1992 Yamaha 55HP on 16 Foot Excel 16DX

abbtech · Jul 20, 2009

I bought this boat last year. It has the original motor on it but the shop I took it to thinks the prop is not the correct pitch. I contacted Wellcraft and they don't know the original prop that would have been provided.

Currently there is a 12.25 diameter X 9 pitch prop on the boat. There is no tach on the boat so I am not sure of the RPM at WOT. The fuel economy is terrible, most people I have talked to think it should get about twice the distance on a tank!

The shop suggested the standard, install a tach then increase pitch to decrease WOT RPM accordingly (1 inch of pitch for each 200 RPM needed).

Here are some pictures of the same boat as mine (except it looks like the outboard has been upgraded).
http://www.en.best-boats24.net/haendler/hellwig/boote/1877125.html

Basics are:
Weight: 1000lbs
Length: 16 feet
Beam: 6 feet
Motor: Yamaha 55HP, model C55ELRQ
Boat: Excel 16DX

I can't find any details on the WOT RPM other than some educated guesses that it should be 5500 RPM.

I did find an unused wire harness by the control box that had a number of wires where the tach would probably connect. I put a frequency meter from a black ground to the white wire and at idle there is 40Hz on it (increases proportional to engine RPM). I am thinking that if I take that frequency and multiply it by 20 that should give me the correct RPM (800 at idle). Does this sound right?

Any help on getting a prop that will improve my fuel consumption is appreciated!

hwsiii · Jul 20, 2009

Re: 1992 Yamaha 55HP on 16 Foot Excel 16DX

Abb, running a 9" Pitch prop on that boat and motor, if you have run it at WOT I can't believe you haven't blown the motor up. When you get a tach and a GPS speed at Wide Open Throttle come back and we will get the right prop for you. Just don;t run it at WOT until then.

H

abbtech · Jul 20, 2009

Re: 1992 Yamaha 55HP on 16 Foot Excel 16DX

Thanks, I am going to try to get it in the water this weekend. I will keep the frequency below 275Hz though since that should be 5500 RPM (if it is a multiple of 20).

Does that sound right that the tach input would need a multiple of 20. Or is there some technical specs that would describe it that you know of? I found some info that indicated it would be a 4 pole alternator in the system.

abbtech · Jul 20, 2009

Re: 1992 Yamaha 55HP on 16 Foot Excel 16DX

I think I just figured it out... Not sure what I was thinking but since 40Hz would be 2400 pulses per minute, with a 4 pole alternator that would mean that it was running at 2400/4=600 (600 RPM).

Does 600 RPM make sense for an idle speed? If so I think I can just use the frequency meter until I get or make a tach.

hwsiii · Jul 20, 2009

Re: 1992 Yamaha 55HP on 16 Foot Excel 16DX

Yes, that is the correct multiples as far as I know, and hopefully you can tell us how many RPM you are turning at WOT when you test it.

H

abbtech · Jul 22, 2009

Re: 1992 Yamaha 55HP on 16 Foot Excel 16DX

hwsiii said:
Abb, running a 9" Pitch prop on that boat and motor, if you have run it at WOT I can't believe you haven't blown the motor up. When you get a tach and a GPS speed at Wide Open Throttle come back and we will get the right prop for you. Just don;t run it at WOT until then.

H

OK, just got back from a run with some surprising results.

6km/h was at a tach frequency of 57Hz. (3.7 miles/h @ 855 RPM)
9km/h was at a tach frequency of 116Hz. (5.6 miles/h @ 1740RPM)
38km/h was at a tach frequency of 200Hz, this is WOT. (23.6 miles/h @ 3000 RPM)

Unfortunately these were done with a half loaded boat (me and one other person) so there was about an additional 200 lbs in the boat.

After seeing these results I am a bit relieved that I wasn't revving the hell out of the motor but I am concerned that the motor is not able to get full RPM with the 9 pitch prop. Could the prop be bent to a different angle?

Would running the motor at 3000 RPM be the cause of the poor fuel consumption?

Thanks for all your help.

Alan

hwsiii · Jul 22, 2009

Re: 1992 Yamaha 55HP on 16 Foot Excel 16DX

Alan, that is a 19" pitch prop you have and NOT a 9" pitch, and you need a smaller diameter prop. LOL Get a real tach and we will get you propped right. You are wayyy overpropped and that is causing your poor gas mileage.

H

abbtech · Jul 22, 2009

Re: 1992 Yamaha 55HP on 16 Foot Excel 16DX

Thanks for your very quick response!

I am quite sure the prop is listed as a 9 pitch, but there is a -G (or possibly a -6) after the pitch. Please have a look at the attached pictures. I have also included some pictures of the prop blades, I am not sure if the original prop could have been modified to change the pitch?

I am thinking that I will probably make a digital tach using a small LCD display and a PIC microcontroller.

Alan

hwsiii · Jul 22, 2009

Re: 1992 Yamaha 55HP on 16 Foot Excel 16DX

Alan, it is impossible for a 9" pitch propeller to do 23.6 MPH at 3,000 RPM with a 1.84:1 gear ratio, that would be a -70% prop slip, and that is impossible. Here are my numbers. So we need to go from here.

I am not trying to be hard or anything else, just factual. And a 55 HP motor can't turn a 9" pitch prop but 3,000 RPM, that doesn't make sense.

Prop Slip

H

abbtech · Jul 23, 2009

Re: 1992 Yamaha 55HP on 16 Foot Excel 16DX

That's fantastic data! So I guess a prop somewhere between a 10 and 13 would be the best? I am wanting fuel economy to be my main concern, would I be correct in thinking that the 13 would be my best bet?

I am thinking of getting a Comprop 10 X 13
http://www.comprop.com/

hwsiii · Jul 23, 2009

Re: 1992 Yamaha 55HP on 16 Foot Excel 16DX

You need a 13" pitch prop and you should be doing about 30 MPH at WOT. I would buy an aluminum prop instead of the comprop, because no matter what all their claims are those blades beak very easily, but it is your boat.

H

steelespike · Jul 23, 2009

Re: 1992 Yamaha 55HP on 16 Foot Excel 16DX

While making a tach is probably fun; why not get a tinytach reasonable,easy to install and will fit any similar 2 stroke motor without regard to stator poles.
Get you on the road to best performance/mileage in an expeditious manor.
Instead of spending time building/testing a tach spend it dialing in your boat.

abbtech · Jul 24, 2009

Re: 1992 Yamaha 55HP on 16 Foot Excel 16DX

steelespike said:
While making a tach is probably fun; why not get a tinytach reasonable,easy to install and will fit any similar 2 stroke motor without regard to stator poles.
Get you on the road to best performance/mileage in an expeditious manor.
Instead of spending time building/testing a tach spend it dialing in your boat.

Very true, why re-invent the wheel... If I don't have the time I will have a closer look at those tachs.

abbtech · Jul 24, 2009

Re: 1992 Yamaha 55HP on 16 Foot Excel 16DX

hwsiii said:
You need a 13" pitch prop and you should be doing about 30 MPH at WOT. I would buy an aluminum prop instead of the comprop, because no matter what all their claims are those blades beak very easily, but it is your boat.

H

The dealer said exactly the same thing about the composite prop, I will stick to aluminum.

I bought an aluminum 13 pitch prop yesterday from my local Yamaha dealer. It is a true Yamaha prop made for that motor. I took it out yesterday evening and couldn't get the boat to plane.

The RPM was low at WOT I think it was 1800 RPM but I can't exactly remember, the sheet with the numbers is in the garage.

I stuck the 9 pitch back on and was back to what I had so the engine hasn't changed.

I did some playing around with the calculator on this site:
http://www.rbbi.com/folders/prop/propcalc.htm

I found that if I enter 3000 RPM, pitch of 9, slip of 0.15 and a speed of 22 I get a gear ratio of 1. The reason for the 22 is that the higher 23.6 was taken when traveling with the river current, the 22 is an average of both directions.

Could that be possible. I have read that the gear ratio is usually 1.84 as you have in your calculation.

Is it safe to spin the prop out of the water at idle speed (with cooling water connected of course). If this is OK I could measure the RPM with the meter as usual and then rig up a circuit to measure how many blades pass through a IR beam every second.

hwsiii · Jul 24, 2009

Re: 1992 Yamaha 55HP on 16 Foot Excel 16DX

Abb, I need to know the exact dimensions of the prop you got.
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.

Anything is possible, but it makes no sense to do that, as you have NO torque at a 1:1 ratio, that is why they gear them with higher gear ratios. And you should only have an allup weight of around 1,600 pounds. If that is your problem then you are in trouble and you will need a 5" pitch to get to 5,400 RPM. And I am thinking you won't have a chance of finding a 5" pitch prop for that motor.

Prop Change

I DON'T KNOW!!!! It doesn't make sense to me.

H

abbtech · Aug 3, 2009

Re: 1992 Yamaha 55HP on 16 Foot Excel 16DX

Hi hwsiii,

You got it, something didn't make sense! I checked the gearbox ratio by slowly turning the motor by hand while in gear. It is 1.84 as you predicted. I also connected IR sensor directly onto the flywheel to count revolutions of the motor and to my surprise it was twice the speed I was expecting. For some reason the alternator provides 2 pulses per revolution (not the 4 that I was expecting).

Based on that the numbers finally make sense, here is the original test with the RPM updated.

WOT = 23.6 miles/h @ 6000 RPM

Here are the specs of the new prop that I got. 10 3/8 X 13. I would think that this should give me 5200 RPM but when I took it out I could not plane and the RPM topped out at 3000 RPM.

I have since done compression tests on the engine (both cylinders are 130 PSI). I have also had both carbs looked at. Both were clean but there was a slight difference on the air fuel mixture setting. There were both within spec but were on the opposite ends of the range. If it doesn't rain I will take it out tomorrow and see what the results are.

hwsiii · Aug 3, 2009

Re: 1992 Yamaha 55HP on 16 Foot Excel 16DX

Your motor has a problem and you need to buy a tachometer, when you buy the tach and run both props again get back with me on the RPM and speed for both props starting at 1,000 RPM in 500 RPM increments all the way to WOT on both props.

H

abbtech · Aug 4, 2009

Re: 1992 Yamaha 55HP on 16 Foot Excel 16DX

hwsiii said:
Your motor has a problem and you need to buy a tachometer, when you buy the tach and run both props again get back with me on the RPM and speed for both props starting at 1,000 RPM in 500 RPM increments all the way to WOT on both props.

H

Thanks again for your help. The testing was done with one adult and one child (70 lbs) in the boat. There was also about 40L of fuel on board.

Even with the carb cleaned it runs the same, so I think you are totally right that there is something wrong with the motor preventing it from providing power at high speed. I didn't take a reading at 1000 RPM, when I made the chart I was sure you wanted to from 1500 but I see now that you wanted it from 1000. I took a speed reading going up stream and down stream and averaged the two.

I don't have a timing light but I am thinking of purchasing one to see if the timing is right. Would this be a waste of time and money or do you think it could be a motor timing issue?

Here are the results of the original 9 pitch prop.
1500 RPM, 4 miles/hr
2000 RPM, 4.6 miles/hr
2500 RPM, 5.9 miles/hr
3000 RPM, 6.5 miles/hr
3500 RPM, 7.1 miles/hr
4000 RPM, 8.7 miles/hr
4500 RPM, 13.7 miles/hr (on plane)
5000 RPM, 17.4 miles/hr (on plane)
5500 RPM, 21.7 miles/hr (on plane)
6000 RPM, 24.5 miles/hr (on plane, WOT, 500 RPM over red line)

Here are the results of the 13 pitch prop.
1500 RPM, 4 miles/hr
2000 RPM, 5 miles/hr
2500 RPM, 5.9 miles/hr
3000 RPM, 6.5 miles/hr
3500 RPM, 7.1 miles/hr
3630 RPM, 7.5 miles/hr (not on plane, WOT)

hwsiii · Aug 5, 2009

Re: 1992 Yamaha 55HP on 16 Foot Excel 16DX

Abb, at this time I would recommend using sea foam and cleaning both cylinders out to see of there is any carbon buildup in them. Some of the time it can cause large losses in horsepower with the carbon and it is a very inexpensive treatment.
See if you can borrow a timing light from someone to check the timing as well.
We definitely have a motor problem that we need to cure.

H

abbtech · Aug 10, 2009

Re: 1992 Yamaha 55HP on 16 Foot Excel 16DX

hwsiii said:
Abb, at this time I would recommend using sea foam and cleaning both cylinders out to see of there is any carbon buildup in them. Some of the time it can cause large losses in horsepower with the carbon and it is a very inexpensive treatment.
See if you can borrow a timing light from someone to check the timing as well.
We definitely have a motor problem that we need to cure.

H

I have treated the engine with normal Seafoam in the fuel and also Deep Creep through the carbs, that stuff sure smokes up a storm! I also purchased a timing light, the idle timing was spot on and since I was running on muffs I just blipped the throttle to see if the WOT timing was correct and it looks like it would get to the right number.

Could it be weak spark that is causing incomplete fuel burning and causing the increase in fuel consumption and lack of power? I have tested both spark plugs with one of those spark testers that indicate that there is spark but not intensity.

Should I start a thread in the Yamaha outboard forum?

1992 Yamaha 55HP on 16 Foot Excel 16DX

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