Surface drive with large diameter propeller

I was originally going to post this on a current thread but decided that the topic/question may be of interest to more people.

Surface drives are typically used in high speed high rpm craft

Large diameter, low rpm propellers are assumed to offer higher thrust and better efficiency and in most cases would require a deep keel for prop clearance.

Ignoring the obvious safety issues of a having a large prop breaking the surface of the water aft of the transom the question is this:

Is it feasible to put a large diameter slow turning propeller on a surface drive with the goal being high thrust at higher numerical fuel efficiencies?
Efficiency being defined as more miles per gallon/ nm per fuel unit


Just happened across this picture but do not have a clue of the application

 

Barry,

Do you have more photos of this set-up?

 

No, I ran across this when I was just looking for picture that I have seen of an unloaded tanker that due to not carrying weight with half of the propeller was out of the water.
Thinking then that perhaps a slow turning, large area/slow turning prop on a surface drive system would be a way of perhaps improving efficiency
This picture may show a high speed prop.

 

Yes, surface piercing props can be designed to be very efficient especially when run at low RPMs, with less cavitation, slippage, etc. Theoretically, the most efficient propeller is a one-bladed propeller, and if a surface piercing prop is lifted High Enough, it essentially becomes just that. There was some experimentation many years back by a chap named Jake Free who used a large diameter 4 ft thin blade in his human power boats, but they never caught on, probably because of the draft required. My guess is the pictured ones are used for pushing a large heavy boat of some kind, at low RPMs to get better fuel efficiency. There may essentially only be one blade that is immersed while under operation because of the lifting Force while under propulsion.

Ps. Here's a link to pictures of some Jake Free creations, scroll down for the surface piercing ones,

Free Enterprises' RESEARCH web page http://www.freeenterprises.net/HPBoats.rsrch.html

 

A few years back I posted a link to Hickman's very first experiment with surface piercing props, in surface props section of forum. He did the work in 1911 and IIRC he was as using around a 25" prop, at 15 mph, and 15hp. On the close order of, anyway. The notion that SSPs are for high speed has no basis in fact. It's like Sea Sleds can't be any good, because they haven't caught on.

 

FWIW, the photo is of MS Tûranor PlanetSolar. Tûranor PlanetSolar - Wikipedia https://en.wikipedia.org/wiki/T%C3%BBranor_PlanetSolar

Yesssss....but there is a lot more going on, just like there is a lot more going on hydrodynamically in that photo than you think. I'll come back to that.

Basically (without getting into the nuances of what happens with a high-speed supercavitating surface piercing chopper props, which this is not) a prop generates thrust by changing the pressure in its swept disk area (surface piercing or not) and absorbs power by the torque required to turn it at a given rpm. For a geometrically fixed blade (pitch, area, and diameter), the change in pressure over the swept area is proportional to rpm and the speed of advance. The faster the vessel goes, the faster the blade has to spin for a given diameter and pitch. The torque, unfortunately, only depends on the rpm and blade shape (pitch, area, and diameter). This means that there is a point where the prop will absorb power without producing any thrust at that speed. To decrease the torque for a given rpm and diameter, you must change the prop geometry i.e. reduce pitch and/or area. Since reducing pitch lowers the speed of zero thrust, all you can do is decrease blade area.

This of course assumes an infinitely strong, infinitely stiff blade. Realistically, there is a material limit of how big in diameter and how thin you can make a blade.

Returning to what else goes on hydrodynamically and mechanically; there are ways to manipulate the inflow to the prop (c.f. above waterline tunnel drives, kort nozzles, and pump jets), hub treatments, tip treatments, CPPs, mechanical blade shape manipulation (used in small thruster motors), the whole issue of thrust pulsing, off center shaft loading and bearing issues, etc...

One last thing, do not equate high efficiency with either high power or high speed.

See the 3rd paragraph of this post... What is world's biggest planning hull boat and how fast? https://www.boatdesign.net/threads/what-is-worlds-biggest-planning-hull-boat-and-how-fast.64476/page-6#post-898807

 

I asked Google if it could 'find' this photo on line, and it did -
Planet Solar Turanor https://www.flickr.com/photos/doverpast/4887937984/in/photostream/

This is one of the two propellers on the solar powered catamaran Planet Solar Turanor.
The boat – Fondation PlanetSolar https://www.planetsolar.swiss/en/world-premiere/boat/

Tûranor PlanetSolar - Wikipedia https://en.wikipedia.org/wiki/T%C3%BBranor_PlanetSolar

 

No it isn't... Tûranor PlanetSolar is 10 knots, 120 kW total.

 

Higher efficiencies than what?

Slow speed boats have low prop efficiencies on paper because of the higher relative slip values of their props. But they burn less fuel going slower than going faster.

As you design for ever slower speeds, almost everything looks better except the paper efficiency of the prop - wetted surface area can be reduced and wave losses can be reduced.

But the trade-offs for going big and slow with the prop can tank the idea pretty quickly.

1. As engines get smaller and less powerful, they rev higher. Which means you have a gearing issue that is quickly getting to be a problem.

2. The inflow perturbations from the seaway-hull interactions don't get any less. If you are running less slip, they become more of a problem because the inflow angles wobble about the average by several degrees.

3. Inflow perturbations are less of a problem as you get deeper, but you aren't getting deeper, you are getting shallower.

4. Engine governors are designed to respond to normal set-ups with normal inertias and small, rapid perturbations which they basically ride through without seeing. When surfing big ocean swells with 8 or 10 second periods, they respond, but most hull responses are too small and short-period for the engine to notice. When you run the gearing way down, you slow the perturbation frequency way down. The bigger prop also transmits stronger pulsations to the engine in spite of the gearing. Bottom line is your engine won't be able to deliver power smoothly and that reduces engine efficiency noticeably. It will always be chasing the governor and hunting around.

This is why props for small, low speed craft are usually sized for 55-60% efficiency on paper. Going bigger than that trying to chase better prop efficiency at reduced slip just creates off-setting losses in prop and shaft friction, greater bearing losses in shafting and transmission, and poorer engine efficiency. And on small systems, the expense of doing this simply can't be recouped.

But surface piercing props are easier to service and keep clean. They may have no shaft or appendage drag at all. And if you have a really slow motor - like a pedal drive - then the gearing issue goes away.

I designed a direct drive 80 rpm pedal drive that had a two-bladed surface piercing prop that was indexed so that the torque ripple of the pedaller matched the torque response of the prop due to buoyancy of the prop blades and depth of immersion.

 

Myself would appreciate any additional information/ links you are willing to release on this project or anything similar..

 

That's pretty sweet, Phil.

 

I only mentioned it because of the picture of Jake Free's trimaran that portacruise linked to. It looks like the same idea. Mine had a sideways seat and the shaft had the pedal cranks on it. I found an extrusion for the smallest helicopter tail rotor and was going to make the prop out of that when life got in the way. I built the boat, and it was about as refined as Jake's stuff - I stretched a 16' canoe by six feet and added an ama over a weekend. I kept all the stuff for about ten years before tossing it. I was trying to baseline human power and electric power options on the same boat.

 

I went back and reread Hickman's article reproduced Professional Boatbuilder. A 20' boat full of snow and leaking badly. A pair of 22" props driven by a single 17hp engine. Did 18-20 mph over a measured mile. Less with three guys on board. 1911. Turning 850 rpm


A pair of 22" props seems big for 17 hp. But that does go some way to answering the question.

 

Hickman's Imagination and the Dawn of Surface-Piercing Props - Professional BoatBuilder Magazine https://www.proboat.com/2019/03/hickmans-imagination-and-the-dawn-of-surface-piercing-props/

 

Here is the designer's site about it: 31m Solar Boat Turanor Planet Solar - Lomocean https://www.lomocean.com/projects/race-for-water

The pic you showed was the first version, when they were aiming for 10-12kn.
If i remember right from a presentation/visit they did in Marseille, they said they were doing quite well at these speeds in calm water. But they swapped that setup out for classic suction props sitting lower - partly because in practice they ended up doing most of the circumnavigation at 5-6 knots, partly because the very ends of these 31m floats just came out of the water too often in a seaway.

BTW, have you seen this thread about Flexitab's attempts to optimize surface drives for displacement vessels 15 years ago ? Surface propeller and low speed displacement boat https://www.boatdesign.net/threads/surface-propeller-and-low-speed-displacement-boat.25284/#post-243723