Props

Steve O. · Jan 24, 2007

Depends

A prop from a inboard power boat with the same size shaft would probably fiton a sailboat with inboard. A splined prop from an I/O or OB ain't gonna fit. Some sailboats have sail drive, and a prop from a small I/O or OB might fit. So there ya go--it all depends.

tom · Jan 24, 2007

Large Prop Slow RPM's

In general a displacement boat type of prop will be larger and spin at lower RPMs than a planning type of boat prop. Sailboats have relatively low horsepower and slow speeds through the water. Even a small powerboat has a lot more horsepower than a large sailboat. Just look at your average bass boat with a 100+ hp and I know of no sailboat with a 100hp engine. A large slower moving prop is more efficent with low hp and slow speeds. The pitch is different also. But remember it isn't sail or power it's displacement or planning types of hulls.

Robert Gainer · Jan 24, 2007

There is quite a bit of difference

There is quite a bit of difference between what the two propellers need to do. A sailboat propeller needs a narrow blade so it doesn’t have a lot of resistance when you are sailing instead of motoring. It also turns more slowly so the profile of the blade is a different shape then a powerboat propeller with the amount of twist from the hub to the tip being different in both types. The relative slip is very different between the two types so the blade shape and pitch start out differently. A motor sailor might have a propeller that looks somewhat similar to a powerboats propeller but even that will have a different amount of rake in the blades and a different blade profile. Its best to use a propeller designed for the type of boat it’s on instead of putting truck tires onto a Volkswagen.All the best,Robert Gainer

Ross · Jan 24, 2007

Dave Gerr NA covers the subject of propellors

extensively in his "Propeller Book". Anyone interested in learning the fine points of the subject will benefit from reading this. The subject is far too complex to treat here in any but expressed opinions.

Robert Gainer · Jan 25, 2007

Manny and EG,

You both have what the propeller manufacturer calls a powerboat propeller. Take a look at www.miwheel.com/MIWheel/products/inboard/mich_prop.htm to see the difference between types. Putting a powerboat propeller on a sailboat is becoming popular and in the future I can see a change in marketing and they might call it a “POWER sailing propeller” but the fact remains the propeller you have was designed for a powerboat and has optimum performance under power rather then minimum drag under sail.Diameter is the single greatest factor in absorbing horsepower in a propeller. If you can’t get enough diameter then you go to a three blade design which is not as desirable on a sailboat as a two blade. If you can’t get enough surface area with three blades then you go to the propeller you have installed. There is a limit as to how much horsepower can be transferred to the water with a given surface area of propeller. With your engine, transmission, available diameter for the propeller etc you have might be the best compromise while giving you good performance under power. The sad fact is you are sailing with a great drag and it hurts sailing performance. The question about which propeller is best for a boat is like asking if a cutter is better then a yawl and it all depends on what YOU want out of a boat. I enjoy sailing and having a powerboat propeller with its added drag is unacceptable to me but if you want to maximize the powering experience then by all means use a powerboat propeller.All the best,Robert Gainer

tom · Jan 25, 2007

When You Start the Motor

You are a powerboat..displacement type of powerboat. A prop's blade can be considered a sail. A long thin blade is more efficent than a short fat blade. A sail can be considerd a wing. You may have noticed that glider's have extremely long wings as they are the most efficent. But short fat wings work if you have the horsepower. Drag is more of a function of surface area if the prop isn't turning. My biggest concern with a feathering or other mechanical prop is marine growth. Get some barnacles or oysters in the wrong place and they probably won't work well. Unless you are racing it's probably not worth the extra maintenance. In freshwater they might be a good thing.

Ross · Jan 25, 2007

If you are greatly concerned about the added

drag caused by a propeller with more blade area and if you can measure your speed to two decimal places, then determine the increase in blade area and find a drogue with that same area and while sailing carefully note your speed without the drogue and then with the drogue. If you find that an extra twenty-five square inches of blade area has an adverse effect on your sailing performance and you can't deal with it then you will have saved the price of the new prop. On the other hand if the effect on your sailing is tolerable and the improvement in motoring is significant then go with a new prop. As robert said it is all a compromise. Propellers are only made for power boats! some of them have narrow blades for reduced drag when the vessel becomes a sail boat, some of them have broad blades for greater thrust at low speed, some look like the blades were hacked off in the middle to reduce cavitation on very fast boats. But nobody puts a propeller on a boat the doesn't have power.

John · Jan 25, 2007

For Echo Gulf-All the Catalina 30's in my yard including my boat have a narrow fixed two blade as the original prop. I don't see any with a fixed 3 blade prop. I also have an M25 on a Catalina 30 with a Hirth transmission. Are you certain your 3 blade is original?

Shippy · Jan 26, 2007

Manny, it was me

I'm the one that reported I had a prop specifically made for a powerboat versus a sailboat. I've added the lick below where we verified the prop (dynajet). It also explains the differences between the sailboat props and powerboat props. The "issue" I was having was my RPMS were too high at open throttle versus what the engine was reated and I could barely maintain 4.5 knots even with a clean hull while under power. Also, when I did open up the throttle, the stern would sink down. Again I thought I just had a pitch problem, turns out the wrong type of prop. I elected to get a new prop and figured I'd go the feathering route with a max prop.

Echo Gulf · Jan 26, 2007

Yes John,

it is a factory installed prop (which would make it a sailboat prop). If you were to investigate further you would find that a lot of C30s could be had with either the three or two bladed prop. In our area with numerous islands and narrow passes, currents of up to 6 knots are common, the added power of the three bladed prop is almost a safety issue. As for the additional drag, sailing in company with other c30s with the two bladed prop, we've never noticed any differance in speed or performance and since we are not in training for the "America's Cup" I feel any possible differance to be a non-issue. I understand that there are other "more efficient" props available ie: The Campbel Sailor Prop, but, for the cost of a possible miniscule gain in speed, I could acquire other instruments or equipment that would greatly increase our cruising comfort. The question should be, "does the prop meet the need and is it compatable with the motor/transmition", not "is it a sailboat or powerboat prop".EG

chuckwayne · Jan 26, 2007

Give it a try!

we love our autoprop, I'm surprised you got a runaround from them-they've always been very helpful to us.If your shaft and strut haven't changed, it should physically fit. check the cruise and peak rpm range for the new engine vs the old, and compare the reduction ratios of the transmissions. the autoprop self adjusts for optimum pitch for the load, so as long as the shaft speed is in the same range you should be fine.We get better performance under sail and power, same speed at lower rpms, and no prop walk in reverse.make sure it's well greased and you install the zinc tailcone.

Props

.

Steve O.

tom

.

.

.

tom

.

John

.

.

.

.