Re: Heavy, beamy boats, and the value thereof
- --- In firstname.lastname@example.org, "John Welsford" <jwboatdesigns@...> wrote:
>...Maybe I was not as clear as I thought I was. I was asking that since a beamy heavy boat already has a poor design as far as hydrodynamics is concerned, how much worse do hard chines make it? My suspicion is that hard chines do not add that much additional drag, no more than about an extra 10%. Yes, hard chines add some drag, but how much? Has that been tested? In fact, depending on the design, could the extra effort to make a multi-chine shape not be worth it, when one includes also the loss of interior space and reduced roll damping? And yet, if the boat is heavy enough, could a hard chine shape add no more drag than a multi-chine shape?
> But the point that I was trying to make was that T had made a blanket
> statement about designers using various ways of replicating rounded shapes
> in plywood by various means not being neccessary, and I was trying to point
> out that sometimes the circumstances dictate that it is.
From a strictly hydrodynamics standpoint, a light and narrow boat is preferable to a beamy, heavy boat: but as I have repeatedly said on this forum, hydrodynamics is not the only issue on the table, and those issues may dictate a different design than predicated on optimal hydrodynamics.
- Moving the crew aft is common practice with the surf boats, however I
did not notice them doing so in the video link that Chris provided
last week. Keeping the bow up is always a good plan at high speeds.
To my mind the basic essential component is a boat that can steer
straight at these higher speeds. That usually means a boat that can
at least semi plane. If the boat can not handle the speed, then the
brakes should be applied. A boat that is already doing 5 knots or
more is going to be forced to surf a little, with waves over about 5
feet, with more of a push from bigger waves. The wave speed / boat
speed differential may cause the back end of the boat to be shoved
around a bit. Lighter DL boats lift off a bit easier. We got kicked
around a lot in the Laser 28 earlier this year. It was during the
early increase in speed from about 7 or so up to about 9 or so. Once
we got past 10 the boat became easier to steer as we had lift off
from the pointy end, and the rudder became more effective. We have
been considering several options to help correct the problem, but a
lighter faster boat may be the best cure. A lot of modern fin keel
boats like to be draged from the pointy end rather than being pushed
by the back end. And most do not like oscilations set up by a
wandering chute. We choke the chute, while trying to get it to fly
high. JW stated a while back that using two poles may help. Another
option is to fly two headsails. Often a reef in the main helps.
In higher winds still, the storm sail can be sheeted to that when
the nose wanders up towards the wind, it is pushed back down.
Boats tend to broach on their ( note the spelling ) deep fore foots
more than anything, some more than others, so any attempt to keep
weight aft, and or speed under control makes sense to me.
- In email@example.com, "graeme19121984"
> 2) As for a "safe way to trim a small boat when running off in athereby
> heavy following sea": C. Andrade, Jr means the boat will (self)
> steer downwind better with the raised bow catching the wind than
> otherwise. He also means, and I think this applies also to surf,
> that the stern down attitude reduces the risk of pearling then
> broaching. Depending on small boat type it may lower the COG
> increasing initial stability, and reducing the risk of rolling thehandling,
> gunnels under. It is more likely the rudder, maybe skeg, will be
> immersed in liqid water and not the froth above it. Better
> and sea keeping.
> His north eastern American audience of 90 years ago would have
> understood him instantly as this was a survival tactic in the North
> Atlantic fishery employed to ride out a blow when boats were
> seperated from the mother ship. That crew could move aft in a dory
> and sink it into this attitude was one more reason it was liked so
> much. A similar approach was used in along shore fisheries when