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Strip test for new cruiser


Scott Dunsworth

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I have done a little test of strip building, for my own education. I first cut two plywood temporary frames in a nice curve as a chine would be. Then I striped it with 1" strips around the curve, and 2" strips on the flatter areas. I used 11/16 cypress to mimic what my build is going to be like. I wanted to try the system that a professional builder in OZ. described. He strip builds all boats with a very thin gap in between the strips. After the boat is completely striped he coats the whole hull using a lightly thickened epoxy. He relies on capillary attraction to fill the voids.

My test piece looks very promising. I have made two slices so far and found no voids. After I do a few more tests on my strip model I plan to slice it every 1/2 inch to further check the process. If I can build this way it would save days and days or weeks of labor, not counting the epoxy savings.

I wanted to check the weight of what I will be building with, per square foot. My three sq. foot piece weighs exactly 6 lbs. So this means I can roughly calculate the bare hull weight minus the cabin and deck. It is covered with 20 oz plain weave and 12 oz on the inside. I don't know yet as to what Graham is going to specify, but this should get me in the ball park for the tests I wanted to do.

Also I want to know how much buoyancy per sq foot the bare hull itself will contribute to the positive flotation, I would like to achieve. Even if the boat is swamped as long as its not sunk is got to be a good thing.

I watched a u tube video of a fiberglass sloop sink in a matter of seconds, something like 15 once it was heavily swamped. About two minutes total. If my boats going down I want time to at least save the rum and grab the life raft.

I calculated that a lead ballast of 2500 lbs would need 37 cubic feet of foam just to float it. Its not going to be easy to get enough foam into this boat with all of the comfort equipment that's going in. But the decks, cabin will all be foam cored. Maybe with everything working together she may be swamped, but NOT sunk :( in a major calamity.

I'll post a picture in a few minutes of my strip model after my camera charges up.

Scott

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I don't have much empirical data, but my gut feeling is that any ballasted boat is going down if it is swamped to the point of sinking. I don't think you can get enough floatation in one and have it remain functional as a boat. So the main objective is to keep the water out in the first place. Meaning upright and watertight. If the vessel is seaworthy, prudence and skill of the skipper will keep her afloat (along with hatch boards and a bilge pump).

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Mike I looked back when I had posted this builders comments in Jan of this year and I didn't make a link. I'll have to Google it and see if I can find it again.

Howard it don't matter who you are, or how careful you are, if your out there enough, stuff happens. Sometimes completely out of your control. The only sure way to keep things from happening is to stay at home.

Oh by the way a bobber floats with a chunk of lead on it!! All of my bobbers leak some. :)

Scott

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That method has been around for some time and it uses lots of epoxy, much more then more conventional strip planking methods. All those gaps eat up goo and once you add them up, gallons of it. For example, lets say you have 3 mm wide gaps on 6 mm thick planks, every 3 or 4 meters of planking will require a half liter of goo to fill this gap. Multiply this by the linear length of your strips and it's a lot of goo.

I've used a similar method, except instead of spacing the planks, I glue and edge set them. Any gaps are filled once the hull is planked up, no need for a cove or bead, just back fill and move on to fairing. At the turn of the bilge and in the fore foot are common areas to find gaps on square edged planking.

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I'd prefer the technique PAR describes over the intentional gap version. PAR's method reduces the amount of epoxy used (probably to a significant degree if you imagine the total savings across the entire hull), and gaps at the turn of the bilge and forefoot should not be difficult to minimize with a few judicious swipes of the block plane or filling and fairing afterward. In strip-built kayaks, I used bead and cove initially and was impressed with how easily it all went together, but when I later tried square edged strips and a few swipes of the block plane as I fit the strip from fore to aft, I was even more surprised at how easily I could get a near perfect fit and how "intuitive" the shaping became after just a dozen strips or so. Of course, those were 1/4" thick strips and I'm guessing your strips are thicker (1/2" or 3/4"?), so the planing might occupy more time. But I would also be a lot more worried about voids with thicker strips using the gap method, and you would need to make sure the epoxy is an ideal consistency if you're going to rely on capillary action--especially on the inside, where the seams between strips will generally be tighter and trapped air may not permit the epoxy to fully penetrate. In your test sample that's not a problem because air can escape out either end of the (relatively narrow) section, but across a whole hull, I'd be worried. Just my two cents. I've never tried that method, nor worked with thicker strips, so take it with a grain of salt. Your sample certainly looks good.

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The reason for this test for me was, to check multiple things. I used about 3 fluid oz. per square foot to fill the gaps. I do have to wonder how much epoxy will be wasted mixing individual batches for each strip. ( Roughly 104 strips) Filling the gaps and wet out the wood for 448 square feet of hull should use about 10 gallons.

About 7 oz. per square foot to fill the weave inside and out. Which I'll have to do no matter how I plank the boat.

Anyway you look at it a boat this size is going to use a lot of goo!

Thanks for the comments. It's always a good thing to hear other peoples thoughts on things, it helps to think about things in a different way.

Scott

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I agree Wes, I've used thick strips (1"x1.5") and the square edge strip is easy to knock a rolling bevel into, once you get a handle on it. The turn of the bilge the gaps open up a bit, but with a plane or even a rasp, surprisingly accurate joints can be quickly made. As you point out, it's fairly intuitive after a while.

The last big strip hull I did was a 25'er with 3/4"x1.5" strips over most of the hull and 3/4"x1" in some areas where twist or bilge turn was substantial. Because the gaps were relatively tight, filling them didn't require much goo.

Scott, let us know about the results of you 1/2" slice and dice of your samples. I'd be curious what you find and it's nice to see some testing, as this is a hallmark of the backyard builder.

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Looks to me that the advantage of Bowdidge's method is that it is possible to use wider strips and still get fair compound curves. The strips in his video look like 1/2 x 2 in. Also with wider strips there should be less gaps to fill, so less goo needed than with narrower strips using the same method. But will this method overall be faster to do than conventional strip planking? If it is then will the time saved be enough to offset the cost of additional goo?

th_Part1StripPlankingvideo-1.jpg?videoplayer=offsite?videoplayer=offsite

More videos here:

http://www.bowdidgem...ing_Videos.html

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G'day all

The plank thickness used in the video was 9mm by 30mm. The glue mixture is 50/50 cabasil and microballons. The design overall is not negineered as a plank boat, but as a composite boat which means we can use a thinner plank as the strength is in the laminate and the plank itself is simply a core of which we're only interested in its core shear strength.

Typically when planking this method, it's easier to do it with two people, one outside pushing the glue through and the other inside telling you when to stop. The preson on the inside wipes of the excess and also fills any voids that was missed. This way the whole gluing process is very quick and easy.

It took me two nights after work to plank this boat on my own and took 4 hours (two people) to glue up.

Hope this helps out

Mark

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It makes sense to minimize epoxy in the gaps since it doesn't add much to the strength. The strength is provided by the outer reinforced layers. Technically, you could get the same strength if you fastened the strips dry and then laminate inner and outer skins without filling any seams. The main reason for filling the seams is to seal the wood and prevent any water intrusion from having a place to run and cause extensive rot.

Also, a thick seam of unreinfoced epoxy (necessary to get the flow to fill the voids) will be brittle and may crack due to flexing while a thin seam will not fail as easily since the stress would be transfered to the wood. The bead and cove strips makes it easy to minimize seam volume but quick use of a hand plane will be almost as effective and waste less wood.

It would also make sense to minimize the number of seams by using the widest planking that will provide the flexibility and make the curves needed. No need to use ten 2" wide strips when four 5" wide strips would do the same.

JMHO

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I must apologize I have unintentionally miss-lead everyone. After doing a search for the builder, I had my memory jogged. The builder that I pick this system of building up from does seem pretty talented, but is not a professional builder. He is from OZ and claims he and many others use this system successfully in OZ. He currently has built a 50 foot power cat using this system.

I do think it is a good system but how proven it is, is another question.

I believe it would work very well. If I did use it, I would glue and screw the planks to the permanent bulkheads. Along with taping them in while glassing the inside.

The gaps would be less on the actual boat, than what were on my test piece. I don't think I'll have many turns as tight as the one on my test. So the 1/64 to 1/32 spaces for the capillary action of the epoxy would be less.

Anyway its something to think about and ask our designer of his thoughts.

Thanks again for everyone's take on this.

Scott

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