1548 SeaArk with 2015 Merc 25 hp new prop

Jeffsam1017

Cadet
Joined
Aug 1, 2017
Messages
6
I just bought a 2015 mercury 25hp 4stroke 1.92 gear ratio. It has a beat up Merc 9.75 x 9.50 AL prop that pushes the boat well. It runs 26-28 mph at 5700 rpms. I am happy with the performance but I think I might can get a touch more out of it. I need a new prop because this one is pretty banged up. Any recomendations on changing pitch or diameter with the new one? Thanks in advance.
 

WesNewell

Chief Petty Officer
Joined
Jan 3, 2018
Messages
497
Minimum pitch for that setup should be an 11P prop for up to 900# people and gear. With a light load, I'd go higher than that. Make sure engine is mounted at proper height. can make up to 20% difference in speed and fuel economy.

Your setup
  • Boat type: Aluminum - Tiller
  • Usage: Top Speed/0 ft
  • Engine Manufacturer:
    Mercury
  • Engine Type: Outboard
  • Engine Year: 2015
  • Number of Engines: One
  • Engine Family: FourStroke
  • Engine: 25 EFI 3cyl (526cc)
  • Drive / Gear Case: 25-30 HP 3cyl 4-S, 1.92 Ratio

Details
  • Calculated Pitch: 10.94
  • Calculated Weight with engine(s): 1,616.20 lbs
  • Calculated Engine RPM: 5,750
  • Calculated Boat Speed: 26.66 mph
 

Jeffsam1017

Cadet
Joined
Aug 1, 2017
Messages
6
I guess I'm not sure how I am running a 9.5 at 5700 and 26 and going up a 1.5 will. It drop my wot and keep my speed the same.
 

QBhoy

Fleet Admiral
Joined
Mar 10, 2016
Messages
8,286
Worth being careful here...

apart from the usual and must have details required, some of which you posted, there will be an ideal RPM rating for your engine. Trick is to get into that, in your case somewhere near the top end of it....but the big thing to watch out for and what often throws people off in their assumptions is....the very fact that you have a damaged prop and are basing your data on its results. Depending on how badly it’s damaged, it can often show results similar to a lesser pitched prop. Just something to watch out for.
 

WesNewell

Chief Petty Officer
Joined
Jan 3, 2018
Messages
497
What I posted was with 900# of people/gear and 42# of fuel. This also assumes the engine is mounted at the correct height, which I doubt as most aren't. With 2 people, say 200# each you'd probably want a 13P for max speed.
 

jimmbo

Supreme Mariner
Joined
May 24, 2004
Messages
12,944
What is your engines WOT range? You say you want to go a little faster. Get rid of unneeded Ballast. That means No WIfe, No Kids, No Mother-in-law, in the boat. Mistresses and Girlfriends, on the other hand, are still allowed aboard. Redistribution of remaining ballast may help. At the speeds your boat is attaining, engine height is not going to be much of a factor, less than a 1/4 mph. If you were in the 45+mph range then it starts to become important.
You could consider getting the current prop repaired, hopefully by a competent shop, and then you will have a better idea of its performance.
How are you getting your MPH? If is by GPS, beware that it will throw MPH off if there is any current in the water, leading to rpm-speed-slippage numbers that don't quite line up
 

Jeffsam1017

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Joined
Aug 1, 2017
Messages
6
5000-6000 rpm range. Gps for speed. So max speed was 29.1 but going down river, 25 mph going up. So I just kinda averaged in the middle. Installed a small tach and it says 5700 rpms but I'm not 100 percent on how accurate that is, seems right because idle rpms are right. Prop is an aluminum one that has seen some rocks and sand, kinda beat up and rounded on the blade. I understand my rpms may be off due to the prop being not 100 percent, what throws me off is the 9.5 pitch and the 5700 but like I said the prop could be acting as a different pitch due to damage. I don't have to run much faster, just wanting to get the right prop for my setup.
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
How many boaters were on board while achieving 5700 wot revs ? If min-max 5K to 6K is the factory wot range, 5700 wot rpm is pretty good. If plan adding more boaters to the equation will need one less pitch whichever diam the prop manufacturer wants to put to prop.

BTW, that 20% drag assumption will only happen if water flow at speed is banging hard right over small upper splash plate, that's at the non edged portion of the lower leg and consequently producing a huge splash out transom or even over it which will inevitably slow combo down as in pic.

Back Water Splashes.JPG

Happy Boating
 

jimmbo

Supreme Mariner
Joined
May 24, 2004
Messages
12,944
Which way the rpms are affected by a damaged prop are hard to predict. Bent blades can increase or decrease pitch. Nicks dings can create Cavitation, which may also affect rpm
You're in the upper half of the rpm range, which is fine. If you can prop it so the engine gets to the rpm where the peak hp is attained, the speed will be the highest. Propping on either side of the peak will result in a slightly lower top speed.
 

WesNewell

Chief Petty Officer
Joined
Jan 3, 2018
Messages
497
Engine height is the number one factor to consider. Mounted too low produces tremendous drag at planing speeds. Raising mine just 1.5" increased my top end speed from 34-35mph to 38-39mph with a smooth ride. To reach those speeds before, I had to trim engine up a lot and the boat porpoised like crazy. Oh, and the rpm also increased when I mounted the engine higher. Get your engine mounted right before you start experimenting with different prop pitches. A 25hp engine is light enough to do without a hoist.
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
If OP is happy running with a beaten prop as loaded and OB revving at 5700 wot revs, should go per same new exact same prop. If a beaten prop achieves less wot rpm, a new one wil surely rev towards its max 6K wot rpm range.

The speed difference between running middle to max wot rpm range is so small that's up to each boater if propping right their OB's for top hole shot losing slight lop end speed or slight top end speed increase with slight hole shot decrease.

Personally find useless to pursue an OB to run towards max speed as few runs their combos to full throttle every time out due to fuel costs everytime you need to fill your tank up unless you happen to be CEO of a Petrol Company. LOL!!

Happy Boating
 

anzomcik

Petty Officer 2nd Class
Joined
Jul 26, 2008
Messages
158
FWIW i ran the numbers on RPM and speed ((((rpm/gear ratio) x pitch) /12) x 60) / 5280= MPH. One of your numbers is off. your theoretical perfect world speed is 26.7 mph with 0% slip, that's impossible to obtain with water propulsion.

So if your speed is correct one should expect your rpm to be around 6400 rpm (roughly 10-15% higher than what your reading on the tach)

If your tach is correct your speed would be in the ball park 22-23mph

My gut is telling me your rpm is incorrect, mainly because i had a 11pitch on a heavy loaded 14' side console with a 25hp mariner that went 22-24mph.

good luck
 

Jeffsam1017

Cadet
Joined
Aug 1, 2017
Messages
6
So I tried the solas 12 pitch today and while it gave me the best top speed 26.5 average it only turned up 5200 rpms. I tried a 9 pitch and it ran 24 but hit my rev light at 6300. The 9.5 pitch ran 5800 at 25.5 average. The 12 was a little Slow out of the hole. I think the 11 or 10.5 would be perfect. Thanks for the info!
 

WesNewell

Chief Petty Officer
Joined
Jan 3, 2018
Messages
497
26.5mph@5200 rpm is a 15% slippage. While not horrible, it's not great. Assuming this test was not with a full load. If it was, that's not too bad. You should still check your engine height. Another thing you could check is to see if there's an air intake restricter plate on your model. If so, removing it should give you a little more power, as the 25&30 hp engines are the same block. A cheap way to get another 15% power. And I'd guess there is one. But yes, the 11P prop is a good all around choice.
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
On those 4 strokes 25 HP ones which are 530 CC rebadged EFI Tohatsu models will need to swap the CPU and probably some other parts too from the 30 HP model for a correct HP conversion which by the way it's not cheap..

If you want to get the max out of that portable OB read this, try to match it :

https://forums.iboats.com/forum/gen...nd-topics/10676402-should-i-raise-to-41-4-ptp

From there can go for a prop maximization playing with pitches..

Happy Boating
 
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