Materials Question - Double Plank Construction

I have a question on materials selection... I am building a chris craft style mahogany runabout. Picture here. I plan to use the double plank method, with the subplanking on the diagonal and outer planks of course running lengthwise. I plan to have the total hull thickness be 3/8" between the subplanking and outer planking. My original idea was to use 1/4" marine fir plywood (ripped into strips) for the subplanking and 1/8" mahogany solid stock for the outer planking. I am having trouble finding lumber suppliers that will mill the mahogany to the 1/8" thickness at a reasonable cost. The quotes are 67% material cost and 33% labor cost.

My question are:
With 1/8" solid stock be too thin to work with? Will it just split in my hands? Should I even be using 1/8" solid stock for the outer planks? Would I be better off using a mahogany plywood instead? Is there a better way to do the double planking than what I am thinking?

 

I think that your use of plywood in strips is defeating the greatest strenght in using plywood. If the hull is a developed shape, the best use of plywood is in the use of large sheets over large areas. If you need to double plank, you would be better off using solid material. I would speculate that you would want planks of similar thickness. I understand your thought process though. If you were able to use sheet ply, a laminated veneer should be fine. On you double plank though, the planks are supporting each other and I would personally feel better about equal planks thicknesses, at least on the bottom.

Others will most likely provide more insight.

Are you planning on using epoxy?

 

I plan to use larger pieces where possible on the sub "planking." The compound curvature at the bow and stern prohibit the use of large sheets in those areas. I expect that I will be able to do the mid sections in large pieces, and that should save me some time and improve strength. My thinking with the two different thicknesses was that the outer planking would conform to the sub planking, rather than the other way around.

The lumber supplier I was talking to seemed very concerned that 1/8" planks would just split on me when I tried to bend them at all. I suspect that a luan plywood would be more resistant to splitting, since it doesn't have unidirectional grain. Does that seem reasonable?

Yes, I do plan to use epoxy, and lots of it!

 

The bottoms of old Chris Crafts were done the way you describe. The sides were usually batten seam construction.
However, your willingness to use epoxy tells me you are already mixing methods. The bottom is easily done with simple plywood, because it is flat enough to make good use of big panels. The sides have some compound to them, so the question does apply there. In that case, the most appropriate method would be cold molded as you describe, but with more layers than two, and no thick layers at all. The purpose of cold-molding is to use thin layers glued together to create a strong and stable monolithic structure. Too thick and any water that does find its way in could cause expansion that adjacent layers can't abide. The hull could distort and tear internally. Plywood is the same, made with a lot of thin layers. Marine ply, in fact, usually has a couple more layers than its Home Depot counterpart.
So you're on the right track, but you need to get a cold molding schedule for your needs (like topsides might be 1/2" thick made up of four layers with the teo outer ones going fore and aft and the middle ones at angles to the outer ones and each other).
Someone here with cold molding experience might advise on particulars.

 

I think 1/2" topsides is way too heavy for a boat like that, which appears to be pretty small.

Both techniques could be made to work: 1/4" substrate with 1/8" fore and veneers or a 3 layer cold molded build with 1/8" veneers (one or two could be plywood). Making at least one of the layers plywood has a lot of merit, but it should be oriented fore and aft (if just a single layer) for best utilization of it's construction. Other wise a double diagonal schedule would be recommended, with a final fore and aft (aesthetic) veneer applied last.

Simply put, you should pick a build method: molded or single planked with an aesthetic exterior veneer. Though the exterior veneer does offer some strength and stiffness, not much unless coupled with additional layers of veneer. To a degree the plywood substrate can offer this, but a single layer of thin plywood, not as much as you'd think.

In the end if you go the plywood substrate route, use high quality 5 veneer 1/4" stock, not the 3 veneer family room paneling stuff. Or two layers of 1/8" plywood.

As to your veneers bending, well, you'll get some breakage and splitting, but not as much as your supplier might think. Mahogany doesn't like to bend, but you can convince it with heat. Most of the planks will be spiled (cut) to shape, not bent or sprung. You really can't edge set veneers, as they'll just cup, twist or break, so they'll need to be spiled. Now, the old mahogany runabouts used lumber thick enough to permit some edge setting, even steam bending, but these too, for the most part were spiled.

 

OK, at least I'm not asking for anything too crazy with the 1/8" solid stock mahogany. I guess lumber suppliers are not used to getting these requests.

I will consider doing more plywood layers.

Yes, the boat is only 14ft long, so I think the 3/8" hull thickness is adequate.

Thank you!

 

What plans are you using?

 

It's my own design. It's based off the smaller Chris craft and hacker craft.

 

How have you developed the scantlings? Did you preform a weight study?

 

Par, what brand of plywood do you use that has 5 layers in 1/4 "? D.F. only has 3 layers
in 1/4". I would love to find a 5 layer ply in 1/4" and 3/8". Thanks, Stan

 

Marine grades other than that sold with the APA stamp on it are usually 5 ply. Hydrotech and Aquatech are common brands, both are meranti and constructed to the BS -1088 and BS-6566 standard respectively. Douglas fir plywood is only suitable for interior work, mostly because it's difficult to finish well. The APA uses it for all their exterior and marine grades, but the construction requirements are quite poor, compared to other marine standards and the price reflects this quality.

 

Thanks much Par.