Stainless steel bottom on composite plywood hull ...

I've designed a small boat to be built in plywood / epoxy / fiberglass and would like to learn more about installing a stainless steel sheet on the bottom for better protection against grounding, etc.

The boat has a box keel bottom which is perfectly flat, so shaping the metal is not an issue. My primary questions are (1) how do I attach the sheet of stainless steel to the flat bottom of the hull? ... and (2) what should I expect as potential problems related to the differential rates of expansion and contraction of the composite bottom and the stainless steel sheet?

 

Hmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm.

I would think the stainless would have to be bedded in an adhesive, because any fastenings would pierce the stainless and be a potential source of water ingress. Yet I don't like no fasteners either.
I gather you mean about 16 ga stainless. If the stainless is wrapped around the chines and drilled/screwed at the margins, maybe the goop need only be at the margins, and something like tar paper could be fitted beteween wood and metal.
I will not go into whether I think this is a good idea, never heard of it, seems to make sense where coral or rocks are concerned. Better if removable without a big fuss, maybe by heating the adhesive after removing the screws.
Salt water or fresh? have you heard of this being done?

Alan

 

Aluminium can be glued with epoxy.
Maybe that's an option?

 

My main concern is what will happen when the steel plate expands and shrinks at a different rate from the composite hull. What I'm afraid of is that the fasteners will bend or break, or the steel plate might buckle ... or maybe the holes in the hull would be stretched and open up.

Gluing the plate on the bottom would eliminate the potential fastener problem, but I cannot seeing myself trusting that the plate would stay there if only glue were holding it on -- unless I used some special kind of glue designed specifically for this type of application.

I don't know what might happen with aluminum any more than I do steel, but I suspect aluminum will corrode faster in the marine environment. If this is true, I would need other reasons to go with aluminum rather than stainless steel.

Thanks for the comments guys, but I'm actually hoping to hear from soneone who has done this before, although I'm not sure anyone ever has.

Phil Bolger specifies adding steel plates to the bottoms of some of his boats I think. Maybe someone who has built one of his boats will tell me how Bolger specifies this type of installation ... ?

 

I think marine grade aluminium will last longer than stainless steel in salt water. If the aluminium is quite thin, I think epoxy glye will be strong enough to keep it in place, and absorb the stress from heat expansion. Just don't put the boat upside down in the sun!

I have read several discussions on stainless steel under water. While it's good for deck hardware, it's not very long lasting under water or inside wood. Maybe a copper plate is the best.

 

This one was done on a small displacement trawler in Australia last summer. I think it's pretty heavy guage SS. Boat normally operates in shallows with oyster beds and coral. IIRC it was bedded with thickened eposy. I don't know if it was glassed over or not but I'm thinking it was. No idea yet how it's holding up. If I can get ahold of the fella I'll let you know.

 

Thanks KnottyBuoyz,

That plate looks about 3/8 inch thick, but of course I'm just guessing so maybe it is only 1/4 inch or so. I would be happy with 1/8 inch simply because my boat is not that big or heavy, so when it bounces on reefs, rocks, etc. there will not be as much pressure on it as a heavy trawler.

From the way they held the plate back away from the edges in your picture, I would guess that it was covered with epoxy and fiberglass too. I never thought of doing it like this, but I suspect that it beats using stainless steel nails, rivets, screws, nuts and bolts, etc.

The boat I want to put the steel plate on will be a trailer boat, so it will probably be subject to greater temperature extremes than a boat that's always in the water. I have to find a resource online somewhere that will give me a clue about how much expansion a sheet of steel like this might undergo ...

 

Bedding in thickened epoxy should be fine...even better would be to drill holes for the bedding to ooze through, then glassing on top to lock it in. I don't think the Philippines has enough of a temperature variation that the metal will expand and contract too much. This might not work in a boat that stays in the water all year 'round in Alaska or Canada but should be OK in the sub tropics.

Steve

 

If the design works I'm going to want to sell these boats all over the world, so I would kinda like a solution that works everywhere.

I think the biggest problem in terms of differential expansion and contraction will occur in temperate climates when it is below freezing on the trailer in mid-winter and very hot in the summer.

If the boat were "in the water" all the time, the cyclical temperature variations its bottom might undergo would be minimal in Canada or Alaska. Granted the temps would be much colder than in the Philippines, but it is the variations not the average temps that would create the problems I think.

 

The thinner the metal, the less the expansion/contraction. What are you looking for the metal to do...and would copper work better?

 

Won't the length of the panel expand and contract the same amount regardless of the panel thickness?

It is for abrasion resistance.

 

What about including some Dynel in the layup? That's provide some abraision resistance without the weight penalty and the trouble of bonding SS or other to the FRP. Besides, if your selling boats you really don't want them to last forever now do you?

 

Fit a few underwater sacrificial strakes on top of the Dynel cloth and mix graphite powder into the epoxy to toughen it. As the strakes wear, they can be built up again and epoxied. If the stainless were plating to be holed, repairs would be more difficult than a laminated bottom.

Pericles

 

A grounding shoe, on the bottom of deadwood is one thing, but a plate bottom sheath would be problematic at best. You'll likely need a reasonably flexible bond, so epoxy is out, polysulfide would be my choice for underwater applications against wood. An adhesive/sealant would more then suffice in holding relatively light gauge plates, though the choice of stainless is questionable in salt water.

Sacrificial shoes (strips), made of high density plastics would be a better choice and eliminate trapping moisture against the wooden hull (like a big plate would). Harwood shoes are a traditional problem solver, as is extra bottom planking.

A Xynole sheathing would be a better choice, if interested in full bottom protection. It has 6 times the abrasion resistance of regular 'glass cloth. Kevlar is an option too.

Stainless just isn't the best choice in the application you suggest. Pound for pound you can do better with less likelihood of other issues cropping up.

If you're married to the idea of a stainless sheet bottom to your box, then use a two part polysulfide or a single part, like 3M 101. Skip the fasteners, they'll just cause issues later and will not permit the dissimilar materials to move around at their specific rates. Try to avoid making a "dish pan" with the plate as it will just trap moisture against the box.

 

Thanks all,

I think you're right when you say epoxy is not flexible enough, so I
would probably go with the recommended polysulfide adhesive. Note that
the entire bottom of the boat will be sheathed in epoxy and two or more
layers of glass BEFORE the metal plate is attached. In other words, the
boat will be completely finished without the metal plate, so the
polysulfide will never be used on wood. The plate is more like
an "add-on" or an "accessory" and would be attached to the finished
epoxy / glass bottom.

I am wondering if it might make sense to cover / encapsulate the metal
plate in epoxy and glass after using polusulfide to glue it to the
bottom of the finished hull? Sure, the glass that covers the metal
plate would get lots of holes torn in it, or worn through it. But even
so, the water that gets to the metal would likely not get through the
epoxy / glass / polysulfide layers and into the wood of the finished
hull ... right?

I know, some of you will say "Why bother covering the metal plate with
epoxy and glass if you're going to glue it with polusulfide anyways?"

Well, I'm thinking that *IF* the polysulfide fails some day, the metal
plate might still be held in position by the external epoxy/glass
sheathing layer, hopefully long enough for the owner to dry dock the
boat and fix it before the metal plate falls off completely. Then he
will not have to get a new one to replace it.

I'm not married to the metal plate concept anyways, I just figured it
would be the hardest material that could take a lot of groundings with
almost no damage -- and no need for constant inspections and frequent
repairs. I tend to like the idea of metal better than Dynel, Kevlar,
etc. for a protective bottom layer on a boat that will undergo lots of
groundings, don't you? Or do you guys actually think Dynel will be more
abrasion resistant than a sheet of metal?

My concern here is not so much the occasional bounce or scrape on the
bottom, but rather a proponged 'wearing' of a focused spot on the
bottom. For example, if the boat sits in one position on the rocks for
any length of time, the Dynel would probably end up with a hole in it
because the rocks would continue their abrasive action in only specific
locations. So the rocks gradually dig holes right through the Dynel in
these spots, then they continue carving their way through the glass and
into the wood. But if I had used a metal plate instead of Dynel, I
suspect that this abrasive action in specific locations might leave
little more than a few scratches on the hard metal plate. This is my
current assumption anyways.

The Dynel solution is something that occurred to be before I proposed
the metal plate concept. I also considered using a sheet of UHMW-PE
plastic rather than strakes. Nevertheless, even if I disregard the wear
issue above, I am still up against the same problem of how to attach
UHMW. I suspect that it is probably even more difficult to glue than
metal since it is a self-lubricating plastic that most adhesives really
do not stick to for very long.

Additional thoughts are welcome.

By the way, I'm not too concerned about the added weight of metal
because the boat has a box keel that gets flooded with water for
ballast anyways. In this case the added weight of a metal plate will
provide even more ballast in the ideal location -- and that's a 'good
thing' -- within reason of course.