long shaft or short shaft

Discussion in 'Powerboats' started by Bill Kubiak, Mar 31, 2006.

  1. Bill Kubiak
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    Bill Kubiak Junior Member

    I have a 12 ft tunnel hull, I have been running it with a long shaft 12 hp motor, it was a bear to get on plane. I added a sting ray stabilizer foil to the motor and now it jumps on a plane right away and will plane at 12 mph, but my top speed is only 17 mph, I am thinking to put a 20 hp on it.
    it seems to me my shaft is way too long creating drag. I trial fitted an old inoperative short shaft motor on it but the cavitation plate is about 1/4 of an inch above the floor of the tunnel. If I run a bigger motor with a short shaft will there still be water at the transom between the hulls to keep the prop from breaking the surface.
    What is the correct way to setup for the prop arc on a tunnel????
     
  2. Corpus Skipper
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    Corpus Skipper Hopeless Boataholic

    Your ventilation plate should be even with the top of the tunnel. Measure your transom and determine which shaft length it was built for, and go with it! Jack plates make tunnel hulls much more liveable as you can "fine tune" the engine to optimal height. Manual jack plates are reasonably priced and work well. 12 HP sounds like you're WAY underpowered, 20 or 25 would be much better, of course not exceeding the boats max. rating. What type of boat is it? Hull material? This info. would let us answer your question much better.
     
  3. Bill Kubiak
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    Bill Kubiak Junior Member

    Scat Cat

    I think it is a Scat Cat, fiberglass, I just bought an old Merc model 200 22 hp on E bay that is a short shaft. I understand it has the Super Hurricane Powerhead so that might be the power I need..
    The transon is funky, it is angled back and is 17 inch overall, so it is closer to 15 inch when measured straight down from the top of the transon to the floor of the tunnel. The hulls are 4 1/2 inches further down then the floor of the tunnel
     
  4. marshmat
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    marshmat Senior Member

    It's probably designed for a short shaft then. Short is nominally 15 but actually about 16 1/2 on most motors. Long is nominally 20 but usually comes out around 21-22. Motor makers build them too long so they won't ventilate; you can always jack up a too-deep motor but lowering a too-high one is a pain.
     
  5. Bill Kubiak
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    Bill Kubiak Junior Member

    I got a 1960 mercury model 200 22 hp and the ventilation plate is just about even perhaps a tad higher then the floor of the tunnel, but the sponsons on each side of the tunnel are 4 1/2 inch's further down then the floor of the tunnel. I guess when I have the motor running good I will find out if it works or not. Any thoughts or suggestion are welcome
     
  6. Bill Kubiak
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    Bill Kubiak Junior Member

    I took some pictures of the motor on the boat showing the position of the ventilation plate in relation to the floor of the tunnel. give me an e mail address and I will send the photos

    Bill
     
  7. Bill Kubiak
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    Bill Kubiak Junior Member

    how do you think this motor will work on this tunnel hull?
     

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  8. Corpus Skipper
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    Corpus Skipper Hopeless Boataholic

    Looks like it will work just fine.
     
  9. Bill Kubiak
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    Bill Kubiak Junior Member

    I was concerned about the location of the cavitation plate with relation the the tunnel floor, I guess the only real way to know is to try it out and see if it does not cavitate and keeps pumping water
     
  10. lewisboats
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    lewisboats Obsessed Member

    For Bill

    Just a bit of a correction in terms here...Technically the "anti-cavitation plate" should be an "anti ventilation plate". It is there to help keep the prop from sucking air in from the surface...Ventilation. Cavitation occurs when there is too low pressure on the back side of the prop blade, actually sucking bits of the prop off and causing pitting and damage to the prop itself. It occurs when the prop is mismatched to the engine (running too fast or not enough surface area to the blades). It usually happens when the pressure differential is greater than 14 or so psi. Some props are designed to run in this environment, but not yours;) !

    Steve
     
  11. Bill Kubiak
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    Bill Kubiak Junior Member

    I really wanted to know what happens to the water between the tunnels, A hull push's water out in both directions as it moves through the water, Since I basically have two hulls connected by a raised floor, what is the water action between the hulls as each hull throws water at the other, is it a solid mass of water for the prop to bite into????
     
  12. Corpus Skipper
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    Corpus Skipper Hopeless Boataholic

    From the picture, your boat is more of a cat than a tunnel. In a well designed tunnel, water is forced inward and upward, providing a fairly solid stream, essentially making the water "deeper". No tunnel is perfect, you will hear or feel the prop blow out on occasion, especially in rough water, as air will get in as the boat bounces around. This is where a jack plate will shine, as you can adjust prop depth for the conditions. A water pressure guage is highly recommended as well.
     
  13. Bill Kubiak
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    Bill Kubiak Junior Member

    Water pressure, first time I heard that, where would one install the pressure probe??? and why?
    Would a picture of the front of the boat looking down the tunnel help??
    Does anyone know the make of the boat, I thought it was a Scat Cat but the rear deck is different from theirs?
     
  14. Bill Kubiak
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    Bill Kubiak Junior Member


  15. ccb
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    ccb Junior Member

    Should a 23 Steigercraft pilothouse (Model Ches.) have a long or short shaft. I went one one for a test ride but it didn't seem right. and it pounded.
     
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