A compact human powered boat drive system
Compact Twisted Chain HPB Drive System
An article by Warren Beauchamp

The four major things that allow an HPB to go fast are: training, hull design, propeller, and drive system. Training is pretty self explanatory, the more you do the faster you go. A fast hull resembles a rowing shell; pointy, long and narrow. Volumes have been written on rowing shell design, and you can see nice examples hanging in restaurants and haute departments stores throughout the country.  Propellers are a complex topic, though decent and affordable designs are readily available. HPB drive systems on the other hand are still in their infancy. In the quest to build an efficient human powered boat drive-train, many things have been tried. Some work well, some don't. Here I will attempt to explain how to construct a relatively simple HPB drive system that works well. 

HPB Drive Types

  • Rider facing forward, bike cranks to 90 degree bevel gears to horizontal prop shaft which angles under the water, prop in rear. (Wavebike, etc)

  • Rider facing forward, twisted bike chain down under the water, prop in front. (Hammerhead, etc)

  • Rider facing sideways, bike chain to horizontal prop shaft which angles under the water, prop in rear. (Sidewinder, etc)

  • Flapping paddles

  • Paddle wheels

  • Various other drive systems using metal or plastic gears, rubber belts, or chains.

While all of these designs work, they each leave something to be desired in terms of efficiency, propulsive force, or rider comfort 

The Compact Drive Systems
The most efficient (fast!), compact and robust fast systems I have seen so far, all use a twisted chain driven directly from a sprocket on the pedal cranks down to another sprocket which drives a short prop shaft to a propeller. These systems include the SpinFin by Bob Stuart, and the Jake Free drive unit

WISIL HPVer Bill Murphy designed and built a compact twisted chain drive unit a few years ago for his Transducer HPB, with which he has had great success at the Hydrobowl HPB races. Bill's drive unit, which I will detail below, is both simple and elegant.

The #25 sprockets and chain in the following description can be obtained from an industrial supply distributor such as McMaster Carr. The #25 chain is more than strong enough for any single rider HPB drive system.

Compact Drive Description
In the picture above, a 45 tooth #25 sprocket is mounted to an MTB crank set. The twisted chain runs inside a 1 1/4" aluminum tube, down to the 10 tooth #25 sprocket, which is bolted to one end of a sealed mountain bike bottom bracket. An APC 16x16 model airplane propeller is bolted to the other end of the bottom bracket. Be sure to add keyways to both the 10 tooth cog and the prop to prevent slippage or loosening under high torque conditions. The bottom bracket is mounted in a section of 1 1/4" tubing which has been TIG welded at 90 degrees to the main down tube. Pick your 1 1/4" aluminum tube thickness so that the bottom bracket slides snugly into the tubing. O-rings can be added to the narrowed middle section of the bottom bracket to further seal the drive unit. A small slit toward the rear of the bottom bracket mounting tube, combined with a tubing clamp secure the bottom bracket into the tube. A rubber expansion plug is inserted into the bottom of the tube to keep the unit sealed, and to provide easy access when threading a chain. The 10 tooth sprocket is just the right size so that the chain is retained by the sides of the tube, preventing derailments. An idler sprocket is used to redirect the return side chain into the tube. This should be a solid idler system, rather than spring mounted to allow backwards pedaling. Being able to go backwards in an HPB is a good thing!

As in the picture above, the crank unit is bolted to the 1 1/4" tube. The whole unit is bolted securely to the boat deck, in a manner which allows it's easy removal. Siding the  1 1/4" tube up and down will adjust the chain tension. The lower section of the 1 1/4" tube, which is in the water, should be faired with a wing section that is sharp in front and in back to ease it's flow through the water, and mitigate any disturbances to the water flow which may effect the rear facing propeller. A skeg should be added to the bottom of the drive unit to protect the propeller from grounding or any underwater obstructions.

Notes on props:

  • The readily available $15 16x16 APC props have good efficiency, and are fairly expendable, making it easy to carry a spare.
  • You may want to go for a higher efficiency prop designed specifically for HPB use, such as George Tatum's Heron prop, or one of Jake Free's carbon fiber props. These props start at about $150, and you will want to protect these beauties as best you can.
  • At this point in time I cannot recommend the Bolly HPB prop.
  • Do not use props designed for trolling motors or other powered boats as they are designed for high RPM and weed shedding, and are not efficient.

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