Props? ugh! Help with info

[tgilmore99]

I have a 1985 Searay Sundancer with twin 5.7 OMC. The props on this are 14 1/2 X 19 . Actually only one prop is stamped. I am not a prop guy. At wto my rpms are at 3600-3800. A tech said they should be at 4600 (wot) Crusing they are about 2800-3000 tech said they should be 4000 to 4200. What am I missing. or does the tech not know what he is talking about. Ask me questions and maybe I can tell you more if that will help. Will different size props change this (rpm)

 

[Cap10Pat]

I'm not familiar with the OMC engines, but I would guess 4000+ should be WOT. Don't worry about cruise - just work on the WOT. First you have to get a definite answer on the correct rpms. Your props don't look to be the problem - at least not initially with almost no information. You should have a good mechanic check the engines and tune them, then make sure they run as they should at the dock. They should be able to reach the max rpm in neutral. Then start working on the outdrives and props. It's important to get the correct props, which will control your WOT.

 

[Cap't Steve]

Is the boat an I/O or V-drive or Straight drive? My 30'er is straight drives with those engines. Prop size is 18X17 & max rpm is 4200. Your max rpm should be the same as mine but I would imagine your pitch is gonna be more & the diameter may be smaller if you have I/O's

 

[rktktpaul]

You can check the prop chart from Sea Ray for the correct prop size/pitch, and what WOT should be. Here is the link:

http://www.searay.com/boat_graphics/sea_ray_prop_charts/1985.ENGINE,DRIVE AND PROP MATRIX.pdf

If I'm reading the chart correctly (270DA w/ twin 260 OMC (260hp = 5.7l engine)), 14.5x19 prop is correct. You should be 4600 rpm at WOT. You're off 800 - 1000 rpms; a bit much to be making up in pitch change only.

I think Pat's right - it doesn't look like it may entirely be your props.

 

[NorCal Boater]

5.7 engines, whether Volvo, OMC or Merc, are all the same GM motor. They should all run in the 4200-4600 RPM range at WOT. But Cap10Pat was right, you need to make sure the tachs are correct first.

I would get, or find a friend that has, an engine analyzer that has a tach function. I had a 4Winns that I thought the props were wrong and it turned out the Tachs were set for 6 cylinder and not 8.

Once you determine that the boat tachs are correct, tune the engines, if it hasn't been done in a while, and then run at WOT and see what you get. If you are not in the above RPM range you may need to change props.

A local prop shop can assist you with what you need. If you can't find one, let me know and I can set you up with a really good one here in the Sacramento area that would work with you on line and over the phone. If your tachs are correct, your props may be over-pitched. Alpha 1 drives almost all had 14 1/2" diameter props so it sounds like you may need 17" pitch.

Shawn

 

[Riptide III]

I agree with norcal boater however, someone please straighten me out if I am incorrect. 1 Determine if you have the correct prop for your setup. 2 Tune up, throttle linkage, Tach accuracy ect. as stated above. 3 (Here is where I differ) if you are still not making RPM's, there is a good chance you have a sick engine. If one is bad it will bog down the other and not allow it to make full RPM's. I would also include a compression check and a knowledgeable mechanic to look it over. If the engine or both engines are sick, just changing the pitch may get your rpms up there, but your speed wont be where it should be. Also if you do have a sick engine dragging the other one down, that is a good way to damage your good engine. IMHO
Phil

 

[NorCal Boater]

You are correct Rip. The engines must be operating correctly or nothing else matters. The only thing I would change in your statement would be to make #1 last.