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Author Topic:   Engine Mounting Height for Single Engine v. Twin Engines
jimh posted 06-29-2014 09:06 AM ET (US)   Profile for jimh   Send Email to jimh  
The textbook or usually recommended position for the Anti-Ventilation Plate (A-V plate) when on plane is for the top surface of the A-V plate to be mostly dry and not have water running over it. However, most of the images I find that show boats with their engines mounted like this seem to be on twin engine boats. I have to wonder if a boat with a single engine might tend to prefer a lower mounting height. My basis for this is the notion that with twin engines any ventilation of the propeller on one engine might be compensated for by the other engine, assuming it is also not simultaneously venting its propeller. Also, with twin engines, the total amount of thrust or power that is developed per engine and propeller may be lower than with a single, as in the case of a boat that was set up with a large single engine, say a 250-HP engine, as opposed to a boat with two twin engines of less power, say twin 125-HP engines.

Perhaps we could have some comments about this theory from people who have experimented with different engine mounting heights on both twin and single engine boats.

Teak Oil posted 06-29-2014 09:43 PM ET (US)     Profile for Teak Oil  Send Email to Teak Oil     
The correct engine height for any boat is as high as possible while still maintaining good grip and handling in any seas that boat may encounter. Certainly some props prefer lower mounting levels like the Mercury Enertia, so in that case a certain prop can help correct a mounting height below optimum for other props.

Engines mounted too low waste fuel, create handling problems, and reduce speed across the board.

jimh posted 06-30-2014 12:29 AM ET (US)     Profile for jimh  Send Email to jimh     
Teak Oil--you made no mention of anything about a single engine compard to twins. Do you have any thoughts about mounting height for single engine compared to twins?
russellbailey posted 06-30-2014 02:36 PM ET (US)     Profile for russellbailey  Send Email to russellbailey     
jimh, my understanding is that twin engine boats have more difficulty with higher mounting heights during sharp turns, where the engine on the outside of the turn is fairly high.

I'm actually experimenting right now with some extra high Hydrodynamics Rapid Jack jackplates on my 1984 Outrage 25 twin twin Optimax 150s (25") on a 20" transom. I recently put some new jackplates on that have a built-in 4.375" rise, and I'm experimenting with how high I can go. The last two weeks I ran them near Morehead City NC fairly high with no problems (mounting hole for max height + jackplate about 2" up) inshore and offshore, and have about 2" more I can play with - the only time I got any slippage was when I had the motors trimmed too high coming out of the hole, and I'm about 2" higher than I was with the old jackplates the prior owner had on the boat. I think on the lakes at least I'll be able to go the extra 2" up, and maybe in the ocean also - TBD.

jimh posted 07-02-2014 02:01 PM ET (US)     Profile for jimh  Send Email to jimh     
Russell--you make a good point about how twin engine propellers might change their relationship with the water line as a boat banks in a turn. But this is another case of the twin propellers working together. If the propeller coming up higher begins to ventilate, the other propeller is going deeper and maintains traction. In contrast, with a single engine, one the propeller starts to ventilate there is not another source of thrust to pick up the lost power and keep the boat moving. This is why I have begun to wonder if these really high mounting heights seen with twin engines--the A-V plate really running dry--are workable for single engines, too.

Now I do recall seeing some very high speed single engine boats running in glass calm water with an adjustable engine mounting height plate, and these could be run with the A-V plate out of the water. But that is only on a straight, high-speed run on a calm lake.

I'd like to see some images of a big single engine, say a 250-HP or more, on a big boat, where the mounting height is such that the A-V plate runs dry.

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