Prop selection problem...

[fenwick]

I'm not quite sure how I should go in getting a different prop.Right now I have a 12.25x15 prop on a 1991 alumacraft Dominator(16 ft.) with a 1991 40hp Evinrude.My hole shot is okay and my top end is 28-29 mph(gps).The problem I'm having is low RPM's, at WOT and trimmed out ,I'm only turning 4600.The motor is rated at 4500-5500 RPM's.I've done a compression test on the motor and both cyl.are at 110,so I know the motor is good.I've also raised the motor so the cavatation plate is ever so slightly above the bottom of the hull.Should I go to a 12x13 prop to get more RPMs and hopefully better performance out of the motor? :/

 

[Ron G]

Re: Prop selection problem...

hows your water pressure with it raised?you need to get your tac verified too.something seems off here to me start with the tac.

 

[Silvertip]

Re: Prop selection problem...

I think your tach is wrong. I had a very similar setup, and my boat was happiest with the 12-1/4 x 15 and ran at identical speeds you have now. The engine turned 5500 rpm. I have a stainless steel Rapture 13P that was on the boat for one week. If you are interested in it post an e-mail address and I'll make you a deal you can't refuse. The prop will go on E-bay this spring.

 

[fenwick]

Re: Prop selection problem...

hows your water pressure with it raised?you need to get your tac verified too.something seems off here to me start with the tac.

 

[fenwick]

Re: Prop selection problem...

Ron,


How would you go about checking a tach to see if it is accurate?Is it something I could do without taking it in to a shop?

Jim

 

[Ron G]

Re: Prop selection problem...

I think a shop would have to do it.you may be able to post the question up in the outboard section and a motor guru will fill ya in.

 

[Dhadley]

Re: Prop selection problem...

Any dealer should have a shop tach that simply clips on like a timing light. They should be able to check it rather quickly.

What brand tach is it?

 

[Silvertip]

Re: Prop selection problem...

I ran the numbers through a prop calculator. At 5500 RPM, a gear ratio of 2.43:1 (which your engine has), 15P prop (16P if prop is cupped), and using a 10% slip factor, your speed should be 30.9 MPH. Those numbers are consistent with my experience using three speed measurements (gps, pitot speedo, and locator speed). So a number of things are possibilities: 1) prop is not a 15P due to mismarking or rework 2) tach is wrong or being misread, 3) speedo is wrong (most are),

 

[fenwick]

Re: Prop selection problem...

Silvertip,
I finally was able to get out and figure out my RPM problem! I went and picked up a Tiny Tach and hooked it up to my motor.I found out my tachometer was 800 rpms low! You were right,with a 12.25 x 15 prop I'm running right about 5400 to 5500 with just myself in the boat.I'm greatly relieved that all this time I haven't been lugging the motor.Those Tiny Tachs are really slick!Thank you to all that helped me figure out this puzzler.

Fenwick