I have a merc 9.9 2stroke on my 16 foot deep vee. I was pleasantly surprised how well it performed eeven with 3 people. How much more ooomph is a 20 4stroke going to have on the same boat? I have been told 4 strokes don't perform like 2 strokes? Better idle, quieter, better on gas but not the power?
It also depends on how you fish if you troll downwind with a 2 stroke you might as well smoke a pack of cigarettes. As far as oomph it might add one or 2 mph it isn't worth it if your motor runs well. Now if your buying new 4 stroke all the way quiet, no fumes and no mixing. When my dad did our 70's Johnson 9.9 carb to a 15 we thought 1/3rd more speed... nope just 1/3rd more gas.
I went from a 9.9 Johnson with a 15hp carb. to a 20hp johnson both 1980's two strokes.What a difference!Went from going 14mph(with prop guard) to 24mph no prop guard.The 15hp did go faster before the prop guard was on, but would not be as fast as the 20hp.
A 20hp merc 4 stroke is a carbureted 4 stroke. The carbed 4 strokes are total junk. Hesitates, have not seen one that didn't and just a poor motor. Find something that is fuel injected or you will not be happy. Or find a 2 stroke 20 merc,much better motor.
I will only peak about the 2 stroke vs 4 stroke motors.
I had 2 200 HP Yamaha's on my boat and changed them out for 2-200 Mercury Verado's.
I needed to watch the added weight the 4 strokes had since the Yammies would have been close to 200 lbs total heavier then the Mercs and would have partially submerged my self bailing scuppers on the deck.
Performance wise they move a 28' Grady through a 2' chop at 50 MPH, they have more alternator to keep battery's charged no matter how much electronics I am running and the fuel economy is what I was after.
With the 2 strokes I was getting less than 1 MPG but the 4 strokes are getting me 1.6 MPG.
At idle you cannot hear the 4 stroke run and there is zero emission smell.
Downside is much of the routine maintenance I did to the old motors I have the dealer take care of and it is not cheap.
Still when I look at my savings in fuel it still out ways the dealers costs.
The shape and design of the boat influences more the speed and way the boat handles than the power-plant on it.
4 Strokes are getting cheaper to purchase every year so if you need to buy a new motor go 4 stroke but if the old one is running well, wait till it wont run anymore before you upgrade. Maybe by then you will buy a complete package of boat, motor and trailer.
I do want an alternator, thats one reason i'd like to upgrade to a newer motor. And although we're talking about a much smaller boat, improved feul economy would be nice, more for the convience than dollar savings. I can get a 20 hp merc or tahotsu for around $3k, anything feul injected will be a grand more, so i'll probably be sticking with a carb.