feel like I might have the wrong prop or be slipping a lot

JoshOnt

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Aug 12, 2013
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So pretty much I can only get 10 mph out of my boat that has a 50hp on it. My boat is 14ft so this 50 is way more than enough to get 25 mph or more but I am not getting there. Had a sting ray added onto the back (mechanic recommended it) and I finally got to about 10 mph compare to my 7 or 8 from before. Anyone have an idea(s) that I could try?
 
Joined
Oct 22, 2007
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Re: feel like I might have the wrong prop or be slipping a lot

The way to check for a spun hub is to mark across the hub and prop with a permanent marker, take her for a run, and see if the mark has moved.

Have you checked all the basics on the engine to have at least a warm fuzzy feeling that it's running properly. Compression check, test the spark with a variable gap tester set at around 7/16 inch, make sure the primer bulb is staying hard, pull the plug wires one at a time with the engine idling and see if you get an equal rpm drop for each.....

Also, did it used to run better, or is this a new to you boat?
 
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Mi duckdown

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Apr 14, 2007
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Re: feel like I might have the wrong prop or be slipping a lot

Are you running on all cylinders???
 

QC

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Re: feel like I might have the wrong prop or be slipping a lot

I am going to move this to the prop forum. What about RPM etc.
 

steelespike

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Apr 26, 2002
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Re: feel like I might have the wrong prop or be slipping a lot

Help us out.
What brand and year motor.
What brand,year and model boat?What material?Aluminum,glass?
The carb(s) opening all the way?Is the timing set right and advancing as it should?
Check that it is running on all cylinders,You wouldn't be the first to not realize your missing a cylinder except for the loss of power.
What size prop? Numbers should be near the base of the blades or under the prop nut.
Is the motor trimmed down for hole shot?
 

JoshOnt

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Aug 12, 2013
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Re: feel like I might have the wrong prop or be slipping a lot

Thanks everyone, as for your questions.
The engine is a 1977 Mercury 500 Series 50Hp. A new starter and all new plugs were put in less than 3 weeks ago. The motor also has a 'sting ray' on it.
The boat is a '65 14ft Scott, it is made out of fiberglass and is a v-hull.
Prop is 19 pitch if memory serves me correct.
I do not think I am able to adjust trim on this motor while it is running so I have it set just a little down.

Steelespike I am not sure about the carbs as I don't have any clue how to check. Also it sounds super good so I would think all cylinders are firing good.
 

steelespike

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Re: feel like I might have the wrong prop or be slipping a lot

A 19" would be off the chart too high. Biggest prop listed here is a 17.
You'll have to verify the prop size. I have the 1965 version of your motor I think the same gear ratio.
I think you need a 12 or 14" prop.Your numbers are near the base of the blades or under the prop nut.
As far as your motor firing good; it may be but even a 2 cylinder 2 stroke is hard for a novice to tell its only on one cylinder except for the lack of power.with a 4 cylinder you could be down 2 cylinders and not realize it.
But for now lets get the prop size.
 

JoshOnt

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Re: feel like I might have the wrong prop or be slipping a lot

A 19" would be off the chart too high. Biggest prop listed here is a 17.
You'll have to verify the prop size. I have the 1965 version of your motor I think the same gear ratio.
I think you need a 12 or 14" prop.Your numbers are near the base of the blades or under the prop nut.
As far as your motor firing good; it may be but even a 2 cylinder 2 stroke is hard for a novice to tell its only on one cylinder except for the lack of power.with a 4 cylinder you could be down 2 cylinders and not realize it.
But for now lets get the prop size.
For the prop I was going by what the guy who I bought it off said and reason I say the cylinders are good is because of my father and uncle who have a combined 80+ years of experience with engines.
I will check prop size later this week when I get out to the boat and where or how would I do a compression test?
 

jestor68

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Re: feel like I might have the wrong prop or be slipping a lot

Your father or uncle ought to know how to check compression.
 

steelespike

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Re: feel like I might have the wrong prop or be slipping a lot

Pull the plugs,Note their condition, brownish tan good,unusually clean bad. black or wet look not good. put the tester in the plug hole and crank the motor till the pressure stops going up.
test each cylinder. Minimum pressure 100 lbs and within 10% between cylinders. Bottom hole will be tough to reach.
 

JoshOnt

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Aug 12, 2013
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Re: feel like I might have the wrong prop or be slipping a lot

Okay so I had my uncle go out to have a look at it since he is retired and has lots of time (lucky man gets to boat often), he did a compression test and he got 123psi and 122psi. That is pretty good far as I can tell. He suggested prop which I will check very soon and hub spin which I will also check very soon. Anyone have a good idea on what the pitch should be?
 

MARV1

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Jun 27, 2013
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Re: feel like I might have the wrong prop or be slipping a lot

15 pitch...
 

phillnjack2

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Apr 30, 2011
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Re: feel like I might have the wrong prop or be slipping a lot

ok first off you need 12 pitch prop to get it on plane. 19 is crazy and will never work.

12 inch pitch should get about 30mph plus,
then you can go up to a 15 inch pitch for a lot more speed on the plane with very good pull away (hole shot).
BUT
you will need to drill 3 holes in the prop at the bottom !!!! yep thats right just like the big boys with big 200hp engines.
DONT PANIC its very easy and very very simple to do and will NOT ruin a prop.
here below is a pic of the very same 15 inch pitch prop you need to get up fast and go fast once up on plane.
this prop struggled with 2 people to get up onto plane, the boat needed to have 12 pitch realy for pull away.
but once it got going it was ok and gave good high top end speed.
I then drilled 3 simple holes at the hub and the prop now gives quick pull away with 4 people in the boat,it realy is that simple.
the pull away is like a 12 inch pitch and the top end isthe 15 inch pitch,so i get the best of both worls in 1 prop.

The holes you drill in the prop work like this.
at idle and slow running speeds they let out a certain amount of exhaust if there is a lot of pressure from the water
at the open end of prop (thru exhaust ).
Now as soon as you hit the throttle the prop trys to move but pressure on exhaust holds it back a touch.
now this escaping exhaust also puts a touch of air to the blades letting them slip a bit, this slip lets the engine rev
and get to power faster.
now as soon as the boat starts moving the holes in the exhaust start to become less open due to water
flow going over them sort of closing them off.
once on plane these holes are now not working at all and in effect are closed off by pressure.
so we now have a 14 inch pitch prop ,but the boat is already on plane and the majority of power needed to get
the boat motivated as already passed,so now its just picking up more speed untill max revs achieved.

anyone who says this dont work , has never tried it, its amazing what it does.
DO NOT BOTHER TRYING THIS WITH 19 PITCH as the prop is just too big for the engine anyway....

please look for the numbers on your prop and let us know what these are, we can then tell you what the
prop size is and what its meant to be on.
i would think a 3 cylinder 75hp on a very lightweight boat.


anyway
here is some pics of the prop i drilled with huge success, just drill 1/4 inch hole in between each blade
then go up a size drill untill maximum of 3/8ths to 1/2 inch diameter.
i think 3/8 inch will be just perfect for your engine with a 14 pitch prop.
The prop in these pics is mercury 10.1/8 x 15p Black alloy prop ..part no 4873140 -15
.
 

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