strip thickness vs. glass

[RDB]

Just a quick question? Can strip thickness be compensated by adding additional layers of glass? I have 3/8" strips and I originally wanted 1/2", kinda wet the bed. Anyways, can i just add more glass or is the core thickness crucial? Thanks for any help.

[L.W. Baxter]

Whether or not it would be structurally sufficient, doing so would make a significantly heavier hull. Also, the extra expense in glass and resin would be considerable; enough to buy all new lumber to make 1/2" strips, I should think.

--Lee

[Bruce Hooke]

How about overlaying the strips with a layer or two of diagonally-laid veneer -- basically the standard cold-moulding approach?

It seems to me that the loss of strength resulting from loosing 25% of the planned thickness would be significant because beam strength in general varies as the square of the beam depth, however, I am not an engineer so I cannot offer anything like a definite answer to your question.

[mmd]

The easy answer is yes, you can compensate with extra glass. The penalty is weight - glass is heavier than wood. As far as strength is concerned, yes, the core is critical. Assuming that a layer of 'glass is 1/16th thick, 3/8" wood with a single layer of 'glass is 60% weaker than 1/2" wood and a single layer of 'glass. To get the laminate back to strength, you will need three layers of 'glass. There is light at the end of the tunnel, though - without getting into the engineering of it, if you put a single layer of 'glass on the inside as well as the outside of the hull, it will probably be as strong as 1/2" wood with a single layer of 'glass, provided that you are using epoxy and not polyester resin. Also, you should consider going up one increment in the weight of the 'glass you were planning to use.

[John Blazy]

Great point on glassing inside as well as outside. You would then be creating a "torsion box" structure, where high skin strength will stiffen, and transfer stresses over a larger area, kindof the way the paper facing strengthens drywall - remove one face of paper and its really weak.

Of course, some would say that the wood will need to "breathe", especially if there is a breach in the outer glass seal from a rock scrape, but I think its fine if the boat isn't stored in the water.

Great idea too on overlaying with thin wood for the cold molded approach, Bruce. Ya gotta look at wood as essentially a natural version of mono-axial reinforcement fiber with the highest strength-to-weight ratio.

[RDB]

Thanks a lot for the advice from everyone. Since its kind of early in the project I may eat some $ and get back to 1/2". Is it frowned upon to glass the inside of the boat? I hear differing arguments, some say the hull needs to breathe, others say lock it up? Anyways, thanks again.

[RodB]

You should email Tom Macnaughton, a perfect question for him with his "Scantling Rules for Sheathed-Strip Construction".

I would think the best way to achieve the stiffness originally specified, would be to add a couple of layers of veneer...to achieve the "core" thickness that comes in line with original specs. The degree of stiffness of the hull is achieved by controlling the thickness of the core material. Additional glass is very heavy and you pay a price in weight to get to where you want to be.

The main purpose of the glass in strip-planked construction is to supply the strength required across the joints...from sheer to garboard. The secondary purpose is abrasion resistance and impact resistance. Of course real impact resistance is a function of the total scantlings, ie., the thickness of the core material combined with a tough "sheath". Using e-glass across the joints on the interior makes for not needing any frames at all... just a strong monocoque structure.

I don't think Macnaughton's rules of Sheath-stripped construction frown on glassing the interior at all. Glassing the interior makes for a "balanced" structure...not a structure that is just sealed on the outside only. If one uses some laminated frames then the interior glassing can be avoided, however the ulitmate strip-planked hull is glassed both inside and out...You get transverse strength and stiffness from the glass across the joints both inside and out, you get longitudinal strength and stiffness fore and aft from the planks. Bulkheads can be positioned just about anywhere you want.

RB

[ 11-30-2004, 07:34 PM: Message edited by: RodB ]

[Venchka]

Further to fabic on the inside...

Somewhere I read that a big bucks Navy project concluded that reinforcement on the inside of a wood hull provided greater impact resistance in a collision. It seems the wood crushed, deformed and spread the impact load over a greater area and the fibeglass/carbon fiber/kevlar/whatever on the inside kept everything from falling apart.

I vote for the two layers of veneer-one diagonal and one fore and aft. Then maybe a layer of Xynole on the outside to protect the veneer.

The 3/8" strips would make a nice little pram.

Wayne
In the Swamp. :D

[ 11-30-2004, 09:52 PM: Message edited by: Venchka ]

[RodB]

I agree Venchka, since the final layer would be veneer, fiberglass would not be necessary to give the transverse strength... the veneer would perform that function. Xynole would give a much better abrasion resistance and supply a thicker epoxy layer on the outside. If this design is on the small side, the Xynole may be overkill. The interior could be glassed with a light fabric to keep weight down but to still give a balanced structure (stable).

RB

[ 11-30-2004, 10:00 PM: Message edited by: RodB ]

[George Roberts]

RDB ---

You don't say what size the boat will be. You don't say what type of constuction this is in enough detail.

You can add glass on both sides or you can add ribs. There must be other ways not yet mentioned.

[RDB]

Yes, quite right. the design is a sailboat LOA is 25', beam = 6', displ=1800lbs. It is a very light design and I am planning a conservative sail area. But still, I would like the boat to be fast, it will look something like an 8 meter, only smaller. Anyways, I guess I have to make the decision to either eat the costs, or try to make up for it down the road, makes me sad. Thanks again to everyone who responded, I really appreciate it.

[Buddy]

One more idea. You could use some of the newer "cored" fabric /mat materials to cover your hull and get the thickness and comensurate panel stiffness all in one easy go, since you were going to sheathe the outside anyway. You can buy the stuff at Defender by the yard. Make test panel of true 1/2" and then your 3/8" with the Coremat/ fabric and then detructive test to settle your mind on it - actual weight, resistance to bending and impact- before you proceed.

[Venchka]

Originally posted by RDB:


Yes, quite right. the design is a sailboat LOA is 25', beam = 6', displ=1800lbs.

Hmmmmmmmmm...with the usual disclaimer that I don't know nothing...sounds a bit too light to me. A typo on the displacement number maybe? Long, skinny and almost no ballast, eh? Should be fun. :D

Wayne
In the Swmap. :D

[Billy Bones]

There are some heavy duty, tried and proven catamarans at 25' with 5/16 cedar strips glassed in and out that have been pounding the oceans for years.

I am puzzled and a little troubled by the disparity of strip plank scantlings around. One tiny sport boat I have plans for calls for half inch strips glassed two sides (!!!) this in a boat with no ballast and 19' overall, replete with bulkheads. I get the sense that some designers have experience as builders and some don't.

Good luck, anyway.