stripper scantlings for a fastrow boat

I'm planning to build a 26ft by 3ft beam sliding seat rowboat out of WRCeder strips. This will have two rowing stations with the option of going single . I have Been reading Gerr's Elements of Strength, but his book really doesnt deal with boats this small. The 1/4 inch diamention of the typical stripper seems too light as the deck of a kayak obviously adds a lot of stifness to the hull. So what to do? Thicker strips? Heavier glass? - Nick

 

Hmmm, I'd build in some scantlings/bulkheads to stiffen it, and disguise them as classy foot-braces for rowing. Actually, they'd work GREAT for bracing your feet against for more rowing power, assuming you make them strong enough, just curve them up to the gunwhale at either side & you've stiffened the whole craft with something that you needed anywise

 

I have been thinking along the line of a couple of widly spaced longtitudinal tall stringers. These would be at the ht for the sliding seat platforms and allow for adjusting trim;switching from one to two rowers, as well as stiffening the hull. But how thick do the strips need to be?

 

I'd be most concerned with the force you'd be putting against them with your feet for that reason. As I don't know, or honestly have any true idea of, the amount of pressure you'd be exerting there, I can't answer that question directly.

I would envision somewhat squared-off "U" shaped stringers about 3" high across the middle 1/2 of the boat (for comfort of bracing your feet against them) and rounding up to the gunwhales from there (to support/stiffen the sides), but have little idea how think they'd need to be to withstand the punishment.

 

The need for additional support involves the loss of longitudinal stiffness only.
Therefore, I'd go for a thicker core, say somewhere around 3/8". The overall weight gain from the added thickness (guessing) could be about 30 lbs. The entire boat might come in at 120 lbs, just quickly estimatimg.
The added thickness would double the longitudinal stiffness (going from 1/4" to 3/8").
I would also beef up the keelson in height (depending on the particular design) and the gunwales in width.

There should be no need to increase stiffness athwartship, as there's no increased stress that way.
A good way to envision this is top imagine a 13 ft boat having the same beam (3 ft). The tendancy for the bottom to "oilcan" doesn't increase if the beam doesn't also increase. However, the fore and aft radius of the boat's shape in plan view has lost half of the curvature of the 13 ft boat. That 1/2 lesser curvature of the 26 ft boat needs twice the stiffness of the 13 ft boat, hence the 3/8" instead of 1/4".

 

Typically with sliding seat rowing , your feet press against an adjustable foot board which bears on the keel and my proposed longitudinal stringers. This means that there is no direct pressure from the rowers feet. This boat will also have bulkheads in front and behind the rowers.
Alan - your logic is exactly what I was thinking about. I just dont have a way to pin down the numbers.

 

I calculate weight based on square area of the hull--- in this case I guessed a boat with 117 sq ft of hull surface. You can figure the actual surface area yourself.
An additional 1/8" of cedar over that area would be based on cedar's cubic foot weight of 26 lbs, and with 117 sq ft at 1/8" increased thickness, at 1/96 cubic ft per sq ft, that comes to 1.29 cu ft total increase. Multiplied time 26 lbs that's 26 x 1.29 = 33 1/2 lbs increase.
It will also be assumed the 1/4" thick hull was 67 lbs, and it increased by 1.5 to 100.5 lbs when the thickness went to 3/8".
Add the glassing and other material and the finished boat weighs maybe another 20-40 lbs if competitively built (120-140 lbs.). I'm assuming the boat will be used in calm water for the most part. Even so, 3/8" makes for a rugged hull.

 

hmmm.... looks like I was off by 90 degrees here. Sorry (boy this is embarrassing...)

 

There are a lot of different ways to increase the scantlings, and U-shaped frames will also stiffen the longitudinal spans by shortening them. I figured the increase in hull thickness was the simplest and most effective way to deal with the longer unsupported spans, basically requiring no more labor.
A friend of mine built a 40 ft trimaran with 1/2" and 5/8" red cedar strips. Doubling hull thickness quadruples stiffness.
A 13 ft canoe with an increase from a 1/4" to a 3/8" hull would be bomb-proof and go from 40 lbs to 55 lbs but the 40 pounder with a 1/4" hull thickness is so much easier to carry.
Increasing the 26 ft hull to 3/8" adds over 30 lbs but there are two people handling the boat in most situations, so why not? Another advantage is added flotation, around 30 lbs in fact.

 

To begin with, assume a doubling of stiffness longitudinally. With that goal in mind, assume an athwartship span that each stringer will support, say 16".
A stringer 3/4" tall by 1" wide should double the stiffness of a 16" athwartship span. The 3/4" height added to the 1/4" hull makes a full inch, and an inch height is 16 times as stiff as a 1/4" height, therefore requiring 1/16 the width to be of equal stiffness, hence 1" wide to double the stiffness of a 16" span.
So the stringer idea has merit, being a very light way to increase stiffness.
Impact resistance isn't increased, and more work is involved than simply using thicker strips. At about 10 lbs additional weight (compared with 33 lbs additional weight using thicker strips), the whole boat might come in at about 100 lbs. Stiff but not rugged, and torsionally as weak as the unstiffened hull. In this case, the gunwales should be considered as stringers, and widened accordingly.

 

Stringers

The stringers will probably be on the order of 6 inches tall. Possibly 1/8 marine ply I beam. This is so the sliding seat platform(s) can be moved fore and aft, at a reasonable ht above the feet. I'm not wild about the aesthetics though. But it does provide a constant width for the platforms as opposed to the varying width of the out side of the hull. There is potential for flotation chambers here too. But again , a very modern aesthetic.
One of the questions unanswered is what are the engineering figures for 1/4 strippers. Is that just a number that results is a fairly light boat with reasonably handled strips? There are articles out there of people building large canoes at 3/16 and even 1/8!
The gunwales will definatly have be rugged as the outriggers will mount there.
Would ribs add tortional stiffness?

 

Thinner strips have to be reinforced by ever more complicated framing and I guess if light and stiff is all that matters, there's no end to the complication. Stiffness carried too far results in fragility. Addressing torsional stiffness is the same. A certain amount of twist is healthy because it releases loading instead of resisting it, decreasing point loading.
I personally would prefer a thicker hull, which is far more resiliant while being less complicated. More practical too---- easier to clean and maintain.
The question is what was originally envisioned. An all out racing shell or a durable and reasonably fast craft. That wasn't really defined.