Mateo
The 72V is nominal for the lithium battery. They would operate closer to 80V.
Power demand is well approximated to the cube power of speed for slender hulls. My estimate is 4kW will give 5kts. You could only get 30kW combined if you are working on the 72V system voltage because you will be current limited on the motors and probably the controllers. Assuming the props are pitched to enable full 30kW (which I doubt) then you could expect 8kts with 30kW - maybe a little more. You may hit the current limit of the motors before reaching the power limit. Meaning the motors cannot rev out.
Speed is very sensitive to surface finish. Wave drag is negligible due to the slenderness of the hulls meaning viscous drag dominates. A poor paint job could double the required power to do a particular speed. So your focus needs to be on getting the hulls nicely faired and smooth then apply a hard antifoul suitable for racing yacht. Attention to detail on the surface finish and keeping it that way is equivalent to tens of thousands of dollars on sails and electric drives.
The bollard pull becomes the important consideration for getting out of trouble in heavy weather. I really need the prop pitch as well as diameter to calculate the bollard pull. Given the high prop rpm I will take a stab at the prop being 18X12. In this case you could expect a bollard pull around 1600N on each unit. The motors will be at their current limit running around 1000rpm. From this perspective there is no advantage in going to the higher voltage battery apart from the current in the battery circuit being lower than the current in the motor circuit.
If you were to get the prop pitch I could be more precise with the bollard pull. The supplier may have bollard pull data. This link has an interesting article on various sailing boat props:
I expect the results surprise many people. Note that the tiny Flexfold 2B produces the 3rd highest thrust of the 15 tested. It also gave the best top speed. The images for the props on the Starboats site suggest the blades are much bigger than needed for best performance on an easily driven boat. The nylon props we have on the 18m proa are 540mm diameter with relatively low blades area ratio and maximum speed is 700rpm. Even with that low speed they still bog the motors down under bollard pull conditions. When hulls are clean they run out close to full rpm and full power at 8.5kts.
With combined thrust of 3200N you might make headway in 50kts wind depending on cabin, rig and deck aerodynamics.
Rick
Ok thanks for the feedback. I should have linked to the genset pdf for more info, here it is. http://www.polardcmarine.com/downloads/PDC-8340VP-40_brochure.pdf. It puts out 48 to 320VDC. The 12 or 24V refers to the regular alternator output for charging the starter battery.
That e-tech pod puts out 10kw if supplied with 48V or 15kw with 80V. I assume 72V would work too.
How fast do you think a combined 30kw would push the boat? I guess you could max out like that for about an hour with batteries plus genset, then power on via genset only. Then what is your estimate for speeds at 20kw with the 18" props at 1200 rpm? Probably faster than necessary but thinking more about power against headwinds in freezing spray.. and currents, at night.
The tube mount that Rob has devised will allow vertical raise/lower and have a tiller for 360 degree steering, mounted on the back of the cockpit bench. Not sure where the other pod will go yet.
I agree more solar is better, and there is plenty of space on the ww hull for 2kw. Was wondering how to string it all together. I guess thicker wires will do it. What brands/shop do you recommend for marine flexible panels these days?