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/fplgtr/fplgtr190/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.

[Skoontz]

Thank you so much for the detail Yellow Jacket! Do you have an old Yellow Jacket? We used to sell them back in the day. I loved their boats and was saddened to see then go when the fiberglass revolution took place in the early 60s.
Essentially, your saying plywood trusses would work then. I see it as being lighte and more rigid nut then use a 3/4 plywood cap about 1.5" wide. I can extend the capday at thethe area where each stringer will intersect and I could dado that an eighth then epoxy. Was also going to cheese punch it in a few areas to loose weight. In the trusses there will be stringers running at 90° angles the length of the boat then it will be sheeted with Okume.
Thanks again for the link and input.

[Yellowjacket]

The Yellowjacket I had was an A/B class runabout built to the plans from Science and Mechanics.. It was a hot boat at the time... here is the

http://www.svensons.com/boat/?p=HydroPlanes/yellowjacket

So long as you put something to spread the load where the stringers meet the bulkhead you can make it work. If the bulkhead is just notched into the cap it's going to pound out locally. With a plywood bulkhead and caps you have a very stiff cross beam. That's good for the overall stiffness of the boat, but local pounding will hammer out the joints. If you just bond the cap to one side of the plywood it will probably work better. I've seen lots of hydro's that didn't have much or any continuous cap on the cross beam where it met the stringers, but they usually have a piece on one side of the plywood (or both) to distribute the load into the plywood bulkhead.

[MN1]

I used a few different woods in this SST-60 I built last year.
Mark N
http://www.woodweb.com/galleries/project/posts/3466.html

[Skoontz]

Mark that looks amazing! I'm not going for ultra light weight here, but obviously weight is an issue. I want to be able to be fast and strong. I think I'm going with 1/2" marine plywood capped with 3/4" marine plywood, which I can use to sandwich the transom. then the stringers, I will use thw white spruce, as that material was used in some of the inlay decks on the old woodies and is readily available at our local lumber yard. Here is the last question.... Okume is expensive compared to marine plywood, for example a sheet of 6mm Okume is dancing at $95 here, and 1/4" marine plywood is $40.00. Rounded figures here....Does anyone know the weight difference per sheet of marine plywood vs okume? I'm only looking to use 6 sheets totel, without cutting for the contours of the boat. I may use a sheet of Okume for the deck, as I want to create a wood inlay look with neon glow paint bloober doodles on the sides.
Thank you once again for the help!

[David Mason]

Obeeche can be used, it is a very good comparrison to Sitka. The cost is lighter on the wallet as well.

If you truly can't find a good source near you for Sitka, you can order from Noahs in Canada. A lot of boat builders order from there.

I suggest getting with Darell Sorenson as he surely has a good source for the Sitka. I believe he also sold it for long time, he might still. Can't get a lot closer to you than Sorenson.

If you must, I have used popular for cross frames and stringers. It is heavier, yes. I don't think it reacts like Sitka either, and might be weaker in hydroplane construction. But, it works for the loads we apply to them in the larger cubic inch classes in hydros.

If you only have access to plywood, you can laminte plys together using west system, to reach the desired thickness, typically 3.4" and cut to length. This will be as strong as the sitka, but alightly heavier. Make sure you alternate the grains to achive the load bearing in all directions. And if possible, use a layer or two of Carbon or glass depending on your strength to weight needs.

As a last resort, send me your specs and I can see what I can do to cut your frames and UPS them out.

[Ed Hatch]

Check out this link

http://www.hainesengineering.com/rhaines/sitka.htm

[Skoontz]

Dave and Ed, thank you. I have a source for Sitka down in Corona, they sell aircraft woods and Sitka is one they stock. White pine, Douglas Fir and Ponderosa Pine are available here at Truckee -Tahoe Lumber and there are a number of wooden boat builders who buy from them. Ponderosa Pine and Doug Fir grow readily all over our area and there are a couple lumber farms within 100 miles of us. What do you think about using 1/4" marine plywood for sheeting over Okume? I'm looking at 15 pounds difference less the cutting from each sheet. I'm thinking I can get this whole rig done around 155 lbs, rigged less engine. I know it ain't hydro or bounceabout weight, but then its not a hydro and was not intended to be one....

[MN1]

Mark that looks amazing! I'm not going for ultra light weight here, but obviously weight is an issue. I want to be able to be fast and strong. I think I'm going with 1/2" marine plywood capped with 3/4" marine plywood, which I can use to sandwich the transom. then the stringers, I will use thw white spruce, as that material was used in some of the inlay decks on the old woodies and is readily available at our local lumber yard. Here is the last question.... Okume is expensive compared to marine plywood, for example a sheet of 6mm Okume is dancing at $95 here, and 1/4" marine plywood is $40.00. Rounded figures here....Does anyone know the weight difference per sheet of marine plywood vs okume? I'm only looking to use 6 sheets totel, without cutting for the contours of the boat. I may use a sheet of Okume for the deck, as I want to create a wood inlay look with neon glow paint bloober doodles on the sides.
Thank you once again for the help!

4' x 8' Okoume plywood weighs 18 lbs. for 6mm and 1/4" Douglas Fir plywood weighs 25 lbs.
If it were me I would go with the Okoume that is BS1088 grade. One way to cut costs would be to find a cheaper solid wood. Obeche is cheaper but it has a low resistance to rot. The Paulownia I used on the SST-60 boat was cheap and has a high resistance to rot but would not work for stringers. But it was perfect for blocking.
I have thought about looking into White Cedar for my next project.
Mark N

[Lil Stinker]

Mastercraft in Reno / sparks has hardwoods and some good ply may be less $ then TT Lumber. No spruce
GB