Hi Ned,
Thanks for the reply. I'll take a look at the reference you provided. You are confirming what I thought about narrow hulls being capable of higher speed, but I am still interested in what happens when two of them are in parallel. Do they interact in some way that matters or do they essentially become two separate racing skulls?
Pat
--- In electricboats@yahoogroups.com, Ned Farinholt <nedfarinholt@...> wrote:
>
> Pat,
> The oft quoted hull speed limit on displacement boats, [1.34*sqrt LWL'] knots, does not apply to very narrow hulls, e.g., cats, racing sculls, destroyers, etc. The effect is most pronounced at 8:1 L/B ratios and above.
> For an interesting insight to speed vs power, I refer you to http://www.atkinboatplans.com/. This is Pat Atkins website from which she sell plans that the Atkins naval architects drew in the 1930's and 40's when affordable marine ICE's were more often in the range of 5 to 15 hp. Many of their boats, especially flat bottom skiffs, ran at twice their displacement hull speeds with 5-6 hp engines. For example, XLNC is a 18' LWL x 3.5' waterline beam with claims of 13.5 mph at 6 hp. She probably displaces 900 lbs [400 hull, 200 motor, 100 fuel, 200 pilot]. That is an S/L of 2.7.
> Ned
>
>
> On Sep 8, 2011, at 1:12 PM, greenpjs04 wrote:
>
> > When I started on this forum almost two years ago, I had never even heard of hull speed. Now I think I understand it on a basic level. It seems the main parameter required to determine hull speed is the length of the boat at the water line. If I understand correctly, the width and shape of the hull affect it, but not very much. That being said, does a dual hull boat (like a cat or a pontoon boat) have a similar hull speed to a single hull boat? I am guessing yes from a practical experience point of view, but how is that explained in theory? I would think a pontoon would be more efficient than a full hull at any given speed, but then, of course, the motor has to drive two of them through the water.
> >
> > My only practical example is my 24' pontoon boat. It's max speed is about 5.5 knots using 4KW (a Torqeedo Cruise 4). A more typical crusing speed is 3.5 knots which requires 1200 to 1500 Watts.
> >
> > The standard formula for hull speed says my boat's hull speed should be a little over 6.5 knots. So, 4KW gives me about 85% of hull speed. From the many posts I have read here, that seems about right.
> >
> > So, back to my question, from a theoretical point of view, does each of my pontoons have a hull speed higher than the standard formula would indicate but after figuring in two of them, the hull speed is similar to that of a full hull boat?
> >
> > Pat
> >
> >
>
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