Engine Mounting Bolts

[Xrturbo]

Q1: how is an engine attached to the transom with bolts?

Q2: what type of bolt is preferred for mounting the engine to the transom?

ASIDE
Hello all. After searching on and off for about 15 years, I am now the owner of a 1980 STANDARD [no model descriptor]. My new boat was owned long-term by a single family and stored indoors most of her life, but unfortunately this boat was a fishing boat and has dozens of holes drilled all over for various gear.

I do not suspect water intrusion as the hull seams to be very light. The boat sits high in the water, and will plane with four people despite only having a 25-HP two-stroke-power-cycle engine.

To be sure [the hull weight was not excessive] I removed the 25-HP outboard engine, I found I could easily off the stern of the boat to clear the trailer.

I am in the process of re drilling all the [existing holes] to a standard size and filling using MarineTex and a syringe.

I discovered the outboard engine was held onto the transom only by the top clamps, even though there were corresponding holes in the transom for bolts.

[Jefecinco]

I recommend 316 stainless steel bolts with self-locking nut. The length should be sized so the ends of the screw threads only portrud by a 16-th to an 8-th of an inch.

[Xrturbo]

Thank you. Your advice is appreciated.

[jimh]

...I am now the owner of a 1980 STANDARD...]

When referencing a particular Boston Whaler boat, please give the model year, the model family, the particular model (usually the length), and any options related to the hull (such as workboat hull, Whaler Drive, and so on.

Q1: how is an engine attached to the transom with bolts?

The transom should be drilled with engine mounting holes that match the pattern on the engine mounting bracket.

If the engine is a modern engine, the hole pattern on the engine mounting bracket will conform to a standard hole pattern known as the BIA engine mounting hole pattern. For information about the BIA engine mounting hole location, and for special situations often encountered on smaller Boston Whaler boats which have a shallow engine splash well, please read the article on that topic at

Transom Mounting Hole Layout for Shallow Splash Well
https://continuouswave.com/forum/viewto ... f=7&t=5808

If the engine is a very old outboard engine, it may have a different engine mounting bracket hole pattern. The BIA pattern became the standard in c.1984.

Also read the three articles that are linked in the above article which give further background on engine mounting:

FAQ: Q8: How does the engine mount ot the transom?
http://continuouswave.com/whaler/reference/FAQ/#Q8

Q9: What is the standard transom hole layout?
http://continuouswave.com/whaler/reference/FAQ/#Q9

Boston Whaler Dealer Bulletin 10-84
http://continuouswave.com/whaler/refere ... 84-10.html

I am certain that after you have read all four linked articles above you will a complete understanding of how an outboard engine mounts to the transom of a Boston Whaler boat.

[jimh]

Q2: what type of bolt is preferred for mounting the engine to the transom?

Engine mounting bolts should be machine screws with fine thread of the recommended diameter as specified by the outboard engine manufacturer. Optimally the bolts should be shoulder bolts, that is, bolts whose threaded area does not cover the engine length of the bold, but leaves most of the bolt unthreaded.

The bolts should be stainless steel of proper marine grade.

The bolts are usually oriented so the bolt head is on the inboard face of the transom and threaded ends of the bolt protrude out of the mounting holes on the outboard face of the transom.

It is essential that the mounting bolts be installed with a marine sealant to prevent any ingress of water into the wood interior of the transom. This is particularly important on the lower mounting bolt engine mounting bolts which often will be continually submerged in water whenever the boat is in use.

One option for the engine mounting bolt holes is to drill them very slightly oversize. Once assuring that the transom wood is completely dry, coat the inside of the mounting holes with thin epoxy resin--definitely NOT with a thickened epoxy like MarineTex. This will reduce the inside diameter of the mounting holes, but completely seal the wood. Then re-drill all the holes to the correct ID to match the mounting bolt size.

If the wood in the transom is wet, you need to wait for it to dry. For advice on that see the FAQ at

Q3: Is there water in a Unibond hull?
https://continuouswave.com/whaler/reference/FAQ/#Q3