jet boat conversion

Had the notion (so i’d like to know where.. you got the notion) to build an electric jet boat. currently in the brainstorming phase but all i’ve been able to generate so far are some light winds.

have an 18’ fiberglass speedboat hull with a jet pump and no motor. lots of these jet boats run a big block v8 that is directly connected to the impeller in the pump. typical motor torque may be around 450 ftlbs and they run around 4500 rpm wot. 50 gallon fuel tank.

i would say a v8 bb engine weighs 700 lbs and fuel would be 300 lbs.

the first thing i wanted to figure out was if i were to replace 1000 lbs of v8 and fuel with 1000 lbs of comparable electric motor, controller and batteries, how long (minutes) this setup could expect to run at full power.

does anyone know how to figure this out!!?

I use a number of 20 pounds per horsepower of motor and one hour of full power running. This is for LiFePO4 batteries.

So your 1,000 pounds gives you 50 horsepower to work with, without either allowing more weight or cutting runtime.

But 50 horsepower is nowhere near the ~400 horsepower of that V8. Batteries are heavy is the takeaway.

The part of that '20 pounds per horsepower...' number that's for the motor is more applicable to outboards than inboards.

One thing to consider is that propeller propulsion is more efficient than the jet pump approach you're looking at. I don't know what the margin is between them, but it's worth considering. I realize that you're wanting to keep the jet pump itself, so less needs to be thrown away.

So just as a though experiment, set aside the jet pump and go pure inboard. Big change, I know.

Say you could get a 100kW inboard motor that weighs 200 pounds. That might be optimistic; I've only dealt with under-40kW motors in my explorations (outboards primarily). So 800 pounds is left for batteries. In my experience, each kWh of LiFePO4 batteries will come to 15 pounds. This is before bus bars, cabling, fuses, disconnects, battery boxes, etc... So 800 pounds allows you a bit more than 50kWh of batteries. That's half an hour of full-power running. And you'll get maybe 120hp out of that 100kW motor.

It's challenging to try to estimate the top speed of such a combination (30-40mph?). There are various boat speed calculators out there. For sure all that battery weight hurts the result. And it might even be that the space those batteries take up on such a small boat (passenger area) is an issue. Of course you've freed up a bunch of space with the old engine and fuel tank (although it's not clear how accessible that fuel tank is for other uses).

I'd be interested to see how this progresses, if it in fact does. I'm facing similar issues with my builds, but my hull approach is considerably different, as are my target power levels.

Why?
Jet pump propulsion is terribly inefficient compared to the ancient propeller.
Your big pump can work on the most powerful engines, and is used for optimal swimmer / skier safety ( enclosed )
You can setup a boat with electric power and relax, or if you want speed good luck.