'22 Mercury 115 Pro XS, best prop?

undercover500

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Engine is 2022 Mercury Pro XS 115hp, the regular version, not the command thrust. Boat is a Tahoe T18, with our current load and gear, we run a top speed of 40.5-41mph at approx. 5300rpm at WOT and fully trimmed out (as far as the engine will allow), engine has the 4 blade 17 pitch SS Spitfire.

On the website for this engine, they list the WOT RPM range as 5,300-6,300rpm. I realize that we are technically at the bottom end of the listed RPM range, but I know that this engine has more to offer. We want to be closer to the 6,300rpm range.

What prop would you recommend? 3 blade, 4 blade, what pitch? I was thinking maybe a 14 pitch 3 blade? Just from my bit of googling, the general rule is that going down an itch in pitch should give us around 200rpm, so if we went down 4 pitches, that would bring us around 6,100rpm. I'm thinking going to a 13p might over rev when we run light, if ever, and to account for if that general rule is not accurate.

The current RPM's and speed are with our typical load, we would rarely run lighter than we did when we got these numbers.
 

Sea Rider

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Sep 20, 2008
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Why do you like runing the combo fully trimmed out ? post how much wot rpm difference and speed is achieved on flat calm water condition with deck load well distributed compared to running the motor trimmed to neutral trim, surely it's not a wow demon speed increase...

Happy Boating
 

undercover500

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Why do you like runing the combo fully trimmed out ? post how much wot rpm difference and speed is achieved on flat calm water condition with deck load well distributed compared to running the motor trimmed to neutral trim, surely it's not a wow demon speed increase...

Happy Boating
Aren't you supposed to "trim it out?" Above a certain RPM range the engine won't let us trim any further. Up until this limit, the RPM's keep climbing until we hit the trim limit around 5,300.

Fully trimmed in is about 4,500 rpm at 30mph. Trimmed as far as the engine will at WOT, we get the 5,300rpm at 41mph. Slightly trimmed in from full to prevent bouncing nets maybe 5,200. It is noticeable.
 
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Sea Rider

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Aren't you supposed to "trim it out?" Fully trimmed in is about 4,500 rpm at 30mph. Trimmed as far as the engine will at WOT, we get the 5,300rpm at 41mph.

What's your current boating location, river, lake, open sea ? There's a lot of tech discussion going on regarding the use or not of the full trimmed down or up running position, the guys that likes running their combos with 3/4 hulls out of the water on flat calm, no wind water loves the latter. Will post some tech information excerpted from most OB's manufacturers regarding trim positions and what to expect once at speed/plane....

Happy Boating
 

undercover500

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What's your current boating location, river, lake, open sea ? There's a lot of tech discussion going on regarding the use or not of the full trimmed down or up running position, the guys that likes running their combos with 3/4 hulls out of the water on flat calm, no wind water loves the latter. Will post some tech information excerpted from most OB's manufacturers regarding trim positions and what to expect once at speed/plane....

Happy Boating
Mostly lakes, and rivers. Typically are not seeing super rough water, the most I'd expect to see would be 3 footers.
 

jimmbo

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I would verify that the Tach is set correct first. Myself, I would ditch the 4 blade, and use a 3 blade, that by itself will raise the PRM, reduce drag, and increase Speed.
Dropping Pitch will certainly make for quicker Acceleration, as for top speed? Dropping one Blade will increase top speed, less pitch will raise RPM, and may or may not change the top speed.
 

QBhoy

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I think something isn’t right here. You should be much higher in the rpms as you’re set up.
 

Sea Rider

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You've posted the min and max wot revs achieved while ridding fully trimmed in/out. With deck load evenly distributed and same number of passengers as in the previous rpm metered runs report max wot rpm achieved with trim set to neutral trim once on plane on flat calm water cond for the combo to ride parallel to the water level, something like this for illustration purposes..
02-Ballance & Trim.JPG
Happy Boating
 

QBhoy

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You've posted the min and max wot revs achieved while ridding fully trimmed in/out. With deck load evenly distributed and same number of passengers as in the previous rpm metered runs report max wot rpm achieved with trim set to neutral trim once on plane on flat calm water cond for the combo to ride parallel to the water level, something like this for illustration purposes..
View attachment 360227
Happy Boating
 

QBhoy

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Mar 10, 2016
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You've posted the min and max wot revs achieved while ridding fully trimmed in/out. With deck load evenly distributed and same number of passengers as in the previous rpm metered runs report max wot rpm achieved with trim set to neutral trim once on plane on flat calm water cond for the combo to ride parallel to the water level, something like this for illustration purposes..
View attachment 360227
Happy Boating
You've posted the min and max wot revs achieved while ridding fully trimmed in/out. With deck load evenly distributed and same number of passengers as in the previous rpm metered runs report max wot rpm achieved with trim set to neutral trim once on plane on flat calm water cond for the combo to ride parallel to the water level, something like this for illustration purposes..
View attachment 360227
Happy Boating
That wee boat in the picture has its engine too low ! The av plate will not be level when in place either 😂
 

Sea Rider

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Hey QBhoy next time will post an illustration showing a boat with deck load well distributed without a motor at the back. Let's hope the OP posts wot rpm numbers and speed achieved while the combo run with neutral trim, at that trim setting probably will dial middle rpm/speed numbers between fully in and fully out trim posted numbers. ?

Happy Boating
 

Texasmark

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Dec 20, 2005
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I guess your gear ratio is 2:1 like my 2002 2 stroke 115.....seems to be pretty much an industry standard for that HP.
If you are running over 35 MPH for a number, trimming out always reduces the wetted area of the boat and increases RPM and speed as you indicate due to less hull drag.

I am running a 19P Solace SS 4 blade at 5250, my RPM limiter limit at 45 with a Crestliner Fish Hawk 1750 running about 4% prop slip. I just ran your numbers and you are at 7% a very respectable slip number: https://www.go-fast.com/Knowledge-base/Find-your-prop-slip/Prop-slip-calculator

From my experience and the slip calculator numbers you are getting all you are going to get out of that engine on that boat with your current load. If you dropped down to 13p to get your RPMs up to the max limit which is usually the desired RPM at WOT without water toys in tow, this would be the result assuming the same prop slip: https://www.go-fast.com/Knowledge-base/Find-your-prop-slip/Prop-slip-calculator

So, my suggestion is to split the difference and go with a 15P and you will be right in the middle of both and have a faster hole shot and have some room for RPM drops with water toys in tow.
 

QBhoy

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I guess your gear ratio is 2:1 like my 2002 2 stroke 115.....seems to be pretty much an industry standard for that HP.
If you are running over 35 MPH for a number, trimming out always reduces the wetted area of the boat and increases RPM and speed as you indicate due to less hull drag.

I am running a 19P Solace SS 4 blade at 5250, my RPM limiter limit at 45 with a Crestliner Fish Hawk 1750 running about 4% prop slip. I just ran your numbers and you are at 7% a very respectable slip number: https://www.go-fast.com/Knowledge-base/Find-your-prop-slip/Prop-slip-calculator

From my experience and the slip calculator numbers you are getting all you are going to get out of that engine on that boat with your current load. If you dropped down to 13p to get your RPMs up to the max limit which is usually the desired RPM at WOT without water toys in tow, this would be the result assuming the same prop slip: https://www.go-fast.com/Knowledge-base/Find-your-prop-slip/Prop-slip-calculator

So, my suggestion is to split the difference and go with a 15P and you will be right in the middle of both and have a faster hole shot and have some room for RPM drops with water toys in tow.
I got 1% for slip. Using the numbers. Gear ratio is 2.07-1
 

undercover500

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Apr 21, 2022
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Hey QBhoy next time will post an illustration showing a boat with deck load well distributed without a motor at the back. Let's hope the OP posts wot rpm numbers and speed achieved while the combo run with neutral trim, at that trim setting probably will dial middle rpm/speed numbers between fully in and fully out trim posted numbers. ?

Happy Boating
When I say "fully trimmed out" I do not mean the engine is tilted beyond what it should be, I mean that we get up on plane, push the throttle all the way down, get moving and then start bumping the trim up. Oddly enough, being at or near the trim limiter on the engine nets us our fastest speed. This engine will not let you trim past a certain point beyond 2000rpms, otherwise you get into tilt range, to prevent blow out. At the very top of the trim range, you do get some porpoising, typically we trim it in just a hair to keep the bouncing down. At this trim range, when the hull is up on pad and running good with no porposing, we are getting our 40.5mph.
 

QBhoy

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When I say "fully trimmed out" I do not mean the engine is tilted beyond what it should be, I mean that we get up on plane, push the throttle all the way down, get moving and then start bumping the trim up. Oddly enough, being at or near the trim limiter on the engine nets us our fastest speed. This engine will not let you trim past a certain point beyond 2000rpms, otherwise you get into tilt range, to prevent blow out. At the very top of the trim range, you do get some porpoising, typically we trim it in just a hair to keep the bouncing down. At this trim range, when the hull is up on pad and running good with no porposing, we are getting our 40.5mph.
Didn’t know they had a pad…anyway…you’re a good bit down on performance. I’ve got the normal version of this merc engine (but with Ct box)…on a 19ft cuddy cabin sports cruiser. I get around 43gps at nearly 6000 rpm with a tempest 19”. And a couple mph more perhaps, with a 20” enertia at about 6000 rpm. My ratio is 2.38-1…but you’d imagine your boat should do a little better than it is now. Although…as far as I know, the CT versions usually run quicker…just with the props they can use.
I saw a huge difference by raising my engine up a couple of holes on the transom. Baystar steering transformed things too.
I alswats use the smartcraft gauges or vessel view read out, just so you know your data is accurate.
 

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Texasmark

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Agree raising the engine off the transom even one hole (3/4") can make a MPH or more difference. You have to have the right prop to raise the engine off the transom since you are inviting ventilation at early trim angles and when up like that you need the trim out to get the hull out of the water and run on the pad only.

When I ran a Ranger (brand) Pad boat as I was getting on plane, acceleration was brisk so topping out in MPH didn't take but seconds, I trimmed out until speed increases stopped and the engine RPMs started rising (ventilating due to prop being too close to the surface). I was running the original Laser 3 blade SS which was built for running such as I said, high rake and cupped blades. Engine was sitting on the transom as the hull had about a 6" offset between the end of the pad and the transom which took care of that. Funny, I could hear the engine singing at 6k and the prop beating the water making a humming noise.....great sport. I had 2 speeds with that boat. Off and 6k RPM.
 
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