Interesting, but it just shows how grossly inefficient these motors are. For reference, my first prototype (far from optimised) electric drive propelled a Storm 15 to close to hull speed on 350 watts (a little under 1/2hp) electrical input power. I will happily bet that the CBL will only need, as an absolute maximum, around 1hp to get to hull speed when driven by a motor with a properly matched propeller - in all probability it needs less than this.
I have spent a great deal of time and effort on measuring hull power requirements, optimising propulsion systems and am overall pretty horrified as to just how inefficient most commercially available outboard motors seem to be. Some of the experiments I have conducted prove, beyond any doubt, that prop efficiency rarely exceeds 50% for any off-the-shelf outboard and most struggle to get better than about 40%. Simply matching the right prop to the hull will make a substantial improvement in almost every case (even small differences in diameter and pitch make large efficiency changes); customising the prop will reap even bigger dividends (albeit at the expense of things like robustness and weed shedding ability).
I am not alone in discovering just how poor most off-the-shelf outboards are in terms of efficiency. Others have similarly performed careful measurements and experiments and come to the same conclusion. It is one reason why electric propulsion systems can deliver the same performance with much lower power levels - when power is an expensive luxury you have no choice but to look carefully at optimising efficiency in every aspect of the propulsion system.
Finally, as an illustration of just how inefficient most outboards are, here is a quick and dirty comparison with the power available from the sails needed to drive the boat to show how little power makes it from the engine to the water. Assuming the boat has a sail area of 10 sq m and is sailing downwind at 4kts in a 10kts breeze (so 3m/S relative wind). The force being generated by the wind on the sails is about 0.5 x 1.224 x 1.2 x 10 x 3² = 66N (assuming an optimistic drag coefficient of 1.2, it will probably be closer to 1 under real conditions).
66N at 4kts (2m/S) is a power of just 132 watts, or less than 1/5th hp. If a 4hp engine is delivering 1/4 power at the shaft (and they are shaft rated normally) to drive the boat at the same speed then the propulsive efficiency is less than 20%, not untypical for some outboards.
If the boat sail area was double the example above, 20m², then that only increases the power from the sails to 264 watts, around 1/3hp, so it still illustrates just how grossly inefficient outboards can be.
Jeremy
