1995 Mercury 15hp mods

[Kennerdell8]

Is there anything i can do to get more out of my 95 15hp merc? I already have carson performance reeds.
Elsewise new plugs, new wires, new carb and coils.

[Roflhat]

Porting, bigger carb, more compression, balance crank and flywheel, velocity stacks, modify exhaust

[zul8tr]

Before going internal with mods suggested by Roflhat let us know some basics:
What type boat are you using it on?
What weight is the rig as you run it?
Do you have a tach? If so what rpm are you peaking at?
Do you have a speedometer, gps or water pressure type? If so what top speed?
What prop make are you using and pitch x diameter?
Where are you running the anti ventilation plate dimension depth relative to the bottom? At or below bottom depth?
Are the Carson reeds single or 2 stage?
Does the carb have adjustable low and high speed jets? If adjustable have you played with them?
What octane fuel and oil ratio are you using?
Is the ignition timing adjustable and where is it presently set, factory spec (what is that) or have you adjusted it (to what)?
What plugs are you using, Brand and plug number?
Have you done a compression test? What are the max readings for each cylinder? They should be close to each like less than 5% difference. My engines are within 1%. I do compression cold to keep temp variable mostly constant. I also do it after running when engine fully cooled down and have carb full open. I also record each rope over pressure. If you have a CD ignition make sure plugs grounded when roping over to protect CD box from probable damage.

[Kennerdell8]

Before going internal with mods suggested by Roflhat let us know some basics:
What type boat are you using it on?
What weight is the rig as you run it?
Do you have a tach? If so what rpm are you peaking at?
Do you have a speedometer, gps or water pressure type? If so what top speed?
What prop make are you using and pitch x diameter?
Where are you running the anti ventilation plate dimension depth relative to the bottom? At or below bottom depth?
Are the Carson reeds single or 2 stage?
Does the carb have adjustable low and high speed jets? If adjustable have you played with them?
What octane fuel and oil ratio are you using?
Is the ignition timing adjustable and where is it presently set, factory spec (what is that) or have you adjusted it (to what)?
What plugs are you using, Brand and plug number?
Have you done a compression test? What are the max readings for each cylinder? They should be close to each like less than 5% difference. My engines are within 1%. I do compression cold to keep temp variable mostly constant. I also do it after running when engine fully cooled down and have carb full open. I also record each rope over pressure. If you have a CD ignition make sure plugs grounded when roping over to protect CD box from probable damage.

Weighs around 400 with me and gear
With the current prop its seeing 4800
Top speed is 21mph solo
Its a 13' Gheenoe
Prop is a solas 9.25/11.5 I've used 9x9 and 9x10 too
Cav plate is 1.5" above plug, any higher it blows out
Carson reeds are 2 stage
Carb is only adjustable for idle
90oct non ethanol with a 50:1 with quicksilver
Comp is 120 top 118 bottom
Plugs are BHS-15s gapped at .060
It runs amazing just not very fast compared to others.

[Kennerdell8]

Also timing is set to what the owners manual says, cant remember off the top of my head what it is

[zul8tr]

Weighs around 400 with me and gear
With the current prop its seeing 4800
Top speed is 21mph solo
Its a 13' Gheenoe
Prop is a solas 9.25/11.5 I've used 9x9 and 9x10 too
Cav plate is 1.5" above plug, any higher it blows out
Carson reeds are 2 stage
Carb is only adjustable for idle
90oct non ethanol with a 50:1 with quicksilver
Comp is 120 top 118 bottom
Plugs are BHS-15s gapped at .060
Also timing is set to what the owners manual says, cant remember off the top of my head what it is
It runs amazing just not very fast compared to others.




The 13ft Gheenoe is a punt rear end canoe type design that is not ideal for speed. Most of your speed loss is that boat design plus the 400 pounds loaded weight is heavy. Videos I watched on these 13ft Gheenoe with a 15 hp shows the boat at speed with a rear squat and a high nose with lots of water drag. There were top speeds noted in the mid to low 25mph range.

https://gheenoe.net/models/gheenoe-13/

1995 Merc 15hp 2 stroke manual included in the 9.9 manual

https://www.manualslib.com/manual/1339757/Mercury-9-9-Twostroke.html?page=28#manual

This Mercury Manual lists using NGK B7HS-10 or Champion L82C both non resister plugs and probably give them a try. They list resistor type but would not use them. I do not know what you list as a BHS-15s plug, what brand is that?

The anti vent plate (not cav plate) is 1.5" above drain plug, but for the bottom design of your boat it is below the static water level when running max. At the height you note the prop still has blades out of water and blow out is still inevitable in that boat design with the service props you use.

Surface piercing props have part of the blade above the static water level for a portion of each prop revolution and they are ventilated with ambient air (this is not the same as a cavitation condition). This prop running condition generally requires cupping the prop on the trail edge to help grip in the air water mix.

Are any of the props you note cupped?

The Merc manual lists full throttle RPM range of the 15 hp at 5200 - 5800 and a gear ratio of 1.84:1. Based on the props you note I calculated some theoretical max speeds with no slip (which there always is) just to see how fast using basic dimensions predicts.

There are many calculators on line to do this, I just used the basic formulas below. I have been playing with props/race engines/race boats/pleasure boats and service engines for many years and although there are many other factors that relate to boat speed but these simple formulas have proven to provide great relative results to evaluate changes. The formula factors for Max mph except pitch can be measured easily. Pitch can be measured several ways and manufactures have their methods, so stamped pitch can be questionable when comparing props and even using the same prop stamped pitch is questionable but if that is all you have use it. I have my pitch measuring process and I always measure the same way to get consistent relative results across the various props used and always with the same gage.

Actual speed noted is also a number that is subject to inaccuracies depending on gage used, water conditions, wind, waves, prop rake, blade shape, pitch progression from lead to trail edge, engine condition and tune, fuel used, rig set up, etc.

Note these formulas do not include prop diameter, rake, blade shape, pitch progression, etc just based on simple screw helix advance theory but still are very useful.

Max mph = RPM x Pitch inches / (gear ratio x 1056) with zero slip
Slip % = 1 - (actual mph/ Max mph) x 100

Using the prop info you provided and the Merc manual max rpm range:

Prop 9.25" dia /11.5" pitch max speed = 30.8 mph at 5200 & 34.3 mph at 5800 rpm
Prop 9" dia/9" pitch max speed = 24.1 mph at 5200 & 26.9 mph at 5800 rpm
Prop 9" dia/10" pitch max speed = 26.8 mph at 5200 & 29.9 mph at 5800 rpm

Based on your reported very low 4800 rpm at 21 mph and using the 9.25/11.5"pitch prop the max speed with no slip is 28.4 mph. This means your 21 mph max speed has 26% slip for your running rig. This indicates too much slip and not a vey efficient rig primarily caused by high water drag and weight. Not surprised with that canoe hull design. I do not know what attitude your boat operates at full speed, a video would help.

What were the rpm and max speeds with the 9/9 and 9/10 props you tried?

You state "It runs amazing just not very fast compared to others" This is subjective and tells me nada. Are you comparing: same boat, same engine, same rpm, same prop, same weight, etc? If not it is not a useful comparison. Your set up is unique and results in your performance.

If you want more speed the first area I would consider is to get the engine in the factory rpm range of 5200 - 5800 and find a prop and set up that does that, if possible? The slip needs to be less.

What I didn't ask but is part of the setup fix is are you running the prop shaft parallel to the bottom or kicked out or tucked in?

There are some subtle things you can do to the engine like: best ignition timing trial and error, the right plugs gapped and indexed, high speed carb jet changes and proper plug reading. But the biggest speed gain is to reduce the water drag and that hull design has its issues.

If you intend to do lots of high rpm running I would use more oil in the gas since more sustained revs with 2 strokes requires more oil (it has been wisely said 'oil is cheaper than parts"). I am not suggesting to use the oil amount I note below, you need to find what works. But 50:1 was for presented for general service use and is ok for that. You also do not need 90 oct fuel. In my race Merc 25ss and others I have found best results with 87 oct non E but I use lots of oil (16:1) since always above 7300 in my 25ss Merc hydro (avatar). These small 2 strokes are low compression(CR) engines when measured relative to the top of the exhaust ports. All the small 2 strokes I have checked are in the 5.8 to 6.8:1 CR range. So 87 works well for them.

Good luck keep us informed

Pete