[Puddin'_Tain]

That the two cylinders have roughly the same compression is more important than the compression ratio per se (if you find more than a 10-15% difference, then I would worry). Getting 270 psi means that you still have about 18:1; not bad for an old engine. Try the "wet" test and see if they are still pretty close to each other. If so, you're probably OK.

 

[Puddin'_Tain]

Puddin', "wet test" by that I suppose you meant squirting abit of engine oil into the chamber. I'll do that.
...

That is exactly what a "wet" compression test is. If there the engine has really low compression, and there is a huge difference between the "wet" and "dry" compression, then the rings are suspect. But given the relatively high "dry" compression that you're getting, I wouldn't worry too much about it.

Why did you do a compression test in the first place? Is the engine running badly? If it seems to be running OK, I would just put everything back together and leave it alone. If it ain't broke, don't fix it.

 

[Puddin'_Tain]

500 naut.miles would be 100 hours at 5 kts. A 2GM20 only holds 2 liters of oil in the crankcase. So, you burned about a liter, maybe a bit more, in 100 hours. That's probably more than a new engine will burn, but not a huge amount. I would keep a closer eye on the oil, check it every 24 hours or so of run time. And try running as heavy an oil as the manual recommends. In Singapore I would think you could run at least 40-weight, maybe 50-weight (but check the manual). In any case, avoid multi-grade oil, as it tends to "thin" more easily over time. And make sure you are using diesel engine rated motor oil, NOT oil for gas engines.

Also, I agree with asdf38. If you can get the revs up, but the boat isn't getting close to hull speed, it's not the engine that's the problem. A 32' Hunter shouldn't displace enough to over-burden a 2GM20, IF everything else is OK (if she's seriously over her designed displacement, that's another story). What size/pitch prop do you have? Are the bottom and prop clean? Is there something else on the bottom that is making it "un-fair"? Are the clearances between the prop and the hull, and the prop and the rudder adequate? Were you towing a dink? Was there a serious headwind?

 

[Puddin'_Tain]

Can you get the engine up to 3400-3600 rpm underway?

I have a 2GM20 on a 27'/6700# boat with a 13x12 2-bladed prop. I have no problem achieving hull speed (~6.3kts) at ~3200rpm. If you can get the engine to rev to 3400rpm, but you are still well shy of hull speed (~7.1kts for a Hunter 326), I would think that you need more prop pitch. In fact, those pitches you list in your post look a bit low to me.

 

[Puddin'_Tain]

I can't swing a larger or higher pitch prop as it would not reach 3200rpm. I forgot to mention I've a 2 blade 15"D x12"P Flex-O-Fold as well. Max it could make is shy of 3300rpm.
So far all checks and feels points to rings wear. Compression test seems to confirm that. Anyhow, I'll try the "Warm" and "Wet" test and post results here. Unfortunately, test analysis of crankcase oil isn't common or known here.

Do the wet test with the engine cold, flip the decompression lever (or leave the injector out) on the other cylinder, and use a fairly heavy oil (40 or 50 weight) to keep the cylinder in question from firing. You only need a squirt, just enough to wet the rings. And only crank it two or three cycles. Crank it too long and you'll build up heat and volatilize the oil, increasing the potential for firing.

 

[Puddin'_Tain]

I can't swing a larger or higher pitch prop as it would not reach 3200rpm. I forgot to mention I've a 2 blade 15"D x12"P Flex-O-Fold as well. Max it could make is shy of 3300rpm....

I just re-read this.

If you are getting 3300 rpm then there is nothing wrong with the engine. You may have too much reduction in the transmission. A 2GM20 with that big prop, with that much pitch, should push an 8000# boat at or near hull speed, IF you don't have too much reduction in the transmission (Yanmar put several different trannies on those engines). In other words, the engine may be at or near max rpm, but the prop may just be spinning too slow. Look on the tag on the transmission and see if you can find the reduction ratio. It should be 2.21, 2.62 or 3.22. If it's 3.22, at 3300rpm the prop would be spinning at 1025rpm. For a prop with 12" pitch that would be about 10kts (rpm x pitch x 60 then convert inches to knots). Since most props have about 45% slip, you can't reasonably expect more than about 5.5kts. On the other hand, if your reduction ratio is 2.62 then you should be able to get pretty close to hull speed (i.e., 7 knots or so).