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Started By
Message
thoughts on a merc 25hp 2 stroke to 20hp 4 stroke swap
Posted on 9/7/19 at 1:04 pm
Posted on 9/7/19 at 1:04 pm
boat is a 14’ flat bottom fiberglass hull, homemade
it’s pretty heavy, I’ve not weighed it but I’m guessing about 700# dry
the 2 stroke pushes it about 25mph lightly loaded at WOT
the 2 stroke weighs 112# with piston displacement of 430cc
the 4 stroke weighs 114# with piston displacement of 351cc
so transom weight isn’t the issue but power looks light I might be. The 25hp 4 stroke is a much bigger block, 157#. Plus it’s much bigger size wise and looks like it would take up a bunch of room at the stern of the boat
I could probably get a grand for the 25 and the 20 is 3300 new. It would be nice to have the quieter engine and not burn up so much gas (boat has a 9 gal tank). It’s a duck boat so I either run wide arse open or putt putt
Anyone have an opinion or experience with the 20?
it’s pretty heavy, I’ve not weighed it but I’m guessing about 700# dry
the 2 stroke pushes it about 25mph lightly loaded at WOT
the 2 stroke weighs 112# with piston displacement of 430cc
the 4 stroke weighs 114# with piston displacement of 351cc
so transom weight isn’t the issue but power looks light I might be. The 25hp 4 stroke is a much bigger block, 157#. Plus it’s much bigger size wise and looks like it would take up a bunch of room at the stern of the boat
I could probably get a grand for the 25 and the 20 is 3300 new. It would be nice to have the quieter engine and not burn up so much gas (boat has a 9 gal tank). It’s a duck boat so I either run wide arse open or putt putt
Anyone have an opinion or experience with the 20?
7
Posted on 9/7/19 at 2:01 pm to cgrand
Keep the oil burner trust me
Posted on 9/7/19 at 9:15 pm to cgrand
I went and purchased a 25 tohatsu 4 stroke and while it is a great engine......it cant hold a candle to a 2 stroke. Leave your motor alone
This post was edited on 9/7/19 at 9:15 pm
Posted on 9/7/19 at 9:40 pm to cgrand
Ran a skiff with a 25 2 stroke Yamaha. Later on I got a 20hp zuke. With the same prop the performance was identical. Only difference was the noise. Top end and hole shot were the same. Jon boats aren't as efficient as my skiff was so you may see a dip in top end speed if you have to run a different prop to keep your RPM's up.
Figure out which prop you're going to be using and the gear ratios of both motors. The gear ratio of the 4 stroke will determine if you can turn the prop from your 25 at the same engine RPM and keep the same speed.
Also consider the new Yamaha 25 4 stroke. I think it's 125 pounds.
Figure out which prop you're going to be using and the gear ratios of both motors. The gear ratio of the 4 stroke will determine if you can turn the prop from your 25 at the same engine RPM and keep the same speed.
Also consider the new Yamaha 25 4 stroke. I think it's 125 pounds.
Posted on 9/8/19 at 9:03 am to HotKoolaid
Thanks I’ll do some math and see what I come up with.
the merc 20 is a tohatsu with merc stickers but it’s got the throttle shifter. Yamaha shifter is on the outboard
I run a 4 blade SS on the 25 I’ll have to go look and see what the size and pitch is
the merc 20 is a tohatsu with merc stickers but it’s got the throttle shifter. Yamaha shifter is on the outboard
I run a 4 blade SS on the 25 I’ll have to go look and see what the size and pitch is
Posted on 9/8/19 at 9:19 am to cgrand
quote:
I run a 4 blade SS on the 25
Why? Is your motor mounted high on the transom or do you routinely carry lots of weight? You will pick up MPH by switching to a 3 blade.
Posted on 9/8/19 at 10:23 am to HotKoolaid
it needs the 4 blade to get a good plane when loaded especially when going upstream on the wax. That current rips down
Posted on 9/8/19 at 10:37 am to cgrand
Keep the smoker my guy, way less things that can fail and all can be repaired by a shade tree mechanic. I've got a 1989 25 merc on my 12ft ouchita skiff and I wouldnt trade it for a brand new 4 stroke. 38 mph with just me and gear = pucker mode
Posted on 9/8/19 at 11:43 am to cgrand
Pushed a hotzu out of a marsh several years ago after it went into limp mode. Give me a 2 stroke in marsh any day over the 4’s. 4’s do not like grass, shell or mud and you can’t work on them.
Posted on 9/8/19 at 1:02 pm to KemoSabe65
that’s a definite concern.
I’m in mud and sand quite a bit
I’m in mud and sand quite a bit
Posted on 9/8/19 at 1:51 pm to cgrand
What exactly made you even start thinking about a swap? Is the 2 stroke giving you issues?
Posted on 9/8/19 at 5:05 pm to deeprig9
it’s a pain in the arse constantly refueling.
burns 3-4 gallons on the way down and 5-6 on the way back. Add in running around and i blow thru 14 gallons or more in a long weekend
plus the motor is 25 years old and I’m constantly replacing parts. I guess I’ll run it till it drops though I did the math and based on gear ratio and weight the 20 will likely be a dog on my boat. As I said it’s heavy and not very efficient but it hauls a load and it’s stable at the blind
burns 3-4 gallons on the way down and 5-6 on the way back. Add in running around and i blow thru 14 gallons or more in a long weekend
plus the motor is 25 years old and I’m constantly replacing parts. I guess I’ll run it till it drops though I did the math and based on gear ratio and weight the 20 will likely be a dog on my boat. As I said it’s heavy and not very efficient but it hauls a load and it’s stable at the blind
This post was edited on 9/8/19 at 5:08 pm
Posted on 9/8/19 at 8:20 pm to cgrand
quote:
so I...run wide arse open
You are going to burn at least 2.5 gallons per hr doing this. That will not change much running either the 2 or 4 stroke at WOT.
If the boat can take it...get a 35 and run at 3/4 throttle.
Posted on 9/10/19 at 11:31 am to cgrand
Why not consider a newer 2 stroke 25hp? Might be a little more efficient than the order Merc. Maybe a 2 cylinder 2 stroke Yamaha 25.
I got a 25 2 stroke 2 cylinder Tohatsu on my 14 ft flat and it rips. 35 all day and I could probably get a little more out of it with some prop tuning. It's not bad on gas at all. Pretty lightweight motor for the power it puts out. It's the same motor as a Mercury SeaPro which are known to be good as well
I got a 25 2 stroke 2 cylinder Tohatsu on my 14 ft flat and it rips. 35 all day and I could probably get a little more out of it with some prop tuning. It's not bad on gas at all. Pretty lightweight motor for the power it puts out. It's the same motor as a Mercury SeaPro which are known to be good as well
Posted on 9/10/19 at 11:35 am to GoAwayImBaitn
I have a 25 etec you should use
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