Twisted belt/chain drives

Hobie do a good sales pitch on "efficiency". Thrust is not efficiency and that is all they prove in the video where they tow the kayak paddlers. Testing I did shows at cruising speed like 4kts they are around 35% efficient and the efficiency improves to 50% at very high power level. You rarely see a Hobie Mirage in a head-to-head race with a kayak because they get blown away.

I doubt that the 4" Minn Kota props will work well for a pedal power boat. I think they will be too small but would need to check this. They would certainly need tall gearing.

If you make a 4ft wide monohull its top speed will be close to hull speed. For a 9ft boat that will be 4kts. It will generate quite a wash at that speed and take a lot of effort. Realistic speed more like 3kts.

Two slender hulls in the form of a catamaran will perform better and be more stable. A 9ft catamaran would glide reasonably well at 4kts with little wash. Hulls can be very simple - made from 3 pieces of ply and deck can just be flat.

Best gear ratio for typical pedal boat is 4 but this needs to swing a large diameter prop. It is easy to double shaft speed using a 2:1 box. This gives a ratio of 8. You could expect efficiency around 80% with an 8" prop. Probably back to Hobie efficiency with the 4" props - or even worse.

A 9ft long pedal catamaran with good drive set up would perform better than a Hobie.

One real issue with a prop is weed. If this is likely then you need to design for it.

Rick W.

 

You can't beat a kayak for efficiency but I find just gliding along in mine can scare the fish even if I am not paddling. I can often see trout scattering when I am in the shallows, I think it is the shadow.

Low RPM should be the key to quietness, but watch for vibration from the drive transmitting through the hull to the water; you may need rubber bushings.

I sympathize about cramp in a kayak. I am considering attaching an empty one gallon (4 L) jug to each end of the paddle so I can put the paddle across the rear deck and sit on it, just for a change of position. Maybe you could try that in your kayak, if you had a couple of clips it may be stable enough to stand. I would try it out but I have a problem with hard water at this time of year!

 

Ancient Kayaker
I know you were talking in general terms about the kayak efficiency so this is just taking at a cheap shot at your use of words.

Indeed kayaks are an efficient hull form but you CAN beat them. Some of the boats that rate better in my view from purely efficiency perspective:
1. The fastest single person human powered craft was Decavitator:
http://www.youtube.com/watch?v=l2UOH65QOI4
It achieved just under 19 knots.

2. Flyaks rate reasonably high on the scale:
http://www.youtube.com/watch?v=U95UReP4mdo
I believe a good paddler can hold these at 23kph over 500m.

3. Then you have rowing sculls. These are optimised for 2000m events and cover that distance in around 6 minutes. They do not achieve their full potential because fixed seating is banned.

4. I believe my V11A design is the best long distance craft:
http://www.youtube.com/watch?v=ckWqIgmVM4Y
It is the black one in the video and is still being developed. It is 24ft long and 9" beam. A lighter version is being built to challange for the 24 hour distance record on water. It is designed for speed of 12kph with 150W input.

5. I place surf skis ahead of kayaks.

6. Outrigger canoes rank 6th in my opinion.

7. Now we get to the kayak - I agree a most capable craft but not something I like in its pure racing form. Requires good technique and upper body strength, both of which elude me.

Rick W.

 

Why do you say this Rick?

Gareth

 

Gareth
My scale is based on the power output of the rider to achieve a given speed over water in the various hulls. It is a speed ranking for a give power input; acknowledging that Decavitator or Flyak would not perform well with only 150W input. So there are crossover points but I am comparing them in the regime where each operates most efficiently.

From my oservations of equally proficient paddlers I believe surf skis do better than racing kayaks at least in endurance events. This guy is the current 24 hour distance record holder for water:
http://www.youtube.com/watch?v=sS-LJs-yt94
He managed 237km in 24 hours. Managed about 13km more than a kayak.

I know olympic K1s will burn down 1000m in 3.5 minutes but these guys are highly trained athletes. I expect they would be producing up to 500W during the event - I have no reference data on this though. I would like to see what an olympic kayaker could do on a surf ski over 1000m.

Surfskis typically have a bit more rocker than K1s for handling waves and this would work against them in flat water at sprint speed. However it will not detract very much at cruising speed around the 7knt mark. My efficiency test would be to put a paddler on a surfski for 8 hours one day under ideal conditions and then same paddler under similar conditions in a kayak for 8 hours a few days later. I think the paddler would go futher in the surfski as I believe it is more efficient.

Rick W.

 

True, I was talking in general terms and what I would term normal craft, but it wasn't a cheap shot, your information is interesting; I had not even heard of a surf ski! Nonetheless, I am sure that I would go further and faster in a kayak rather than in the decavitator or flyak, and I suspect I would be more likely to survive the attempt.

The counter cheap shot would be "where do I buy one" and "why don't they have them in Walmart" but enough ...

150W sounds like quite a lot to sustain over 24 hours, I can't imagine anything I could sustain for that long except maybe sleep. I am obviously out of my league.

 

I thought I had, but I was thinking of those sit on tops that look like a surfbooard, which is why I asked the question.

I had imagined that Rick was talking about a planing kayak, so thanks for the video that cleared that up.

Gareth

 

Human power output is an interesting topic. The fellow I do HPB designs for is a Canadian. Not a spring chicken. He can hold the 150W mark for around 10 hours but then fades - as most would. Gets down to about 80W towards the end. So the 150W is the design condition that things are optimised around.
http://www.adventuresofgreg.com/HPB/2008_02_02_archive.html

Tests done with Lance Armstrong showed he could sustain over 400W aerobically. Olympic scullers get up around the 500W mark for their 6 minutes over 2000M.

If I do a bit of training I can get to around the 130W mark for 4 hours duration.

I guess you have to know about the engine if you want to make pedal boats.

Both pedalling and paddling are similar biomechanical efficiency but driving a prop in a boat is arounf 20% more efficient than using a paddle. Rowing is least efficient but you have more muscles involved. The advantage of pedalling is offset a little by the extra weight of the drive system so the aim is to make these parts very light.

The only pedal boat that has a reasonable market is Hobie. They have built on the Hobie sailing name and done a really good con job on selling the "efficiency" of their flapping foils. However the Mirage hull is well matched to the flappers. If you want to do a comfortable 7-8kph on water they are a good choice. I go to a lot of effort to get someone like Greg up to 12kph at continuos power output. You would need Lance in good shape to get this speed with a Hobie.

I personally like the feeling of power and control in a pedal boat. Using your leg muscles allows rapid application of power. I would not have a hope of getting over 18kph in any paddled or rowing craft but I can do it in a nice pedal boat. Having hands free and laying down on the job are also decisive benefits.

Rick W.

 

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

Good question, can anyone help here?

 

From my limited experience low RPMs need a bigger prop, deeper draft, and
high RPMs smaller prop. A 4:1 gearing needs a 400mm dia prop, a 8:1 gearing needs a 250mm to 300mm dia prop, higher revs would give you an even smaller prop. Its like a dog chasing its tail, low revs, lower noise but deeper draft and high revs more noise lower draft.

 

My Idea

Just giving you the update on my latest project, I've finally succeded in making a twisted chain drive that works. That is one where the chain stays on and dosen't slip off all the time. The problem I've had in the past is going from the large chain ring with 52 teeth at the top and then tuning it 90 degrees and going to a 13 teeth gear at the bottom. This time I have used a 13 tooth gear at the bottom turning 90 degrees to a 21 tooth gear at the top. This is fixed to a bike front wheel axle with the 21 tooth on one side and a 13 tooth on the other, the 13 tooth gear is connected in-line to the 52 tooth gear at the cranks. I've cranked it up fast and the chains still stay on track although it hasn't been tested under load. Distance from prop to centre of cranks is 650mm, this is the shortest distance where the chain runs smoothly.

Ian

 

Ian
My twisted chain drives are 700 mm from prop to centre of cranks but I use a .025 " pitch chain. I believe that the most important part of these units is a chain tensioner on the return side as this helps to guide the chain and keeps it on.

Dennis A

 

Tensioner

Dennis A
Since that last post I have fitted a guide at top and bottom to keep the chain tracking onto the gear. Without the guides the chain was coming off when cranked fast, this way it is staying on and works ok. Haven't got around to putting the fairing on yet, or fitting it to a boat. In the mean time I have been using Ricks idea of a flexible steel shaft and have found it works great. It gives you low resistance and springs up to allow you to beach the boat or get over snags. When I first saw it I wondered how it worked but since using it am sold on the idea, one day will get around to fitting the twisted chain drive to a boat.

Ian

 

Flexi-Steel shaft

Ian

The one thing that confuses me on this system, is additional support for the shaft. Looking at your thumb nails of your proa it does not show any additional support.
Rick however says he uses a light folding tension strut to prevent the prop swining around and hitting the hull due to waves or turning. Do you have one of these installed and does it not catch weed.

Dennis

 

Flexible Shaft

Dennis A
The system on my proa uses an Involute 3.3:1 gearbox, this is a complete unit with pedals that eliminates the need for chains. The 8mm dia spring steel shaft is attached directly to the gearbox, the prop is fitted at the end and when the shaft rotates the spinning prop levels itself to the horizontal. Rick can give you a better technical description, the system does away with the need for shaft supports and universal joints. The purpose of Rick's tension strut is to pull the shaft up in shallow water and also to prevent the loss of the prop if the shaft snaps. The shaft can be lifted by hand and weed cleared from the prop without having to get out of the boat, I haven't fitted a strut to mine as yet.

Ian