3.0 over reving with 19p

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atistang

Petty Officer 2nd Class
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I got a 1989 cheetah 1800ls, alpha 1, 130hp 3.0 from a friend last year. it was in need of a gimbal bearing among other things. any way last year when i took it out before doing any work it would hit 35mph at around 4500rpms with just me aboard. this spring i replaced the gimbal and rebuilt the carb. took it back out with 10 gal more fuel than before, one extra person (me and my friend each weigh 220ish) and now it still does 35 mph, but at times i can hit 5000rpm. currently the prop is a 14" aluminum 19, and the drive has a whale tail.

most of the time it will be 2-4 people total on board, but there will be times when i will have the 7 people on board. probably never pull any skiers, just a tube here and there. should i stick with the same diameter and go to a 21? or maybe just increase the diameter a bit? my friend i bought it from claims on a different lake (lower elevation) he had the boat running 47mph without the whale tail, do the tails slow you down that much?

another note I can't go to much bigger, i tried my friends stainless 21 i think it was 14 3/4" but it hit the little aluminum fin on the drive.

in my head now i'm thinking aluminum 21 14"1/4, then keep the 19 around for when i plan to have a full boat.



EDIT: max operating rpm is 4400-4800rpm
 
SparkieBoat

SparkieBoat

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Re: 3.0 over reving with 19p

how much are you over revving?? what is the best WOT RPM for your motor?? the 21 should reduce your rpm about 400..the diameter will not make much difference. if the 14 3/4 hit your alum fin you will have to stay with smaller diameter. your fin is not on backwards is it???
 
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atistang

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Re: 3.0 over reving with 19p

the fin is in the right way

max operating rpm for the engine is 4800rpm
 
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atistang

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craze1cars

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Re: 3.0 over reving with 19p

Is it possible you have torn exhaust bellows and your prop is ventillating due to exhaust blowby?

If your speed stayed the same, and your RPMS have increased, it usually means something is wrong. Slipping hub, slipping engine coupler, inaccurate tach, inaccurate speedo, torn exhaust bellows, damaged propeller from impact...etc.

It is also possible the whale tail is drawing air into the prop at certain speeds. To answer your question about whether the whale tail robs speed, just take it off and do a before/after run to compare. Some boats work well with them, some work absolutely terrible with them....
 
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craze1cars

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Re: 3.0 over reving with 19p

Diameter is pretty meaningless. I say ignore it. Pitch changes speed/RPM. Plus every brand and style of prop is slightly different.

And lower elevation will always be faster. High elevation reduces engine HP. How much elevation difference is there between your locations?
 
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atistang

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Re: 3.0 over reving with 19p

craze1cars said:
Is it possible you have torn exhaust bellows and your prop is ventillating due to exhaust blowby?

If your speed stayed the same, and your RPMS have increased, it usually means something is wrong. Slipping hub, slipping engine coupler, inaccurate tach, inaccurate speedo, torn exhaust bellows, damaged propeller from impact...etc.

It is also possible the whale tail is drawing air into the prop at certain speeds. To answer your question about whether the whale tail robs speed, just take it off and do a before/after run to compare. Some boats work well with them, some work absolutely terrible with them....
Click to expand...

i just had all that stuff off, didn't replace the exhaust bellow, it looked pretty good. i just figured after the carb rebuild the engine was running a bit better, but the extra weight was why i wasn't getting any more speed. i do suspect the tach to not be the most accurate tach in the world
 
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atistang

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Re: 3.0 over reving with 19p

craze1cars said:
Diameter is pretty meaningless. I say ignore it. Pitch changes speed/RPM. Plus every brand and style of prop is slightly different.

And lower elevation will always be faster. High elevation reduces engine HP. How much elevation difference is there between your locations?
Click to expand...

about 1200ft
 
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craze1cars

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Re: 3.0 over reving with 19p

1200 ft isn't anything to worry about. Not high enough to kill power substantially.

I will say that I believe 47 mph would be likely a world record for a 1989 3.0L equipped 18 footer....I'm not calling your friend a liar but hmmmmm.....

35 to 40 top speed is pretty typical for that type of boat.

Need accurate data to really give any accurate answers. Need EXACT top speed RPM (confirm with a differen tach), EXACT GPS top speed, your exact outdrive ratio, and you have already provided 19 inch pitch prop data. With those 4 data points you can use a prop slip calculator to see if you have good slip or excessive slip. Guesses and estimates are not good enough for any of that data...if you put garbage into a calculator you'll get garbage out. Without 100% of those 4 pieces of data it's almost impossible to troubleshoot a potential problem or recommend a good prop.
 
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atistang

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Re: 3.0 over reving with 19p

craze1cars said:
1200 ft isn't anything to worry about. Not high enough to kill power substantially.

I will say that I believe 47 mph would be likely a world record for a 1989 3.0L equipped 18 footer....I'm not calling your friend a liar but hmmmmm.....

35 to 40 top speed is pretty typical for that type of boat.

Need accurate data to really give any accurate answers. Need EXACT top speed RPM (confirm with a differen tach), EXACT GPS top speed, your exact outdrive ratio, and you have already provided 19 inch pitch prop data. With those 4 data points you can use a prop slip calculator to see if you have good slip or excessive slip. Guesses and estimates are not good enough for any of that data...if you put garbage into a calculator you'll get garbage out. Without 100% of those 4 pieces of data it's almost impossible to troubleshoot a potential problem or recommend a good prop.
Click to expand...
i checked it with a gps last year, the speedo was reading right with the gps, so as far as i can tell it should still be accurate, right? how much slip is to much? rough numbers in a calculator show i have 20%. I'm basing that on a 1.98 gear ratio.

I'll try and confirm the ratio this week
 
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craze1cars

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Re: 3.0 over reving with 19p

That's a fair amount but not terrible. I'd expect closer to 15% to be good for a boat like that.

You can reduce RPMS and gain efficiency/speed/holeshot all a bit by going to a more aggressive prop that has more cupping and more rake. Incidentally, frequently these props will be SMALLER in diameter...probably 14 to 14 1/4", but will still possibly hit your factory anode due to the extra rake.

Have no idea what kind of prop you have now...if it is not cupped/raked you can likely reduce your RPMS a few hundred while gaining a little bit everywhere else by simply adding cup & rake....while keeping the same 19 inch pitch.

I will be real surprised if your 3.0L is capable of pushing a 21 pitch prop. Most 3.0's run 17s and 19s.

I'm confused of when you added this whale tail. If it was after your run last year with 4,500 rpm, then I suspect the whale tail may be contributing to your higher RPM problem and creating some slip somehow. 4 bolts usually? Easy to remove. The holes left behind will hurt nothing. So I still suggest a before/after run and see if it's really helping or hurting you. Those things really get mixed reviews and sometimes do not work.
 
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steelespike

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Re: 3.0 over reving with 19p

Depending on the setup a whale tail could lose a couple of mph. I would try it without the tail.
Whale tail is a bandaid over a setup problem.Some I/Os are slow out of the hole.imo a prop change is a better choice than a tail or passive tabs.Its not unusual for any boat to have to move people forward till on plane.
My experience the 19 should be just about right.Is the prop in excellent shape?You need to verify the tach.
 
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atistang

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Re: 3.0 over reving with 19p

the prop is a stock mercruiser prop i believe. 48 78120 A40L 19P

the whale tail was on it when i got it, if i get a chance to launch it this week then i'll take it off and see how it acts
 
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atistang

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Re: 3.0 over reving with 19p

ok, got the tach reading correctly, and

the drive is 1.98:1

4800rpm (this is the max rpm in the max range)

and 35mph (gps)

this is with a full tank of gas and one passenger (about 450lbs of people)

19 pitch prop

so is there a better prop out there for me? maybe not a 21, but one with more cupping?

this figures up to be 20% slip in the calculators
 
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