Alternative propulsion

[JeffM]

I've done a bit of reading on recent research into alternative propulsion. Seems the lowliest fish is WAY more efficient swimmer than the most sophisticated prop-driven boat. Seems that a lot of engine power is spent creating vortices (whirlpools) at prop edges that stir up the water and waste energy -- doing nothing to drive the boat forward. Part of the answer is blade area: the bigger the blade, the smaller (proportionately) the length of the edge. Hobie Cat's "Mirage Drive" takes advantage of this in their kayaks: two large blades pivot back and forth under the hull, twisting at each push of the pedals so that they drive the boat forward on every stroke. (Yes, kayakers pedal the things!) Put another way, it is more efficient to move more water slowly than to move a little water fast. For sheer power, Hobie Cat says that this propulsion beat another, two-man, paddled kayak in a tug of war. It is also about 10% more efficient than paddle propulsion (=going a bit farther at the same speed without getting more tired).

I wonder how hard it would be to adapt a Byrnes hull to this drive? I expect a critical question would be the depth of trunk it could work in (A kayak doesn't draw much, so the pedals aren't far above the blades.) I wonder how fussy it would be to maintain in a salt water boat? If it were practical, I wonder if it would ever be worth the cost? (The drive alone sells for about $400.) And, of course, my Core Sound 20 is much more boat than a one-man kayak!

The mirage drive is only one step along the way to something more radical: fish are far more efficient still -- something about creating vortices at one stroke, then recovering their energy by "pushing off" the same vortices on the return stroke...

Back to the practical present: at the very least, it would seem that oars with big blades moved slowly are more efficent than smaller blades move more briskly.

Ideas? Experiences?

[Frank Hagan]

Sculling takes advantage of this same kind of action; using a single oar off the transom of the boat. But the sculling oar does come up out of the water on the return stroke.

Closer to the idea is the Scully Fin, as seen at http://www.scully-fin.com/, which is attached to the rudder of inflatable dinghies to allow some propulsion:

Its a flexible plastic fin. Not too far off how I have "pumped" my rudder to make it through a tack in light airs in my Weekender. Being able to raise and lower something like this on a sailboat would do away with the disadvantage of increasing drag when you are sailing.

I remember steering our soap box derby cars by putting pedals on the front axle; a similar motion might be able to pull something like a Scully-Fin from right to left without having to conjure up a bicycle sprocket and chain type of affair.

BTW - I find the Scully-Fin site kind of funny. Inflatables are notoriously hard to row, so the Scully company also makes a centerboard you mount on the bow :!: and a sailing rig :!: :!: Practical Sailor reviewed it in their December 2004 issue and came away with mixed feelings, but mainly because of the inflatable dinghy's built in disadvantages when rowing.

[JeffM]

Frank, thanks enormously -- this is very near to what I was looking for! I wonder how their "scully fin" would work on something as big as a CS20? Their "retrofin" is clever, too, but I couldn't see spending half your effort pushing water SIDEWAYS with the submerged rudder blade. Maybe something mounted to the rudder cheeks that could be used when the rudder is up? But, then I don't think the CS rudder actually ever clears the water... Anyway, I'd love to experiment with it, but at $250 a pop, I might wait.

[Brent]

Jeff, do a www search on the word 'yuloh" and see if this inspires. The prototype I made pushes the boat with a very sustainable amount of human effort. However, the yuloh ends up being problematic to store on board due to ite length.

[JeffM]

Thanks, Brent. It's already on my list. I'm not sure I understand how the thing is moved to produce thrust. Anyway, oar/yuloh length is one of my problems with the whole business. I just read Michalak's new book ("Boat Building for Beginners" or something close) where he comments that any boat he's ever rowed would be well-served with 7 foot oars -- but with significant freeboard and 6' of beam, I think the geometry won't work. On the other hand, if I use his style of oarlocks (metal loop secured on a heavy pin secured in the socket), maybe longer oars could be conveniently stowed on the side decks by securing only the blades on the foredeck.

Even so, I wish I could get someone to try out scully fins on something as big as a CS20...

An outfit called Pacific Tailboat has a nifty design, too, though too ungainly and impractical to be alternative propulsion: a vertical "wing" on a long stern boom sweeps back and forth -- "flying" through the water -- and lifting the boat forward.

[PAR]

There are still engines that don't require the prop to spin the water to a froth. Unfortunatly, slow turning, heavy reciprocating mass engines are being replaced with low internal mass, high revving engines that spin smaller props at higher RPM. These engines get better economy, are lighter, smaller and have many more parts, but reliability isn't an issue so much any more. On the down side, these fast spinning engines generally can't be started without electricity are much more difficult to repair and have a lot more that can go wrong with them.

The most efficient prop has a single blade, but does have some engineering and balance issues to address. As blades are added, efficiency goes down a little, but a happy medium can be had with 2 or 3 blades.

I think a sculling oar is a very easy way to move a boat with reasonable effort. I use to scull a Crocker Stone Horse around, because I was to lazy to fix the engine. I think the best use of man power would be turning a crank driven generator so you can charge batteries or run a trolling motor. The crank needs not be turned very quickly and with reasonable reduction gears doesn't have to be a lot of effort.

Anyone seen those shake it and use it flashlights they've been selling on the toob? How about a real big version, using wave action or the bobbing motion while on a mooring or underway to charge up battery driven motors?