cavitation plate depth

Discussion in 'Outboards' started by valvebounce, Jan 31, 2019.

  1. valvebounce
    Joined: Dec 2010
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    valvebounce Senior Member

    My transom was a 15"-16",I have added 3" to the height of the transom.

    If I use a longshaft motor,the cavity plate is 3"below the bottom of the transom.
    Would the drag be too much,or would I get away with it.?
    The boat is a broom 14ft speedboat that has been converted for sea fishing.
    (back seats removed) splashwell at the stern.
    The boat is very stable.
    I'm not looking for speed,it's just to get out to the fishing spots.
     
  2. kapnD
    Joined: Jan 2003
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    kapnD Senior Member

    Clamp it on and try it out.
    Other than gaining a little draft, at slow speeds it won’t make much difference at all.
    Bringing it up on plane may have some interesting effects!
     
  3. valvebounce
    Joined: Dec 2010
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    valvebounce Senior Member

    Thanks for your reply,I have a 1999 Johnson longshaft that I am going to try on it.
    If it handles that ok,I'll be getting a 40hp Yamaha longshaft.
     
  4. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    V, is it possible to bolt it to the transom, rather than use thumb screws on the bracket ? If it is able to be done safely and securely, that might be a way of gaining the height you need, you certainly don't want the water stream behind the boat being higher than the splash plate, situated above the cavitation plate.
     
  5. kapnD
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    kapnD Senior Member

    I’m not following your line of thought here.
    You have a short transom and a long shaft motor, which is a bit of a conundrum, but you said you just wanted to go slow, albeit in a speedboat hull, so why would you want to purchase yet another motor that doesn’t fit the boat?
    Why wouldn’t you look for a short shaft motor or open discussion on raising the height of the transom to accommodate a long shaft motor?
     
    Barry likes this.
  6. valvebounce
    Joined: Dec 2010
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    valvebounce Senior Member

    Hi Mr E, I'll have a look at that idea,thanks.
    Clear blue sky here,but snow on the ground,I believe you are sizzling over there.
     
  7. valvebounce
    Joined: Dec 2010
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    valvebounce Senior Member

    I'm inbetween with the transom height because I raised the transom with 1.5" thick GRP.Used SS bolts.(laminated-3/4" x 2)
    So the transom is now between 18"-19".
    It seems used longshaft motors are more available than shortshaft in my area.
    I'm just seeing what choices may be available to use.
    Seeing as I already have the 25hp longshaft Johnson,I may do as you suggested and clamp it on and try it.
    I may get a 40hp for the extra torque if it works out.
    Seeing how I'm 74yrs in march,I'm looking for an electric start rather than the pullstart I have now.
     
  8. valvebounce
    Joined: Dec 2010
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    valvebounce Senior Member

    The boat planes with an 18hp Evinrude on it,but there is no tilt tube on the motor because it's a 1964 model.
     
  9. kapnD
    Joined: Jan 2003
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    kapnD Senior Member

    Short shaft motors are somewhat of an anomaly nowadays, but they do exist, and often sell for cheap, due to lack of demand.
    Slip a spacer in on top of the transom, no problem as long as the motor is thru bolted securely.
     
  10. valvebounce
    Joined: Dec 2010
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    valvebounce Senior Member

    Spring is just around the corner here in the UK so it will give me the opportunity to get out and experiment.
    I live 50mls from the sea and have to trailer the boat.
     
  11. Magnus W
    Joined: Nov 2017
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    Magnus W Senior Member

    I gained 4 knots (17 to 21) just by raising my Suzuki 15 from just a tad low to just a tad high (one bolt hole) on a 4,25 meter flat bottom boat. No negative side effects so a win-win. So why not fit a proper engine (or modify the transom to handle the longer leg)?
     
  12. valvebounce
    Joined: Dec 2010
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    valvebounce Senior Member

    To tell you the truth MW,I'm 74yrs old,and I'm not fit enough to change things much.
    If I can get out fishing in the boat I will be satisfied with that.
    I'm a long way from the sea and there are not many boat chandlers in the Area,those that are think they are God's gift to boating and charge too much.
    The fishing season around North Wales where I fish has started with a bang,and I want to get out there.
     
  13. Magnus W
    Joined: Nov 2017
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    Magnus W Senior Member

    Well, I'm not educated as many others here are so I can only offer speculations based on experience. But I'm sure you boat will work and I don't think it matters much how long the leg is as long as the prop is well in the water and you don't intend to go fast and/or expect high efficiency.
     
  14. valvebounce
    Joined: Dec 2010
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    valvebounce Senior Member

    Thanks for your interest MW.
    Sometimes experience is as important and has much or more to offer than what's in the book.
    I know the boat will perform ok,It's well built with a deep freeboard and very stable on the water,
    It has safety rails all round and plenty of rod holders.I have got a Lowrance GPS fishfinder to fit and a ship to shore radio.
    I am considering Mr E's idea of slipping a piece of timber under the motor on the transom,and bolting the motor on
    to make it more secure.
     

  15. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    Probably the easiest way is a small slab of aluminium at least half an inch thick, (preferably a bit thicker) you bolt that to the outside of the existing transom, it would need to be say 18" x 9" or thereabouts. Naturally the height of the plate has to be set correctly to get the cav plate level you want, and you can't have bolt heads standing proud, that might foul the outboard's bracket. You could use large countersunk stainless screws to avoid any problem with that. Alloy is light, the plate won't be weighing things down much, and your engine size should not strain it too much. 3/4" would probably be ideal. A local metal fabrication shop might even have an offcut you can use. you will have to do some measurements to see what the minimum size you need is, but use 4 bolts to fit it to the transom.

    B Transom.png
     
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