How to Service Strut Blocks Under Cabin Sole

[albinalaska]

Good Day!

We are in the middle of a refit of the aft head on our Trawler and have come across these strut blocks under the subfloor. It's my understanding that anything bolted through the hull needs to be serviced and resealed over time but it is very clear that this area hasn't seen the light of day since the boat was built in 1979.

The area is dry (on the stbd side at least as that is as far as we've got so far) and show no signs of weeping or seeping but before we hauled out we were made aware of salt water intrusion observed on the port side. We thought these were the culprit but now we're scratching our heads.

In light of being safe and thorough while we're under the floor we'd appreciate any advice or experiences with these that may help us better care for the vessel.

Thank you!

 

[Comodave]

My struts have bolts coming up through the hull where the struts are mounted. Not sure about yours since I don’t see any bolts coming up through the hull. Not sure what you have there.

 

[fgarriso]

Look under the hull in the same spot.

 

[albinalaska]

Apologies for any confusion- those are the top side of the struts. Verified by accounting for what’s underneath and also a small nick from the angle grinder revealing a bolt.

The dark circular spots in the first photo are the protrusion of the four bolts under the fiberglass.

The photo including the shop vac show the aft strut blocks which are wider so have two blocks also encased in fiber glass.

I attached a photo showing the smaller sliver of silver from the bolt.

 

[Comodave]

I have never seen them glassed over like that but then again I have not seen everything. So I guess if they are not leaking then I would leave them alone.

 

[fgarriso]

Are they tight and water tight?

Are they loose and moving or leaking?

Proceed as needed!

 

[kchace]

Can you tell if the bolts are bronze or stainless steel? Considering the age of your boat I would image they’re bronze which is much better than having SS encapsulated like that. Don’t know why they were sealed over like that, but I agree, if they’re not leaking and not corroded, I’d leave them and just keep an eye on them. Though if they’re fine after this many years they’ll probably keep doing their jobs.

Ken

 

[tiltrider1]

Can you tell if the bolts are bronze or stainless steel? Considering the age of your boat I would image they’re bronze which is much better than having SS encapsulated like that. Don’t know why they were sealed over like that, but I agree, if they’re not leaking and not corroded, I’d leave them and just keep an eye on them. Though if they’re fine after this many years they’ll probably keep doing their jobs.

Ken

If they are stainless, and a zinc has always been bolted to the strut then I agree with above. If they are bronze, I agree with above. If they are stainless and there is no zinc attached to the strut then they are gone. I can not tell you how many times we have found both ends of a stainless strut bolt with everything between the ends eaten up. Fortunately, struts are usually attached with 3m 5200 which is almost as good as bolts.

 

[Comodave]

If the bolts are S/S then they may be suffering from crevice corrosion. In that case I might feel like I had to look at them and see what condition they are in. Unfortunately you probably can’t tell unless you pull the bolts out. I also really don’t understand why they glassed them over. That may help promote crevice corrosion.

 

[tiltrider1]

I should have stated that when we find strut bolts deteriorated we also find a bonding issue. Comodave is correct that crevice corrosion is also something to be concerned about.

 

[Comodave]

I should have stated that when we find strut bolts deteriorated we also find a bonding issue. Comodave is correct that crevice corrosion is also something to be concerned about.

Agree on the bonding issue since mine are bonded inside the boat on the strut bolts. These can’t be bonded since there isn’t any bonding wire going to the bolts. So that is another issue besides the crevice corrosion.

 

[bgillroy]

Struts typically are through bolted. We replaced all our 1976 trawler strut bolt sets in 2017 with 316L shoulder bolts. Old bolts were SS and about 30 percent wasted (deep pitting craters) along surface of bolt . Once bolts were pulled internal rot on 2 blocks required partial renewal of block. Then re-glass. Proper inspection requires removing surface glass.

 

[Comodave]

Maybe they glassed over the bolts to prevent leaks into the boat. I don’t see any other reason to do that. Personally I would grind off the glass covering the bolts and include the struts into the bonding system if you have a bonding system.

 

[tiltrider1]

It’s hard to say. If the strut has had zinc on it all these years and the bolts were bedded with 5200 then all should be fine.

 

[Comodave]

That is the ideal situation for crevice corrosion. No oxygen getting to S/S is a prescription for crevice corrosion.

 

[tiltrider1]

No oxygen yes but if no water then no corrosion.

 

[Comodave]

I had a 1/2” S/S bow eye get crevice corrosion inside the fiberglass and inside the 5200 that it had been bedded in. It broke under load and you could see the crevice corrosion had gone about 80% of the diameter of the bolt. And that was well above the waterline. The struts are below the waterline and therefore have water pressure on it to help force water inside the strut mounting area. Not saying it has happened but it is certainly possible to happen. And S/S is much worse below the waterline than bronze bolts.

 

[tiltrider1]

Our experience with SS strut bolt deterioration is directly related to lack of bonding. I am not discounting crevice corrosion, it’s just something we rarely see on strut bolts.

 

[Comodave]

But have you ever seen strut bolts glassed over like this? Unique situation.

 

[Pete Meisinger]

If everything is tight and dry, leave it alone.

pete

 

[RT Firefly]

Greetings,
Mr. aa. Although an added expense, I'm in the at least "grind off the top and take a look" camp. It also may be cheaper, in the long run, to remove the whole block and the retaining fasteners and replace all with new PLUS you've got all that space to work in, at least on one side.

 

[boatpoker]

That is the ideal situation for crevice corrosion. No oxygen getting to S/S is a prescription for crevice corrosion.

Agreed and it is highly unlikely that no water has gotten in there after how many years ?

Only way to be sure is pull the bolts and inspect at which point it makes no sense not to replace them. Nothing lasts forever.

Keep in mind that crevice corrosion is much worse on the inside than is visible on the outside. This is an x-ray of a single pit in an ss shaft. The photo is where the shaft broke. You can clearly see the inside of the shaft looks like an Aero chocolate bar.

 

[albinalaska]

Thank you for the insight and info. We'll have to make some decisions about whether or not to start digging into the strut "boxes". But I completely agree with the need for bonding and the implications for it not being so. It's slightly intimidating because we just had the shafts and stuffing boxes replaced in Seattle before we travelled 800nm away from anywhere that does this kind of work.

 

[albinalaska]

Anyone have any pictures of their strut bolts from the inside of their vessels? I know it's thru-bolted and that there is some form of backing block in there. Trying to research before I bust out the old angle grinder. Really appreciate the collective expertise here!

Kyle

 

[Bustlebomb]

Here’s mine. There is a much larger backing block that appears to be glassed in to the hull. You can see it at the edge of the pics.

 

[albinalaska]

Here’s mine. There is a much larger backing block that appears to be glassed in to the hull. You can see it at the edge of the pics.

Thank you for those! I see you've got every bolt tied together for bonding which I didn't consider so that is helpful. Have you serviced this yourself? I assume once I was through the blocks and down into the actual bolts (if they are even still there) that I would use 5200 to seal the hull when replacing the hardware?

 

[Tator]

Thank you for the insight and info. We'll have to make some decisions about whether or not to start digging into the strut "boxes". But I completely agree with the need for bonding and the implications for it not being so. It's slightly intimidating because we just had the shafts and stuffing boxes replaced in Seattle before we travelled 800nm away from anywhere that does this kind of work.

You should be able to find someone in SE Ak. to do any of this work including turning new shafts. Just had a new generator put in by Piston and Rudder in Petersburg. They have a complete machine shop. I know that Wrangell has a lot of shipwrights as well. I've had custom exhaust work done in Juneau. Not as familiar with what's available in Ketchikan or Sitka, but as they have large fishing fleets I sure it can be done there as well. I'm planning on pulling my strut bolts this May when I haulout in Petersburg as I have noticed some weeping.

Tator

 

[Art]

Thank you for the insight and info. We'll have to make some decisions about whether or not to start digging into the strut "boxes". But I completely agree with the need for bonding and the implications for it not being so. It's slightly intimidating because we just had the shafts and stuffing boxes replaced in Seattle before we travelled 800nm away from anywhere that does this kind of work.

I'd take into account the condition of all other below water metal items and bolts; as well as and especially the condition of the underwater bolt heads of the through bolts you are questioning... before making decision to leave things as is or to replace.

Why were the shafts replaced? Corrosion?? Also, condition of anodes and any record of anode replacements over the years/decades.

If inspections prove to make things seem ok, and the questioned bolts are still well tight. Then I'd lean toward leaving as is if I was not planning to be in heavy seas on a regular basis.

Whether replacing or leaving as is: I'd best as possible have future access through floor engineered in.

 

[albinalaska]

I'd take into account the condition of all other below water metal items and bolts; as well as and especially the condition of the underwater bolt heads of the through bolts you are questioning... before making decision to leave things as is or to replace.

Why were the shafts replaced? Corrosion?? Also, condition of anodes and any record of anode replacements over the years/decades.

If inspections prove to make things seem ok, and the questioned bolts are still well tight. Then I'd lean toward leaving as is if I was not planning to be in heavy seas on a regular basis.

Whether replacing or leaving as is: I'd best as possible have future access through floor engineered in.

Thanks ART-

There is no documentation on this vessel but from what I’ve seen on our refit process thus far the last owner knew precious little on how to properly care for this vessel. There is evidence of galvanic corrosion and various stages of mitigation measures over the years to prevent it; at various degrees of success. The shafts were replaced as the previous ones needed to be cut to replace the stuffing boxes with dripless seals. I have no faith the bonding system is sufficient to return to sea regardless of sea state. It needs to be completely replaced. I appreciate the advice and agree about future access under the subfloor.

My main concern with pulling the new shafts out is the strut alignment work ahead of putting the shafts back in. I’ve read that struts can be removed fine if there hasn’t been a grounding event or another reason they are considerably out of wack. Any info or experience with this would be helpfully We’d need to remove the struts from the hull, clean and 5200 the surface before bedding the struts back on which is concerning as the margin for error is exceptionally low.