Cold Molding- how much wood?

[TomFF]

I'm considering the Marsh Cat- 15 foot LOA; beam 6'11". I haven't ordered the plans but believe it calls for 4 layers of 1/8" mahogany cold molded. I will likely stick with the cold molding.

Can anyone give me a good estimate of how many board feet of mahogany it would take to do the hull?

How much epoxy?

[Bob Cleek]

Yes and no... there are various formulae for calcuating the surface area of a boat, or any other round object, but they are all in the realm of "squaring a circle." Boatbuilders have various rules of thumb. Take the length times the the depth times two and maybe fudge a little based on your experience with the hull shape. This gives you a "long" estimate that should cover your waste. Of course, a "board foot" estimate is less accurate because with 1/8" laminates, you can figure another 1/8" between each one for your saw kerf and so on... Maybe you might ask the designer what others have discovered was necessary. Don't forget that every construction method is a compromise. Cold molding's downsides are the costs of the adhesives and the cost of the wood... figure you'll be making as much sawdust as you do boat!

[TomFF]

I wasn't thinking of cutting the veneer myself. 1/8" stock can be had from a couple of sources. But how much is needed?

[Nicholas Carey]

Originally posted by TomFF:
I wasn't thinking of cutting the veneer myself. 1/8" stock can be had from a couple of sources. But how much is needed?Well...veneer is usually sold by the sq foot or the 'flitch' (a log sliced up into veneer). If I had to do a rough guesstimate as to how much veneer was needed I'd model the hull as a 1/2 of a ellipsoid prism whose length is the overall length of the boat.

Ellipses are defined by two measurements (A) the major, or long axis and (B) the minor, or short axis. A circle is a special case ellipse where the major and minor axes are the same length (congruent).

For the boat-as-ellipsoid-prism model, one axis will be the maximum beam of the boat and the other will be the vertical distance, in profile of the highest point on the sheer to the lowest point on the keel.

The formula for the approximate perimeter of an ellipse is this:

</font><blockquote>code:</font><hr /><pre style="font-size:x-small; font-family: fixed;">pi * [ 1.5 * ( a + b ) - sqrt( a * b ) ]</pre>[/QUOTE]where a is 1/2 the major axis and b is 1/2 the minor axis.

So, for the March Cat, we have an LOA of 15 ft. You list the beam as 6 ft 11 ins, which we'll round up to 7 feet for convenience. You don't list a depth, so for the sake of argument, I'll assume the sheer-to-keel distance is 4 ft.

The above numbers give us an a of 3-1/2 ft (1/2 the major/long axis) and a b of 2 ft (1/2 the minor/short axis).

Plugging these values into the formula (and using 3.14 for pi), we compute p, the perimeter of the ellipse as:

</font><blockquote>code:</font><hr /><pre style="font-size:x-small; font-family: fixed;">p = 3.14 * [ 1.5 * ( 3.5 + 2 ) - sqrt( 3.5 * 2 ) ]

p = 3.14 * [ 1.5 * 5.5 - sqrt( 7 ) ]

p = 3.14 * [ 8.25 - 2.64575 ]

p = 3.14 * 5.60425

p = 17.6 (approximate).</pre>[/QUOTE]So the area of the ellipsoid prism is ( 17.6 * 15 ) square feet or about 264 square feet. We're only interested in 1/2 this area since we're modelling the hull as 1/2 of the prism (a longitudinal slice to cut it in half).

Therefore, each layer of veneer will be about 132 square feet. For 4 layers of veneer, then, you'll need about 528 square feet of veneeer.

I'd probably up that by some factor (10%?, 15%?) to allow for waste/miscalculation/etc.

Hope this helps.

[Art Read]

... Or you just do it like I did with my carvel planking. Just buy only as much wood as your checking account will bear in the first order. By the time you've used it all up, you'll have a much better idea of just how much more you're likely to need! ;)

[B. Darrah Thomas]

Some creative guessing :D put me at 400+.

[Ross Faneuf]

Do you have the body plan? If so, measure the girth of each section; measure with a marked batten, or a map wheel if you're working from a plan, or something similar. Use the station spacing and one of the simple integrating rules (such as the trapezoidal rule) and get an approximation of the area of 1/2 of the hull. Work from there to the total area of veneer you need, and add 50%-60%.

If you haven't encountered the trapezoidal rule before, it is:

s * (1/2G(1) + G(2).... + G(n-1) + G(n))

where: s = station spacing
G(1) is girth of station one
G(2) is girth of station two
more stations
G(n-1) is girth of next-to-last station
G(n) is girth of last station

There are more sophisticated and accurate integrating rules, but this is plenty good enough for estimating how much veneer you'll need.

[George Roberts]

Ross Faneuf ---

last term should be 1/2 G(n)

TomFF ---

You might try

3/4 x (beam + 2 depth) x length x number of layers

This will be high but you will have a lot of offcuts.

[Pembroke Faucette]

I'm currently in the 4th layer on the Marsh Cat. Each layer has taken approximately 90 boards 1/8"x4"x5'6". I've bought 140bf of 8/4 mahogany and had it re-sawn to .160 on a Baker saw, then plane to1/8".

[TomFF]

Thanks for your thoughtful answers.

Pembroke- Glad to hear of your progress. At the start of this thread I planned to buy veneers. But on investigation the cost seems pretty steep. Your way sounds like the way to go. How long have you been working on the Marsh Cat? Any trouble spots? Any pictures of your progress?

BTW welcome to the forum.

[Buddy Sharpton]

The plans call for 900 square feet in the hull, 100 more if you mold the coaming rather than bend in solid stock. Mine was molded.

[Buddy Sharpton]

Hey, Pembroke, I'm an old Savannah boy ( Savannah High Class of 66) who's been making a living in Atlanat ever since I started at Georgia Tech. But my oldest daughter lives in Savannah ever since she graduated from SCAD. The point is, now that it's finished, I plan to bring my Marsh Cat down to Savannah lots- coastal sailing is why I built it; we keep an auxiliary here at Lake Lanier that's just to much work to trailer. May be we could get together; I'd love to see your's. How far along are you?

[mhoffman]

Tom-
do you have an aversion to cutting your own? I did mine and saved two or three truckloads of money. I only used mahogany for the outer layer and cypress for the internal ones. Good excuse to buy a decent bandsaw, if you don't have one already...
Resawing is pretty easy operation, and you can cut exactly what you need. Look around your area, you should be able to find some local lumber that would work for you. The mahogany for the outer layer would only be requisite if you are finishing bright, strength being compensated for with glass and epoxy.
I did a mahogany runabout cold-molded- shoot me an email off line if you like, if you have procedural questions.
Just my two cents.... :rolleyes:
Buddy- ever make it to St Simon's?
Matt

[Buddy Sharpton]

'Bout every other year. My bride's from Screven, outside of Jesup ( and for everybody else thats near Brunswick and that's on the mainland opposite Saint Simon's). We occasionally drag the Whaler or take kayak done. I'm betting this summer we might bring the MarshCat. Love the Crab Shack.

[mhoffman]

Buddy-
No kidding! My wife is from Jesup- she knows a lot of folks in Screven (all three families...). We both used to live up in Atlanta- did not know her then- met her on St Simon's then bought the property over on this side a few years ago.

I built a kayak a couple of years ago- if you make it down this way, give me a shout and we could go for an outing if you like. I'm on the water (Buffalo River) and can put in here. Have a marina in the neighborhood too when you get that cat ready... ;)
Matt

[TomFF]

No aversion at all. I orginally figured that cutting my own would take alot of time and produce truckloads of sawdust. But after pricing veneer, it's looks like it's worth the effort.

Unfortunately I don't have a Baker sawmill and can't convince SWMBO of the need. Sooo many tools.... sigh.

[mhoffman]

Tom-
I just used a standard 14in bandsaw. Trick is to get as wide of a blade that you can fit on your machine to prevent drifting. I found that Constitution saw blades (back of WB) had good prices and great product. I would strongly recommend not using a table saw due to waste of the kerf thickness.
Really, if you are cold-molding 'glassing, using mahogany thoughout should not be required- just my thought.
What type of glue are you planning to use between layers? Epoxy would be pretty expensive- but has really good gap filling capabilities, and long set-up time. I tried to use cross-linked PVA glue, but ended up using thickened epoxy- used sawdust from sander and talc (baby powder). The talc helps it smooth out better- sawdust alone doesn't spread as well. Cheaper too than straight epoxy.
I tried using clamps for the whole project, but gave up for staples- lots faster. Just pull them out with a good staple puller before the epoxy cures fully. The holes are filled with the next coat of epoxy and are not visible outside of five feet.
Just my experience- might not be the best way to go, but its what worked for me... :rolleyes:
Best-
Matt

[mhoffman]

Oh one other thing- sorry- I found a guy who had a sawmill that I was buying my lumber from. I asked him to mill the bords I was buying to 3" thick. After I re-sawed them, I ended up with 3" wide, quarter sawn strips. Got lucky with the boards though- he was cutting a huge cypress tree (like, 40 foot bole, 40 plus inches in diameter...) he pulled out of his swamp that was felled around 1900. I don't think they make those anymore- too bad.
Matt

[Buddy Sharpton]

Tom-a few thoughts to add to what Pembroke has said about cold molding this hull. I have cold molded other projects, but I bought my Marsh Cat built and it had some real problems I had to correct concerning fairness and inter ply adhesion in the tight compound turns in the bilges.It was built by a professional small craft builder for his own use and I suppose he was not that experienced with this technique. I understand another MarshCat he built had similar problems.

Don'tscimp on you glue or staples and end up with voids. You neep to wet out both the hull and the next layer of veneer's mating surfaces with regular resin, let itsoak in a bit, then slather on a layer of thickened ( I like to use wood flour- and you can get this cheap as a bakery food product-no kidding, not $9 a quart boat supplies) epoxy before you put the veneer on the hull. Staple the bejezus out of it in the tight turns at the bilge particularly. I would try to learn vacuum bagging for the next one of mine to ease back on the number of staples. I have not tried the plastic staples people say you can just leave in all bit the last layer but its an idea. I think, particularly when you are laying the firat and every layer on the diagonal, you have to put LOTS of ribbands on your mold in the turn of the bilge area on this curvy catboat to ward off problems as the individual bands take the shortest straight line route with out a ribband and create flat spots here. Think of a fabric awning stretched over a curved bar frame. You can see the ribs stand proud- the cloth doesn't hold the shape of a perfect cylinder, it draws into scallops. Lots of ribbands will prevent this. Otherwise a lot of fairing like I had to "redo."
Another thought or two from reading Paul Gartsides work. He suggests that no matter what you do, the seams of the last outside veneer layer will inevitably print through to the surface of the hull.
The one's on my hull, some 11 years old, certainly did if you looked carefully. Paul suggests these seams will look better if you run your last "ply" fore and aft. He actually recommends a technique using "lightweight" strip planking going on first to use the natural tendency of straight stock to bend to a fair curve ( won't need those ribbands), then a couple of diagonal layers to lock in rigidity, then an outside layer on vennner run fore and aft. I imagine on a cruiser, this would give you the effect of bright finished parallel planking or ceiling boards in the cabin, at least above the berths if not in the bilge where the strip pattern would become funky with pointy ends and stealers.

I solved the fairness and visible seam problems on my hull by laboriously sanding off the 6 oz glass and thru the wood "humps" a bit, and filling the hollows with WEST 410 a bit. Lots of longboarding, again only really bad in a ten inch band at the turn of the bilge from the stern to about 2 1/2 feet back of the bow. I took the refairing all the way to the gunnel and reglassed 6 oz cloth. Epoxy primer and 2 part LPU rolled and tipped for a finish. No seams now, but will they return in time?
Nice and fair though. The shallow curves of the entire bottom and the flat topsides forward were never unfair. Only the tighter curves, so that's where I recommend you watch out.
I do think that Paul Gartside's alternate method of coldmolding is worth your consideration. Or using a lot of ribbands and vacuum bagging.

[Ross Faneuf]

George - thanks for the correction.

[Nicholas Carey]

Originally posted by Buddy Sharpton:
You neep to wet out both the hull and the next layer of veneer's mating surfaces with regular resin, let itsoak in a bit, then slather on a layer of thickened ( I like to use wood flour- and you can get this cheap as a bakery food product-no kidding, not $9 a quart boat supplies) epoxy before you put the veneer on the hull. Who would spend $9 per quart for wood flour?

Fiberlay will sell you 5 gallons (8 pounds) for US $27.14

[Buddy Sharpton]

Dunno, but that's what West Marine sells the Epiglass branded product in their stores for. I guess somebody does.