Proper prop pitch

My Westebeke 18W died a couple of years ago. At the time, I couldn't afford a new inboard, so I had the yard plug the prop shaft hole and I put on an outboard. This year, I purchased an electric engine and learned the painful lesson of why mechanics get paid so much for installing inboards. :-/ When I purchased the new engine, I had to give the company the size of my prop. Not knowing that I could just open up this Maxprop and find out what the pitch was, I looked online, took all sorts of measurements, and calculated what the pitch must be. For the sake of this story, understand that I don't know for sure that how the prop was put together off of the boat is the same way it was together on the boat. Before putting back on the prop, a friend was by helping me with another part of the project, saw the prop, explained what a wonderful toy I had and showed me how it worked. He also pointed me to the manual online that explained how to install it. Fast forward. The hole is redrilled in the bottom of the boat, the engine and prop are reinstalled and all is right with the world. Well, not quite. When the speed of the boat reaches about 2 knots, there's a noise that my former Navy crew member tells me is cavitation. When I get it above 3 knots, the cockpit floor starts to vibrate. Someone suggests that I decrease the pitch. So we haul the boat out and do so. Now it takes an extra knot of speed to get the cavitation to occur, but at least there's no vibration. I could find at what RPMs the problem occurs, but I don't remember offhand. The boat is a 1975 2-29. The prop is a 13" 2-bladed Maxprop. We originally set the prop at a pitch of 12. We reduced it to 10. My question is: does anyone have any idea what it should be? Or should I keep reducing it until the cavitation goes away? How will I know, besides the fact the boat won't move, that I've reduced it too far? Anne Racel 4 Degrees http://boatjournal.stuffandjunk.net

Anne, I replaced the prop on my 29 last season, there are a lot of folks in this group that have far more understanding of this than me so I'll leave them to comment on how to calculate, however, on the recommendation of the prop people in Michigan I went from a 6pitch 12" 2 blade which topped me out at about 3-4kts to an 8pitch 12" 3 blade (because I mainly cruise not race) which works pretty well and gets me to 6kts easily (approx 3200 rpm) providing the hull and prop are clean, if they're not it labours very badly so I have to assume it's a little 'over propped' and may opt for an 11" or a 7pitch next season (any comments gladly received). One thing that was pointed out was the gap between the hull and the blade tip and from what I was told there needs to be around the 2" mark to allow for 'flow', with the 12" on mine the gap is nearer 1.5" so maybe 13" is a little close - thoughts anyone ? Cheers, Grae Morrison 1972 Cal 29 #435 'Destiny' Red Bank, NJ On 12/01/2011 06:29 AM, Anne Racel wrote: > > My Westebeke 18W died a couple of years ago. At the time, I couldn't > afford a new inboard, so I had the yard plug the prop shaft hole and I > put on an outboard. This year, I purchased an electric engine and > learned the painful lesson of why mechanics get paid so much for > installing inboards. :-/ > > When I purchased the new engine, I had to give the company the size of > my prop. Not knowing that I could just open up this Maxprop and find > out what the pitch was, I looked online, took all sorts of > measurements, and calculated what the pitch must be. > > For the sake of this story, understand that I don't know for sure that > how the prop was put together off of the boat is the same way it was > together on the boat. > > Before putting back on the prop, a friend was by helping me with > another part of the project, saw the prop, explained what a wonderful > toy I had and showed me how it worked. He also pointed me to the > manual online that explained how to install it. > > Fast forward. The hole is redrilled in the bottom of the boat, the > engine and prop are reinstalled and all is right with the world. Well, > not quite. When the speed of the boat reaches about 2 knots, there's a > noise that my former Navy crew member tells me is cavitation. When I > get it above 3 knots, the cockpit floor starts to vibrate. > > Someone suggests that I decrease the pitch. So we haul the boat out > and do so. Now it takes an extra knot of speed to get the cavitation > to occur, but at least there's no vibration. I could find at what RPMs > the problem occurs, but I don't remember offhand. > > The boat is a 1975 2-29. The prop is a 13" 2-bladed Maxprop. We > originally set the prop at a pitch of 12. We reduced it to 10. My > question is: does anyone have any idea what it should be? Or should I > keep reducing it until the cavitation goes away? How will I know, > besides the fact the boat won't move, that I've reduced it too far? > > Anne Racel > 4 Degrees > http://boatjournal.stuffandjunk.net > >