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Donar kayak and mechanical efficiency

The Hyak donor kayak from Nimbus Kayaks arrived last week!

It is in pretty decent shape, but does not include a top or any bulkheads - just the bottom hull. That's OK because I would have just cut out the top anyhow before adding my own top. Ben came over yesterday and build a little Styrofoam stand for the kayak, then proceeded to take measurements so he can model the kayak hull in SolidWorks. We'll use the Solid Works model to build the canopy top, and then use that 3D model to generate sections which we will cut out of foam.

Check out Rick Wianecki's Frank-n-liner streamliner at Warrens recumbents.com site:

This is exactly the male mold construction method that I plan to use for the test boat. I am a little concerned about wetting out fiberglass and not using a vacuum bag, as the vacuum would surely deform the thin foam strips, but Rick maintains that he never had any problems with the wetted glass bubbling or peeling up. I'm thinking we could use stretchy film wrap to press down the wetted fabric without distorting the mold.

TB-1 test boat ("TB" for 'Test Boat" - not a very inspiring name is it....)


Testing the mechanical efficiency of 3 types of drives

There has been some discussion at HPV boats forum regarding the efficiency, pros and cons of various mechanical drive approaches. The task with regard to human powered boats is to transmit the direction of power input from cranks which rotate in a plane that aligns with the boats length, to a prop which rotates in a plane perpendicular to the boat and the cranks. Rick Willoughby typically uses two right angle gear boxes and a shaft to transmit power from the cranks to the prop. Others such as Warren Beauchamp use a chain that twists 90 degrees from the chain ring on the cranks down to the prop.

The advantages to using two gear boxes and a shaft are mostly that it is structurally very solid and strong - probably a good combination for an ocean crossing. The disadvantage to this approach is potentially less mechanical efficiency than a twisted chain due to the heavy gear box.

The advantages to using a twisted chain is light-weight, possibly good mechanical efficiency and easy to replace standard bicycle parts if something goes wrong. The disadvantages to using a twisted chain is related to the fact that the chain isn't really designed to twist, and it may be difficult to replace a broken chain in the middle of the ocean. I'm just not certain how long a twisted chain drive will last.

So, at least with regard to the mechanical efficiency questions, I thought I would spend some time and conduct a few experiments designed to elicit exactly what the power 'cost' is for each approach.

To summarize, here is the mechanical power efficiency % for each drive configuration for average resistance of a prop spinning in water at 80 rpm at the cranks (relative to a straight chain at 100%):

STRAIGHT CHAIN = 100%

RIGHT ANGLE GEAR BOX = 94.1%

TWISTED CHAIN = 93.3%


THE STRAIGHT CHAIN

As a baseline - or control, I set up a drive that was a straight chain running from my 39 tooth SRM watts meter chain ring to an 11 tooth cog mounted on a bicycle bottom bracket. On the other side of the bottom bracket, I mounted a standard bike chain ring. A chain runs from the large 53 tooth gear to a 12 tooth gear on the rear wheel of a road bike on a magnetic resistance trainer stand.

The lower chain ring and bike wheel simulate similar resistance of a prop spinning in water. In this case, I am looking for an average resistance of around 150 watts of power to turn the cranks 80 rpm. The photo above shows a magnetic resistance roller and a wind resistance roller (red) on the rear wheel, but I found that neither was required to maintain about 150 watts of power to turn the cranks at 80 rpm. For this experiment, the rear bike wheel was freely spinning - this had the added benefit of being more consistent between drive leg configurations, as the magnetic and wind rollers change resistance slightly due to various pressures against the tire and temperature.

The control using the straight chain would be a best case scenario, as there is very little mechanical loss from a straight chain. Obviously, it is an unacceptable drive option because the direction of power transmission is in the same plane. (I would have to be sitting SIDEWAYS on the boat in order to use this).

Here are the results:

FREE SPINNING = 0 WATTS
No chains at all, just the main crank and pedals spinning freely. The SRM power meter was calibrated to measure 0 watts from this free spinning condition.

NO RESISTANCE = 2 WATTS
The bottom chain was NOT connected to the bike wheel. This test measures the no-resistance mechanical loss of the SRM 39 tooth chain ring with a straight chain to the 11 tooth gear mounted on the lower bottom bracket turning a pedal and chain ring with no chain or resistance.

RESISTANCE AT 80 rpm = 135 WATTS
This configuration is as shown in the photo above. The bike wheel is linked to the drive and simulates typical resistance of a prop spinning in water. It required 135 watts of power to turn the SRM cranks at 80 rpm.


THE TWISTED CHAIN

The chain twists 90 degrees from the 39 tooth SRM chain ring down to the 11 tooth gear mounted on a bottom bracket. The return side of the chain is tensioned and positioned with a chain guide (orange roller). A free spinning bike wheel with a 12 tooth gear provides the same resistance as the control. Note that gearing and therefore, the resulting resistance for both this configuration and the control is exactly the same.

Here are the results:

FREE SPINNING = 0 WATTS
No chains at all, just the main crank and pedals spinning freely. The SRM power meter was calibrated to measure 0 watts from this free spinning (same as the control)

NO RESISTANCE = 8 WATTS
The bottom chain was NOT connected to the bike wheel. This test measures the no-resistance mechanical loss of the SRM 39 tooth chain ring with a twisted chain to the 11 tooth gear mounted on the lower bottom bracket turning a pedal and chain ring with no chain or resistance.

RESISTANCE AT 80 rpm = 144 WATTS
This configuration is as shown in the photo above. The bike wheel is linked to the drive and simulates typical resistance of a prop spinning in water. It required 144 watts of power to turn the SRM cranks at 80 rpm. The twisted chain required 9 more watts of power than the straight chain to turn the cranks 80 rpm.


THE RIGHT ANGLE GEARBOX

The chain runs straight from the 39 tooth SRM chain ring to an 11 tooth gear mounted on one axle of a Mitrpak right angle gear box. A 53 tooth large chain ring is mounted on the other gear box axle which turns a chain connected to the 12 tooth bike wheel. Note that gearing and therefore, the resulting resistance for both this configuration and the control and the twisted chain is exactly the same.

Here are the results:

FREE SPINNING = 0 WATTS
No chains at all, just the main crank and pedals spinning freely. The SRM power meter was calibrated to measure 0 watts from this free spinning (same as the control)

NO RESISTANCE = 8 WATTS
The bottom chain was NOT connected to the bike wheel. This test measures the no-resistance mechanical loss of the SRM 39 tooth chain ring with a straight chain to the 11 tooth gear mounted on the right angle gear box turning a pedal and chain ring with no chain or resistance.

RESISTANCE AT 80 rpm = 143 WATTS
This configuration is as shown in the photo above. The bike wheel is linked to the drive and simulates typical resistance of a prop spinning in water. It required 143 watts of power to turn the SRM cranks at 80 rpm. The right angle gear box required 8 more watts of power than the straight chain to turn the cranks 80 rpm. The gear box required 1 less watt than the twisted chain, but 1 watt is easily within the margin of error, so I would consider both drives equally efficient.


The nitty gritty details

To start with, I used an old exercise bike with a roller pressing down against the rubber wheel. This didn't work at all because the bearings in the wheel are old and changed resistance periodically.

The SRM power meter computer showing crank rpm and power in watts.

This is a third drive option that I tested, but it didn't work at all. It's a flexible drive shaft from my Shuttlebike kit. There was far too much resistance and the shaft just twisted up inside the housing.

I had to fabricate a connection for the flexible drive shaft.

The flexible drive shaft connection to the SRM cranks and the lower bottom bracket

The Mitrpak right angle gearbox.

To make a collar to fit onto the gearbox shaft, I cut an axle from an old Shimano Octalink bottom bracket in half and inserted a smaller diameter stainless steel tube into it and welded it in place. This allowed me to use the standard spider bolt to mount the chain ring. The smaller diameter tube was a press fit onto the right angle gear box shaft. To hold the collar in place, I drilled and tapped a hole for a set screw.

The small 11 tooth cog fit onto the gearbox axle the same way - I welded a short tube to the back plate of the cog and press fit that onto the gearbox axle. It is held in place with a set screw.


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