Join Date: Aug 2005
Location: Ellicott City, MD
Re: off topic jet boat (electric) questions
If you can get up to 15kt you should be able to get up on plane. My father-in-laws 17ft center console can get up to about 25kt and at around 12kt it is just getting up on a plane. If you can really get up to 15kt with a 15ft depending on the shape of the hull and how much those batteries are weighing it down I think you could just get up on a plane. If you can't get up on a plane that is going to restrict your speed a lot from water resistance. I'd think if you really can get 90hp out of the controller/batteries then you'd be able to manage 15kt, my father-in-law's boat has a 150hp Yamaha on it to get up to ~25kt.
Unfortunately I can't help with the controller issues.
I can tell you that for 90hp you're going to need around 69,000 watts of power (69kw). For 69kw using lead acid batteries, depending on the voltage the controller can handle (I'd assume no more then 250v) you'd want the lead acid batteries in either 10 or 20 battery series to get 120 or 240v from them supplied to the controller. At 120v and no more then 20amp, that only gets you 2.4kw of power, no where near the 69kw you'd want for a full 90hp. Considering the weight of batteries I think you're going to be hard pressed to put together an electric powered boat with any kind of real speed.
A car battery of average size has around 500w/hr based on a reserve capacity rating (25 amp draw, a little more then the 20amp I used as an example). So a single array of to achieve 120v with car batteries will net you a total capacity of about 5kw/hr based on the reserve capacity, able to put out around 3kw or about 4hp for around an hour and a half. You could certainly up the draw, but at 10 times draw you're probably only going to have about 1/3rd the rated capacity (the higher the draw, the lower the actual total energy in the battery), so you could maybe manage 40hp out of the setup, but you'd drain the batteries dry in probably 3-4 minutes.
The 240v setup would get you around 8hp also for about an hour and a half, or around 80hp for 3-4 minutes. A 240v setup with car batteries is also going to probably weigh about 600lbs. The lightest weight batteries compared to the best reserve capacity is probably still going to run you over 500lbs for that setup.
Your 15ft boat is probably only rate to handle something in the 800lb range for engine and passengers. So the electric motor, wiring, controllers and battery array for a simple 240v setup is going to be pushing that weight with just the boat captain.
A dual 240v setup (IE 40 batteries, 2 sets in parrallel of 20 batteries in series) would be pushing half a ton, but that would at least get you around 16hp for an hour and a half (probably enough for 4-5kt) and you could goose it up around that 90hp maximum of the motor for at least a few minutes (5-6 minutes).
There is a good reason that those party whale barges don't go very fast. Lead acid batteries weigh a lot and don't deliver a lot of stored power. Gasoline is several times more efficient based on volume and many dozens of times more efficient based on mass then lead acid batteries for storing energy.
If you could score the Li-Ion batteries to power your boat you'd be better off since they have better then twice the storage efficiency of lead acid, but even then your still talking fairly limited endurance (maybe call it an hour) and not that high a power output (maybe talking 20-30hp in that scenario and again you could goose it, at the downside of reducing capacity).
Electric cars work because they have significantly less resistance then a boat. 20-30hp in a car weighing 3,000lbs can get you cruising at 60mph, 20-30hp in a 2,000lb boat might only get you going 8-9mph.