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rswenson

3/4" PLYWOOD V. 5/8"

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General question.  I'm building a Princess 26, Hull #25.  The plans call for 5/8" plywood for the bottom.  My local supplier, Macbeath Hardwoods, only carries 3/4" marine ply.  If I make the substitution, will there be enough bend in the wood?  I would then use 1/4" ply (three layers)  for the curved section through the bow.  Do you think I can make this work?

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3/4" is tough to bend, though some cursing can usually get it done. Two layers of 3/8" would be much easier, assuming the forward sections can accept this level of panel stiffness. Maybe 4 mm plywood would be a better choice and much closer to the desired original dimensions. Additionally a single layer of 1/4" over the molds, followed by an additional layer of 3/8" will get you the actual desired panel thickness, again if the forward sections can tolerate bending 3/8" around it. My way of looking at these "engineering on the fly" moments, is to keep it is as simple and practical, preferring two layers over three and trying to nail the exact scantling dimension, rather than being 20% over, like the 3/4" or three layers of 1/4" would be. If you do use two layers instead of three in the forward sections, make the inner layer the thinner of the two.

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It is tough when you cannot get what you need. As Par said 3/4" ply can be difficult to bend, so is 5/8" but that little extra 1/8" will make make you work for it. Fortunately the bend is not too severe. The brake from sheet the diagonal planking was chosen for economy and convenience. The sheet planking (white)is two sheets long and the forward edge is cut back at 30 degrees to reduce the amount of bend required at the chine and it is about the ideal angle for the bow planking (see arrow in picture). The bend should not be a problem at the keel line as it is less than at the chine and you have something solid to clamp to.

 

The chine edge will be more difficult. One tactic would be not cut back the 30 degree angle until after the bottom is glued to the chine. This will give you a lot of extra leverage and keep the bottom fair as it is almost impossible to get a curve in the end of a stiff board. If you cannot get it to bend for you you can cut back the line of the solid bottom to where it will work for you. Your other alternative is to double or triple plank the entire bottom. Or you can put down a layer of 3/8" ply and then glue on a 1/4" layer.

 

Botom planking.jpg

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Thanks men.  I won't be using the 3/4" plywood.  Using 3/8" glued up with 1/4" sounds very easier and more reasonable.    Then I had another thought.  Can I use 1/2" ply on the bottom without compromising the structural integrity of the boat?  I could sheath the bottom with 1708 bi-axial  mat or some other heavy fabric.   Any thoughts?  Thanks!

 

Ron

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Ron,

 

You could use 1/2" ply on the bottom with 1708 if you use meranti instead of okume. Besides being stronger than okume it is also more durable and cheaper. It will probably come out slightly heavier than 5/8" okume with 10 oz glass but you will have a tougher bottom skin and if you have to add some weight at least it is as low as you can get it.

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