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Yes. It is not very efficient. It is sobering to see how much power a human can sustain. Average person off the street will tire quite quickly producing 100W. Surface piercing props enjoy an advantage in high power, high speed applications where appendage drag and cavitation are seriously detrimental to conventional marine propellers.

Surface props do not work the same way as conventional propellers unless you are comparing with a fully cavitating prop.

At very low human power level it is possible to use very high aspect propeller blades similar to those used on aircraft that can get up around 85% for average power level and near 90% at elite power level. The best a surface prop will get is maybe 70%. This is good compared with a heavily loaded submerged prop but awful compared with typical HPB prop.

The attached picture gives an idea why I know this stuff off the cuff. It was one of my most disappointing trials - much splash with little dash. I could have done better but the potential is constrained.

The folding prop is close to weed beating.

Rick W

 

Thanks for that Rick. I did wonder, because the main advantage they talked of was diameter of the prop and revs.

That's an amazing getup you have there! hnnnh so much effort to find out it did not work! So yeah, you have tried it and FWIW you have my gratitude. I also wondered about splash, not only from an aesthetic POV, but because every bit of splash is waste.

One thing that shot brought to mind was that a two-blade prop has one great advantage for beaching in that you have almost zero draft or even clear by simply setting the prop as you show and lifting on the curved shaft, even with a 16" + prop.

Yeah I was looking at your folder. Nice work, and food for thought indeed. You mentioned really spinning to get reverse. For my project that would be important. How effective is it? For my purposes, I was wondering if maybe I should look at some sort of locking system.

 

Cable driven

Here are some notes from "Mike C":

It worked Ok at "casual high speed" but at my top effort (not an athelete) would kinda bind and go feeling like the cable was winding up and unwinding.

Note that it was a VERY thin cable, maybe 3/16", out of a small gas powered weed wacker. I have since found weed wacker cables that were nearly twice as thick and may do better.

I ran it through some heavily greased plastic (I think it was Polyethelene) plumbing tubing that fit just right like the original plastic guide tubing. The tubing showed no signs of wear that would indicate friction with the cable was a problem.

I was using a plastic 4 blade automotive electric radiator fan as a prop that may have loaded up a lot under high torque in water.

You have to observe the wrap on the cable so it tightens in the direction turned under load and not kinda "unwrap".

I abandoned any further research in favor of the simplicity and effectiveness of the twisted chain design.

I have given some thought to using regular 3/8" or 1/2" cable in plastic plumbing tubing. Braided cable may be better than twisted lay cable in terms of not being directional dependent in rotation but I don't know. Might make a handy "clamp on" or "bolt on" design so you don't have to cut a hole in the boat for a twisted chain drive. I figured that I'd use steel bushings (3/8" ID X 1/2" OD) crimped on one end to hold the cable and bearings on the other end of the bushing.

The flex cable allowed me to rotate the prop for steering which is handy as a rudder adds drag and don't work well at low speed.

I have also crudely (prototype) modified a twisted chain for transom mounting (peddal it facing backwards like rowing, with feet & pedals over the transom) AND to allow the prop to be rotated for steering. Haven't had time to put it in the water yet.

Mike

 

Thanks for posting that rambat. And thanks 2ndhand to Mike!

If you have to put the cable is housing that has got to add losses, greased or not.

All goes into the data base.

I am happy to use the trials of Others at this stage (a) no use reinventing the wheel (b) the better I go first time, the more likely I am to keep trying stuff.

Nick

 

My folding prop is not very effective in reverse. At full spin I can get up to about 3kph. It is OK for backing out of things but limited for stopping quickly or easy reversing in a strong current. A fixed bladed prop is almost as good in reverse as ahead.

I am using very light blades. They are 25 x 3mm stainless steel. With a heavier boat you would need bigger blades simply to take the loads so the throw of the blades in reverse might produce a better result.

I used to like my last fixed bladed prop. I even carried it to Canada to test alongside the precision milled prop. Mine hand fabricated prop was fractionally worse but still around 85% efficient. However I now have a strong preference for the folding prop. Being able to drop weed just by a momentary stop in pedalling is a noticeable benefit in the lake where I exercise. It will also be beneficial in the first few hours of the race next week because there is a lot of floating weed growing in the upstream lake that flows over the weir into the Murray at Yarrawonga.

Rick W

 

OK. In the Swan, weed is not usually a problem in the more pleasant parts. When it's weedy it's usually pretty dank and grotty.

Jelly fish might be though. Seriously, in the lower broader parts, the wind blows millions of them to the lee shore and it's like sailing through soup.

Floating and submerged logs would be my biggest hurdle I would think in the upper reaches.

Nick

 

If floating weed is not a frequent occurrence then a folding prop does not offer any great benefit and it has downsides. That said you will notice weed much more if it is present.

I have felt the blade slice into jellyfish like it is mud but does not hurt the prop. For a moment you wonder what was struck. I am certain I have hit fish with the prop; too solid for a jellyfish or bag but not as hard as a log. I often see swirls in the water just ahead of the boat as fish scarper. I have come close to hitting diving birds as they emerge.

Being able to easily reach the prop and inspect it is an advantage.

In the Murray the visibility through the water is about 60mm. I watch for logs by the current eddies. Sometimes I miss them. Quite a few boats get damaged one way or another each year.

Rick W

 

Interesting. Thanks.

So jellies no problem, although if it's like mud then there are places here where it will be like mud. Stay away I guess.

Fish and birds! I suppose a big slow revving prop and no motor makes you a bit stealth-like.

I have a pretty fair idea of where the logs are worst in the river, so I will stay away from that as well, or proceed with caution. What I am going to do is make the prop stanchion as flexible as possible and I am looking at some way to force the prop to rise over solid stuff, without too much drag. The flex shaft is great for that.

I have to remember that at best I will probably be doing half your speed. Having said that I have thought about either building a couple of hulls for a one-man + dog catamaran, or using a surfcat. One is more efficient, the other less work and very cheap, hulls only.

 

Its a joke right? What are you trying to achieve

 

The best way to go over solid stuff is not to have a skeg with the flex shaft. Also the prop is more efficient without a skeg also because the prop is free to seek an equilibrium angle under forward thrust instead of being held in a firm position where it cannot respond and adjust instantly to forces. A skeg also tends to collect more weeds and immobilize the prop as the tangle grows, and untangling is more time consuming than with no skeg.

A hinged or some kind of flexible skeg might be a better compromise than fixed. It might allow some freedom for the prop to wander, but within limits so that the prop doesn't hit the side of the hull. That way you could still have the advantage of reverse and being able to retrieve the prop if the shaft breaks.

See http://www.boatdesign.net/forums/inboards/prop-shaft-systems-24636-4.html post Rick's #53, open the bounce prop attachment for one of my short videos.

 

Oops, Rick's post #54 shows the prop when it encounters a hit with solid object.

 

Pardon?

 

Ok. Yes I see what you mean. So ironically, maybe the best thing to use is a coiled shaft from a weedeater, with a little collar for the main (curved) shaft. It would just hold the prop in place, and be springy, but have the bend to let the prop move?

 

I don't think a coiled shaft will work because of internal frictional losses, rust, and lack of torsional rigidity, etc. as has been mentioned. I don't see how it can be configured to go over solid objects unless the jacket itself has a measure of flexible rigidity comparable to Rick's flex shaft.

I don't know what your boat looks like or how you would be using a flex shaft, but the best way is shown on the video. On a state of the art boat like Rick has, I would start by using a forward inclined somewhat flexible skeg made of rugged, springy fiberglass which could still be removed for weed clearing. That way, if a protruding log or boulder happens to thread its way to the prop suddenly, you don't have to worry about a damaged skeg or broken shaft, since the whole setup would just kick up out of the way. Of course, some tinkering would be needed. This idea might already have been considered and rejected as unnecessary or unworkable for the type of boating Rick does.

Porta

 

Sorry Porta and thanks for your time.

When I referred to the coiled stuff I was referring to the stanchion/skeg for the prop, not the driveshaft. I am still not sure how , but I am convinced that the curved spring steel shaft works, and is is the way to go. I just want to constrain it enough for reverse and as you say to prevent it banging the hulls.

The irony I saw was that the coiled stuff had pretty much eliminated as a driveshaft, but may have a use after all.

The coiled stuff strikes me as ideal as it can bend quite sharply without breaking or irreparable distortion. I was thinking of having it slide in a tube or narrow box, so I can lift for beaching/sailing, as well.

I do see rust as being a problem though. I can of course completely sheath it in plastic soft tube with silicone....but given that it will lift in the narrow box, I may as well save myself the trouble and do as you say and use thin whippy fibreglass.

My boat will be very like Tiny Turnip's: 18' cat for easy cruising, two people driving with various picnic gear. Definitely not a speed machine. However I realise the principles of good efficient design are still very important.

Nick