Considerations in Engine Mounting Height

Optimizing the performance of Boston Whaler boats
jimh
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Considerations in Engine Mounting Height

Postby jimh » Sun Aug 08, 2021 9:10 am

Engine mounting height is often raised with the the goal of improving boat performance on the basis of reducing the amount of the engine's gear case that is actually in the water, thus reducing drag. A visual observation of the position of the Anti-Ventilation plate to the flow of water around the engine gear case is often suggested as an method to assess proper engine mounting height. The usually stated goal is for the top of the A-V plate to be clearly visible above the water flow. However, there are limits to how high the engine mounting can be raised. The limiting factors on how high the engine mounting can be raised are really two: the cooling water pressure and the propeller design.

As long as the engine cooling water pressure stays within the recommended range as the engine mounting height is raised, the engine will not be affected. However, at some point with the typical engine gear case and the typical engine gear case water intake port location, the cooling water is going to be affected by the raised engine height.

Some newer engine gear case designs have moved the location of the water pickup to the nosecone of the gear case bullet, which improves cooling water pressure at high boat speeds with elevated engine mounting. I have no idea if your engine has that sort of cooling water pickup location. Some engines offer optional cooling water pickup screens which will improved water flow when the engine runs at an elevated position. Cooling water pressure must be maintained as the engine mounting height is increased.

The second factor affecting engine mounting height is the propeller. Modern propeller are often designed with the idea that the blade tips might be running very close to the water surface or for the whole propeller to be running in aerated water. In some cases the designated pitch for the propeller is stated for the propeller being run in those conditions, rather than for the propeller being run with much greater immersion and operating in solid water. For an example, the Mercury REVOLUTION4 propellers seem to have a pitch number that is appropriate only if the propeller is being run at elevated engine mounting height. If the REVOLUTION4 propeller is run with the blades fully immersed in solid water, the effective pitch of the propeller is much larger than the declared pitch stamped on the hub.

Another factor in the propeller design is the diameter. The smaller the diameter of the propeller the greater the distance of the blade tips from the Anti-Ventilation plate. The smaller diameter performance propellers are often actually only smaller in diameter because their blades have a high rake angle. Propellers with higher blade rake angle can often be operated with higher engine mounting heights because their blade tips are perhaps 0.5-inches farther away from the A-V plate than on another propeller with less blade rake and thus larger diameter.

Another propeller factor is how well the propeller maintains good thrust in rough seas or in tight turns. Propellers running at elevated engine mounting heights may tend to ventilate in rough seas or in tight turns. For those reasons, a lower engine mounting height might be more appropriate for those sea states.

Generally if you raise the engine mounting so the A-V plate is running dry and above the water, you will improve the boat speed in calm water and straight line running. Just how high you can go will be determined by the two constraints mentioned: cooling water pressure and the particular propeller being used.

jimh
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Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
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Why Raise Engine Mounting Height

Postby jimh » Wed Jun 15, 2022 12:06 pm

Raising the engine from the lowest mounting position as it is now to a higher engine mounting may improve the performance of the boat in certain situations. Typically the reason for raising the engine is to reduce the drag of the gear case as it is pushed through the water by the propeller.

When the engine mounting height is raised, the drag will reduce. This can cause the load on the engine to decrease. With less load on the engine, the engine will tend to continue to accelerate until the engine power output reaches the same power as the propeller is putting on the engine. The greater engine speed will mean greater propeller shaft speed. Generally greater propeller shaft speed produces more thrust, and more thrust produces more boat speed.

In some instances raising the engine mounting height reduces the lever arm by which the propeller thrust is applied to the hull with regard to fore-and-aft trim and may affect the tendency for the bow to oscillate, and with the hope that the effect will be favorable

Other effects of raising the engine can be a loss of cooling water pressure and volume, leading to engine overheating. The propeller may begin to operate in airy water, which will reduce the thrust produced by the propeller accelerating less dense water astern.

There may be a loss of sternward propulsion because the propeller thrust when directed toward the bow may be hitting the transom.

Exactly what outcome will occur in a particular instance is hard to say, but generally an engine mounting height of one-hole-up is quite normal and in some instances two-holes-up mounting is also deemed as useful.

jimh
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Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
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Re: Raising Engine Height

Postby jimh » Sat Jun 18, 2022 10:06 am

Before the BIA standard transom hole layout and BIA engine mounting hole layout came into use in c.1983 and caused Boston Whaler to sent out Dealer Bulletin 10-84 to advise dealers on how to drill the engine mounting holes in the transom of their boats with shallow engine splash wells, there is a very reasonable basis to infer that OMC outboard engines were routinely being installed on Boston Whaler boats by using the blind mounting holes. Those engines would have been installed at what is now called the lowest possible position but what at that time was just thought of as the normal mounting position.

That maybe 40-years later there is a school of though that an engine mounted with the blind lower hole location must no longer be “normal” and that the “right” way to mount engines on these boats with shallow engine splash wells is to use a method that makes mounting the engine at an elevated position of two-holes-up the lowest possible mounting height seems a bit revisionist.

There might be a Boston Whaler boat that has been operated for decades with its engine in what was once the normal manner and is now to be considered an inappropriate manner, and no one ever gave a thought to it for all those years.