Motor Placement

Discussion in 'Multihulls' started by Fanie, Mar 4, 2012.

  1. Fanie
    Joined: Oct 2007
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    Fanie Fanie

    Hi Guys, a bit of a stupid one :(

    I made a foldable cat using two Tri-Yaks, so it's 4m500 long and 4m wide. I have a 6hp Parsun two stroke rowing it that used to sit on the frame center.

    The problem is it's ok at low speeds, but the moment you throttle up a huge wake builds behind the motor and spray starts flying around. Not very productive. It won't get on the plane either. I need the power and speed for surf launching. The very last thing you want is for a wave to overtake you when you come out.

    Sooo what's the problem, get another same motor and just mount each motor on each stern right ! Right.

    Look at the stern of this darn Tri-yak. The bracket is not the problem, it's a simple matter of laser cutting the parts and weld and bolt in place.

    Where would the best place be to have the motor sit ?

    Attached Files:

  2. keysdisease
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    keysdisease Senior Member

    Is that floating in a swimming pool?? If the last picture is of it in the run location/height it looks way too high. Your description of excessive spray and inability to get on plane may indicate aeration of the prop, it needs to be deeper.

    At rest the leg needs to be in well past the cavitation plate. In a catamaran install with a single centered engine the inboard wave patterns coming off the hulls and meeting in the middle typically work in favor of the center mounted outboard by raising the water level where the outboard is.

  3. Fanie
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    Fanie Fanie

    Hi Keysdisease,

    The spray comes from water coming off the motor from ahead, it's not the prop. As you see it in the pool there is no load.

    We took it out and by shifting your weight forward or aftward you can control the motor's running depth.

    That is why I want a motor on the stern of each hull. The hull keel is 55mm wide (2 inches), which I suspect may not allow enough water to get to the prop.

    The question is where should the motor's be placed on the hull butts.
  4. Fanie
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    Fanie Fanie

    We finished the trampoline over the weekend, you can see the rear of the tri-yaks here as well.

    The idea is to sit on the trampoline with your feet on the tri-yaks to keep your butt dry and it's more comfortable as well.

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  5. PAR
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    PAR Yacht Designer/Builder

    It's likely a combination of things; trim angle once the hulls settle with speed increase, ventalation plate and leg making a plow at the now incorrect thrust angle, the engine mounted forward of the stern being levered out of the water by the squatting kayaks, converging wave trains of the kayaks, etc.

    Rake the outboard with 3 or 4 degrees of downward trim, while the boat is at rest (trimmed out). This would be the starting point, you'll probably need more then this. Those hulls will permit quite a bit of bow up trim, so you may want to address this as well.
  6. Richard Woods
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    Richard Woods Woods Designs

    You need a fairing in front of the leg, otherwise you won't do more than about 5-6 knots, too much aeration.

    See the central 25hp engine on my Skoota 20 powercat. As an experiment we took the fairing off. Water went everywhere and the max speed I dared go was 5 knots, which was at about 4000rpm. With the fairing we could do 15

    Richard Woods of Woods Designs
  7. philSweet
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    philSweet Senior Member

    The water jetting up the front of the leg is a huge PITA. It can drown the engine and flood a boat. The cav plate and transom usually stop this but you have a lot of water above the cav plate that the transom normally displaces. I had a Craig Cat that had this problem, and my little cat dink does to a lesser extent. You need to put a vertical board in front of the leg. Shaped like a wedge and pretty darn fine. It doesn't have to be as wide as the leg. 70% or so should do. 6 hp ought to work Fanie. Apply more finesse, not more gas. Screw some door skin tabs into the 'yak's bottoms and play with wedges to get her on top. Good luck.

    edit sorry, crossposted with RW
  8. Fanie
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    Fanie Fanie

    Thanks guys,

    Ok this is more what I had in mind.

    If the motor sits behind the tri-yaks butt, how bad would it turn out with that keel in the way ? or is it again a matter to compromise by lowering it more into the water and trimming to where one gets best results ?

    Attached Files:

  9. catsketcher
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    catsketcher Senior Member

    Not with those hulls

    Fanie - I am going to rain on your parade here but those kayak hulls will not plane easily. They have too much rocker in them. They really need to be almost dead straight stern sections to plane well. You can put heaps of horsepower into those hulls and make lots of waves. I have a dinghy that rows beautifully that won't tow a damn due to its pronounced aft rocker. It makes bigger waves than our 38ft cat at 10 knots. I used to try and tow it sometimes before I built a back deck. Now it always comes on board. The tow line was really tight at speed - heaps of load and drag.

    Try two Tornado hulls or similar. They would work well. I can't think of any kayaks that would plane well. They are designed for a whole different speed and power range. 6 knots in a kayak is really fast.


  10. Fanie
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    Fanie Fanie

    Phil, I agree, they're not displacement nor planing, they're floating :D

    With one motor I get 12km/hr at half throttle, flat out with spray flying I get 18km/hr, but it's not planing and this is with two of us on it. If I add another same motor we should have a bit more speed for surf launching and it may actually go on the plane. It's not nice if the waves overtake you from behind when you go out... A second motor will also make it more manueverable.

    I have to get another motor in any case, these same two motor's will go on the other cat, can just as well make the most of it.
  11. garydierking
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    garydierking Senior Member

  12. TeddyDiver
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    TeddyDiver Gollywobbler

    The reason behind the spray is the bow wave interference, in front of the engine at certain speed, and a bit more throttle and it meets the engine. If you mount the engine it a bit to port might help.. me thinks..
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