Crank-shaft centre-line

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Mike Rometer
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Crank-shaft centre-line

Post by Mike Rometer » Sun Jan 06, 2019 11:22 am

Is there any convention (or preferred position) about where the crank-shaft centre-line should be in relation to the engine bearer mounts, i.e. above, on, or below, the bearer mount? Currently I'm looking at level for my twin.
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Re: Crank-shaft centre-line

Post by barts » Sun Jan 06, 2019 8:32 pm

Well, first order concern is lining up the prop shaft and other things (pumps) driven by the engine, so that usually takes precedence. Otherwise, there's a side-to-side vibration caused by the crankshaft counterweights (unless you have a counter-rotating balance set up ), so keeping the center-line close to the engine bearers is a good idea. A bit above or below won't be noticeable. On Sea Lion's engine, the crank is well below the bearers, but that's to keep the crank and prop shaft out of the way. Physical design issues rule here according to me.

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Mike Rometer
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Re: Crank-shaft centre-line

Post by Mike Rometer » Mon Jan 07, 2019 9:06 am

Finding the line of the prop-shaft is tricky when there is no boat yet. Obviously existing bearers in a boat can be packed to a suitable height, but only if the engine bearer plates allow. If there is no convention I'll go with in-line as probably giving the best options.
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Re: Crank-shaft centre-line

Post by DetroiTug » Mon Jan 07, 2019 8:17 pm

If given the choice, I would set the crank position as low as possible in the engine assembly. 1: It lessens the angle of the propshaft to the line of thrust, setting the prop blades at more of a right angle to the line of thrust. 2: It allows the connecting rods to be longer without raising the cylinder assembly, longer rods mean less angle at the crosshead reducing the side forces upon it - less resistance means more power. Negatives, it makes the bearings more difficult to service and also any water in the bilge gets thrown around by the crankshaft.

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Re: Crank-shaft centre-line

Post by Mike Rometer » Mon Jan 07, 2019 10:45 pm

Ron, that was pretty much my figuring as well. Good to hear I'm not too far out. A good oil-pan under the engine should keep out most of the bilge water, and I'm using plummer blocks for outer main-bearings, so the grease nipples can be remoted.
Last edited by Mike Rometer on Tue Jan 08, 2019 8:30 am, edited 1 time in total.
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Re: Crank-shaft centre-line

Post by Lopez Mike » Tue Jan 08, 2019 3:54 am

Correct me if I'm wrong but did you say that you didn't have a boat yet? That opens a possibility.

My new hull has what seems to be called a 'box keel' that provides much more room under the engine for bilge water and allows me to mount my engine much lower with all of the advantages previously mentioned.

In my old boat the clearance under my flywheel was less than an inch. The least bit of water spilled or leaked in and instant mess.
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Re: Crank-shaft centre-line

Post by Mike Rometer » Tue Jan 08, 2019 8:29 am

I'm not sure where that's taking you Mike, but if the old boat had a 'box' around the engine base, higher than the normal bilge water level, that should keep the flywheel out of it. There may be occasions where the level gets too high and spills over, but hey, later is better than sooner for getting drowned!
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Re: Crank-shaft centre-line

Post by Lopez Mike » Tue Jan 08, 2019 10:32 pm

That's just it. The old hull had no box at all and very little clearance between the flywheel and the hull. My new hull has a box keel with several inches of room for water accumulation. By the time I might get that much water in the boat I would probably be dealing with a number of urgent issues!
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