Propellor for Cape Dory 27

[James Scotney]

Would anyone have the original propeller for a CD 27 using a Yanmar YSB 8. Mine was changed for a 2 blade at some point and I now go backwards in any more than 20 knts of wind on the nose! Yours James Scotney British Cape Dory Owner in the Bahamas.

[Ron Churgin]

Hi,

I have a CD27 which dates from 79. I have a YSM 8 engine. My boat has a two blade prop which I am guessing is the original (cannot be absolutely positive).

I get good performance out of the two blade prop, often fighting 3 knot currents going out of Jones Inlet on Long Island. The prop I have is about as large a diameter as one could fit in the available opening.

Don't know if this is any help, but I think I am getting as much power as the 8hp engine can provide with the two blade. I have heard other owners who prefer a three blade.

[Tod M]

James, given Ron's reply, my first inclination would be to check the prop to make sure it is free of barnacles, etc....

[John Vigor]

James, if you want to change to a three-bladed propeller, I would recommend a 12-inch diameter by 9-inch pitch as a starting point.

If you can't get top engine revs with this prop, a propeller shop can easily re-pitch it an inch or two less.

But I wouldn't expect to make much headway with such limited horsepower against a 20-knot breeze, if I were you, especially if there's a decent chop.

Cheers,

John Vigor

[Ron Churgin]

John,


What improvements would a three bladed propeller offer on the underpowered 27? Would there be more thrust? Greater speed? Is there a trade off with more drag than a two bladed prop?

I am not familiar at all with this area and would appreciate your expertise.

Thanks

[Bob Owens]

James,
My CD 27, with a YSB 8, has what I am certain is the original two-blade prop. When I replaced the shaft a couple of years, the very fine Yanmar specialists who did the work (Precision Marine in New Rochelle, NY) strongly advised against changing the prop. I have always been pleased with its power. When it and the boat bottom are both clean, it gives me plenty of push.
Bob

[John Vigor]

John,

What improvements would a three bladed propeller offer on the underpowered 27? Would there be more thrust? Greater speed? Is there a trade off with more drag than a two bladed prop?

I am not familiar at all with this area and would appreciate your expertise.

Thanks

Ron, in theory a two-bladed prop is the best bet for a sailboat. Each blade is working in water less disturbed than the blades of a multi-bladed prop, and a two-bladed prop offers less drag when you're sailing, especially if you line it up vertically behind the sternpost.

But the diameter of a prop is often limited (as it is in the case of the CD27) by the angle of the prop shaft and the size of the aperture in the rudder. There might be times when a two-bladed prop does not present enough surface area to the water to absorb efficiently all the power the engine can produce. In that case, a three-bladed prop will provide the necessary blade area without increasing the prop diameter. (There should always be a clearance of at least 10 percent of the prop's diameter between the tips of the prop and the hull.)

But the most compelling reason for making a change is that, to quote naval architect Dave Gerr, the standard three-bladed prop "has proven to be the best compromise between balance, area, and efficiency."

Another reason is that three blades are smoother than two. They reduce vibration. "Every time the blades of your prop pass under your hull, or by the strut, they cause a change is pressure that generates a push," according to Gerr in his book The Nature of Boats. "If the push is strong enough, you get a bang. Lots of bangs--vibration." The more rapid the cycles, the smoother the feel--and the less likely your hull is to resonate with the vibration.

I would not expect a three-bladed prop to give you more thrust, unless your two-blader is sadly under-pitched or lacking in diameter. The thrust available (and therefore speed through the water) is determined by the engine power available, and the big trick is always to match the propeller with the engine. It doesn't matter whether you have two blades or three, if often takes a lot of experimenting to find exactly the right pitch. Even yacht designers find it difficult to hit on the right combination on launch day, and have to conduct sea trials for new designs.

I wouldn't change from two blades to three on a CD27 if there was no problem with vibration. You can swing a 12-inch diameter prop on that boat, which should be plenty. It is simply a question of experimenting to find the right blade area and pitch. And pitch, as I've said before, can be altered relatively cheaply by any good prop shop, within limits.

Just find the pitch that will allow the engine to reach full advertised rpm at full throttle, or maybe 100 rpm less. Too little pitch, and the engine will reach full speed too easily, and boat speed will suffer. Too much pitch and the engine will labor. It will not reach its top rpm and the exhaust will belch black smoke. Not good.

There must be plenty of CD27s with YSB 8s who are getting good results with two-bladed props. What is the pitch you guys are using?

Cheers,

John Vigor

[VegaIII]

Vega III is 1978 CD-27 (#109) that I have owned since 1993. I do a lot of cruising in coastal New England and at times run into some fairly difficult sea/wind conditions. I had several experiences in such conditions in which I felt the the boat was seriously under-powered. One such experience occurred while coming out of the Cape Cod canal with the tide but against a strong SW wind. Those who have had this experience will know what a difficult experience this can become. We started losing headway and were completely unable to keep the bow in the wind--the engine finally simply lost compression and died. It was a memorable few minutes as we ended up having to sail out of the channel to the nearest harbor of refuge at Onset.

This experience along with the need to undertake some other major maintenance chores on the engine led me to consider re-powering.

Since I love the boat so much I decided to try and re-power her with a Yanmar 2GM-F and add a three-bladed prop. It was a big job fitting the larger engine in the engine compartment and getting the prop pitched correctly.

It has made an enormous difference in terms of my ability to handle almost any sea/wind/current conditions that come along. The added benefit is that the vibration from this engine/prop combination is vastly reduced, while one can achieve hull speed at 2000 RPM without difficulty in most wind conditions. However, when needed there is plenty of reserve power.

I can only guess at fuel consumption but my guess is that it is about 1.5 liters per hour at 2000 rpm.

[Ken Textor]

My experience in going from a two-blade prop to a three-blade (12x7) confirms John Vigor's technical discussion of the switch. I might also add that the three-blade makes operations in reverse much more certain. In flat seas, my YSM-8 will push the boat forward at four knots into a 15-knot breeze. (I only do this to go the last quarter mile or so to my mooring.) In a calm, it pushes the boat (lightly loaded) at five knots at 2600 RPMs. To me, if you can afford the new prop, this is a no-brainer: Go three-blade.

Ken Textor, s/v Marie Rose (CD 27 hull #139)
Arrowsic, ME
________
Yzf-r6

[James Scotney]

Many thanks to all of you who responded to this question. I think my 2 blade is short of diameter she seems to rev far too easily. As I'm not with the boat at present I'm not sure by how much ( 10 1/2 inch rings a bell ) I think a 12 x 6, 2 or 3 blade is the way to go. Does anyone have one kicking around? I don't expect her to become a greyhound but a little performance directly to windward would make me less nervous!