Adding backing plate to Prop Strut - Remove strut?

[chowdan]

Hey all,

I had noticed my prop strut is through bolted with just some small normal washers and zero backing plate. The bolts on the inside of the boat are quite rusted looking and the washers also are quite rusted.

I'm doing a haul on March 2nd for a week and was thinking while i'm at it, I might want to think about replacing the strut bolts + re-bed the sucker and add a backing plate.

I had two different thoughts though - if i re-bed it the alignment has to be damn perfect and I don't know how i feel about taking that project on. That brought me to the idea of replacing 1 bolt at a time and adding a backing plate to each bolt that is a G10 plate.

I could loosen 1 bolt - pound it out(hopefully i dont need to drill it out), stick in a new bronze(?) or stainless bolt with a backing plate and then tighten it down and repeat until I have done all of them.

I worry about this latter process causing some over tightening of the existing sealant which will cause a weep.

If i use Bronze, does anyone have any sourcing recommendations? I assume due to lack of oxygen that stainless should be off the table?

[Shrew]

I think you might be over thinking this. You're not lifting the boat by the strut. Cleats hold boats with just washers. What contingency are you planning for? The strut is going to take almost no force pulling away from the hull. Lateral loading and movement are going to be minimal.

Now that being said, I'm certainly a fan of getting the absolute largest washer I can install to distribute any potential load. However, in this case, a backing plate for a shaft strut might be overkill.

Just my humble .02

[chowdan]

Originally Posted by Shrew

I think you might be over thinking this. You're not lifting the boat by the strut. Cleats hold boats with just washers. What contingency are you planning for? The strut is going to take almost no force pulling away from the hull. Lateral loading and movement are going to be minimal.

Now that being said, I'm certainly a fan of getting the absolute largest washer I can install to distribute any potential load. However, in this case, a backing plate for a shaft strut might be overkill.

Just my humble .02

Now that you say this, that does make sense - I was thinking of a collision - ie a log hitting it and causing damage, but thinking about this further - the force of that isn't going to damage the hull as the thickness is quite a bit. The strut or shaft or prop is going to be the thing that is likely damaged first.

That said, replacing bolts still seems logical if they are stainless and been in there for 42 years now. Replacing it 1 at a time, or just pulling it, and replacing all of them and re-bedding/sealing it back together and hoping to god i can get the alignment just right?

[boatpoker]

Originally Posted by Shrew

I think you might be over thinking this. You're not lifting the boat by the strut. Cleats hold boats with just washers. What contingency are you planning for? The strut is going to take almost no force pulling away from the hull. Lateral loading and movement are going to be minimal.

Standing at a fuel dock about 12yrs ago a lovely CS 36 approached. He was dragging a line in the water. When he hit reverse it got around his prop and tore the strut and a big chunk of the bottom from the boat ... it sank before we could get the emergency pump to it. The pump was 20' away.

There was no backing plate, just nuts & washers. I know cause' I investigated the insurance claim.

[Rumrace]

It seems to be a prime spot for failure when neglected. I’ve seen major glass repairs to repair a little wiggle, crushing glass.
So yes anything you can do to spread the load makes sense.
You might try a touch of navel jelly to break the rust on the bolts. I’ve put on top of bilge nuts another zinc nut. Interior anodes get cheap jokes but work.

[sinnerman]

My Cat 30 strut has thru bolts through the hull with washers (flat & lock) and double nuts on the bolts. No backing plate. The bolts have sealant.

The strut assembly fits into a shallow cavity in the underside of the hull and the entire cavity is filled in / glassed over.

The nuts on top are also glassed over, ( I guess as a precaution?).

The fiber glassed-in cavity adds side-to-side rigidity.

https://www.catalina36.org/sites/def...stallation.JPG

cheers

[Minggat]

My strut was never aligned with the shaft log when the boat was built.
You’re right. Getting that alignment right now would not be easy.

I chopped off the hub of the strut and encased the remaining tab in FRP. Basically made a little mini-keel which starts at the shaft log. Far stronger than the original strut.

I made sure that the shaft was centered in the shaft log and held steady by simply putting wraps of blue tape around it. Then proceeded with the built.

It was a gut wrench decision, but very happy with the result.

[Minggat]

I did extend the shaft log itself with a piece of FRP small diameter exhaust pipe. All of this became encased in the solid “mini keel”.

I would not have done any of this if there had been a long run between the shaft log and the existing strut.

[Boatyarddog]

Originally Posted by chowdan

Now that you say this, that does make sense - I was thinking of a collision - ie a log hitting it and causing damage, but thinking about this further - the force of that isn't going to damage the hull as the thickness is quite a bit. The strut or shaft or prop is going to be the thing that is likely damaged first.

That said, replacing bolts still seems logical if they are stainless and been in there for 42 years now. Replacing it 1 at a time, or just pulling it, and replacing all of them and re-bedding/sealing it back together and hoping to god i can get the alignment just right?

If you've done this, before you can ascertain where the "sweet spot" is with regard to rotating the shaft while connected to the gearbox, this is still a ballpark measurement.

Definitely use big flat washers, or backing plate.
If using either, be sure they lay flat on the surface, they'll get pulled around by the contour of the surface and can effect alignment.

If you remove the strut and rebed, be sure to check the alignment in the water after doing so.
.003 is your target measurement, rotating 90 degrees, at each measurement point.
Boatyarddog

[Boatyarddog]

Originally Posted by sinnerman

My Cat 30 strut has thru bolts through the hull with washers (flat & lock) and double nuts on the bolts. No backing plate. The bolts have sealant.

The strut assembly fits into a shallow cavity in the underside of the hull and the entire cavity is filled in / glassed over.

The nuts on top are also glassed over, ( I guess as a precaution?).

The fiber glassed-in cavity adds side-to-side rigidity.

https://www.catalina36.org/sites/def...stallation.JPG

cheers

Funny Catalina doesn't think pre alignment of the strut is important.
There's zero reference to this.
Your supposed to "Eye" it?
Or what?

There's a shaft log there to set the shaft into, and very little clearance all around it.

Make sure your shaft is bolted to the engine coupler before bedding in your strut, rotate the shaft till you find the best spot that feels the smoothest while turning it, a little soapy water on the cutlass helps with a dry bearing.

This will assure that when you go to align your engine to the shaft, you'll have enough room without interfering with the shaft tube.
When tightening the strut bolts, do it evenly, as the strut can slide around in that cavity.
Be sure the shaft can easily rotate, by hand in that strut, before aligning the engine to the shaft.
If you don't you'll wear that cutlass out prematurely.
Boatyarddog

[nortonscove]

I wouldn't oversize anything here. As stated previously there isn't much force at play other than directly on the forward or after end of the shaft. I'd also go with bolts that would shear if hit over tearing a piece out of the bottom of the boat. It's for that reason that a backing plate would be a no go for me.

[Boatyarddog]

Originally Posted by nortonscove

I wouldn't oversize anything here. As stated previously there isn't much force at play other than directly on the forward or after end of the shaft. I'd also go with bolts that would shear if hit over tearing a piece out of the bottom of the boat. It's for that reason that a backing plate would be a no go for me.

You might benefit from post#4, boatpoker,

He relates a story of an insurance claim that shows why one needs to spread the load with respect to strut bolts, washers, and backing plates.
In this case the strut plate tore the bolts out of the boss.

If it had a backing plate, it's much harder to do this, as the load is dispersed.
These are meant to keep strut plates in their positions by spreading load over a larger area.

This also assures the strut will have sufficient attachment if the bolts loosen.
Strut bosses on most vessels are reinforced for this reason.
It would be better if the strut shears off than the entire strut plate pulling out of the boat.

This is the reason for the load dispersing backing plate, reinforces that area.
Keeps smaller bolts from pulling out easily.
Now it can be darn near impossible to fabricate something that fits well, because these areas are so inaccessible in a lot of cases.
Boatyarddog