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hull strength


JeffM

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Don't know much about boat design, but...

I just read something in Devlin's Boat Building that gave me pause: he uses 1/4" plywood for the hull only in small skiffs; 1/2" for 20-footers. The Core Sound 20 I'm building is designed to be seaworthy enough for coasting, yet has bottom panels of 3/8" and 1/4" ply, and sides of 1/4" ply. All bulkheads and deck are 1/4" ply. Is Devlin over-building or is Graham under-building, or am I missing something?

What is the contribution of epoxy and fiberglass to hull strength? When I first began to learn about stitch-and-glue I thought of it as wood-cored fiberglass, but I find many builders don't even bother with the fiberglass if the plywood is high-quality okume. If I expect to take my Core Sound 20 out of Narragansett Bay or out to Nantucket, would I be well-advised to reinforce the hull somehow?

Jeff

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When plywood is stressed such as how the core sound and Stevenson's boats are made, it becomes very strong. Take a pc of paper, fold it in half, now the paper becomes very strong and won't bend easy across the fold. Plywood sorta works the same way but different. (don't use the paper to make your boat) There is a name for it but it escapes me at the moment. Epoxy and glass will add a little to the strenght but not much. It mainly is for protecting the wood from dampness and water. Also prevents checking of Doug Fir plywood.

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"Torsion box" and "stressed skin" is sometimes used for this type of construction. Its amazing how strong a box made of stressed skin 1/4" plywood really is ... it feels like a brick.

Devlin makes some very hardy, semi-displacement hulls in his larger boats. He has thick panels to allow them to ground in the un-friendly waters of the Pacific northwest. His Surf Scoter, for instance, is a 22' boat that has a displacement of 3800 pounds.

His 19' Sea Swift sailboat is closer in intent to a Core Sound, and is a lot lighter than the Surf Scoter type of boat, with a boat weight of 750 pounds. I suspect it is built with lighter plywood.

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Sam builds all of his boats with heavier scantlings than that recommended by most designers. I'd say that most are overbuilt for anyone who is performance minded. He describes his boats as solid, comfortable and quiet, all of which tend to increase with scantling dimensions. Different market. A power boat with power enough to punch into waves at more than twice sailboat speed experiences shock loads much higher than a slower boat and needs stronger scantlings.

Around here, B&B's Core Sound boats run in water at least as rough and challenging as anything Narraganset Bay will toss at you. These boats are plenty strong enough as designed. Added weight is just more weight.

If you plan on running into things more solid than water, then you should think about beefing up any possible point of contact. Otherwise, the boats are fine as is.

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Thanks, Arnie, Frank, Tom and Ray. I'm feeling more confident in these floppy panels (once folded, of course). I'll glass the bottom for those times I'm not completely successful at "avoidin the rocks." Thanks, Ray for the invite to look at your boat. I've got an invitation to take a ride with a new CS20 owner who's local -- amazed to have one so close!

Jeff

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A 1/4" plywood panel laminated with epoxy and directional glass is stronger and lighter than a thicker plywood panel with only one layer of glass.

With the glass on either side, you have a composite sandwich. You might do a search on this "I-beam" property of sandwich cores.

Trust the designer :D

Joel

Boatbuildercentral.com

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Trust the designer :D

Joel

Boatbuildercentral.com

Depends on the designer I guess' date=' but I feel pretty comfortable with this one.[/quote']

One big problem in plans for amateurs is that some designers never build the boats that are purchased on the plans. Yes that actually happens.

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Thanks, Joel. I am familiar with the I-beam concept of cored fiberglass. It's nice to know it pays off in strength-weight terms. Although I like the idea of avoiding glass on the interior (both for wood inspection and looks), I'll probably glass the interior forward of the fore bulkhead, and maybe the inside of the bottom panels as well. I'm likely to beach a lot, sometimes on gravel.

Jeff

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You don't want to go too far with the sandwich-strength thing. The stiffness of a beam or panel is much more a factor of the thickness than anything else. Given two same wood panels of the same wood and equal weight, the solid wood one will always be stiffer than one with fiberglass laminate on one or both surfaces.

Wood is good stuff.

When talking about strength, you have to define whether you are talking about breaking, impact, tensile, compression or some other factor.

You have to get pretty high tech into fibers and cores (beyond glass) to beat wood as a boatbuilding material. I have built wood racing sailboats and had to add lead weights to meet the class minimum weight restrictions while fiberglass builders have to fight to keep the weight down to the minimum.

Plywood is good stuff.

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Thanks, Tom. I'm learning a lot! In discussing my needs with Graham a few days ago, he confirmed that his design is well able to handle the pounding and stresses of coastal sailing, but did say that fiberglassing inside and out would considerably reduce damage from impacts. He said that an impact with, say, a rock that made a small hole in the exterior of the hull would make a much larger hole in the interior -- unless the surfaces were glassed. I don't want a racing boat; I do want a boat I can beach with confidence.

Jeff

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Jeff,

A layer of glass on the inside will greatly increase resistance to hull rupture from impacts on the outside. Actually better in this regard than glass on the outside. Because of its toughness and high tensile strength, a layer of Kevlar on the inside might be the ultimate for this purpose.

For abrasion resistance on the outside, an epoxy/fabric sheath of Xynole or Vectra polypropylene is excellent. A layer of 6 oz glass cloth in epoxy is about twice as good as epoxy alone and a single layer Xynole gives about 6 times the abrasion resistance of the glass cloth. These numbers are based on some objective tests I made a few years ago. On a light boat, such a sheath will stand dragging over sand, rocks and oyster shells with no more than minor scratches.

The down side of these sheaths is extra money, time, labor and a bit more weight. If the boat is expected to see rough use as with a beach cruiser where the boat will be drug around in less than idea conditions, they are worth it.

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Tom, to get a little off topic here, would the Blue Jacket benefit from interior glassing as well? Foregive me as I haven't been at the plans for a while now. I don't recall the plans calling for interior glassing. would it be a good idea????

This seems timely as I am thinking more about it as my shop is coming to be.

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John,

I think your question is spot on the topic.

A few weeks ago, we were cruising on Bay River at about 16mph when the boat hit a submerged something under the starboard side of the hull. Just a loud clunk and when the boat was taken out the next day I found some paint missing about 30% aft of the stem and further aft on the chine. No physical damage at all. In the Pacific Northwest, which we hear has quite a bit of debris in the water, this may happen more often. While I don

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The PNW where Devlin builds his boat does have its share of rocks, submerged logs, etc. And the weather is cold, so they build stout and hardy boats up there. The designs of Edwin Monk found favor there (for production boats of his design, look at the older Tollycraft boats).

Tom, you'd be interested to know that the PNW, and on up through the inside passage of Alaska into Anchorage you see a lot of pilot house cruisers. Your Bluejacket would be at home there. My wife went on a cruise with her sisters and she noticed quite a few of the C-Dory's up in Anchorage.

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Tom, you'd be interested to know that the PNW, and on up through the inside passage of Alaska into Anchorage you see a lot of pilot house cruisers. Your Bluejacket would be at home there. My wife went on a cruise with her sisters and she noticed quite a few of the C-Dory's up in Anchorage.

Aye! That is why I intend upon building one! Thinking about a cold moulded layer of 1/4" on the forward portion. ??? And/or the interior lasing re-enforcement. ?

Thoughts.

Yes, there is lots of 'stuff' in the water here. :shock:

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John,

There are a fair amount of lateral and longitudinal bulkheads in the forward area. Even without stringers or interior fabric reinforcement, the scantlings meet normal practice. The curvature of the bottom panels in this area create great rigidity. Belt and suspenders will help if your pants have a habit of falling down. :)

The entire hull bottom is extremely strong as a result of the torsion box construction with longitudinal bulkheads on 7 1/2" centers bonded to both hull bottom and cabin/cockpit soles. The only issue is penetration of the hull by objects the boat may encounter. After looking at the beaches and marinas in Florida this morning, it appears that it may be impossible to guarantee against that. Gonna be some cheap boats down there.

Adding 1/4" ply to the forward sections might be a difficult fairing job unless it is put on the inside. There, it might be a physical problem to fit in unless it were done in sections like cold molding. It would be impossible to layer it before the hull is formed up since it would never accept the curvature. If you did decide to add the layer, we will discuss a procedure later.

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3/4" plywood will not take the bend in the bow, John. The required bend is near the limit for 1/2" material. I suggest fabric on the inside as the easiest solution, if you think that reinforcement is necessary for higher impact resistance. That is what I did on my new runabout, Scamp. I had to reduce the forward bottom thickness to 1/4" for the curvature on this boat.

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