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
--- In electricboats@yahoogroups.com, "Jeremy" <jeremy_harris_uk@...> wrote:
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> .... For example, my solar powered river boat is 18ft overall and displaces about 600lbs with two people on board. The hull resistance data shows that at 2kts it needs about 7N of thrust, which is about 7 watts of power (at the output of the prop, not the motor input, power being simply speed (in m/S) x thrust (in N)). Increasing the speed to 4kts increases the thrust to 26.3N and the power required to about 56 watts. At the fastest practical displacement speed of 5kts the resistance increases to 46.4N and the power increases to about 119 watts.
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> If the propulsion system has an overall efficiency of 50% (allowing for propeller efficiency, motor efficiency, drive train losses and electrical losses) then the power from the battery/solar panels to do 5kts will be around 238 watts. To this has to be added a margin to allow for wind, manoeuvring etc. Doubling the cruise power requirement for sizing the system maximum capability is probably a good idea to give a big enough buffer of power in reserve for a low power boat, so making the total power capability required around 500 watts. This doesn't match the "1000 watts per ton rule" very well, in fact it is around 2000 watts per ton, so illustrates why it is useful to use a pretty good estimate of hull resistance as a starting point.
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> Jeremy
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