what is proper rpm to run

[DaveS.]

I have a '93 20ft w/a with a 175 Johnson on it. WOT is right at 5500 rpm. I rarely run it at that. Typically in open water i run it around 34-3600 rpm and in the intracoastal and rivers around 4000-4500. Reasoning is fuel consumption/ ride vs. speed and engine wear. Is this okay or should i open her up more often?

 

[Jack Shellac]

Re: what is proper rpm to run

Although all boats are different, you're usually most fuel efficient in a rig like yours at about 4000 - 4500 RPM. Understandably, you can't always run this fast offshore in big seas, but it's a fairly good reference point. At 5500 RPM at WOT, it sounds like you're propped right, too. It won't hurt to open it up at times, but there's no real mechanical need to do so.

 

[Dhadley]

Re: what is proper rpm to run

Sounds like youre real close on set up. 5500 with an average load is not bad but I would like to see it at 5800 with an average load. <br /><br />I wouldnt suggest changing props -- thats a last resort unless yours is terribly ineffeceint. Whats your top speed (accurate)? Post that and we can calculate slip%. <br /><br />I would play with your set up. If the motor is mounted all the way down, you might try raising the motor on the transom or bracket (whichever way you have it mounted) which will change your X dimension.<br /><br />You should pick up some rpm's, gain speed and holeshot plus get better fuel economy. Also what brand and type prop?<br /><br />Let us know!

 

[DaveS.]

Re: what is proper rpm to run

Thanks, Jack. Have always been of a mind that if you don't beat something it tends to last longer. I'll take reliability over speed any day.

 

[DaveS.]

Re: what is proper rpm to run

top speed is 39-40 mph as per my GPS. the motor is all the way up on the slides and I have Doel Fins on it.

 

[DaveS.]

Re: what is proper rpm to run

Sorry, the prop is a s/s 15 x 17. I think it is a Michigan Wheel but not sure. The symbol is a slanted box with a capitol M inside.

 

[Dhadley]

Re: what is proper rpm to run

Not bad. Youre around 18% slip. I like to see them between at least 12 and 16% but youre real close. The slanted box or oval may be the OMC logo. Too bad the bolts are in the bottom holes. I would like to have seen what it would do an inch or so up higher but that would require a jackplate.<br /><br />Good luck!

 

[DaveS.]

Re: what is proper rpm to run

What is the formula for calculating that? next time I get it in the water, I will record the info. and check it. Would trim tabs help at all?

 

[Dhadley]

Re: what is proper rpm to run

There is a formula but I have a calculator wheel that computes the slip %. <br /><br />Most times, not always, trim tabs slow you down. They are great for leveling the boat. <br /><br />You're sure the motor is as high up off the transom as it can go? Where's the cav plate in relation to the bottom when they're parallel?

 

[DaveS.]

Re: what is proper rpm to run

The cav plate is about even or slightly above the bottom of the boat. The upper mounting bolts are in the second of four holes but the lower is just slots in the motor bracket and those bolts are all the way to the bottom of the slots. I don't see any way to raise the motor short of drilling new holes in the transom.

 

[Dhadley]

Re: what is proper rpm to run

Youre right. I hate it when they dont use the template to drill the holes. Bummer. I'm sure you would gain some if we could raise the motor easily.<br /><br />Oh well, nothing else is going right today either! LOL!<br /><br />Good luck!

 

[DaveS.]

Re: what is proper rpm to run

Would taking the Doel fins off help?

 

[Dhadley]

Re: what is proper rpm to run

It might. Thats pretty low to be running a fin. Slip her off and see. That'll be interesting. Let us know! You can always pop 'em back on real quick.