Suggestions on hull scantlings for pocket cruiser?

[RWL]

I am new to this forum, but have read many interesting comments. I am designing a 22 foot displacement cruiser for wandering about the protected waters of the Pacific Northwest. This will be a minimal cruiser, fairly narrow beam (6 ft. at DWL) that I can store/move around on a trailer. It is a single chine, plywood over sawn frame hull, and will be powered by a 9.9 Yamaha high thrust OB in a well. The swim grid covers the extension of the hull to give me maximum waterline length and a low platform for entry and exit from a kayak. If I have done the upload properly, there should be a picture of the design in progress attached. I am thinking of using 3/8" plywood for the hull, attached to sawn yellow cedar frames. This should keep it lighter, which should help my efficiency. (I am shooting for 1 litre/hr fuel use at 6+ knots). Any one have experience with hull scantlings such as this? I should mention that my design (called a Spirit Bear 22) is planned for 3000 lb. displacement, fully loaded with 2 people, 2 kayaks, food, fuel, etc.

[Eric Sponberg]

RWL,

Your boat is too small for any reasonable calculations by a scantling rule, so you tend to go by the seat of your pants and pick what is reasonable. That said, to me, 3/8" plywood is a tad too thin. For the sides maybe it would be OK, but certainly for the bottom I would go for 1/2" thickness.

Also, you have to consider the spacing of the framing, which you don't give. Structures are critical to both strength and stiffness. That is, they can be plenty strong, but bend or deflect too much. On boat hull and deck panels, one can increase the stiffness by 8 times simply by cutting the frames spacing in half (twice as many frames.) You want to be sure that the hull and deck panels don't deflect too much when they are loaded. As a practical test, if you build the hull upside down, for example, you would like there to be practically no deflection under foot when you walk on the upturned hull. If the hull deflects, either add more frames or increase hull thickness. Since the structural weight of just about any boat is about 2/3 due to the hull skin, and 1/3 due to the internal framing, it is more efficient cost, labor, and weight wise to increase the number of frames. The same applies to the deck--if the deck deflects when you walk on it, it is too thin, or there needs to be more frames.

Also, I would not use cedar for the framing. In general, frames and structural members need to be stronger woods that are not susceptible to rot and take fastenings well. Strength and stiffness vary directly with wood density in general, and certain hardwoods are better for internal structure. I usually specify honduras mahongany for my internal framing on wood boats. It is great for structure on strength and stiffness, it resists rot, and it glues really well with epoxy.

Eric

[mackid068]

Gorgeous boat. Any way you could make that a steel aft pilothouse a'la nordhavn 72 that is 32 ft?

[RWL]

Thank you for your suggestions. I will try and get a drawing of my framing ideas posted. As for the hull thickness, would two layers of 1/4" plywood, screwed and epoxied for the bottom be any better than one layer of 1/2"? I was thinking it might take a little longer, but be easier to bend up near the bow.
Just to expand on my idea, the plan for this boat is to make it a "kit" with all major wooden parts cut and shaped on a CNC router, so that a person with few major tools and not a lot of space for lofting etc, could build one in a reasonable length of time (say 4 to 6 months). Not sure if this is possible at this time, but feel comfortable enough that I want to build a prototype to prove out the design and then fine tune the fabrication details.

[RWL]

If and when I think I have this boat figured out, I might be inclined to attempt what you suggest. Right now, I would be kidding myself, and anyone foolish enough to believe me.

[RWL]

attached is a quick drawing of my ideas on framing. I was planning on spacing them approximately 30" on center, fudging them to line up with bulkhead positions if needed. Using 1x4's on the chine rail and shear and two 1x2's spaced equally over the floor and one 1x2 stiffener half way between the chine and the deck, I thought I would have sufficient support to stop the bottom and hull sides from flexing since this is not a planing hull. Maybe I should shorten the spacing to 24"? I just don't want to add lots of extra weight if it is not needed, as it will lead to more power required, more fuel tankage, heavier trailer, stronger towing vehicle etc. etc. Open to suggestions.

[dionysis]

See if you can have a look at Dave Gerr's book "Boat Strength". It has a wealth of details on scantlings and construction.

[dionysis]

By the way, his plank thickness for your size boat is about 0.7 in. This is for plank-on-frame.

[Raggi_Thor]

Take a look at
http://boatdesign.net/forums/showthread.php?p=41636#post41636

LWL=7m and Deppl=1.5 tons (m3) gives a skin thickness of 12mm (1/2 inch).
You can use a thinner skin if you have more stringers.

[Eric Sponberg]

RWL,

Your longitudinal frames should lie short side against the hull, not the wide side. Flip them 90 degrees, they are stronger and stiffer that way. The transverse web frames then should then be twice as tall as the longitudinal frames.

A transverse frame spacing of 30" may be OK, and then to make it easy, make the longitudinal spacing 15" (half the fransverse spacing). Build a test panel and have someone stand on it while you measure the deflection in the center of a hull panel. As mentioned before, you want little to no deflection when someone stands in the middle of a panel.

Another thing to consider. If you make the web frame spacing 32", then the longitudinal spacing 16", your interior woodwork fits very well with standard 4'x8' sheets of plywood. By working in multiples or sub units of 16", you minimize wastage considerably. As a matter of interest, I design the Moloka'i Strait motoryachts all in multiples or subunits of 16". This makes the spacing between decks also very convenient because the rise of stair treads becomes 8", and this makes for an easy-to-climb stair. All of the vertical stiffeners on bulkheads at 16" then line up perfectly with both hull and deck longitudinals, also at 16", and lots of whole sheets of plywood, and convenient subsheets of plywood, fit the interior very well. Just something to consider.

As for layering the plywood, definitely a good idea. That is how I designed my Delft 25 pocket cruiser (stock plans section of my website www.sponbergyachtdesign.com/DelftPlan.htm). It has a multichine hullform. The very bottom panel on centerline is two layers of 3/8" ply. The next two panels are 3/8" and 1/4" ply, and then the top two strakes are two layers of 1/4" ply. That way, the hull thickness varies from 3/4" total thickness to 1/2" thickness. The rest of the construction follows the Gougeon Brothers methods of stringer/frame plywood construction.

There are no shaped chine logs in the boat. That is, the longitudinal frames nearest chines are actually 1" inboard of the chine corner. That way, the long'l frame does not have to be shaped to fit the bevel of the corner--just cut it square and mount it in place. The bottom panel lays from the edge of the previous panel, lands flat over the next long'l frame and passes it by 1", enough to hold it in place. The next panel down repeats the procedure, again with no shaping of the faying surface of the long'l frame. Simple, fast construction. If you look at my website for the Delft 25, you'll see a midship section there of the hull structure and can see what I am talking about.

Eric

[RWL]

Thank you again for some very positive comments. I will definitely look at your website. Just to let you know, I am an industrial designer, and have worked on this design a little bas acwards, starting with the flow shape I wanted for the hull. I then developed the outside hull surfaces, stitched them into a solid and then used the solid to work out CofG, displacements etc. since my software will do it automatically. I am now starting to project the frame and stringer placements onto the inside of the hull. This might explain why I am asking questions like this at this time. Thanks again.