Electric Jet Ski Conversion

I am in the process of converting a XL800 Yamaha Waverunner jet ski into a kids friendly boat for my 10 year old son.

I have removed everything above the gunwales and built a forward deck, cockpit and rear engine compartment. The boat now resembles a small racing boat that you sit in and not on.

I plan to use an Etek or Lynch motor with a 48v system to power the boat using the original 155 mm jet pump.

The original 800cc Jet ski motor produced about 6500 RPMs and a maximum speed of 50 mph.

I have been told that the Etek motor will produce 3 times the amount of torque produced by a gas engine of equal size. So if the Etek produces 22hp then the torque would be equivalent to a 66hp gas motor. Jet skis with 66 hp engines can go about 30-40 mph.

I have also been told that the Etek will generate 72 RPMs per volt or 3456 RPMs (48v x 72 RPMs). I planed to use a belt drive system just to keep the engine out of the bilge but I could use a belt reduction of 2:1 which would give me the same RPMs at the jet pump as the original engine. I find it hard to believe that an electric engine can produce the same performance as it's gas guzzling cousin.

Can anyone give me an idea of what performance I can expect from this configuration. I don't need speed I would prefer longer cruising times but it would be nice for an adult to ride once in a while.

 

The weight of the batteries from a golf cart will put a end to the power system. Stick with gas. You do not have enough boat to support all the battery weight for the power and duration you want. Leave the original motor setup in for every good design reason. Limit the boat speed by limiting the throttle travel for him in the boat. As he gets safer you can give him more throttle travel. I am doing this on a 18' training boat for kids. 5.7L Merc/ shaft & prop. I will use a intermediate throttle shaft and bellcrank with different holes in it to go from 10% to full just by moving clevis pins around.

 

Weight of Batteries

I considered the weight of the batteries.

The weight of the 26 gal gas tank, oil tank, engine and excess superstructure is far more than the weight of the 4 batteries and the 20lb electric engine. The boat will only be carrying 1 perhaps 2 youngsters and not 3 full grown adults.

 

You were not given accurate Etek motor information. Enclosed motor is rated at 48vdc @ 60amp = 3.8hp or open motor frame of 48vdc @ 150amp = 9.4hp. You could get bursts of more power if you don't mind cooking the motor a little and risking a fire. No way will the motor and batteries ever come close to a gas motor in any application. Electrics do not come close to a gas job. I hope you still have the motor and controls. I loved electrics in factories.--------------------You can verify this by web search of--- Etek motor--- Pick the Briggs and Stratton site. It's there.

 

I know you have your own idea about this little kid boat.But i have a idea how about geting a 60 or more pound thrust troling motor wack the stem off of it and jam it up the nozel of the jet pump then you could stere it with the handel bars. all parts could be had localy and you would be done in a weeked.the boy will be bored with it in less time it takes to build anyway.well good luck :idea:

 

If we use your number of 9.4hp continuous that is still quite a bit of HP for such a small boat. Briggs and Stratton also says that the Peak hp is 15.

Having said all this. The Etek will also generate 72 rpm per volt and I have heard estimates that it will deliver 2-3 times the torque of a internal combustion engine.

If I use a 2:1 belt reduction. How will this effect my performance? My ultimate goal is to be able to run longer and not necessarily faster.

 

You are trying to get the Etek motor horsepower and torque up to the gas jobs. Not going to last at those power levels. At 15 hp you need a 48 vdc speed control that can handle 225 amps. continuous. Not cheap. You should have a severe overheat problem. Dc motors can easily generate 20 X the rated torque at 20 X the full load current. Gas engines stallout with a jam. Dc motors pull more and more current, some can go to 40 X full load. Now you are in a fire area and unlimited twisting forces area also. Life is now 1 to 5 sec and a major motor and or controller fire. You will need forktruck controls and fuses with people 3-5" away. A REAL FIREPROOF wall is mandatory. Your kid is going into this boat ALONE. Do something safer. The dangers are out of your control. I have not listed all of them. This is dangerous jury rigging by anybody.

 

I'm not actually !

I'm trying to determine what configuration will give me the best performance with the least strain on the engine. I want the battery charge to last as long as I possibly can.

This is why I have asked about gear ratios and torque. If I can use some of the additional torque to drive a 2:1 gear ratio and get a little more speed with out draining the batteries faster then I will be happy.

Because the boat is for a kid the fastest I would ever want it to go is about 10 mph. I will use the controller like a govenor to keep the speed down and maximize the battery charge.

 

A jet-ski is by nature a planing hull, and a very inefficient one at that. It's designed to jump wakes and slam into waves at 50 mph. There's a reason nobody ever goes slow in these things- the hull's simply not designed for it, and is not very manoeuverable at low speeds.
Your proposed electric setup would be perfect for a pure-displacement hull of about twice the length of your jet-ski, something like a pair of canoes made into a catamaran. In the jet-ski hull it is unlikely to give you your desired performance, and is downright dangerous.
Whatever this motor puts out- 3hp, 8hp, 12hp, whatever- it is well below the threshold needed to plane the boat. On your ski that would take a minimum of 10hp to begin to climb 'over the hump', and it will be sluggish and unresponsive with any less than 25.
Cramming all those lead-acid batteries into such a small hull, so close to a high-power motor, is a recipe for disaster. If you enclose the battery bay, hydrogen will build up inside; if you don't, water will come in. I've worked on electric vehicles a fair bit and the danger posed by a large battery bank must not be underestimated. There's a lot of very difficult engineering problems involved in setting this up, that could be very dangerous if neglected.
You're correct that an electric motor produces more torque than a gas engine of equivalent hp and rpm. What that means is that your motor/pump set will reach full rpm and flow faster. It will NOT propel the boat faster than a gas engine of the same hp.
I'd take the advice of others who have responded here- put the original gas engine in, and put a flow restrictor or throttle stop in to keep speed down. Putting an electric setup in a boat like this is not a wise or safe endeavour.

 

The jetski hull and pump were free which is hard to beat. I wanted the water jet for safety more than any thing else.

The jet ski is a very stable foam filled boat that is ideal for conversion into a kids "Putt Putt". It has now been rebuilt so it now looks like a really cool little racing boat.

I think that properly ventilated batteriers are always going to be safer than the same boat filled with 26 gallons of fuel and a hot engine.

 

You now know most of the feelings of others. Do as you please, and safe boating.

 

on second thought get the boy a real boat he can grow with like a little whaler or a gheeno.get a 8 D battey like i had as a kid with a troling motor .and it will run all day long no lie.and then the boy can get a 6 hp-15hp-25hp and then he has graduated.and he will ask you for a biger or better boat not a better damn jet ski.

 

In response to your question:

For electric motor input: Power (watts) = Amps x volts

For motor (any kind) output:

power (watts) = torque (Newton-meters) x RPM x .10472
and: 746 watts = 1 HP

An Etek is about 87% efficient at peak efficency, meaning you'll get 870 watts out for every 1000 watts you put in. (Gasoline engines are about 25% efficent.)

A given motor will operate at a given RPM and torque, generating a given power. If you use a belt or chain to DECREASE the rpm of a driven shaft by a factor of 2, you will INCREASE the torque by a factor of 2. If you INCREASE the rpm of the driven shaft by a factor of 2, you will DECREASE the torque by a factor of 2. Belts and chains WILL NOT change the power, just the rpm and torque components of power.

A propeller or jet-type impeller is designed to operate at a given "advance ratio", or ratio of forward speed to rpm. If you know the forward speed and rpm of your impeller under gas power, you will want to aim for the same ratio with electric drive. You won't have as much power under electric as under gas, but that's OK, since you don't want to go as fast.

As a rule of thumb, the SPEED of a boat varies with the CUBE of the amount of power. So if you want to double the speed, you'll need 8 times the power. Since the Yamaha 800 has a peak HP of 120, and the Etek has a peak HP of 10, you therefore should expect to have 44% of the top speed under gas. And since the impeller is designed to operate at a given advance ratio, you should turn the impeller 44% the rpm of the Yamaha at 120 horsepower -- which means you should turn the impeller at 2840 rpm with the Etek.

But the Etek at 48 volts is already running at nearly this speed -- between 3500 to 3100 rpm, depending on the load -- so there should be not much speed reduction at all between the Etek and the impeller shaft. In this example, a 6:7 ratio would work just fine, if you can find pulleys that size. If not, a no-reduction setup would work nearly as well.

But I'd really recommend running the Etek at 24v instead of 48v, which is safer and will double the battery endurance; and also at more moderate amperages than peak, which will mean much less heat and less chance of fire. Both of these will mean lower power too. For example, at 24 v and 55 amps, the Etek is at nearly its peak efficiency, and producing 1.5 HP. The rule of thumb says that would give you 23% of the top speed under gas (about 15-17 mph), but here's where rules of thumb break down, since the boat will probably not be on plane at that speed, and therefore will run slower -- say 8 or 10 mph. (That still may be plenty fast for a 10-year-old.) At 15 mph, that would mean an impeller rpm of 1508. But since the Etek operates at 1571 rpm at 24v, once again you don't need much of any speed reduction. But if you're not on plane and only getting 8 mph, you might try reducing the impeller rpm by as much as 2 from the motor.

Of course, all the usual safety measures should be taken: proper fusing, circuit breaker, and use of a motor controller. NEVER wire a DC electric motor directly to a battery!! This is a sure-fire way to cause fire for sure!

One final note: DO NOT use standard lead-acid batteries, since there is chance of leakage and/or spillage when the boat capsizes (and it will, sooner or later). AGM (Absorbed Glass Mat) batteries are the way to go, they are spill-proof.

 

I put together an electric boat last year, from a 14 ft O'Day Javelin, turning a 14 x 17 prop, using two 125 Amp-hour deep cycle marine batteries. The boat works fine (at sailboat speed) even with 4 adults aboard. Endurance is about 3-4 hours. I think your project would be satisfactory, but you should design for just a few horsepower, you will not get a fast boat with the kind of power realistically available in the Etek motor. We enjoy the electric, no noise, no vibration.

Fred Rosse

 

very interested

hello i am the owner of enve marine, we build a 10, 12 and 20ft hovercraft and are getting into larger custom yachts that are hybrids, I believe their to be a market for what you are doing with this electric drive, I believe you need lithium polymer batteries which have have great connections for,, if you can get one proven , I am sure we could do something on a larger level, we are looking to mass produce a electric waverunner , with performance and good battery life. Environmentally friendly ,, respond back and well get to talking if their is interest,