Help please - WOT RPM too high

[Steelhd]

As the title says the problem is that my WOT RPMs are too high. The boat is a 17 foot 1991 aluminum Spectrum Fish & Ski with a dry weight of 930 lbs that I guess is normally loaded to about 1200 lbs. Power is a 90 HP Merc 2 stroke that I think is a 1992 or 1993 model. Prop is an aluminum 12-3/4" with a 21 pitch (PN 48-77348-440L-21P)(Merc?). Tested today and the WOT throttle RPM is 7,000 at 35 mph. Merc's recommended range is 5000 to 5500 RPM. Ideas?

Thanks

 

[hwsiii]

Re: Help please - WOT RPM too high

Steel, either your tach is wrong or you have a spun hub on your prop, if this is not a pontoon boat, but is a regular hull. You are very close to the speed you should be getting, but my Prop slip shows you are showing about a 41% prop slip at 7,00 RPM. My first thought is probably the tach, try turning the switch on the back of the tach 8 or ten times to clean the contacts, and then bring a mechanics tach on the boat to double check the RPM.


Prop Slip

H

 

[jkust]

Re: Help please - WOT RPM too high

With at 21 pitch, that boat should get into the mid 40's. That said a 21 pitch just seems a little on the high side. For comparison, my 16 foot, Bayliner is 1050 lbs dry including the weight of the 85 hp Force, 2 stroke engine. With a 19 pitch, three blade, aluminum it will gps into the low 40's anytime not just in perfect contitions. It doesn't have a tach. I just have to think a 21 pitch would lug, not overrev. I don't know what the hull differences are with your aluminum versus my example boat.

 

[Gary H NC]

Re: Help please - WOT RPM too high

That light of a boat with a 90 Merc should be flying at 7000 rpms.

I suspect the prop hub also.
Check rpms with a mechanics tach then look at the prop next.
Mark the hub of the prop,give it a run and see if the marks are still lined up.
That motor should be fine running up to 6k but 7 is too much.

 

[Texasmark]

Re: Help please - WOT RPM too high

I've said this 50 times and the old salts are tired of hearing it, but this guy is not an old salt.....bear with me.

I have the same engine on a 17' alum BB, 700# dry, about 1400# in my run mode, low drag hull. I run GPS 48-50 mph (ambient temp related) with a 24p XP Ballistic SS @ 5600 rpm. As far as over reving Mercs, within reason, forget it. For the past 10 years I have run towers and 3 cyls to 6 (or a tad over) grand in normal, every day fishing/playing (no hot dog contest racing stuff), and have had nothing but "give me more bubba, I love it".

The problem I had on this rig running alum was an rpm reduction, not increase. I will go with the spun hub senario.

Mark

 

[Steelhd]

Re: Help please - WOT RPM too high

Thank you all!

Will check out the hub next time the boat is on the water. Am a little suspicious of that diagnosis though. The boat had a 19P on it that was also badly over revving so I put on the 21P spare (it looked almost brand new) but the reduction in RPMs was very minimal. Maybe both are spun? Will also toggle the tach (it is brand new) as suggested and try to find a mechanics tach. Not sure where to get one or how to use it but will figure that out. Hope to post results in a week or two if not on Monday.

Edit - will also borrow a hand held GPS to verify speed.

 

[hwsiii]

Re: Help please - WOT RPM too high

I may be wrong, but in my opinion that just reinforces my thinking that you may have the wrong tach for that motor or it is set to the wrong position on the back.


H

 

[mpsyamaha]

Re: Help please - WOT RPM too high

i agree, sounds like an inaccurate tach

 

[Mi duckdown]

Re: Help please - WOT RPM too high

Tach. If you have been running at 7000 rpm your motor would be smoked.
Almost all motors run @ 5000 to 5500 plus or minus.

 

[Steelhd]

Re: Help please - WOT RPM too high

Tach. If you have been running at 7000 rpm your motor would be smoked.
Almost all motors run @ 5000 to 5500 plus or minus.

I don't run it wide open for that reason.