Not reaching proper rpm or speed

tpgrealty

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Sep 13, 2005
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I have a 1997 Yamaha 150 pro-v with a 25 pitch prop (can't find make,)5 inch jack plate on a 1983 Ranger 350-v. I am only getting 52mph @ 5000rpm wot. Motor is setting 3.75" above transom and as best as I could measure cavatation plate is 1-2" above step on transom. I have a slow hole shot, mid acceleration to top speed. A friend says I am throwing to much water and losing a lot of power when he rode behind me. <br />I know That this boat and motor set up should pull 5400 -5600 rpm and hit 60+mph. I would like to get a better hole shot and higher top end.<br />Thanks in advance for your help.
 

Ron G

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Apr 28, 2005
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Re: Not reaching proper rpm or speed

Is your exsiting prop a 3 or 4 blade?someting doesnt sound right to me.what does he mean your throwing to much water as a rooster tail?hows your water pressure with it trimed all the way?the experts should be by and give you alot of help.
 

tpgrealty

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Sep 13, 2005
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Re: Not reaching proper rpm or speed

Prop is a 25-m 3 blade. I think he means in the rooster tail. The rooster tail reaches about the height of the motor at wot with it trimmed out and I am getting 17-21psi. Motor does not sound like it is bogged down. All the cylinders compressions are within 5lbs. The motor has had very little use the last three years. I bought the boat in June of the year. The prop is in good shape and has no knicks or dings. A local prop shop said there is nothing wrong with the prop and he suggested to use a 23p-3 blade or a 22p-4 blade. Hope this helps
 

QC

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Mar 22, 2005
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22,783
Re: Not reaching proper rpm or speed

I was going to suggest that pitch appears to be your problem as you are not reaching WOT RPM. However, this assumes that it is running correctly and you also note that it has had little use. Do you know for a FACT that the engine EVER ran at the correct WOT RPM with this prop?<br /><br />BTW, it is "bogged down" or in a full lug condition if it is not reaching WOT RPM . . . ;)
 

walleyehed

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Jun 29, 2003
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Re: Not reaching proper rpm or speed

25M sounds like a Turbo for a Merc...maybe, maybe not...<br />Rooster tail at engine heigth is about as high as you want it...anything higher is a sign of lost efficiency.<br />You say 17-21 on the water pressure...it's not fluctuating that much is it????<br />Also, as quitecat has asked, has it ever run at 5500+ RPM????
 

tpgrealty

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Sep 13, 2005
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Re: Not reaching proper rpm or speed

wallyhead<br /><br />I bought the boat in June. The most rpms I have gotten out have been 5000rpm. The water pressure @17psi all the way trimmed out, @ 21psi just short of all the way trimmed out. Either way I am turning 5000rpm. I am trying to find a 23p to borough and try out. Tough to find here
 

LubeDude

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Oct 8, 2003
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Re: Not reaching proper rpm or speed

If Im not mistaken, and I could be, I think that those year Rangers had built in wedges built right into the corners of the hull. That slowed them down some. They were put there to illiminate chinewalk. Many had them removed. Also, those old Rangers were very heavy. I still think you have more in it though and Walleyehed will guide you though it. 60 MPH isnt an unreasinable goal.
 

tpgrealty

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Sep 13, 2005
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Re: Not reaching proper rpm or speed

OK tried a 23 tempest plus. Got rpm up to 5300 wot but lost 3mph (48 gps) and gained huge rooster tail ( 10ft+) when motor trimmed out. Hole shot improved to 6 sec with two people in boat. Down from 8-10 sec.<br /><br />What to do now? Lower motor and try 23p again?
 

Dhadley

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Feb 4, 2001
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16,978
Re: Not reaching proper rpm or speed

Nope. Look at the hull like LubeDude suggested. What he said makes a lot of sense after your test results. Spend some time under the hull with a 4' straightedge.
 

tpgrealty

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Re: Not reaching proper rpm or speed

ok today lowered the motor 1 and 5/8ths inches. No more huge rooster tail. Ran 23tempest plus and got 53.88 (gps) @ 5100 rpm. The 23 has been worked and has a good bit of cup to raise bow. Hole shot 3 sec with just me and fishin gear.<br /><br />ran 21m yamaha prop and got 51.8 (gps) @ 5500 rpm. Not as much bow lift (as 23) and felt like too much hull in water. Hole shot was 2 sec again with just me and gear. <br /><br />Motor seems to be at right height and both props shot rooster about half as high as top of motor. Both had good acceleration 23 had better. <br /><br />Where to go from here? Agian I would like to get to around 60mph if possible.
 

walleyehed

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Re: Not reaching proper rpm or speed

If you keep lowering the engine, forget about 60.<br />The reason you got 5300 but dropped 3mph and added a 10ft roostertail is because the engine is too low, and max trim is required to get the R's up....higher slip ratio..<br />You're going to need to consider the hooked hull as well. That may be your speed limiting factor.<br />If Dhadley would offer some suggestions in this case, I would pay very close attention as "Speed" is his middle name.
 

tpgrealty

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Re: Not reaching proper rpm or speed

Walleyehead<br /><br />I'm confused. I thought a tall rooster tail is from the engine being mounted to high. I was told lowering the engine puts the prop shaft lower in the water thus lowering the height of the rooster tail. I may be backwards on this. Please explain.
 

walleyehed

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Re: Not reaching proper rpm or speed

Yea, you can get too high, but if the boat is still accelerating with the RPM gain, you're good. if all you get is, say 5200RPM at 60mph, and you trim a bit farther and get 5300-5400 with no speed gain, but DO gain a roostertail, you are lowering efficiency. The more trim used beyond what results in max fwd speed or max RPM, the more power you have that is un-usable.<br />You want the engine as high as possible that will allow the needed lift with the minumum amount of trim.<br />It may require a higher quality prop to achieve this.<br />The problem you have, is the faster you go, the more lift the stern has because of the hull design, which in turn, requires more positive trim to keep the nose up...The prop is fighting the hook and the hook is fighting the prop <br />
No more huge rooster tail. Ran 23tempest plus and got 53.88 (gps) @ 5100 rpm. The 23 has been worked and has a good bit of cup to raise bow. Hole shot 3 sec with just me and fishin gear.<br /><br />ran 21m yamaha prop and got 51.8 (gps) @ 5500 rpm. Not as much bow lift (as 23) and felt like too much hull in water. Hole shot was 2 sec again with just me and gear. <br /><br />Motor seems to be at right height and both props shot rooster about half as high as top of motor. Both had good acceleration 23 had better. <br />
With this info, you answered your own question as to the affect of efficiency...the roostertail was not an indicator of the perfect set-up, as your speed increase tells us that especially with a lower RPM...Problem is, 5100-5200RPM will kill your engine over a shorter period of time that you may like.<br />You have hull issues that a prop may not fix, but the RPM needs to come up even if the speed doesn't.
 

tpgrealty

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Sep 13, 2005
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Re: Not reaching proper rpm or speed

Thanks Walleyehed<br /><br />Talked to a prop shop this afternoon and he explained basically the same. He told me that I needed to have the prop worked to fit my boat setup. He said that the prop was the reason for the tall rooster. He said I probally had the motor maybe an 1/8" to 1/4" high at the start of this whole process(1st post). He told me to raise the motor to 3.5" below pad and have prop worked and blueprinted.<br /><br />Would you agree with his suggestion?<br />Again thanks.
 

walleyehed

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Re: Not reaching proper rpm or speed

I'm all for blueprinting...ya get a "true" prop this way..<br />You want to be pretty close to what you want, and explain in detail to the person working your prop, what the boat is doing and what you want, and don't be surprised if you have to adjust some.
 
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