Replacing water-logged bench seats

[wayne kevin]

GL
Just a quick question--how did you go about looking up the info on Divinycell, with regard to densities and relative strengths? And I'm wondering if I should go balsa again, but I doubt I can even get it in 1' x 12' lengths, much less afford it. Anyway, thanks for your advice.

Wayne

[GreenLake]

I believe I just googled. "Divinicell specific weight" was the search string I used (it's still stored in my search box). I hear you on the balsa.

I think that you'll find that by simply by making a sandwhich glass/foam/glass, the whole structure gets to be very stiff, that's the way a sandwwich works. The middle (core) needs to have good compression strength (and shear strength) for the purpose.

Since the divinicell is made for that purpose, you should be fine, the question is just which grade is sufficient.

I don't know how what the minimum requirements are, but I guess if it can hold your weight over a sufficiently small area you should be fine. I would always make a sample, before you do the actual bench. That allows you also to experiment with the minimal amount of glass - you really don't want to overdesign here, because that adds weight. You would take an oar and try to puncture the test panel, if it survives, and is stiff enough, that's enough glass

If the result is fine in the short dimension but too bendy in the long dimension you can always add a hollow stringer, or use an extra strip of foam to make an L shape with the short leg of the L below the seat edge. (Because I don't really understand your layout, I assume that the space below your seats is currently open, if there is a glass panel then that would give enough support...)

[wayne kevin]

GL
Excellent. I really appreciate it. Best,
W

[K.C. Walker]

Wayne,

According to the Georgian Brothers in their West System manuals, you're unlikely to save weight by using cored composite construction as opposed to using the plywood when using small dimensions. Plywood would be far easier to construct in this situation. I can't imagine that you would need to go thicker than 1/4 inch plywood for the sides of the seat tank and 3/8 for the seat. I would suggest that you investigate the possibility of using plywood.

Any foam core or balsa wood core becomes pretty heavy by the time you put the required resin and fiberglass on each side to make it as strong as plywood. The benefits of using cored materials are for compound curves and for thicker composites. I'd have to go look it up but I believe their breakeven point for strength versus weight ratio is about 1/2 inch plywood versus composite.

The volume of flotation of a cored material is minimal and not really relevant. It's the volume of the enclosed flotation tank that is relevant.

If you do decide to use core materials I found Noah's Marine to be a good source. They often have odd sizes and cutoffs that can be substantially less than retail. A lot of the boat builders in my area use this for decks and cabin tops http://www.noahsboatbuilding.com/noahus ... =0&Tp=&Bc= .

KC

[GreenLake]

K.C., as usual spot on. I confess, I'm still unsure what Wayne wants to realize: a set of benches on supports, or a set of boxlike tanks. For the latter, you are right - nothing beats plywood. And he could go down to something less than 1/4", say for the verticals, if he can source it.

The trick with plywood is to coat it 100% and on all sides with epoxy, so it's sealed against moisture entering the wood.

[K.C. Walker]

GreenLake,

I agree that 1/4 inch verticals would be more than enough. Also, for sure encapsulate the plywood with epoxy. No matter the brand of epoxy that you use, the West System technical manuals are a valuable resource. http://westsystem.com/ss/how-to-publications/

KC

[GreenLake]

@K.C., are you familiar with the Mirror Dinghy? It's built entirely from plywood. The kits for that boat come with 4mm or 5mm strong plywood (not sure which, but it's definitely thinner than 1/4" which would be a little over 6mm). Its cockpit seats are box-like structures that serve as flotaion tanks.

So Wayne, if you build your seats from 1/4" plywood, they would be plenty strong, and, as long as they are, would help stiffen the boat.

Normally, in that kind of construction, one would reinforce the seam, or corner, with a bit of glass tape and a fillet on the inside. If weight allows, a very thin glass cover on the seat surface would provide abrasion resistance to withstand any imaginable abuse. An extra coat of epoxy followed by PU (see for example the info on the System Three website) might be enough.

If you just built the seat from two plywood strips, one horizontal and one vertical, it would be the simples. The one downside would be that the edge of the seat would be a bit sharper than what you find on the molded seats of later iterations of the DS. It might be possible to make the edge out of a strip of light wood, so that it would be possible to round it by router or sanding. But that makes the design more complex.

[K.C. Walker]

GreenLake,

I'm not specifically but only generally aware of the mirror dinghy. It makes sense that these building principles would work well in this application. I found an article that I thought was interesting and you might, as well. It could also apply to the building methods for this project. I think that you are right about the possibility of not even needing reinforcements on the corner but just using fillets. http://www.duckworksmagazine.com/10/how ... /index.htm

If one does not mind painting surfaces you could round the corners quite generously for comfort and still have a very professional look.

KC

[GreenLake]

K.C.,

I don't know whether Wayne is still following this discussion or whether he's decided on a course of action for his project already. In any case, the Mirror is very much an example of the kind of construction techniques for lightweight boats that are discussed in the excellent article you linked. I must have read it at one point, because the images looked familiar....

I would not be surprised to find out that re-building say a set of molded seat flotation tanks in this technique were to end up lighter and stronger than the original.

If I remember this correctly on my DS, the molded seats seem to have an angle that's slightly more acute than a right angle, either giving a tiny bit of extra space for the feet, or tilting the seat surface back a bit. But it appears not to be 90 degrees.

Since the molded seats of later generations of DS are white or colored, painting would not seem out of place, and the epoxy needs UV protection anyway. Paint is superior because you can notice should it wear off. So there would seem not to be any reasons not to try the kind of rounded corners.

[wayne kevin]

K.C and GL,

Indeed, I am following and MOST appreciative. My goal is to replace the long boxlike seats on my old DS and to save weight. My sense is that the old seats use thickly coated balsa on top and plywood for the verticals. The seats--12' worth--are glassed only where they attach to the floor and side of the boat, I think. Gosh, lighter would be better. But the boat is very stiff--and my whole goal is not to mess with the basic structure, because it sails nicely; I just can't stand being 35 pounds heavy. My fear is that I will do all this and still be heavy. If I might ask one more question of both of you. I've looked at the article and am convinced going 1/4'' marine ply would work, but what steps would you take? I planned on removing the seats with a small rotary saw and then building the seats to match in my garage, then attaching them. That way I could coat the inside of the structure and make sure it's all sealed before trying to glass them back into place. Thanks again you guys.
W

[K.C. Walker]

Wayne,

Fitting a 12 foot seat tank is going to be tricky. It might be worth breaking it up into 2 sections. You could build a class legal seat size for the aft section and then build the "extension" seat portion forward of that separately. Of course you could join them together for ultimate stiffness and still be illegal.

If you think it's just the bench seat that's waterlogged you could just replace the top. If you have flotation foam inside the tanks you might want to check that, as well. The fiberglass hull itself is somewhat hygroscopic and can take on weight. 35 pounds could be lurking in all kinds of places in the boat.

You could always take your boat to the desert to get it to lose weight. I forget where I read the account, but somebody from Texas drove to a regatta in California and by the time they got there their boat was 20 pounds underweight or something like that.

KC

[GreenLake]

Wayne,

for the weight, you need to simply run an estimate. I know that one can build a Mirror, which is 11' x 5', but a whole boat, not just benches, so that it comest to about 100lbs. So you would need perhaps 50% of that.

A 1/4" plywood panel, is estimated at 25lbs, for a half box of 1' x 12' with 1' of vertical, you'd need 1.5 panels. So you are approaching 40 lbs.

Because of the way I visualize that your seats taper, I'd expect that you save enough wood to roughly offset the weight of the epoxy coat, fillets, and glass tape for the corners. Your existing seats are unlikely to be lighter than that, but perhaps not much heavier (in their dry state).

If you have 10% of the volume of 1" balsa filled with water, that's 1-2cubic ft of water, given your seat geometry....

So, if your diagnosis is correct, that you have waterlogged benches, and if my estimates are realistic, I would expect a weight savings from their replacement.

Stiffness: duplicating the existing structure should ensure that the stiffness remains the same. These plywood constructions are plenty stiff, and the corner helps.

Putting it together: since your benches are longer than a plywood panel, you need a joint. I would start with making the four panels, by joining them with a scarf joint or lap joint. Then you can cut out the shapes (I assume that they have curved edges) and dry fit them. At about that stage I would epoxy coat the panels on both sides.

Now comes the question how to assemble them into a seat bench. I would be afraid that, despite best efforts, a pre-assembled seat might be harder to fit. Here's a suggestion for an in-hull assembly method for your consideration:

First, I would epoxy a few small wooden blocks along the side of the hull and the bottom. These would serve as supports/stops when fitting the panels. They would be fully epoxy coated.

Then, I would epoxy something like a 1'x1' triangular cross section along the inner upper edge of the vertical panel. That way, you can fit the vertical panel first, then lay the seat on top. (you can epoxy one or two "stops" underneath the seat surface, so your vertical panel has something to rest against if it should flex, or you can use nails that you remove before the epoxy has fully hardened.)

Finally, you would fit the seat. It should rest securely on the wood blocks on the hull and on the 1x1 for the vertical while you glass tape the connection to the hull. After the epoxy joint to the vertical is cured enough, you can sand the seat edge to perfection, and re-coat with tape and epoxy.

Even though, with this method you assemble in the hull, you can have everything fully epoxy coated. And by using the stops you can provide some pressure/weight to firmly hold the panels into place.

The one downside of the method is that the work of sanding the edge has to take place in the boat.

With a bit of forethought, you might even be able to have the epoxy coat on the panels still be in the "green" stage when you glue them together, so that you get the strongest bond.

Also, this approach could be used to have the vertical panel slightly curved - rather than having the seat edge a straight line, it could be a constant offset from the gunwale, for example, before it starts tapering. A slight bend in the vertical panel would make it stiffer in the other direction.

OK, that was a bit detailed. I hope this can help you clarify your thoughts on the matter.

Oh, and whatever you do, fill the void with blocks of foam. That way you will have positive flotation even if your seats don't make an air tight seal with the hull... Yes, the foam weighs something, but from a safety point of view it's not optional.

[Jett]

For an interesting take on what to do with seats, check out the current issue (May/June) of Small Craft Advisor for an article on modifying a DSII for cruising. The author removed the seats entirely (among other unique modifications: boom gallows, running headstay, etc.).

There is also an second article on crusing a Spindrift DSI on Lake Superior, which you may or may not want your spouse to read! Good issue for Daysailers.