Two blades or three?

[Tom Spohn]

I have had both a 2 blade folder (Martec) and a three blade folder (flex-0-fold). I have a 35' boat that weighs 10K lbs. and a 29 hp motor. They both moved the boat at hull speed, but the 2 blade vibrated more. The water flow was in pulses which impinged on the spade rudder making steering tiring when hand steering. Since the prop was worn, I can't be sure if it was the design or the state of repair. The 3 blade is much smoother and is better powering into a current or high head wind. Both did a good job in reverse, with the edge to the three blade.

[Alan Wheeler]

Will I really take a significant hit in sailing performance with a 3-blade feathering prop over a 2-blade?

That really is down to one thing. How well the manufacturer has desinged and built his prop. It depends on how it folds up and allows water to flow over it with as little drag as possible. Certainly the poorest folding is always going to be better than the best fixed.
As for the difference in performace of two or three blade. At full RPM or at least at high RPM, there should be little difference. Yes there WILL BE a difference, but not so promounced as at lower RPM. Lower RPM is where a three blade works better, simply because it has more surface area pulling it through the water. It has three wings, not two. Simple really when you think about it. But once a prop starts getting up in speed, the slip becomes far less. Infact, a three blade can become a problem in high reves. So if you have an engine that does 4K RPM, then a two blade is better suited. 2K RPM, a 3 blade is better suited. HOWEVER, that doesn't mean you HAVE to use only a three or a two at any set RPM and in all situations. Diesels get no where near fast enough to worry about "blade intrusion" or where the turbulence of a blade washes across the next blade. In race boats, it is the number one reason why they use 2 blade. We don't have that issue, so the number one issue is the folding and presenting the least drag in the water, the "bite" one needs to control his boat and of course, Cost.

[cooper]

On my 30 footer l have a folding prop (2 blade) of the ancient kind, that is un-geared. To be blunt it dont go backwards.....l have found that from time to time one of the blades gets fouled in position and l have to dive and clean it off. There is a distinct "Thhrumming" sound which is created by the slack water dead behind the keel as the blades line up with it. given that the engine vibrates all over the place (single cylinder) l have not worried about this lesser vibration. l do think that it may have a bearing on the life of your bearings. l would love to try a 3 blade fixed prop to compare both the under power performance and see what reduction in speed under sail. Can anyone quantify an actual change in terms of boat speed from a 2 to 3 , or visa versa ?

[Heron]

I don't know about the comment that there is "slippage" on the three blade prop if you up the RPMs. I know on my vessel, the max RPM on my Yanmar is 3800. For every increase in engine RPM, my boat travels a bit faster. After I have exceeded her maximum hull speed, the boat continues to increase in speed right to the bitter end of the RPM. I have seen my boat do 7.9 knots at max RPM with my MAX prop. I don't think there is any slippage going on.

The last comment I will make is that the folding props still end up with a ball of metal that the water needs to pass through which provides turbulence and drag. The MAX prop merely presents her blades in the most aerodynamic way to the water pressure and angle of lean of the vessel. Lastly, there is no lound "clunk" when going from forward to reverse as you would experience in the folding prop. Also, the thrust in reverse is as dynamic as forward motion. I have never tried to measure hull speed backwards but I know she pulls the boat with authority.

HERON

[fstbttms]

Originally Posted by Weyalan

Will I really take a significant hit in sailing performance with a 3-blade feathering prop over a 2-blade?

I have no insight to give you other than this: The owner of PYI Inc., (the U.S. distributor of Max Prop) has done the Pacific Cup several times. He typically brings his 45' cruiser/racer down from Seattle with a 3-blade Max Prop on the boat, whereby I swap it out for a 2-blade Max Prop for the race. There is enough difference in sailing performance for him to feel that this is important to do.

[Alan Wheeler]

"Thhrumming" sound which is created by the slack water dead behind the keel as the blades line up with it.

That noise is caused by a shockwave. Water can not be compressed. As the blade comes past a solid unmovable object, the water being displaced around the blade tries to be compressed between the blade and the keel for instance. As it can not be compressed, that energy is transmitted into the hull as a thump. The closer the two are together, the louder or harder that thump. It is why a set of minimum distance rules for desing exist.

Can anyone quantify an actual change in terms of boat speed from a 2 to 3 , or visa versa ?

Not easily. Firstly hull speed is hull speed. Once you reach it, that's it. unless the hull is light enough and flat enough that it can be easily pushed into planning and you have enough Hp to do so, coupled with the fact that you are most likely over proped if you can do so anyway. The engine should be matched to the prop as such, that when you reach full RPM, the boat is at approx hull speed. (Unless you own a McGregor)
All propellors have a certain amount of "slip" in the water. This is because water is a fluid. But the faster a propellor spins, the more resistance the water has to being moved and the more efficient the propellor is at coupling to it. There will always be a certain amount of loss of energy due to the water having movement within its'self. It's like us trying to run up a sand dune. The sand keeps moving away under our feet. However, becasue a 3 blade has more surface area, or in other words, it can couple to a greater area of water, it can work against a greater mass of water and movement of mass equals movement of boat. (for every action there is an equal and opposite reaction) So simply looked at, at lower RPM a two blade is simply poor at coupling to the water mass. A three blade is better. Byut as boat speed and RPm increases, the water have a greater fluid resistance, offers its'self as move of a solid and the propellor is able to couiple more energy into the water. So the difference between two and three blade at speed is not so large. YES there is a difference because we are not creating RPM's associated with racing boats, but the losses are not as pronounced as at idle speeds.

I have purposely not gone into greater details. This is both helpful and not so helpful. But the reason is that there is no hard and fast rule. That's why so many propellors exist. And it is why most manufacturers say, try these and see which is best. The manufacturers that have been around awhile, usually build up a data base and can suggest which is the best choice. But the science is such that no one propellor is perfect. Each has advantages and disadvantages and the final choice comes down to what the boat owner wants the most from his boat.

[Weyalan]

My venerable 2-blade feathering prop would not, I think, push the boat to hull speed (31'6" LWL = 7.5kn) even in dead flat water with zero breeze. Whether this is becasue the propr is undersize or nmerely worn is moot. I generally motor at about 2100 revs, and although the engine will rev a lot higher, the prop starts to cavitate at about 2400, and the boat doesn't go any faster.

Frankly, against current & wind, with a bit of chop, I think I would really struggle to motor off a lee shore, which is why I want a better prop. If I could be convinced that a 2-blade feathering would do the job, I might go with that, but it seems (in a "gut feel" sense) that 3-blade feathering is the way to go.

[cooper]

Alan ...so l guess its yet another of the classic "boat Compromises"Weyalan...you have my total sympathies. l can get to just under 3 knots flat knacker..no wind, no tide, We do not do lee shores !! Unfortunately in my case re-proping 40 year old 7hp single is not worth it and seeing how l am building another boat, l think that this "method of improving ones sailing ability" will be passed on to the new owner.

[Alan Wheeler]

Older folding props did have a lot to be desired. Unfortunetly old folding designs are still being sold as new. The difficulty for the ones like Weyalan is finding the wheat from the chaff. Certainly a prop should never cavitate. Sadly some do. But I would also suggest that Weyalan, your prop is either very badly designed or very badly worn. In theory, a 2 blade folding should work close to a standard 2 blade. In theory, a 2 blade should also be close to a 3 blade. Not the same, but close and of course, in theory.
Coop's, sadly you can't do a heck of a lot with 7Hp in the tonnage you have. The blade is most likely not the issue I would imagine. But what I am not sure of, is how do you stop that weight when coming into a birth. I could imagine running to the bow and using mind power to will the boat to stop would have more affect than 7Hp in that boat.

[cooper]

Alan you are absolutely right.......she dont go backwards,......the anwser to the questions how do you come in is ...SLOWLY. Always head to wind or stream. As l sail by myself a lot l also make sure l have all lines to hand , even to the point of bringing a bow line back to the cockpit so l can stick on a springer and then leap out run forwards and tame the bow before it decides to head off up the channel (or were ever). l have a bit of a reputation of being one of the gentlest boats coming along side in my patch ......(lm not going to tell them its necessity)

[Boracay]

That's not a 7hp Yanmar that you have?

Sorry for taking so long to jump back in here. I am traveling, and work takes all my waking hours these days. To clarify, I am talking about folding, not feathering props. Great responses, and, ultimately, I am now sold on the two blade for my application. I suspect there will be some pulsing, but the prop shaft is supported by a strut, so it shouldn't be too bad. As for speed, I was referring to speed under power. I have heard guesses of 1-2kts difference under sail with folding vs fixed, hence my choice to go with folding, but that decision of 2 blade or 3 is stricktly about benefit vs compromise under power. Thanks all for the great input.