Merc 50 hp RPM Issues

[Buckem]

I have been trying to get the correct prop on my 16' Key Largo Center Console. It has a 2003 50 hp Merc 2 stroke and the prop that was on it was 10 3/8x12 and the rpm's would be pegged out past 6000 wrong about 32 mph. I swapped it out for a 10 3/8x14 and did not make much difference. I am still at about 6000 and dropped speed to about 28 mph and my holeshot is worse. Both props are stainless three blade. Anybody have suggestions as to a good prop to run? Thought about 11x15 which is about the biggest I could find to fit but really think my getting on plane is going to suffer. I sure would like to get to about 35 mph and more importantly my RPM's at a tolerable 5500.

 

[QBhoy]

I’d make sure your tachometer is good before going any further. I prefer your first prop, but not the rpm if corrext.

 

[WesNewell]

First thing to check is engine mount height. If it;s mounted too low, you're just pissing in the wind. Might want to use the mercury prop finder to get an idea of where to start. It's pretty accurate if you have the engine mounted for max speed performance.
https://www.mercurymarine.com/en/us/propellers/selector/#!/step-one

Hwre's what I come up with assuming 700# noat, 400# people and gear, and 12 gal. gas.

• Boat type: Bay Boat
• Usage: Top Speed/0 ft
• Engine Manufacturer:
  Mercury
• Engine Type: Outboard
• Engine Year: 2003
• Number of Engines: One
• Engine Family: 2-Stroke
• Engine: 50 Classic
• Drive / Gear Case: 40-50 HP 3cyl 2-S, 50-60 HP 4cyl 4-S, 1.83 Ratio

[h=3]Details[/h]

• Calculated Pitch: 15.82
• Calculated Weight with engine(s): 1,372.40 lbs
• Calculated Engine RPM: 5,375
• Calculated Boat Speed: 40.08 mph

 

[QBhoy]

First thing to check is engine mount height. If it;s mounted too low, you're just pissing in the wind. Might want to use the mercury prop finder to get an idea of where to start. It's pretty accurate if you have the engine mounted for max speed performance.
https://www.mercurymarine.com/en/us/propellers/selector/#!/step-one

Hwre's what I come up with assuming 700# noat, 400# people and gear, and 12 gal. gas.

• Boat type: Bay Boat
• Usage: Top Speed/0 ft
• Engine Manufacturer:
  Mercury
• Engine Type: Outboard
• Engine Year: 2003
• Number of Engines: One
• Engine Family: 2-Stroke
• Engine: 50 Classic
• Drive / Gear Case: 40-50 HP 3cyl 2-S, 50-60 HP 4cyl 4-S, 1.83 Ratio

[h=3]Details[/h]

• Calculated Pitch: 15.82
• Calculated Weight with engine(s): 1,372.40 lbs
• Calculated Engine RPM: 5,375
• Calculated Boat Speed: 40.08 mph

I’m a big fan of the prop calculator but these figures are never going to happen. Unless I’m totally thinking of the wrong boat...that pitch is likely too much and the speed estimation would be a record for a 50hp on anything other than a zap cat or similar haha.
Most boats similar and with a 50hp merc I know run a 13”p usually bit as we know...all boats are different ! Who knows.

 

[Buckem]

I agree no way I’m getting those speeds with this motor. I did the Merc prop selector and came up with the prop I have now, supposed to hit 38 mph. I’m at 10 less than that. I’m on last holes for raising motor and actually looks good. Possibly lowered a hole due to the V configuration of the boat. This will help RPM but hamper speed. I have Merc gauges but haven’t calibrated. Runs about 750 at idle in water in gear. Boat is rated for 75hp and I’d be better off getting a different boat before switching motors but 25-28mph is not ideal.

 

[Buckem]

[No message]

 

[Buckem]

My explanation didn’t show up on pic. Tach is set at 6p(pulse) which I believe is correct for this motor. Spacing on motor mount holes is 3/4” and I can only lower 1/2” before hitting top of transom. I’m running out of options.

 

[WesNewell]

I highly doubt you want to lower the engine.I'd bet money you need to raise it, as most engines are mounted way too low from the factory for max speed. You can lose up to 20% in max speed with the engine mounted too low due to lower unit drag in the water acting as a speed brake. With the av plate in line with the hull bottom, you don't want it below the hull. On a flat or mod v hull, about 1 "above will offer max speed. And because of reduced drag, engine rpm will also go up. Now if this is your boat, dry weight is 975#. In that case you probably want a max pitch of 14". That's what I run on my 995# boat with a 60 hp 4s merc. 13P may even be better for 50 hp.
http://boatspecs.iboats.com/Key_Largo_by_Marine_Mfg.__160_Center_Console__2003/bp/66b138262

 

[WesNewell]

This is for your 975# boat with 600# people and gear and 12 gal. gas. [h=3]Your setup[/h]

• Boat type: Bay Boat
• Usage: Top Speed/0 ft
• Engine Manufacturer:
  Mercury
• Engine Type: Outboard
• Engine Year: 2003
• Number of Engines: One
• Engine Family: 2-Stroke
• Engine: 50 Classic
• Drive / Gear Case: 40-50 HP 3cyl 2-S, 50-60 HP 4cyl 4-S, 1.83 Ratio

[h=3]Details[/h]

• Calculated Pitch: 14.00
• Calculated Weight with engine(s): 1,847.40 lbs
• Calculated Engine RPM: 5,375
• Calculated Boat Speed: 35.49 mph

 

[jimmbo]

A cheap Induction Tach can verify if your tach is accurate. If it is, and you are consistently getting higher Revs than what would be expected for the boats actual speed given the prop pitch, then yes I would be checking to see if the engine is indeed mounted too high for the design of prop in use. If an elevated engine height was fine with one prop, a prop of a different Brand, or Model might require an adjustment.

32mph on a 16 ft with a 50 isn't too bad

 

[QBhoy]

A cheap Induction Tach can verify if your tach is accurate. If it is, and you are consistently getting higher Revs than what would be expected for the boats actual speed given the prop pitch, then yes I would be checking to see if the engine is indeed mounted too high for the design of prop in use. If an elevated engine height was fine with one prop, a prop of a different Brand, or Model might require an adjustment.

32mph on a 16 ft with a 50 isn't too bad

Agree. It not a bad turn of speed at all.

 

[Buckem]

A cheap Induction Tach can verify if your tach is accurate. If it is, and you are consistently getting higher Revs than what would be expected for the boats actual speed given the prop pitch, then yes I would be checking to see if the engine is indeed mounted too high for the design of prop in use. If an elevated engine height was fine with one prop, a prop of a different Brand, or Model might require an adjustment.

32mph on a 16 ft with a 50 isn't too bad

I agree - my main issue is getting the RPM's down. I'll pick up an induction tach to compare numbers. The info previously provided is my boat and I plugged in all the numbers for correct prop and comparable mph values. This made sense if my speed was about 32 with 12P and 35-38 with 14P and would also explain my high RPM's. Maybe I should just take to a prop shop. I will if the tach comparisons match.

 

[WesNewell]

32mph with a 12p @6000rpm is a 14% slip. which isn't great, but not real bad. 28mph with a 14p@6000rpm is a 35% slip, witch is terrible and doesn't make any sense with the same load.

 

[jimmbo]

I agree - my main issue is getting the RPM's down. I'll pick up an induction tach to compare numbers. The info previously provided is my boat and I plugged in all the numbers for correct prop and comparable mph values. This made sense if my speed was about 32 with 12P and 35-38 with 14P and would also explain my high RPM's. Maybe I should just take to a prop shop. I will if the tach comparisons match.

If every thing was working fine, and say you were getting, say 30mph at 5400 rpm with a 12 in prop. Switching to a 14 is not going to give you 34+mph. You would probably remain at 30mph +/- 1mph. Your engine would be turning fewer RPMs because of the the greater load and have a slower Hole Shot.

Once the tach is verified and/or adjusted, an inspection as to how your engine is mounted on the boat will need to occur. More info on the props will help fill in some gaps. Speaking of gaps, can you post a pic showing the fit of the prop in relation to the gearcase, from the side?

 

[jimmbo]

32mph with a 12p @6000rpm is a 14% slip. which isn't great, but not real bad. 28mph with a 14p@6000rpm is a 35% slip, witch is terrible and doesn't make any sense with the same load.

The numbers make some sense if the prop has lost its bite from ventilation.

 

[Buckem]

My main goal was to drop my RPM's into a more tolerable rate/factory specs. The person we bought t he boat from hardly used it and pulled grandkids around in tubes mostly when they came over. I thought possibly that this was re-propped for this purpose for better low end torque. After looking at the prop charts and knowing I had RPM's to give I thought I would at lease gain a few mph and dropped into a tolerable zone on the engine. My thought was since I actually dropped speed and revolutions barely dropped that maybe I was getting some ventilation. Engine is mounted on its highest setting but in relation to the hull it looks decent. However a few inches higher than the lowest part of the V hull. As I stated in previous posts I thought I would drop the engine a hole but I don't have the clearance on the transom to do that unless I redrill mounting holes. I'll get a pic loaded to show this in a few.

 

[Buckem]

[No message]

 

[WesNewell]

From the photos it appears you can't raise are lower the engine at ll because of where the mount holes are drilled. Can't really see the hull bottom shape in either photo very well but it appears to have a pronounced v at the center, which would mean you want the av plate at least an inch above the v bottom when the engine is trimmed even with the bottom. Problem is, in the photo, the engine is not in line with the bottom of the hull and appears to be in a full down postion. Trim the engine so the propeller shaft is parallel with the hull bottom. That would also put the av plate in line with the hull bottom. Then take a photo from a 45 degreee angle to show both the hull bottom and the av plate of the engine. Similar to one below. Looks to me whoever drilled the engine mount holes drilled them too low to be able to make any height adjustments. That sucks.

 

[Faztbullet]

The small diameter gearcase is killing ya as RPM is slip and cavitation. Go with the largest diameter prop to increase bite like a 11.5x14

 

[WesNewell]

I get good performance from 1331-111-14 Solas ss 14p prop. It's 11 1/8 x 14.