Keep the weight down

[Kudzu]

My favorite thing about a Fuselage Frame is the light weight and how easy they are to handle.  I have always been interested in keeping the weight down as low as practical.  Never really studied where the weight is but it was brought home this week when I made a set of stringers for a client. 

 

Stringers were made from Western Red Cedar for a Ravenswood.When I bundled them together and put them on the scales they weighted 20+ lbs. I was surprised and had never realized that the stringers make up over half the weight of the finished boat!

 

So if you are looking to keep the weight down of your boat choose your stringer material accordingly. Also keep in mind that a multi-chine boat such as Long Shot, Short Shot, will be heavier since they are multi-chined.

[Hirilonde]

  I have always been interested in keeping the weight down as low as practical.  Never really studied where the weight is but it was brought home this week when I made a set of stringers for a client. 

 

Stringers were made from Western Red Cedar for a Ravenswood.When I bundled them together and put them on the scales they weighted 20+ lbs.

 

I checked on-line for the strength characteristics and relative graivty of both WRC which Jeff specs his designs on and Alaskan Yellow Cedar which I wanted to use.  I concluded I could scale the ALC down to  9/16" and achieve approximately the same strength and weight.  That is what I did.

 

If you ever meet a fanatical cedar strip canoe builder (old before epoxy type) they are constantly trying to get the weight down without the canoe collapsing when they get into it.  If one were to take a conservative approach to this idea, use Okoume BS 1088 ply which is as strong if not stronger but lighter than Baltic Birch, maybe even go to 9 mm instead of 12mm.  Push the dimensions Jeff recommends and see if you can live with the results.  Use the lightest cloth which will absorb less paint. Some day some one will do this, not sure it will be me though.

[Action Tiger]

A more severe chamfer or round over on all the stringers would help a bit, too. Just taking a little extra sliver from the corners of all those sticks will add up, and not reduce strength. Grams count!

Start with good wood. If you are going for strength to weight, you need to start with the strongest stuff. Pick tight grained wood, say 15 rpi minimum, to pick an arbitrary goal.

Also, if we're talking about THAT boat, you really should probably make up a bunch of stringers and weigh them. There can be a substantial difference in the weights of two pieces of wood, even ripped from the same board from the same tree. Pick the lightest ones with the tightest grain.

If you've gone this far, shuffle the stringers around a bit until you have the lightest ones, AND the weight is balanced on both sides of the boat.

Nerd.

[Action Tiger]

I am not at all weight obsessed, but I have been thinking about it since the op.

Perhaps the stringers could have coves cut into some of the faces, to remove that little extra material not really needed for strength?

Also, perhaps some, or all, of the stringers could be tapered toward the ends?

I'm not trying any of this, but do think about stuff, sometimes. Right now I'm thinking about building a new gate.

[BigD]

I weighed my firefly frame last week after I got her lashed together. 21.5 lbs + 2 lbs for the combing. Not bad!!

[andy00]

Missing from this discussion is analysis of failure modes. We need information on how SOF boats have failed:

• Did a stringer break? At a frame? Between frames? At a knot? At a joint? What species of wood? Dimensions?
• Did a frame break? Where? What type of plywood? Dimensions?
• Did a lashing or other fastening between a stinger and frame fail?

This forum might be a way to collect some information of this sort. Then some ideas on where weight can be safely reduced and where to keep things robust can be developed.

 

Paddle on, Andy

[Kudzu]

As an FYI, I must point out, that if you choose to alter the design, you are on your own. Once you change something it is no longer my design and you are on your own.

[Joe Underwood]

Missing from this discussion is analysis of failure modes. We need information on how SOF boats have failed:

• Did a stringer break? At a frame? Between frames? At a knot? At a joint? What species of wood? Dimensions?
• Did a frame break? Where? What type of plywood? Dimensions?
• Did a lashing or other fastening between a stinger and frame fail?

This forum might be a way to collect some information of this sort. Then some ideas on where weight can be safely reduced and where to keep things robust can be developed.

 

Paddle on, Andy

Consider milling the the two gunwales (heaviest boards in the kayak) into I beams. This was a common practice in older high performance aircraft where weight and strength were very important.  This was done to the wing spares with great success and a part where failure was not an option.  You would leave the gunwales at full size where the frames and lashings need to be.  You would need the largest diameter router bit you could use so the radius on the webb where you lash the framing would be as big as it could be. You could easily take 1/4 of the weight of the gunwales away and not lose any strength. Think ounces and you"ll lose pounds in the end.

[bylawren]

Agree with Mr. Underwood's comment about ounces adding up. For my Short Shot, I routed a radius on the length of the western red cedar gunwales/stringers where they would interact with the skin. I haven't compared it with a "standard" frame; however, my finished frame, excluding coaming, weighed 19.5 lbs. YMMV