Boat outboard - 2t vs 4t power output?

[hotponyshoes]

So I currently have a 19' boat fitted with a 90hp 2-stroke outboard.
It has a 13x19" propeller fitted.

I am looking to change it to a boat that's more suitable for fishing as that's about the only thing I have ever used it for so looking for plain trim, empty decks etc.

I have found one locally, this has a 75hp 4-stroke on the back.
It is also a bit longer at 22'
I just presumed it would be slower (not really a problem for fishing) but then noticed the prop on that one is 19x19"

I was going to download the manuals for both engines and look at the specs but we still dont have any electric back on yet!

Just going off prop size i am presuming the 4-stroke has a lot more torque than the 2t but I know very little about outboards and nothing about 2-stroke engines!
Both rev counters red line roughly the same.
I have been very impressed with the fuel consumption of my 2t so far but they are both fairly modern electronic injection units so I dont expect the 4t to be much worse?

Has anyone else run both?
I always have the option of swapping the engines if one is massively "better" than the other but I would rather spend that weekend fishing!

 

[Migmac]

Mate changed from 2 stroke to 4 stroke, big advantages are as i see it. Uses way less fuel, a lot quieter. We could use nearly 20l of fuel a trip, only out for a few hours. The same trip with the 4 stroke the gauge ( a stick) is nearly sitting at 3/4 full

 

[Barking Mat]

4 stroke more economical. That's all I know.

 

[Rannsachair]

I have not run a modern fuel injected 2 stroke, though have run old carbed 2 strokes up to 20 years ago when I went 4 stroke. The 4 strokes are heavier, way more fuel efficient, much quieter and nicer to trawl fish with. The 2 strokes offer more exciting acceleration, I assumed more torque or perhaps just a steeper power curve.

My experience of sizing props for boats is that yes you can sometimes live with a prop that will not hit max revs for better fuel economy, though it it may not be as fast, I ran a 20 foot rib, I could do 42 knots at full revs with a 115 hp 4t, though boat ran fine and was more economical with a prop that only did 38 knots and would not hit max revs flat out.

 

[Lazurus]

I put a four blade prop on my little 15hp suzuki for the Broads, gives better early bite and low speed handling, economy not a problem on the broads with a 4mph limit

 

[addjunkie]

I have not run a modern fuel injected 2 stroke, though have run old carbed 2 strokes up to 20 years ago when I went 4 stroke. The 4 strokes are heavier, way more fuel efficient, much quieter and nicer to trawl fish with. The 2 strokes offer more exciting acceleration, I assumed more torque or perhaps just a steeper power curve.

My experience of sizing props for boats is that yes you can sometimes live with a prop that will not hit max revs for better fuel economy, though it it may not be as fast, I ran a 20 foot rib, I could do 42 knots at full revs with a 115 hp 4t, though boat ran fine and was more economical with a prop that only did 38 knots and would not hit max revs flat out.

In this day and age Id be going 4 stroke too, I loved the old carburated 2 strokes, easy to mess with an increase power, but for fishing you dont really need speed, just a decent pace.

You can massivly change a boats performance, by changing props. Assuming the props are the same diameter, reducing the pitch increases acceleration, but reduces top speed. Take the pitch to low and you can kill your extra acceleration by having too much cavitation, this can be exacerbated turning too. If its cavitating you can reduce this by cupping the blades slightly more. This can also effect how long the boat takes to lift up and drop on to the plane. Increasing the pitch gives you better top end therefore fuel economy, but reduces acceleration.

You can also alter how the boat travels on the water by lifting the engine slightly, or putting caps on your trim rams to trim out the engine a touch more, this inturn if the engine is run hard, in roughish water puts additional load onto your bottom mountings, and can add instability at high speed on the plane, flatter bottom boats roll from chine to chine.
In simplistic terms you want the tip of the top blade litterly just below the surface of the water when flat out.

You could then go daft and look into kort nossels and those funky scrolled propellors, forget their name now.

 

[dannyp]

my view with 2 v's 4 strokes 2 strokes are much more fun when they are running. 4 strokes are much more likely to start, happy running at any throttel use less juice and generaly dont **** about and last longer. but 2 strokes are fun