5.92 kts
> > > > > > > > for a
> > > > > > > > 19.5 ft waterline. I'm not a math major or engineer but
> > by
> > > > > > heeling and
> > > > > > > > extending the waterline somewhat , you are right that
> > the
> > > > hull
> > > > > > speed
> > > > > > > > would change
> > > > > > > > but only increase by a hundredth of a knot for each
> > tenth
> > > of
> > > > a
> > > > > > foot of
> > > > > > > > waterline gain. Probably the engineers and other
> > technical
> > > > > folks
> > > > > > will
> > > > > > > > chime in here
> > > > > > > > and set me straight if I'm wrong. I also know that
> boats
> > > can
> > > > > > exceed
> > > > > > > > their
> > > > > > > > technical hull speed coming down the face of waves or
> > > under
> > > > > other
> > > > > > > > optimum
> > > > > > > > conditions. This is a function of hull shapes of which
> I
> > > > have
> > > > > no
> > > > > > > > business delving
> > > > > > > > into.
> > > > > > > > George #18 "Spaghetti"
> > > > > > > >
> > > > > > > >
> > > > > > > > [Non-text portions of this message have been removed]
> > > > > > > >
> > > > > > > >
> > > > > > > >
> > > > > > > >
> > > > > > > >
> > > > > > > > Yahoo! Groups Links
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