prop pitch/material

[chad lester]

Hello all just wondering what would be the best prop for my 1977 yarcraft 15 yankee runabout with a 74 johnson v4 85hp. currently I have a 13.75 15p aluminum prop which turns about 5100 rpms and 38mph on speedo. I also have a 13.5 17p ss prop which turns 5000 rpms and about 5000 rpms. Was wondering if I were to change to a cuped prop or one with the ventalation holes if I could improve overall performance. Oh and the stamp on the motor says 85hp@5000 rpms so I know I'm@ the right rpms just wondering if I could improve on performance or not. Thank's agian Chad

 

[steelespike]

Re: prop pitch/material

I think we need to examine your setup.That boat motor combination should run over 40 and in the 5,500 rpm range,the 15 would probably be in the 6,000 range and the 17 in the 5,500You should determine the motor is healthy,(compression) properly tuned,As a starting point the anti vent plate(just above the prop) should be even with the bottom of the boat.Prop in excellent shape.Have you carefully adjusted the trim?Is the boat bottom clean?

 

[walleyehed]

Re: prop pitch/material

The tach and/or speedo is way off...
to get 38mph with your 15 you would be turning just over 6000rpm.
If the tach just happened to be right, your top speed with the 15 would be about 32.
If you want to get serious about changing props, verify or replace the tach first.

 

[Lakester]

Re: prop pitch/material

Hello all just wondering what would be the best prop for my 1977 yarcraft 15 yankee runabout with a 74 johnson v4 85hp. currently I have a 13.75 15p aluminum prop which turns about 5100 rpms and 38mph on speedo. I also have a 13.5 17p ss prop which turns 5000 rpms and about 5000 rpms. Was wondering if I were to change to a cuped prop or one with the ventalation holes if I could improve overall performance. Oh and the stamp on the motor says 85hp@5000 rpms so I know I'm@ the right rpms just wondering if I could improve on performance or not. Thank's agian Chad

hello,

i do not think u will improve overall performance, as, imo... sounds like u r running very good currently... at least based on the specs of the Lakester Special... if u run 38 with the 13 3/4 x 15 that is pretty good! u can chase the tail...but i think u are basically there now.

regards,
lakester

 

[walleyehed]

Re: prop pitch/material

People....SPEED is only a RESULT of how well you set up your rig...it is an END result....AND, unless it's GPS, it means about zero...the same as an older tach, especially a Teleflex.
Why does everyone want to walk past the fact the RPM is the single most important factor in this equation... As a Professional at what I do....if you have an inaccurate tach, you are lost in telling or giving any information that would lead a consultant, such as myself, to give good advise.
To simply say "you've reached the top" is a very bold statement.
I get upset with these situations when we have only inaccurate variables, and if someone really "wants" to have the best performance and fuel economy why would anyone be expected to recommend something with no constant values?
Everyone's opinions are heard here...that's great, but let's keep in mind what we need to know to make an accurate diagnoses.

 

[chad lester]

Re: prop pitch/material

The tach is nos omc the motor has 125 125 122 120 on new compresion guage new power pack new coils new voltage regulator and rebuilt carbs the speed was on speedo will bring gps with me next time and when I did the testing had about 600lbs people 3 adults 15 gal fuel and about a case and half of beer tackle box and poles.

 

[chad lester]

Re: prop pitch/material

oh and the tach is set for 6pole

 

[walleyehed]

Re: prop pitch/material

Sounds like you have the bases covered there...
Is there any way you could get a P/N from that prop....If it's in the same vintage as the engine, it may or may not be cupped, which on that rig will mean the difference between 2" of pitch...Based on numbers for RPM, I'd say you have a cupped 391198, but just a guess there.
It sounds like we'll be lifting the engine a couple of holes.

 

[chad lester]

Re: prop pitch/material

Do you mean the trim holes or the mounting holes walleyehead?

 

[walleyehed]

Re: prop pitch/material

Mounting holes....get some gearcase out of the water so we can pick-up some speed and RPM.

 

[chad lester]

Re: prop pitch/material

Finaly got the prop numbers for you the s/s is 389948 the is the one that turns 5000 rpms aluminum is 011002 the one that turns 5100 rpm's also have a alumunum 391198 that is beat up and needs recomditioning anymore input would be apprediated. Thanks Chad

 

[chad lester]

Re: prop pitch/material

bump

 

[chad lester]

Re: prop pitch/material

Hello again all just got done raising motor up two holes was on lowest setting. Now anticavitaion plate is about 1/2-1 inch below middle of boat. Is this where it should be or should I take it up or down a notch? All the rivers are closed right now around me, and lake michigan is not is this boats cards so have some time to do setup work. Thanks all Chad Lester

 

[chad lester]

Re: prop pitch/material

guess this thread is dead?? no-one?

 

[ivar]

Re: prop pitch/material

If you raised engine and its now 1/2-1" below the bottom of the boat with the the engine trimmed so the cav.plate is parallell with the bottom, it was way too low, costing you rpm and mph. I'd think you could raise another hole without problems, only one way to find out...

 

[steelespike]

Re: prop pitch/material

I think we need to examine your setup.That boat motor combination should run over 40 and in the 5,500 rpm range,the 15 would probably be in the 6,000 range and the 17 in the 5,500You should determine the motor is healthy,(compression) properly tuned,As a starting point the anti vent plate(just above the prop) should be even with the bottom of the boat.Prop in excellent shape.Have you carefully adjusted the trim?Is the boat bottom clean?

raise the motor to a point just before ventilation becomes a problem.Be sure it pumps water.

 

[chad lester]

Re: prop pitch/material

Thank's for all the help guys think local river will be back open this weekend will go out and give it a shot, and let all know how it went when I get done. Thank's again Chad Lester