Small Inboard Electric Lake Boat

With due respect to your engineer friend, the numbers you have quoted are only for a narrowly defined set of input factors. to make the boat plane cleanly it must have sufficient bottom area and sufficient dynamic pressure to hold the boat up near, not necessarily at, the surface of the water.

Unless you boat is a feather weight you are not likely to have it plane at 5 knots. ( I think that we would have to define the meaning of Plane in more precise detail before we get into a debate. That has been the subject of discussion on many occasions here on the forum and elsewhere)

A 10 foot boat with two adults and some machinery is not going to be a lightweight. Your best bet is to go ahead and consider that your little boat will be a displacement type. So lets try to make it as efficient, in terms of power requirement, as we can. Take a look at small dinghys that are to be propelled with economical input such as rowing or by very small outboards, say 2 horse Honda or similar. Those boats invariably show that the bottom is curved upward as it goes from somewhere from about midships toward the transom. It is curved upward an amount that is sufficient to raise the bottom at the intersection of the transom at or above the waterline. If you fail to do that then the immersed transom, as with the straight bottom, will drag a bunch of turbulent water behind it. It sucks!

OK now lets go really fast. We have a planing boat, it has a straight aft bottom profile. The boat goes fast enough that the water does not fall back onto the back of the transom. It has gobs of power input. Now it does not suck. All the while the difference between displacement operation and planing operation is largely dependent on power to weight ratio.

 

I presumed all along that welding would be in order, but while I was focused on thinning the material to save weight, I forgot about the weldability of the material. Thank you for the tip (reminder).

 

I’m not against this being a displacement-only boat. Or, for that matter, choosing a different motor/battery setup. I want to stay with 48V, for safety reasons, but different batteries can provide higher current, therefore more power (at the top end). Of course, it’s unlikely that I’d get more than 10 kWh in the same space, so if it was set up to do 400A of current, doubling the power to 16 kW, it would only go flat-out for about 36 minutes, which still might be fine.

 

A useful discussion of rocker is only a few clicks away. Go to Aerodynamic and hydrodynamics tab, scroll down to the subject: Quantifying drag of rocker. There is some good and useful information in that thread.