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Old 26-11-2014, 07:41   #1
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Thru hull backing blocks

Hello all, I recently had a survey done on my 73 Morgan 36 OI. He found no major issues, but one thing he did mention is the backing blocks on many of the thru hulls were "not in the best of shape" and I should prolly replace them while its on the hard. These backing blocks, are they referred to as ferring (sp), fairing blocks

What should I use for sealant/adhesive? Ive read about 3M 5200 4200 4000. Also "Life Seal"?

He said to use teak for the backing blocks, also said I could even use plywood (wont last long, so I wont), but teak is the best. I am in the wood flooring business, and have alot of 5/8" x 3" bamboo flooring around. Would bamboo be a suitable replacement to teak? it is very waterproof. opinions?

Any tips or pointers to mention before I start replacing these? If the pic apears, it is of the grounding plate block, and another thruhull.

Thanks in advance, Moparots / Craig
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Old 26-11-2014, 07:52   #2
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Re: Thru hull backing blocks

Oh man...

Use starboard or a glass laminate panel... You'll never have to worry about them again... I'll find some links...
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Old 26-11-2014, 07:56   #3
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Re: Thru hull backing blocks

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Originally Posted by HappyMdRSailor View Post
Oh man...

Use starboard or a glass laminate panel... You'll never have to worry about them again... I'll find some links...
Here's a Defender search link... It's got both starboard and glass lam panels in the results...

Defender.com Search Results: fiberglass board
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Old 26-11-2014, 08:43   #4
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Re: Thru hull backing blocks

Quote:
Originally Posted by Moparots View Post
Hello all, I recently had a survey done on my 73 Morgan 36 OI. He found no major issues, but one thing he did mention is the backing blocks on many of the thru hulls were "not in the best of shape" and I should prolly replace them while its on the hard. These backing blocks, are they referred to as ferring (sp), fairing blocks
Fairing blocks.

Quote:
Originally Posted by Moparots View Post
What should I use for sealant/adhesive? Ive read about 3M 5200 4200 4000. Also "Life Seal"?
This is one time 5200 would be OK, assuming you use a "lifetime" material like HappyMDRSailor said, and never plan to have them apart again. I wouldn't be against 4200 or even 4000 though. I don't have enough experience with Life Seal to answer that one.

The bond should be mostly mechanical, so the sealant is not really as important as one would think. I've never seen a leak around a through-hull because of the sealant used. It's always a structural problem in my experience. But then again, someone here might have a different story...
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Old 26-11-2014, 09:22   #5
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Re: Thru hull backing blocks

Go to this website:
Compass Marine How To Articles Photo Gallery by Compass Marine How To at pbase.com

There you will find a number of truly excellent articles, several of which pertain specifically to installing through-hulls and the proper backing plates for them.

Good luck.
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Old 26-11-2014, 09:39   #6
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Re: Thru hull backing blocks

You can get starboard pretty reasonable on Ebay sometimes in smaller sizes. I'd use that. Sealant most any good one ought to work. I use 5200 but that's just me. Marine ply is OK if you saturate it with epoxy. I'd worry about any solid wood cracking.
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Old 26-11-2014, 09:51   #7
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Re: Thru hull backing blocks

Don't use solid teak !
It won't rot, but it has grain and can split, unless you carefully select a piece where the grain is running parallel to the hull. It is also too oily for most sealants to stick to.

Teak plywood, or marine ply would be OK, but as others have said fiberglass, G-10, or Phenolic are all better choices.

I think Starboard or other plastics might be a mistake since most sealants will not stick to them; They can also creep or deform over time.

Getting the thru-hulls out may be very difficult.

Cheers,
JM.
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Old 26-11-2014, 10:14   #8
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Re: Thru hull backing blocks

You don't need o use wood or starboard! Go to a fiberglass supply store get a yard or 2 of stitch mat. get a qt. or gal of resin, and some chemical gloves at home depot. Make form out of a piece of plywood and 3/4 by 3/4 the size of the largest backing plate/fairing block, with a cover piece the same size. Cut the mat to size with scissors, cut 2 large ziplock bags along the edges, leaving the bottom edge untouched. As you wet out each piece of mat put it in the mold you made on 1 of the plastic sheets, position it so you use only 1/2 the sheet!, your using the other 1 for wetting out. Wet out 7 to 8 pieces, flip the excess plastic flap, put the plywood loose plug on top of the flap, weigh it down with a couple of bricks, push them down gently to get some of the resin to ooze out around the edges. Wait an hr. and you have your 1st plate. The rest are done the same way. I cleaded the edges with a cut off wheel. now get the right size whole saws cut your holes. If you clean the hull around the thru hull holes with acetone, all you need to do at this point to bond them is to make a resin bog with milled fiber in the resin. Put enough bog on the plate so that when you push it down on the hull, it squeezes out around the edges. I did mine this way and made and mounted 4 in an afternoon. When I had the boat surveyed I got a thumbs up from the surveyor.
On another note DO NOT use 5200 to mount your thru hulls! 5200 is a permanent bonding glue. you should only use polysulfide. Clean and service your thru hulls. If they are thru bolted put them in loosely, with a bead of poly on the mating surface, put a 3/8 inch bead of poly sulfide on the inside corner, and lip of your thru hull then thread it up sort of snug, load up your screw heads with poly tighten them up, and finish tightening your thru hull. If you did it right you should get about an 1/8th inch bead of poly to squirt out around the head of the thru hull
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Old 26-11-2014, 10:19   #9
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Re: Thru hull backing blocks

I also would not use King Starboard. Instead I use Bruynzeel marine plywood, epoxied with WEST epoxy, and carefully fitted to the curve of the hull. Bed with 3M 5200.
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Old 26-11-2014, 10:48   #10
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Re: Thru hull backing blocks

It is my understanding that 5200 or any of the other products will not adhere to starboard so I would cross that one off of my list. I would think that fiberglass lam is the best but teak or plywood would also work. IF I used those I would coat them with a penetrating epoxy.
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Old 26-11-2014, 11:16   #11
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Re: Thru hull backing blocks

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It is my understanding that 5200 or any of the other products will not adhere to starboard so I would cross that one off of my list. I would think that fiberglass lam is the best but teak or plywood would also work. IF I used those I would coat them with a penetrating epoxy.
I don't know that it has to adhere... just needs to seal. You are not gluing it, just sealing it. Just like an o-ring doesn't "adhere" to mating surfaces.
Fiberglass lam is good, but a whole lot of messy work....
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Old 26-11-2014, 11:21   #12
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Re: Thru hull backing blocks

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I don't know that it has to adhere... just needs to seal. You are not gluing it, just sealing it. Just like an o-ring doesn't "adhere" to mating surfaces...
Not so fast.

An O-ring seals because
a) it is trapped in a groove so it cannot squeeze out
and
b) because it is under compression

Neither of these conditions are met with sealant against Starboard.
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Old 26-11-2014, 11:26   #13
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Re: Thru hull backing blocks

I am just about finished doing all of my thru-hulls. I used 1/2" G-10 and bedded the G-10 plate in Epoxy. The Epoxy was thickened with a West Systems product for adhesive applications. The G-10 base plates can be removed by hitting laterally with a heavy hammer. It breaks loose, but with much force.

The G10 was tapped for 5/16 machine screws. The bronze screws are bolted in through the base of the seacock. So, the screw heads are inside the boat. The seacock base is attached to the G-10 with screws prior to installation. A good amount of thickened epoxy is laid down and then the based is pulled down by screwing the thru-hull into the seacock base. Keep the thru-hull flush with the hull...this is important, since the outside of the hull does not necessarily match the contour of the inside hull.

Once the epoxy is set, then pull the thru-hull out, caulk it (SikaFlex 291 used caulk), put Teflon tape on the threads and screw back into the base. Works great.

Disclaimer: Almost all of the information is well documented with pictures and text in Maine Sails "How_To" articles.

A side note: Fiberglass release wax was used in places where contact with epoxy was not desired. So, the threads of the screws and the threads of the thru-hulls had release was applied. This way, these items are easily removed just in case epoxy might get on them.
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Old 26-11-2014, 11:27   #14
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Re: Thru hull backing blocks

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Not so fast.

An O-ring seals because
a) it is trapped in a groove so it cannot squeeze out
and
b) because it is under compression

Neither of these conditions are met with sealant against Starboard.
well.... if your sealant is not under compression between your seacock/backing block and the hull... YES... you have a problem... you forgot to screw it on!
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Old 26-11-2014, 11:49   #15
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Re: Thru hull backing blocks

If you're using Groco or Apollo Seacocks the easiest and best backing block is made by Groco.



Link: Groco Seacock Backing Block


It comes with threaded inserts. You glue it down with epoxy and bolt the seacock to the threaded inserts. Easy Peasy.
Works with the Groco Flanged Adapters as well.
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