Best way to strengthen plywood hull?

[JimD]

Specifically, I have just put this 15 footer back in the water. The bottom panels are 3/8 fir ply with an fg sheath. Its a displacement sailboat hull so its slow moving, presently powered by five ponies.

http://im1.shutterfly.com/media/47b9d804b3127ccec727212a090800000040O00CbOGrVu4cMQ e3nw8/cC/f%3D0/ps%3D50/r%3D0/rx%3D720/ry%3D480/

Now that Prairie Lass is going to be plying the briny sea and not just putzing around little ponds in Alberta I want to add another layer of something to the bottom. Strip off the paint and add either a couple layers of fg cloth or another layer of plywood, perhaps 4 or six mil. And I don't know if I can get the boat right off the trailer and onto its side to do it. More cloth sounds like by far the easiest way to go. But will it add the strength? The sort of strength I have in mind is mainly surviving collisions with floating logs. They're everywhere around here. Any ideas one way or another on what method will add the most strength for the least money and hassle?

[paladin]

If you have some method of turning it over and sanding the paint off the bottom, I'd be inclined to just sand off the paint to raw glass and resin and add another 1/4 inch ply in epoxy, then a couple layers of glass and repaint...I did something similar to a 27 footer but with lotsa help.

[donald branscom]

Logs in the water. You will need a steel boat.
Usually after a storm you see stuff floating in the water.

[spirit]

How about some kevlar on the bottom, instead of more plywood, perhaps under some strong hardwood battens?

[JimD]

Logs in the water. You will need a steel boat.
Usually after a storm you see stuff floating in the water.

Yeah, there's limits to what any hull can hold up to, for sure, Donald. I'm not aiming for indestructable, just to improve my chances.

[JimD]

If you have some method of turning it over and sanding the paint off the bottom, I'd be inclined to just sand off the paint to raw glass and resin and add another 1/4 inch ply in epoxy, then a couple layers of glass and repaint...I did something similar to a 27 footer but with lotsa help.

Chuck, what's the best way to stick the plywood on to avoid voids between the original and the new layer? Lotsa staples?

[JimD]

How about some kevlar on the bottom, instead of more plywood, perhaps under some strong hardwood battens?

Not sure I know what you mean by battens. Where would you place them?

[Dusty Yevsky]

Can you access the hull from the inside? If so, this is the place to put FG for strength. FG is far stronger in tension than compression. Inside=tension loads, outside=compression loads.

[JimD]

Can you access the hull from the inside? If so, this is the place to put FG for strength. FG is far stronger in tension than compression. Inside=tension loads, outside=compression loads.

Can't access the inside. Its gotta be the outside. That's why another layer of plywood sounds like a good idea. That way the hull would end up being plywood/glass/plywood/glass which sounds like the best alternative.

[David G]

I'd be thinking a layer of polyester cloth - like xynole - in epoxy chosen for toughness, not hardness. The xynole - and, AFIK, all polyester cloths - is stronger in compression than tension. Or, another layer of plywood and then xynole.

Cheers,
David G
http://www.harborwoodworking.com/index.html


"All progress is precarious, and the solution of one problem brings us face to face with another problem" -- (http://www.quotationspage.com/quote/3264.html)Martin Luther King Jr. (http://www.quotationspage.com/quotes/Martin_Luther_King_Jr./)

[Todd Bradshaw]

I don't care what type of cloth you apply on the outside, the "strength" gain is going to be so minimal that it's probably not worth doing due to the compression/tension thing. Polyester, nylon, Kevlar, etc. might give you somewhat more tear strength than fiberglass cloth, but if you get yourself into a situation where that matters, you are already going to have a really big problem on your hands. Other than possibly being able to salvage the boat with its bottom in one piece, I don't think these coatings are likely to save your bacon.

More plywood, applied with thickened (microfibers) epoxy and then glassed over might be a good option if you can't get inside. I might also consider a layer of UHMW polyethylene, like they sometimes use on river dories and drift boats - high impact strength, high abrasion resistance and slick enough to slide over most things if needed. I believe tubes of goo and counter-sunk screws are still the chosen method for applying it.

[Thorne]

(snip) I might also consider a layer of UHMW polyethylene, like they sometimes use on river dories and drift boats - high impact strength, high abrasion resistance and slick enough to slide over most things if needed. I believe tubes of goo and counter-sunk screws are still the chosen method for applying it.

Sounds like it would do the trick to me. You'll have to be careful when launching, as it might just skid right off the trailer unless carefully secured.

[Krunch]

I think I'd be tempted to take my chances and leave it as-is.

First off, with 5hp, it's not like you're going to be up on plane and hitting something at 20 kts. You're not going to be able to hit anything that hard.

And if you do hit something hard enough to damage the boat, well, chances are, you would also damage it with another layer of glass or whatever on it.

Also, adding more plywood or FRP is going to add substantial weight, it seems to me, that would change the boat's behavior and possibly her trim. You would sort of be "going off the reservation" as far as how the designer meant it to be, scantling-wise...

[paladin]

Tests by the U.S. navy types in vietnam showed that a layer or two of kevlar INSIDE the hull was a better option to prevent/oppose puncture..If I were going to do it externally, I would cut the ply to size (assuming that it's a "V" bottom, then I'd add a thin layer of polypropylene cloth or kevlar in epoxy to the inside of the plywood...then a good mix of epoxy/microballoons to use between the existing bottom and the new piece, well stapled dowmn.....and I'd use straight staples over the heavy package banding material to pop the staples out....if the staples were to stay in, I'd use bronze or Monel staples if you can find them...if you decide to use staples let me know, I have a few boxes of Monel....you can probably rent a gun....or I may be able to find mine...
After all that, I'd add a layer of xynole/epoxy on the outside.....this will really stiffen the bottom, and the kevlar on the inside id better puncture proofing...

This may be overkill.....
The short way around it may be to just add a good layer of xynole over the existing bottom, then 4 battens, about 2 inches wide by 3/4 inch thick, epoxied/screwd to the outside using care to keep them aligned with the water flow...

[JimD]

I find the most comfort in the idea of another layer of plywood. But its a next year project. For the rest of this year I wanna use the boat as is. Last couple days I cut a hole in the sole to create a foot well in the bilge and I'm raising the wheelhouse addition about three inches. With the footers now down as far as the plywood bottom I will have finally achieved a long time ambition - full standing headroom in a fifteen foot boat! :D

[alkorn]

http://www.glen-l.com/designs/sailboat/sailboat-images/dsn-mina1.jpg
If you hit a log head-on, the greatest stress will be on the stem and/or forward part of the keel. Based on the pics on the Glen-L website, the Minuet doesn't have much meat there. Perhaps you should plane a flat on the stem going back along the keel as far as the centerboard and then laminate on several layers of a hard wood. It would increase drag a little, but it would help in a head-on collision.

[JimD]

If you hit a log head-on, the greatest stress will be on the stem and/or forward part of the keel. Based on the pics on the Glen-L website, the Minuet doesn't have much meat there. Perhaps you should plane a flat on the stem going back along the keel as far as the centerboard and then laminate on several layers of a hard wood. It would increase drag a little, but it would help in a head-on collision.

Yes, that's another idea. But possibly the worst logs are the submerged deadheads you can't see, that can come up under the boat and ram into the bottom.

[Don Kurylko]

I’ll second what Krunch said. I’ve sailed many a mile in my cedar strip Whitehall on the B.C. coast. The ½” WRC hull is only protected with a couple of coats of epoxy resin and 4 coats of paint – no fabric. Any damage I have incurred from flotsam was so minimal and easy to repair, that it isn’t even worth mentioning. The real damage occurs if you intend to ground the boat. Then you’re just asking for it. Avoid that and you will be fine. At low speeds, it’s not that hard to spot deadheads and stuff. Watch for seagulls seemingly standing on water. There’s usually something big under them. You get to know the signs after awhile, no worries. Enjoy yourself and stop fretting about such things, or buy an old log salvage boat that’s designed to take a s**t kicking.

Prairie boys…. :rolleyes: :p

[P.L.Lenihan]

Third on Krunch. Also, get your lovely crew to earn her keep by maintaining a sharper look out:D:D


Cheers!


Peter

[The Bigfella]

I'd leave it as is too. That's plenty of meat there already.

I hit a 16' Hobie "log" with my wooden Tornado once - at about 20 knots (a big southerly had hit the race fleet - 30+ knots and it was survival conditions - fortunately as I screamed over the top of a 10-12' swell, I was on starboard and he was on port)... the first either of us knew was when we hit and I punched a hole in his very solid fibreglass hull and ripped a big hole in his mainsail with my port hull.

There was minimal damage to the bow of the Tornado. I patched it with a bit of gunk and completed the regatta. The ply on the Tornado is either 3/16" or possibly 1/4". If it can take a 20 knot hit like that, your hull can handle the odd log too.

Adding a whole heap more mass might actually increase your chances of damage - if you follow the scientific laws.

[brad9798]

I'll agree with the 'leaving it be' crowd.

The boat was designed as it was for a reason.

Hull size and weight are compatible with the build specs.

If something punches through that hull with that boat's size and displacement, it would certainly punch through a larger vessel's hull.

Ply like used in your boat is about the strongest thing you can get for its light weight!

[brad9798]

Nice little boat, BTW!!

:)

[James McMullen]

You've already started making mods. . . .why stop now? Make that sucker really tough with a few inches of boiler plate. You'll probably be the first on your block to have your very own ironclad.

Here's a pic of another former sailboat that got the treatment:

http://americancivilwar.com/pictures/CSS_Virginia_3.jpg

[spirit]

What I meant by battens was the kind of full length runners used on river boats to help protect the bottom from rocks. A Kevlar sheet underneath these runners would add strength with minimum added weight.

[JimD]

The boiler plate idea has merit. But I don't want to make the mistake they made on the Titanic and use lower grade rivets, which could cause the seams to open catastrophically in the event of a collision.

[JimD]

What I meant by battens was the kind of full length runners used on river boats to help protect the bottom from rocks. A Kevlar sheet underneath these runners would add strength with minimum added weight.

Thanks. Or I could just put them on without stripping the entire hull and resheathing. Now somebody's gonna say something about extra wetted surface...Time also to start a new thread about retro-fitting bilge keels.:)

[BBSebens]

indeed, i think some runners will let you slide over the driftwood. you could also fit the bow with a nice cutlass piece to save it from the banging.

at 5-7 knots, even if you do hit a dead-head, you'll probably bounce off.

[Candyfloss]

"Love the one you're with". (Neil Young).
Jim, go sailing. Insure your boat well & tow a good dinghy, but go sailing. Standing headroom in a fifteen footer? Hell! I didn't have that in my 25 footer, nor did I expect it. But I loved that boat to bits. They said the Titanic was unsinkable. No wooden boat ever will be (except maybe the Legend's catboat). Go sailing. Your boat is as good as it ever will be & it looks bloody terrific. It's summer where you are. GO SAILING.

[floatingkiwi]

The scariest thing I ever remember seeing at sea. Down in the Coromandel, ( Graemes stomping grounds),some years back,full speed a mile off in my 3 metre, glass bottom, inflatable, Aquapro, a very dark night , small chop, about 60 kilos of snapper slidin around on the floor and saw what I thought was a big bird sleepin on the water as it flew past my starboard bow. It didn't startle so on inspection I stared in horror as there, bobbing menacingly quiet, was the snapped trunk of a seventy foot tree,sticking up out of the water a foot or two, inverted, with its barkless trunk and branches spreading downward into the inky depths.
2 or 3 feet to the right and the thing would have stopped me dead in my tracks, thrown me into a centre console arrangement, probably breaking both my legs and then tearing the boats airfilled compartments clean from the hull, to skim for a few seconds before disappearing around my pained mortal being,Oh the thought, I sure slowed down a little out there from that moment to this so as to avoid anything visible on the surface. I knew that tree was 70 feet because I saw it a few weeks later, against a cliff that plunged straight down to the sea floor, jarring a sick thud each time it was dropped with the swell action.I remember seeing the damn thing quite some time after that as well, haunting me.

[JimD]

The scariest thing I ever remember seeing at sea.... was the snapped trunk of a seventy foot tree,sticking up out of the water a foot or two, inverted, with its barkless trunk and branches spreading downward into the inky depths...

Yup. Those are the ones. And they don't always stick up out of the water at all. They rise and sink like huge pistons in the swell and if it comes up while your boat is over top of it it could easily be good bye boat. Not that it happens very much but like Stephen says I'm a nervous Nellie about dying of hypothermia.

[JimD]

"Love the one you're with". (Neil Young).
Jim, go sailing. Insure your boat well & tow a good dinghy, but go sailing. Standing headroom in a fifteen footer? Hell! I didn't have that in my 25 footer, nor did I expect it. But I loved that boat to bits. They said the Titanic was unsinkable. No wooden boat ever will be (except maybe the Legend's catboat). Go sailing. Your boat is as good as it ever will be & it looks bloody terrific. It's summer where you are. GO SAILING.

Just one salient point I'd like to remind. The standing headroom thing is for the motor version of this boat. I don't intend to sail with that thing sticking way up like that. Its a convertable boat. If I could I'd have a second boat such as a Devlin Dipper but since that's not reality I've built this little windowed box to stand up in during the many months of cold rain we get here in the winter. The idea is a year round boat. I will be sailing really soon. Honest.

[jclays]

Here's a thought from left field... Sorry my mind just works this way.
What about adding several "battens only"no additional glass to the bottom. Make these battens out of a plastic material such as a "Starboard" type of material. Taper the leading edges, drill and counter sink stainless fasteners. Maybe apply a 3m 4200 type product to help attach and seal the installation. Not much added weight to change the boats handling and tough as heck. Also no finish needed on the battens. You could probably get away with 3/8 thickness and get good protection.

Just a thought from left field.........

[Candyfloss]

Sorry Jim. Didn't mean to upset you. I hear tell the Vancouver area is as beautiful as the Hauraki Gulf but the winters are real harsh. Good luck mate.