Material Selection

[Skoontz]

If you were going to build a boat would you use mahogany planking or Sitka Spruce for the framework? Or in a yet other harebrained scheme would you use 3/8 Marine plywood for the bottom bracing and dado cut a 1" wide piece of mahogany and bond it around the edges of the plywood sort of like a floor truss for the Okume plywood to fasten to?

[zul8tr]

You didn't say what kind of boat. For the small hydros and runabouts I would use the spruce for the framework. While a great choice Sitka is expensive and not easily available so other choices are available that work well. I have had good performance with #1 spruce (white wood) available at specialty lumber yards but not Home Depot or Lowes. This is a straight grained wood (desirable) with essentially minimal to no knots at reasonable cost. Other frame wood choices are available that have been used. Mahog is a heavy wood. Your 3/8 ply with mahog added for a nailer is to much extra work for my taste.

[Skoontz]

Its going to be a 12' mod VP 54" wide with a 14" wide pad. The tunnels on each side will be 12" wide, 4"thick deep at the front and 3" deep at the transom. The boat will have right hand drive steering, and use a 31cucumber cubic inch two cycle platform with foot throttle and power trim. In order to run on Boca Reservoir and Lake Tahoe I'm installing 4 cycle conversion kits on the motor hood. I don't mind the extra work of the plywood truss, as I got all kinds of time before I can run it but for feasibility do you think it will work? Last thing I want is the make some cool whatchamacallit and break it the first run.
Thanks for the input.....

[Lil Stinker]

Remember the Water Craft Check Points at Tahoe, don't get caught running 2 cycle, big fines. Tahoe is heavy on Boat wakes all summer long and beats the s.. out of small Hulls.
GB

[Skoontz]

I should be OK at the check points I'm putting a valve cover on the head. We've also got Donner Lake which thinking about is a better choice and free launching. If I like the boat I'm building 5a more and ill drop the $7500 om an E Tech and look at a Merc for brand warfare. Something missing for 20the year
s.

[Yellowjacket]

I found a good resource on wood strenth resource in the US Forest service Wood Handbook. Chapter 5 has strength, density and other properties that let you compare different woods. Here is a link to the PDF file:

http://www.fpl.fs.fed.us/documnts/fp...chapter_05.pdf

A good thing to download and keep...

African mahogany has a specifc gravity of .42, a compressive strength of 3730 psi, a shear strength of 931 psi and a side hardness of 640. Sitka Spruce is lighter with an SG of .35, a compressive strength of 2460 psi, a shear strength of 290 and a side hardness of 630.

Bottom line is that the mahogany weighs about 20% more than the spruce, but it's also stronger in every category except side hardness. Typically compressive and tensile strenghts of wood are very similar, so that means that in bending the mahogany is going to be a good bit stronger than spruce for an equal cross section section, Since the section properties of the built up part really determines the strength you really have to look at each area and see what you are getting and giving up. Most likely if you use thinner mahogany you can end up with a comparable structure in terms of weight, and it might be a bit stronger. You do need to be careful at the joints however. The mahogany has the same side strenth as the spruce.

If your design is such that it puts a lot of stress on overlaping frame joints then mahogany will get pounded out just as easily as spruce. In restoring my old runabout I found that many of the joints in the framing were pounded and loose. This happened because most all woods aren't very strong in side loading. That is, where a cross memeber went over a stringer, both pieces of wood got pounded out over time. To address this a good design will use angle blocks to increase the surface area that is in side loading (perpendicular to the grain) and convert that into a shear load that is more easily accpepted by the cross beam... JMHO

You can very easily use a truss type construction for bulkheads like you have described. Again you have to be careful that you don't exceed the side compressive capability of the mahogany. There are two kiinds of loadings that you bulkhead is seeing. One is the beam loading, and your concept will work great for that. The other is the loading from each local stringer and your shear web is thin. I fear that where ever you cross a stringer your cap (the mahogany) will get pounded loose where the plywood is cut down into the cap. Better would be to put the ply on the side and bond it to the mahogany. The shear strength is much much higher and the surface area would be larger. If you look you will see a lot of bulkheads and cross beams for hydro sponsons done that way. Just cutting a small groove in the mahogany cap doesn't seem like it's going to work all that well.. The local loading is high and the area is too small.