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post #1 of 9 (permalink) Old 12-03-2009, 04:50 PM Thread Starter
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off topic jet boat (electric) questions

Ok heres the deal my lake is electric only and the party whale barges that lurch along the lake (2-3mph) are not for me.  I have an idea to take an old 15 ft jetboat hull (lightest hull i can find that is a bow rider) and convert it to electric drive.  I am set on a briggs and stratton etec permanent magnet motor and 36v controller.  I have all the electronics worked out and correct wire gauge for draw.  played it safe all 2 gauge    My question is if anyone here can help me figure out the load on the batteries so I make sure I dont burn up the controller.  I will be using the jet drive that was coupled to the sport jet 90hp motor.  I already know i am not going to get up on plane but am hoping for 15 knots.  any input appreciated.
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post #2 of 9 (permalink) Old 12-03-2009, 05:44 PM
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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.
-Matt
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post #3 of 9 (permalink) Old 12-03-2009, 06:07 PM Thread Starter
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Re: off topic jet boat (electric) questions

Yea the batteries will be 48V (4x-12v) not 120V  There isnt a controller that will go that high I am talking dc not ac.  I can get 3k rpm on the etec motor at 48v but I will gear it down 1.5/1 to make max output around 1500 rpm at the pump shaft.  The gas engine was 90hp the electric motor will be about 8hp at 48 volts.  but insane amount of torque  The gas motor usually runs around 2,500 rpm so i should get 3/4 throttle with the etec motor howerver I dont know what the draw is.  If the draw is like 60 amps the batteries will be dead 5 min out.  even with marine deep cycle.  I can get 200lb thrust electric trolling motors  that will push her at about 7-8 knots but I was hoping to use a high rpm motor direct drive to the existing impeller.  was hoping someone might know.  the boat will do 40 mph at full throttle so i am hoping for 15-20 at 3/4.
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post #4 of 9 (permalink) Old 12-03-2009, 08:52 PM
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Re: off topic jet boat (electric) questions

The torque doesn't matter in this case, it is the horsepower that does. If the electric motor puts out 8hp your only getting 8hp, if the electric motor is spinning at 2,500rpms then its putting out around 16lb-ft of torque to get that 8hp, or 1/12th what the gas engine produced. So probably something like 1/6-1/4 the maximum speed, probably more like 1/4 or maybe 8kt if you can get about 35kt out of 90hp.

4x12v marine batteries with the same reserve of a car battery with a 60amp draw gets you to almost 4hp, so your looking at 120amp draw on 4x12v batteries to get to 8hp. If you can do 3 sets of battery arrays in parallel to get the draw down to 40amps per array you'd probably be looking at 20 minutes or so of battery life at those draw levels for 8hp.
-Matt
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post #5 of 9 (permalink) Old 12-04-2009, 11:03 AM Thread Starter
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Re: off topic jet boat (electric) questions

Thanks looks like I wont be using the jet drive then. I can get 200lb thrust trolling motors on 24V *traditional prop" that will push this boat at those speeds 8-10kts according to thrust to weigh calculations I measured using manufacturers statistics on size and weight of boat. I really didnt want props due to the kids. but if I have to put in 3 banks of batteries even with removing the engine and tank etc. the boat will be too heavy. God I hate electric lakes. The 200lb thrust motors are also rated at 2hrs of constant use befor recharge. the marina said it will give a great thrust taking off but will still only max out at like 8-10kts. maybe i can even drag a donut behind the boat for the kids to sit in. wont be skiing lol.
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post #6 of 9 (permalink) Old 12-04-2009, 12:39 PM
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Re: off topic jet boat (electric) questions

In either case your still going to be limited to what the batteries can put out. Whether it is a 200lb trolling motor or a water jet if the batteries can only put out 3,000w for 10 minutes, then you only have 4hp for 10 minutes no matter what propulsion you are using.

Not sure the draw that a 200lb trolling motor has, but if it equates out to say 6-8hp then I'd think that 8kt is possible. It takes the cube of the horsepower to double your speed, so for half the speed of 35kt from a 90hp motor should take around 11hp or so to get you about 17kt...of course that is also if the water resitance remains the same and your probably going to be off plane which is going to increase it. I really don't have any equations for it, but I'd say if you can find a way to get around 4-5hp you should be able to get 8-10kt from the boat. Back tracking a little from what I was saying before (because I didn't bother to really do the math before).

4-5hp is still going to be around 3,000 watts, which is going to drain 2 12v 500w/hr batteries in probably under 10 minutes. Now if you had something like 3 of these setup in parallel you'd probably boost it closer to 45 minutes of course longer if you weren't throttled all the way up. 6 car type batteries isn't going to weigh the boat down too much (200lbs or so).

For safety, I sorta agree, but with a prop just make sure to cut the throttle when the kids are in the water right around the boat. If you're towing them they shouldn't be to near the boat.

Anyway, with more thinking you can probably do it, but you're still going to need a pretty sizable battery array and aren't going to get anything like the speed you would with the old gas engine (though 8-10kt is a heck of a lot better then 2-3kt). Plus if the controller can handle the wattage, you can always up the array later, or if nothing else get more full throttle endurance by just adding on another pair (or foursome) of batteries.
-Matt
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post #7 of 9 (permalink) Old 12-04-2009, 01:01 PM Thread Starter
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Re: off topic jet boat (electric) questions

yea i went back and the motor is actually rated at
Torque constant: 1.14 in-lb/Amp (0.13 Nm/Amp)
12V to 48V (has been run up to 96V / 45 sec. max. / 25 hp)
72 RPM per volt (3456rpm @ 48V / continuous duty)

10 horsepower continuous duty at 48 volts.
Max motor currents: 330A for 2 minutes / 480A for 45 seconds.
10 horsepower max at 48 volts continuous duty!
14 horsepower max at 60 volts continuous duty with optional air cooling port.
19 horsepower max at 72 volts for 2 minutes only!
28 horsepower max at 96 volts for 45 seconds only!
but at 48V I should be able to make constant 8kts and have a reserve of 20 min. then keep a 12v system up as standby for the 2kt ride home lol. again i hate electric lakes.

i understand the electric drive would be less powerfull but i figured if i can keep the rpms the same it would get about the same performance
i am going to gear it dow too because peak is almost 3500 rpm with a pulley drive I am hoping to max out at 2500 rpm so the acutual relative torque will increase as well.

thanks for all your help. and brainstorming.
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post #8 of 9 (permalink) Old 12-04-2009, 03:24 PM
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Re: off topic jet boat (electric) questions

Sure thing. You could if you wanted to make a 60v power pack and run it at 48v normally through the controller, but step it up higher if you wanted a little boost temporarily.
-Matt
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post #9 of 9 (permalink) Old 12-04-2009, 04:17 PM Thread Starter
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Re: off topic jet boat (electric) questions

[quote author=azazel1024 link=topic=162136.msg3443506#msg3443506 date=1259954675]
Sure thing. You could if you wanted to make a 60v power pack and run it at 48v normally through the controller, but step it up higher if you wanted a little boost temporarily.
-Matt
[/quote]

ahahaha I will run 72V and put a momentary switch to a 200amp relay and mark it "nos" or "turbo"!!!!! After I get it up and going I am planning on using the impellers water port for cooling (the factory motor is cooled by lake water) wrap a few rotations of aluminum tubing around the body of the motor to transfer heat.
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