1636Re: Pop Pop 16' canoe
- Aug 2, 2009Additional comments to message #1632 and 1634 from Sydney and Donald.
1°) The speed of a boat is stable when the hydrodynamic resistance is equal to the propulsive thrust. The hydrodynamic resistance is the sum of frictional resistance, wave resistance, and air resistance.
The frictional resistance and air resistance evolve with the square of the boat velocity.
The wave resistance is something much more complicated. For light displacement boats, when planning conditions are met it can decrease (and then increase again with the boat velocity). Unlikely to decrease for a canoe due to the shape of the hull.
Whatever, pop-pop propulsive power is so minute that there is no chance of planning of a canoe.
2°) To get the same thrust, 2 (or more) small engines are preferable to a big one because multiple engines are lighter and smaller, and because they develop a higher jet speed, which means a possible higher boat speed.
Go to www.eclecticspace.net (it is not a commercial site), click on "pop-pop", then on the English flag, then on "To know more " and scroll down to open the 3rd document from the bottom of the list: It is entitled "Engine/hull adaptation". Look at the graph at the end of this document. It expresses clearly the limits of pop-pop propulsion.
--- In firstname.lastname@example.org, Donald Qualls <silent1@...> wrote:
> sydneygreenestreet wrote:
> > So a
> > longer boat of similar beam to a shorter boat will require less power
> > to get it to go faster that the shorter boat. ie a boat 12' by 4'
> > will need more power that a 20' x 4' boat to travel at the same speed
> > ergo a 16' x 3' canoe (you call them Indian canoes made famous by
> > Nelson Eddy and Jannete MacDonald to me they are classed as yes you
> > guessed it Empire Canoes)will need less power to travel at the same
> > speed as an 8' x 4'6" snub nosed dinghy....
> There are two factors at work here (maybe three). First, fineness
> ratio; generally, for a given displacement, the "finer" hull (longer and
> narrower) will require less power at any speed than the less fine,
> though refinements in the shape beneath the waterline can reduce power
> requirement, up to a point, for any fineness.
> Second, hull speed; there's a speed, for any given waterline length of
> displacement hull (that is, a hull supported only by buoyancy, not by
> dynamic forces), above which the power requirement for greater speed
> increases very sharply, and that speed is the one at which the trough
> following the bow wave just reaches the stern (since the speed of common
> waves in water is fixed, this comes to a specific speed for any given
> hull length). Hull speed is why the aircraft carrier is typically the
> fastest ship in a fleet, even though destroyers have much higher power
> to weight ratio; a modern supercarrier will have a hull speed in excess
> of forty knots, while a destroyer is limited to not much over twenty,
> and a tender (basically a miniature tanker, not much bigger than an
> oceangoing tug) may have to plug along at twelve to fifteen. A transom
> stern can help here, because it provides an effective lengthening of the
> waterline at speed when the water takes time to fill in behind the boat.
> The potential third factor is the one mentioned relative to hull speed:
> if a hull planes, that is, is partly supported by dynamic forces, it can
> go faster than hull speed without the precipitous increase in required
> power. This is why a tug with five thousand horsepower on tap can still
> only churn away at ten or twelve knots, while a runabout with as little
> as fifty horsepower can manage thirty-five (and with a couple hundred
> can go sixty): the runabout is using hydrodynamic lift to climb over the
> bow wave instead of wasting horsepower just building it higher and higher.
> This is applicable to pop-pop boats because every one I've seen is a
> displacement hull; even if the hull is correctly shaped to plane,
> pop-pop motors don't have enough power to climb up onto the bow wave, so
> none of them will go faster than hull speed -- which is pretty slow for
> a boat with a waterline length of, at most, a couple feet.
> For the application of a people (and dog) carrying canoe, it's unlikely
> an reasonably sized power plant will push a canoe significantly faster
> than a couple strong paddlers. Beyond that, wind forces on the hull's
> freeboard can be high enough to upset a paddler (I recall it being quite
> difficult to paddle upwind with a single person in the stern, because
> the available steering force was barely able to overcome the weathervane
> effect) and are likely to swamp the thrust available from even a fairly
> large pop-pop. On the other hand, if you have a canoe to experiment on,
> and the tubing and burner to play with, the worst you'll likely do is
> set fire to the canoe, sink an open bottle of LPG (which will pop up
> again when it gets sufficiently empty), and have to swim for land with
> your water dog towing you...
> If, through hard work and perseverance, you finally get what you want,
> it's probably a sign you weren't dreaming big enough.
> Donald Qualls, aka The Silent Observer http://silent1.home.netcom.com
> Opinions expressed are my own -- take them for what they're worth
> and don't expect them to be perfect.
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