CMC 35 install ?

Hey all new to the site form the PNW with a question regarding a recent install of an aftermarket tilt trim unit.

Mounted on a 2015 starcraft Alaskan with a 20 hp Yamaha hanging off the back.

Added the trim tilt plate over the top of the screw heads that secure the transom support to the transom.

When mounting, the plate sucked up to the edges where the screwed weren't but appear to have deflected the transom inward where the screwed heads are proud of the skin.

I woke up thinking I should have added spacers so the bracket floated above the screw heads.

I am worried about damage and wonder if I should reinstall with spacers or let is sit as is, thinking the on/off pressure then a no pressure may me worse.

First tin boat and a little leary.

Thanks for any assistance

headduck said:

Hey all new to the site form the PNW with a question regarding a recent install of an aftermarket tilt trim unit.

Mounted on a 2015 starcraft Alaskan with a 20 hp Yamaha hanging off the back.

Added the trim tilt plate over the top of the screw heads that secure the transom support to the transom.

When mounting, the plate sucked up to the edges where the screwed weren't but appear to have deflected the transom inward where the screwed heads are proud of the skin.

I woke up thinking I should have added spacers so the bracket floated above the screw heads.

I am worried about damage and wonder if I should reinstall with spacers or let is sit as is, thinking the on/off pressure then a no pressure may me worse.

First tin boat and a little leary.

Thanks for any assistance

Click to expand...

May have placed this in the wrong forum. Please move if necessary. Apologies.

Welcome to Tin Boats! This forum is fine for your question.

I think these mount on the outside of the transom. I also think you are saying there are screw heads on the outside of the transom that are now pushed in along with the surrounding transom skin. I'm not sure what you mean by transom support. Are these through bolts to an inside knee brace? I think I would remove the CMC to relieve the stress on the transom skin and better evaluate the problem. I think I would also back out the screws to see what happens. Probably wishful thinking that things would return to normal. If they are through bolts, then you might be able to lightly tap them back from the inside and hope the transom skin follows - again might be wishful thinking.

If installing the bracket over these screw heads, could you drill a matching hole in the CMC bracket and reinstall new screws through the bracket? If feasible, I think I would do that rather than leave the screws as is.

Pic's would help folks understand the issue.

Picking up what I'm laying down pretty well.

Upon closer inspection, looks like less deflection in the skin and more so in the bracket. I may have chicken littled it a bit.

Here a pic or 2.

I lean towards the addition spacers at the attachment points. A washer would me 90% there.

The screws heads are through bolts to a transom bracket. There appears to be no movement in the inside of the transom.

My concern would be that the bolt heads that are trapped are now under a lot of stress, or the transom skin itself is. I think I'd make up a spacer plate, thicker than the screw heads and at lease as large as the CMC bracket. Then cut reliefs in the space plate for the screws.
I'd also either heavily paint or grease the panels to prevent corrosion or salt build up.
Aluminum does bad things when stressed for long periods, if the stress point turns out to be the transom skin or inner bracket its going to be a lot harder to fix than making a spacer plate.

I don't have the ability to fab a plate but understand the idea of pay now or pay a bunch more later.

The washer spacers may leave too little of the plate in contact for my liking.

Perhaps aluminum strips as a spacer along the 2 vertical outside edges ... But then this will concentrate loads in an odd way.

There is about 2½" of wood sandwiched between the inner and outer skins.

Does the bracket need corrosion protection? I do the salt but give a decent rinse every time.

headduck said:

I don't have the ability to fab a plate but understand the idea of pay now or pay a bunch more later.

The washer spacers may leave too little of the plate in contact for my liking.

Perhaps aluminum strips as a spacer along the 2 vertical outside edges ... But then this will concentrate loads in an odd way.

There is about 2½" of wood sandwiched between the inner and outer skins.

Does the bracket need corrosion protection? I do the salt but give a decent rinse every time.

Click to expand...

Maybe relieve the plate itself. The few dimples or even holes should not creat too much of a compromise, it's pretty stout.

headduck said:

Maybe relieve the plate itself. The few dimples or even holes should not creat too much of a compromise, it's pretty stout.

Click to expand...

This is a good option. Just to make sure we are thinking the same thing, mark the location of the bolt heads on the inside of the CMC bracket plate. Then drill some dimples (or even thru holes if needed) so when installed the plate will sit flat against the transom. In the meantime remove or loosen the bracket to take the stress off.

BTW, probably a typo -- your transom wood is likely 1 1/2", not 2 1/2".

Looked again... sometimes it takes a few good looks and thinking in between...

The transom side plate is 2 pieces bound by the ram mount in the middle. The plates flexed a bit and the ram bracket bent to accommodate.

I'm gonna sit on it for a moment and think through y'all's advice.

Thank you.

I had that situation once in a boat. I first put dabs of paint on the bolt/rivet heads and pressed the plate into place to mark the back exactly. Then I used a Dremel tool or a die grinder and a round ball stone to put dimples in the plate. It worked awesomely!

If I recall, I had to do it more than once, as the bigger bolt heads held it away from the smaller ones, but In the end, I was super proud of the “factory” results. It came out pretty perfect.

Just an idea/suggestion that others have probably given already. (Sorry, haven’t read all the replies yet - busy day!). I hope it helps.

One other warning I just thought of is when I had a transom skin deflect like that, it turned out that the transom wood was mulch inside. The transom support made everything feel solid, so it was sneaky.

Eventually, I replaced the core, but thought I would share that experience.

Appreciated.

The transom is super solid. All layers visible and nice shaving when drilled.

The more I look, the less it seems the transom moved and its robustness seemed to force the plate to bend instead.

The plate now has an obvious bend which would need to be remedied if I dimpled the plate for a flat fit ... One thing leads right into another...

With apologies, I think I may be misunderstanding. I think you won't go wrong following @thill 's advice.

All advice/ideas appreciated.

Thank you.

headduck said:

Appreciated.

The transom is super solid. All layers visible and nice shaving when drilled.

The more I look, the less it seems the transom moved and its robustness seemed to force the plate to bend instead.

The plate now has an obvious bend which would need to be remedied if I dimpled the plate for a flat fit ... One thing leads right into another...

Click to expand...

Wow, you must have really cranked down on those bolts. Impact gun?

Either way, I hope you get it worked out.

headduck said:

I don't have the ability to fab a plate but understand the idea of pay now or pay a bunch more later.

The washer spacers may leave too little of the plate in contact for my liking.

Perhaps aluminum strips as a spacer along the 2 vertical outside edges ... But then this will concentrate loads in an odd way.

There is about 2½" of wood sandwiched between the inner and outer skins.

Does the bracket need corrosion protection? I do the salt but give a decent rinse every time.

Click to expand...

You could use wood instead of an aluminum plate but making a plate isn't that difficult. I'd measure the rear of the CMC mounting surace, and go get a piece of 1/4" plate at your local metal supplier. then using a hole saw make some relief holes for the bolt heads your covering up.
Its not so much the salt in between that's the issue, its the likely hood that any two pieces of metal will be disimilar enough to induce some galvanic corrosion, and that will eat through the thin transom skin faster than it will eat in the plate or the CMC frame.

If you use wood, I'd use plain exterior grade wood, cut it the full size of the bracket or slightly larger and drill the needed relieve holes and bolt holes. Then paint it well. I'd consider painting or at the very lease greasing the mating surfaces to help stave off corrosion over time. I would not leave the bolt head pressing against the plate or transom like that, they will eventually either pull through or crack. At this point, if the transom skin has been sucked in by the bolt heads, I'd even consider some larger washers over that area to redistribute the load.
Do not use silicone or any sort of caulk, it will only invite corrosion since it'll make the area harder to flushout or air dry.

As posted above, aluminum doens't like to be under stress for long periods. It fatigues in ways you don't want to deal with on the water. It may be next year, it may be 10 years from now but it won't be good when the time comes. Do not use pressure treated wood, the cupric sulfate in it will eat the aluminum fast. On my boat I cut a 1/4" plate of aluminum, marked and masked off the area to be covered, then sprayed that area with some Rustoleum bed liner in a can, then did the same to both sides of the plate, let it dry, then mounted my motor to the plate On mine, was also replacing the transom wood so I installed new bolts, that went completely through the new plate, and through to the knee braces. I also cut strips of stainless steel to reforce the edges of the knee braces since one had a few small cracks in it. I wasn't using a CMC jack but having that plate there helps spread the weight and torque of the heavier motor out a bit and it acts as a buffer or wear plate against the transom. I did the same thing to back up the trolling motor bracket I added as well.

No impact gun. The center tie is thin and deformed fairly easy. Should straighten easy too.
As the transom side is made of 2 half's, the inside plate edges tilted away from the transom easily.

I did use 4200 at the through holes and between washers. None along the bracket.

I don't really understand the stress aspect of it all. I understand how it's loading etc but not how the aluminum responds. In essence I'm increasing bolt head force... Could it push through?

Love to read a bit more of there's a good source.

The bracket bolts are installed as manf directs, but it's generic of course.

How would the inside assembly (sealant, washer, lock nut) be improved? Was a plate used both interior and exterior of transom in the example above?

Thanks. Learning stuff.

headduck said:

I don't really understand the stress aspect of it all. I understand how it's loading etc but not how the aluminum responds. In essence I'm increasing bolt head force... Could it push through?

Click to expand...

If I'm understanding what is happening, you put a lot of stress on the outside aluminum transom skin around those bolt heads that pushed in under the CMC mounting rails. You run a good risk that will cause the transom skin to crack around those bolt heads.

However, I'm just not clear what exactly now has a bend in it. When you say "plate" are you talking about the CMC mounting rails that are bolted to the transom? Those are pretty robust. It would be pretty hard to bend those. I suspect they may have just kind of racked sideways because they were not sitting flat. I hope you have removed them already, but if not, remove them and check. Then remount the CMC rails so they sit flat on the back of the transom. I personally would use the "dimple" solution provided in a couple of different posts above.

That's right the bracket halves are tilted and maybe slightly bent with the "connecting plate" bending the most to accommodate.l the irregular surface

Ill go through it again an see what the most direct way to relive the stress. There are bolts for the plate that connects the 2 halves, right where a dimple would go for a pair of the transom bolt heads. Adding a little complication for the dimple process, or even negating it.

A composite/plastic plate gasket would get there most easily while keeping materials separate.

Do y'all know of good options?

The bracket and the boat are both aluminum so is the corrosion risk still as severe? Bolts are SS.

I have a 1/2" piece of starboard that could be easily made into the ice cream between two aluminum sandwich pieces.

But it's a soft material and may move too much over time.

Something harder but just as durable would be nice