Loose Prop Strut

Apr 24, 2020
40
Hunter Cheribini 37 Portland
on my 83 HC 37 I wrapped a line and loosened the strut. It leaks badly. it has two bent 3/8" throuh bolts holding it. I removed it and see what looks like, 4 embedded threaded nut plates underneath. What's the story? Has my strut been replaced with a two bolt model
What's common knowledge? I don't want the strut to bend again and have the boat leak badly.
Dan
 
Jun 5, 2010
1,067
Hunter 25 Burlington NJ
I am always wary of any metal ‘embedded’ in fiberglass. This is nearly always a recipe for later disaster.

The correct way (and a decent fix for you) is to drill right through the boat from outside. If you drill through the plate, without removing it, so be it. Locate the holes inside and lay up some extra ‘glass - going wider than you think you need to. I wouldn't bolt a prop strut to anything less than 1/2” of glass that extends a good 4” or more beyond the bolt holes.

Then use a strong fiberglass backing plate (see McMaster.com) like 3/8” thick, NEW bronze bolts (NOT brass - see BoltDepot.com), wide bronze fender washers and bronze locknuts (or lock washers if you can’t find them and use ‘large-pattern’ hex nuts).

And - in spite of what anyone else here says - assemble the whole thing smothered in 5200, which is the only thing the factory would/should use. The 5200 will hold better than the bolts do (it’s what holds your keel on), will not fail underwater and admit water to your newly-constructed structural sandwich, and will provide the required vibration-dampening flex without breaking its seal.

I have done this job often in production. And I know I get known as ‘Dr Doom’ on here; but it’s because my standards are noted in the industry for being consistently higher than those of people who think an amateur’s assumption of ‘good enough’ is good enough for a space capsule taking you into an alien environment that can kill you (i.e., a boat going to sea). There really are some things some boat factories get right; and one of them is a 50-plus-year track record of relying on 5200 for structural integrity.

Backing plates ‘embedded’ in fiberglass is how motorboaters mount their seats’ bases, which accounts for why (with the typical motorboaters’ fetish for silicone) most of them over 20 years old have rotten cockpit soles. I could explain why they think this is the only way (since it’s almost 100% universal); but the answer would come down to poor design, poor production techniques and a lack of caring - none of which any sailboater should embrace, tolerate or deserve.
 
Jan 1, 2006
6,100
Slickcraft 26 Greenport, NY
Once the strut has changed position doesn't alignment of the shaft become an issue? For my situation removing strut bolts to re-seal, I didn't get the strut into the exact same position which led to further problems.
 

Johnb

.
Jan 22, 2008
1,296
Hunter 37-cutter Richmond CA
on my 83 HC 37 I wrapped a line and loosened the strut. It leaks badly. it has two bent 3/8" throuh bolts holding it. I removed it and see what looks like, 4 embedded threaded nut plates underneath. What's the story? Has my strut been replaced with a two bolt model
What's common knowledge? I don't want the strut to bend again and have the boat leak badly.
Dan
Dan, does it look like mine - wish I had a better picture.

1619465961383.png
 
Apr 24, 2020
40
Hunter Cheribini 37 Portland
Yes!..but I put on a folding prop. I'm hoping the folding prop did not cause the problem!
 
Apr 22, 2011
735
Hunter 27 Pecan Grove, Oriental, NC
My 84 H27 prop strut is a Buck Algonquin. It is mounted with four 1/4" flat head bolts. There is a ss backing plate for each set of 2 bolts. I don't think that Hunter would have mounted your strut with only 2 bolts. I suspect that the strut has been replaced by a PO and they bought a replacement strut that had no holes predrilled.

 
Apr 24, 2020
40
Hunter Cheribini 37 Portland
I am always wary of any metal ‘embedded’ in fiberglass. This is nearly always a recipe for later disaster.

The correct way (and a decent fix for you) is to drill right through the boat from outside. If you drill through the plate, without removing it, so be it. Locate the holes inside and lay up some extra ‘glass - going wider than you think you need to. I wouldn't bolt a prop strut to anything less than 1/2” of glass that extends a good 4” or more beyond the bolt holes.

Then use a strong fiberglass backing plate (see McMaster.com) like 3/8” thick, NEW bronze bolts (NOT brass - see BoltDepot.com), wide bronze fender washers and bronze locknuts (or lock washers if you can’t find them and use ‘large-pattern’ hex nuts).

And - in spite of what anyone else here says - assemble the whole thing smothered in 5200, which is the only thing the factory would/should use. The 5200 will hold better than the bolts do (it’s what holds your keel on), will not fail underwater and admit water to your newly-constructed structural sandwich, and will provide the required vibration-dampening flex without breaking its seal.

I have done this job often in production. And I know I get known as ‘Dr Doom’ on here; but it’s because my standards are noted in the industry for being consistently higher than those of people who think an amateur’s assumption of ‘good enough’ is good enough for a space capsule taking you into an alien environment that can kill you (i.e., a boat going to sea). There really are some things some boat factories get right; and one of them is a 50-plus-year track record of relying on 5200 for structural integrity.

Backing plates ‘embedded’ in fiberglass is how motorboaters mount their seats’ bases, which accounts for why (with the typical motorboaters’ fetish for silicone) most of them over 20 years old have rotten cockpit soles. I could explain why they think this is the only way (since it’s almost 100% universal); but the answer would come down to poor design, poor production techniques and a lack of caring - none of which any sailboater should embrace, tolerate or deserve.
Thus sounds good. Thank you!
 
Jun 5, 2010
1,067
Hunter 25 Burlington NJ
Would you put four bolts in or use the two that are there now?
I would always use four. I don’t even know how two bolts would be configured (side-by-side? -tandem?) to handle the torque loads. Most (if not all) bronze prop-shaft struts I have seen have four bolt holes.
 
Last edited:
Jun 5, 2010
1,067
Hunter 25 Burlington NJ
For my situation removing strut bolts to re-seal, I didn't get the strut into the exact same position which led to further problems.
If this happens usually you can ‘float’ the whole motor on its mounts or shim or sand the mating surfaces. If you cleaned out old bedding material and replaced it, know that quite often in production the builder will make up any gaps by filling in with more 5200. This is actually not a bad idea as 5200 is very good in both tensile and shear loads, as well as very durable (flexible) in surviving vibration. I’m afraid my usual way with ensuring very tight, solid fits and damning the consequences may not be the best tactic with shaft struts!

If you loosen all the engine-mount bolts (and this assumes you have rubber-cored mount fittings, which you should - it’s not a motorboat) you can try spinning the prop by hand and listening for scraping, etc. Given new components you can learn where it sounds & feels happiest and then tighten it home where it most likes to be. At worst you might replace engine mounts with any other type, hoping for new bolt holes, and remount the motor.

As I said, this is worst-case. If possible your best hope is to sand or shim the hull-to-strut-flange fit - find its happy place and wedge in or remove material as needs be.

And if you have a stainless-steel shaft (and why would you?) DON’T NEGLECT THE ZINCS. Between the two metals, underwater, the bronze will lose. We broke the strut on our Raider 33 (after my dad died) because no one replaced the zinc shaft collar. There’s an example of ‘further problems’!
 

RoyS

.
Jun 3, 2012
1,218
Hunter 33 Steamboat Wharf, Hull, MA
While aligning your motor and the strut, pay attention to the shaft in the stern tube. It should be centered. Alignment after strut replacement is extremely difficult. As mentioned above turning the shaft by hand will indicate any binding at the strut. Bent struts can be straightened by propeller shops. Exact replacements may be available as well. Start with a new cutlass bearing also. My 1980 H33 had a two bolt strut.
 
Jun 5, 2010
1,067
Hunter 25 Burlington NJ
What Roy says is absolutely vital and though I was thinking of that the whole time I totally forgot to mention it in my contribution! In fact this is the ONE aspect you are looking at if you are wiggling the motor and shaft to find that ‘happy place’.

A little bit of misalignment may be tolerable but remember that, though that’s what the Cutless bearing is for, too much misalignment will destroy the flexible innards of the shaft bearing in no time at all. That leads to wear-and-year, direct abrasion between shaft and strut insides, and eventual leaks. Properly aligned, these should last a dozen years easily. Any prolonged policy of ‘oh; it’s not too bad; it’ll last another season’ will lead to an ugly inevitability (it’s not ‘if’ but ‘when’).

Leaking shaft bearings is probably the number-one cause of inboards sinking at the dock.