RPM too high at WOT

[fastempo]

Re: RPM too high at WOT

Sorry, I haven't answered what I should check per SS and topglock's suggestions, but I have not had time to put boat back in the water these last two weeks, as I'm preparing to take it "across" the country from here Nor. Cal. to MO/ARK border where I'm Going to reside for a few months. So as soon as I can reach Norfolk Lake, in Northern Arkansas, and do the testing, which will probably be in about two weeks, I will report back my findings, if you are still interested in hearing the results. In the meantime, thanks for your interest and sound advice. ps I'm a little dumb, but what does S&Gs mean?
Until then Regards, U3 (my first name begins with a U, and my last name begins with a W, hence U3)

 

[aerobat]

Re: RPM too high at WOT

something is clearly wrong here. on the one hand a 3.0 should not be able to rev past 5000 rpm ( on 1.98 ratio) with a 19pitch prop attached, on the other hand- when for some reason it truly would , you should go with this pitch faster than 30mph.

you tachometer is wrong or your prop is slipping i would say.

cheers !

 

[steelespike]

Re: RPM too high at WOT

something is clearly wrong here. on the one hand a 3.0 should not be able to rev past 5000 rpm ( on 1.98 ratio) with a 19pitch prop attached, on the other hand- when for some reason it truly would , you should go with this pitch faster than 30mph.

you tachometer is wrong or your prop is slipping i would say.
Tach was changed a few posts back.3900 was the max I believe with a 23" prop. He was going to change to a 17.I suggested the 19
would be closer seeing the motor seems healthy. A 19" would add about 600 or 800 rpm over the 23" prop.
putting the rpm at 4600 or 4700.

 

[fastempo]

Re: RPM too high at WOT

you tachometer is wrong or your prop is slipping i would say.
Tach was changed a few posts back.3900 was the max I believe with a 23" prop. He was going to change to a 17.I suggested the 19
would be closer seeing the motor seems healthy. A 19" would add about 600 or 800 rpm over the 23" prop.
putting the rpm at 4600 or 4700.

Here are the facts as they now stand, since two weeks ago: I have not installed the 23" pitch prop; I installed a new Tach, and I used the old somewhat beaten up 19" pitch prop, I took the multi-meter along to compare rpm's between it and new tach and these are the results; at WOT the new tach showed 3900rpm, the multimeter 5400(obviously wrong)and the gps speed was 32mph. Which checks out with say 10% slip and 1.98:1 ratio and 3900rpm, and it calculates to 31.9mph. So now what I got left to do is to exchange the newly bought but not used 23"pitch prop for a new 19" pitch prop and somehow figure out why my WOT is only 3900rpm, when it should be abt 4600rpm. And then, if I can get there, @ 4600rpm and new prop (19"p) and say 6% slip, I might see 39.3 mph, which is close to the 41.3mph @4700rpm Scott Danforth achieved, see above. And I'll be happy camper, and I'll let you all know what the Arkansas lake water will do for me when I get there, in abt two weeks, and have the time to tweak the motor to rev to 4600rpm. Happy driving and boating, U3

 

[aerobat]

Re: RPM too high at WOT

uh, sorry- i did not see your answer where you figured it out due to a false rpm reading.

now your results seem correct with a 19pitch prop until the false reading messed all up. but bring it back and buy a 17pitch ! even a 19p is to big for the 3.0 , you will never achieve correct WOT rpm,s .

 

[steelespike]

Re: RPM too high at WOT

I feel a 140 in a 17ft should normally easily reach rated rpm range with a 19" and while the motor appears healthy something is holding it back.
At that age the timing could be set wrong and may not be advancing as it should.The throttle may not be opening all the way.
The bottom could be fouled or distorted.
I've seen lots of 3.0's run fine with 21" and rarely 23" props. Might struggle with a load or water sports.

 

[fastempo]

Re: RPM too high at WOT

I feel a 140 in a 17ft should normally easily reach rated rpm range with a 19" and while the motor appears healthy something is holding it back.
At that age the timing could be set wrong and may not be advancing as it should.The throttle may not be opening all the way.
The bottom could be fouled or distorted.
I've seen lots of 3.0's run fine with 21" and rarely 23" props. Might struggle with a load or water sports.

Thanks SS for reply. Yesterday I snuck the boat out on the water one more time, before I have to head out on the highway for Missouri. I didn't have time to check the timing etc but I did check the linkage for full throttle, and the bottom is smooth and does not appear to be distorted, but the old 19"p prop is a bit rough around the edges, due to an encounter with a submerged old car, near he launch ramp, which becomes a hazard at low tide. At this part of the S.F. Bay we have a 6' to 8' tide differential.
This time, as I'm getting a little better at trimming for max speed, I achieved 4000+ rpm and again abt 32mph, as I didn't pay that much attention to the Garmin. The motor sounded good at full throttle, and at one time I got almost completely air-borne, when I was checking instruments and didn't pay attention to the oncoming wake from a passing tanker. Spilled my beer needless to say over everything including self.
I don't think I have mentioned that I have a whales tail mounted on the anti-cavitation plate (from previous owner), and I have mounted a Briggs&Stratton 4 cycle air cooled kicker-motor and outboard bracket on transom. It's kinda' heavy, around 60lbs plus the bracket. Could those two things contribute to keeping the motor from developing the desired 4200 to 4600 rpm at WOT?
Thanks again, U3

 

[aerobat]

Re: RPM too high at WOT

you can try to remove the plate and the kicker motor, but you will not not gain 600 extra rpm with this. remember that running a 19pitch on a 1.98 ratio with 4600 engine rpm calls for going roughly 40mph which is a little optimistic for the 3.0 .

i had a volvo 3.0GS with 150hp on a 21footer cabin boat with a laserII 17pitch. it reved about 4000-4100rpm and 28-29mph. for this 3000lbs boat the prop was a little to big.

i think a 17pitch stainless would match to your smaller boat perfectly, not a 19pitch.

beyond that i,m wondering whatkind of boats run a 21 or even 23pitch with the 3.0???

 

[steelespike]

Re: RPM too high at WOT

Prop calculator indicates that a 3.0 I can't tell you specifics But these props and speeds came up in discussions about 3.0 setups and trying to help folks understand
what there 3.0 can and can't do. I can't tell you if the speeds were by gps as I don't remember.
I have seen many posts of 3.0's running at 39-41 mph And at least one individual claiming a 23" prop.
there is no doubt that a 3.0 set up for best speed will lack pulling power.Typically it would be suggested to run a 17" prop for heavy water sports with the caution to watch the rpm.
Calculator indicates a 3.0 with a 1.98 ratio 19" prop 4800 rpm 8% slip will make 40.1 mph.
Keep in mind there are 3.0,s rated at 115 up to I believe 145. many of them at 4800.
I don't think its a stretch that a setup with a 21" prop might might make over 40 to 43 mph.
with little ability to easily ski behind.

 

[aerobat]

Re: RPM too high at WOT

I don't think its a stretch that a setup with a 21" prop might might make over 40 to 43 mph.
with little ability to easily ski behind.

well, "overpropping" does not give better speed. since an engine does not develop its rated power at every rpms but only on its peak point ( lets say 4600-4800 here) you waste horses when you prop that the engine runs max 4000rpm- at 4000 rpm it develops less power. the same is true when you "underprop" so the engine revs more than its rated rpm. on the one hand its over the peak and starts to loose power again , on the other hand you may be forced to reduce throttle for preventing damage- in both cases you push less horses in the water and will go more slowly.

in my opinion you should prop whatever is needed to reach with your boat the rated rpm- not more not less.

the correct pitch highly depends on your boat ( mass, hull design), and of course on the gear ratio.

when the thread opener lacks in rpm the boat is simpy to heavy for a 19pitch with the given gear ratio. ( of course assuming the engine is healthy and the boat does not carry a ton of water inside)

 

[steelespike]

Re: RPM too high at WOT

I'm not recommending over propping If you note my number of posts I think you can realize that I should understand prop selection.

I've never looked it up but have had others report 3.0's boat companies suggesting 21" props for maximum speed.
So I think it is unlikely all 3,o's would be over proper with a 21.Surely some would be.
I fully agree to prop for max rated rpm.I know full well you can get beyond best rated hp both above and below certain rpm.
I'm not advocating a 21" prop for every 3.0 It would depend on the boat,the activities, typical load,gear ratio.and of course hp
and base line performance numbers.

 

[aerobat]

Re: RPM too high at WOT

I fully agree to prop for max rated rpm.

yep... so the best quess for the thread opener would be to verify a "healthy" engine and then reduce pitch when he is unable to reach more than 4000rpm,s .

in most cases its a boat/engine problem when the boat reached correct WOT,s and somedays with the same prop not anymore and in most cases just a wrong prop when initially after a new prop installation it struggles to rev up correctly.

cheers !

 

[Philster]

Re: RPM too high at WOT

Is it time to ask, "How far down is the prop sitting in the water?" or "Where is the antiventilation plate, which has the whale tail on it, sitting in relation to the keel?"

On plane, the antiventilation plate should be visible and skimming near the surface. If you have the prop buried in bad water, you are adding drag. And man, if you drag that whale tail through the water, it's like a water parachute -- it magnifies drag by a factor of god-knows-what.

Don't we need to clear this up, too?

 

[aerobat]

Re: RPM too high at WOT

a good point philster !

 

[steelespike]

Re: RPM too high at WOT

Good idea.
It is an i/o should be the right height but bears checking.

 

[jimbo_jwc]

Re: RPM too high at WOT

My 1987 3.0 L 130hp mercruiser w/17 P exceeds WOT ratings with only 2 guys full tank /supplies ,fishing gear on a 19'but not shure with full boat of riders . Thought of geting a spare w/19p for more mph but the original 17 pushes it as fast as I want to go 45+. I have asked about overeving damages but not read about any doing it or damages for this little 4 cylinder.

 

[Philster]

Re: RPM too high at WOT

An I/O with a whale tail? I actually lost track that we are discussing an I/O and am confused from reading some other threads. :redface: I chimed in about the motor height because when I picture a whale tail, I just immediately picture an outboard.

But a whale tail on an I/O can cause more grief. On an outboard, most people consider them worthless or potential problems, but the outboard position can be adjusted somewhat to compensate. On an I/O, I just see that thing as dragging through the water.

 

[fastempo]

Re: RPM too high at WOT

Is it time to ask, "How far down is the prop sitting in the water?" or "Where is the antiventilation plate, which has the whale tail on it, sitting in relation to the keel?"

On plane, the antiventilation plate should be visible and skimming near the surface. If you have the prop buried in bad water, you are adding drag. And man, if you drag that whale tail through the water, it's like a water parachute -- it magnifies drag by a factor of god-knows-what.

Don't we need to clear this up, too?

Thank you all for all the input and advise. Presently the boat is bundled up and wrapped for the long trip to MO/ARK, which I'm embarking on in a couple of days, so I can not take it out on the water for further testing until I arrive my destination and unpack. And I will do additional water testing with other propellers to see if we can achieve recommended rpm at WOT, (4200-4600 for the 140- 3.0, w/ ser.# below 3826282).
In the meantime I took a few pictures, that show the following:
1- The long (rusty) stl straight edge is placed parallel to bottom of hull, with its bottom edge about 1/2" above bottom of keel.
2- From the pictures we can see that the anti-ventilation plate is located about 3/4" below bottom edge of this steel pc.
3- We can thus conclude that anti-ventilation plate (on this boat)is located just about 1/4" below bottom boat keel.
4- I also included a picture of the rough edged 19"p. prop.

 

[steelespike]

Re: RPM too high at WOT

You could easily have that prop repaired and do some test runs.If a prop change is in order you will have a dandy spare or a very good prop to sell.It appears in the pics you have removed the whale tail, thats a good start.
I've seen a number of older I/O's that the timing was stuck.Good luck with your trip.Keep us posted.

 

[fastempo]

Re: RPM too high at WOT

You could easily have that prop repaired and do some test runs.If a prop change is in order you will have a dandy spare or a very good prop to sell.It appears in the pics you have removed the whale tail, thats a good start.
I've seen a number of older I/O's that the timing was stuck.Good luck with your trip.Keep us posted.

Thanks SS,
I will do that, keep you posted that is, and thanks also for the good vibes for my trip. Also, I have not yet removed the whale tail, but I intend to do that as a comparison on my next test run in ARK. If you look closely on right side of one of the pix you can still see it. And thanks for the advice on the possibly stuck timing advance. How would I check for that?
Again thanks to all for your input. I'll be back soon. U3