Prop for Mercury 500 on Cutter Cricket

lankadank

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Oct 4, 2015
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Hello all. I am new to the forums and the boating community, and I have just purchased my first boat. It is a 1981 Cutter Cricket with a 1971-1973? (I think) Mercury 50 2 stroke outboard on the back.

I have taken the boat out a couple times now and it seems that I can only get it up to about 38kph or 24mph. I feel like I should be able to get some more speed than that with this motor on such a small boat. The prop that it came with serial # is 48 56244 a1 11p. I believe this is a 11 pitch prop? It seems to plane out nice and quick, but the top speed is just not there. I also have a hydrofoil. Unfortunately I do not have a tachometer yet to see what my rpms are.

Just wondering what your suggestions would be for a better prop for top speed, but at the same time, I am using the boat on Lake Ontario so I need a prop that will handle some waves as well. I would appreciate any feedback you guys have.
 

steelespike

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Apr 26, 2002
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Is the speed by gps?
​I have a much bigger boat but quite light 335 lbs dry weight ,with an all up weight of about 1,000 lbs including myself at 200 lbs.
To do this right you need a tach.A induction tach will work.It has used an 11" prop, 4 total; since 1965.
If we guess about slip and assume the gear ratio is 1.64,11" prop 24 mph calculates at about 4300 rpm.
Rpm range is 4800 to 5500 at 5500 rpm it would be at about 30 mph.The foil is probably costing some speed.
Is the anti vent plate (just above the prop) about even with the bottom?
I would start with the motor,a compression check,It may have a Magneto distributor. Be sure its firing on all cylinders.
Don't do any trouble shooting without a service manual. There are 2 carbs and it may have 2 fuel pumps.
 

lankadank

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Oct 4, 2015
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Thanks for the reply, steelespike.

The speed is by GPS with 3 people in the boat. The motor sounded like it was running great, it gets up on plane quickly, but I have not taken it out with a tach yet. I am hoping to over the next few days once the weather warms up a bit. Yes the anti vent plate is about even with the bottom, maybe an inch above the bottom of the transom. I have a feeling the prop pitch is the reason I am not getting a very high top speed. I have looked on the mercury prop finder on their website, and it suggests a 15-16 pitch prop, and mine is only an 11. I believe this motor does have a Magneto distributor and I have downloaded a service repair manual, however I am not very educated on boat motors. Also, what would your guess be on the weight of this boat? It is 14', I am assuming around 400-500lbs? I cant seem to find any information on this model other than a couple of pictures on google images.
 

steelespike

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I think you are right if its glass 400 to 500 seems reasonable.You could go to a scale and weigh the outfit then launch the boat and weigh the trailer.
I think you will find your all up weight is close to what mine is. Boat,motor,gas and tank,battery,yourself.misc.gear.
We need a lightly loaded wot rpm and gps speed.We can go without a tach initially to see if wee asre in the ball park.
If you needed a 15 or 16" pitch the motor would be screaming with an 11" pitch.If you put on a 15 as it stands the speed would be even less with
horrible hole shot, if any.
You can manually check your gear ratio. remove the plugs and mark the flywheel so you can
count the turns for the prop to turn once. I think 1.64. (slightly more than 1 and 1/2 turns. Other ratios are 1.83 and 2.00.
These ratios might use a higher pitch prop.
Increasing pitch 15" is like selecting a higher gear in a car,harder to turn but more speed if the motor can turn it. If it's a 1.64 ratio it won't turn it
fast enough.
 

lankadank

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Oct 4, 2015
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so, after doing a bit more research it seems that the prop I currently have is more suited for a pontoon boat. I have used the mercury websites prop fitter and played around with the numbers and it is suggestin I drop down to a 10" prop.

with a gross weight including boat, gear, motor and passengers of 1267lbs it suggests a 10" prop with a 16.74 pitch and calculates my top speed should be 42.24mph with engine rpm at 5,375. this is assuming I have a full boat of 4 people.

does this seem reasonable as far as overall weight? should I go with a 10" prop with a 17 pitch since I will want better performance when I have 2 or less people on board? I played around with the numbers on the prop selector quite a bit and basically it suggested a 10" prop for all gross weight between 1100- 1500lbs.. however the pitch would lower with increased weight. when I punched in the numbers I thought I would be at with just myself on board it suggested a 19 pitch prop.

so I think I am going to purchase a new aluminum prop this week, just need to decide whether or not I should go with a 16 or 17 pitch..

should I go with a lower pitch if I plan on trying to wake board or ski behind it? I weigh 200lbs.

I'm leaning towards the 17 pitch.
 
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steelespike

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Are you sure you have the correct gear ratio? If it is 1.64 you will be very disappointed in a 16 or 17" prop. It appears that
you may be using a 1.83 ratio in your calculations.That would make a 17 more reasonable but I think too much for the load.
Your little 14 footer may struggle to get up on plane with that load?42 mph is out of the question for your full load.
In fact 42 may be unlikely with an empty boat. Is there a capacity plate on the transom?
Any marine engine is working very hard,like a car pulling a trailer up a never ending hill. When you increase pitch its like putting the car in a higher gear
maybe even like over drive. Rule of thumb a 1" increase in pitch could lower rpm "about" 200.Not carved in stone but will definitely lower rpm.
You can't replace your prop without a tach. we don't know your present rpm and we can't tell the results of the new prop.
We need the lightly loaded test run with gps speed.You should confirm your gear ratio.
 
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steelespike

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The Merc 500 appeared in 61 ran through to 1965 mine I believe the 65 may be the last year for the 1.64 ratio.
Your right your ratio probably is likely 1.83.
66 through to 78 was the last 500. 74-76 are the props available here at iboats.
Sizes 7" to 17" An 11 "is just on the light side of the middle sizes. The 17 is the highest pitch available at iboats.
Popular sizes would usually come from the middle sizes 10-13.
Your motor is suspect at this point apparently making only 4300 at 24 mph.
Although with 3 or 4 aboard its not terrible. your little boat may struggle with 600# of people aboard.
Going from an 11 to a 17 will lower rpm by about 900 to 1200 rpm. And ruin any performance you had
with the 11"You need that tach and determine the motor is at full power.a lightly loaded test run.
 
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lankadank

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Oct 4, 2015
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So my girlfriend and I went out on my boat this evening and I brought a tachometer. At WOT the RPM's were about 6050. Way too high, I just hope the previous owners and myself didn't do too much damage to the motor from reving it that high for extended periods of time.

I went into a boat dealer today to ask about props and they suggested a 10 1/8 x 15 pitch prop from Turning Point Propellers. This seems about right if I want to reduce my RPMs by about 800, correct? Or maybe I should go with a 14...
 
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steelespike

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The 15 should knock off about 600 to 800 rpm. If it can make 5400 speed could be about 36 .probably with you and the girl friend.
 

steelespike

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6000 at 24 mph calculates to 29% slip This could be from a bad prop,a poor setup or the tach set wrong.
Normal slip is about 10 or 12%.
 

lankadank

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Oct 4, 2015
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update: I put a new 15 pitch prop on the boat and now the boat revs up to about 4750rpm but I'm still only getting about 23mph. is it possible that I'm not getting full throttle? the throttle cord is definitely stretched. . also, after removing the old 11 pitch prop I noticed that it had a decent Crack in it. could this have caused the high rpms?
 

steelespike

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The numbers don't work.
4750 at 23 mph calculates to 37% slip, If we take the speed. gear ratio,and prop size and use more normal 12% slip we get 3300 rpm.
37% slip is highly unlikely . We know 1.83 should be right,15" is right 23 should be right This makes the tach suspect as 37% slip
is higher than that for a pontoon or other very hard to push boat.If your using a induction tach you should use the bottom plug and get the wire outside the cowl as soon as possible.Should be calibrated for a 2 stroke motor.
 

steelespike

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It appears that if your only getting 23 mph and the slip is at 37% one or more of your inputs are wrong.37% slip is not accurate.
It appears that your actual rpm is about 3300 rpm. so if the 15 is an accurate selection your not getting full power.
It's possible that the 11" prop was the there because the power was down.
See if a little choke helps at speed.If the choke helps it could indicate a lack of fuel (dirty carbs)perhaps.
Do a compression check. Note the condition and location of each plug.
An induction tach will get stray pulses if connected at the distributor, needs to be connected at a plug preferably the bottom plug.
 
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