Through - Hull Backing Block

[Don Guillette]

Normally, I don't write about maintainence things on the sail trim forum but this one is important. Last week I was at an Alamitos Bay (So Ca) boat yard visiting a friend. The mate with the boat next to us was installing a through - hull. What caught my attention was he was scurrying around looking for a piece of plywood to use as a backing plate. He told me any piece of wood would do. That is a BIG mistake and here is why. Not all pieces of plywood, even marine, are the same. Regular plywood is full of voids and is not bonded together with marine glue. Plywood marked marine could be made from a lot of things (Douglas fir, tropical hardwood, Okoume or Mahogany). For plywood blocks, I prefer Mahogany plywood.Why do you need a backing block in the first place? The reason is to spread the load. Without a backing block, you are creating a hard spot in the hull which can easily be overloaded beyond a safe limit. If the sea cock becomes stuck and you try to force it with a wrench you could crack the laminate on the hull. That is not a good thing.What shape of block (round or square) is best for a fiberglas hull? The answer is round. On wood boats, square would be the best choice. With the round block you should take it one step further to improve its usefullness by tapering the edges.Now that you have cut the block to match the seacock - are you finished? The answer is NO. A seacock could last 20 years but an untreated piece of wood may last only 5 years. To extend the woods life you should saturate the blazes out of it with several coats of epoxy.Actually, I don't like to use plywood as backing blocks. My wood of choice is either mahogany or white oak.Do you think the sailor installing the sea cock listened to my backing plate advise? Not at first because he told me he was in a big hurry (like most folks in So Ca) to get the boat back in the water. Fortunately, his wife was listening to my 2 minute pitch and she thought it made sense. A couple of days later my friend called me and told me the husband and wife installed the mahogany block and they both said to tell me thanks for the advise.

 

[K9piper]

WOOD or WOOD not

Don,Fantastic advice for all to take notice of. Why do thejob half way why not do the job right to last the first time for almost all time.We spend a heck of a lot of money and time making necessary repairs as well as modifications so why not do them the right and long lasting way. That in itself would save money and time, the time we could enjoy under sail.Good advice Don!K9piperK9piper

 

[Ross]

When we put Bietzpadlin in the water last spring

we had a small seepage around one of the thru hulls, Yes I do need to rebed it bu more to the point after about a week the wood had absorbed enough water to swell and stop the leak. You must use durable wood.

 

[patrick]

Only thing I would add

is that you should thin the heck out of the first coat(s) of epoxy. Use lacquer thinner or alcohol and it will penetrate into the wood

 

[Bob]

Good point, Patrick

And make sure the wood is warm, too. A thinned epoxy mix will penetrate better into warmed wood than if it is room temp or cooler. Another trick is to keep the epoxy mix relatively cool until you use it - this way it will not set up nearly as quickly and allow you to paint numerous layers onto the wood, each layer penetrating and setting up fairly quickly (because the wood is warm) and each succeeding layer adhering well because it is applied to the not-quite-hard layer under it. A bucket with ice, a-la champagne, will delay the mix from going off, but be careful not to use open flame or too hot a source to heat the wood. Use twice as much on the edges (end grain) of the wood - they will really suck it up.

 

[Don Guillette]

Another choice

A sailor named Peter Milne from Sidney, BC over on the "ask all sailors" forum gave a great substitute for wood and that is a product called StarBoard, which you can buy at West Marine. It is the same stuff they make the stern perch seats out of. I've used it for bow seats, stern seats, cockpit tables etc. I started using it because I got sick of varnishing oak.

 

[Ken]

Backing plate

I do a bunch of on line seminars. One of them discussed through hulls. The are presented by a marine repair company, that seems very knowledgeable. The purpose of the backing plate is to distribute loads, especially shock loads to a larger portion of the hull thereby increasing the srength. They state that the backing plate should be 3/4" grade A marine plywood. Do not use solid wood, it may crack along the grain line, especially under a heavy shock load. The backing plate should be saturated with epoxy as previously stated. The prefered installation includes bonding the backing plate to the hull, rather than using it like a big washer. I can't find the reference, but Starboard is not recommended. It is "slippery" and doesn't bond to the hull well, it is also soft, over time it will conform to the uneveness of the fiberglass hull, what was initially a good fit may become loose. This is all based on larger boats that have engine rooms and bilges that can be walked around in. For our sailboats with the through hulls hidden in closets and cubby holes the chance of tripping over the though hull is almost non existant, therefore any backing plate is better than no backing plate. But doing it wrong may give a false sense of security as the years go by.

 

[RichH]

Why make things 'complicated'?

Its easy to construct backing plates for through hulls out of Fiberglass cloth/roving, filled (chopped fibers or collodial silica - for the 'mush'). Just cut the squares/circles of cloth or roving and on a work bench make a 'mold' of the final shape, and lay-up the part , then expoy the finishd part to the hull. Fiberglass (installed with fillets (rounded or tapered 'edges') to prevent further hardspots) wont have the 'problems' as the 'other' materials and will be pretty close to the composiition of the original hull laminate. If you dont want to be 'fussy' simply build the backing plate at the spot that you want, then apply epoxy 'mush' to add the tapered fillets, then drill through from the outside to cut the correct sized hole for the throughhull. You have to set the wood or polymer backing plates with polyester or epoxy 'mush' etc. anyway to make sure of the even-ness of applied strength of the assembly is correct anyway ... so just build the backing plate from FRG at the same time. By the time you screw around matching (grinding, sawing, chiseling) a wood or plastic block to match the hull contour you can make your own FRG backing plate. The advantages of fiberglass backing plates are: doesnt rot, crack, wont creep, has the same thermal expansion as the hull, and is fairly easy to learn to do. Fiberglassing 'entry' level manuals abound on the net, the resin manufacturers websites and bookstores.