How important is WOT?

[FreeBeeTony]

Now that my Mercruiser 165 IL6 is running where I think it should be, I changed to a 17" prop from a 15". My WOT is ~3500RPM. Is this ok? The spec is 3800-4200. Is there any harm using this prop or should I go back to the 15" where I get closer to the 3800 mark? I am not worried about "hole shots". After I installed the trim tabs it gets on plane nicely. The boat is a '78 20' Grady White.

 

[highaltitude]

Re: How important is WOT?

the prop is basically acting as a rev limiter. without running in the specified rpm range you run the risk of engine damage-detonation.

 

[TwoBallScrewBall]

Re: How important is WOT?

You're close...... can you trim the outdrive to get the 3800RPM? If not, I would go back to the 15". If you really want a 17" for the better top end, try to find one with a smaller diameter. For example, if you're running a 14X17, try to find a 13.25X17 or 13X17, should help to get the RPM's up to where you need them.

 

[LubeDude]

Re: How important is WOT?

You are lugging your engine with this prop, I would go back to the 15P..Its like driving up a hill all day in high gear. It causes undue stress on your engine and can create more heat also. You will probably use more fuel also. Did you gain anything with the 17P prop?

 

[Bondo]

Re: How important is WOT?

Yep.....<br />You'd better put a Smaller Prop On..........<br />You're going to put Big Holes in the tops of those new Pistons....................

 

[FreeBeeTony]

Re: How important is WOT?

I will change back to the 15".<br />Why would the 17" burn holes in the pistons if the timing is correct?<br /><br />ALso, let me describe the "throttle position".<br />After I reach 3200RPM's, the throttle still travels about 20 deg. For example: at 3200 the throttle position is at 11 o'clock, at WOT the position is at 9 o'clock. The RPM's only increase about 300 RPM's in the last 20* of travel of the throttle. Does this sound correct? Or do I still have another problem other than the prop?<br /><br />In another post daveswaves said maybe my high speed jets are clogged. Would this be noticable anywhere else? Idles fine.

 

[Bondo]

Re: How important is WOT?

Why would the 17" burn holes in the pistons if the timing is correct?

Even with the timing Perfect,<br />Lugging the motor Causes Detonation............<br />And,<br />High Cylinder Heat.......<br /><br />As far as the Throttle position,<br />Forget about the Lever.........<br />Go look at the Carb....<br />Is it Open All the way ???<br /><br />If your high speed jets Were clogged,<br />You'd Never see 3200rpms........<br />You're running on the High speed circuit Way before that...................

 

[FreeBeeTony]

Re: How important is WOT?

Thanks bondo.....<br /><br />And yes, the carb is wide open.<br /><br />Guess it must be the prop.<br /><br />Seems like other similar boats with the same engine/outdrive are turning a bigger pitch prop and able to get the RPM's. Maybe I'm just thinking too much? <br /><br />I will change the prop back this w/e and go fishing!<br /><br />Tony

 

[John Carpenter]

Re: How important is WOT?

It is also possible that your tach is not reading correctly...have you checked this?

 

[FreeBeeTony]

Re: How important is WOT?

yes, tach has been verified.

 

[Doug Durako]

Re: How important is WOT?

Tony---that 20-foot Grady runs with a whole lot of boat in contact with the water all the time. <br /><br />I ran the same motor for years on a much shorter 17.5 foot Cobalt runabout---I had to trim out as far as I could to get enough boat out of the water to run right at 4100-4200 rpm with a 17 pitch prop.<br /><br />My guess is the best you will ever get is 3800-3900 at WOT due to the Grady's hull design. Even if you do spin faster, you aren't going to go much faster, as you probably already know---the extra rpms are lost pushing water across a larger contact area on the bottom of the hull.<br /><br />I know your boat is clean, but barnacles and scum will also drop you another 300 rpm or more.<br /><br />Good luck!

 

[FreeBeeTony]

Re: How important is WOT?

Thanks ddboater, that's kinda the conclusion I am making. Just want to make sure I'm set up optimumly.

 

[Cobra SX]

Re: How important is WOT?

Go back to 15 dude , or Take your 17 pitch to a local prop shop and have them PITCH it to 16(they can do that) <br /><br />Because at this point you need to Beable (even if you dont run WOT) To reach WOT so as to burn of harmful carbon. and reduce detionation.<br /><br />Summed up GO BACK to 15 (but get a 16p trust me)<br />getting a 16 will put you Right at the Point you need to be (3,800)