MONTAUK 17 Engine Mounting Height

Optimizing the performance of Boston Whaler boats
JJ Donovan
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Joined: Tue May 08, 2018 7:29 pm

MONTAUK 17 Engine Mounting Height

Postby JJ Donovan » Sat May 26, 2018 6:00 pm

I have a 1990 Boston Whaler Montauk 17' with a 1990 Yamaha 90-HP two-stroke-power-cycle engine. When I purchased the boat the engine was mounted in the top hole [which is probably the lowest position--jimh]. Judging by the installation it might be the original mounting height and probably was used that way its entire life.

I removed the engine over the winter so I could clean it, paint it, and clean the transom. The boat was a mess and not well taken care of. Now that I have everything on the engine painted and cleaned up, I am ready to install the engine.

Today I experimented with the different [engine mounting positions or] holes to see how far off the bottom of the boat [keel] the [anti-ventilation] plate is at level trim. The third hole down from the top [this is probably what we would refer to as two-holes-up mounting--jimh] puts the [anti-ventilation] plate 2-1/4-inches higher than the bottom of the boat [keel].

The second hole down from the top [this is probably what we would refer to as one-hole up mounting--jimh] puts [the ventilation plate] at 1-inch above the [keel].

And the top hole [the lowest mounting position] puts [the anti-ventilation plate] about 3/4-inch below the [keel].

Some people have recommended [two-holes-up position].

Others have said to have the [anti-ventilation] plate no more than 1-inch from the [keel].

One person said the boat porpoised in the [two-holes up position] with the engine trimmed in all the way.

I appreciate everyone's opinion and would like to get this engine re-mounted the first time at the correct height, to avoid doing it over.

Thanks John Donovan

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Phil T
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Re: Engine mounting height confusion

Postby Phil T » Sat May 26, 2018 6:25 pm

John -

I had the exact same setup albeit a 1987 vintage.

Mount the motor 2 holes up and you will be fine. See the article below

http://continuouswave.com/forum/viewtopic.php?f=7&t=739
1992 Outrage 17
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jimh
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Re: Engine mounting height confusion

Postby jimh » Sat May 26, 2018 9:15 pm

Because the spacing between the hole centers is only 3/4-inch, moving between adjacent holes should only change the engine anti-ventilation plate position relative to the hull keel by the same 3/4-inch. The measurements reported here seem to have a variation in the position of the anti-ventilation plate that is much larger than 3/4-inch. The data:

Lowest mount = -3/4-inch relative to keel

One hole up = +1-inch relative to keel
Engine moved up 0.75-inch but plate moved 1.75-inch; this is impossible)

Two holes up = +2-1/4-inch relative to keel
Engine moved up 0.75-inch but plate moved up 1.25-inch; this is impossible)


I don't understand how moving the mount in an increment of 3/4-inch moves the anti-ventilation plate in increments of 1-3/4-inch or 1-1/4-inch. I would double check your measurements. What you describe is just not possible, nor is there any consistency to the distances moved.

Because the transom is usually slanted back at a slight angle, moving up the transom 3/4-inch will not actually raise the anti-ventilation plate at a vertical trim position by the same 3/4-inch. The plate will move less. If the transom angle were 15-degrees, then the anti-ventilation plate at a vertical trim would only move up at a rate of cos15-degrees. The cos of 15-degrees is 0.97. So a 0.75-inch rise on the sloping transom should make a vertical lift of only 0.72-inch.

As Phil mentions, mounting at two-holes-up is often a good spot. The boat performance will depend on the propeller you have. Some propellers don't work as well as others with high mounting height. If you want to avoid making a change, be more conservative and mount at one-hole up. Or, start at two-holes up, and if the propeller ventilates too much, lower one-hole. It is easier to move the engine down once it is on the transom than to move it up; gravity will do most of the work.

JJ Donovan
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Joined: Tue May 08, 2018 7:29 pm

Re: Engine mounting height confusion

Postby JJ Donovan » Sat May 26, 2018 10:30 pm

Jim--I have the K17 steel propeller.

When I did the measurements I put a level on the -[anti-ventilation] plate and leveled the trim. I then took a straight edge laid it on top of the cavitation plate and extended it to the back of transom, then I measured from the straight edge to the bottom of the boat.

If you think my propeller will work [mounted two-holes up], I will start there.

Phil T had the same setup and this setup worked fine for him.

I wonder why the previous owners or maybe the original installer used the [lowest mounting].

I read one of the old posts that stated a Yamaha mechanic recommended the cavitation plate to be within 1-inch from the bottom, which would put me at the [one-hole up].

I will take Phil's and your advice on the [two-holes up] and hope it works. Thank you both again for taking the time to advise me on the best way to do things, it is appreciated.

John.

jimh
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Re: Predicting What Mounting Height Works Best; How to Describe Mounting Height; Goal of Engine Mounting Height

Postby jimh » Sun May 27, 2018 7:22 am

PREDICTING WHAT MOUNTING HEIGHT WORKS BEST
John--I can't say for certain on your boat if a two-hole-up mounting height will be the better for the Yamaha K17 propeller than one-hole-up might be. But I am confident that the lowest position won't be the best. Again, If you want to avoid making a change, be more conservative and mount at one-hole up. Or, start at two-holes up, and if the propeller ventilates too much, lower one-hole. It is easier to move the engine down once it is on the transom than to move it up; gravity will do most of the work.

HOW TO DESCRIBE MOUNTING HEIGHT
Please use the method of referring to the mounting height as recommended by describing the number of units of 0.75-inch or holes the engine is raised from lowest. We don't talk about mounting height in any other way because of ambiguities. We don't count holes from top or bottom. See

Engine Mounting Height
viewtopic.php?f=7&t=739

GOAL OF ENGINE MOUNTING HEIGHT

The goal of adjusting engine mounting height is have the anti-ventilation plate in a position relative to the water flow around it that puts the anti-ventilation plate atop the water or just at the top of the water that is flowing over and around the plate. The goal IS NOT to wind up with a particular distance between the keel and the anti-ventilation plate. The plate should be just at the top of the water, not buried below the water.

Compare at A-V Plate Pictures
http://continuouswave.com/ubb/Forum4/HTML/006491.html

Below is a Yamaha engine with a good mounting height; the propeller was a K17. Note that the anti-ventilation plate is not buried in the water, and the rear of the plate is clearly visible just at the waterline:

Image
As seen here the engine mounting height has provided for the anti-ventilation plate to be running just at the top of the water. With this rigging there was no tendency for the propeller to ventilate, and the there was never any instance of the propeller blowing out or losing grip, no matter what sort of seas were being encountered. The engine was a Yamaha 70-HP and the propeller was a K17.

NAME OF THE PLATE BEING DISCUSSED
The plate or foil on the engine gear case that forms the upper boundary of the propeller aperture is a called the anti-ventilation plate. It IS NOT called a cavitation plate. The purpose of the plate is to prevent ventilation of the propeller. The purpose IS NOT to cause cavitation. Cavitation is a boiling of water due to low-pressure areas developing on the blade tips. The action of cavitation will tend to erode any paint on the propeller blade tips.

REPEATED UP-DOWN OSCILLATION OF THE BOW OR PORPOISING
Some combination of settings of engine throttle, boat speed, engine trim position, weight distribution, propeller type, engine mounting height, and perhaps other factors WILL produce a tendency for the bow of a boat on plane to oscillate up and down. This can occur on almost every boat with the proper circumstances. Some boats will have a greater tendency to get into an oscillation than others. Weight distribution on the boat, total transom weight, engine trim, and propeller type all tend to affect how easily the boat will be to get into an oscillation. The oscillation can usually be suppressed by careful adjustment of engine trim, and often the tendency can be suppressed further by weight distribution.