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Mercury 115 2-stroke outboard RPMs too high

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Mercury 115 2-stroke outboard RPMs too high

Old 11-24-2020, 10:34 AM
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Question Mercury 115 2-stroke outboard RPMs too high

17 ft bowrider with a 1999 Mercury 115 2-stroke outboard, a sticker on the engine shows the RPM limits to be 4750-5250, im getting 5500+ RPM with speedo showing 42mph, it would actually climb higher if not for the rev limiter. Im thinking the prop is underpitched? The stamping says Quicksilver 48 16988 18P, the prop is in great condition with no dings at all. If i trim it down it will slow down to 5000 but the speed drops significantly, which tells me its not trimmed up too much to cause the overrev. I would like the RPMs to peak right in the middle of the engines Max range at 5000, how much of an RPM reduction can i expect out of each degree of prop increase? such as a 19P or 20P prop?

Last edited by AceSpades; 11-24-2020 at 01:45 PM.
Old 11-24-2020, 12:12 PM
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Your RPM are not too high, the next size prop will probably drop around 200-400RPM, which will put your max on the low side perhaps.
Old 11-24-2020, 01:10 PM
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Previous owner may have towed a skier or tube often.
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Old 11-24-2020, 02:04 PM
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What 17’ bow rider? Brand? Model? Year?
Old 11-24-2020, 02:08 PM
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Yes, that motor has a 5250 RPM redline, and you want to be at or near that at WOT.

With its 2.07:1 gear ratio, the prop is yielding about 7 percent calculated propeller slip @ 42 MPH and 5500 RPM.

For most props, that might be normal, but you have a Mercury / Quicksilver Vengeance model which typically yields very low, or even negative calculated propeller slip figures.

This makes me wonder a few things:

Has the prop ever been worked on? What is its condition?

How are you measuring boat speed?

What type of tachometer do you have?

How is the motor mounted in the boat?

Remember, there is a lot more to propeller selection than just pitch. There is no 19” Vengeance fir this motor, and the 19” Laser II yields more slip, so until we figure out if the data is correct, we can’t really make any solid recommendations.
Old 11-24-2020, 02:41 PM
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It is a 1990 Searay with a 99 Mercury 115. The prop looks absolutely perfect, Speed and RPM are going by the original dash gauges, next time i will bring a GPS for accurate speeds. How is the motor mounted? i dont know how to answer that, its on there with 4 bolts at the transom I believe there is a 19P Vengeance prop (48 16316 A5 19P) available, and the Laser i believe is rated for higher HP engines?
Old 11-24-2020, 02:49 PM
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Originally Posted by AceSpades View Post
It is a 1990 Searay with a 99 Mercury 115. The prop looks absolutely perfect, Speed and RPM are going by the original dash gauges, next time i will bring a GPS for accurate speeds. How is the motor mounted? i dont know how to answer that, its on there with 4 bolts at the transom I believe there is a 19P Vengeance prop (48 16316 A5 19P) available, and the Laser i believe is rated for higher HP engines?
The question about mounting is referring to height. There are a series of mounting hole on the motor. Is the top bolt in the top hole? Or one of the others? That will give Tom and other prop guys a better idea how to help you.
Old 11-24-2020, 03:07 PM
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I believe there is a 19P Vengeance prop (48 16316 A5 19P) available...
No, that Vengeance is the large tube version (made for larger motors with a large 4.75 gearcase)

...and the Laser i believe is rated for higher HP engines.
The Laser II is only available for intermediate gearcase (4.25) Motors like yours, the large tube Laser II was discontinued over a decade ago.
Old 11-24-2020, 03:14 PM
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Some photos of the boat might be helpful. That way we may be able to see how high the motor is mounted.
Old 11-24-2020, 03:30 PM
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Yes i will take some photos when i get home this evening.

...For reference, my current prop model is a Quicksilver 48 16988 A5 18P

Last edited by AceSpades; 11-24-2020 at 06:20 PM.
Old 11-24-2020, 06:13 PM
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Here are some pics. The height isnt adjustable because the upper mount uses the highest available hole while the lower mounting uses the lowest available hole




Old 11-25-2020, 10:31 AM
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Thats weird. Did someone repower and redrill the transom holes for some reas
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Old 11-25-2020, 10:38 AM
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Yes, it is unfortunate the lower holes through the transom were drilled that way. There is no easy way to raise the motor now.
Old 11-25-2020, 10:53 AM
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Yes it has been repowered, the boat is a 90 and the engine is a 99. I was happy to learn about that after going through it because the seller did not mention having a newer engine The engine is not height adjustable, but does it look too low or high? It never occured to me that the mounting height could be the problem
Old 11-25-2020, 11:01 AM
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There is no way to check the mounting height from that picture, do it just like this with a straight edge or long level on hull bottom and engine plate trimmed parallel to it



Old 11-25-2020, 11:56 AM
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One reason a propeller might slip more than expected is a motor mounted too high. The photo confirms that is not the case; if anything, it is too low.

This just makes me more suspicious of the data, though usually when an old fashioned pitot tube speedometer is inaccurate, it reads high, not low.

Tachometers can be off too. If that’s the case here, you may be fine, but who knows until we rule some other things out.
Old 11-25-2020, 11:57 AM
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You should loose that foil.
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Old 11-25-2020, 03:55 PM
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On my previous boat with a stern drive it didnt have the foil and people pointed out that it should be installed. Now this boat came with one i should remove it What exactly is its function? Is it the stern drives that should have one, and outboards shouldnt?

If its not mounted too high, then the only other variable is the prop pitch or diameter. With a target reduction of 400 RPMs, maybe a 20P would be a better fit?

Last edited by AceSpades; 11-25-2020 at 04:24 PM.
Old 11-26-2020, 10:24 AM
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Originally Posted by AceSpades View Post
If its not mounted too high, then the only other variable is the prop pitch or diameter. With a target reduction of 400 RPMs, maybe a 20P would be a better fit?
No, that is not true. Again, there is far more to propellers that just pitch and diameter, and until you can verify your speed data is correct, there is no reason to change props.

I’m still trying to figure out why your propeller is yielding more slip than I would expect. A motor too high can be one reason, but I think we can rule that out now. A disturbance of water flow on the hull’s bottom is another possibility. Are there any through hulls on the bottom ahead of the motor?
Old 11-26-2020, 02:28 PM
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Originally Posted by AceSpades View Post
On my previous boat with a stern drive it didnt have the foil and people pointed out that it should be installed. Now this boat came with one i should remove it What exactly is its function? Is it the stern drives that should have one, and outboards shouldnt?

If its not mounted too high, then the only other variable is the prop pitch or diameter. With a target reduction of 400 RPMs, maybe a 20P would be a better fit?
The foils are just a bandaid attempt to try and raise stern and drop bow on power up. On trouble boats.
Im not a fan but some like them

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