Transom Mounting Hole Layout for Shallow Splash Well

Optimizing the performance of Boston Whaler boats
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Transom Mounting Hole Layout for Shallow Splash Well

Postby jimh » Mon Aug 24, 2020 6:18 pm

A diagram has been frequently posted by co-moderator Phil T that shows the typical engine mounting bracket hole layout, called the BIA (Boating Industry Association) layout. The exact author of the diagram is unknown to me.

I am not sure if the BIA still exists as an association, but its standard engine mounting bracket hole layout certainly does. It is in wide use on almost every modern engine. I believe it came into use based on what was being used by OMC.

I have emended the drawing that was being posted to have improvements. Below is the improved drawing that explains the BIA engine mounting hole layout for a transom and the options available for boats with shallow engine splash wells as is often found on 13 to 17-foot older Boston Whaler boats. My hope is the new drawing will be easier to understand that the one often posted previously, which to me was only comprehensible if you already knew the information the old drawing was trying to illustrate.

Fig. 1. Transom hole layout for Engine Mounting Brackets with BIA Hole Layout.
biaHoles.png (74.5 KiB) Viewed 9935 times

The drawing above shows several variations for the transom hole layout. The location of holes in the transom is chosen to match the location of holes in the engine mounting bracket. The drilling of holes in the transom always follows the horizontal spacings and centerings shown for the engine mounting holes in the engine bracket as illustrated above, but the vertical position of the transom holes is often done with some variation from standard.

Sometimes the top pair of holes is not drilled at the 1.875-inch distance from the top of transom. The standard vertical offset is highly recommended, but some installers may have drilled the transom hole slightly closer to the top of the transom. If so, the engine mounting height will be slightly raised from the standard position. If the top holes are spaced farther than 1.875 below top of the transom, there may be interference in mounting the engine. The engine bracket often has a horizontal lip that will rest on the transom. The top pair of holes are shown in black in Figure 1 above. All other hole vertical spacing is measured from the existing top hole location on the transom.

Only ONE set of lower holes will be drilled in the transom. In order to understand what the diagram is trying to show, some additional explanation will be provided.

The "standard" pair of bottom holes is spaced 8-inches from the top pair, and in the diagram above those holes are shown in red. This is the normal location for the lower mounting holes. On all modern Boston Whaler boats, the engine lower mounting holes in the transom would typically be located in this position. (I believe the red color was used in the original drawing as a warning that this hole location would end up under the splash well on older Boston Whaler boats.)

Non-standard vertical spacing of the lower transom holes may be used in order to work around a shallow engine splash well on the transom of an older Boston Whaler boat, usually a 15-footer or a 16-footer. This occurs with the smaller sink-type engine splash wells. Drilling the transom for a lower set of holes at 8-inch spacing often results in the inboard end of these holes exiting BELOW the bottom of the engine splash well and thus being inaccessible.

As a remedy, the transom holes for the lower pair of engine mounting bolts can be drilled at a non-standard and shorter vertical spacing. This tends to solve the problem of the hole inboard exit being below the splash well. However, if a shorter-spaced non-standard bottom transom hole location is used, it will limit the engine mounting height range.

In the diagram in Figure 1 there are four sets of lower engine mounting hole locations shown. Again, only one set of holes will actually be drilled in the transom.

If the lower pair of transom holes is spaced 7.25-inches as shown by the yellow holes in the diagram, this raised location may allow the inboard end of the hole to be above the bottom of the engine splash well. However, this hole is often drilled at an angle upward to give the inboard end more clearance above the shallow engine splash well. If this position is used then the engine mounting height must be at least one-hole-up from lowest. There will be a slight misfit of the nut onto the engine bracket and a similar misfit of the bolt head at the inboard end onto the transom. See the Dealer Bulletin 10-84 for more details, linked below. (I believe this hole was denoted in yellow in the original to warn that the inboard end might not quite clear the bottom of a shallow splash well.)

If the lower pair of transom holes is spaced 6.50-inches as shown by the green holes in the diagram, the inboard end of the hole should clear the bottom of a shallow engine splash well. With this lower hole position the the engine mounting height must be at least two-holes-up from lowest. The hole can be drilled without an angle, so the fit of the bolt and nut will be uniform. (I believe the use of green color for this hole was to indicate it would be certain to clear the bottom of a shallow splash well.)

The "blind hole" location that was provided on older outboard engine mounting plates but typically not found on modern engines is shown with no color in Figure 1 above. The blind-hole mounting was used to mount older outboard engines on older Boston Whaler boats with shallow engine splash wells. If the engine has blind hole mounting threaded bosses, then once the engine is mounted with this option its mounting height is fixed at the lowest position and cannot be adjusted. For this reason, using the blind hole mounting is not recommended. You would very seldom if ever use a blind hole mount today.

A further variation in engine mounting brackets themselves--not in the transom---is found quite often on modern engines. Instead of four pairs of lower mounting holes drilled separately in the engine bracket, there are instead two long vertical slots in the engine mounting bracket that extend over the range where the individual mounting holes would have been. This helps to cure problems if the transom holes were not drilled at exact 8-inch vertical spacing or exactly at 0.75-inch offsets from 8-inches.

If is now often seen that engine mounting brackets have five sets of hole spacing, thus allowing the engine mounting height to be raised to a four-holes-up height in some cases.

The FAQ explains in detail all the variations in transom hole locations for various situation. Please also refer to the FAQ for advice. There are several ways to work around the shallow splash well problem.

The engine mounting variations are described in Q8.

The standard hole layout is described in Q9.

Also see the 1984 Boston Whaler dealer bulletin about engine mounting problems which is reproduced at ... 84-10.html

The Boston Whaler dealer bulletin offers further advice about engine mounting for shallow splash well situations, particularly if an angled hole is drilled.