HPB - looking for input

twin prop human powered boat

I don't sleep much lately. My brain won't shut off because it's designing a propulsion system to be added to a sit-on-top kayak.
It would be bolted on and could be removed easily too. Instead of going through the hull like the Nativecraft and the Hobie, it would be a twin prop system that would "fold" over the sides.
I was hoping to get input on a side-by-side, twin-prop, human powered boat.
Here's my napkin sketch. It's clearer in my head than on paper.
I wondering about thrust vs. drag with a twin prop. Anyone have input?

 

Center of gravity of the boat, occupant pedaling and propulsion system will be considerably higher than of the boat with an occupant paddling, partly because of the high CG of the propulsion system and partly because the occupant will have a higher CG with their legs in the air to pedal. Stability will be reduced as a result.

The bicycle wheel will be a gyroscope when the boat rolls the wheel will try to yaw it and when it yaws the wheel will try to roll it..

 

The system could spread the two propellers further from centerline and provide outriggers above the propellers?

 

The wheel is light, gyro effect should be minimal.

R/C props are great for this. One would be better than two.

What are your objectives?

You only have 120 - 250 watts to play with from a healthy person.

-Tom

 

Objectives: Catch big fish.

Really though,
-I want to be able to just slap a prop system on top of my boat and cruise around the lake without having to pack a big, heavy bettery.
-I want to be able go faster than I would with a paddle.
-I want to have my hands free to be able to fly cast.
-I want some good excercise (cardio) in addition to fishing.
-I would like to be able to propel it by hand while I'm standing to make fine adjustments forward and reverse.

This will go on a very stable platform - a Nu Canoe; beam at the water level is 36". It's only 10', so it'll be slow (unless I go crazy and put a 5hp outboard on it).

Thanks for your interest and input.

 

I'm having a hard time finding a pair of counter rotating 14"-16" RC plane props. Does anyone have a link? I'll keep looking, but if you have a favorite, please post it.

Thanks.

 

I too had a hard time but you may get lucky if your specs are more common.

If not, you wouldn't suffer from two RH props at your power level.

You could also make your own from SS, mild steel, f-glass, f-epoxy, carbon-f or kevlar by reverse copying the purchased RH one.

What pitch are you thinking? RPM?

-Tom

 

I've seen a bunch of the folks on HPB websites use 16x16 props for their single prop rigs. I was thinking of trying to get as steep as I could, being that we're talking about low RPMs.
What I don't want to do is pedal like a madman. I would like to be able to cruise at a modest speed. I have 2 assumptions in my head: 1-Larger prop=more thrust with the same # of RPMs but more resistance and 2-steeper pitch = more resistance = more thrust with slower RPMs?
The best I've found so far is a pair of counter rotary 12x12's.

My best idea for making them is 1/4" bar stock aluminum or steel. I figure by heating and twisting a piece of a stock so it makes a 180 degree twist, I should be able to shape two props that are close to being mirror images. As for making my own with cf, glass or kevlar, I've never used any of them before.

One crazy idea I had was to do a bunch of wooden props in different geometries, sizes and pitches. I wouldn't have to worry about the durability, just use them as testing mules. What would be really fun is to mount different size/geometry/pitch props on opposite sides and see which direction I turn = better thrust. I could then use that either to make a mold or just a pattern.

Thanks again

 

Rick Willoughby (who used to post here but has sadly now left) came up with a very neat way for making stainless steel props with high efficiency. Here's a link to his method: http://www.recumbents.com/wisil/hpb/Prop_Fabrication/Propfab.htm

I've made folding stainless props using his technique and found them to be excellent, very efficient and easy to make.

I think you'd probably find that a single prop over one side would work OK and be a lot simpler. Rick has used drive from one side on several of his pedal powered boats and reported that the asymmetry didn't tensd to make the boat turn a great deal at all. His flex-shaft drive tends to align with the direction of flow, can be easily pulled clear of the water and would seem to suit your idea of a bolt on module.

The pedal boats thread here: http://www.boatdesign.net/forums/boat-design/pedal-powered-boats-23345.html has a lot of useful information, but is a bit long (take note of posts in that thread by Guest625101138 as they are by Rick, who learned, and shared, a lot from experiments). Here's a link to Rick's simple website that has some photos of his boats - the propulsion systems might give you some useful ideas: http://www.rickwill.bigpondhosting.com/

Jeremy

 

Thank you Jerry. I found Rick and have been in contact with him as I work out the physics. He's a HPB genius.