Electric Ski Boat

Discussion in 'Powerboats' started by Adman, Oct 17, 2019.

  1. Adman
    Joined: Oct 2019
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    Location: Canada

    Adman Junior Member

    New to the forum and first post :)

    I'm looking for a winter project with my Dad and few friends and we're kicking around trying to convert an older direct drive ski boat into electric. My back of the napkin calculations haven't told me that this can't work but the basis of the design hinges on the electric motor and battery - Obviously... I know!

    Motor - Electric motors have massive torque and efficiency benefits when compared with standard ICE engines but what I'm trying to understand is just how much power do I need to get the boat up on plane and pull some average skiers at ~30mph. When looking at standard ICE engines they're 350 HP and I know I don't need that much equivalent to move at 30-40mph but what I can't determine is just how high that number is? I can get a decent 134 HP/220lb-ft (at zero rpm) electric motor that I suspect is enough to do the trick but it's hard to know what the power requirements are for a few 2500 lb boat with a few people and a skier.

    Battery - Once I know how much power I need I can work backwards on the battery. Ideally I would like to size it for 1 hour of whatever my cruising requirements are. A lot of the conversions I have seen are for more traditional displacement hulls and I believe that a ski boat should be more efficient but I don't have much to quantify. My current model is this amazing project Scott's 2016 Bayliner 175 http://www.evalbum.com/5217 where he converted a Bayliner 175 and when I look at reported GPH for the bayliner compared to ski boats it shows roughly 40-45% efficiency improvements on the ski boats. His 24 kWh battery gets him 25 miles of cruising range, which I am assuming would get me about 35 miles, all things being equal. If I had more clarity on the hull efficiency of a ski boat compared to a bayliner that could help me calculate my battery pack requirements.

    Thanks and any help is appreciated. Even if it doesn't have to do with the electric components as long as I can get a good idea of the:
    • power required to not go crazy fast or pull 5 barefooters
    • comparable "efficiency" of a ski boat as compared to either a surf boat or bayliner
    Thanks!!!
     
  2. Rumars
    Joined: Mar 2013
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    Location: Germany

    Rumars Senior Member

    Do you know what the GPH and rpm is for the boat when doing it in ICE config?
    In order to get to the speed you need the rpm's regardless of source. The GPH and the engines power/fuel chart give you the HP.
     
  3. Adman
    Joined: Oct 2019
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    Location: Canada

    Adman Junior Member

    Thanks Rumars.
    I don't have that information but I will start looking for it. Even if I can't find it on the exact model I'm looking at maybe it would be available for a current version. I will do my best to find this information but do you know any places that may have those charts?
     
  4. Rumars
    Joined: Mar 2013
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    Location: Germany

    Rumars Senior Member

    What engine are we talking about? What's the prop on the boat? I understand there is no transmission yes?
    You should also be sure the boat had the performance you are looking for before conversion.
     
  5. Yellowjacket
    Joined: May 2009
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    Location: Landlocked...

    Yellowjacket Senior Member

    Ski boats don't use nearly their maximum power when pulling a skier. They have big props and big motors with a lot of torque so that the skier doesn't drag down the speed when the skier cuts hard, and they can pop a skier out of the water quickly without dragging them for a long time. They're heavier than an outboard because of that bigger and heavier motor, but that also makes the boat less likely to get pulled around by the skier, and that's also why the ski post is in the middle of the boat. Typical skiing speeds are 20 to 30 mph, the boats are designed to have a good bit of weight forward so that they plane efficiently at moderate speeds and have a small wake. You can easily pull a couple of skiers with a 50 hp outboard on a reasonably light 16 foot runabout. Putting a lot of weight (batteries) in a classic ski boat hull is going to require more planing surface or you're going to get more wake as the hull is going to have to run at higher trim angles to support that higher weight. In short you're not going to get a very good ski boat if you take an existing Correct Craft or Ski Nautique one and covert it to electric power the higher weight and no more hull area will result in a lot bigger wake. You would want a wider hull with a very flat bottom and perhaps even some hook trailing edge of the bottom to generate more lift at lower speeds. This is another one of those "just because you can doesn't mean you should" kind of things. If you want to repower a boat with the weight of a battery and electric motor system you should steer away from the "classic" ski boats like a Ski Nautique and look for something like a vintage V drive ski boat that has a lot more beam. Those boats were designed to have a 600 pound motor in the very back of the boat and were light to start with, as well as having a lot more beam and have a flat bottom to give you good efficiency. You don't need the V drive, you can just go with a straight inboard layout, but you could redistribute the weight so that it wasn't as aft loaded as the original and you'd have something that could handle the weight of batteries a lot better than the relatively narrow beam "classic" inboard ski boats. Lots of those boats also had a large trim tab across the back that would allow you to add hook as required to get your efficiency up at lower speeds.
     
  6. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    Unless you change the battery out at the dock between skiers.
    Or quick charge at the dock between skiers.
    You could even end up lighter than ICE but with a hugely reduced run time.
    EV forums may prove more fruitful than this one.
    Post what you end up doing, with pics please.
    Good luck!
     
  7. Adman
    Joined: Oct 2019
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    Location: Canada

    Adman Junior Member

    Thanks again Rumars. I don't have the exact boat in mind so no more details on the engine and prop but I may just have to base my design off a modern configuration if they're the best numbers I can cobble together.
     
  8. Adman
    Joined: Oct 2019
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    Location: Canada

    Adman Junior Member

    Thank you Yellowjacket for the detailed response.
    I will look into a vintage V drive ski boat and pull out the V-transmission.

    With regards to weight, and this is just looking at the big systems but roughly:
    Motor = 500lbs, Fuel Tank = 75lbs, 100L Fuel = 160 lbs, TOTAL = 735 lbs
    Electric roughly:
    134 HP Motor = 110 lbs, 50kwh battery = 580 lbs = 714 lbs

    There is obviously some extra support electronics but those are the large systems, plus the batteries can be easily distributed throughout the boat. I'm not looking for hours of riding time so if I can get away with a 30kWh battery pack, I would drop 200 lbs. Either way the weight of the replacement system would not be very different from existing. If a 100kW motor can pull a skier at 30mph with a 50-60kW draw then I can "reasonably" size a 40kWh battery pack and be comfortably in the same weight.
     
  9. Adman
    Joined: Oct 2019
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    Location: Canada

    Adman Junior Member

    Thanks Bluebell.
    Definitely not pulling out a battery pack beside the water plus I'm not that strong...

    The size of the battery pack is still my big unknown. Trying to get my head wrapped around those numbers right now and think honestly about the run time that would work for me.
     
  10. Yellowjacket
    Joined: May 2009
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    Yellowjacket Senior Member

    Battery capacity is your friend. If you try to discharge at much higher than the one hour rate you're going to kill battery life. And remember that you really shouldn't discharge more than 80%, so really if you're using a 50 kw pack you are really going to get just over 45 minutes of operating time if you had a 50kw pack and were drawing 50kw all the time. In a boat like a Rayson Craft they were using big block motors and the weight of dressed motor was nearly 700 pounds. Add the V drive stuff and a bell housing and clutch and it would have been even more, add fuel and a tank (fuel tanks aren't going to be 75 pounds, they're a lot less) and you'd be well over 850 to 900 pounds. This is why those boats had a wide beam and had flat bottoms. If you go closer to 60 kw of battery you won't be overloading the hull. And remember that if you're skiing you aren't cruising at steady state. You need more power to pull up the skier, and if he falls you turn around and go at high power to get back to the skier, and then you idle for a bit then you go to big power to pull him up again. If you want an hour at 40 to 50 kw draw you need a 60 kW battery pack at least. If you short the battery pack you're not going to end up with a very useful boat after you've spent a good bit of money. I looked at the link you provided for the electric Bayliner and a smaller, lighter hull with a flatter bottom won't draw as much power at that hull did. I'd say you're in the right ball park, but I'd be looking at a bit more battery capacity, on the order of 60 kW would be my starting point.
     
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  11. Adman
    Joined: Oct 2019
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    Location: Canada

    Adman Junior Member

    Thanks again Yellowjacket.
    I have been hoping I could get away with a smaller battery pack as it drives up cost but I didn't really think through full depletion. It isn't impossible to start with a smaller battery as proof of concept and add packs to build out more range as needed - Looking at roughly $1300 per 5.2 kWh Tesla battery pack.
    I found this great reference project (It's in german so I did a google convert but there's a nice table showing output at different speeds)
    http://www.piktronik.com/images/media/yare1209_mastercraft.pdf
    Also the boat is powered by a 100kW electric motor so that reassures me that the motor I was looking at should be sufficient for my needs.
     
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