Max RPM?

[Marlinman10]

I have a 1995 Yamaha 85 two stroke on a 17 foot glasspar that weighs about 850lbs with no motor on it. It has a 13.25 x 17 prop. I just installed a tach and was surprised to find out it only turns 4000 rpm at WOT. Is it possible the tach isn't correct. What rpm would one think it would turn with this prop? The motor is in like new condition and runs excellent. The boat goes 29 kts at wot.

Could the prop be to big ?

Mike

 

[countvlad]

Re: Max RPM?

I have a 1995 Yamaha 85 two stroke on a 17 foot glasspar that weighs about 850lbs with no motor on it. It has a 13.25 x 17 prop. I just installed a tach and was surprised to find out it only turns 4000 rpm at WOT. Is it possible the tach isn't correct. What rpm would one think it would turn with this prop? The motor is in like new condition and runs excellent. The boat goes 29 kts at wot.

Could the prop be to big ?

Mike

17p .... you might be overpropped...it is possible the tach is not on the correct seting... follow the instructions make sure the tach is on the right setting/pole....

next find out what is the recomnd manufct WOT range...

 

[Marlinman10]

Re: Max RPM?

I know the RPM range. 5000-5500.
The directions don't tell you which poll to put it on. I have it on 12 which is what a mechanic told me but that doesn't mean much. The other position is 6 but I haven't tried it yet. I need someone who could hook a tach right to the motor and than I can check the tach. It just doesn't seem like I could have 5in too much pitch.

 

[junior1113]

Re: Max RPM?

a 17 should do more than 4000 on a 17' boat. tach is off or there is more issues with motor performance. how it hole shot?

 

[hwsiii]

Re: Max RPM?

Definitely don't buy a new prop, Paul is right something else is the problem.



H

 

[Marlinman10]

Re: Max RPM?

Hole shot is fine. I need to take it in and check the tach first. It runs excellent. It does 30kts with two of us on board and 24gals of fuel.

 

[hwsiii]

Re: Max RPM?

Mike, I can tell you the boat can't be doing 35 MPH at 4,000 RPM, as it is showing a -9% prop slip, with a true 17" pitch prop and a 2:1 gear ratio, if I have the gear ratio wrong please let me know as my numbers will be wrong. What make and model of prop do you have on it now.

Prop Slip

H

 

[Marlinman10]

Re: Max RPM?

H,
It's a solas prop 13.25x 17

I'll look up the gear ratio and post.

 

[hwsiii]

Re: Max RPM?

Ok


H

 

[wca_tim]

Re: Max RPM?

Someone referred to this above. there should be a switch on the tach that allows you to set it for say 4, 6 or 8 cylinders. what is that set on? Without knowing the number of pulses per rpm from the rectifier on your outboard, I'm guessing it should probably be set on 6 cylinder (currect me if I'm wrong, but I am assuming that your engine is a 3 cylinder? 6 cyl is the only multiple of 3 option and would be the onlny setting that makes sense as a first thought) It sounds like your engine is running ok, and that your propped ok.

if for example the tach is counting 4 pulses as one revolution and it is supposed to be three, your tach would be reading 3/4ths of the actual value. putting your max rpm at wot right now around 5300 rpms.... and right where you would want it.

Hope this is helpful.

 

[Marlinman10]

Re: Max RPM?

Someone referred to this above. there should be a switch on the tach that allows you to set it for say 4, 6 or 8 cylinders. what is that set on? Without knowing the number of pulses per rpm from the rectifier on your outboard, I'm guessing it should probably be set on 6 cylinder (currect me if I'm wrong, but I am assuming that your engine is a 3 cylinder? 6 cyl is the only multiple of 3 option and would be the onlny setting that makes sense as a first thought) It sounds like your engine is running ok, and that your propped ok.

if for example the tach is counting 4 pulses as one revolution and it is supposed to be three, your tach would be reading 3/4ths of the actual value. putting your max rpm at wot right now around 5300 rpms.... and right where you would want it.

Hope this is helpful.

H,
It's 2:1


It is a 3cly. There are 3 switches on the back. One switch is for 4 stroke or two stroke. That's easy.

Another is for the trim indicator for two wire or three wire. Mine is three.

The third switch is for 6 pole or 12 pole. I have mine set to 12. That's what the Yamaha mechanic told me to set it too. I'll try the 6 position and see what it says. I just want to know the RPM before I do anything with the prop. I would also like to put a stainless prop on it and keep this prop for a spare.

Thanks for your help.

PS Would this have any effect on the hour meter being correct or not?

 

[hwsiii]

Re: Max RPM?

Mike, my assumption is you are not using a GPS for speed, the Solas prop has a heavy cup on the trailing edge or the tach is wrong. My assumption is the tach is wrong. And it really should have no effect on the hour meter, as the hour meter should only work when the motor is running, not just when the key is on.
When you get it sorted out please let us know.


H

 

[Marlinman10]

Re: Max RPM?

I am using a GPS for speed and also have run the measured mile. I switched the tach to the 6 pole position and all that did was double the RPM. It turns about 8000 rpm now. NOT. I don't think the tach is the right one for the motor. I'm taking it in for a tune up and having it checked out. I'll keep you posted.

Mike

 

[hwsiii]

Re: Max RPM?

Let us know Mike, and then we can see what needs to be done, if anything does.


H

 

[Marlinman10]

Re: Max RPM?

H,
Well I finally got it in the shop.

1. My tach is only off 40 - 60 RPM they say.
2. They suggested a de-carb so I had that done. They couldn't find anything else wrong. After doing the de-carb he said it had quite a bit of carbon in it.

3. This weekend I'll open it up and double check what max RPM is all trimmed out. The best I've seen so far is 4100 and 30 Knots.

4. I was wrong what the rpm range should be. It's 4500-5500 RPM.

5. I'll weigh the boat soon and let you know what it is for sure.

Mike

 

[hwsiii]

Re: Max RPM?

Mike, this is the reason I get people to fill out my form, so I an do a omplete analysis on the omplete setup, this is my fault. So I have done the best analysis I ould with the information I ould find myself, sine we have this problem. Please go over everything that I have below to see if you find anything wrong that you an see. But please weigh the boat as you intend to already.

Boat and Motor

Prop Piker

Prop Pith hange

If you an only get 4,100 RPM out of a 17" pith prop then to get about 5,400 RPM you would need to hange to a 13" pith prop, and there is something wrong with having to use a 13" pith on a boat that has an allup weight of only 1,800 pounds with an 85 HP motor and a 2:1 gear ratio. I have a 16' Sundance Flats Boat with a 1995 Force 40 HP outboard and I run a 14" Apollo SS prop on it with a 2:1 gear ratio. When I am loaded up with 4 adults and 3 children my WOT RPM are at 4,450 and I am doing 24.5 MPH with an allup weight of right at 2,019 pounds. I don't normally run with that many people, but I take my son his wife and teenage daughter and the 3 grandkids. And when I only have an allupweight of 1,450 pounds I am able to run right at 30 MPH.

Prop Slip

Number 1 is what you show your boat is doing at this time, and that is 34.5 MPH at 4,100 RPM with a prop slip of -5% whih is impossible.
Number 2 is what I think you are probably doing if the speed on your GPS is atually showing MPH and not Knots and shows a prop slip of 9% whih is attainable.
Number 3 is the speed you say if you atually have a 19" pith prop instead of a 17" prop maybe beause the prop has been repithed, and it shows a prop slip of 6% whih is low but is attainable. If it atually is a 19" pith then by hanging to a 15 pith we would attain an inrease in RPM to about 5,200 RPM.

Prop Pith by Speed

This analysis shows the theoretial prop pith needed with the prop RPM and the speed shown plus a 10% prop slip.
The first numbers are the Theoretial Speed that your boat ought to be able to attain with the Weight, Length and HP shown using my alulations and the prop pith required.
Number 1 Real Speed is the atual speed you say it is doing and the prop pith required to do that with a theoretial 10% prop slip.
Number 2 is the speed if it is MPH and not knots and the prop pith needed to do that with a 10% slip fator.

Motor Height

I would like you to measure the antiventilation plate and see how high it is ompared to the keel of the boat.

Mike I don't really know what is going on but it doesn't make sense unless the speed is in MPH, the boat is full of water, the 17" pith prop is atually a 19" pith, the motor is NOT produing the rated HP or the tah is wrong or something I haven't thought of.


H

 

[Marlinman10]

Re: Max RPM?

1.Weighed the boat. 2050 lbs.
- 300 trailer
1750 lbs.
This was full of fuel and what I had on board during my last test. I weigh about 280 so 2030 lbs is very close.

2.With trimming I reached 4500 RPM and 32 knots on the GPS. It wasn't the smoothest in the bay but this is very close to max.

3. The boat is 17'6'' and has a deep v but is pretty flat from midway back to the transom. I haven't had a chance to measure the bean yet.

4. The antiventilation plate is about level with the bottom of the keel.

5. The boat performs just fine but I just want to make sure I'm not lugging the motor.It seems I'm a little over propped. I would also like to buy a s/s prop one day and keep this one for a spare. I would normally have another adult and my bait tank full which hold 22 gals of water. Also about another 100 lbs of gear on board.

Also what would less pitch and more RPM do for top speed and fuel economy?

What do you think? Thank You

 

[hwsiii]

Re: Max RPM?

Mike, if you buy a 15" pith prop, in the exat same prop you have now, your RPM will go up to about 5,100 RPM whih is a little above the middle of your operating range and if you go to a 14" pith your RPM will be right at 5,400 to 5,500 RPM. You shouldn't lose muh speed and your fuel eonomy will go up.

Prop hange

H

 

[Marlinman10]

Re: Max RPM?

Thank You,
What if I wanted to go with a S/S prop. Would I go with the 15in pitch?

 

[hwsiii]

Re: Max RPM?

No, you will want to go with a 14" pith in a stainless steel prop. And if it was me I would go with a 14" even in the aluminum prop to get maximum RPM and fuel eonomy as well as long engine life.


H