Sizing up from 2GM20 to 3GM20

[JohnnieBsailing]

Hi, any help/ advice will be appreciated.
I’m looking to replace my 16 hp Yanmar 2GM20F with a 3GM20.
My boat is 33’ Carter built in ‘74. The 2GM is slightly under power for my 11000lb boat but if I could find the exact match I would jump at it. The Yanmars that I have come across have less hp [usually 13 and I don’t want less power). I may have found a reasonable 3GM20 and I’m trying to find the exact measurements. Anyone have any thoughts about sizing and replacing with a slightly bigger engine. I guess my first concern is, will it fit?

[MikeHoncho]

I’d consider your current prop and if it maximizing your current engine prior to repowering. My 2gm20f pushed my 10000 boat at hull speed at 3200 and the prop is only 13”.

[Wotname]

Originally Posted by JohnnieBsailing

Hi, any help/ advice will be appreciated.
I’m looking to replace my 16 hp Yanmar 2GM20F with a 3GM20.
My boat is 33’ Carter built in ‘74. The 2GM is slightly under power for my 11000lb boat but if I could find the exact match I would jump at it. The Yanmars that I have come across have less hp [usually 13 and I don’t want less power). I may have found a reasonable 3GM20 and I’m trying to find the exact measurements. Anyone have any thoughts about sizing and replacing with a slightly bigger engine. I guess my first concern is, will it fit?

Will it fit should be your second concern but let's address that first. Attached are the principle dimensions of both the 3GM30 and the 2GM20. BTW, I am assuming you realise that "3GM20" is a typo as there isn't any such Yanmar engine.

Now back to the first concern. As MikeHoncho alludes to, is your existing prop correctly sized to absorb all the power from your existing engine?

To elaborate, can you reach max rpm (3600) with a really clean hull and prop in flat calm water? If not, why not?
What rpm is needed to reach hull speed with a with a really clean hull and prop in flat calm water? The difference between this 'hull speed' rpm and max rpm (3600) is the reserve power you currently have available with your 2GM20 if you are propped correctly.

As an example, with my previous 31' boat (powered by a 2GM20), I could reach hull speed at about 2200rpm at which point the engine was delivering about 4 hp. So there was about 12/14 hp still available to deal with adverse sea states and head winds. Note, max continuous rom for the 2GM20 is 3400 rpm (16 hp) and max 2 hour rpm is 3600 (18 hp).

If your reserve power is too low for your liking and you proceed with re-powering, make sure your current prop aperture is suitable for whatever prop is needed with the increased power (usually you will need a larger diameter prop).

[roverhi]

If your engine space is limited in forward room, the extra cylinder may not work. PO replaced the A4 with a 3GM in my Pearson 35 but had to move the engine a foot forward to have enough width for the aft engine mounts. It was a bitch to make the modifications to the mounts and engine bed to slide the engine back 8" so it didn't stick out into the galley.

Check to see if there is enough width aft of the engine to move the mounts aft if you don't have enough room in front to go with the current 2GM rear mount location. Believe the mounts are the same width on the 2GM as the 3GM just spaced further apart. Own both but they are 2,000 miles apart so can't compare.

If you are limited in width here's my solution to get a bit more room for the aft mount. Took the port engine mount bracket and mounted it reversed on the stbd side. Used factory drilled and tapped holes in side of block to mount an 'L' shaped bracket that we made to fit the mount on the port side. Used after market engine mounts which were a lot smaller than the huge Yanmar mounts to pick up another 1" or 2" of room.

[JohnnieBsailing]

Thanks for the input. All very helpful. I should make clear that I am replacing my older 2GM. I just haven’t been able to find a replacement. There are 2GMs with less hp around but i don’t want less power. My prop situation is fine. I came across a 3GM that seems in reasonable condition and if it fits it would be nice to have just a bit more power. The 2GM has served me well and there may be more life left in her, but I rather tinker with her on own time and get my boat up and running sooner.

[hugosalt]

Originally Posted by JohnnieBsailing

Thanks for the input. All very helpful. I should make clear that I am replacing my older 2GM. I just haven’t been able to find a replacement. There are 2GMs with less hp around but i don’t want less power. My prop situation is fine. I came across a 3GM that seems in reasonable condition and if it fits it would be nice to have just a bit more power. .

The 3GM... is 22.5 hp made from 1980-83
The 3GM30... is 27hp 1st made in 1983
Both came in raw and fresh water cooled and both very good/reliable.

[Cheechako]

They are nearly the same engine but with one extra cylinder. So you need more engine room space going forward from where your 2GM sits now.
Good notes on if your prop is right etc. But yeah, the 2GM is a wimpy little engine and what HP it has is from being rated at high rpm.

[dhmhtrhs]

i havethe same engine YANMAR 2GM 20 F and my boat is SUN ODYSSEY 32 MODEL 2002.I run the engine at 3000 rpm and i have a speed SOG about 6 knots.a fried of mine hase a similar boat DROMOR 31FEETS and the YANMAR 3GM 30F. he runs the engine at 2500 rpm and the speed SOG of ΤΗΕ ΒΟΑΤ is 6,5 with a maximum at full power not exceed 7,2 knots.

[NorthCoastJoe]

I put a Yanmar 3GM30F in my Pearson 30, which I also re-propped to match.

The 3GM30F fits with not much room to spare in the forward aft dimension. Leaving about an inch of the shaft between the packing and the coupler in the rear. I had to do some reworking the woodwork forward to get clearance.

Also, I put a 1/4 inch steel plate under the motor mounts to make up for the forward overhang of the motor mounts. The overhang was about 1 1/2 inches.

Otherwise, I love the power and have had no problems. Purrs like a very
happy kitten.

[Searles]

If you can swing a larger prop simply go to a 3/1 gear box ideally keep max shaft to 1200rpm or close to it ,reduces chance of cavitation in a displacement hull ,gets power into the water better ,probably simpler to .⛵️⚓️