Luckily, I had tow test data, as well as data from Michlet and Freeship, for my hull, plus there was a fair degree of experience with using the same hull for a human powered boat, so the power requirements were pretty well understood before I started experimenting with electric propulsion.
Overall it's pretty close, although my total system efficiency is better than 50%; I have a prop efficiency of just under 86% (measured), a motor efficiency of 91% at cruise (it's massively oversized - I'm using a 2.8kW rated motor and significantly under-running it for low losses), a controller efficiency of 99% and high capacity LiFePO4 batteries that have an internal resistance that's low enough to ignore any losses from them (the batteries will run the boat at cruise for around 8 hours with no solar panel input). Overall, my system efficiency is better than 75%.
Normal cruise speed is 4mph on our inland waterways (which happens to be the speed limit in most places) and the system will sustain this with an input power of about 70 to 80 watts, although this can easily increase to over 100 watts with any sort of head or side wind.
When it comes to light displacement, long waterline length, boats, they often need little power to get close to maximum displacement speed. This isn't that surprising as the average person struggles to sustain a power output of more than around 100 to 150 watts, and paddle or oars are unlikely to be better than 50% efficient. There have been a lot of tests on canoe and kayak hull resistance (Winter, et al) and typical figures would be around 25N to 30N at 5kts, which equates to a power into the water of between 65 and 77 watts, or a power input to the paddles of between 130 and 154 watts. The downside is that wind and waves have a significantly greater impact on overall power demand on such light craft.
Jeremy
--- In electricboats@yahoogroups.com, "Eric" <ewdysar@...> wrote:
>
> Hi Jeremy,
>
> I'll have to qualify my often stated rule of thumb as being accurate for displacement auxiliary sailboats in the 27' to 32' range. I'm guessing that boats under 1000 lbs may need a different rule. But for mid-sized displacement sailboats, the 1kW per ton works very well and 2kW per ton would be oversized. Smaller displacement power trawlers like Myles' boat seem to fit the 1kW rule too.
>
> I appreciate the calcs that you've put into predicting your boat's performance. So, since you've got the boat to collect objective data, how close does it come to the predictions that you state below?
>
> You state that your hull should take 119W of power to cruise at 5kts. Your assumption of 50% total efficiency sounds good to me, so you should be able to measure a 250W draw from your batteries while cruising at 5kts. What power consumption have you observed while motoring at those speeds?
>
> I know that my boat is very different from yours, but I pull about 2500W at 5kts, so I am very interested to hear if small, light boats are that much different. I believe that the 17' ocean kayak that I built that displaces about 225lbs with me in it would take more than 115W of motor (1kW rule) for acceptable performance, given my experience with the Torqeedo 403 product, that number should be closer to 400W. If I extrapolated that performance to tons, then 3.6kW per ton looks right, but we know that this is not true.
>
> This is similar to the conversation that we had about the accepted power predictions from the diesel powered world. Those predictions were overstated, through observation we could see that some of the assumptions were obviously not valid for our boats.
>
> So to move our predictions from theoretical to practical, they should be validated with real world observations. Your predictions for your boat seem understated to me, but if you've got the objective data to back them up, then I'm a believer.
>
> But you raise a good point, I'll make sure that I qualify that the 1kW per ton rule is for boats that exceed 1 ton, just to be on the safe side.
>
> Fair winds,
> Eric
> Marina del Rey, CA
>
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