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Started By
Message
anyone have experience going from a 2 stroke 25 to a 4 stroke 20?
Posted on 8/21/25 at 9:25 am
Posted on 8/21/25 at 9:25 am
pulled the duck boat out of mothballs yesterday and cleaned it up, i want to start using it for easy trips out of manchac. its a homemade flat bottom fiberglass 14' with 4' beam, pretty overbuilt and heavy. the (very) old mercury 25 will plane it empty at about 30 mph. i want to repower
mercury 25 4 stroke is quite a bit bigger and heavier than the 2 stroke, while the 20 4 stroke is about the same size and weight. it appears that the 20 would fit better on the transom. without a load in the front the boat wants to porpoise a bit, so i'm concerned about the extra weight of the 4 stroke.
anyone have the 20 or the 25? i dont want to be severely underpowered because as i mentioned the boat is pretty heavy
the 20 is 333 cc, 100lbs
the 25 is 500 cc, 132 lbs
also, the 25 has shift on the throttle, the 20 does not
mercury 25 4 stroke is quite a bit bigger and heavier than the 2 stroke, while the 20 4 stroke is about the same size and weight. it appears that the 20 would fit better on the transom. without a load in the front the boat wants to porpoise a bit, so i'm concerned about the extra weight of the 4 stroke.
anyone have the 20 or the 25? i dont want to be severely underpowered because as i mentioned the boat is pretty heavy
the 20 is 333 cc, 100lbs
the 25 is 500 cc, 132 lbs
also, the 25 has shift on the throttle, the 20 does not
This post was edited on 8/21/25 at 9:29 am
7
Posted on 8/21/25 at 9:41 am to cgrand
quote:
the 25 has shift on the throttle, the 20 does not
I'd get the 25 just for this if nothing else, and stack whatever I need to stack under the front deck to make it run right. Moving the gas tank and battery to the bow should offset the extra 30-40 pounds.
You can always add buckets of water to the stern to approximate the weight change and go test run it first. The CG will be off a little bit but it'll be good enough to give you an idea of how the boat will run. 14' is a short boat and it's always tough to get the porpoising out of them. The whale tail cavitation plates tend to help a good bit with them but rearranging the static weight in the boat helps the most.
Posted on 8/21/25 at 9:51 am to cgrand
I have a little fiberglass double hull 16' Triton Skiff. 40hp 2 stroke Merc. did about 34mph. I put a 60hp Tohatsu 4 stroke on it and didn't pick up any speed. The 4 stroke was about 40lbs heavier.
Just my experience with my boat.
Just my experience with my boat.
Posted on 8/21/25 at 10:14 am to cgrand
quote:
the 20 is 333 cc, 100lbs
the 25 is 500 cc, 132 lbs
Also, it looks like the 25 has a tank that's 3.4 more gallons so depending on your setup you may need to also address the additional ~20lbs when full.
Of course, if you already use something larger or plan to use your own tank, that may be a moot point.
Posted on 8/21/25 at 10:21 am to DownshiftAndFloorIt
it does look like mercury has put the new tiller handle on the 20, so thats good. its center mounted, adjustable up and down and side to side and does have the shifter on it. that wasnt the case last time i looked and i do want that.
heres the boat...gas tank and battery already in bow compartment. i'm going to run over to basspro and have a look, it just will be a bit of a leap of faith not being able to see them each on my boat
heres the boat...gas tank and battery already in bow compartment. i'm going to run over to basspro and have a look, it just will be a bit of a leap of faith not being able to see them each on my boat
Posted on 8/21/25 at 12:17 pm to cgrand
Sweet tub. Ours is similar size but aluminum and is also very heavy.
You could do something with the big tab on the back if you need to. Would be easy to glass in a small hook at the back if the porpoising remains a problem. One time I made a cavitation plate extension from an old aluminum trailer fender and bent the trailing edge down a little bit and it worked extremely well. Now we have a store-bought whale tail thing and it looks stupid as hell but it does work. That and a little bit of hook at the transom by bending down the little tab extensions from the bottom plate make it run great.
Since you've rearranged everything to the bow already, i'd do the buckets of water thing to see how much more weight it can handle at the transom and still run well. Super easy to do and way better than guessing.
You could do something with the big tab on the back if you need to. Would be easy to glass in a small hook at the back if the porpoising remains a problem. One time I made a cavitation plate extension from an old aluminum trailer fender and bent the trailing edge down a little bit and it worked extremely well. Now we have a store-bought whale tail thing and it looks stupid as hell but it does work. That and a little bit of hook at the transom by bending down the little tab extensions from the bottom plate make it run great.
Since you've rearranged everything to the bow already, i'd do the buckets of water thing to see how much more weight it can handle at the transom and still run well. Super easy to do and way better than guessing.
Posted on 8/21/25 at 12:54 pm to DownshiftAndFloorIt
quote:thats a good idea, 5 gal = 40lbs. although i did just google the weight of my 2 stroke and it says its 100lbs, i thought it was lighter than that so maybe it doesnt matter about weight and it will be just size on the transom. i'd prefer the 25
i'd do the buckets of water thing
Posted on 8/21/25 at 1:06 pm to cgrand
i'd go with the 25 me
Posted on 8/21/25 at 1:24 pm to cgrand
Something to consider, that old 25 might be flywheel rated and the new one might be prop rated, so you might actually end up in the same place with a new 20 as you are now or maybe even pick up a few more hp depending how tired your old engine is. A brand new 25 might feel like a turbine on there.
In general though, I've never heard anybody say "damn this engine has too much power I wish I had gotten a smaller one." If the ~ 30 pounds extra is a problem you can do some things with weight distribution or the hull to sort it out.
Main thing is, you want that twist shift on the throttle. Being able to drive and maneuver fully one handed is ultra beneficial in a small boat like that.
The weight difference going from a 2 stroke to a 4 stroke of similar hp usually doesn't make any real world difference.
In general though, I've never heard anybody say "damn this engine has too much power I wish I had gotten a smaller one." If the ~ 30 pounds extra is a problem you can do some things with weight distribution or the hull to sort it out.
Main thing is, you want that twist shift on the throttle. Being able to drive and maneuver fully one handed is ultra beneficial in a small boat like that.
The weight difference going from a 2 stroke to a 4 stroke of similar hp usually doesn't make any real world difference.
Posted on 8/21/25 at 1:39 pm to duckdude
quote:is the new motor under propped? Doesn’t make sense.
I have a little fiberglass double hull 16' Triton Skiff. 40hp 2 stroke Merc. did about 34mph. I put a 60hp Tohatsu 4 stroke on it and didn't pick up any speed. The 4 stroke was about 40lbs heavier. Just my experience with my boat.
Posted on 8/21/25 at 4:43 pm to cgrand
4 stroke HP is different (lower) than 2 stroke, or has been traditionally...may not be so with modern technology but it used to be about 80% or so so a 25 4 stroke was about the same as a 20 horse 2 stroke due to weight and other factors.
Posted on 8/21/25 at 5:02 pm to White Bear
I had a diagnostic ran on it towards the end of break-in and the RPM's looked good. It doesn't seem to wind up overly high at full throttle and it gets on a plane just fine so I believe the prop is pretty close.
I did think it would be a lot more power going up 20hp but that's not the case.
I did think it would be a lot more power going up 20hp but that's not the case.
Posted on 8/21/25 at 5:06 pm to cgrand
You're going to be disappointed with the performance of the 20hp 4 stroke when compared to that old 25hp 2stroke in my opinion. You're probably spinning a 10-3/8 inch prop while the new 20 has a 9-1/4. The new merc 25 has a larger prop (10-1/2) and an extra cylinder (3 on the 25 vs 2 on the 20). The new 25 vs the 20 aren't even comparable, the similar hp ratings just make it seem like they are. Outboard hp ratings are notorious for being misleading.
Posted on 8/21/25 at 5:46 pm to cgrand
Keep the 2 stroke
Posted on 8/21/25 at 5:53 pm to Tangineck
i had not considered the prop thats a good point. i use a 4-blade prop now on the 2stroke to help keep the bow down (powertech 10x13). i did look up the SN on the 2stroke, its 3 cyl, 500cc and weights 114lbs i guess i should have done that first 
so the 25 4stroke seems to be the ticket.
thanks all
so the 25 4stroke seems to be the ticket.
thanks all
Posted on 8/21/25 at 6:47 pm to cgrand
Would keep that merc, freshen it up and bolt some 40hp carbs on it.
I bet it has more years left than you have.
I bet it has more years left than you have.
Posted on 8/21/25 at 6:54 pm to KemoSabe65
I’m going to keep it as a spare. That motor has survived a lot including decades of duck seasons, a loss at sea, a hurricane, a theft and a lightning strike rescue op (with sheriff supervision)
Posted on 9/25/25 at 12:27 pm to cgrand
so i've decided on a 20hp 4 stroke; the 25 is just too big size-wise for my transom. there are a couple of other things i didnt like on the 25 (no pull start for example). since i'm not planning regular long rides over open water i'm confident the loss in top end speed will be fine.
anyway, i've got the old 25 off and i'm not going to keep it, no point. i would like it to go to a good home, preferably to a young man getting his first boat. it will need a prop, some minor skeg repair (or not), a general service once over and probably a carb clean/replace. if anyone knows of kid who could use it, please let me know here and we can talk. its in my barn in hammond
anyway, i've got the old 25 off and i'm not going to keep it, no point. i would like it to go to a good home, preferably to a young man getting his first boat. it will need a prop, some minor skeg repair (or not), a general service once over and probably a carb clean/replace. if anyone knows of kid who could use it, please let me know here and we can talk. its in my barn in hammond
Posted on 9/25/25 at 12:32 pm to cgrand
(no message)
This post was edited on 9/25/25 at 12:35 pm
Posted on 9/27/25 at 8:22 am to PocketLab
I would just like to say that I’ve met a lot of cool dudes from this board and PocketLab is definitely one of them. Enjoy the motor brother and let me know how it goes with the young man. You raised him right
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