ST2 v's SR2

[claudia]

Is one better than the other, do they perform as well as each other is one easier to maintain than the other. Just looking for peoples thoughts.

[bizzard]

Is one better than the other, do they perform as well as each other is one easier to maintain than the other. Just looking for peoples thoughts.

The ST2 is about 20hp compared with the SR2'S 13 or so. The ST is a little noisier and higher revving. But is virtually identical in looks and size,just a touch taller. Maintenance is similar to the SR. They will more or less strait swap with an SR2. But again there are the industrial ones which can run in the opposite rotation.

[claudia]

Hi Bizzard, why would you choose one over the other and which one?

[bizzard]

Hi Bizzard, why would you choose one over the other and which one?

Although i've put an ST2 in my boat i really prefer the SR2 as they are smoother,quieter idle more slowly and not so harsh,otherwise the ST2 is also an excellent engine.

[alan_fincher]

How big / heavy is the boat, and where do you plan to use it ?

 

I'm no means an advocate of unnecessarily large engines, (quite the opposite in fact!), and acknowledge that SR2s over the years have been used perfectly successfully in fifty-something or even sixty-something foot narrow boats.

 

However, by modern standards, if you had a large boat, and maybe planned to do a lot of river work, then an SR2's 13 horsepower might be considered inadequate by some.

 

As Bizzard says, yopu would find the ST2 a significantly more powerful engine, though might lose a bit of "refinement", (an odd word to use with an air-cooled Lister!), over the SR2.

 

In round numbers, power-wise a two-cylinder ST2 is equivalent to the three-cylinder SR3 - often the choice of hire boat firms in the past once boat length went much over 50 feet.

[bizzard]

Ian you are propped for an SR2! I changed mine from an SR2 to an ST2 because i do towing now and then.

[claudia]

You are right Bizzard all is right for an SR2 on my boat as its only a 36 footer and propped for the SR2 its just that I can't find a good one and I do want to go on the Trent.

[bizzard]

You are right Bizzard all is right for an SR2 on my boat as its only a 36 footer and propped for the SR2 its just that I can't find a good one and I do want to go on the Trent.

The SR2's prop will be ok for an ST2 although under pitched the ST will play with it a bit,but you would have no fear of overloading and overheating the engine.But because the ST develops its maximum cruising torque at a few hundred more RPM than the SR there will be more than enough power and will whizzzzzzz along. Your boat will also be able to go dead slow too if needed.

[claudia]

Would I be right in assuming that the air cage manifold thing with the gear box on would bolt onto a ST2 the same? From what I can make out with my gearbox it would not matter which way the engine spun it just determines which way I push the lever to go forward. Would what ever connects the gearbox shaft to the engine be the same for both engines? I really don't know alot about this do I, never mind I have learned to type quicker.

[bizzard]

Would I be right in assuming that the air cage manifold thing with the gear box on would bolt onto a ST2 the same? From what I can make out with my gearbox it would not matter which way the engine spun it just determines which way I push the lever to go forward. Would what ever connects the gearbox shaft to the engine be the same for both engines? I really don't know alot about this do I, never mind I have learned to type quicker.

Without checking in manuals ect i'm pretty certain it'll all fit ok. The engines front mounting maybe a different width but that should also swap over.

Because your boat is 36'long, the SR2's prop would be a better bet anyway,because the ST's idle speed is higher than the SR's,so you will be able to go dead slow too if needed which is important.

[claudia]

This is starting to sound good, can I strip the engine down enough to man handle it out of the boat or am I being silly?

[bizzard]

This is starting to sound good, can I strip the engine down enough to man handle it out of the boat or am I being silly?

Yes but they're mighty heavy.It would need about three or four of you.

These engines usually come with the flywheel fan housing ect already on.

Edited May 23, 2012 by bizzard

[claudia]

Some would say I am 3 or 4 of me but I don't think they would mean in strength. How hard or complcated is it to change one of these from fixed speed to variable?

[Timleech]

The SR2's prop will be ok for an ST2 although under pitched the ST will play with it a bit,but you would have no fear of overloading and overheating the engine.But because the ST develops its maximum cruising torque at a few hundred more RPM than the SR there will be more than enough power and will whizzzzzzz along. Your boat will also be able to go dead slow too if needed.

 

Are you certain about that? I would expect an ST2 to need a smaller prop than an SR2, working from some common;y quoted ratings.

The SR2 is often listed as 13 bhp @ 2000 rpm. If you spin a prop faster, the power needed goes up with approximately the cube of the rpm, so for a prop matched to that rating to run at the (again often quoted) 2600 rpm of an ST2 would need (2600)cubed/(2000)cubed times the horsepower, which is roughly 2.2 x 13 = 28.6 bhp, whereas the ST2 rating at 2600 is only 20 bhp. Yes the ST should work with that prop, and probably give more than the SR, but not up to its full speed and horsepower.

Of course these engines were shipped with a variety of ratings, and you can probably play with the figures to get a match - it depends what rating the existing prop matches, and what rating you're aiming for with the ST.

 

Tim

[bizzard]

Are you certain about that? I would expect an ST2 to need a smaller prop than an SR2, working from some common;y quoted ratings.

The SR2 is often listed as 13 bhp @ 2000 rpm. If you spin a prop faster, the power needed goes up with approximately the cube of the rpm, so for a prop matched to that rating to run at the (again often quoted) 2600 rpm of an ST2 would need (2600)cubed/(2000)cubed times the horsepower, which is roughly 2.2 x 13 = 28.6 bhp, whereas the ST2 rating at 2600 is only 20 bhp. Yes the ST should work with that prop, and probably give more than the SR, but not up to its full speed and horsepower.

Of course these engines were shipped with a variety of ratings, and you can probably play with the figures to get a match - it depends what rating the existing prop matches, and what rating you're aiming for with the ST.

 

Tim

Ahoy Tim. Yes the 'hp rev range' can vary around the 5hp mark a according to build.Although the ST develops its maximum HP at higher rpm they do have larger cylinder capacity 77.3 cu ins compared with she SR's 67.3 cu ins so plenty of power to play with.

Indeed my own boat is a 35'trad but heavy for its size with an ST2 that i installed in place of an aging SR2 some years ago and it certainly plays with the SR's propeller,it will rev out and cause the boat to speed at 7mph or more.I've docked it twice since and pondered whether or not to fit a heftier prop but changed my mind as i do like to be able to go dead slow and the ST's higher idle speed might have prevented this,but the extra prop bite would be handy when towing.So i'm always in a quandary over the propeller when i dock,which after another 4 years is booked for this coming August,so the proponderance will again be presant. And of course its important not to over prop an air cooled engine as they're cooled by rpm and not load unlike a thermostatically controlled water cooled engines,which have more control over their ''rev-load'' cooling.

[claudia]

If it spins the prop faster on tick over how does this make it go dead slow?

Hi Tim I have read over your post several times and I still can't get what you are saying is it that the prop will slip or are you saying the rpm will cause the prop to give more drag leaving it short of hp?

[Timleech]

If it spins the prop faster on tick over how does this make it go dead slow?

Hi Tim I have read over your post several times and I still can't get what you are saying is it that the prop will slip or are you saying the rpm will cause the prop to give more drag leaving it short of hp?

 

No, just that the prop will probably put a slightly bigger load on the engine than would one to which it is properly matched. It'll just mean that the engine might not reach its maximum governed rpm (and therefore maximum horsepower) and might smoke a bit on full throttle if everything isn't exactly spot on in the fuel injection department.

Actually quite similar to what I understand to be your position now, except that the whole thing should be much more useable and it'll only show up near to maximum revs. Propeller matching is not a terribly exact science anyway, at this sort of level.

 

If the idle speed is higher than that on the old engine, you'll go faster through the water at tickover than you would with your existing engine.

 

Tim

Edited May 24, 2012 by Timleech