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Measure TWICE, weld ONCE!!


dumb, dumb, dumb, dumb....

For those of you who haven't been following this along in a super technical way, the "drive leg bay" is a water tight frame that is built into the kayak hull with a hole in the bottom so that I can insert and pull-out the propeller/pedal unit - called the "drive leg". The reason I don't simply build the drive leg into the kayak hull as a permanent part of the boat is because I need to be able to pull the drive leg out to service it (lube the chain, replace the chain, etc), and also I need to be able to raise it during testing in case I need to beach (and for transportation). Don't ask me how I plan on raising the rudder yet... It's pretty dam tough, and it's way in the back, so I will be able to remove it from the outside of the boat once close to shore.

And while we are in 'refresh mode', I want to go over some of the reasons I decided to go with a chain and gear box for the drive leg rather than one of many other approaches to a drive, like a shaft, or twisted chain.

When I started to design the drive unit, I set a few design constraints:

1. I wanted to use as many standard bike parts as I could. The reason is that I simply trust these bike parts that I have been using for years and years. I have thousands of miles on some of my chains and they are still in VERY good shape. They take water, dirt, mud and abuse on my bike they would never see enclosed in a stainless steel housing at sea. I can buy lightweight bike chains, stainless steel bike chains, carbon fiber bike cranks, a few dozen different types of pedals, various sizes of chain rings, gears, etc, etc. Replacement parts are easy to find, and fairly easy to replace with standard bike tools. I have plenty of experience working with bike parts.

2. Re-build. Although the plan is to bring a couple of spare drive legs, I also wanted to be able to completely overhaul one of these on the support boat if I absolutely needed to. I can pull every single component out of the drive leg and replace it with a brand new one in less than 30 minutes.

3. SRM watts meter. I wanted to build a drive leg that would work with my SRM power meter. In training and testing, it is VERY important to me to be able to monitor and measure the level of power I am putting into the drive, and resulting speed I am getting out of the human powered boat Within. This was a vital aspect of the human powered vehicle 24 hour record I set in Critical Power HPV. The EarthRace guys
constantly monitor how much bio fuel their record boat is consuming and are constantly calculating the on-going efficiency of the engine and props and systems. My 'engine' is me, and I need to do the same. I constantly monitor and record my heart rate, cadence, power output, hydration, speed, etc. The SRM makes all of this possible, and you can only use it on a Shimano bottom bracket.

OK - on onto my dumbness.... When I first designed the drive leg bay, it was fairly short - only long enough to slide the drive leg forward, then pull it up and out the top. I may decide to NOT cut out that huge canopy top on Within, so pulling the drive leg straight up and out is not an option. So, I redesigned the drive leg bay to allow the drive leg to be rotated out of the water.




This was really slick and worked really great. I welded on a hinge and was very happy and proud of my wonderful creation.


I failed to place the new longer bay into the hull to see if it would fit!!! Duh! Well, it doesn't fit. It's too long. The bottom of the hull curves up and the drive leg bay won't sit flat on the floor. One of the reasons I built the bay out of stainless tubing is that I didn't want to compromise the structural integrity of the hull when I cut a giant hole in the bottom of it to stick the prop strut through. I wanted it to be super strong. Once a flange is fitted to the hull bottom, all of the stresses on the boat will be routed around that steel frame.


So, I either need to move my drive leg way back (toward the stern), or cut it up to allow for the hull curve. Moving it back too far is not an option because it starts to really mess with the weight and balance estimates, my head will be too far from the front window, and I start to greatly reduce my sleeping cabin in the rear.


It looks like I'm going to need to cut the bottom tubes of that frame out and re-fabricate to allow for that hull taper. Not a huge deal, but will definitely require some delicate measuring, cutting and welding.




Well, I think I will blow off my training for today (did a real hard 5 hours yesterday and have a 6 hour day planned for tomorrow), and start the drive leg bay operation. I'll go prep the operating room now. Stand by and say a prayer.

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