Proper Prop Pitch

[seldomseen]

Re: Proper Prop Pitch

Keep in mind that the production two stroke outboard is a very low compression machine with equally low volumetric efficiency. The things that work on a four stroke have only a little corelation to a two stroke. With a four stroke over proping to yeild a WOT rpm of 20% less than the manufacturers reccomendation would have one good result on an average boat instillation and that would be increased gas milage. The four stroke is most efficient at WOT and low rpm. This is why gas engine airplanes run WOT at cruise but run the prop pitch so high that the RPM is LUGGED down such that the HP output is approx. 70% of the HP at WOT and max RPM. The overproping on the four stroke could have bad results also and the worst is an inability to acheive a planing speed on a planing hull.
The two stroke overproped 20% will not increase the MPG to a marked degree except to the extent that the slower speed on the boat might put it at a more efficient speed. Do not be afraid to "LUG" a two stroke as regards damaging your engine.From the engine damage standpoint I think that an extreme overprop on todays two strokes such that it's WOT RPM was 50% of reccomended WOT RPM would not hurt the engine at all. Just make sure your water pump is good.

 

[191Seanymphstriper]

Re: Proper Prop Pitch

Ok, my bad, I didnt supply the needed info. Currently I have a 14x13P Blackmax prop, and WOT is about 5300 RPM with normal load. Now here is my personal strange "thing". I have never, nor will I ever run any engine at WOT for any extended period of time, I never have and never will. What I am looking for is not top end speed, but maximum mid range speed. I have done the WOT test just to see what it achieved. I normally run around 4000RPM.
I currently get about 15 MPH at 4000 RPM. This was clocked by a boat running nest to me, I will get GPS speeds today, as I am headed out to the water early this morning. I am also used to working with 4stroke engines, as I am a Honda technician on motorcycles and ATV's, and a 35+ year veteran mechanic on all things motorized.
I have worked on the 2 strokes mainly as maintenance and repair but not performance. I assume, which is bad to begin with, that like 4 strokes, they produce maximum torque in a specific RPM range, that is not WOT, but I could be wrong. That is why I am asking here. Is there a taller prop that will give me better midrange performance. Im not interested in WOT performance.

I have found on my application with the 13 1/2 x 17 at 3500 to 4200 rpm it was quick to get on plan and was a good crusing speed. Also with the 13x19 and again im running a 90hp mariner 2 stroke on a 19ft deep V aluminum boat... Now for max speed that you said your not looking fo. Onmy boat was 13x18P

 

[191Seanymphstriper]

Re: Proper Prop Pitch

Keep in mind that the production two stroke outboard is a very low compression machine with equally low volumetric efficiency. The things that work on a four stroke have only a little corelation to a two stroke. With a four stroke over proping to yeild a WOT rpm of 20% less than the manufacturers reccomendation would have one good result on an average boat instillation and that would be increased gas milage. The four stroke is most efficient at WOT and low rpm. This is why gas engine airplanes run WOT at cruise but run the prop pitch so high that the RPM is LUGGED down such that the HP output is approx. 70% of the HP at WOT and max RPM. The overproping on the four stroke could have bad results also and the worst is an inability to acheive a planing speed on a planing hull.
The two stroke overproped 20% will not increase the MPG to a marked degree except to the extent that the slower speed on the boat might put it at a more efficient speed. Do not be afraid to "LUG" a two stroke as regards damaging your engine.From the engine damage standpoint I think that an extreme overprop on todays two strokes such that it's WOT RPM was 50% of reccomended WOT RPM would not hurt the engine at all. Just make sure your water pump is good.

Yea but he is running a 2 stroke and so am I?

 

[Traceyh]

Re: Proper Prop Pitch

Ok here it goes. I have a 19ft 191 seanymph striper cc deep V aluminum boat and i to have a 90hp mariner on it. I learned this the hard way. When I purchased the boat the tach was not working correctly. It had the same size pitch your boat has. It would over rev and do about 24 mph. I then bought a brnd new 13 1/2 17P ss stainless prop 15 spline prop for the 90hp mariner. Put it on. Installed a tach. Ok she would rev up to 5900 rpm and do 36mph. My motor is a 1989 so they say it should be between 5K and 5500 rpm. So I was over reving because of the size and wieght of my boat a little. So I purchased a 13x19 stainless steel prop brand new. i was asured this was the right prop. Well it toped out at 5K rpm and did 39mph but I felt It was working to hard to spin that prop at WOT. I wanted the sweet spot 5500 rpm at WOT.... So I went onto this website http://www.mercurymarine.com/propellers/prop-selector/

Put in all these rpm's and speeds etc.... And it told me I needed a 13x18P prop So I got a new 13x18P ss prop...... The boat did 5500 RPM WOT and hit 42mph


So it was a expensive lesson. And now im stuck with 2 brand new stainless steel props ! Hope this helped... If the prop selecter tells you that you need a prop with the sizes that I don't need then shoot me a PM with your name and number and Ill send you out a prop if you need 1... Hope this helped you! And I think your prop is way off!

The problem with Mercurys prop site is that they dont cover a 1988 engine, they only go back to 2000, I dont expect that a 2000 Mercury 90HP is exactly the same as a 1988 Mariner 90HP, but I could be wrong.

 

[lmuss53]

Re: Proper Prop Pitch

It's my impression from reading here, and personal experience, that overpropping a 2 stroke will cause a lean condition resulting in too much heat and one dead outboard.

I pulled an OMC V-4 85 hp off of a 17 foot Viking deck boat and put it on my 24 foot Manitou pontoon without changing the prop. 10 minutes into the test run the motor, which had run like a charm on the Viking, ate up a piston and never ran again. There was no tach but you could tell the motor was struggling at WOT to catch up. That was many years ago and an expensive lesson. Today, I would never run a 2 stroke below the recommended RPM range at WOT, or run one with a prop that would not let it reach the recommended range at WOT.

 

[seldomseen]

Re: Proper Prop Pitch

Imus53; Today we have fixed jets in the carbed two stroke motors. The main,and secondary's are fixed and only the idle jet can be set lean or rich and idle jets only function with butterflys closed or almost closed.Piston burning can be caused by lean conditions but WOT at mid rpm's does not result in a lean condition with todays carbed two strokes. The easy way to burn pistons is to advance the timing beyond mfg. specs. I have personaly witnessed a 3 cyl yamaha two stroke run at WOT for an hour at 2000 rpm with no damage. This was an unusual happenstance with the event caused by only one cyl. running of the three. Each cyl is entirely seperate and the result of this was the same as running a 24"p prop on it with all 3 cyl. working.

 

[Traceyh]

Re: Proper Prop Pitch

It's my impression from reading here, and personal experience, that overpropping a 2 stroke will cause a lean condition resulting in too much heat and one dead outboard.

No, If the carb is properly jetted for 5500 RPM, and you load the engine to where it would only turn say 3500 RPM, you would end up with an overly rich situation, not lean. As the jets would still have the potential to allow enough gas to pass for 5500 RPM, but you would decrease the air moving through the venturi by the low RPMs would cause a potentially rich situation because the jet would be to big for the RPMs at hand. Now taking into account that the lower speed of the air through the venturi would also lower the vacuum on the main jet would also decrease the amount of fuel "pulled" through, it would still be too much for the RPM of the engine.

 

[seldomseen]

Re: Proper Prop Pitch

Since You may soon be replacing the 13p prop that is old and worn down, I can offer an easy way to experiment with higher pitch props. The aluminum alloy used on modern alu props is maleable to a degree. Just pull the prop and using a 10 to 40 lb chunk of lead chucked up in your bench vice as a backing, just beat the trailing edges of all blades with a brass hammer, more or less the same ammount, to increase the pitch. If you are a gym rat don't bother removing the prop, just hold the 30 lbs of lead firm against the back side of each blade and wack it. Should you have a SS prop you will have to remove it and go to the vice as some serious hammer blows are needed to cup SS props and you do not want to bend your prop shaft. Eyeballing a cupped prop will give you a visual idea for the profile. Bending close to the prop hub has little effect as the number of feet per minute there is much less than near the tip. I have been cupping and decupping, brass, stainless and aluminum props since 1957. In 1957 my race boat was slower than the competition. I went to a local outboard dealer who had won many races in the late 40's and asked about cupping props which although rather new was beginning to gain adherents. He turned to the wall behind him and pointed out a brass 2 blade and he said" I cupped it myself with a pair of pliers and it won the race around Manhattan". I was taken aback and dissapointed as the cup job was so crude. I went home and cupped mine carefully and it was a lot better looking than his and it dropped my rpm by 400 and gave 3 MPH more speed.

 

[lmuss53]

Re: Proper Prop Pitch

Read the attached htread on lugging your motor guys. I was wrong about the lean condition, but over propping resulting in lower than recommended RPM at WOT is a motor killer. The attached thread explains why.

http://forums.iboats.com/showthread.php?t=106974&highlight=lugging

 

[lmuss53]

Re: Proper Prop Pitch

Here's another Joe Reeves reply that explains it well.


Joe said:

"The term of "Lugging" pertains to a engine which has a mismatched propeller in relation to the craft it's installed upon, a prop with usually too much pitch. This result is that, at full throttle, the carburetor butterflies are wide open and the timing is at the full spark advance point, but due to the prop mismatch the engine cannot obtain the proper rated rpm. In short, what's being taken in by the engine cannot be expelled.The result of the "Lugging" is a thick sort of white waxy like substance build up on top of the pistons..... very thick! This substance might resemble a thick carbon build up except for its color and the fact that it is not as hard. Whatever the case, the fact that it's there decreases the engine's performance.Although the OMC engines are designed for full throttle performance of a unlimited time span, when properly matched (Engine/Boat/Prop), it is fully acceptable to run any of them at a lesser rpm also for a unlimited time span."

 

[Boss Hawg]

Re: Proper Prop Pitch

just beat the trailing edges of all blades with a brass hammer, more or less the same ammount, to increase the pitch.

I think i'd pass on that advice but thats just me

 

[seldomseen]

Re: Proper Prop Pitch

I suspect that Joe Reeves has not disassembled many two stroke outboard engines. His knowledge of pistons and rings that are "coked up" are not from "hands on experience". There may be others on this forum who can support my contentions but they are not reading or replying to this thread. Help out there,... isn't there someone with knowledge of brake mean effective pressure, specific fuel consumption, and volumetric efficiency, who can contribute?

 

[lmuss53]

Re: Proper Prop Pitch

SS, I won't claim to know Joe Reeves at all, let alone well, but his profile says he is a retired OMC outboard tech and he has over 7000 posts here helping folks with OMC problems. From reading any of his posts it is obvious to me that he literally knows these engines like the back of his hand. I am an OMC owner and I will heed his advice.

 

[seldomseen]

Re: Proper Prop Pitch

I53; I suspect that Mr Reeves is close to my age. I became a JohnRude "certified" mechanic in 1959. I agree with 99% of his advice. He has not weighed in on this thread concerning the correct prop for the two stroke 90 hp mariner on the 24' pontoon, but I am confident that he would agree with me that the best prop fof that toon is one that at WOT gets the RPM within the MFG's recco. range. He obviously does not agree with me that you can prop a two stroke so big that it will only turn 3500 RPM with no bad effect. He states that the 6000 max RPM motor when proped so that it could reach only 4900 RPM was "LUGGING" it to the point that proper exaust was impossible. Well it an accepted fact that two stroke motors , both cross flow and loopers have poor exaust at any rpm. The major reason is that they have no exaust stroke. As far as one and a half to two inch thick coke build up on the pistons that his post says is the result of this 4900 RPM LUGGUNG..... well for the combustion chamber to have this much room between the piston top and head bottom at TDC... it would indicate an extreme modification to the crankshaft or piston had been made and the power output of that engine would have been a small fraction of its manufactured specs.
Unlike the Admiral, I did not continue as an outboard mechanic subsequent to 1966 and in the intrim between 1959 and 1966 I was a jet engine mechanic.

 

[191Seanymphstriper]

Re: Proper Prop Pitch

The problem with Mercurys prop site is that they dont cover a 1988 engine, they only go back to 2000, I dont expect that a 2000 Mercury 90HP is exactly the same as a 1988 Mariner 90HP, but I could be wrong.

Actaully it is! Its the same gearing and the same compression! From 1978 to 2010 on all 90 hp elpto merc mariner outboards!

 

[191Seanymphstriper]

Re: Proper Prop Pitch

If you do want to play around I do have these 2 new props here! Add a tach and grap your car portable gps if you dont have an onboard and go at it! You need to be within 5000 and 5600 Rpm Recomended 5300 to 5500RPM is ideal at WOT!

 

[Traceyh]

Re: Proper Prop Pitch

Actaully it is! Its the same gearing and the same compression! From 1978 to 2010 on all 90 hp elpto merc mariner outboards!

Now that is some useful info. I was unaware of that. Thank you very much!

 

[191Seanymphstriper]

Re: Proper Prop Pitch

Yup the only differents on the 1978 to 2010 is in 91 they changed to fuel injection. So Some are fuel injected and some are carberated!