Engine Mounting Height

Optimizing the performance of Boston Whaler boats
jimh
Posts: 5445
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
Contact:

Engine Mounting Height

Postby jimh » Fri Apr 15, 2016 12:51 pm

When discussing engine mounting height, the engine height is described in units of 0.75-inch or "holes" and the reference is to the lowest possible mounting position. The possible mounting height descriptions are thus:

--"all the way down" or "the lowest": the mounting location when the engine is mounted to the transom so that it is in the lowest possible position;

--"one-hole up": the engine has been raised by 0.75-inch or the spacing of one set of the pre-drilled holes from the lowest position;

--"two-holes up": the engine has been raised by 1.5-inch or the spacing to two sets of the pre-drilled holes from the lowest position;

--"three-holes up": the engine has been raised by 2.25-inch or the spacing of three sets of the pre-drilled holes from the lowest position;

--"four-holes up"; the engine has been raised by three inches or the spacing of four sets of pre-drilled holes from the lowest position; only possible on engines with five sets of holes.

Visually one can reference the mounting height as follows; imagine the bolts are installed in the indicated hole:

o -- lowest position
o
o
o
o

o
o -- one hole up
o
o
o

o
o
o -- two holes up
o
o

o
o
o
o -- three holes up
o

o
o
o
o
o -- four holes up (only possible on engines with five sets of holes)

Generally if an engine mounting plate has only four sets of pre-drilled holes, then "three-holes-up" is as high as the engine can be raised without adding some other bracket or mount. Some engine might have five sets of pre-drilled holes, so they could be raised to a "four-holes up" position, which would then be 3-inch higher than lowest.

Because of the ambiguity of how many holes are in a particular engine's mounting plate, four or five sets, engine mounting cannot be clearly described by offering descriptions of which hole the bolt fasteners as passing through and counting up from the bottom set. For this reason, using the engine mounting height descriptions above are much preferred. They will directly communicate to readers the engine mounting height without any ambiguity.

Any reference to engine mounting height assumes that the holes drilled in the transom are in the standard locations as specified by the BIA mounting template layout. For more information about the standard location of outboard engine mounting holes in boat transom, see the FAQ Answer #9, "What is the Standard Transom Hole Layout?"

There are three additional variables affecting engine mounting height: the height of the transom, the length of the engine's shaft, and the propeller.

The height of the transom on a particular boat is where the manufacturer intended it to be, but this might vary among various models from the same manufacturer as well as among different manufacturers. What one boat maker thought was a transom height appropriate for a "25-inch-shaft" engine might be slightly different from another manufacturer.

Similarly, among outboard engines there can be some slight variation in exactly where the propeller shaft will land relative to the mounting bracket, and one should not assume these dimensions are always exactly identical among all outboard engines.

Different propellers also perform differently at different mounting heights. Some propellers are designed to perform best if mounted to engines at elevated engine mounting heights, and other propellers may not tolerate such high mounting. Propellers are also affected the the gear case shape ahead of them in the water, so propellers can perform differently on different engines.

For these reasons it is common to find that engine mounting height can vary according to boat brand and model, engine brand and model, and propeller brand and model. So it cannot be said that a particular engine mounting height, say one-hole-up mounting, will always be the best for every boat, every engine, and every propeller.