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I doubt naming the prop will help since there was only one brief response to my query here for any experience with the prop sometime ago (Search on Slipstream prop) . I did not describe the specifics since the theory of what causes a prop/boat to act like this is generic in my opinion. However here are the details:
It is a relatively new design, "Slipstream" folding 2 blade prop made in Australia. the boat is a 1987 Ericson 34 shoal fin keel, exposed rudder with a the prop a large distance from the keel and rudder. (photos are available but will confirm this) It is a 15x9 prop. Engine is a Universal M35 4 cyl 30 hp engine 1.8:1 reduction. Mounted on the center line. Rudder is perpendicular and is a balanced spade rudder. Boat goes straight under sail with no wheel pull.
Former prop was an older model 3 blade feathering prop. I sold it since I would stay with the better performance of this prop even with the problem, but I am not certain if a prospective buyer would agree. The new prop gets 2950 of 3200 rpms. no smoke, no overheating.
I now get 6 kts at 1800-1900 rpms, with a noticable pull on the wheel to the left (with the boat wanting to go left). At 6.5 at 2300 rpms the pull is very considerable.... In either case if I let go, the boat goes quickly to the left in a tight circle - hardly a safe situation. These rpms/speed are about 1/4 to 1/2 knot faster than before
BUT the powering performance is truly superb - I would not trade it.. and of course when racing the folding prop is as expected. There is no noticable prop walk in reverse and the stopping ability is quite sufficient. In general a very superior folding prop.... except for this one problem. The manufacture has been very helpful but has not run into this before after many sales worldwide... although the problem in general is not unheard of. He says various subtle factors can be working... placement of the zinc, distance of the prop from the boat fore and aft, etc.
So he is supplying relpacement 15/8 blades (just have to replace the blades, not the core) which will get the rpms up to 3200 max and hopefully will help. But it requires a haul out because of the use of duplicate set screws using loctite as a final backup. It will be next year before my normal haulout and I would pay for a haulout now if the problem could be resolved. (my wife is not fond of holding the wheel under pressure while motoring)
I suppose my concern is that with a reduced pitch, I might get the same degree of pull at the same speed as before (while at a slightly higher rpms). Since I would like to go 6 to 6.5 knots, there might be no real changed in the pull. Or will there be a change??? Or would a reduced diameter be better... as my local marine mechanic says.
One Internet resource indicates that with power boats at load speeds, an over pitched prop will cause the boat to turn. But that is mainly a power boat web site.