Pedal Powered Boats

Rick and others, fascinating thread.
I used to compete on road bikes twenty years ago.
Rick, you mentioned you were chasing down losses on the order of .25 watts. I would guess all the bioefficiency and mechanical efficiency work was done using road bikes. Pedaling produces all kinds of harmonic forces and stray torques affecting all six degrees of freedom (the three axes of rotation and the three axes of translation). Bikes and boats resist these forces very differently and I was wondering if you had compared the power difference between your boat being pedalled and being propelled by a small motor that doesn't cause any bobbing and rocking and yawing as it goes? How far did you get with this? were you able to distribute the proportion of losses among the six degrees of freedom?

If your purpose is to be able to compare different hulls with out resorting to elaborate testing apparatus, the dynamic losses could be related to an easily duplicated and measured static configuration. (that came out like gibberish, didn't it ) example- The yaw caused by pedalling could be equated to propelling the hull in a turn of a given radius buy a motor that doesn't produce cyclical yaw. Like wise for the other axes. To the extent this is valid, you can use static tests to predict relative performance in a more dynamic environment.

My other thought for some do-it-yourself science would be to consturct an eccentric flywheel in the manner of a harmonic ballancer and play with its phase angle as the boat is pedalled. Should work well to counter vertical movement.

P.S. I always wondered if a sidesaddle bicycle wouldn't be a more efficient machine. Probably not faster due to windage, but maybe a better setup for a two wheeled pedicab.

 

Phil
The most accurate power testing we have done was taken using an SRM metering crank. So biomechanical effects are not measured.

I usually do a calibration of my heart rate versus power at a local gym each year so I can get numbers to compare with the boat so this takes biomechanical factors into account. At low power level my gym numbers translate well to the boat at sprint type levels I can get about 20% higher in the gym than the boat. This is because of the flywheel on the gym machine.

The lack of flywheel effect is a factor in biomechanical efficiency. In fact with the thin shafts I have been using it is quite noticeable because they feel distinctly rubbery. If I was sprinting I would use a thicker shaft. However adding a thicker shaft adds drag through the water so it is a compromise.

With the flywheel you add weight and this adds drag plus any associated gearing also adds to losses. So flywheel does not provide benefit - at least for systems I can engineer and for the cruising power level.

Most of my effort has been directed at long range cruising. My own power level target is between 120 and 130 watts. So things I observe relate to that sort of level.

A stiff frame is a benefit. You need to be able to lock in the forces between backside and cranks. I am working on a new carbon fibre frame and seat that will have a total weight of 3kg and be very stiff. It will also have the gearbox mount. I will post some pictures when finished.

If you want to go as well as you can on the boat you need to train on it. I am not sure if you have watched the videos I have posted or linked to but you should be able to see that the boat moves without much pitch, yaw, roll or speed variation. It is far more efficient in this regard than a rowing scull or kayak. The more you train on the boat the better you are able to spin and keep the body aligned.

Biomechanical factors are important and things to work on.

Rick W

 

Ian
I have a recent arrival of spring steel for shafts. This is really good stuff with UTS around 1600MPa although it does corrode. I paint it to reduce corrosion.

You can use machinable aluminium but it would need to be thicker to avoid the rubbery feeling. It also corrodes but not as fast as the spring steel. I tried to get some high strength stainless steel but there is nothing readily available near me.

I can give you design calculations on the shaft if you want it and provide the spring steel at my cost if you are interested. If you try to curve the shaft too severely it will fail in fatigue. Also the stern tube through the hull should follow the intended curve so you need to have this sorted before you place the tube. I have used the 19mm poly pipe used for watering plumbing watering systems.

 

Rick, have you tried using oval sprockets (do you even use sprockets)? They were being tried twenty years ago, with out much luck, on street bikes. On a boat, they could be used to match the prop slip to the differences in torque as you crank. That might get you some of that 20 % back.
Good luck on your future endevors.

 

If you go down this blog you will see a photo of the oval chainring Greg K got for his boat:
http://www.adventuresofgreg.com/HPB/2008/08/gps-distance-measuring.html
He was experiencing knee problems and this helped.

If the shaft is stiff enough it does not make much difference. If the shaft is borderline then it will make a difference as was Greg's situation. He should have been using an 8mm shaft not a 1/4" shaft. If the shaft is too flexible then it will not help much. I have had shafts that are so rubbery that you virtually stop rotating at the crank deadspots until the shaft unwinds. It is very awkward. The difference is significant. With one of my boats using an 8mm aluminium shaft at 3.75 step up I could get it to 12kph at full tilt. By going to spring steel I could get to 17.8kph.

A few recumbent riders use small cranks so they spin faster but with the same force. These would reduce the range of movement with knees as well. There has been a lot of testing on these aspects but I think it mostly gets down to what feels nice. I tend to set my clips toward the instep to reduce calf strain. Others like to have them more on the toes and they really stretch out. Greg traded biomechanical efficiency for reduced muscle strain. However he did pull back on cadence a little by increasing the gearing from the original design of 90rpm.

Rick W.

 

Curved shaft

Rick
I have some 10mm aluminium tube which I am using for the shaft, do you know what the minimum radius would be?

Ian

 

Ian
The tube will need to have at least 8m radius to survive. If you can fit this in then there will be other options available if you find it too rubbery. I expect it will be borderline.

At that radius you could use ordinary stainless. It will have ample torsional stiffness. With the spring steel I get below 3m radius but this is so I can locate the prop beside me. Running it aft makes it much easier to get the large radius.

I figure the shaft of the gearbox will need to be angled around 20 degrees with the required vertical offset over the shaft length. You need to lay it out showing the height of the prop and the gearbox.

Rick

 

Boat Update

Just an update on progress with the new boat, hull is progressing and have made the skeg and the seat, camera still not working. Got hold of some 6mm galv. steel rod and will try using that as a flexible shaft, length is 2100mm and connected to 3.3 step gearbox. Prop is 450mm dia and the thrust will be taken at the skeg, approx. radius of shaft is 4m. Found that with the thicker aluminium shaft prop would be way past the boat to get an 8m radius.

Ian

 

Ian
The galvanised shaft should not fail but could feel rubbery. I have the spring steel if you find it does not feel nice. It can go up in diameter and still survive.
Rick

 

inside a seacycle drive

I thought a couple of shots down the inside of my seacycle drives might be of interest. I've had the tops off to inspect, as after my first outing, I hit an underwater obstruction with the prop, and the unit started binding. I can't see anything out of place, and the drive rotates perfectly when not under load. I think I may just have the chain tension too tight - I hadn't realised that it was a twisted chain unit till I got it apart - I'd assumed it was a bevel. I can understand that if the chain is too tight when twisted it may bind. Interestingly, the chain is not bike chain, but is quite a lot smaller, with a link wavelength of only about 8mm.

Anyway, I've slackened the chain off, and I'm trying to find the time for a second outing.

 

TT
The short pitched chain will twist easier than long pitch. Normal bike chain is not the preferred choice for twisted chain drive.

The short links work OK and some say they will last a long time. The are arounf 97% efficient or even worse so there is a lot of extra rubbing going on compared with a straight chain run at 99% efficiency. I get around 96% efficiency with my chain/right angle gearbox combination and they last a long while without any deterioration.

You can get around 97% efficiency with a big gearbox without chain but the boxes are quite heavy- up around 4kg. Both Ian and I have boxes from India that have 1:3.3 step up that you just fit cranks too. The housing is purpose built but the gears are the same used in large angle grinders. Ian really likes his. I have not had the need to use mine yet but the price was too good to pass up.

I only ever tried one twisted chain and after looking at the torture that the chain goes through I chose other methods. Mine had a heavy duty chain and it did not like to be twisted, at least while new. My main issue with them is that they cannot be engineered to chain data because they are using the chain in an unintended way.

Rick W

 

thanks, Rick- that's an interesting insight. Re reading the seacyde webSite, they recommend tensioning the drive when there is more than 2" of movement in the pedals, and I've still only got 18-20mm slack at the pedals, so I might slacken the chain off further- it does feel pretty tight at the moment.
looking at the efficiencies of a straight chain drive- do You think there is any mileage in a 2 person double outriggEr monohull, a larger version of your series, with 2 sideways facing engines seated in the main hull, facing out on opposite sides, straight chain drive, and the props combined with the outriggers? There was a boat using the sideways seating position on the IHPV website. I imagine that the neck twist issue would put the mockers on it, without mirrors (or cameras and displays!) but the curvature possible in your flexible shaft might alleviate the issue?

 

a lousy sketch of the idea

a cruddy sketch of the idea (photographed and submitted by phone! )

 

The efficiency is not a really big deal. Difference is 2%. The real benefit of the straight chain is reliability - low wear rate, low stretch, no risk of jumping off.

I have attached a picture of Close to Perfection from Duisburg. It was a good performer in sprints which is impressive since it is a catamaran. It has a direct chain drive to a prop in each hull.

With two people in a monohull you would have a design speed around 14kph if you wanted to optimise. This would require a long hull say 9m.

There are advantages in having people sitting behind each other to reduce windage. At 14kph it will be significant. Also two people cranking a common drive with each pair of cranks offset by 90 degrees improves efficiency. My choice would be two little boxes to a single shaft. One of the boxes would be a "T" box:
http://www.mitrpak.com/product_datasheet.php?product_id=66
The shaft could be set up under the seat for inline drive or beside the hull to avoid hull penetrations.

Rick W

 

More time on the lake yesterday. The weather was overcast with some very light rain. Wind was variable. Some gusts to 40kph but mostly 15 to 20kph from the southwest. Most of the weed has been pushed to the northern end so this was not a serious problem - just a clean each loop.

I did 29.3km in just over 3 hours. I started well but the effort of pushing into the wind caused muscle fatigue after a couple of hours. Still not able to average 10kph.

Rick