Low rpm's is it the prop?

[fluidjuice]

I have a mercury classic 40 hp 1991,on a 14ft speedboat wired up a new tacho and found the rectifier needed replacing, done that and now it works , so i took it out for a spin at wot and im only getting 3000rpm's at wot.
The prop i have on is a mercury 10 inch diameter , 3 blade with a 13 inch pitch. is this over propped? and is it ok to still use? as before i was oblivious to the low rpm's. wot rpm's should be 5000-5500 as spec in manuel. i get about 30 mph at wot. tacho is registering around 650 to 700 rpm's at tickover. so if this is the prop what size is needed or standard.
I have read that by changing the pitch size up or down by one inch you can see a 300-500rpm diffrence at wot, surly my prop isnt that far out????
Thanks

 

[steelespike]

Re: Low rpm's is it the prop?

First of all the rule of thumb is about 200 rpm per inch of pitch change.
Define Speedboat.
I think your tach is wrong.Your speed is in the ball park.30 to 35 with a stock setup.Hard for me to believe you would be over propped at 13 on a 14 ft "speedboat"Is the antivent plate about even with the bottom of the boat?

 

[fluidjuice]

Re: Low rpm's is it the prop?

Yes cavitation plate level with bottom of boat.
Do you think tacho needs calibrating even though it registers correct idle speed???
I think mercury reccomend a 12 pitch would that make so much difference??

 

[Randybeall]

Re: Low rpm's is it the prop?

I suspect you have running problems. Perhaps a cylinder not running at speed, water or dirt in a float bowl. Could be as much as ignition failing under load. That boat should fly with 40 hp. 3000 rpm 30 mph does not sound too out of line just needs all its power to go on up.

 

[steelespike]

Re: Low rpm's is it the prop?

If your dragging the whale tail through the water thats good for a couple of mph.I'm sure it helps you plane easier but it probably makes the boat lay down too much.
Rule of thumb 1" change in pitch nets about 200 rpm.
Disregarding the small boat a 40 is usually good for 33 to 35 mph ideal stock setup.I think if you were short 2,500 rpm You would be going a heck of a lot faster than 35 at 5,500.Possibly it is capable of 40 or so but not with a stock 13 prop.I think 30 at 3,000 would be unusual as well. Boat appears to have a
fairly deep V ,not conducive to high speed at relatively low power.
By all means confirm throttle opens all the way.And it operates on all cylinders.

 

[fluidjuice]

Re: Low rpm's is it the prop?

this engine is manuel trim, when i took it out i had the trim pin in the second hole up from the bottom, would this make a difference to wot rpm,s????
If so what is the reccomended position for pin???
Thanks

 

[steelespike]

Re: Low rpm's is it the prop?

Generally the antivent plate parallel with the bottom, but sometimes other positions work best. Try them all.Usually trimming out ,prop away from the boat ,will improve speed.Sometimes one notch may not be enough but one more is too much.You can buy or make wedges for between the mount and the transom to give you half a notch.

 

[GiMLit]

Re: Low rpm's is it the prop?

Yes cavitation plate level with bottom of boat.
Do you think tacho needs calibrating even though it registers correct idle speed???
I think mercury reccomend a 12 pitch would that make so much difference??

Not sure if it just me but that engine seems to be a bit high doesn't it?
Anticav plate looks like it is above the boat bottom?
If I recall from previous readings even or up to an inch below boat bottom is ideal. correct ?????????

 

[fluidjuice]

Re: Low rpm's is it the prop?

Its just the angle pic was taken i can confirm cavatation plate is level with bottom of boat.

 

[steelespike]

Re: Low rpm's is it the prop?

Its just the angle pic was taken i can confirm cavatation plate is level with bottom of boat.

Correct term is "antiventilation plate".cavitation is a different animal.
Anti vent plate even with the bottom is considered a starting point.
If you have no ventilation issues you are probably too low.Raising the motor a little at a time to a point just before venting becomes a problem .
Could get you a couple of mph and some rpm.May improve hole shot.
A stainless or a prop like a Hustler turning point may allow even more height.
Just be sure it pumps water.Most folks tolerate occasional venting to get that fine point height.

 

[fluidjuice]

Re: Low rpm's is it the prop?

Ok i took it out today after re-calibrating tacho,and results were>>>>>>>

third trim hole up and tacho showed 4600 and 30mph at WOT.
Fourth trim hole up and tacho showed 5300 and 35 mph at wot but then it would rev out so i guess i was ventilating to much i think???Still had water coming from pee hole but water seemed a bit hotter and engine sounded alot louder, Engine has a whale tail fitted and was traveling on the surface of the water / spray wil i harm the engine using it at this trim point or should i try and trim it inbetween these two pin holes with wedges.

Mercury 40hp classic 1991 stock prop manuel says 5000/5500 rpm's at WOT

 

[j_martin]

Re: Low rpm's is it the prop?

If you're looking for performance,
1. Ditch the whale tail. It's just drag when you need a clean, slippery foot.
2. Install a water pressure gauge. You can run the motor as high as you want as long as you have sufficient water pressure. The gauge is an instant indicator for that. They don't cost much, especially compared to engine repairs.
3. Systematically experiment with your prop and setup.
4. Use a cheap hand held GPS to check your speed. The water driven speedo's are notably unreliable, and usually pretty generous.

There will be an ideal setup of engine height, prop, and trim angle that will give you max speed and run in the upper part of the WOT rpm range. You may want to trade off a bit of it for handling.

I run a v6 on a bass boat, and trim it out till the water pressure starts to flutter, then back in a small amount. At that point the plate is way above the water, and about 1/4 of the propeller is out of the water.

The cupped high performance stainless prop stays hooked up because it's designed to do that. It's also ventilated so it has a neck snapping hole shot.

Looking at your results, it looks like your engine is a little too low, and the trim angle has to be too high. I'll bet it's throwing a pretty good rooster tail, which is wasted energy if excessive.

hope it helps
John

 

[steelespike]

Re: Low rpm's is it the prop?

Ok i took it out today after re-calibrating tacho,and results were>>>>>>>

third trim hole up and tacho showed 4600 and 30mph at WOT.
Fourth trim hole up and tacho showed 5300 and 35 mph at wot but then it would rev out so i guess i was ventilating to much i think???Still had water coming from pee hole but water seemed a bit hotter and engine sounded alot louder, Engine has a whale tail fitted and was traveling on the surface of the water / spray wil i harm the engine using it at this trim point or should i try and trim it inbetween these two pin holes with wedges.

Mercury 40hp classic 1991 stock prop manuel says 5000/5500 rpm's at WOT

LOSE THE FIN. It will plane slower but its the nature of the beast.
I think trim results will be better as well.I think its possible the jump in rpm and speed is the fin clearing the water better in the 4th hole.
Fin is probably making it harder to get the bow up as well.

I think the Hustler turning point AL.

 

[CaptAhab]

Re: Low rpm's is it the prop?

S.S. is right the whale-fin is prolly hurting you'r top end..cause if you had the right prop, maybe a stainless 12..[if they make it]you might be able to run the motor higher eliminating the whale tail problem at high speeds, no fin in the water but it's there when you have to go off plane and launch again.
I'm pessimistic about how much you can achieve with that motor..a classic Fifty like I got or a classic 65 with racing foot and prop that boat would be scary..you would want the deep vee at that point.

nice boat man.