Correct prop size?

[Peter Hodges]

I have a few questions concerning the correct prop for my oday 28 sailboat. The engine is a 11hp universal. Currently I'm using a Martec 14x10 folding prop and can reach about 5 knots at moderate rpm. I don't have rpm meter so I don't know the rpm. I would like to increase cruising speed if possible via changes to the prop. I don't currently race this boat so speed under sail is not a major consideration. I have a recommendation to change to a 12x12 2 blade fixed since this would be more reliable for cruising under power and would provide about the same speed. I've also been told that a fixed prop can be cupped and that this might help.- is 5 knots about the best I can do with this boat and engine?- how would thrust of 12x12 fixed compare with 14x10 folding?- can the 14x10 folding be pitched; what pitch would you recommend?- what about cupping a 12x12; would that make sense?- other thoughts on what's the best prop for my situation?

 

[RonD]

Couple of thoughts

1. Your boat can generally only reach its displacement hull designed hull-speed unless you apply massive energy to overcome the standing-wave action and "plane". The approximation is:hull speed = 1.34 x SQRT(waterline length in feet)Example: A Catalina 28 has a Lwl = 23' 10" (23.833 ft) Its max displacement hull speed would be 6.54 knots2. The horsepower required to move a boat up to its displacement hull speed is dependent on the the actual displacement (assuming a good low-friction hull shape, which most sailboats have). A rough rule-of-thumb is 5 HP per long-ton (2,240 lb) of displacement. That rough r-o-t has some allowance for various power losses expected through the drive chain, I believe.Example: A Catalina 320 has a displacement of 11,900 lb (5.3 long-tons)Its engine needs about 26.5 HP to move it at hull speed. (A C320 is equipped with a 27 HP diesel engine.)3. Prop dimensions are based on the physical shaft-to-hull space available (prop diameter) and matching the engine to the load (pitch, blade count, blade shape). The engine/load matching (pitch) should account for the max engine RPM and the transmission ratio.Having said all that, its best to contact the manufacturer (or engine dealer) and tell them the boat model, engine model, and transmission model & ratio. They can suggest an optimum prop size for your needs, or comment on the likely effects of the switch you mentioned. As to getting past 5 knots to displacement hull speed, make sure your boat has sufficient HP for the displacement at max RPM. Then make sure the prop is sized for running at that RPM. --Ron

 

[Steve Dion]

Cupping and winglets.

Peter:Cupping is generally thought of as something for power boats. There is a guy in California that does some tricks with props that is suppose to be the cats ass. He cups the blades and makes winglets on the prop tips. This is suppose to give you more thrust and improves backing ability too.His name is Bob Kilian at Bay Props (510)337-9122. Talk to him about what you have. He will need to engine model/hp and transmission ratio too. I think that he needs to know about the displacement of the boat too.If you can find a used 2 bladed prop around he charges about $135 to fix it up for you. Otherwise you are looking at the price of a new prop plus the service.

 

[Kevin]

prop

Dear Peter,Thanks for the note. A properly sized propeller will limit the engines capabilities of obtaining Rpm’s greater then the rated amount for the engine. To adjust and to size a propeller to a system that does not utilize a tack is a guess at best. One thing that we need to keep in mind is that the propeller is only as strong as the engine driving it. If you have a great deal of running hours, poor fuel or items attached to the motor, these will heavily influence the amount of torque being distributed to the propeller. The more items robbing power the less torque being supplied to the prop. This is why it’s difficult to say if the achieved 5 knots is in fact slow or fast for your particular boat and combination of gear. On average I would say that 5 knots is good, not excellent. I hear of Swan 38’s and 40’s that motor between 5.75 and 6.2 knots so speed is not always related to the size of the vessel.In my opinion I would not use a 12 x 12 two blade with a cup. The 12 x 12 would most likely not distribute the amount of thrust your folder does currently. I would also think that the 12” would also have a problem when you are confronted with heavy winds or seas. The cupping you are talking about is a method of absorbing HP and producing load without adding pitch to the propeller. I would not believe it would increase your speed drastically. You current folder is not able to have its pitch changed unless you buy and fit new blades with a greater pitch.I think you should first look into determining how the engine is reacting to the current prop by using a hand held tack and then make your adjustments from that if they are needed. Hope this helps. Kevin