OMC 400 increase pitch?

superbenk

superbenk

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I have an 18' 1980 Century with an OMC 140/400 Stringer. At WOT with my wife & 2 young kids (6 & 9) in the boat I run about 5000-5100 RPM according to the dash tach. GPS reads about 32-33 mph. According to my Solec manual the engine should only do 4500-4600 RPM at WOT. My current prop is a Michigan Wheel model: 011003 (Diameter: 13.25"/Pitch: 17"). Should I go to a 19" pitch or higher? What kind of top end speed increase should I expect?

Also, I've read a few good things about how smooth & fast a 4-blade prop is. Should I consider that?

Finally, I read about the Pro-Pulse prop (I think) which has adjustable, replaceable blades. Is it any good?
 
jestor68

jestor68

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Re: OMC 400 increase pitch?

The Solas Amita aluminum 19 pitch will drop you rpm 300-400 and likely give a boost in speed(2-3mph) across the rpm spectrum.

Order the Solas thrust washer with it to be sure.

Unless you intend to do a lot of water sports pulling, the 4 blade might not be for you; as you lose some top end speed compared to the 3 blade.
 
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steelespike

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Re: OMC 400 increase pitch?

5100 at 33mph indicates 33% slip, very high. Using a 1.71 gear ratio 5100 with a more normal 10% slip would produce 43 mph.
I would suspect your tach is not accurate. Does the boat seem to respond well to up trim?Do you have a strong hole shot?
Perhaps you can borrow a shop tach. I suspect your actual rpm is closer to 4000 unless there is some other outside force in play like
too much boat in the water.
 
superbenk

superbenk

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Re: OMC 400 increase pitch?

steelespike said:
5100 at 33mph indicates 33% slip, very high. Using a 1.71 gear ratio 5100 with a more normal 10% slip would produce 43 mph.
I would suspect your tach is not accurate. Does the boat seem to respond well to up trim?Do you have a strong hole shot?
Perhaps you can borrow a shop tach. I suspect your actual rpm is closer to 4000 unless there is some other outside force in play like
too much boat in the water.
Click to expand...

Good points. I'll bring my tach/dwell meter along on the next trip and see (it is a 32 yr old tach in the boat after all). It does respond to trim, but I don't have the same amount of trim that a Merc or Volvo would have since I have the Stringer. Hole shot feels pretty good. What do you mean by too much boat in the water? It certainly doesn't seem to sit low at rest:

IMG_3676.JPG


Best speed I've ever gotten was 35-36 (GPS) by myself in the boat on smooth water.
 
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steelespike

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Re: OMC 400 increase pitch?

Too much boat in the water, I mean the boat doesn't respond enough to up trim.
I couldn't find any 18 footer with a 140. weights seem to be about 1600 up to about 2300.
a 2300 has a 7'7" beam.Boat sure looks big for a 18 footer.
 
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