Propeller Considerations for a shaft-driven Alerion Express 28
November 16, 2015
From the log of Gene Knapp’s Keuka
I purchased a 1994 Alerion Express 28, Hull #31, two years ago and sail the boat on Lake Michigan out of Montrose Harbor in Chicago. The previous owner took excellent care of her and I have done my best to continue to maintain the boat in Bristol fashion. One improvement I was contemplating is to switch from the 12x9, RH Martec folding prop which is currently mounted on the ¾” shaft drive Yanmar 1GM10, 9HP diesel engine and replace it with a feathering propeller.
My experience under power has been poor. The engine will only come up to 2600-2800 rpm and, when pushed, there can be a lot of smoke. Top end motoring speed is 4.7 to 5.0 knots. When I purchased the boat I noted the pin on the Martec prop was worn and the blades were quite loose so I replaced the pin and re-bedded the strut, but the blades continue to be pretty “floppy” and there is no question the prop has experienced significant wear (20+ years will do that!). As to reverse . . . let’s just say it can be a “Spiritual Experience”.
When reading the Chesapeake Alerion Log online yesterday, I noted there was mention made that several boats in the San Francisco Bay fleet with similar propulsion gear switched from Martec folding props to Max Prop feathering props. I was interested in knowing what size 2 or 3 blade prop they switched to and how things have worked out using the new feathering prop?
The Chesapeake Alerion contact put me in touch with Ralf Morgan, a member of the SF Bay fleet and a contributor to the original log entry. He promptly replied with the following observations:
The size of the Martec prop, 12x9, is probably the cause of my motoring problems. The pitch may be too low.
A prop with too large a pitch can cause similar problems. When he first got his Alerion, it had a Martec 12x12 folding prop. The engine would not come up to rpm and would smoke quite a bit. The local Yanmar/prop guru suggested reducing the pitch to 12x11. He ordered new blades from Martec, put them on and solved all the problems. Good speed under power, engine comfortable at 2800 rpm and no more smoke stains on the transom. Engine rpm can now go up to its designed 3200 rpm maximum.
With the 1GM10, there is a real danger in putting on too much prop. While the feathering props may be a little better in reverse, they tend to have significantly more drag under sail and, given the limited horsepower of the 1GM10, don’t add much, if anything to forward motoring while adding a fair bit to engine load.
Unless I spend a lot of time in reverse, I might want to stay with a Martec feathering prop but change the pitch to 12x11.
Also, on the Martec props, eventually the pins and blades do get loose and it pays to send the prop back to Martec to have them recondition and rebalance it.
From the experiences Ralf related, I plan to purchase a new 12x11 RH Martec folding prop and see what difference this makes in achieving better RPM performance and improved forward speed. After some time with it next season, I’ll update this log entry with my observations and conclusions.