I just got finished reading and excellent article in Professional Boatbuilder about propulsion efficiency. It involved a very comprehensive study in Sweden utilizing a Malo 46 sailboat that had a 2.2 liter turbocharged Volvo D2-75. For the test they used a total of eight different propellers, a torque & RPM meter, a fuel flow meter, a fuel cooler for constant fuel temperature and a custom data logger that included a GPS link to the nav station and an accelerometer which recorded the boats motion. They even disconnected the alternators for more consistent loading.
The objective was to perform in-the-water testing at engine speeds in 200 RPM increments. The props included:
1) Volvo Pentas newest 22" x 18" 4-blade folding
2) Bruntons 22.3" Autoprop
3) Bruntons 23.2" Autoprop
4) Gon 22" x 17" 3-blade folding which the blades rotate to two different pitches, normal & overdrive
5) MaxProp 23" 4-blade feathering propeller with adjustable pitch set to 20", 22" & 24"
6) Varifold 22" x 15" 4-blade folding
7) Flex-O-Fold 22" x 17" 3-blade folding
8) Conventional fixed 19" x 14"
The article is loaded with graphs and data. This test was very comprehensive with very little left to chance. They even calculated every imaginable parameter of the Malo 46 right down to the diameter of the rigging and even the total length of the rigging.
The conclusions were striking. Fuel efficiency fell dramatically as hull speed increased regardless of propeller. For instance:
At 6 knots fuel consumption was 0.6 gal/hr. At 7 knots it was 1.0 gal/hr. At 8 knots it was 2.0 gal/hrs. At 8.6 knots it was 4.3 gal/hr.
To put this into financial perspective, at $3 a gallon, the last 2 knots cost around $8 per hour, or $4 per mile, whereas the first 6.5 knots cost 35 cents per mile.
They also ran a performance test with a mildly barnacle fouled propeller and then the same propeller polished. The results were similarly shocking. At any given speed, the fuel consumption was approximately 50% higher with the barnacled prop, while for any given level of fuel consumption the speed fell anywhere from half a knot to a full knot.
The conclusion they came to was for those interested in achieving higher fuel economies, keep the propeller clean and slow down a knot or two. I was suppressed that it didn't seem to matter on the design of the prop, pitch or even the size.
Dale Tanski
I am not suprised by the difference in fuel consumption as related to speed. As the boat speed comes up, more and more energy is burned up trying to push the bow wave out of the way. The Stars and Stripes syndicate spent a lot of time and money on the drawing board, in Cray Research's computers, and in the test tank trying to either suppress the bow wave or produce a more spread out waveform on the model that later became Stars and Stripes 87. They eventually came up with something that is still secret. Apparently it works, at least somewhat.
As for the propulsion test, It is suprising that there didn't seem to be much difference between the different props.
Indeed it is somewhat illogical that the type of prop did not have a direct effect to the miles per gallon at a specific speed. I would guess and it would indicate that all of the props reached their design efficiency sooner or latter. That being said, they were analysing fuel consumption at the maximum hull speed at a given RPM that each of the props would provide. The laws of physics are nonnegotiable and the amount of BTU's in a gallon of diesel fuel is a given therefore the amount of work output is limited. More work requires more energy and the faster one tries to drive a displacement hull the bigger the hole the boat digs. In our case for every boat length we must move 18,000 pounds of water one way or the other.
What they didn't mention was if certain props accelerated the boat faster, plowed through a head sea better or backed down with more control. I can be certain that if that were the case, energy is transfered into motion and the money meter is running. I think where you and I are coming from is dollars per mile are only part of the equation, clawing off of a lee shore in 40 knots of line squall when your anchor starts dragging or stopping 6" too long at the dock trumps a greenback.
Dale
Seems like common sense to me.
We are running the conventional 3 blade fixed pitch prop. Our other boat had the fixed 2 blade. Several times coming into the slip with the other boat I thought we were not going to get stopped. We never did hit the bulkhead, but I sure thought we were going to. The 3 blade backs down MUCH better. Unless the Westerbeast quits, we are going to get stopped with plenty of room.
We also have a Martec Elliptec 2 blade folding prop. I pulled it off the boat before we first launched. A little research revealed that to get any significant reverse thrust out of it you have to zing it up to 2,000 rpm. We have a 3,000 rpm engine and a 2:1 reduction- 2,000 rpm prop speed is not attainable. I'm real leery of that thing for that reason. I may someday decide to try it, but given a choice, I would prefer a MaxProp.
Just ran across a discussion of the Kiwiprop (http://www.kiwiprops.co.nz/) from New Zealand. I've been happy with the performance of my three bladed prop but would certainly appreciate the added performance (and quiet) under sail of a feathering prop. The price is attractive and the bronze prop could be kept as a spare.