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hull strength


JeffM

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Not to throw a wrench into the mix but, my friend and I both built the bottom with 3/4 acx. Never once had a problem with it bending and mating to the keel. :? We started at the front and worked our way to the back. We just stand on it while we screw it in place with 4 inch screws. You do have to put the screws in at a bit of an angle to the keel so you grab a bit of each lamination in the keel. Otherwise it might want to pull away from the keel a bit. We're 10 months into construction without any pull-away.

We're thinking it might also help the stability a bit when we float them.

Ulysses Stockdale

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  • 3 weeks later...

How hard do you want to hit things with your CS20? I was looking at my CS17-in-progress yesterday with a friend and we were talking about catastrophic stuff. I feel pretty good that if the hull is breached I won't drown. There are 2 watertight (or nearly so) compartments under each seat, a watertight forepeak and a watertight lazarette and there's the main cockpit area for a total of 7 distinct areas (No it's not unsinlable like the Titanic :) ). I think that's a positive thing if you're thinking about catastrophic events. I've never hit anything hard enough in ANY boat (and I've built some rickety boats) to breach the hull so I think we're really talking about a survival event here, not the normal wear and tear of beaching.

Any home built boat can be a home repaired boat and so as long as I can limp home with wet feet I'm happy.

The cockpit sides add a lot the the structural strength of the stressed-skin/torsion-box model so the boat shouldn't fall apart if holed.

Does anybody out there have good stories about boats that actually have been holed and what might have prevented the damage?

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  • 2 weeks later...

Back to the BJ 24; Tom I had a nice conversation with another person here in Olympia who is getting ready to possibly start building. He expressed the same concerns as I in regards to the possiblity of hitting a deadhead in the water.

He was considering a 2 part urethane foam in between the stringers to increase hull rigidity and strength. I was wondering what your thoughts (as well as others) are in regards to this. My main concern would be the possibility of trapped water in the foam, but I suppose that is a mute point since it is a sealed compartment anyway. ;):)

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Capt Jake, you know the BJ24 is one of my "dream boats", I have plan set #3, and if I ever get a chance I will build it! So this thread is like torture ...

But anyway ... I don't think the addition of foam will help any, other than providing flotation (which is not a bad idea in any case). With submerged logs, etc., I think you need more strength against something poking a hole in the boat. Rigidity is not a problem in a "torsion box" or stressed skin plywood boat.

Cold molding layers of ply on the inside might help, but would be very tedious. I don't think it would be worth it.

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Capt Jake, you know the BJ24 is one of my "dream boats", I have plan set #3, and if I ever get a chance I will build it! So this thread is like torture ...

It is getting to be torture for me as well. :) The call yesterday out of the blue (I have had several in past months) are getting me excited. I have a space problem at the moment I am trying to overcome before I can build. ;):)

I had thought of cold molding, but as Tom points out, it would be a tough thing to do. I am leaning towards his recommendation of Kevlar; seems to make the most sense to me and fairly easy also.

Turns out the fellow that called me lives around the corner from where I used to live. :shock:

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Frank & John,

I have been away for a while and went to the Annapolis boatshow while visiting #1 son. Real crowded and pretty much a waste of time and money unless you are interested in big expensive fiberglass or aluminum boats. Anything under 30' was stuffed away behind the tents. Some interesting stuff in the tents but not enough to get me there again.

I do think the risk of hitting submerged objects may be creating a bit more concern than it warants. Just how much impact would be experienced by hitting a log in the bow area is unknown. There are just too many variables like speed, size and weight of the deadhead, just where and at what angle the impact occurs, etc., etc.

In the PNW or flooding inland rivers where semi-submerged trees and logs are sometimes found, some extra caution in both cnstruction and boat operation may be called for. The hull of the BJ24 is very strong but any hull can be penetrated, given the right conditions. Looking at photos of boats following the Florida hurricanes should be enough to convince anyone that no boat is immune from such damage. I think the best route is to use the normal construction in most areas and to put a layer of Kevlar on the inside of the bottom from the stem back to the forward bulkhead of the pilothouse in areas where there is much concern with debris. I have used 18 oz biaxial cloth which might be as good but I have no comparative data. Kevlar has extremely high tensile strength which would prevent penetration by all but the most sharp and heavy blows.

I would prefer this to just making the skin thicker since it would allow some flexing under impact but would be very hard to actually penetrate. Some tests I have seen of this kind of sandwich show that the plywood can fracture while the structure remains intact. The Xynole on the hull exterior is tough and flexible which will also help maintain an intact skin even though the plywood is fractured.

"Liz" has over 1500 lbs of positive floatation in the form of blue waterproof styrofoam in addition to the natural buoyancy of the wood so sinking is out of the question.

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Well, I have sit by reading this exchange, thinking of my reply. I keep coming back to my original thought on kevlar, double skin and resistance to submerged objects. I have boated on the Mississippi River, all up and down the east coast, yes in the mud banks of Georgia, through the dismal swamp, up through the Pungo, Alligator River, which is absolutely the worse for logs, second to the ICW at Myrtle Beach. Next wrose use to be the East River in New York City, where bodies are mixed in with rims and tires. Maybe I have been lucky, or maybe it just wasn't my time to go down. I have hit a lot of stuff and have never puntured a hole in both wood and fiberglass hulls, even my own mixes, which is no less or no more than the specs on the Bluejacket 24. At the designed speed of the boat, you will need to hit something pretty solid like a tree dug into the land, to hole here. Now this is my rant and my opinion for the year. A floating object at even 20 knots, which is probably more than you will be doing under most conditions, you can say that your chances are greater on the highway. Thank you for the chance to give you my opinion, biaxall cloth is the trick. :wink:

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I think you are both right. It isn't really one of my main concerns anymore. Thanks. :)

Tom, #1500 of foam flotation; I skimmed the plans and I am not seeing this, other than in the cabin and deck tops. Where am I missing this?

Oh, BTW, I will be building out of Meranti when the time comes. Will this affect the performance much? I know that Meranti is a bit heavier than Okume.

edited to say;

I just found it, the flotation is under the cockpit sole. Found it in the pics.

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I think you are both right. It isn't really one of my main concerns anymore. Thanks. :)

>>Good. My thoughts also.<<

Tom' date=' #1500 of foam flotation; I skimmed the plans and I am not seeing this, other than in the cabin and deck tops. Where am I missing this?

Oh, BTW, I will be building out of Meranti when the time comes. Will this affect the performance much? I know that Meranti is a bit heavier than Okume.

>>>The difference is significant but not catastrophic. I will need to look up my measurements of the difference.<<<

[i']I just found it, the flotation is under the cockpit sole. Found it in the

pics.[/i]

>>>Also look in the back of the galley ( I hope I mentioned that) and don't forget the ice box, which gives about 800 lbs. It's a w-e-l-l insulated ice box.<<<<

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  • 2 weeks later...

I have no real problem with meranti except for the extra weight. I recently used some (3/8") that I got from Graham on a runabout. I thought the surface was rougher than most occoume I've used but that caused no problem. It is stiffer and probably stronger.

Some of the floatation I put in was an afterthought.

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I realize the weight is the major concern with Meranti. I am guessing in teh order of an additional #400. Sound about right?

Only reason fo thinking Meranti is it is stronger, more rot resistant and a whole lot cheaper. Don't know about bending it though.

From the local supplier (same place Hugh M. bought from)

Okume 1088, 9 plies and #40

Okume 6566, 7 plies and #40

Meranti 1088, 9 plies and #56

Meranti 6566, 11 plies and #56 (these Meranti weights listed are higher than other suppliers).

Number of plies seem odd to me regarding the Meranti.

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  • 5 months later...
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Having read all the posts on this thread a couple times, I have a question about plywood. They say old fiberglass boats get heavier over time due to water absorption. How well would a plywood boat hold up with a skin coat of glass cloth/epoxy, if the boat is left in the water year round? Will it absorb enough moisture through the skin to wet the plywood and start trouble? I don't see plywood being a problem on a trailer sailed boat that is only in the water for a few days or weeks at a time. But year round?

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I've read that epoxy is not as waterproof as polyester resin, so I think a lot of this depends on the thickness of the epoxy coating. You have to get to a certain thickness before it is as waterproof as a gelcoat on a boat.

I think the main danger with epoxy encapsulated plywood on a boat in the water all the time is the water build up inside the hull due to condensation. An amature builder is less likely to seal the inside surfaces as well as the outside, and then the water can work its "magic" as the universal solvent.

For the fiberglass boats out there, I know mine has a pretty good gel coat on it, but the polyester resin on the inside of the hull is pretty rough. I'll bet a lot of that water infiltration into the glass mat is from the bilge, and not the water outside the boat.

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