Transom Bolts question

[ucb4ume]

What kind of bolts should I use to mount my outboard motor to my transom? Should I use stainless steel bolts or some other material? Would stainless steel react with the aluminum on the engine mounting bracket? The boat will live on a trailer and only be in the water for hours or days, not weeks or months.

The boat is an 18' deep-V constructed of wood. You can see pictures of it here: The Turn Over!

The transom is 2.25 inches thick and is made up of three layers of wood laminated together with epoxy and stainless steel screws. It's made up of two layers of 3/4" marine ply and 1 layer of 3/4" thick of solid mahogany.

The engine is a 2000 model Mercury 125 inline 4 cylinder that weighs about 350 pounds.

[marshmat]

The owner's manual for the engine should specify what grade and type of bolts are required. Stainless steel does have some crevice corrosion issues in certain applications, especially when embedded in underwater wood.

On my own engine the mounting bolts are double hot dipped galvanized, and they are holding up just fine. Granted, that boat does live on her trailer 98% of the time.

Make sure the strength grade of the bolts you buy is equal or better than that called for by the owner's manual! Having the motor shear off a bolt and jump ship when you gun the gas would not be pleasant.

[ucb4ume]

Marshmat,

Thanks for the information.

Unfortunately, since I purchased the engine used, I don't have the owners manual. I will use the strongest bolts I can find. I just haven't decided on the material yet.

[PAR]

The usual choice is stainless. Take a magnet with you when you my them. If the magnet sticks to the stainless then it's not good enough, especially if you're heading into salt water.

Also consider "bonding" the bolt hole locations with epoxy, to prevent moisture getting past any bedding.

[alan white]

Just buy 303, 304, or 316 ss bolts. Check all bolts to make sure they aren't magnetic. Bed them in polysulfide or any good polyurethane caulk (Life Caulk from Boat Life is fine) . Bolt strength isn't too much of an issue, as any bolt is strong enough considering the bracket is pressing and not pulling on the transom. Sealing them is important. I would go as far as to say I'd epoxy-jacket the holes in the transom too, to ensure no water could find its way into the core.

Alan

[ucb4ume]

Thanks for the help.

I will plan on using stainless steel bolts.

I also plan on using penetrating epoxy inside the bolt holes to keep the moisture from entering the transom.

Thanks again.

[alan white]

West or any laminating epoxy thickened with high strength filler makes a putty that can create a jacket around a bolt hole. You can bend a nail at the tip 90 degrees about 1/8" to 1/4", chuck it in a drill, and stick it in between the inner and outer layers of glass. That clears the ply away. Then cover one side and push the putty in and cover that side. Afterwards, drill the original hole out.
Many ways to do this, but the effect is an epoxy jacket that permanently seals around the hole.

Alan

[Fanie]

You will probably use 12mm SS bolts there. 4 off is fine, they won't break.

Put some grease on the bolts thread if you use SS nuts or nylocks. SS is a bugger to freeze the nuts up if there isn't any lube on them. If they do freez up you'll have to cut them off and replace. On the transom inside put some nice big (penny) washers to prevent the bolts from sinking into the transom, they do after a while. Some guys make a 2mm SS plate with holes in them to prevent this. On the motor side standard flat SS washers otherwise the nuts will damage the bracket when you tighten them up.

The motor bracket has a lip or hook shape to the top where it fits over the transom, it is good practice to have it hang on the transom when you let the ss bolts through. The slots in the motor bracket should be at the lowest position so you have a bit of play to adjust upward if need be.

Make sure the cavitation plate is at the right height, usually the cavitation plate should be about level with the bottom of the hull. If too high or low you won't get proper performance, and you could get lots of spray behind the boat or severe drag if the cavitation plate is under water when you plane.

Nice size motor. Nice performance while not too wild on the fuel either. If you don't floor it all the time and just cruise you can go quite a ways on a tank. If you're not used to a big motor, get to know it a bit and how it behaves with the boat.

He he... enjoy while there's still some fuel left

[powerabout]

Buy the factory bolts as there will be none finer on the market.
These bolts are mission critical so I doubt they will give you a spec
which means if something goes wrong you will not be able to prove to your insurance company you have adequate bolts!