Filling Old Thru-Hull hole

[nwboater]

We are replacing the old Raritan head with a new composting toilet. NiSa is a 34' 1985 Monk Trawler built from cedar in Oregon. I need to fill the 2" hole where the sewage outlet thru-hull was. The hull is 1" thick at this point and the old 1" backing block is still in place. Am looking for guidance as to how best to plug the hole.

My present idea is to keep the backing block in place and epoxy in a 2" diameter cedar plug that goes through the hull and backing block. I would then put an aprox 1/2" or 3/4" wood cover piece over the top of the backing block and put screws through to the hull and shorter ones to the plug while the epoxy is still wet.

I was originally going to remove the old backing block, but it's in a very awkward place to work on and I'm a little concerned about hull damage from the process. It also seems that having the additional gluing surface of the backing block may be helpful.

Is my approach a reasonable one? Might there be issues from wood swelling too much and causing damage at the hull/plug junction?

Thus far all I have done is to remove the old through hull and do a lot of cleaning of the hole.

I tried to search the forum for similar posts but couldn't find anything. Any help would sure be appreciated.

Thanks,
Rod

[boattruck]

NWB, I think you will sleep better if you get at the backing block, remove it, fabricate a new one much like it,( without a 2" hole) install it, then build a 'plug' for the remaining hole in the plank...disregarding the difficult access, it is a pretty minor chore. Use a good adhesive bedding, and screws of the same flavor as the rest of the boat...Cheers, Bt

[J.L.L.]

My recommendation would be to replace the backing block with a new one without the hole if at all possible. A well bedded block gives good security to the plugged plank. Taking a look at the screws fastening the block would be good too, they have some age on them and its good to check. my 2 cents.

[Peter Malcolm Jardine]

I agree with BT, but I know how awkward some of these places can be. The problem is bonding in wood that could be quite high moisture content. I had the same issue with an old generator exhaust this spring, but I did put in a new backing block. The real security is putting in a new backing block.

[sealrock]

Prefered method of blanking out a thru hull around here is to hammer a tapered plug into the hole with your favorite epoxy glue and grind the outside flush. Then nail a lead tingle over the end grain of the plug to keep the worms out. Bed the tingle in fibregum and fasten with bronze or galv. ring nails around the perimeter with the heads of the nails almost touching. Don't use brass carpet tacks or use those pine plugs from the marine store, they have too much taper and are for emergency use. You can turn the plug on a lathe or cut it on the band saw or cut off a piece of round stock and taper it with an angle grinder. The right amount of taper is about 0.25'' per 1.5'' of planking thickness. If you feel that the backing block is sound enough leave it in place, if any doubt throw it out. On the other hand you can hammer the plug right into the thru hull from the outside, wire the valve shut, remove the nipple and put a bronze pipe plug in its place.

[George Ray]

If you trust the current backing block, planking and existing bedding and fastenings:
You can plug the hole most any way you want and then add a solid backing block on top of and fasten to or through the existing backing block, all well bedded and well fastened, your good to go. A fancy thing might be a shallow graving piece let into the planking but smaller than the original backing block and well bedded and lightly fastened. Water has to get around graving piece and past the plug and then either behind the original backing block bedding (perhaps the weakest link) or by the new blanking block (least likely).
Consider the mode of failure. Boats occasionally sink when a plank comes loose during severe stress or spits out a large bit of caulking on an old and way too large seam, but seldom due to a slow seep which is more than likely the worst case if you have misjudged the condition of the area you are patching over or somehow massively botch the repair(seems very unlikely).
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The selection material for plugs, blanking blocks and bedding is topic for discussion.
I like waterproof ply for plugs and blanking blocks (laminated w/ epoxy for larger pieces) and tend toward 5200 but oil bedding putty has worked well for hundreds of years. Epoxy works but is less forgiving of a lifetime of shrink and swell ..... but any little cracks in 20 years will likely to have been very well sealed by the the various inside and outside paints and putties.
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The perfect is the enemy of the good.
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You approach perfection asymptotically, the trick is not knowing what is prefect, many know that, the trick is knowing what is 'good enough', very few know that.
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http://t2.gstatic.com/images?q=tbn:ANd9GcSO3hqRq7wKhB064uYzpAlkEvTQ8LMuI jTAmI55rYRx-3fWCjsO

[Jay Greer]

My recommendation would be to replace the backing block with a new one without the hole if at all possible. A well bedded block gives good security to the plugged plank. Taking a look at the screws fastening the block would be good too, they have some age on them and its good to check. my 2 cents.
This would be my own choice. Been there done that.
Jay

[nwboater]

Wow - What a great wealth of knowledge and good ideas here. Thank you all very much!

Tomorrow I'm going to try to get the old backing block out. It does make sense to have it gone if I can. Good points on the wood moisture content and epoxy. I took a meter to the boat today and it's way too high. Think I will use 5200 or similar. Also interesting on using a tapered plug. I guess the idea is to have it very securely in place. I should have mentioned that the valve is also out.

Will sleep on the different ideas here and hope that I can get the old backing block out tomorrow.

Thanks again,
Rod

[George Ray]

Pictures are fun.
Having the old block out and seeing how the hidden bits are holding up will be fun, educational and you will sleep better.

[nwboater]

I just realized that I had not made a post here about my 'final solution'. Sorry bout that!

I removed the old backing block. Then with a hole saw and guide slightly over-sized the old hole. Wanted clean wood to bond the plug to. I then made a plug to fit the new hole and a new 1" thick backing block. Both out of yellow cedar, the same material as the hull. I screwed and epoxied the plug to the backing block. Then took that assembly and installed it in the boat with 5200 and screws.

I feel good about how I did this and NiSa is happily afloat again with no leaks. One less large hole in the boat and the composting toilet installation is almost done.

Thanks again for all your help and advice here.

Rod

[Dan McCosh]

Might note that leaving an old backing block in place in this situation is an invitation to rot--an old block with lots of end grain exposed that has no particular function.

[nwboater]

Might note that leaving an old backing block in place in this situation is an invitation to rot--an old block with lots of end grain exposed that has no particular function.

Dan - not sure if that was a comment to me, or advise to others. I did remove the old backing block and replaced it with a new one screwed and epoxied to the plug.

Rod