Help sizing prop

[drtmnky]

First a short run down. 1979 Sea Ray 260 Sundancer with 5.7 OMC 800. Before I had the OD worked on I could not get the engine past 3400 rpm at wot. Then the upper gear cluster failed. (how do you break one tooth off a gear?) That was repaired. Took her out for 2 hours, then another OD failure. The shop said it was the lower gear set. They replaced it. Put the OD back on, ran it, same noise before they repaired the lower gear cluster. Took it back, the shop said the upper gear cluster failed. How in the h** does that happen??? They repaired the upper gear cluster a second time. Took her out today and she ran like a champ, which leads me to my present issue.

Now the engine revs to who knows how high above 4000 rpm. I only hit 4000 for a few seconds. I was afraid of breaking something again. Some one told me the prop should be sized so that the engine will not rev above 3800 to 3900 rpm. Is this correct?

How do I determine the size of my current prop? I already checked 6 times, there are no markings on it to indicate size. Diameter is easy, how do you determine pitch?

Should I run aluminum? (current prop) or stainless steel? How do I figure the right size?

 

[coolings]

Here is a previous thread about WOT level:
http://www.clubsearay.com/forum/showthread.php?t=21555

Here is an article that outlines how to measure prop pitch:
http://www.propellerpages.com/?c=articles&f=2006-03-27_manual_pitch_measurement

You are probably better off having a prop shop put it on their machine. For your size boat I would say you need a stainless for best performance.

 

[tobnpr]

First, you need to find out the manufacturer's recommended WOT rpm range for your engine, to determine whether "someone's" 3800 to 3900 specification is correct. Sounds low to me...

Once you know what the upper end of that range is, your prop needs to be sized to allow the engine to turn up to those RPM's with the boat loaded the way your normally use it. Adding pitch (or cup) to a prop will lower max rpms, removing pitch will increase rpms.

Unless your boating is done in shallow, rocky areas a SS prop is preferable. It's heavier weight and thinner blades with less flex will provide better performance. Aluminum props are preferred if there's a reasonable chance you're going to hit something- aluminum will "give" and protect the drivetrain in the event you strike an object. SS will not...

 

[searayjoe250]

If its the original engine check the valve cover, there is a plate with the engine's serical number and there should also be a WOT range. My 1987 250 Sundancer has a range of 4400 to 4800. I'm at 4300 which is a little low so I will try cleaning the bottom or repitch a size lower. Did you try looking for the prop serial number on the inside of the hub?