RPM too high at WOT

[fastempo]

I have a '73, 3.0- 140 motor in my Silverline Comoro 17T, which is running real well, but at WOT and trimmed out it is developing 5400 RPM, which is abt. 800 RPM more than the recommended max. Presently, I have a 15-1/2" diam., 19"pitch prop. , giving me a top speed of abt. 30 mph (gps). I have bought (thru iboats) a 14" diam., 23" pitch prop. in order to develop more speed and to reduce the WOT to approx. 4600 RPM, but have not tried it yet. Am I going in the right direction, or am I all wet? Your knowledgeable input would be greatly appreciated, thanks. U3

 

[Don S]

Re: RPM too high at WOT

First thing you need to do is is verify the tach is reading correctly. 3.0L engines do not use 23" pitch props.

 

[fastempo]

Re: RPM too high at WOT

First thing you need to do is is verify the tach is reading correctly. 3.0L engines do not use 23" pitch props.

The rpm is taken off my new multi-meter, hooked to neg coil term. and good ground, and set for 4-cyl engine. The tach in instrument panel is non-responsive, and needs to be replaced. So, I guess I'll have to go out and buy a new one and thus verify if multi meter's rpm are correct. My thought was that going up 4inches in pitch would reduce the RPM by 2x400=800rpm. BUT, If I can't use 23" pitch prop, I better cancel my iboat order now.

 

[cyclops2]

Re: RPM too high at WOT

dropping a 1 1/2" in diameter will allow the prop ro really unload & spin faster. The 4 extra inches of pitch may not be enough to bring down rpms.
A prop shop could be your best friend. Maybe work a deal on your props. He gets your RPMS at peak & works out a good price on the left overs.
Are you planning on carrying other people ? Yes ? they will pull the rpms down. Alone ? Back off the throttle.

I would not be concerned about the 800 extra rpms. UNLESS You flip the keys to other people & do not go along.

 

[Don S]

Re: RPM too high at WOT

. BUT, If I can't use 23" pitch prop, I better cancel my iboat order now.

At this point, I would cancel the 23" prop, you may need a prop, but probably not a 23. Find out what the problem is first, then go from there.

PS: 800 extra rpm is something you should be worried about, regardless what some people say. They obviously don't understand what is going on.

 

[fastempo]

Re: RPM too high at WOT

PS: 800 extra rpm is something you should be worried about, regardless what some people say. They obviously don't understand what is going on.[/QUOTE]

Thanks Don S.
I'll do just that. I'm going out to buy a new tach. for the instr. panel this eve. and tomorrow I'll do some add'l. testing.
Question: How come my Mercruiser serv. Manual #01 and the iboat propeller table both list 23"pitch propellers for this 3.0L (4cyl)
140HP motor as an option ( for 17'+/- long boats)? U3

 

[Scott Danforth]

Re: RPM too high at WOT

my 17' 3.0 liter with an OMC had two props. a 17p for pulling skiers, and a 19p for running the bay (top speed 41mph). I tried a 21p once however it was too much prop for the boat, so the 23 will really be too much prop.

 

[fastempo]

Re: RPM too high at WOT

my 17' 3.0 liter with an OMC had two props. a 17p for pulling skiers, and a 19p for running the bay (top speed 41mph). I tried a 21p once however it was too much prop for the boat, so the 23 will really be too much prop.

Well, I'm starting to see that 23"p is most likely going to be too much prop. I sure would have liked to try it out though, but that will be an expensive experiment as you can't return it, once it, the prop that is, touches water. $150, incl freight and misc hardware.
What puzzles me is that I'm only getting 30mph at 5400rpm with a 15.5x19p prop.
What rpm were you pulling at 41mph and 19p prop?
Thanks for responding. U3

 

[Scott Danforth]

Re: RPM too high at WOT

4700 RPM according to the tack on the dash, 41.3mph according to garmin. it was a stainless prop on back of a stringer 400.

If your hitting 5400rpm you WILL warp the head if running at long period of time.

 

[cyclops2]

Re: RPM too high at WOT

What is The actual GEAR RATIO between the engine shaft & the prop in foward gear. The ratio may be the cause of the over revs or a badly reconditioned prop from a prop shop.

If possible. Pull the plugs out of all cylinders. put a piece of tape or magic marker on the engine shaft. Put tape on 1 of the prop blades. How many engine revolutions are needed to make 1 full prop revolutions ? That will get you to a correct prop faster.

Older boats do get none standard parts ALL the time to save a buck & keep running.

 

[fastempo]

Re: RPM too high at WOT

4700 RPM according to the tack on the dash, 41.3mph according to garmin. it was a stainless prop on back of a stringer 400.

If your hitting 5400rpm you WILL warp the head if running at long period of time.

Thank you for the warning, but I'm aware of that danger, and I have only had it up at those high rpm's for short bursts.
But, I would like to be able to use WOT, at times when the occasion calls for it, hence the reason I'm looking into larger pitch dimensions as a means of reducing the WOT rpm to something in the 4400 to 4600 range, and gain a little more speed at the same time

What is The actual GEAR RATIO between the engine shaft & the prop in foward gear. The ratio may be the cause of the over revs or a badly reconditioned prop from a prop shop.

If possible. Pull the plugs out of all cylinders. put a piece of tape or magic marker on the engine shaft. Put tape on 1 of the prop blades. How many engine revolutions are needed to make 1 full prop revolutions ? That will get you to a correct prop faster.

Older boats do get none standard parts ALL the time to save a buck & keep running.

As soon as I get the new Tach installed (I know I don't need the tach for this verification, but I still want to complete that previous task), I'm going to perform that gear ratio test, and I'll report back, some time later today. It's only 6:05 am here in Calif. right now. And to answer you on the prop., it is a "several times" reconditioned one, and it was pretty beat up before the last prop. resto.
just after i bought the boat. so you may be right on that one.

Thanks to both of you for your attempts to educate me, U3

 

[Don S]

Re: RPM too high at WOT

You don't have to waste your time with the gear ratio check. If you had any other ratio but the 1.98 on your 3.0L, then you would have a low WOT rpm problem, not high rpm.

 

[cyclops2]

Re: RPM too high at WOT

On another site they show a 1.98 as the primary gear ratio. No optional gear ratio. High Altitude is showing 2.4 gear ratio.
Since we do not know what has been done or how correctly it was done. We really can not rule out anything due to the age of the boat.

 

[steelespike]

Re: RPM too high at WOT

Based on figures given & 1.98 ratio and a guess at 6% slip 5400 would make about 46 mph.
Using the same figures and 30 mph indicates about 3500 rpm.
I think your meter is off. Typical speeds with a 3.0 in a 17 or 18ft boat are around 40 mph
Your 30 mph I think indicates issues other than prop selection.
Calculator indicate 39 at 4600 rpm.

 

[cyclops2]

Re: RPM too high at WOT

I have never seen this in real or unreal life.

The prop is slipping on the prop shaft ?

 

[steelespike]

Re: RPM too high at WOT

I have never seen this in real or unreal life.

The prop is slipping on the prop shaft ?

Sometimes the hub will slip but when it does it usually pretty much fails
after a few previous slips.With each progressive slip eventually reduced to sub planing speed.

 

[UncleWillie]

Re: RPM too high at WOT

Trouble shooting rule #2... Keep asking yourself, "Is my test equipment lying to me?

5400 rpm does not sound anything like 3500 rpm.
When the tach says 5400 rpm, does the motor sound like 5400? Is it screaming?

 

[fastempo]

Re: RPM too high at WOT

Well last night I went out and bought a new Tachometer, ad to work work during the day, but I installed it this evening, and at 8:30 pm I Launched the boat and went out for several high speed test runs on the Upper San Francisco Bay, Sacramento, San Joaquin Rivers Delta Region, keeping a watchful eye out for the various Tug Boats, Tankers, Barges, Freighters, Dredges, Nunn Buoys, and Commuter Ferries, which there are plenty of in these waters and especially at night. Oh, and before I went out I performed a Compression test, and the results are as follows: #1- 142psi, #2-150psi, #3-155psi, and #4-145psi, which is pretty respectable for an almost forty yr old engine, and I brought along the multi meter, which had previously given me the high rpm's, which caused me to purchase the 23"pitch propeller from iboats. Well guess what, I was not able to achieve any higher rpms than 3900, based on the readings from my new Tach, while the multi-meter was still showing 5400 to 5600 rpms at the top end. Interestingly enough, the rpm readings between the the new Tach and the multi-meter, were consistent up to abt 1800 rpm, after which the rpms from the multi-meter started to take off. I was never able to achieve any higher rpm than 3900 based on the new Tach, trimmed out to the best of my ability and the surface glass like, possibly due to the fact that I damaged the existing 19"pitch prop pretty badly upon take off from the launch area, as I hit a submerged object (at this super-low tide), alleged to be an old auto. So, I'm gonna exchange the 23" pitch prop for a 17"

 

[steelespike]

Re: RPM too high at WOT

Your motor appears to be healthy Which imo would make a 17" pitch unecessary.
Are you sure the throttle opens all the way,Is the timing set right and advancing as it should?
Is the bottom clean and smooth?Does the boat respond to up trim?
Does the boat lay down flat like its stuck to the water?

 

[topglock]

Re: RPM too high at WOT

I'm with SS, here. I'm no prop guy, but I've been an engine guy all my life and it sounds like you're not getting full power from your engine. Check timing, spark advance and carb for full open and see if you find anything. While you're at it, check for fuel flow and exhaust restrictions. ...and just for S&Gs, verify the tach is set properly for your engine... I'd kinda like to know what you find...