GOP Home Build

Discussion in 'Boat Design' started by schultzfactor, Oct 25, 2013.

  1. schultzfactor
    Joined: Oct 2013
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    Location: Rotorua New Zealand

    schultzfactor Senior Member

  2. Mr Efficiency
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    Mr Efficiency Senior Member

    It does not porpoise, which is a bit surprising considering how stern heavy it seems to be.
     
  3. serow
    Joined: Mar 2016
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    serow Junior Member

    Well done.
     
  4. schultzfactor
    Joined: Oct 2013
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    schultzfactor Senior Member

    Porpoising

    The stern does look heavy, although I think that is because there is absolutely no weight in the bow, no anchor no electronics. The fuel tanks and battery where all in the stern, plus a bit of water which came in through the outboard bolt holes. My concern is there is cavitation, and I don't really want to drop the motors anymore. With two people on the bow it flattened out quite well. It also needs hydraulic steering as the cable is really difficult to turn, but the motors move freely when not connected.

    I will definitely remove the old centre pod as this will definitely be obstructing the tunnel quite badly. Chines are working well and by the aerated spray I'd say the dimple finish is doing its job too.

    Any thoughts on these outboards?
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    Cavitated running in a straight line ? That suggests the engines are set too high. If it does it in turns, that is not unusual in cats, especially with alloy props, even if the engine height is OK. Stainless props work better. Your boat seems super light, a cat of any weight carves the water up noticeably. This thing seems to create little disturbance. And I hope you have taken the precaution of fitting bouyancy into those hulls, a cheap way is to fill it with plastic drink bottles. You don't want it to sink.
     
  6. schultzfactor
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    schultzfactor Senior Member

    The whole Hull is going to still be fibreglassed internally so this should add quite a bit of weight. It will be foam filled and deck sealed. I prefer not to use bottles as it adds no structural rigidity to the hull where as closed cell pour foam should strengthen everything heaps.its also very compartmentalised so shouldn't be able to take on water in any great volumes?
     
  7. schultzfactor
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    schultzfactor Senior Member

    Centre console Hard Top

    This is my final sketch for the console.
     

    Attached Files:

  8. SamSam
    Joined: Feb 2005
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    Location: Coastal Georgia

    SamSam Senior Member

    It seems to run Ok so far.

    I'm curious, what brand of potato did you use to record the video? ;)

    Being elevated so high in the boat and no substantial sides to keep you in, you should have a kill switch connected to you. I'm sure the motors came with them. In the video, when you went back to pick up something red, it doesn't look like you have them attached to you. If you fall overboard, the boat will keep on going leaving you amongst all those sharks and tiny deadly jellyfish you have down there. Seriously, the boat will be gone and it won't come back. Well, if you happen to grab the steering wheel as you go over, it will keep coming back in a tight circle until it runs out of fuel. In the meantime you have to keep dodging it. I'm not sure if it will keep the sharks away or attract them.

    In these photos you posted above, the cavitation plate on the motors is 4 inches or so above the water when planing, which is why they cavitate.

    [​IMG]

    [​IMG]


    This is how they should be, with the cavitation plate more or less flush with the bottom of the hull and not the bottom of the pod...

    [​IMG]

    The pods don't add any flotation once underway, and only move the motors back, meanwhile leveraging their weight and making the stern ride lower. If the bottom of the pods came off level with the bottom of the hull, they would actually become a part of the hull, lengthening it and supplying more flotation for the stern. If the cavitation plates were then as the above picture, it wouldn't cavitate and the increased length of the hull might make for a more balanced, faster hull.

    When at rest, it doesn't look like you can lower the motors much without putting the cowling into the water. How long of shafts do they have? They go from short- 15", long- 20", Xlong- 25" to XXlong- 30". How long should they be? (That measurement is from the top of the transom to a straight line off the bottom of the hull, not off the bottom of the pods)

    [​IMG]

    This might help a little, especially the "other variables" part....

    http://www.boats.com/how-to/the-outboard-expert-boost-speed-with-outboard-engine-height-adjustments/

    The console sketch looks good.

    .
     
  9. schultzfactor
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    schultzfactor Senior Member

    Awesome feedback thanks

    Hi Sam Sam
    You're absolutely correct, about the kill switches and everything else. I was waiting to be pulled up on that!!
    I do have the clips yes and they will be used I promise.
    The motors were really high, and I have since dropped them about 4 inches and increased the angle by about 30deg. I have not yet run a test again, but will do so soon. I also moved the gas under the console and the batteries to the bow.
    Your idea on extending the hulls is a really good one and I have been thinking about this for a while. I'll see how we go with the next trial and go from there. Cowlings are still ok for water clearance. The potato filming was my mates son with a phone. Hopefully next one will be better.
     
  10. schultzfactor
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    schultzfactor Senior Member

    Cavitation plates

    I have also been reading that the cavitation plates on a cat need not be as low as on a mono, the consensus seems to be higher is better?
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    Where have you read that ? Experience will tell you what is best for your set-up, but I see no reason why you could get away with mounting higher, compared to a mono. They need good water to work properly.
     
  12. schultzfactor
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    schultzfactor Senior Member

    Forum info

    A number of the other forums, speak about a visible cavitation plate sometime above the water when planing.
    Agree though that trial and error will dictate what best in this case.
     
  13. schultzfactor
    Joined: Oct 2013
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    schultzfactor Senior Member

    Deck laying

    I've been fibreglassing the forward deck and foam filling the voids.
     

    Attached Files:

  14. DennisRB
    Joined: Sep 2004
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    DennisRB Senior Member

    Congrats! What kind of pour foam will you be using? How about adding a few milf bottles before pouring to save on foam cost and weight?
     

  15. schultzfactor
    Joined: Oct 2013
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    schultzfactor Senior Member

    Polyurethane foam

    Hi Dennis, thanks. I have been using polyurethane foam, which is closed cell and really strong. I did think of using milk bottles, but they don't give any rigidity to the hull. The pour foam is very strong and adds considerable strength to the hull. What I have found Is, just how important the mix is, it isn't exactly 1:1, but slightly more monomer, gives a much stronger outcome. It also negates the foam collapsing, which can happen when the mix is wrong.
     
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