Drive leg bay frame

Since weight down low in this human powered pedal boat is a good thing, I figured that I might be better off building the drive leg bay out of stainless steel square tubing. That way I can be assured that it will be strong enough to efficiently transfer power from my seat, to the cranks, through the drive leg, and drive leg well, then down to the prop. The seat will be bolted to two stainless steel rails which will be welded to the drive leg bay frame.

I originally had the drive leg pull straight UP and out of the bay, and in order to do so, I required the canopy top on Within to be open. We are now talking about NOT cutting out the canopy top and leaving it as a permanent part of the deck. I would add a small standard ocean sail boat hatch on the top for fresh air, and a larger standard hatch on the side to enter and exit.

Reason? Well, I'll get into this a bit more when I have a plan to show, but basically I am really nervous about building an adequately strong latch hinge for that huge canopy top. I am familiar with a few cases of the ocean ripping off large overhead custom hatches. In my imagination, I can see a monster wave ripping that over-sized canopy top right off of Within. With the smaller commercial hatches closed, Within will be more like a submarine than a boat - much like the EarthRace boat - Speaking of which - have you checked that baby out yet? That thing is fully sick man! The EarthRace crew are attempting to break the round-the-world power boat record with their wave piercing bio diesel speed boat.

So, if I decide not to cut out that canopy top, I am going to have to come up with some other way to remove the drive leg for servicing, or for beaching (during testing and training). What I came up with was pretty neat. I extended the length of the drive leg bay and added a hinge to the drive leg. The leg will now simply rotate out of the water like the gif animation below:

I designed the new bay with a computer model to get the basic dimensions and clearances right.

Then I constructed a cardboard mock-up of the bay and tested the drive leg in it. I even made a cardboard propeller based on the prop dimensions Rick W has calculated for me.

Then I cut a few dozen sections of 3/4" square stainless steel tubing, assembled it all and spent the better part of a day welding it all together.

Wow - this thing is majorly solid! I am going to weld a nut into the drive leg to bolt it firmly to the well frame when peddling, but it fits so nicely into the slot right now, that I don't think it will require any additional fasteners. Once that hinge is on, it will probably only require a lock to hold the other end down to the frame.

To bond the bay frame to the kayak hull, I will first screw through the kayak hull into the stainless frame (counter sink the screw holes and fill with micro), then wrap fiberglass around the bottom tube and around to the underside of the deck. I'll insert Styrofoam tiles into each open side of the frame, then glass around the whole thing to make it water tight. I DON'T want to weld plugs onto all of the tube ends on the frame - I wonder if filling them all with micro/epoxy, then giving the whole steel frame a few good coats of epoxy will make it all water tight?

To better secure the drive leg bay to the kayak hull, it will be bonded to a bulkhead at the bow end of the bay, and a partial (short) bulkhead at the stern end of the drive leg bay. The seat back will be mounted to the rear bulkhead.

This was the view outside my shop window this morning. I was outside on Tuesday for a 6 hour bike ride - the first outdoor ride of the season. By the time we rolled in, it started snowing. No outside rides this week. I need to get outside to get some rides in! Ironman Arizona is in 5 weeks.


3 Responses to “Drive leg bay frame”

  1. # Blogger peter

    It looks like you are tackling the tough parts and making good progress!

    I'm looking forward now to the next update. How about something on the drive leg fairing profile, anything you have planned to seal closed the bottom of the drive bay and how it all fits together?


    Peter Raymond  

  2. # Anonymous Anonymous

    I really like your tilt up design! There is not much space, so it was good that you could work in the tilt design. Even better (from the standpoint of hitting solid objects)if a tilt away from the direction of travel could be incorporated, but I don't see how it could be done.... If you can access the prop for clearing of weeds, line, etc. it would help, seems to me.  

  3. # Blogger Michael Lampi

    The idea is good, but the implementation leaves a bit to be desired.

    Having a big gearbox in the water will create a lot of unnecessary drag. Even if you fair it with a big bulb it will still be a log of drag.

    I understand that you want something that is bulletproof and still easy to access for maintenance, and a tilting drive leg is a good way to go. So, have you taken a look at the drive leg that WaveWalker makes for their pedal boat? is their web site.

    It has a much slimmer drive leg that is well faired and much more compact. The engineering appears to be excellent. While it doesn't use bicycle chain it does use a highly durable #35 chain and has an integrated oil bath.

    I have previously spoken with Harry Howard, the owner, and up until now he has been unwilling to sell drive units to hobbyists or possibly competitive companies. Still, for your case he might make an exception.

    The beast is small enough that you could carry complete spare units.  

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