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  1. #1
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    Default Cavitation plate depth

    1958 Alumacraft 12' boat with 1973 Merc 7.5 outboard.
    Transom length: 15"
    Shaft length: approx 17" (top of transom clamp to bottom of cav. plate)

    The best I could measure the bottom of the a/v plate is approx. 1-3/4" below the bottom of the hull. The attached Pix will illustrate that the top of the 2x4 resting on top of the a/v plate is even with the bottom of the hull.
    The 2x4 is 1-1/2" thick plus approx. 1/4" to the bottom of the a/v plate = approx. 1-3/4" below the hull bottom.
    Reading other posts, the bottom of the a/v plate should be even with the bottom of the hull. That said, I have read that on older motors since the water pump is not self priming and located above the a/v plate, that the motor should be low enough that the water pump is below the hull bottom.
    The top of the water pump is nearly flush with the rib above the a/v plate where the lower unit attaches to the upper unit.
    I am confused which is correct. Do I need to raise the motor on the transom?
    Another pix illustrates the motor raised 1-1/4" which would raise the bottom of the a/v plate to approx. 1/2" below the hull bottom.
    I would place a reinforced spacer to accomplish the raising of the motor and also bolt the motor to the transom.
    I have read many threads concerning these issues, but have only come away with more questions and confusion.
    I have recently replaced the water pump and had a broken skeg repaired. The motor runs great in a barrel with plenty of flow through the pee hole and I would like to resolve the motor depth issue before I take it to the lake or river.
    All advice will be appreciated.
    Chief Bob
    Attached Images Attached Images

  2. #2
    Petty Officer 2nd Class
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    Default Re: Cavitation plate depth

    Personally I run most of my motors about 1.5" above the bottom of the boat, checked with the anti-cavitation plate level with the bottom of the boat.
    However in most cases though, on a boat and motor that size, I doubt you'll see much of a difference. Your main concern is first and foremost that the lower unit is able to pick up water at all times.

    Too much of the motor hanging down below the boat will create more drag.
    The owners manual for one motor I have here, a Sears 9.9/15hp, says to mount the motor so that the anti-cavitation plate is just below the bottom of the boat. You basically want the prop in clean, undisturbed water, but on a small aluminum boat that's rarely a problem. Hang the motor, take it to the water and try it both ways, make up a few shims to go on top of the transom to make it easy to change.

  3. #3
    Vice Admiral 5150abf's Avatar
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    Default Re: Cavitation plate depth

    Start all the way up and drop it it till it doesn't cavitate, there really isn't one measurement that works for all boats.

    I have a CMC TT on mine that gives me a 6'' set back so I run mine about 1 1/2 or 2'' above the bottom of the boat.
    GO IRISH!!!!

  4. #4
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    Default Re: Cavitation plate depth

    Not much help here!

  5. #5
    Moderator Bob_VT's Avatar
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    Default Re: Cavitation plate depth

    Quote Originally Posted by chiefbob View Post
    Not much help here!
    That's pretty rude. Why not visit staples and buy an easy button!

    The primary concern is to maintain the proper cooling. There would be a negligible performance difference with a 7.5 Hp motor.
    This is a great link to boat specifications http://boatspecs.iboats.com/
    Please, shop iboats first!!


  6. #6
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    Default Re: Cavitation plate depth

    I was rude with my comment and I sincerely apologize to all for my insesitive comment. I guess I was frustrated and impatient when I made the comment and this very valuable forum is not the place to vent.
    Again, I apologize to all and thank iboats.com for providing this very valuable service and all who respond and provide guidance.

  7. #7
    Rear Admiral
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    Default Re: Cavitation plate depth

    The cav plat is supposed to be even with the keel of the hull, give or take an inch up or down. Run it as is and see how it performs. Then just for curiosity's sake, shim the motor up a half inch at a time and try each shim setting. By the time you decide which you like, you'll forget all the questions you had about this in the first place and go fishing.

  8. #8
    Cadet
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    Default Re: Cavitation plate depth

    Thanks tmcalavy!

  9. #9
    Petty Officer 2nd Class
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    Default Re: Cavitation plate depth

    Most (if not all) portable outboards have longer shaft lengths than advertised. It might be a safety thing about keeping the water pump primed.

  10. #10
    Rear Admiral
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    Default Re: Cavitation plate depth

    Depends on product engineering, too. Older outboards, especially OMCs from the 50s and 60s, have the water pump higher in the leg than those made later. As a result, they are set/run lower to ensure good water intake.

  11. #11
    Supreme Mariner
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    Default Re: Cavitation plate depth

    Bottom line is for best speed, run it as high as you can without cavitating. With a motor such as yours, that generally is about even with the boat bottom or a hair below. Some motors have to run deeper, resulting in more confusion.

    Water pump pickup really isn't much of an issue because when at rest, boat not planing, the motor will be plenty deep. At planing speeds, the water intake is strategically located by the engineers to be force fed by the prop. If you were to get rediculously high, it may be a problem.

  12. #12
    Petty Officer 2nd Class
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    Default Re: Cavitation plate depth

    Then why do the OMC 9.9/15, and other motors of similar hp, have shaft lengths of 16.5 inches?

  13. #13
    Moderator Bob_VT's Avatar
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    Default Re: Cavitation plate depth

    Quote Originally Posted by ChrisAG View Post
    Then why do the OMC 9.9/15, and other motors of similar hp, have shaft lengths of 16.5 inches?
    Those are short shafts for smaller boats with a short transon.
    This is a great link to boat specifications http://boatspecs.iboats.com/
    Please, shop iboats first!!


  14. #14
    Petty Officer 2nd Class
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    Default Re: Cavitation plate depth

    Quote Originally Posted by Bob_VT View Post
    Those are short shafts for smaller boats with a short transon.
    What?

    I was asking (thought it was obvious) why they weren't 15 inches, to match that of boats that have 15" transoms. That extra 1.5 inches creates a lot of drag.

  15. #15
    Cadet
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    Default Re: Cavitation plate depth

    I finally had the boat in the water on Saturday and it runs great. With the 1-1/2" shim on top of the transom I did get some cavitation. I will now replace the shim with a 1" shim, lowering the motor 1/2" and try it again. The important thing to me is that the motor runs great with plenty flow thru the pee hole. The cavitation can and will be corrected.
    As far as the depth of the cav, plate goes, it is about 1/2" below the keel and will be lowered another 1/2" to remedy the cavitation. The cavitation was slight, but I will correct it. BTW, when cruising the cav. plate seemed to be just under the water surface. This old Merc did a great job handling my 12'
    Alumacraft boat with plenty of power. I was pleasantly surprised.
    Thank for all the advice.

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