wrong pitched prop

[muskie1]

I have a 1985 375V Ranger (18'10" length) with a 1992 150 hp Mercury Outboard. I recently purchased it and took it for a test run this week. I noticed the RPM's were red-lining at 6700 rpms when I held the throttle wide open. I was traveling around 52-55 mph and the trim was up all the way (optimum angle) and the lake had white cap conditions. I'm guessing that the pitch of the prop is at least 2-3 sizes down too low due to the high rpms. When I later examined the prop pitch, the only identifying marks I could find did not clearly indicate to me what the pitch or size of the prop was. All it showed was 48-832832 c121 (the prop is aluminum). Since I do not own a spare prop I would like to purchase a high performance stainless steel or aluminum prop that will get my rpms down to the recommended 5000-5600 rpms. Any recommendations on what pitch I should be running with would be greatly appreciated and also any recommended high performance props (alum or SS)for this style boat. Thank you.

 

[JB]

Re: wrong pitched prop

Hi, Muskie1.<br /><br />I think you are on the right track, but:<br /><br />Verify that your tach is accurate. I am not certain what the switch should be set at for that engine, but 6700 rpm is really screaming for a stock engine and I would think that you would have been alarmed by the engine noise. If it was really turning 6700 you are very fortunate that you didn't hear a grenade behind you.<br /><br />If your tach is set for a 5 cylinder engine it was really turning just over 5500, which would be a bullseye. Your top speed sounds about right for that rig with the right prop.<br /><br />Let us know what you find.

 

[ob]

Re: wrong pitched prop

Musky,Your current prop 48-832832 is a 14 1/4 X 21 made by Merc.

 

[muskie1]

Re: wrong pitched prop

Thanks for the info - I'm thinking you're right to say that the tach may be set for a 5 cylinder motor. I know at one time there was a different motor on this boat. The engine sounds nice and strong when running the throttle wide open, it doesn't sound like it's winding up too high, so I think we might be onto something here.<br />I was also wondering when I go to purchase a high performance prop, would you suggest 3 blades or 4 blades? and if my current 21 pitch (alum. prop) turns out to be the perfect fit, should I drop down 2 inches of pitch if I should decide to get a stainless steel prop? I enter fishing tournaments frequently and boat speed is relatively important. Once again your input is very much appreciated.

 

[muskie1]

Re: wrong pitched prop

I was also wondering when I go to purchase a high performance prop, would you suggest 3 blades or 4 blades? and if my current 21 pitch (alum. prop) turns out to be the perfect fit, should I drop down 2 inches of pitch if I should decide to get a stainless steel prop? I enter fishing tournaments frequently and boat speed is relatively important. Once again your input is very much appreciated.

 

[mattyc]

Re: wrong pitched prop

I've read that on an outboard, you don't typically need to change pitch when going from aluminum to SS.

 

[seadiva]

Re: wrong pitched prop

on a ss prop you will probably drop rpms, i understand the aluminum prop, the blades flex when under way at high speeds, and the ss does not.

 

[JB]

Re: wrong pitched prop

No, aluminum props do not flex.<br /><br />SS props will have thinner blades and offer less drag, but the differences in rpm are trivial unless the SS prop has radical cup and/or other drag reducing features.

 

[Dhadley]

Re: wrong pitched prop

Well....weve found that they (ss blades) actually do flex and eventually lose the original "foot print". <br /><br />I wish they didnt.

 

[ooootis]

Re: wrong pitched prop

If SS props really do flex how long does it take for this to happen. Kinda seems like a waste of money if this happens. Have no experiance with al. or stainless just Bronze.