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Old 26-02-2008, 11:59   #1
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Rebuild 2GM vs repower with 2YM15 or 2YM20

Hi Folks,
My mechanic brother is about to check out my 25 year-old (raw water cooled) Yanmar 2GM. The engine was smoking a bit at high RPMs / High load, but otherwise worked well. I did have an issue where the raw water flow stopped a couple of times, but when I pulled off the water hose on the exhaust elbow, there was a poof of dry air, then the water flow came back right a way, and worked fine after that. May be an intermittent failing water pump??
Anyway, he will do a compression test, possibly an oil analysis, and check the head (valves, valve seats, etc.) If all check out OK, He will replace gaskets, and I may still spend about $500-1000 for new engine mounts, and a rebuild (or gasket kit) on a 25 year old engine - that has been raw water cooled, in salt water, for 25 years.
I don't have extra cash!! But, I can get a new 2YM15 for $5800 + tax. or a new 3YM20 for $6900.
Any advice would be appreciated! If my engine does not test well (compression, oil analysis, or look at water jacket, etc), do you think it would be wise to replace with an engine of approx same HP (2 YM15), or, as they say most boats are underpowered, and we have strong tides in the area (35 ft tides, on the Bay of Fundy) - Should I consider the 21 HP (3YM20).
I assume that I would have to look at re-propping, or at least re-pitching, if I went with more power. I have an old fixed 3 blade prop, on my current 13HP (2GM)
The boat is a 1978 Seafarer 30 (book disp 8600lbs, crane said it weighed 10,000, with gear onboard)
- A new 3YM20 is only 38lbs heavier than a new 2YM15!
- I do plan on keeping the boat for the next few years, at least.
- My water heater would be much more efficient with a fresh water cooled engine
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Old 26-02-2008, 17:34   #2
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I ran the calculator on the 30' length 10,000 lb and it likes the 20 hp motor better then the 14. Recommended prop 14 x 10
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Old 26-02-2008, 18:12   #3
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Inlet - Thanks for the calculations.
It looks like my engine beds widths are OK, as the 2GM and bothe 2YM15 / 3YM20 have the same width mounts. IF I go with the 20hp, the front engine mounts are in the right place, but I will have to move the rear mounts back 3". That would not be a big job, as I have good access.
Also the 20hp is 3" longer, so I may need to shorten the shaft, while going with the larger prop.

I will have to check on the recommended exhaust hose size for the 20HP. Although, it would be easy to run new hose now anyway.

Quick question - On my old 2GM, I do not have a waterlift muffer - all that I have is a small rubber silencer, in the middle of the exhaust hose. Then I have a high loop in the hose, before it exits downward, through the hull, just above the waterline - not out the back of the transom. Would this setup be OK, or should I look at a new waterlift??
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Old 26-02-2008, 18:28   #4
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is the bottom clean?
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Old 26-02-2008, 18:55   #5
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I don't mean to be stupid, but... If you mean Is the bottom of the hull clean, vs dirty from exhaust smoke - Yes it is clean! There is always a little soot on the lower transom, as I assume that the smoke, (when I over rev the engine), swirls a bit, as it exits under the boat, and lands on the transom.

If that's not what you meant, please ask more specificaly - I will catch on eventually!

I DO appreciate the help!!
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Old 26-02-2008, 19:14   #6
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what amount of marine growth is attached to your wetted surface?
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Old 26-02-2008, 20:47   #7
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Sorry, very minimal slime. Boat was freshly bottom painted last spring, then in fresh water last year. Only spends 6 months in water.
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Old 26-02-2008, 21:45   #8
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Originally Posted by Northeaster View Post
Anyway, he will do a compression test, possibly an oil analysis, and check the head (valves, valve seats, etc.) If all check out OK, He will replace gaskets, and I may still spend about $500-1000 for new engine mounts, and a rebuild (or gasket kit) on a 25 year old engine - that has been raw water cooled, in salt water, for 25 years.
Skip the compression test and oil analysis. order a gasket set, oil pan gasket and a set of standard rings. I'm only saying this because of your free labor. Pull the head and relap the valves. pull the pan and both pistons. Hone the cylinders and reinstall with new rings. You could replace the rod bearings too if you wanted. Send the injectors to a reputable injector shop.
This is an inexpensive in frame rebuild.
I'm recommending this because the boat has served you well so far from your post. Repowering is a large expense and will ad some value to the boat but not enough to break even. Your going to spend 60-75% of the value of the boat with a new engine.
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Old 26-02-2008, 23:10   #9
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I have always suggested that if you replace rings, you replace the bearing shells as well. The increased load on old shells can cause problems not too far down the track. If you have gone to the trouble of pulling the piston, the shells are easy to flick out and install the new ones. I have never had any issue with shaft wear causing any problem, so you don't have to fret about getting the shaft ground. Unless there is evidence that the wear is excessive of course.
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Old 27-02-2008, 05:35   #10
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Thanks to all for the advice!
We will pull off the head, and oil pan, and check for any deal-breaking damage / corrosion, ect. If it looks OK, I will order the gaskets, rings, rod bearings, and rebuild as suggested.
If there are major issues, I will opt for the new 3YM30, as I believe that most space, mounting issues, prop, etc, can be dealt with.
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Old 08-03-2008, 10:22   #11
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NeverM - My current prop, with the Yanmar 2GM, is a fixed 3 blade 15 X 10, which is larger than the 14X10, that you estimeator liked with a bigger engine. Just wondering what the implications of that are?..
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Old 08-03-2008, 11:22   #12
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going to the 14x10 will allow the engine to rev about 300RPM higher. Is the engine able to get to 3600 RPM in gear now?
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Old 15-03-2008, 06:36   #13
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NeverM - Great question. I only bought the boat last year. Didn't have a tach. I bought one, but never hooked it up, as it was the wrong kind. I intend on hooking one up this year! The boat had been like that for probably 10+ years.
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Old 18-03-2008, 09:51   #14
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before you size up to the larger yanmar from the 2GM20, get the prop diameter for the bigger engine then check whether you can get proper propeller tip clearance between end of blade and hull and proper driveshaft angle.. i re-engined a seafarer 30 from the farymann 11 hp one-lunger with a new yanmar 2GM20F . we had a lot of space for the engine in the engine compartment, and an easy prop shaft angle, but we had a devil of a time getting proper tip clearance. had to cut out, grind and then patch part of the hull in the vicinity of the prop to get proper clearance. as i recall, the larger yanmar 3GM would not fit in the SF30, as there was an issue with the prop drive angle exceeding the recommended 7 degrees, which would result in oil starvation.
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Old 18-03-2008, 10:48   #15
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Mitch - Do you know off hand, what diameter propeller you used with the 2GM20F.

Just to be accurate, I am considering repowering, from a 13hp 2GM, (not a 2GM20), to a 3YM20. I am also considering a Beta Marine 16hp, or 21hp (BD722), as they both can be ordered to fit the 2gm beds. I would have to drill and tap new bolt holes in the glassed over metal beds.

re: prop size, I have a 15" diameter 3 blade fixed now, that, based on info from others, is probably too large for the 13hp current engine. It has 1 1/2" of tip - hull clearance now. Since one of the guys ran my numbers in a prop calculator, it seems like I should only have a 14 x 10 prop, even if I go to the higher (20 hp) engine.

So, I would think that I would be OK, as long as I stay with a 14 or 15" prop. Also, since the larger engine is a couple of inches longer, if I leave the shaft length as is, the prop will sit a couple of inches further, towards the stern, and that would give about an inch more clearance - as the down angle moves the prop father from the hull, the more sternward it is.
Adding a drive saver coupling would do the same thing.

If I am grossly missing something, or not taking something into account, please let me know!!!!

We have taken the trannie, starter, alt, etc, off the old engine, and my brother will be checking it more in the next few days. I have decided only to repower, if the old engine needs more than a few hundred in gaskets / parts.
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