Steve,
I would be careful making this extrapolation. Your ultimate top speed, (velocity), is more a function of the hyro and aero dynamics of your vessel, and not necesarily it's mass, or displacement. With the same waterline length and similar hull configuration your boat might require pretty similar power requirements to reach the same velocities even with quite a bit less mass,(displacement). What will be effected is your accelation. You' might get to that speed a lot faster if you are lighter, but it could take almost just as much power to maintain it. You will also have a lot more control over your boat in heavy seas with less power if you have less weight with a similar hull. But just because you have half the mass, you will not necessary be able to get away with half the power and achieve the same terminal velocity results. Maybe. Maybe not.
Although displacement does indirectly take into account the wetted surface area of your boat which has a large determining factor on your hydrodynamics, hydrodynamic charactistics can vary considerably from one vessel to another with the same displacement or be quite similar from one vessel to another with wildly different displacements. Consider that you can add a significant amount of mass to many boats and have very little effect on their terminal velocity in calm conditions. Your top speed will primarily be affected by the additional hull surface area that is coming in contact with the water which could be a quite minimal percentage for a considerable increases in mass. It will take you longer to get to your terminal velocity with the additional mass and you'll be a lot slower in heavy seas, but in calm water and wind, you will often see very little change in your terminal velocity per unit power input.
A comparison of your boat's wetted surface area and streamline characteristic to Eric's boat is also very important in accurately predicting your power requirements. What is your draft, beam, and projected surface areas compared to his and what are the specs on your prop. These parameters will be more important in determining your ultimate power needs than a simple comparison of length on the waterline and mass. Although mass is very important parameter, particularly when considering performance against wave action, unless you want to apply some really nasty mathmatical modeling to the problem, just take into account as many things as you can and stick with some of the rules of thumb as a guide for sizing.
I don't agree with the old adage, "use your thumb if your dumb" because your thumb is a very useful tool. Just don't let it get in the way of good design. The rule of thumb of 1 kW/ton is an excellent generalized guideline but there are many other factors if you want to split hairs to achieve the most cost effective results. Always try to consider as many of these factors as you can, then use your best judgement and you'll usually get good results.
Carter Quillen, P.E.
From: Steve <sstuller@netzero.com>
To: electricboats@yahoogroups.com
Sent: Friday, April 5, 2013 4:48 AM
Subject: [Electric Boats] Re: Measured performance - even more data
To: electricboats@yahoogroups.com
Sent: Friday, April 5, 2013 4:48 AM
Subject: [Electric Boats] Re: Measured performance - even more data
An interesting correlation between displacement, speed and KW input for your boat: at 3 knots, KW input is 100 watts per ton of displacement; at 4 knots, 200 watts per ton; at 4.5 knots, 300 watts per ton; at 5 knots, 500 watts per ton. My boat is half the displacement of yours on the same waterline length so I think I can extrapolate from this data that the power requirements will be the same per ton of displacement. Thanks. Steve S.
--- In electricboats@yahoogroups.com, "Eric" <ewdysar@...> wrote:
>
> Last weekend I realized that it had been about 2 years since my last performance test, and I wondered if anything had changed over time. I was at the marina on Easter morning and the conditions appeared to be ideal for another performance test. I ran through the same process listed below in about 100 minutes, covering a little over 7nm in total.
>
> Here's this weeks results:
> 497W = 3.1kts
> 1000W = 4.0kts
> 1500W = 4.5kts
> 2500W = 5.2kts
> 3975W = 5.7kts
> 5000W = 5.9kts
>
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