Diesel - Two Cylinder vs Three Cylinder

[Surveyor]

Background: I am about to install engine motor mount beds in my Alberg 30.

Reason for asking: I have read that a 2 cylinder diesel will vibrate more than a 3 cylinder.

Comment: I have procured a 16hp Beta (2cyl) but it is not too late to upgrade to a 20hp (3cyl). The engine has not yet been shipped. The 16 will push my slow boat at hull speed but at a higher rpm in relation to the 20hp with nearly the same fuel consumption. The only experience I have with diesels of this size are diesel light plants and they are all 2 cyl and do vibrate heavily until they get going. Vibration is a concern because I am installing the beds myself and I want to be sure I dont pay for something I hate later or on the other hand paid too much for.

Questions: Is there a notable difference in vibration between Beta (or any other) 2 cyl engines and 3 cyl engines? Should I be concerned?

Thank you in advance,

[saben]

I would go for the 3 cly. They are much smother. I went from 3-4 cyl on my repower and glad I did. Don't think 20 hp would be overpowering your 30 ft.

[Geoduck]

by all means - go for the 3 banger - much smoother! You'll be glad to have the extra HP for not much more weight. Most 3, 5 & 6 cylinder engines run smoother than 2 & 4 cyl engines no matter who's it is.

[Dick Pluta]

HUGE difference. A 2 cylinder engine can hang up if the piston rods stop in the up and down position. If this happens the piston pushes straight across the crankshaft and doesn't move. To eliminate this the crankshaft has to have the throws placed a few degrees off 180 degrees, let's say 185 and 175 degrees apart.. The power pulses come in an irregular sequence that sets up a harmonic that vibrates a lot. A 3 cylinder has the throws placed 120 degrees apart and can't lock up, so you get the power pulses at a nice even pace.

Dick Pluta
AEGEA

[Starbuck]

20 hp is not too much for your boat. You'll be glad you have it the first time you motor into a swell and headwind.

And you'll be glad you have it every time you start it and your coffee cup doesn't dance to the edge of the table.

A little more money up front, and nearly the same fuel consumption, for significantly smoother operation, and confidence in your pocket for snotty conditions = win.

[S/V Alchemy]

Get the 3-cylinder (if it will fit as easily, and then you can throw in a nice big alternator for faster charging without worrying you're "parasitizing" your thrust.

I don't know how the Alberg 30 is for engine "cave" space, but if you have to, consider cutting panels on the sides if needed for the different engine.

[markpierce]

I always liked the sound of one-cylinder engines and thought that was how "real" boats sound.

[Hydra]

On my 30', 3.5t boat, I have a 2-cyl Nanni 2.50 HE engine, made on the same Kubota base as the Beta. IME, it runs fairly smoothly.

However, it is rated at 14HP and I find the boat slightly underpowered, e.g. for maneuvering in windy conditions. Unfortunately, there is no space for a (longer) 3-cyl engine (and no money, either).

Alain

[Dockhead]

Originally Posted by Geoduck

by all means - go for the 3 banger - much smoother! You'll be glad to have the extra HP for not much more weight. Most 3, 5 & 6 cylinder engines run smoother than 2 & 4 cyl engines no matter who's it is.

Well, a 3 cylinder is not smoother than a 4. But a 3 cylinder engine has a nice loping rhythm. It's incomparably better than a twin. Go for the 3.

[manitu]

The reason a 2cyl 4stroke has more vibration than any other engine constructions is easy to explain. if you wanted the duty cycles to be 180 cam degrees apart, the pistons would have to be paralell. wich wouldn't work. So to get the crank balanced, the duty cycles has to be 90 and 270 cam degrees apart, wich makes the engine go bang bang pause, bang bang pause.

And a 3cylinder 4stroke IS smother than a 4cylinder whithout a counter-rotating balance shaft.


.manitu

[Key Wester]

I think the real question is this - concerning your intended use of the boat, what is the correct engine for the application? If you are contemplating long distance cruising, the Beta 20 is recommended. It will provide reserve power to keep your boat moving against wind & tide, while also charging batteries. The Beta 16 will drive her to hull speed [assuming the correct reduction ratio transmission and prop are installed] but will have less reserve power. Joe D

[Compass790]

Go for the 3 cylinder, you'll be glad you did.
Dock neighbour has a 20hp Kubota in a sistership ( 26ft fin keeler) & absolutely loves it. He reckons he can motor at 7knts all day & he only motors AFAIK.
Its a beautiful sounding thing as far as a stinking diesel goes & is smoother than a twin for sure.

[Pete7]

Since the thread dates from 2011, I guess the decision of which engine to fit has already been made.

Pete

[Compass790]

[Cheechako]

Originally Posted by Surveyor

Background: I am about to install engine motor mount beds in my Alberg 30.

Reason for asking: I have read that a 2 cylinder diesel will vibrate more than a 3 cylinder.

Comment: I have procured a 16hp Beta (2cyl) but it is not too late to upgrade to a 20hp (3cyl). The engine has not yet been shipped. The 16 will push my slow boat at hull speed but at a higher rpm in relation to the 20hp with nearly the same fuel consumption. The only experience I have with diesels of this size are diesel light plants and they are all 2 cyl and do vibrate heavily until they get going. Vibration is a concern because I am installing the beds myself and I want to be sure I dont pay for something I hate later or on the other hand paid too much for.

Questions: Is there a notable difference in vibration between Beta (or any other) 2 cyl engines and 3 cyl engines? Should I be concerned?

Thank you in advance,

I would go with the 3 cylinder. Generally they are smoother, the 2 cyl is a bit small for your boat anyway it seems to me.
I had a 2 cyl Kubota (not used as a powerplant though) and it was a bit rough at some rpms. In an Alberg 30 I would want AT LEAST 20 hp at reasonable rpm. 25 would be my goal.

well crap, just saw it's an old thread!