Twisted belt/chain drives

[grob]

Why do most human powered boats (pedal boats) use a twisted chain drive, surely a twisted belt drive would be better as it is better suited to the marine enviroment.

Gareth

[Fanie]

Hi Gareth,

The sprocket belt would work much better, quieter and will last longer. Even the bikes use them nowadays. I suspect the belts are not as well known as the chains. Who-ever built them probably hasn't seen a belt before so did it as grampa would have.

[grob]

[quote=Fanie;174295]The sprocket belt would work much better, quieter and will last longerQUOTE]

Yes you are right , the sprockets tend to be made from aluminium, there are some new belts that can take very high loads so can even match the width of chains not that that is an issue in this application. And the belts can run in salt water all day.

Gareth

[Fanie]

There are plastics that will do better than the alluminium. Have a look at Ertalite, Ertasetel, high strength, high wear resistant, should last (almost) forever

[Guest625101138]

Originally Posted by grob

Why do most human powered boats (pedal boats) use a twisted chain drive, surely a twisted belt drive would be better as it is better suited to the marine enviroment.

Gareth

Gareth
Most of the commercially available HPBs use other than twisted chain. The Hobie use flapping foils, the Wavewalker and Cadence use straight chain and gears and the Nauticraft use a twisted belt.
http://www.nauticraft.com/propulsion.htm
http://www.hobiecat.com.au/kayaking/i14.html
http://www.wavewalker.us/products/index.aspx
http://www.openwatercycling.com/owcpropulsion.html

A good proportion of the home made ones use twisted chain with bike components simply because of the ready availability of the components. A visit to the local tip can often provide the necessary components for a twisted chain drive.

You will get debate on the topic but my opinion of the best solution is gearbox in the attached photo. This accepts bicycle cranks, fully enclosed and has a 1:3.3 step up. The next best is probably two gearboxes - one with cranks and the other underwater with the prop, each having 1:2 step up to give 1:4 overall. Third choice is a straight chain for the speed step up driving a right angle drive.

Rick W.

[ancient kayaker]

I assume Grob refers to notched belts used with specially designed sprockets providing positive non-slip drive like a chain. Just as efficient as chains, some are more so when operated twisted, but maybe not as long-lived which explains why they are less common if cost matters. Power loss is comparable in a single sprocket or a gear stage so theoretically a single stage gearbox should be more efficient than two sprockets and a chain/belt, but the chain/belt drive offers design flexibility. Pure belts relying on friction can be quite lossy.

[FAST FRED]

With simple construction it is far easier to change the length of a chain than order a different belt at a store.

Since the losses are similar EZ is king.

FF

[ancient kayaker]

A NY company called Berg markets several types of flexible plastic chains and belts; they will customize the length for you or a field spice is available for each type. The field splices are not as strong as the belt.

Whatever; the technology exists and is well established, and I just wanted to mention it. If I were building a human powered boat that required a twisted chain drive I would give serious consideration to using a belt of suitable type. Personally I'm far too lazy ...

[I57]

I am going to make a twisted chain drive and would like to know the min length between the prop centreline and the centre of the crank. I have estimated a 3:1 gear step up and a prop of 500mm dia.

[Guest625101138]

Originally Posted by I57

I am going to make a twisted chain drive and would like to know the min length between the prop centreline and the centre of the crank. I have estimated a 3:1 gear step up and a prop of 500mm dia.

If the drive is to be mounted through the hull then you need around 300mm above the hull to the crankshaft so you get heel clearance. You need about 20mm clearance between prop tip and hull, then 250mm to centreline of prop using a 500mm prop. So total is 570mm to get adequate clearances.

This will be tight for a bike chain resulting in heavy rubbing but you can get 3/8" pitch chain that should not complain to much.

The losses in a twisted chain are not insignificant. Actual testing that has been done shows they have chain losses of around 3% under load compared with less than 1% for a straight chain.

The other problem is the drag from the drive leg. This could end up being as much as 30% of the total drag of the boat if you have a good hull.

I have found small gearboxes to be worth their weight in gold for pedal drives. They cost somewhat less than gold. Overall you end up with a much more efficient system. I have attached some images of my latest design. The side mounted box has a lot of benefits. Very low appendage drag; the prop is visible, no risk of chain coming off; very durable and reliable system; can go full power astern, light weight; no hull penetration. On the downside the gearboxes cost around USD250.

I have also found 1:4 gear ratio with a 400mm prop to be close to optimum but this depends on the boat and desired cadence.


Rick W.

[I57]

Thanks for the reply Rick, its me Ian. Thought I would try the cheap alternative of a twisted chain drive before spending the money on gears would like to try the concept of a dropped drive first. I have started work on it and the chain looks like it will take about 1 metre to twist through 90 degrees. It will be deep in the water, the middle of the crank has to be 300mm above the keel leaving about 700mm to the centreline of the prop, overall about a 1 metre deep drive leg under the keel. By adding a bit of weight to the bottom of the drive leg, probaly only a few kilograms, I can get rid of the outriggers. The drive will fit through a centreboard casing making transport easier, boat on roof rack and drive in boot.

[Guest625101138]

Ian
It is certainly worth a try. I could do a stability analysis on your hull but with the weight well down you will not need a lot.

Getting in and out from the beach could be a problem because you will need to have the leg raised and this will lift the CoG. Having the leg rest on the bottom might stop it from rolling.

You could still use a couple of floats that sit above the water level for ultimate roll control.

I think a ballasted boat will be better in waves than the outriggeres providing the righting moment is good enough.

Rick W.

[Guest625101138]

Ian
I checked your hull data. The KMt is 240mm above the keel.

From my measurements I have determined the CoM of my body in a recumbent position is around the 350mm mark. This is in a low back position. The hull CoM is around 180mm high.

I have attached the righting moment curve for your boat will a 10kg weight mounted 700mm below the keel. Maximum stability is achieved at 20 degrees and stability diminishes above 35 degrees. This is probably practical for reasonably rough conditions. The hull swamps at just over 50 degrees. It would take 12kg on the keel to get diminishing righting and swamping to coincide.

Rick W.

[grob]

Originally Posted by ancient kayaker

A NY company called Berg markets several types of flexible plastic chains and belts

thanks for that the 3D belts look very interesting

and they look well suited to the marine environment.

I even already had one of their catalog's from years back, and had never noticed those belts before.

Cheers

Gareth

[skullhooker]

Today I bout two minn kota weedless props for 11 bucks each, thinking about using them as replacement ones for my trolling motor, or possibly for a human powered craft. They are 4 inch diameter, with swept tips.

This thread is current and interests me. I would like to investigate a very shallow drive, possibly tunnel hull flats boat like craft, that does not need to be fast or designed for maximum efficiency. I would like a stable platform for stand up style flats & dock fishing. There are a lot of launch locations in my area, where I would only need to travel a quarter to half mile to get to good fishing areas.

I have a kayak, and though hobie mirage drives are very efficient, being a tall guy, I get real cramped sitting down all day. A paddle boat style drive would make too much noise, scaring off the fish. The width of a gheenoe, with a pocket or tunnel, would be nice, say 30 to 48 inches wide, with room for a cooler, a 5 gallon bucket, and possibly an elevated seatpost. In Dave Gerr's book the nature of boats, he has a design fo a two-part dinghy, glue - stiched plywood, which has good dimensions. A lightweight craft that can fit in the back of a pickup or SUV, with a good amount of clear deck, would be a trick setup.

Would threading two or three props on the shaft enable lower RPMs and less noise for the amount of thrust generated?

I could probably sell of bunch of these if they worked well enough!

Any suggestions will be greatly appreciated. Thanks!