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  Effect of Motor Mounting Height

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Author Topic:   Effect of Motor Mounting Height
The Chesapeake Explorer posted 03-22-2005 09:57 PM ET (US)   Profile for The Chesapeake Explorer   Send Email to The Chesapeake Explorer  
I took delivery of my new 2004 Yamaha 90-HP to re-power my 1987 Montauk after 1,800 hours on the V4 Evinrude and a load of questions here over the last few years. Now more questions.

How does motor mounting height affect performance?

I understand that the 1988 Evinrude 90-HP and the 2004 Yamaha 90-HP have the same mounting holes. My current OMC motor is set with about a 3/4-inch gap [between] the bracket and the transom. The mounting holes are way down in the well. When they mounted the first motor the washers cut into the glass on the inside of the transom to the bottom of the washers splitting the gel coat a bit because they were very low to the radius of the well--not good rigging. I made custom aluminum oversized bolt mounting plates and sealed the area as soon as I got the boat. There been no enlarging of that damage. (That is the way it was when I bought the boat as it was already rigged, brand-new, ready to go. and I was green. Know what I mean? Not a clue!) Maybe the original motor was mounted too low.

Where should the [anti-ventilation plate]--that flat surface on the lower unit--be in relation to the bottom of the keel at the rear of the boat?

I bought the Whaler brand new in December of 1988 for $12,500 with 90-HP Evinrude-3 year warranty and EZ loader trailer from a famed long-time Chesapeake Bay Whaler dealer that no longer sells Whalers.

jimh posted 03-22-2005 10:45 PM ET (US)     Profile for jimh  Send Email to jimh     
If the engine mounting holes have been drilled on the transom in the proper position, the usual factory recommendation for a 20-inch shaft motor on a single engine hull like the 17-footer would be to mount the engine one set of holes above the lowest point. From your description is sounds like the current engine is mounted in that way. This is often referred to as "one hole up".

You can check the mounting hole location in the transom against the suggested template location by reference to my FAQ article on precisely that topic. (See URI and hyperlink below.)

The mounting height is a balance between two factors:

--the higher the engine is raised the less resistance created by the lower unit because there is less of it submerged. This tends to increase the top speed potential, however it may make the boat more difficult to get on plane.

--the lower the engine is mounted, the longer the lever arm of the engine's thrust acting on the boat's center of pitch resistance or center of motion, and thus the more the engine can control the bow's rise/fall and the overall angle of the boat hull in the water when on plane. By trimming the engine in, you can get the boat on plane faster, and by trimming the engine out, you can control the bow angle when on place. You can do this more effectively the lower the engine is in the water because of the greater lever arm.

In addition, there is a limit to how high the engine can be raised because of two additional factors:

--the tendency of the cooling water pick up to begin to have problems providing water to the engine;

--the tendency for the raised propeller to suck air down into it from above and ventilate the propeller. For engine mounting heights which are more than one or two holes up, it is often necessary to use propellers which have a positive rake angle to the blades. A raked propeller will tend to hold better and not ventilate as easily as a propeller of the same pitch which is not raked.

In most cases the factory suggested position will probably be the best place to start for you.

Q9: What is the Standard Transom Hole Layout?
http://continuouswave.com/whaler/reference/FAQ/#Q9

jimh posted 03-23-2005 07:08 AM ET (US)     Profile for jimh  Send Email to jimh     
[Moved]
The Chesapeake Explorer posted 03-23-2005 11:03 PM ET (US)     Profile for The Chesapeake Explorer  Send Email to The Chesapeake Explorer     
Thanks Jimh!

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