Repower with 2GM?

Thinking caps everyone. I’ve got the 3GM out of Whisper and have started looking at putting in a 2GM instead. It’s practically a bolt in, 3 inches shorter, less fuel consumption, 13 HP instead of 20, 60 pounds lighter. I’ll take any input, and would be interested in what props you’re swinging if you have comparable HP. I haven’t contacted SLM yet. Any thoughts?

Maybe I missed an older post – but why out with the 3GM30?

To me, replacing with the 2GM20F sounds like bad idea, I had the 2GM20 on my Nor’sea and it was OK, but that’s a much lighter boat. 3GM30F certainly runs much smoother.


IDUNA’s Sabb engine develops 10 hp at 1,800 rpm measured at the propeller. At this engine rpm the 17" propeller is turning at 900 rpm. Speed is 5 knots. The Sabb is a torque engine and can swing a very fat 17" controllable variable pitch prop. We can set the optimum pitch for a given rpm and speed.

Yamaha engines are not torque engines and 13 hp is not enough to push a BCC through the water at reasonable speed or against current and/or wind.

Think of the one or two situations when you absolutely have to rely on an engine for the survival of your craft and the souls on board.

One of those situations is moving off a lee shore against a steep chop with waves in excess of 1 metre (each of which slows your forward motion by 0.7 knots), against say 25 knots or more of wind, and in a situation of embayment where you cannot find a close reach of respectable length.

That’s just one situation that is easy to imagine. That’s when you need more than 2 horsepower for each ton of displacment. A 13 hp 2GM might do the job, but you’ll be running it at max rpm with nothing in reserve. A 3GM with 27 hp should do the job.

You want another? Try being anchored in a river that is in flood and rising. Fast. Zygote was in the Endeavour River (just a few boat lengths from where James Cook repaired his Endeavour), with the river flowing at 7+ knots, rafts of logs being washed down and snagging our anchor chain, other boats dragging all around (and some in destruction), and not much river downstream of us. We dragged one boat length and then re-set, but knew that sooner or later I was going to lose the battle of fending off log rafts. That’s when I gave up relying on the 27 theoretical horses generated by the 3GM30F and happily took a towline from an wonderfully overpowered Coast Guard RIB.

In calm conditions, a sculling oar with the manpower you can develop will move a BCC. In nasty conditions, you need all the horsepower you can find.




Well written with good examples based on sea miles of experience. We regard an engine as safety equipment.

In reference to a sculling oar, agreed one can power a BCC with one, but not for long and at a very slow rate of speed.


i am repowering with a yanmar 3ym30. i am replacing a 2 cylinder volvo engine. the engine will be matched to a 16" max prop. the reasoning behind the switch included the need to have an engine with readily available parts, a quieter engine with considerably more power. i second the need to have that extra power in less than ideal conditions. currents can be a bear.

Now that’s what I’m talkin’ about. Thanks for all the feedback, guys. Thanks for your input, Rod, good info. Our 3GM is a 20 HP, not 30 and it’s out as part of a major refit in prep for a return to the real world(at sea). In 7 years of full time cruising(Med, Carib, US east coast, Panama Canal, South America, we can only remember a couple of times that we MIGHT have needed the HP. We tend to be very conservative sailors and we found(for us anyway) that if we used our heads or were patient we didn’t wind up needing it. We admit a bias in that we seriously considered going engineless and still like the idea of pure sail. We do love our amenities however and there are undoubtedly times when it is just good to have the option. We have good friends who are also very accomplished BCC sailors who had only an outboard. Are there other experienced owners with less HP that can kick in their 2 cents? Thanks also for your input,Bil. Always appreciate being able to benefit from someone else’s experience.

My response is similar to the other responses already posted. By BCC (hull #64) came with a Volvo 17hp engine. I recall one occasion when we were sailing to Victoria, B.C. with very light winds but extremely strong currents. We were literally moving backwards, despite using maximum power. We eventually had to reverse course because we simply could not make any headway.

I have since re-powered with a 34 hp Westerbeke and love the excess power available from that engine (with a MaxProp). Where I sail (in B.C., but generally described as the Pacific Northwest) the currents are strong and it is comforting to have the extra power.


Morning Star has a 16hp Vetus with a Maxprop 13x8 and there have been many times I wished for extra power. Trying to power into 15-20 with 2-3 foot chop slows me to around 2 knots max. There is much weeping and gnashing of teeth. Add a small current to that and its time to submit. I make the same VMG under sail tacking which of course is the point but with more power I’d be in that cozy anchorage before dark asleep with a full belly. Full cruising speed 3000rpm in flat water clean bottom no wind or current maybe 5.2kts. If I had a suitcase full of money Vetus makes a 27hp that would bolt in with just a change in front mounts. Sigh

We also are a 10hp BCC in the Pacific NW. For cruising I have to go with the currents and it means altering plans now and then and occasionally being late for work as the wind and currents were not to our liking. But that in itself has value to remind us why we have a sailboat. It would seem with your experience and modest approach that your proposed changeout would be a good fit. Cheers,


My thanks again for your responses. You all make valid points and it goes to show there is no one way to do things. Ben, I’m interesested in your comments about vibration. Can you talk about RPM ranges or load, no load. Ron, I’ve tried to follow you posts for a while, we lived in Anchorage for 15 years before cruising. Still miss it.


What I meant was the engine rocks back and forth alot more, as a result of their only being 2 pistons in opposition. You often hear of broken motor mounts on the 2GM20’s because of the rock. This is only really an issue at idle, and you can adjust the idle up a little bit to help reduce the rocking. As far as RPM range and load, the engine performs just fine.

Here’s a good video that illustrates the rocking I am talking about. Also watch the end, when he shuts the engine down. I don’t think this is a reason not to get a 2 cyl. engine, but just something to be aware of.

My opinion on the 2 or 3 cyl in a BCC is that if you are going to have an engine, have one that will do the job in all situations. Saving 60lbs and a few gallons over the course of a year isn’t worth the swap, but if price of a new engine plays into it, then you’ve got something to consider.

I seem to remember that the Yanmar 3GM30F has 3 ratings: 30hp for 1 minute, 27hp for an hour, and 24hp continuous. Another thing to consider–a cruising BCC will probably displace at least 8 tons, maybe more. I vote for the 3GM30.
Dan sv Shaula

I completely agree with Dan. “Gertrud” came with a Yanmar 2GM20F, essentially 18 hp. It’s fine except when you need the power to slog to windward in a heavy chop and headwind. Then, I couldn’t help but notice our 2 knots average speed over the ground. I’ll stay with this engine (it’s got only 1000 hours on it) but I’d go with the 3GM if I had the choice.

Derek Lundy

I completely agree with Dan. “Gertrud” came with a Yanmar 2GM20F, essentially 18 hp. It’s fine except when you need the power to slog to windward in a heavy chop and headwind. Then, I couldn’t help but notice our 2 knots average speed over the ground. I’ll stay with this engine (it’s got only 1000 hours on it) but I’d go with the 3GM if I had the choice.

Derek Lundy

Whisper, you say your 3GM is a 20 hp engine. Could it be that you have a 3YM20? If so, the weight difference is only about 36 lbs (the 2GM’s 287 vs the 3YM’s 251). The hp must be about the same? My mechanic thinks the YM engines are a big improvement on the GM series–much easier starting in cool weather due to glow plugs, better access to the raw water impellor, etc.

If you do have a 3YM20, maybe you should consider changing to a 3GM30 or a 3YM30. What part of the world do you plan to sail Whisper in?

Dan Shaula

Hi Dan. I have a 3GMF, rated at 20 HP max, 17 continuous. I wasn’t aware there is a 3YM20, only 3YM30. The 3GM30 really isn’t a game changer for me as the only real difference is the added HP and a slight increase in fuel efficiency at top end. As far as plans, we were in Ecuador, prepping for S. Pacific when we came home to help aging family. When our obligations here are finished, we’d like to pick up where we left off. Cheers, Ray

Your situation is becoming clearer! Your 3GM20 must be fairly elderly? How many hours does it have on it? We bought our 3GM30F in 1987 and there’s no mention of the 3GM20 in the workshop manual–just the 1GM10, 2GM20, 3GM30 (and the 3GH35?). I guess the 3GM30 is a developement of the 3GM20.

If you’re comparing (and considering?) just the 3GM20 and the 2GM20, their HP must be about the same–about 17-18? Since the 2GM20 isn’t in production, are you thinking of a used/rebuilt engine? What’s the availability of used (or new?) Yanmar diesels in Ecuador? Are you now considering the 3YM20 (or30)?

Does your 3GM20 need a major overhaul? What does the overhaul situation look like where Whisper is located in Ecuador.
Dan Shaula

Dan, the 3GM30 is simply a 3GM with larger bore(72/75mm). The same is true with the 2GM and the 2GM20(72/75mm). I am considering the 2GM(used or rebuilt). We are currently in Texas caring for aged family and the boat is in the back yard. As for work in Ecuador, Guayaquil has the largest fishing fleet in the world and diesel work is very easy to arrange.

The instruction manual of Yanmar 3GM30F recommends 5 different makes of engine and transmission oil. Shell, Mobil, Esso etc. I have hard time finding them.
I wonder what you use… Do I have to purchase them from marine stores?