Outboard Motor Mount: Butyl tape or caulk?

[RhythmDoctor]

My C250 does not have a retractable motor mount. Instead, the outboard (a 15hp Honda) mounts directly on the transom. The fiberglass is protected by a SS plate on the front of the transom, and a ~10"x10"x1/8" piece of starboard on the back. I recently removed the starboard because there was some moisture behind it. It had been attached by four screws at the corners, with some caulk bedding between the starboard and fiberglass. The starboard popped off the caulk easily ('cause starboard don't stick to nuthin'), and I was able to scrape the caulk off the fiberglass easily with a razor blade scraper. Based on the properties of the film, I think it was 4200 that had been used.

I would now like to re-mount the starboard onto the transom. What do you suggest that I use? I'm considering butyl tape, but have some concerns. This plate will be subjected to fluctuating pressure and vibration from the outboard, so I am concerned that the butyl tape might "ooze" out over time, causing the starboard to loosen under the brackets - not something you want happening to an outboard. But on the other hand, if I could use a rolling pin to get the butyl tape down to about 1/16" thickness and coat the whole board with the stuff, the hydraulic lubrication forces to squeeze it out would be HUGE, so the bed should be stable. And its viscoelastic properties would be a nice shock absorber. (sorry about the engineer-speak)

On the other hand, traditional caulk would cure, so that it would never ooze out. In the case of the caulk, I would be more concerned about how to squeeze out the air from behind the starboard so that it does not accumulate water back there again. If I squirt a bead all around the perimeter of the board before placing it on the boat, there is no place for the air to escape. Should I use a slotted trowel to apply it (like flooring guys use)?

Which do you guys think I should use? Butyl or caulk? What type of caulk?

[LakeSuperiorGeezer]

I think the viscoelastic properties of butyl tape will more likely result in the the material being slowly forced from behind the new plate over days time until there is no cushion left. Better have a cable hooked to the outboard in case it comes loose. Keep tightening until all the oozing is done with. If you go with caulk, put a coating on the transom and the starboard material and keep checking the clamps until all excess material is forced out as with the butyl tape precedure. I think either way is alright because the clamp on the outboard is what will hold it on there.

[deniseO30]

Rick.. I dunno.. but is starboard even appropriate for backing up the twisting induced by the weight of an outboard on the mount? 3/4" plywood or even an aluminum plate comes to mind when I think about it.

[Maine Sail]

I would not use Starboard as a backing material as it adds little to no benefit and as you found out nothing sticks to it. Marine ply or solid fiberglass are better solutions.

[RhythmDoctor]

Starboard has worked fine for 13 years. No reason to change. The moisture was minimal, but I did not know that until I pulled it off to inspect.

I actually prefer poor adhesion - I want to re-bed the SS plate but still can't get it off because it's stuck too good.

[Ulladh]

If the goal is poor adhesion but limited capillary wicking then try liquid gasket from an auto parts store.

[LandLocked66c]

Interesting question. How do people mount the outboard brackets then. On my boat someone used thick rubber mat on the outside between the mount and transom. What's the proper method?

[RhythmDoctor]

Originally Posted by Ulladh

If the goal is poor adhesion but limited capillary wicking then try liquid gasket from an auto parts store.

Colin,

That's exactly my goal. I'll consider the liquid gasket. But I think I'm leaning toward the butyl tape instead. After your post I realized that I care more about eliminating capillary wicking between the gasket and the transom (and the screw holes in the transom) than I do between the gasket and the starboard. I don't care if there's water under the starboard - I just don't want it getting inside the transom through the screw holes. So I think what I may do is to countersink the screw holes, fill with butyl putty (by wrapping the screws as described on Maine Sail's site). Then I will use a pin roller to flatten out some butyl tape into a very thin mastic that I will roll directly onto the transom (to ensure no air bubbles). Onto this I will apply the starboard, which has a somewhat pebbled surface that will allow most of the air to escape.

I still have a couple days to think this over, as my first priority is to try to get the SS plate off the other side of the transom. I really want to countersink the holes under that and re-seat it to, so I'm going to try to use a heat gun to get it off. First, I will try the heat gun off the stuff I scraped off the transom to see how much it softens, since I assume they used the same stuff under the SS plate.

[RhythmDoctor]

As I stated before, I am going to stick with the starboard protective cover over the outer part of my transom. I finally managed to get the SS plate off the inner part of my transom by wedging a putty knife behind it as I poured a few drops of acetone into the gap to soften the mastic (which I believe was 4000 or 4200 - but am not at all sure). That SS plate was REALLY anchored on strongly.

I'm still looking for a sealant that will get me a good water-tight seal around the screw holes (which I have now countersunk). Adhesion to the starboard is not essential IMO, but compatibility with it is since I don't want to compromise its integrity. I would like to have some UV resistance so the edges don't dry out and crack like they did with the prior mastic.

I'll look into the liquid gasket suggestion, but not sure of its compatibility with the starboard. Any other suggestions of what I should use?

Ideally I would like something that I can apply with a slotted trowel, since I think that would get the most even film coverage. The 4200 would definitely not work for this, because its way to sticky coming out of the tube.

[eziron]

I am very new to sailing, but I know butyl tape and sealents. They both have there purposes and I don't think this is one for butyl. If you use a caulk stay with a good polyurethane base (sikaflex 201, vulcam 116, ) are excellent. If u decide to go with butyl I can give u the name of my supplier to get a good grade and 3 inch wide.