Merc 25 4 Stroke Prop Question!

[YSAF]

I'm getting a 25 Mercury 4 Stroke that I will be putting on a 1542 Jon Boat, What prop would you all recommend for this application?

I usually have another guy, a cooler and fishing gear in the boat. (total load af about 500#) The boat itself only weighs 200 pounds.

Thanks!

 

[steelespike]

Re: Merc 25 4 Stroke Prop Question!

If it has a prop you should get the prop size , under the prop nut, and do a test run for the wot rpm and speed.
Something like a Tinytach is easy to setup and will work on almost any motor,
If I had to guess an 11" prop should work well.

 

[Jdeg]

Re: Merc 25 4 Stroke Prop Question!

Hey, I'm a newbie also. I've got a 16' that weighs 300lbs with a 2 stroke 25hp yammi. The prop I have is 9 7/8" dia x 11 1/4" pitch which is how it came from the factory. I don't have a tach, but with that prop it seems to rev high with an empty boat. I got a 13" pitch and that has dropped the RPM to where the engine doesn't sound like it is over-revving. If I had to guesstimate, a 12 or 13 pitch should be adequate and should push your boat at 25+ MPH.

 

[jestor68]

Re: Merc 25 4 Stroke Prop Question!

If it has a prop you should get the prop size , under the prop nut, and do a test run for the wot rpm and speed.
Something like a Tinytach is easy to setup and will work on almost any motor,
If I had to guess an 11" prop should work well.

That's what the Merc prop selector recommends. Details: 11.5 pitch at 5500 rpm = 28.1 mph.

But after reviewing some of the motor tests on the Merc web site, I'm thinking a 10 pitch to get the rpm close to 6000.

 

[jestor68]

Re: Merc 25 4 Stroke Prop Question!

Hey, I'm a newbie also. I've got a 16' that weighs 300lbs with a 2 stroke 25hp yammi. The prop I have is 9 7/8" dia x 11 1/4" pitch which is how it came from the factory. I don't have a tach, but with that prop it seems to rev high with an empty boat. I got a 13" pitch and that has dropped the RPM to where the engine doesn't sound like it is over-revving. If I had to guesstimate, a 12 or 13 pitch should be adequate and should push your boat at 25+ MPH.

The most experienced person cannot tell the rpm an engine is turning by sound alone.

Every motor has a recommended WOT rpm range that the motor must be propped to operate within. Too fast and it's over speeding. Too slow and it's laboring. Either situation will shorten the life of the motor.

Some kind of tachometer must be used to select the proper size propeller; unless you're feeling lucky. :joyous:

 

[BF]

Re: Merc 25 4 Stroke Prop Question!

I have an 11" pitch solas on a 16' tinny. (pretty bare bones boat, no wooden floor, but does have a battery) My cousin has the same motor (25 hp merc 4 stroke EFI) on a comparable boat as mine with a 12" pitch. My rig is propped better. Side by side mine rev's a bit higher, and is a mph or two faster, particularly when there's a couple guys in the boat. I do not hit the rev limiter when alone. With pretty much an empty boat, I get ~25/26 mph. Maybe (?) a 10" would be better, but I doubt it. Sorry, no tach.

 

[YSAF]

Re: Merc 25 4 Stroke Prop Question!

With pretty much an empty boat, I get ~25/26 mph.

Using a GPS I assume?

I was hoping to get closer to 30mph. I had a '92 Evinrude 25 2 Stroke on it previously, and it ran 30-31 on the GPS.

 

[YSAF]

Re: Merc 25 4 Stroke Prop Question!

BF

What kind of boats are you guys running exactly? V-Hull? Flat Jon? Mod-V?

 

[piranha propellers]

Re: Merc 25 4 Stroke Prop Question!

I'm getting a 25 Mercury 4 Stroke that I will be putting on a 1542 Jon Boat, What prop would you all recommend for this application?

I usually have another guy, a cooler and fishing gear in the boat. (total load af about 500#) The boat itself only weighs 200 pounds.

Thanks!

look up piranha.com

 

[BF]

Re: Merc 25 4 Stroke Prop Question!

speed has been checked with GPS, but to be honest, it could easily be a mph or two more. Just don't remember. Since it's my "fishing" boat, speed isn't a big thing. Here's a pict of my boat, before I gave it a make over. It's a 16' Edson, V-hull nose, but flat bottomed at the transom. Not really much of a modified V in the mid section. No wooden floor. split pedestal seating at the back, 2 benches and a triangular seat in the bow. Very basic. I replaced the wood on the seat tops, added 2 seats on swivels, added a hidden compartment for a battery and wired the boat for lights, horn, bilge pump and fish finder. My 25 hp Merc EFI has electric start. I'd guess the empty boat with no battery, no fuel, no motor would be ~275 lbs. Would be pretty comparable to a jon boat I think. My cousin's is comparable, but has removable plywood floor sections that might add a few lbs.

 

[YSAF]

Re: Merc 25 4 Stroke Prop Question!

Nice set up BF...I just ordered a 9 9/10 X 12 pitch Solas, I'll use that prop as a starting point. I should have my tach in a few days, and the prop shortly after. I'm going to install the tongue hinge, and try to have the boat on the river in just over a week. Ill report back with my findings!

 

[BF]

Re: Merc 25 4 Stroke Prop Question!

Ya, let me know how it turns out. I like the engine... runs smooth throughout the rpm range. Only knock against it is that the idle seems to hunt up a bit sometimes, particularly when just shifting in and out of gear. That can mean that when quick shifting fwd to N to rev (like when wanting to slow the boat when docking), that a momentary idle surge can make it harder to shift in/out of gear. I have a feeling I'd like it better if it had a separate shift lever instead of the merc twist-shift throttle combo.

 

[YSAF]

Re: Merc 25 4 Stroke Prop Question!

I had it on the water this evening and was pleasantly surprised! I was alone, so I put a 50lb bag of salt, an anchor and a tub misc. things up front....for a total of about 90lbs. since I was on the mississippi the current played a role on the following... down current (very little flow) and against the wind I hit 31.8 on the GPS, up current I hit 29.69. I didn't move the pin to adjust the motor at all. From a dead stop this motor is insane! I was AMAZED how fast it gets up to top speed. I'm curious to see what it tachs out at with the current prop..... My tach hasn't arrived yet! Over all I couldn't be happier!

 

[steelespike]

Re: Merc 25 4 Stroke Prop Question!

Looks like a 12 might be the one. Only worry might be the blistering hole shot could lead to over revving the tach will tell us where your at.Actually if its overrevving an increase in pitch could squeeze out a touch more speed.
Seems to me that if you stopped and measured your drift with the current it would be easier to evaluate your actual speed.
If there is room I wonder what running across current might indicate?
If I use 30.8 for your down stream run and guessing 10% slip I get 5782 rpm.
Certainly in the ideal range.
Obviously the tach will help to pin down your setup.

 

[BF]

Re: Merc 25 4 Stroke Prop Question!

Glad to hear it... my cousin's set up with the 12" pitch is good, and he's happy with it. It's only when we had the boats side by side that we found out mine was a bit faster... it was close with a light load, but an mph or two when there were 3 in the boat. It could be that if we had the boats really empty of gear and were riding solo, that his might pull away... (?) Steele... I don't think over-reving is an issue. Pretty sure there's a rev limiter... I think mine kicked in a couple times when I picked up some weeds while blasting at WOT. I've never hit the limiter otherwise.