Question: My friends and I are having a heated debate over what type of bolts to use when mounting an outboard engine. Some say the answer is to use galvanized regular steel bolts, grade five or better. Others recommend using stainless steel. I’m planning a repower on my center-console this winter and want your take on it.
Answer: This is a pretty common question and you are right, everybody has an opinion. Your question came in while I was at the recent Miami Boat Show, and I had the opportunity to check about 15 new boats on the show floor to see what outboard mounting hardware they used right from the factory. The answer? Stainless steel, with either a double-nut locking method as shown in the photo here, or in many cases using stainless steel Nylock nuts. So it certainly does seem that stainless steel is the favored choice of material for the bolts in factory installations.
All that said, there is an important caveat to using stainless steel bolts for this purpose, which is really the origin for the galvanized bolt side of the debate.
Stainless steel is very prone to what is known as crevice corrosion, which is caused by any situation that causes oxygen starvation to the surface of the metal when it may be exposed to seawater nonetheless. If the stainless steel bolts are not thoroughly sealed to ensure that no sea water migrates along the shaft of the bolt into the transom or mount bracket, then danger lurks. Crevice corrosion on these bolts will typically cause the bolt to “hourglass” inside the transom or mount bracket while the visible bolt and head of the bolt will look fine. Enough hourglassing and the bolt will break. Galvanized bolts are not susceptible to this phenomenon and that’s why some folks use those instead of stainless steel fasteners. My choice is the stainless, but make sure that you use plenty of marine grade sealer such as 3M 4200 or polysulfide sealer on the bolts before you push them through the transom. This, by the way, is also important to ensure that water doesn’t migrate into the transom core, which is sure to damage the core material, eventually necessitating a major repair to your boat’s transom.