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 Is my outboard long enough?		  New Topic  Topic Locked
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Tahoe Cruiser
1st Mate

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USA
72 Posts
Initially Posted - 09/13/2013 :  22:16:13  Show Profile
I have a Honda 9.9 on the transom. I measured 28" from transom to top of cavitation plate. This seems to be the longest shaft by doing research on various manufactures websites. My problem is only when I am turning hard in my buoy field the motor seems to start running at hard rpms like it is coming out of the water. I worry that I might get the same results if I have to motor in rough water. What are your experiences with this motor shaft length and how does one remedy it? I had a Tohatsu extra long shaft on my catalina 22 with the lowering mount and never had this issue. My model is a Honda 9.9 BFA9.9. Thanks.
1998 250 WK/TR #355 "Trail Break"
Lake Tahoe California
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GaryB
Master Marine Consultant

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USA
4312 Posts
Response Posted - 09/14/2013 :  08:08:02  Show Profile
My motor will ventilate if I turn really hard above a certain speed.

I know someone can say this better than I but... it's caused by the water "separating" (creating a bubble of air) on one side of the motor above a certain speed and rate of turn (kind of like sweeping you arm through the water just below the surface - we used to call it "making George Washington's hair"). The bubble of air is sucked into the prop causing the motor to "rev up" and loose some or all thrust.

If you remain below a certain speed and/or rate of turn the water will remain "attached" to the side of the motor and the prop will not ventilate.

To correct the issue when it happens, reduce the rate of turn by straightening the motor some while also reducing the throttle and the water will "re-attach" to the side of the motor and the prop will stop ventilating.

To correct permanently, learn what speed and/or rate of turn produces the condition and then stay below those numbers!

Also, the make & model of the motor, RPM, and the rate of turn combined with the design of the hull will effect whether a given combination of these will cause this condition to happen or not.
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Stinkpotter
Master Marine Consultant

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Djibouti
9087 Posts
Response Posted - 09/14/2013 :  09:09:52  Show Profile
28" is indeed the "extra-long" shaft--the longest you'll find. (They call it 25" because that's the recommended height of a powerboat transom so it extends slightly below the bottom when on plane.) If your anti-ventillation plate is 5" or more below the surface when the boat isn't moving, you have enough depth for all but the biggest chop, in which you might get some occasional "spins." The boat will squat a little under way, so the prop will be deeper.

In addition to your turning experience, I also experienced some "ventillation" in reverse gear, where the prop created a vortex that sucked a little air down into it. No big deal.
Edited by - Stinkpotter on 09/14/2013 09:11:40
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Notice: The advice given on this site is based upon individual or quoted experience, yours may differ.
The Officers, Staff and members of this site only provide information based upon the concept that anyone utilizing this information does so at their own risk and holds harmless all contributors to this site.