Bulkheads, bay glass, and mechanical efficiency

Drive leg bay

I started the fiberglassing of the drive leg bay. There are two
purposes for adding fiberglass to it, one is structural - to make the
drive leg bay a structural part of the kayak hull, and the other
reason is to water proof it. The first layer is water proofing and
runs around the kayak hull bottom, up and over the DL bay walls and
then a few inches onto the floor.

The bay is a very complex shape, and the glass fabric running along
the inside of the narrow part of the bay frame has to be very thin
because the drive leg struts fit in there pretty tight. I used 3M
super 90 spray glue to hold the fiber glass fabric down to the form,
then whetted it out with epoxy resin. The resin does not seem to
dissolve this glue, so it holds it's position VERY nicely - almost
like it was vacuum bagged. The only issue that I have, is that I'm not
positive that the epoxy is fully saturating all of the threads in the
fabric due to this spray glue. However, a tight fitting glass covering
is also a very important aspect of sealing and structure.

I plan to add a least one more layer, then some thicker roving to the
edges for structure.

Adding the glass have me an opportunity to finally insert the drive
leg, prop the seat up and get in for a quick spin. Everything feels
just great!

Mechanical efficiency test

I was also able to test the mechanical efficiency of my drive. The
last time I did it, I determined that there was a 8 watt loss due to
the chain / cog / gear box. I repeated the test, this time with the
completed drive leg and the loss was 7 watts. So, it does not appear
that the chain clanging in the stainless tubes is responsible for any
measurable losses.

I need to point out that this 7 watt loss is 7 watts from free
spinning of the crank and chain ring without any chain. 7 watts of
power is required to turn the chain, turn the small cog on the gear
box, and to turn the gear box. All of this work needs to be done
regardless of what kind of method you have to get the power down to
the prop, so it's not really a 'loss' so to speak. It's just a cost of
getting the power to the prop - watts that won't directly be producing
any forward thrust.

For comparison purposes, Rick has a drive now that takes 5 watts, and
has made a gear box that took only 3 watts. But that gear box was too
small for this design, and was filled with a high viscosity
lubrication - again, not applicable to my drive. 6 or 7 watts could be
normal, and I think with a double right angle gear drive with a shaft,
it could be as high as 10 watts.


I cut out 3 Styrofoam bulkheads and 2 wood bulkheads. The two small
1/2 plywood sections fit on the sides of the drive leg bay and run out
to the kayak walls. These will add structure to the drive leg. The 3
Styrofoam bulkheads are for the small solid buoyancy compartment in
the bow, the bow compartment (which also acts as further structure for
the drive leg), and the main cabin bulkhead behind my seat.

I measured the curves using my curve guide, then traced the shapes
onto cardboard and messed with the cardboard shapes until they fit
nicely into the kayak hull. Then I traced the shapes onto some 1/2"
thick Styrofoam and carved and sanded them.

Next, I will cover them with fiberglass and then glass them into position.


Here are some shots of the rudder in the rudder steering shell tube. I still need to sand down the rudder top more so that it fits flush to the bottom of the hull


1 Responses to “Bulkheads, bay glass, and mechanical efficiency”

  1. # Blogger warren

    Hi Greg,

    One thing I noted when racing the Necky HPB, was that even thoughmy drive unit fits into the drive well in a reasonably tight mannner, at high speeds water still pushed it's way up the back side of the drive well on burbled like a little spring into the kayak. Because of this I'd suggest making your drive well plug completedly water tight.


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