Golf cart motor for small boat.

Discussion in 'Powerboats' started by xx123j, Nov 24, 2009.

  1. xx123j
    Joined: Nov 2009
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    xx123j Junior Member

    Hey all,

    I'm very new to the boating world, in fact I had a hard time answering the 'no spam' questions for registration here haha. I'm very good at building stuff, but am absolutely clueless when it comes to boat design.

    I was wondering if anyone could answer a few questions I have. First, I plan on building a boat that uses an electric motor from a golf cart. I know this is possible because of this company uses the same method. I'm shooting for this to fit 3-4 people. The smallest boat that company builds holds 7.

    I was wondering what kind of hull I should be looking at?

    Can you guess the length the boat will probably have to be?

    How do I go about figuring out the shape of the hull?

    More questions to come.
  2. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    I once used a washing machine motor for a 24' sailboat. This was long before it was politically popular to do so (late 80's). In reality, I was too cheap to replace the Atomic 4, so I yanked it out, used the shaft and 4 batteries. I sucked down two on the way out, then two on the way back (24 VDC motor), they charged at the dock. The boat was only used a few times a week to tune up for weekend racing and I just need to motor down a short river to the gulf.

    As far as designing the hull yourself, you have much to learn and frankly, it would be a whole lot easier to just build a known design. I say this because . . .

  3. Guest625101138

    Guest625101138 Previous Member

    What speed do you hope to achieve?

    How far do you expect to go on a set charge?

    Rick W
  4. Tim B
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    Tim B Senior Member

    I would suggest that the simplest way to achieve what you want would be to find a cheap sailing dinghy, and use that to test the concept. That will give you a hull-form that is a reasonable size (say 14 feet long) and shape. Dinghies tend to be fairly easily powered, so that means a (fairly) small motor and light battery setup. Remember though, that the batteries and motor and controls will probably weigh about the same as one person.

    You might want to look at the motor control system and rudder control system (is a tiller with a throttle lever ok?) in some detail, as the hull is rarely the difficult bit to design.

    A small electric boat is quite possible. I have looked at the feasibility myself, several times. At the moment, the difficult decision is with the battery technology you use. LiPO is light but expensive. Lead-Acid is heavy and cheap. The decision here is forced by a few factors:

    What range do you want and at what speed?
    How much weight can you carry? (remember that more weight = more drag = less range)
    How much are you willing to spend?

    Hope this helps,

    Tim B.
  5. xx123j
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    xx123j Junior Member

    Thanks for the replies. I see what you mean Par. I suppose I'll build a well known design. Any recommendations?

    Par, How fast did you get in that washing machine powered boat?

    I don't expect to go very fast, hopefully equivalent or fast than rowing speed, and won't get very far, just a few hours per charge. And since it'll be small I hope that takes strain off of the motor.

    I think it'll be 48v motor with what I'm guessing to be 8 batteries. My plans aren't final.
  6. xx123j
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    xx123j Junior Member

    As for weight, I'm not looking to carry much. Just 3-4 people and if it fits, a small water cooler.

    My budget is open.
  7. Guest625101138

    Guest625101138 Previous Member

    The speed will be 4 to 5 knots. This means you need something at least 4m long to avoid significant wave making. It will not take much power to do that speed. Lets stab at 500W - more than enough I expect.

    You could use quite a slender hull that will improve efficiency because the batteries will help stabilise it. A large canoe would be most efficient. Something about 3ft beam would offer adequate stability using the battery weight as ballast.

    Lets say taget speed of 5 knots.
    3 people weigh 220kg
    boat weighs 70kg
    motor controller and batteries weighs 100kg

    Hence total weight is say 400kg.

    Taking these figures the lowest drag hull is 8m long and 550mm wide. It will take 100W on the hull to do 5kts.

    If you constrain length to maximum of 4m and beam to minimum of 1m then the lowest drag hull with these constraints requires 360W on the hull. This will end up at about 500W on the motor shaft allowing for prop losses.

    You might find wandering through this thread of interest:
    There is a lot of information you might find of benefit and some test data on different aspects.

    This thread may also be of interest to see how long slender hulls are being incorporated into more practical hulls:

    Rick W
  8. PAR
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    PAR Yacht Designer/Builder

    Actually, I did a little better then 6 knots in flat water and calm. I rarely ran at full speed though because it ate the batteries quickly.

    As far as controllers, batteries, motors and other choices, there are much better qualified then I to speak up.

    The hull choices are all over the place. The canoe idea Rick has is a fairly good thought, though if any of you guests are bothered by "tippy" canoes, a skiff may be a better choice. The skiff should be narrow to get the best from her, considering the limited propulsion.
  9. xx123j
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    xx123j Junior Member

    I see what you mean Rick. I understand that a canoe would be most efficient, however I cannot use one. As PAR said, guests would not want get on haha. I am extremely interested in the "faux-tri" hull, the idea blew me away. However, I have to consider size. To be honest I'd prefer it shorter, but I know it won't be as good the shorter it gets. If there was a way I could make a faux tri into like a few components that come together, it would be more space efficient and a lot better for me. I know it would complicate things, so I'm entertaining the idea.

    I'm not sure how HP or any type of motor power converts into watts, but this is the exact motor I'm planning on using. Its rated at 7HP at 4200rpm. So I did a conversion through google and got this, 7 hp = 5 219.8991 watts. I'm thinking that's incorrect, correct me if I am wrong. However, I'd like to point out though that Mike Menezes was able to run a 590kg curb weight car at 45mph top speed.
  10. tkk
    Joined: Jul 2009
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    tkk Junior Member

    Your hp to watt conversion is correct. That is a big motor, much bigger than you need and thus also heavier. I donĀ“t know about the efficiency of an electric motor when run at less than 50% of max, but I suppose you would get better mileage with a smaller motor running more or less 100% power.

    On the other hand, some extra reserve power for nasty current, head wind or an emergency need to get out of the way of something big might be handy at times.
  11. xx123j
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    xx123j Junior Member

    Yeah, I'd really like the headroom just in case. It's about 27kg. I'll pick out some batteries to go with the motor so I can get a better idea of how much it'll weigh if I actually use it.
  12. Guest625101138

    Guest625101138 Previous Member

    When I said canoe I was referring to the underwater shape. You can get an idea of what a boat with this shape can look like if you go through posts #186, 191, 192, 196 on this thread:
    All those are reasonably easily driven. The proportions you are looking for are reasonably slender WL beam of about 1m on at least 4m WL length without a wide submerged transom. There are quite a few designs that will have these qualities.

    As noted the weight of the motor is an issues if bigger than needed. A typical car battery may give maybe 50Ah. So allowing for voltage drop you will get about 500Wh from a typical car battery. If you have a 7HP motor running up near rating it will chew 5000Wh per hour. Hence you will need 20 car batteries to get 2 hours operation. (By the way car batteries are not recommended for deep cycle use.)

    You can see that is a lot of weight. So while the motor has the ability to make big waves it is wasteful and will take a lot of energy to go anywhere. It is best to operate at a speed below significant wave making. That speed depends on the hull length and its slenderness.

    A catamaran would also be something to consider but I do not know of any standard designs that suit your requirement. These get you narrow hulls and are stable.

    Rick W
  13. portacruise
    Joined: Jun 2009
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    portacruise Senior Member

    With an open budget, there are many possibilities for increasing speed, range and carrying capacity by improving efficiency. You can do much better than a series wound motor to start. If you look at the solar boat post by Dr. Bob, you will find a brushed motor that is up to 93% efficient, compared to maybe 60% for your series wound. Then if you use Lithium batteries for power, the weight savings will allow for a smaller boat of same capacity. There have been some good suggestions on hulls already.


    QUOTE=xx123j;318145]As for weight, I'm not looking to carry much. Just 3-4 people and if it fits, a small water cooler.

    My budget is open.[/QUOTE]
  14. TollyWally
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    TollyWally Senior Member

    Not to mention the obvious but copy the Duffy. I've rented those before when touristing in SoCal, they work well for their intended purpose of modest daytrips with several people aboard. I rented one in Costa Mesa, youu're not supposed to go outside the harbor. LOL, I can say with confidence that they can handle modest conditions in the ocean on calm days :)

  15. kerosene
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    kerosene Senior Member

    as rick pointed out 7hp is way too much. The battery technology simply isn't there for for having compact power storage for several kwhs (1000w power for an hour). Prius has 2kwh battery pack just give perspective - that can give you 2000w for one hour - not much.

    Also having plenty of spare capacity in an engine is rather useless as displacement hull will have max speed defined by its hull shape and length - adding more power will only make more waves with very little increase in speed.

    what kind of waters are we talking about here?
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