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Author Topic:   Engine Mounting Height
billmontauk posted 04-04-2004 09:05 AM ET (US)   Profile for billmontauk  
[What is the proper height for mounting an engine on a classic 16-foot Boston Whaler boat?] [My engine is] a Yamaha 90 2-stroke 2004. Thanks Bill
peetmin posted 04-04-2004 09:45 AM ET (US)     Profile for peetmin  Send Email to peetmin     
Bill- I think in most cases you want the anti-cavitation plate on the motor to be level with the bottom of the keel of the boat. My motor was an inch too low and it was spraying water all over against the transom. I raised it one hole, got better performance and much less water deflecting all over.Pete
jimh posted 04-04-2004 10:51 AM ET (US)     Profile for jimh  Send Email to jimh     
Using "hole" as a unit of measurement, the typical mounting height of an engine on most Boston Whaler hulls is "one hole up." This will usually put the anti-ventilation plate on the motor even with or slightly higher than the bottom of the boat.

There have been many reports of engines mounted higher than this. If raising an engine to greater height, it is wise to monitor the cooling water pressure to be certain the engine is able to draw sufficient water.

With a 2004 model engine, you may have additional water intake ports on the lower unit. Many engine makers now include a lower water intake port to insure better water flow when the engine is mounted at elevated height.

The type of propeller being used also affects the ability to run the engine at elevated mounting. Your propeller may not "hold" the water as well if mounted too high.

JohnJ80 posted 04-04-2004 02:19 PM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
BW recommends that they AV plate be 0-3/4" higher than the bottom of the boat (hull). Often/usually you will be able to mount it higher. Typically this yields a better ride, better performance. Each hole that you raise the motor removes 3/4" of the lower unit from being in the water - this is considerable drag.

Factors such as seaway, prop selection, loading and driving style can all affect the performance at a particular vertical height setting.

Basically, the advice that I have collected, followed and now promote is to raise the motor as high as you can such that you still maintain a cooling water stream and you do not get prop blowout.

Ride harshness, porpoising and top end speed are all factors that are improved (typically) by raising the engine on the transom.

When I repowered last year, the dealer mounted the motor all the way down. The degradation in the ride and performance was scary. I was all but ready to take the whole thing back when I started working on the tuning and especially vertical adjustment.

What you should do is to plan on about 2 hours with the trailer boat at the ramp to move this around and evaluate how it works for you (its easy to do this at the ramp with a few tools). Once you dial it in, you will be - I think - pleasantly surprised at the improvement in performance.


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