Prop Question on Pitch 50HP Evinrude

[carbineone]

OK we have a little GW Invader 10 foot boat and have been asking questions on selecting a prop.We got a tach installed yesterday and took it out with the GPS.It only ran 30MPH and only reading 4200RPMS with me at 220lbs and my Son at 130lbs.The Tach if its accurate as it should be is a Teleflex. The prop that came with the motor is a 13x19 Pitch and I will include a pic of the prop and the boat as maybe that will help.These readings are with the motor trimmed up to the second hole.Get a little less speed on the lower one but very little and really no RPM change either.If I trim up to the third hole its completely unacceptible as it Chinewalks I believe is what you guys call it.The motor height is set so that the cavitaion plate is even with the bottom and I really cannot get anymore height of it..So running 30MPH with us both in there at 4200RPM I would think we are overpropped but this small light boat should in theory take a pretty high pitch.I know the motor rides a little lower than it should but based on my readings what do you think my prop options may be?I know the RPMs need to come up for sure but I am afraid if we lower the pitch too much it may run in the RPM range but go even slower.My manual says the ratio is 12:32 and it says the full throttle operating range is 5000 to 6000 RPMS.Thanks.Oops forgot for those who did not know this is a 1974 50HP Evinrude...Bruce

 

[solar7647]

Re: Prop Question on Pitch

Re: Prop Question on Pitch

I would say you need to get in your proper RPM range. Shouldnt take gut maybe a 2 inch drop in pitch, but you should gain some bite so I would think your speed may not very.

 

[carbineone]

Re: Prop Question on Pitch

Re: Prop Question on Pitch

Thanks,the thing is this boat should easily do 40 plus MPH with the 50HP on it,there has to be something we are missing here...Just to add something here to get it out of the way.We did a compression test and did it about 4 times on each cylinder and came up with 155PSI on each.Sounds really high to me but thats what it says.Let me correct myself.Its running 160PSI on each right on the nuts,no variance at all.It says in my gauge instructions that a 9 to 1 compression motor should be 160 pounds.Thanks

 

[trendsetter240]

Re: Prop Question on Pitch 50HP Evinrude

Hey carbineone, something is not coming up right with the numbers. At 4200 RPM, with 2.66:1 gear ratio and 19p prop at 30mph you have -5.6% slip; which is not possible.

SO. One of the numbers is not correct. Since you have GPS then it's either the tach, the gear ratio or prop pitch.

Have you checked the back of the tach to ensure you have the correct setting for that motor?

 

[carbineone]

Re: Prop Question on Pitch 50HP Evinrude

I set the tach to the 6P setting.In gear in the water it shows around 800RPM at idle..Maybe its off but I do not know how to calibrate it other than putting it on 6p.Maybe its just plainly drag on the motor thats throwing off the numbers..I do not know what comes into play with all the figures you have.The prop pitch is clear on the Prop.13x19

I did want to add something though and maybe you can tell me if this would even have any relavance to the current setup.Before this we had a 1963 Johnson 28HP.We never had the Tach then but that motor was only running a 11 pitch and it went pretty darn good.I have a 13 1/4 x 17 pitch here and a stainless 13x15 pitch too off some bigger evinrudes and am going to try each one tommorow just to see if I get much differance..In theory the 11 pitch we were running on the older 28HP would be about what we would need if I understand the pitch stuff correctly.I have read that each inch lower in pitch equals around 200 RPM drop or so.So to get from 4200RPM up to 5500RPM I would in theory have to drop around 7 pitch sizes to the same as the old Johnson 11 pitch...I am confused and want to figure this out.

Oh trendsetter.I am no math guy for sure but how does that ratio thing work?You must have converted 12:32 down somehow...Thanks

 

[trendsetter240]

Re: Prop Question on Pitch 50HP Evinrude

Is that motor a 2 cylinder or 3-cylinder? The 2 cyl needs 5p.

12:32 is 2.66:1. (32/12)

 

[carbineone]

Re: Prop Question on Pitch 50HP Evinrude

Its a 2 cylinder.Someone here told me it should be 6p setting...So if the setting I have is incorrect and needs to be on 5p instead which way would it theortically throw the tach off?Read higher or lower than it is? Thanks

 

[trendsetter240]

Re: Prop Question on Pitch 50HP Evinrude

Its a 2 cylinder.Somone here told me it should be 6p setting...So if the setting I have is incorrect and needs to be on 5p instead which way would it thoretically throw the tach off?Read higher or lower than it is? Thanks

2 cylinder 50's were 10 pole so if you had it set on 6p (12 pole) it would read lower. My guess would be that you are around 5200RPM not 4200. You should run it again to be sure.

Here is what I used as a reference for the pole setting:

http://www.jamestowndistributors.com/userportal/woeimages/howtos/Large/837i.jpg

Cheers.

 

[carbineone]

Re: Prop Question on Pitch 50HP Evinrude

It does not say anything year specific for sure but it does say Less than 70HP 2 Cylinder 2 strokes are 10 pole 5p...Thanks very much for bringing that to my attention..Will set it to 5p tommorrow and see what happens...

 

[trendsetter240]

Re: Prop Question on Pitch 50HP Evinrude

It does not say anything year specific for sure but it does say Less than 70HP 2 Cylinder 2 strokes are 10 pole 5p...Thanks very much for bringing that to my attention..Will set it to 5p tommorrow and see what happens...

You're very welcome, glad I could help. And even if you are at 5200 - 5500 RPM you have a ways to work with up to 6000. You want that motor wound out as close to 6000RPM as possible.

That 15p stainless might be the perfect prop. I would start with that one and try the 17 too.

Going down in pitch can actually increase your speed if the prop is more efficient.

 

[carbineone]

Re: Prop Question on Pitch 50HP Evinrude

Well I give up! I took the boat out today and first tried the Stainless 15 Pitch.It would sure burn rubber with that one if it was a car..Lots of throttle snap for sure but lost about 4 MPH down from the other speed of 30MPH with the 19 pitch..I then swapped on the aluminum 17 pitch.Still great plane time,under 3 seconds but the MPH remained the same as the original 19 pitch one...The 15 just seemed to scream and is definitley to low...

I really believe that 30MPH is all we are going to get out of it.At 30MPH way down on the floor you feel like your really moving anyway.I do not mind the 30MPH and my theory is that the motor just plain needs to come up higher.The cav plate sits even with the bottom now and the transom is 20 inches but maybe next year we will try to raise it....The dang boat sits so low in the water as they are designed to so I think the motor would have to be atleast 4 inches above the keel to get the cav plate up high enough to avoid drag.The cav plate runs atleast 2 to 3 inches below the water line now I believe.I really do not understand this because it should not be doing this with a 20 inch transom but its just the boat design I guess..Anyway just frustrated and going to run it as is.

I cannot get the tach to work correctly either and have given up on that too...From past boats I have owned the RPMs sound real nice..Definitley not lugging and not overevving with the 17 pitch like the 15 seemed to be...Thanks

 

[Silvertip]

Re: Prop Question on Pitch 50HP Evinrude

IF --and I repeat IF the engine is actually running 5000 RPM instead of 4200, AND the gear ratio is actually 2.66:1 then the slip number isn't too bad at 11% at 30 MPH. IF (there it is again) you get the engine to spin at 6000 RPM speed would go to 36 MPH. The next step is to ensure you really have a 19P prop and then verify that the gear ratio is 2.66. I don't think it is. Later 40/50 HP engines had 2.43 gears but I can't verify the 1974 did.

 

[carbineone]

Re: Prop Question on Pitch 50HP Evinrude

Says right on the prop,19...Unless theres some way someone could alter them from factory.Why does the 17 pitch off the other Evinrude I have here really make no differance in speed though?Just seems the 17 makes the RPMs a little higher but no speed differance...Maybe someone has a manual that will tell the gear ratio but thats what my owners manual say...Thanks

Oh if I turn the flywheel one revolution and count the number of turns on the prop will that tell us the ratio if the manual is a misprint?

One other thing that may or may not be worth mentioning,when we had the little old 28HP on here we moved as much weight to the bow as possible.Mainly the fuel tank and battery to get a better plane.Could that just be too much weight on the bow maybe?Causing the front to drag? Thanks

 

[carbineone]

Re: Prop Question on Pitch 50HP Evinrude

Well my owners manual is more likely correct on the ratio..I marked the prop,marked the flywheel and removed the plugs and turned the prop one revolution and the flywheel traveled 2 1/2+ turns and the best way I can tell you is the flywheel traveled a little more than 2 1/2 turns plus between just over half way between 2 1/2 turns to 2 3/4 turns so by my calculations that right at 2.66.Half way between 2 1/2 and 2 3/4 would be 2.62 turns so just over half way would be right at .66 My thinking here may be flawed but that should be right...So assuming with these figures so far disregarding the innaccurate tach and knowing if I want the engine to run at 5700RPMs and the gear ratio I have is there any formula for figuring what a close guess to prop pitch may be?I am more likely going to have to buy one of those little tiny tachs and I actually have one here from a go cart with a 4 cycle briggs and maybe that one may work.I do not know for sure..I will have to call them and ask....Thanks

 

[Silvertip]

Re: Prop Question on Pitch 50HP Evinrude

You have verified that the ratio is 2.66 so that's good. I, like others feel you have the tach set wrong. As for why the 17P increases rpm but you don't gain speed is very simple. Just because the engine revs faster means the prop must spin faster to keep the SAME speed. Next time you are in your car, cruise at 30 MPH and then simple move the shift lever down one gear. Note that the tach goes up but speed stays the same. The prop on your boat is the transmission and it has only one gear. Changing pitch has the same effect as changing gears. 17P means for every prop revolution, the boat moves forward 17 inches (theoretically) minus the slip. For a 19P it means the boat moves 19 inches. All props are not made equal and all of them have varying "designs". Rake, blade shape, number of blades, stainless vs aluminum, vented vs unvented, cupped and uncupped are all factors that influence prop performance. That is exactly why prop selection is such a crap shoot.