So, I am replacing all of my thru hulls on a 1994 192 Tournament and noticed that none of them have a wood backing plate. The inner nut is threaded straight against the fiberglass on the inside of the hull. They are the original Perko thru hulls. Any idea why Grady did it this way? I am assuming I should install wood backing plates that have been epoxied to prevent rot, right?? I am just surprised Grady didn't do this. Anyway, thought I would share this observation.
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Thru hulls with no backing plate
- Thread starter mboyatt
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DennisG01
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I assume you're talking about the hull bottom (under the waterline)?
Are you sure this was done from the factory and not somewhere along the line? Yes, best practice is to glass in a wood block. Another best practice is to make the hole in the wood about 1/2" larger than the hull hole. Then fill the gap with thickened epoxy. There's also a good chance that the existing hole in the hull was never coated with epoxy. Give the hole a sanding then do that, as well. By doing that, even if some water does migrate through, it's not going to leach into the hull/gel nor the wood block.
Are you sure this was done from the factory and not somewhere along the line? Yes, best practice is to glass in a wood block. Another best practice is to make the hole in the wood about 1/2" larger than the hull hole. Then fill the gap with thickened epoxy. There's also a good chance that the existing hole in the hull was never coated with epoxy. Give the hole a sanding then do that, as well. By doing that, even if some water does migrate through, it's not going to leach into the hull/gel nor the wood block.
Dennis, these are all above the water line. They are the drains for the cooler/live well and also the exit port for the bilge pump. Again, all are above the water line. Having said that, I always just assumed one would install with a backing plate. I am quite certain they are original to the boat.
DennisG01
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OK, then. In that case, they're fine as is. You could add a plate if you want to, but it's not necessary.
Interesting. I guess they have lasted this long without any backing plate. I removed them today, so this will make my install easer. I suspect I will reinstall the new stainless ones without a backing. That will save me some time and effort. Many thanks for the advice!!
wlewis
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ElyseM
GreatGrady Captain
the thru hulls don't have a "backing plate". the nut that holds them on should have an oversized flange attached to it.
Guys, thank you for the responses. I am aware of the oversized flange nut that holds the thru hull in place. I just always assumed that proper installation required one to place a piece of wood (with hole, of course) between the hull and the flange nut. I guess this is overkill for above waterline installations, as some have noted. I will forego the extra work since all of mine are above the waterline. :mrgreen:
ElyseM
GreatGrady Captain
i'm not sure i would want a compressible back plate on a below the waterline thruhull; could allow it to become loose. back plates are usually used to distribute stress (eg cleats, stanchions, motor mounts, etc). good luck , ron.
I didn't make this up, guys. Do a google search on the subject. I found an article in Boaters Magazine and other publications just now with a cursory search. I would post a link, but I am not a computer guy. Google search it and you will find articles on this. You do NOT just stick a piece of wood between the hull and nut. You have to prepare the wood with fiberglass or epoxy (or something to this effect) so that it will not break down (rot) when subjected to constant moisture. I assume the backing plate is used to spread out the force when you crank down the nut on the thru hull. As I indicated earlier in the post, I think I will reinstall as she came from the factory - sans the backing plate. From what I have read though, this is a process that some use when installing thru hulls and sea cocks. :hmm
DennisG01
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When below the waterline, it might even be a CG requirement, but I'm not positive on that. What I do know is that from my personal experience and 25+ years in the industry, it's common/best practice to do this.
See my post above regarding the general procedure.
See my post above regarding the general procedure.
ElyseM
GreatGrady Captain
http://www.pbase.com/mainecruising/replacing_thruhulls
this may be one of the articles you looked at. don't know who this is. based on this, if you think you need a back plate inside the hull, i'd would wonder why you wouldn't need one on the outside. the flange nut is generally the same diameter as the external flanged collar, so reason would say if the inside doesn't distribute adequately, the outside won't either.
not looking to get into anything, just an interesting thought. ron
this may be one of the articles you looked at. don't know who this is. based on this, if you think you need a back plate inside the hull, i'd would wonder why you wouldn't need one on the outside. the flange nut is generally the same diameter as the external flanged collar, so reason would say if the inside doesn't distribute adequately, the outside won't either.
not looking to get into anything, just an interesting thought. ron
Ron, thanks for posting the link. I hadn't read that one, but it is similar to others I have read. This article is perhaps more detailed. I sit behind a desk 5 days a week, so I am by no means an expert on this stuff. I have been fixing up my 192 tournament for 2 years now, and I try to use the best materials and best process possible, based on what I learn here. Based on what I am seeing and hearing from folks here, I will reinstall my shiny new thru hulls without any backing. Heck, that's how the originals came and they were in pretty good shape (1994 Grady). Now I need to learn how to post these links to other web pages and how to upload photos!
DennisG01
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ElyseM said:http://www.pbase.com/mainecruising/replacing_thruhulls
this may be one of the articles you looked at. don't know who this is. based on this, if you think you need a back plate inside the hull, i'd would wonder why you wouldn't need one on the outside. the flange nut is generally the same diameter as the external flanged collar, so reason would say if the inside doesn't distribute adequately, the outside won't either.
not looking to get into anything, just an interesting thought. ron
Ron, did you mean me? No, I have not seen that article. All I can say is that it makes sense to me and this is the way I've learned to do it over the years and "best practice" from the manufacturer's side is that they do it, too. I'm definitely not going to second guess the engineers at top-tier companies. Especially on something that's "tried and true".
I see where you're coming from with your line of thinking. But adding the inside block does actually increase the strength on both inside and outside. The larger the block is on the inside... the "stress load" area becomes larger, which decreases the stress at any one point. This comes to a point of diminishing returns, though. In other words, it doesn't really help to just keep making this block bigger and bigger. Approximately 3 times the size of the inside nut is more than enough.