Optimal gearing for human powered watercraft

Hi there,

I am a mechanical engineering student from Exeter University and have been tasked with designing the fastest (greatest top speed) watercraft over 100m. Having already decided upon a general design with rowing boat hull and recumbent bike ergonomics (pedal powered). Since finding this forum we have realised the design is much like Rick W's V11 (Rick's Boat Pages - V11 http://www.rickwill.bigpondhosting.com/V11.htm). We would like to see what peoples answers may be to the following questions:

What gearing system would be optimal for a maximum speed over this short distance for a cadence of 90/100 rpm at the main sprocket (was thinking single gear of ratio of either 5:1 or 6:1 however have not seen any information to back these), would two or three gears be better to reduce initial turbulence when accelerating?

Also, would two right-angled gear boxes containing beveled gears be appropriate for transmitting this power to the prop or is there another more weight-saving method that we may have missed?

Would be grateful for any help!

 

You only need one mitre/bevel gearbox to turn the drive from pedals to prop. Gearing depends on propeller size, there should be some information in the long human powered watercraft thread on this forum. Are you familiar with Mark Drela's record and "Decavitator"?

 

First determine the propeller characteristics including diameter, pitch, area ratio and shaft speed. Then design the gearing with the required ratio to go from pedal speed to shaft speed. If you are the drive designer and someone else is the propeller designer then tell the propeller designer you need an estimate of the propeller shaft speed.

Each set of gears will increase losses. Research efficiency of different types of gears. Noise and vibration are probably not a consideration. Will you need to use commercially available gears? You may want to incorporate a quick change system t allow optimizing gear ratio during on-the-water testing.

 

Rick's design probably won't be the fastest over such a short distance as 100 meters. If the rules allow it, maybe use a hydrofoil, hydrocopter, or some air injection or air support like hovercraft system. The lightest drivetrains are probably Twisted chain systems, and some avoid the bevel gear losses, with the twist. Agree that you have to start with the prop and work backwards. There is very little if any advantage to using shifting multiple gear ratios. All of these hints are based on my humble experience and opinion.

 

And what are the vessel characteristics?
Weight, dimensions, etc.

5 or 6 : 1 is in the ballpark, depending...

Prop diameter? How much room will you have? 20" diameter?

The gearing system that doesn't break due to torque and provides the least resistance will be the best one providing it is as light as possible.

No right angle gearbox will offer the least resistance. You can achieve this by sitting sideways, athwartship to pedal.

 

Rick takes into account the shape of the boat when designing his props. He might still sell his custom-made props which I think were tailored to the individual boat. A prop from one of Rick's racing boats might not work well in a non racing shaped boat, like a human powered dinghy.

Click on pics for more info about different foil designs on this link:

Haiflyike - Human Powered Hydrofoil http://human-powered-hydrofoils.com/hydrofoils/haiflyike/

I believe the record speeds listed are measured based on a 100m trap, the speeds would lower from a standing start for 100m, if that is what you are doing.

 

I think for a project like this, variable pitch prop would be a much better cost/complexity investment than variable bearing.

Your description is a little unclear. You state the goal is highest top speed, but you also mention 100m. Is it highest speed sustainable over 100m, or the actual peak velocity, that the boat must accelerate to within a 100m course?

 

It would also help to know how many in the crew.

 

A hydrofoil is going to be the best solution. Here is one. That's for sure the most efficient way to move quickly across the water by human power. The human powered speed record is held by a hydrofoil. As to gear ratio, it depends entirely on the prop pitch and diameter and blade area that you select. If you select too big a prop you won't have any slip so it would be hard to start and you'll turn it more slowly. A smaller prop is probably what you want since that will allow slip on starting and as you go faster the drag won't go up. If you're going to cruise along you want more prop area to transmit the power, the faster you go the smaller prop you want and you'll turn it at higher rpm's.

The current human powered speed record is 18 knots, and his held by a hydrofoil that is powered by a propeller that is running in the air and not the water...

Manta5 Hydrofoil Bikes https://manta5.com/

 

No it doesn't. Watch the video.

 

Apparently "decavitator" had up to a 3 mile per hour Tailwind, when it set that record. The large air Propeller which is well above the waterline acted as a sail. The decavitator was slower in a side-by-side race with the "flying fish" which has a conventional submerged propeller.

Human-powered hydrofoils have a more limited range when flying compared to Conventional hulls, and more hazardous consequences if they strike debris while flying.

The water speed record that's surprisingly hard to break https://www.bbc.com/future/article/20170921-how-fast-can-a-watercraft-powered-by-humans-go

 

Thank you all very much for your responses!

In answer to a few of your questions, we are able to make hydrofoils and will probably try and change to a more decavitator style as recommended by most more experienced people we talk to however I realise this is still up for debate somewhat due to the tailwind. The watercraft would be a single-person crew.

We have no set physical constrictions when it comes to the watercraft, only that it has to be human powered and reach 'a maximum top speed over 100m', I would interpret this as trying to break the 1991 record set by Drela. In regards to the propeller, we were thinking skinnier longer blades as this would allow us to have a higher top speed (high speed, low torque) so I think 20" may be appropriate?

 

Clmanges, Yellowjacket is referring to Decavitator as driven by an air prop, not the boat in the video he shared. The video available of Decavitator's record run is very poor quality, and appears speeded up, possibly because of low frame rates:

 

G00M, Rick W is usually amenable to talking about designs, (be sure to cite him if this is coursework!) and should be contactable through his website, or possibly facebook. Rick uses right angle bevel gearboxes, (by mitrpak). The ones from grinders are generally thought not robust enough. Consensus seems to be that 1:6 and 20" diameter is about right for maximum speed, but if you are looking for maximum acceleration from a standing start this may be different. Rick chases down small improvements in efficiency in every aspect of his designs. With respect to the gearboxes, sitting at right angles to the prop is most efficient, followed by a right angle bevel box, followed by a twisted chain. IIRC the differences come down to fractions of a percent, and most feel that it is worth compromising the highest efficiency available for the big advantage of looking in the direction you are going.
I would add that if this is an actual build and record attempt, then on top of getting the design right, recruiting the most athletic cyclist you possibly can will make a huge difference. It is likely that someone competing successfully at high club, county, or national level will be able to put out 5 or more times the power that an averagely fit person can produce.

 

I will reiterate my point, that a variable pitch prop gives a better return than variable gearing, and if you're expecting to be anywhere near 18 knots, surface piercing will be better than conventional.

Unless you're considering a prop in air, rather than water.