Resizing a propellor

Advice requested. Asked by Jeremy Parrett (#101, Two Pelicans (ex #101, Pelican 1 / Two Pelicans) )

a. Hi Jeremy, I have a Pathfinder 50 engine, rated at 42 HP. It drives through a 2.77 gearbox ratio, with a 1 1/4″ shaft. Normal max engine rpm is about 3000-3500. Originally I used a Gori 2 blade folding propeller, 16.5″dia x 11″ pitch. It worked OK, even in reverse, but maximum speed was only about 6 knots. I have now fitted a fixed 3 blade prop 3bladeprop.jpg and I BELIEVE it is 18″ dia x 11″ pitch but I can’t locate the reference details. The blade tip clearance is about 2″ from the hull . I have a prop strut that I designed myself, offset to port so that the prop shaft can be withdrawn to miss the skeg. It may be different from your strut My shaft slope is 1.375″ per foot, according to my sketches made long ago. I had my prop shaft and propellers machined so that the Gori can still be used on the same shaft, as a spare. Speed at max rpm is now over 7 knots, with great acceleration. Of course the 3 blade prop gives significant drag while sailing compared to the Gori and I have to lock the gear in reverse to stop it spinning while sailing. There are different opinions on this procedure but the one I go by is that there is insufficient splash lubrication of the gearbox at the revs caused by the prop windmilling.
David Salter (#050, Opportunity)

b. (May 15, 2013) This response might be a bit late to be of much use to you but here is my experience with Autoprop… “#155, Blue Run” has a Westerbeke W58 with a Hurth V-drive transmission (15 degree down angle). Engine under the companionway steps. I installed an 18″ Autoprop in 2006. Under flat water no wind conditions we get 6.5 knots at about 1800 rpm. At 2100 rpm (the engines torque peak) we will make 7+. With wind we lose about a knot and with waves in the 3 to 4 foot range about 2 knots all at the same rpm. I do not usually run at higher rpm however the engine is rated for continuous output at 3000 rpm and a maximum output at 3600 rpm so there is some reserve available. I have been very pleased with the performance of the propeller.
Regarding backing: the Autoprop has lots of bite in reverse however the geometry of the hull, the 15 degree downward slope of the propeller shaft, and the shaft offset to starboard combine to create a substantial prop wash effect due mainly to the wash from the propeller hitting the hull and pushing the boat sideways. This effect will vary from boat-to-boat depending on the geometry of the drive-train. With a sail drive the prop wash effect should be near nil and only the prop walk will be present. A boat with a flat or near flat bottom will be little affected by the prop wash.
Something unrelated to the propeller that can affect backing is the installation of a hydraulic autopilot servo. If it limits rudder angle on the wrong side it may make it impossible to compensate for the prop walk/prop wash effects.
George Weeks (#155, Blue Run )

c. I hauled out in June and besides doing the bottom, polishing the hull, and installing a boarding ladder, I fitted a used autoprop. It has a diameter of 18 inches with 14 inches of pitch at max rpms. You will not believe the figures we obtained after launching the boat. Bearing in mind that I have a Yanmar 3 GM 30 F, at 2800 RPM (max is 3400) we achieved a straight line, flat calm speed of 8.2 knots (incredible). On easing back to 2200 RPM she was still doing 6 knots. The previous maximum speed on the fixed 15in X 14in pitch was 6 knots at 3400 RPM. Now for the best part…….backing out of the haulout slip I managed to reverse all the way out of the marina into the channel. Almost all the prop walk has gone. Leaving the dock is now a breeze. The only downside to this is I really miss the ability to throw the boat into the dock when reversing to a stop coming alongside (joke). I guess I am going to have to get used to a boat that goes, stops, and reverses in a straight line???!!
Jeremy Parrett (#101, Two Pelicans (ex #101, Pelican 1 / Two Pelicans) )

d. I tried the new ” J Prop “, has a real good bite and I had to reduce pitch considerably for the 33 HP of ours. As is now, the new prop is at 20 * 11, In forward I can get close to full RPM but in reverse the prop bites too much. I wonder if it’s the water flow difference under the boat ??? We played quite a bit with this new prop, and end result seems to be that with our engine, (Vetus 33 HP ) there is NO acceptable compromise with respect to pitch. Eg. if you choose a good forward performance, you end up with a SUB standard reverse, or vice versa. Seems that there is a different gearbox reduction in forward ( 3:1 ) to reverse ( ?? ). ( Our box. Hurth HBW 150 V ) My conclusion to date: Other than the feathering feature under sail, it was not worth the effort or expense, certainly not with our type of engine setup. It is worth to mention that the prop when set up for either direction bites VERY WELL, but not in both, as is our case. Considering the price of J Prop, and I really like their concept, why would they not allow for different pitch adjustment for Forward & Reverse ???? I will ask the factory in Italy that.
Regards, Frank Bryant, (s/v #186, Visitant)

e. We have just had #050, Opportunity away from the dock for the first time since installing the Autoprop, autopropinstalledshowingboatclearances.jpg . It was a quick motoring trial in rather lumpy conditions but seemed to perform well although it felt quite different from the fixed blade prop. We are off for a few days and will have a chance to get some real experience with the prop.
David Salter (s/v #050, Opportunity)

f. Our new 18 ” AutoProp must be helping with our improved sailing speed (previously a 17″ fixed 3 blade prop, autoprop.jpg ) A couple of days ago we achieved 7.2 knots by GPS with a beam reach, winds to 18 knots and somewhat rolly conditions (about 2 ft seas) with full main and one reef in our 135% genoa. We have generally found that reefing the genoa first (it’s easy!) moves our centre of effort forward and eases the rudder angle. Also the new (2nd season) fully battened, loose footed mainsail (from Quantum Sails) is easier to get flat and reduce heeling, hence reduce excessive rudder.
David Salter (#050, Opportunity)

g. You may recall that I fitted an Autoprop 3 blade folding propeller a couple of seasons ago. I have been pleased with the performance and now the maker, Bruntons, have come out with a newsletter. Here is the latest, .
David Salter (s/v #050, Opportunity)