15HP vs 20HP for Jon Boat

Twitches

Seaman Apprentice
Joined
Jan 30, 2018
Messages
36
I have a decked out 14ft flat bottom jon boat that I am currently running an ,'83 Evinrude 15hp two stroke. Motor runs well and gets me up to 19 mph per GPS. I would like to go a bit faster if possible and the boat is rated for up to 20hp.

My question is if the added weight of a 20, or even 25 would be worth it.

My current motor weighs about 70lbs and between it, me, and the gas tank, the rear end already sits pretty low.
 

TOHATSU GURU

Admiral
Joined
Jul 22, 2004
Messages
6,164
Slight increase in top in by going to a 20...Maybe two to three mph increase if everything else is equal.
 

robert graham

Admiral
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Apr 16, 2009
Messages
6,908
If you're talking about a newer model 4 stroke then the extra weight will be noticeable.....stick with lighter 2 stroke motors and if it's rated for 20 HP max then best to stay at 20 or less
 

Sea Rider

Supreme Mariner
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Sep 20, 2008
Messages
12,345
Other nice alternative :

If plan keeping the old 2 strokes 15 HP Evi can go for a prop maximization. Will need to know min-max wot rpm range factory stated, install an induction tach, go for a wot spin and check achieved max wot revs.

If the OB revs towards the min wot rpm range can install a 0.5 or 1.0 less pitch from current prop to pull wot revs bit higher and slightly increase max top end speed. If revving towards the middle wot rpm range, nothing can be done as the current max top end speed is already being achieved.

Happy Boating
 

Twitches

Seaman Apprentice
Joined
Jan 30, 2018
Messages
36
Other nice alternative :

If plan keeping the old 2 strokes 15 HP Evi can go for a prop maximization. Will need to know min-max wot rpm range factory stated, install an induction tach, go for a wot spin and check achieved max wot revs.

If the OB revs towards the min wot rpm range can install a 0.5 or 1.0 less pitch from current prop to pull wot revs bit higher and slightly increase max top end speed. If revving towards the middle wot rpm range, nothing can be done as the current max top end speed is already being achieved.

Happy Boating

This is basically where I was headed with the question. I have a tach on and the motor and it settles in at about 5500 rpm WOT. The stated range for it is 5500 - 7000.

The prop on it is not labeled in any way so I am not sure what pitch it is. So the question at the end of the day is if it is worth messing with the prop maximization or just look for a 2 stroke 20hp and maximize that motor? Or, if the 20 is only going to give me 2 more mph just leave it the way it is.
 

jimmbo

Supreme Mariner
Joined
May 24, 2004
Messages
12,944
Going to a 20 from a 15, the MPH diff will be 1 or 2 mph, but it will give you a better capacity for greater loads,
 

Sea Rider

Supreme Mariner
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Sep 20, 2008
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12,345
At 5500 wot rpm achieved as loaded for that test the motor is running towards the lugging side of the factory range which is nada bueno. How loaded, passengers on board were there when tested the motor at full throttle grip ?

Remove current installed prop and take to a prop shop to have it measured as to know the current diam and pitch. From there can go for a prop maximization which is a day night boating difference, take my word for granted LOL!! How's the hole shot right now, is it pleasing and fast enough for you, would like a faster hole shot if possible ?

How's the hole shot right now, is it pleasing and fast enough for you ? With a under powered motor powering a 14 footer boat it's very important to state the load you usually go boating with to take the max out of that horse. Can go from there...

Happy Boating
 

Twitches

Seaman Apprentice
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Jan 30, 2018
Messages
36
HEAVY load for the boat. Yes tested at the full throttle grip. What I really need is a bigger boat lol, but not an option right now. Me - 200, boys - 120 combined, 2 deep cycles - 120, cooler -30, motor and tm -100, decking and lights - 70. So I am right around weight capacity of 650lbs.

This is the only way the boat ever runs as I don't go out without the boys so it's always heavy. Hole shot is a little sluggish but I am not too worried about that. More concerned with top end to get to the back of the lakes in my area. 7ish miles..

I have been shopping around for an older 2 stroke 20 but they seem to be a lot less common than the 15s. At least in my local trade area. In the meantime I guess I might as well try to get the right prop on there. So have it measured and then get a prop with 1 inch less pitch?
 

hardwater fisherman

Lieutenant Commander
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Mar 19, 2010
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1,725
I think just about every prop I have has diameter and pitch stamped on them somewhere....Did you take it off and look inside to see if it is marked?
 

Sea Rider

Supreme Mariner
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Sep 20, 2008
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12,345
If with no info stamped on the prop and once the prop shop provides a proper diam/pitch answer need to know and do :

1-The min-max wot rpm range factory stated, should be a sticker somewhere middle motor, check that data.

2-Install an induction tach, would recommend the Hardline HR-8061-2 model. Check Iboats, Amazon for them.

3-Run a wot test as fixed loaded and report max achieved rpm at max grip.

4-Subtract the max wot rpm achieved from the max rpm range factory stated.

5-With that data will know how many pitches needs to go down to run middle to max wot rpm range, it's a matter of preference. My boat being powered by a under powered motor was propped right to run at max rpm range as fixed loaded.

6-Did you know that a correct prop maximization can save lots of cash by not buying next HP motor. With a 15 will not have a speed demon but will work much better and pleasing than if doing nothing...LOL!!

Don't play with prop pitches without a tach installed, are cheap and excellent to count with. With that load I'll bet that motor is currently running at the nada bueno lugging side of the factory stated rpm range.

Happy Boating
 
Last edited:

Faztbullet

Supreme Mariner
Joined
Mar 2, 2008
Messages
15,589
Max RPM is 6250.so your close at 5500. Luckily you have thru hub exhaust unless commercial model. You can get a SS prop and have it cupped for a little better performance.You biggest problem is its a flat bottom and has a high drag ratio.
 

Twitches

Seaman Apprentice
Joined
Jan 30, 2018
Messages
36
I found a 1972 merc 20hp in near mint condition in my area for $450. I know I can get $500 for my 15 so maybe I will get the 20 and then maximize that prop????
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
If max rpm is 7K, at 5500 wot rpm motor is being run extremely lugged, if adding more load worse. If going for the 20 HP motor, need to perform a wot test as loaded, check max rpm achieved to see if motor revs at least at middle of the wot rpm range factory stated before going for a prop maximization...

Happy Boating
 
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