Lots more good info, Jerry. Your numbers as well as Thomas' match my experience with my 24V system pushing my 2,000 pound boat, which is on the efficient side with it's narrow, 18' double-ended hull.
I also found David Gerr's equation quite useful and I used it to calculate power requirements in my book on electric propulsion, even though his equation is mostly based on empirical data. Your data confirm that David's equation seems to work well. I normalized the equation to plot power requirements per 1000 lbs of displacement. What I found was that it takes 1500 watts to push 1000 lbs of displacement to hull speed regardless of the length of the boat. For some reason I can't put a picture of the graph here, so this table summarizes the data per 1000 lbs of actual weight on the water.
50% hull speed = 200 watts
80% hull speed = 700 watts
90% hull speed = 1050 watts
100% hull speed = 1500 watts
At first it seems odd that the results do not have the LWL in the final summary. But, it make sense on second thought. A long narrow 1000lb boat (image a 30' rowing scull with 5 people on board) will take less power per mph than a 1000 lb 10ft scow. If you normalize the equation to hull speed, the LWL cancels out.
Thanks for your comments, Dave and Thomas. It's clear that keeping the displacement and weight low is a key element to low-power operation. It's not too surprising, but when you can actually measure the power (unlike with a gas or diesel engine), it really drives the point home. And it's always kind of amazing when the theory jives with reality. Now I'm working on trying a different prop -- bigger diameter and lower pitch -- to see what happens. Stay tuned!
Lots more good info, Jerry. Your numbers as well as Thomas' match my experience with my 24V system pushing my 2,000 pound boat, which is on the efficient side with it's narrow, 18' double-ended hull.
Thanks Jerry,
I also found David Gerr's equation quite useful and I used it to calculate power requirements in my book on electric propulsion, even though his equation is mostly based on empirical data. Your data confirm that David's equation seems to work well. I normalized the equation to plot power requirements per 1000 lbs of displacement. What I found was that it takes 1500 watts to push 1000 lbs of displacement to hull speed regardless of the length of the boat. For some reason I can't put a picture of the graph here, so this table summarizes the data per 1000 lbs of actual weight on the water.
50% hull speed = 200 watts
80% hull speed = 700 watts
90% hull speed = 1050 watts
100% hull speed = 1500 watts
At first it seems odd that the results do not have the LWL in the final summary. But, it make sense on second thought. A long narrow 1000lb boat (image a 30' rowing scull with 5 people on board) will take less power per mph than a 1000 lb 10ft scow. If you normalize the equation to hull speed, the LWL cancels out.
Thanks for your comments, Dave and Thomas. It's clear that keeping the displacement and weight low is a key element to low-power operation. It's not too surprising, but when you can actually measure the power (unlike with a gas or diesel engine), it really drives the point home. And it's always kind of amazing when the theory jives with reality. Now I'm working on trying a different prop -- bigger diameter and lower pitch -- to see what happens. Stay tuned!