# Ski Boat Conversion?



## madderscience (Jun 28, 2008)

I know nothing about boats, so I will chime in. 

Your biggest challenge isn't going to be the mechanical conversion (I imagine hooking a big honkin electric motor to the stern drive in place of the ICE isn't any worse than adapting one to a car transmission. It also isn't going to be fitting batteries, I imagine there is lots of room for them in a boat.

Your biggest challenge is probably going to be the engineering work to figure out how much battery you will need to get some amount of run time, and ensuring you get the performance you want. How many gallons per hour does your boat use? What will that equal in KWH per hour? How big of a battery do you then need for X hours of run time, plus safe margin? How many horsepower are you putting out at cruise? Will the EV motor and drivetrain be able to sustain this? how many HP does it take to get the boat up to speed? Does the EV drivetrain have the same peak power? How much more power might it take with the extra weight of batteries?

I imagine so far as controllers and components go, a DC motor and a high power controller like a soliton (available!) or zilla (not available right now but hopefully soon again) would work. You will probably want to look at waterproofing and weatherization and such of all the EV components and the batteries, especially if you will be running in saltwater. Doubly so if you want a high voltage (greater than 144v) system, which is pretty much a requirement for high performance. This might of course present a cooling challenge. As for the battery, you will probably discover that you will need a rather good sized lithium battery pack to do the job. Ski boats aren't built for getting good mileage.

For safety and reliability, you might also want a secondary "emergency" battery pack and controller that you can switch overt to, so you could power yourself back to the dock in the case where something goes wrong with the main system or you just run yourself out of juice. Assuming you are zooming around only a couple of miles from the dock pulling a skiier, a 72V secondary system would be fine and would give enough power to putter back to shore. You could switch the motor over with a set of contactors, or just a big ass anderson connector. of course a little ICE outboard would work too for backup propulsion.

I suppose a series hybrid boat could work too. get a diesel generator and use it to top off the battery pack. Might work better in a boat than in a car due to extra space and easier to set it up for constant running. Still would add a lot of complexity and engineering effort though. I know a few hybrid sailboat drive systems have been done but of course their goal is silence and efficiency, not pulling a skier.

Good luck.


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## ewdysar (Jun 15, 2010)

madderscience said:


> I know nothing about boats, so I will chime in.
> 
> ... Your biggest challenge is probably going to be the engineering work to figure out how much battery you will need to get some amount of run time, and ensuring you get the performance you want. How many gallons per hour does your boat use? What will that equal in KWH per hour? How big of a battery do you then need for X hours of run time, plus safe margin? How many horsepower are you putting out at cruise? Will the EV motor and drivetrain be able to sustain this? how many HP does it take to get the boat up to speed? Does the EV drivetrain have the same peak power? How much more power might it take with the extra weight of batteries?...
> 
> ...


I do know boats and the issues related to converting them to electric. 

Madderscience nailed the ski boat issue on the first try. How many gallons of gas do you typically burn in how many hours. A gallon of gas contains about 33kWh of energy and weighs about 6 pounds. Let's say you use 5 gallons when you take the boat out (I'm guessing more, but let's stick with this for now.) That's 165kWh of energy. But you get a break on this. The thermal efficiency of an ICE engine is about 25%, which means that only 1/4 of the energy in the gasoline goes towards moving the boat, the rest is dissapated as heat through the cooling system and exhaust. Electric motors can be above 80% efficient, so you only need 1/3 the usable energy from your battery pack.

So let's build a pack that will be just like putting 5 gallons in your empty tank. We need 55kWh usable so that means 68,750 total rating for an 80% DoD. We'll ignore Peukert for now because things are already looking bad. If we go with Lithium, the batteries alone will cost around $30,000 and weigh more than 1700 pounds. Lead acid will cut the cost but double the weight. That's for a tank that only holds 5 gallons of gas. You may not be able to pull a skier any more because of the ten extra people on board (lithium) that never leave the boat. BTW, how much does you boat weigh now?

If you can get past that, let's talk motors. Ski boats don't care that much about torque, HP is what gets the job done. Again you get to take that 3 to 1 advantage for electric. I'm guessing your engine is about 130-150 HP, marine engines are often de-rated for continuous duty, so all you need is a motor rated for 45-50hp continuous.

As far as a hybrid, it's even more money, weight and you lose efficiency creating, then using electricity from a ICE generator. Driving the prop directly saves fuel and delivers more power.

Sail boats are better candidates because the motor is used much less, the max speeds are below 10mph and the engines that we replace are 20hp or less. Plus most sailboats are built to carry more weight. I rarely used more than 1 gallon of fuel in a day of sailing, usually much less. 

Fair winds,
Eric
Marina del Rey, CA


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## bliksem (Aug 3, 2009)

there is an electric speedboat on the Metricmind website:
http://www.metricmind.com/boat.htm


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## ewdysar (Jun 15, 2010)

bliksem said:


> there is an electric speedboat on the Metricmind website:
> http://www.metricmind.com/boat.htm


There's other examples as well. Here's a site that comes up periodically on the electric boat board. http://www.seattleoutboard.org/electric.html 

I'm not saying that electic can't be done. But converting a Bayliner while retaining the performance and usability of the ICE drive is pretty difficult.

Eric


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## TX_Dj (Jul 25, 2008)

Yep, the biggest problem for the boat won't necessarily be the runtime/power, but the weight.

Let's say your ICE engine weighs 400 lbs (probably less) and lets say you have a 50 gallon tank, full, removing both of those would be about 750 lbs removed.

Now lets say you add back in 200 lbs of electric motor (probably less) and electronics needed to drive that motor and a 1700 lbs battery pack.

You just cut into your passenger capacity by 1150 lbs.

If your boat is 20-23 ft, that could be all or most of your passenger capacity.

You'll now be running like you've got a full load of passengers all the time, even when it's just you.

Now you need MORE power and MORE energy storage to try and make up the loss caused by extra weight (which is extra drag in the water). There is no way to make this up.

There are a lot of electric boats out there, some are built for racing. Few are "practical" in the same way as an ICE boat.

Not to mention, as others pointed out, you'll spend as much on converting the boat to meet your design goals as a brand new ICE boat would cost you, and the brand new ICE boat will give you years of service and will perform correctly out of the box.

Win some, lose some.


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## esoneson (Sep 1, 2008)

This thread had some interesting efficiency data.

http://www.diyelectriccar.com/forums/showthread.php?t=47329


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## mutt lover (Nov 18, 2010)

ewdysar said:


> I do know boats and the issues related to converting them to electric.
> 
> Madderscience nailed the ski boat issue on the first try. How many gallons of gas do you typically burn in how many hours. A gallon of gas contains about 33kWh of energy and weighs about 6 pounds. Let's say you use 5 gallons when you take the boat out (I'm guessing more, but let's stick with this for now.) That's 165kWh of energy. But you get a break on this. The thermal efficiency of an ICE engine is about 25%, which means that only 1/4 of the energy in the gasoline goes towards moving the boat, the rest is dissapated as heat through the cooling system and exhaust. Electric motors can be above 80% efficient, so you only need 1/3 the usable energy from your battery pack.
> 
> ...


Wondering what you think of this, Three of us go skiing for 1 1/2 hr, actual run time for boat is 35-40 min @ 32mph using maybe 4 gals of gas. Boat is a 94 malibu at 2800lbs with a 260 hp @ 4400rpm 350 chevy. 32 mph is about 31/3200 rpm (maybe 150hp? ). I'm thinking of using a older smaller lighter ski boat that would be maybe 1500lbs W/O engine. I was going to use those 850a 12v deep cycle batteries @ 65 lbs apiece. 14 batt = 910 + motor 200lb ? = 1100 lbs, chevy 350 with manifolds = 700 lbs ? So 850 x12=10200 x 14=142800 x75% discharge =35700 which is getting pretty close no ? Also have the option of going slow to and from the ski course 2 miles away shaving 7 or 8 min off hi speed time. Thanks for any imput


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## ewdysar (Jun 15, 2010)

mutt lover said:


> Wondering what you think of this, Three of us go skiing for 1 1/2 hr, actual run time for boat is 35-40 min @ 32mph using maybe 4 gals of gas. Boat is a 94 malibu at 2800lbs with a 260 hp @ 4400rpm 350 chevy. 32 mph is about 31/3200 rpm (maybe 150hp? ). I'm thinking of using a older smaller lighter ski boat that would be maybe 1500lbs W/O engine. I was going to use those 850a 12v deep cycle batteries @ 65 lbs apiece. 14 batt = 910 + motor 200lb ? = 1100 lbs, chevy 350 with manifolds = 700 lbs ? So 850 x12=10200 x 14=142800 x75% discharge =35700 which is getting pretty close no ? Also have the option of going slow to and from the ski course 2 miles away shaving 7 or 8 min off hi speed time. Thanks for any imput


I'm not sure where to start. The first problem is that the important rating for the battery is the Amp Hours, not Amps. A 65 pound deep cycle battery will be rated for about 120Ah, maybe less. This means that a single battery will hold 1440Wh.

Then why 14 batteries? Are you planning on a 168V system, or maybe 84V? 12 batteries, 144V or 72V, would be easier to find components for. But let's stick with 14 batteries for now. 14 times 1440Wh is 20,160Wh and using 75% of your rated capacity is 15,120Wh usable, or the equivalent of a little less than 1.4 gallons of gas out of 900 pounds of batteries. I would suggest 24 batteries (34.5kWh or 26kWh usable) at 144V for 1560 pounds that is equivalent to 2.4 gallons of gas. 

The next problem is getting the skier out of the water. I've been on boats with only 100hp that can't quite get bigger skiers up. If you use 150hp today, however briefly, then you'll need a drive that can deliver 150hp to get the skier started, so I would expect 780A at 144V (112.5kW)  for 10 to 15 seconds before you can back off the throttle and cruise. For just cruising around, you could get by with just a 50-75hp motor, but I doubt that you could launch a skier. 

Now let's look at your cruising scenario. 4 gallons in 40 minutes is 6gph. 6gph at 25% thermal efficiency is 66hp (6gph * .25 TE/0.0226). So lets use 50hp for your cruise power requirement. 50hp is 37,500W or 260A @ 144V. Because of the heavy constant load, the 24 batteries will go from completly charged to 3/4 empty in less than 20 minutes.

I just want to say that all of these calculations have been made favoring the electric drive, your results would probably not turn out this well.

I don't mean to be negative, but this project would be a lot of work and fairly expensive. You should have a good idea of what your results would be before you start your investment.

Good luck,
Eric


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