Propellor for pedal powered catamaran

that unit isn't cheap is it? What's a mirage drive go for? I think I'll piece together my own and save 600 bucks or so.

 

Peddle Powered Raft - Prop help

Hello

I'm a youth worker for the Salvation Army, we are working with a group of young people to build a raft for a raft race in a couple of week. Actually we built the raft last year, won and now want to put a couple of bikes on it to get some leg power in action.

I have 2 tandem bikes, and a small right angle gear box, plus a pile of old bikes to use a few extra gears etc...

The raft itself is a cat, made of 2 poontoons, about 6.5m long, 40cm wide at the top and 10cm wide at the bottom, they are about 40 cm deep. We had 8 adults on it last year for the race, so its large.

Our idea was to run a single prop on the back, we have the ability to get some gearing into the system, but I am really unsure what the idea spend of the prop would be, as well as the size, shape etc...

Any thoughts, ideas or wisdom that could be shared would be much appreciate. I suspect I will either have to make something, or use a boat prop, but from the reading I have done a boat prop sounds like a bad idea.

thanks

Darren

 

read this whole thread and follow the links, many of your questions will be answered. props are designed around a specific hull speed, power input and rpm, taking a prop from a power boat would not be optimum. If you want just buy something off the self you might consider buying one of the props for small electric trolling motors, it will at least be closer to your needs. Optimum performance means making your own.

 

I would look for a 16" X 16" or even up to 20" X 20" airplane propeller from a hobby supplier.

They are inexpensive.

The ratio you're looking for is 4 or 5 : 1 for prop RPM : foot RPM.

 

thanks, will check out some hobby shops

 

Lets review:

You have four boys on two tandems.

What size/age are the boys?

Could you post a picture of what you have now, i.e. your boat?

It would be great to just fix the forks to the hulls and put the back tires on a common roller that drives the gear box.

What's the gearing in the box?

How robust is it?

Pictures of the box?

Any video of the boat in last years race?

 

Hi

Yes we have 4 peddlers on the 2 bikes, plus I think we will have 4 paddling sitting on the sides as well, especially if we don't get a rudder built in time.

We have 2 races, an under 13 and an over, would assume an average of 45kg each for the younger race, and 80kg for the older race.

The bikes are now attached to the frames, and secure next job as you say is to get them running onto a common shaft, then into the gearbox, hopefully will have that sorted over the weekend.

The gear box is 1 to 1, but we have the bike gears we can use as well, we can in theory with the gearing we have get anywhere up to 1620 rpm based on 60 rpm of the peddles. But unsure if we would have the power to keep that up. The way the gears will be set up we could easily have it running at 540 rpm as well. So that's our rpm range.

Its a solid frame, perhaps too solid!

I will post a pic later they are on my old pc, and put a link up to a video as well

thanks

Darren

 

the video is at https://www.youtube.com/watch?v=wwVAaf1q3J8

 

Hi Darren,

Ah, a good looking rig.

So RPM's you're refering to are the rear wheel I presume.

Other than the large roller under both rear tires, or a shaft connecting the two rollers, I don't have any bright ideas for you.

It would be good to step that gearing down 2 : 1 so you have a range of 270 - 810 RPM?

The roller would create a ratio as well of course but in the wrong direction.

I don't think you'll be changing gears while underway but it would be great to have it as an adjustment option to select from.

I suspect turning the prop in the 400 realm to prove beneficial.

Do you know the speed of the boat from last year?

Any luck on your prop search?

Are the tandems mounted forward, mid or aft?

What's the brand name on the gearbox?

Do you know what it weighs?

I'm concerned it's going to break.

What did you have in mind for a drive shaft to the prop from the gearbox?

Pictures of any of this?

Great video!

 

Hi

The RPM would be the prop speed. The plan is to chain drive from back wheels, via the chainrings on the bike, which leave us the other 7 gears which we can use on the bike. This will run onto one shaft which will then run onto the gear box. have all the parts sitting in the workshop for this, just need to put it together. Wont be back there till Sunday now.

400 RPM for the prop is pretty easy to achieve with the set up.

No idea what the speed was, other than being much faster than anyone else in the race, we finished when second was at half way. But cant rest on last years win, as others are building new rafts I suspect based on what we did.

The bikes will be mounted in the middle, side by side, the frame is 2"x1" box steel, so its pretty solid, and the mounting is looking just solid, the load is spread across the poontoons evenly, so I don't think it falling apart is an issue.

The gearbox, came off a ride on lawn mower, so unsure of the brand, didn't even see the mower, but its fits in my hand so its not big, but its still pretty solid.

I found a hobbie shop, and have orders a 20" wooden prop, should be here on Monday, so hopefully that will do the trick.

Overall weight of the raft now, two of us can pick it up and move it, so I would guess maybe 60-70kg, most of the weight is the frame. There will be another 20-30kg with the bikes and stuff I suspect. A bit of a guess with that.

The shaft for the prop I was just going to get a bit of steel rod from the local engineer suppliers, they will cut it to length for us, and not 100% sure of the length yet, so was going to get that on Monday. Due to the size of the prop, we are making it so we can lift it up, when in shallow water, and for transport on the trailer.

No pics of any of this yet but once we have it together I will post some.

Darren

 

Over the weekend, if you get the bikes to a common shaft other than what I've described, have the peddles 90 degrees out of phase from one bike to the other.

Better yet, split the chain on each bike connecting the two chainwheels (peddle gear rings) together and set them 45 degrees out of phase.

THEN set the two bikes 45 degrees out of phase from each other.

If I have my numbers right you'll end up with each of the four peddlers applying power during the push stroke at different times from each other.

Think of it like a four cylinder engine and each cylinder fireing at different times from each other.

Bike 1 forward, bike 2 aft, bike 1 aft, bike 2 forward, etc.

Fore and aft isn't so important (depending on the shaft to gearbox configuration) but getting the bikes to alternate would go a long way to reduce loading on the gearbox.

It would also smooth out loading on the prop which will make it deliver power much better.

 

great idea, and pretty easy to do, thanks

 

Our posts crossed so I only just saw yours.

Sounds like you've got a plan.

The 20" prop is 20" pitch as well?

Two blade I trust although you may be able to turn a three blade.

Two is more efficient.

I'd be curious on the brand name of the prop.

Pictures, think pictures.

If the prop is too big, you can try slowing the RPM's or trimming the diameter (but don't rush into that one).

 

the prop I got is a 20x 10, was the only option sadly, http://www.trademe.co.nz/Browse/Listing.aspx?id=564548816 is a link to it.