Propeller Recommendation - 1998 Mercury Force 50 HP

[Steve Fournier]

I have a restored aluminum 1973 MirroCraft Fisherman 16-foot, powered by a well preserved 1998 Mercury Force 50 HP outboard. On my first outing at WOT the engine ran the boat up to 31 MPH and slightly over 7000 RPM on the tach, recommended operation is around 5300 RPM. I made sure the tach is properly calibrated and started to investigate the propeller and it seems to have a diameter of around 10", which seems to be a bit small, I researched online to find out what the prop should be and came up with a Solas 13-spline, 3-blade 12.1 x 9 Amita.
The boat weighs 350 LBS per factory specifications, loaded maybe 800-1000 LBS.
Can anyone make a recommendation?

 

[jimmbo]

What Prop is currently on it? If the Pitch greater than 9 in, then you would be going the Wrong Direction. Also, you are 100% certain that the Tach is set correctly?

 

[Steve Fournier]

Do not know the pitch, there is no stamp on the prop, just measured the diameter. The tach is calibrated correctly.

 

[jimmbo]

The info is on the Prop somewhere. Remove it and look it over. That will also enable you to see of the Propshaft Seal has Fishing line against it

 

[Steve Fournier]

Okay, I'll check it out.

 

[Steve Fournier]

The info is on the Prop somewhere. Remove it and look it over. That will also enable you to see of the Propshaft Seal has Fishing line against it

Okay, I found the number it's 48 73138 A40 14P

 

[Steve Fournier]

Okay, I found the number it's 48 73138 A40 14P

I also checked the tach and confirmed it is properly calibrated.

 

[airshot]

Bad prop, bad prop hub, motor mounted to high, faulty tach reading....those are your choices !! At 7K rpm, you should be well over 31 mph....gps confirmed speed ??
My guess would be spun hub or faulty tach reading...fact is your engine won't last long unless you figure it out quickly.

 

[Steve Fournier]

Bad prop, bad prop hub, motor mounted to high, faulty tach reading....those are your choices !! At 7K rpm, you should be well over 31 mph....gps confirmed speed ??
My guess would be spun hub or faulty tach reading...fact is your engine won't last long unless you figure it out quickly.

Tach is fine, I only tested the engine once after a 3-month rebuild of the boat. Engine does not have many hours, compression tested both cylinders at 160 PSI, no variation. The engine should only run at 5300 RPM, not 7000, so my opinion based on some research that the prop is too small at 10.25" x 14 pitch. It should be around 12" x 15 pitch; I'm looking for other opinions or people who have a similar setup and what they are using.

 

[jimmbo]

I researched online to find out what the prop should be and came up with a Solas 13-spline, 3-blade 12.1 x 9 Amita.

So now you are saying your research is pointing to a 12" X 15"?
Shooting for a specific Rpm, like 5300, might not be achievable. What is the WOT Range for your engine?

Does the current Prop have a Rubber Cushioned Hub, or does it have one of those Sacrificial Hubs? Post a Pic of the end of the Prop that is against the Gear Housing

 

[Steve Fournier]

So now you are saying your research is pointing to a 12" X 15"?
Shooting for a specific Rpm, like 5300, might not be achievable. What is the WOT Range for your engine?

Does the current Prop have a Rubber Cushioned Hub, or does it have one of those Sacrificial Hubs? Post a Pic of the end of the Prop that is against the Gear Housing

The WOT range is 4500-5300 RPM
The 12" x 15" was established using one of those propeller sizing apps on-line.

 

[airshot]

One ince of pitch equals about 200 rpm...so to drop from 7K down to 5300 rpm means an 8" change in pitch.not gonna happen, still willing to bet the tach is off or a spun prop. Can't believe your engine stayed together at 7K rpm......

 

[jimmbo]

That looks like a Rubber Hub Prop, and the Face of the Prop where the Thrust Washer goes isn't worn away from a Slipping Hub\
Can you provide a couple Pics of the Prop on the Engine, One from the Side, and the other also at that height, but at a 45 Degree Angle looking toward the rear of the Gearcase, from behind.
I ask this cause I think the 10X14 is for a Lower HP engine that uses a Smaller Prop, and the Hub of that prop is too small to Funnel all the Exhaust Gases thru, it and some are going out over the Blades, resulting in Ventilation

 

[QBhoy]

I also checked the tach and confirmed it is properly calibrated.

Hi. Not at all questioning you…hope you don’t mind if it appears so..but you mind me asking what kind of tach you’re using ? And the means of making certain, its calibration and value is accurate?
Just asking to double check…I really honestly don’t think it’s possible for you to have seen 7000 rpm or anything close. Not with that pitch. Obviously assuming that all else is as it should be.

 

[QBhoy]

Tach is fine, I only tested the engine once after a 3-month rebuild of the boat. Engine does not have many hours, compression tested both cylinders at 160 PSI, no variation. The engine should only run at 5300 RPM, not 7000, so my opinion based on some research that the prop is too small at 10.25" x 14 pitch. It should be around 12" x 15 pitch; I'm looking for other opinions or people who have a similar setup and what they are using.

Hi Steve.
Genuinely trying to help here. But I need to mention a thing or two first. So if you only go up one inch in pitch (even with more diameter), you are never ever going to drop the rpms to anything close to what you are aiming for. Nothing like it.
I think it’s likely that your compression gauge is a little optimistic too. Could be wrong. But she’s very high for a two stroke like that.
Also. As I said earlier. You really are probably getting an inaccurate tachometer reading too.
I won’t bore you with the mathematical numbers that undoubtedly will hold testimony to this…but instead maybe suggest you make sure she’s reading accurately. Maybe I can help with an opinion, if you tell me what kind of tachometer you are using and what its set up or installation is like.
All the best.

 

[airshot]

We are not questioning your ability to setup your tach properly, but the info that comes from Force is not always up to date or accurate. The parameters given to you for setup may not be correct. Based on the numbers posted, something is definetly off...

 

[Steve Fournier]

That looks like a Rubber Hub Prop, and the Face of the Prop where the Thrust Washer goes isn't worn away from a Slipping Hub\
Can you provide a couple Pics of the Prop on the Engine, One from the Side, and the other also at that height, but at a 45 Degree Angle looking toward the rear of the Gearcase, from behind.
I ask this cause I think the 10X14 is for a Lower HP engine that uses a Smaller Prop, and the Hub of that prop is too small to Funnel all the Exhaust Gases thru, it and some are going out over the Blades, resulting in Ventilation

Here are the pictures, let me know if they are okay?

 

[jimmbo]

Well the Prop Hub looks like it is the correct Diameter for your motor.
Sorry to sound like a Broken Record, but the Tach is still suspect. What Setting on the Tach did you select?

 

[Steve Fournier]

Well the Prop Hub looks like it is the correct Diameter for your motor.
Sorry to sound like a Broken Record, but the Tach is still suspect. What Setting on the Tach did you select?

It is a Faria Tach, with the Deutsch Connector Case, setting 5 or 12-pole according to their instructions for that motor. See the attached PDF file.

 

[jimmbo]

This pic shows you engine mounted pretty much even with the Hull Bottom which is normally the correct height, but yours as a lot of Aluminium Hulls do, has a protruding Keel directly in front of the Motor. Perhaps that is creating a less than solid supply of water to the Prop, causing ventilation and higher revs. I can't tell by the pic if that Prop has any Cup. If not, a cupped blade will handle a bit of air a lot better

How far ahead of the Transom does the Keel, diminish?