excellent point but the weight of the boat still doesn't really have too much to do with the hp requirement does it? instead it is a matter of fighting a current which amounts to how closely one needs to approach hull speed OR how much windage a propulsion system must fight against
my favorite approach to figuring what you need is to get a hundred foot line and a linear scale (fish scale), then you find some rather calm water and tow the boat that you wish to eventually power making note of knots attained and the weight recorded as the force pulling on the line... if you plot it out you will get a really good picture of you boats hull speed and a pretty much dead on account of what various thrust levels do to your boat.... it will usually be remarkably linear until you get outside about 70% of what you can eventually see as hull speed AND the acceleration attained will be pretty much all that is relevant within that 70% range.
From: Arby Bernt <arbybernt@yahoo.com>
To: electricboats@yahoogroups.com
Sent: Wednesday, September 14, 2011 10:59 AM
Subject: Re: [Electric Boats] Re: 1000w per ton?
I just changed our Ericson 27 (6800lbs) from a 2kw 24v system to a 5kw 48v system. At 2kw, the boat was fine in the harbor, but right at the practical limit for use in SF Bay. Add a stiff wind on the nose, however, and 2kw wouldn't move the boat. With 5kw on line, heavy seas and wind aren't a problem.
The power ratio isn't for flat water, but rather to give you an edge when the going gets rough. The top speed has gone up a little, but the real advantage is being able to maintain control in adverse conditions.
Be Well,
Arby
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