Carbon Boom Reinforcement

Floppy booms are not good.  I've complained before that the boom on the Goat is too flexible.  This prevented me from applying full downhaul for fear of breaking the boom. 

The original banana boom in light air

The bendy boom is a Douglas fir constant width box section, 37mm wide by 52mm deep tapering to 45mm at the front and 43mm at the back.  The Boom started out as a solid two piece laminate but it was very heavy and not stiff enough.  So rather than building a new boom I hacked this one as an experiment.  First I routed out the middle and capped the resulting U shape with a new piece of 7mm thick Douglas fir thereby removing some weight and increasing the depth which should increase the stiffness.  This lighter/stiffer boom is shown in the pic above and was still much too flexible.  

 Cross section of all wood boom:

I did not feel like building another boom and had a lot of carbon tow left over from another project so I decided to add a layer of the tow to the top and bottom of the boom.  Laying up tow can be a real pain unless you have a way to control it's placement.  So I routed 2mm deep channels on the top and bottom of the boom to contain the tow.  Since the highest load on the boom is applied at the downhaul I doubled the depth of the tow channels to 4mm for the front ~1.5m of the boom.

2mm deep tow channel and beginning of 4mm deep tow channel.

The tow was wrapped back and forth around 4 small nails at each end of the 1.5m channel to a depth of 2mm.  I made sure to work epoxy into the tow every few wraps to avoid dry spots.

Nails used to wrap tow around

Beginning to wrap tow back and forth on the boom, I ended up suspending the spool of tow from the ceiling to facilitate the weaving. 

The tow was compacted by clamping with a packing tape covered batten.  Picture taken after epoxy had set and batten was removed.

Closeup of cured first layer. 

Once the epoxy set the process was repeated for the entire length of the boom first top then bottom.  I used clear packing tape to compact the layup.  Then a bit of sanding to smooth it out plus a UV protection clear coat.

 Top layer of carbon tow sanded and clear coated

Holes left over from the 4 nails and carbon end wraps still visible after finish sanding.

So what did all of this farting around do to the stiffness?  First the boom weights:

Hollow wood boom:                  2815g
After routing carbon channels:  2460g
With carbon added:                   3150g

I very carefully measured the deflection of the hollow wood boom before and after the carbon was added using a bucket and water as weight.  The chart below shows deflection vs. weight of the boom before and after carbon. 

For a weight increase of 335g or 12%  the stiffness increased 65% correction 40%!  The boom now feels indestructible and no deflection is noticeable under sailing loads.  The boom is heavier than I'd like but that is mainly because I believe there is too much wood left.  The 690g of carbon and epoxy is doing most of the work and a lot of the wood is just along for the ride.  A thinner wall wood box with the same amount of carbon would most probably work just as well.  How much thinner could it be?  I don't know.

Deflection of wood carbon boom with 14kg load in the middle.

The finished boom.