# mounting an inboard engine

Discussion in 'Boat Design' started by Bal 66, Feb 28, 2010.

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### Bal 66Junior Member

Hi everyone, I have just registered so am still finding my way around.
I am building a 24ft (LOA) full planing steel hulled criuser and it has an 8ft beam. I wanting to know if there is a formula or a way of working out how engine location will affect the longitudnal bouyancy. The centre of bouyancy is 2ft past the centre frame. Any help would be appreciated.

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### Scott CarterSenior Member

Bal,
In fact the location of any weight in the vessel will not affect its center of buoyancy directly, except that it will cause the boat to trim and thus affect the underbody volume and thereby affect the center of buoyancy. You may be confusing this with its center of gravity. The engine, combined with all equipment and the hull structure itself makes up the whole boat's weight. The centroid (geometric center) of all of these little and big individual weights, along with their positions in space, determine the boats center of gravity. As viewing the vessel in profile (from either side) the method of determining the center of gravity (very briefly) is to define first a horizontal baseline (can be arbitrary, or the waterline) and then a vertical baseline (again, anywhere, maybe the boat's midship station). Use the object's relative distance from these lines (called an arm, units perhaps in feet) multiplied by the mass of the object (say, the weight of the engine in pounds) to yield the objects "moment" about the reference line (now in ft-lbs). Add all of these moments up. The sum of the moments divided by the sum of all of the individual weights (ft-lbs divided by pounds gives feet) yields the distance (above or below the horizontal reference line and forward or aft of the vertical reference line) of the center of gravity. This procedure needs to be done one more time as viewing the vessel from above to determine the transverse center of gravity. This position needs to end up on the centerline of the boat or the boat will list (lean one way or another). Moving various weights around on the vessel will affect this.
The vertical center of gravity is used for stability analysis, while the longitudinal COG, along with the center of buoyancy, is used to determine the boat's trim characteristics, which is what you want to do.

The center of buoyancy is simply the centroid of the underwater volume of the hull, called the underbody. Be sure to make the distinction between the underbody volume and the volume of the entire hull. The buoyancy characteristics of the boat don't really care about what happens above the waterline. It's just the volume of the hull below the waterline that we're concerned with here.
This is a much simpler point to determine as it is simply the center of one big volume, and not the combined effort of many smaller forces. Hull form software will give this to you automatically, but if you want to do it yourself you'll need to read up on Simpson's rule or the trapezoidal rule to manually calculate the geometric center of an irregular 3 dimensional object/space. There are also various rough estimate rules-of-thumb for this purpose.

Then, the way these two points (C.O.G. and C.O.B.) interact is very intuitive. They want to align vertically. This is why the boat lists if the COG is off center. The boat will list until the points align vertically. It's automatic. The boat will also trim bow up or bow down in order to vertically align the two points. It's dependent on the hull form (which determines the distribution of the underbody volume and the distribution of the hull structure's weight) and the location of the various (significant) weights aboard the boat as to how much trim or list will occur to bring these into vertical alignment.
Hope this helps.
Scott

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### Hunter25Senior Member

Most designs are very specific about where the engine should be located and how it is mounted. Have you made enough changes to the original plans, that you now need help locating the engine properly or is this a wing and prayer sort of thing?

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### Bal 66Junior Member

thanks very much for the info Scott. As to your question Hunter, the boat plans don't allow for a transmission (motor is connected direct to prop shaft via a dog clutch). I am planning on fitting a velvet drive trans to motor. Hence wanting to know if the motor needed to be repositioned to compensate for the additional weight of trans.

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### gonzoSenior Member

The transmission is not heavier than an anchor and chain. It shouldn't make much difference, specially since it is so close to the center.

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### Scott CarterSenior Member

Bal, you're welcome. I agree with Gonzo. The weight of the transmission is likely not going to affect the trim appreciably. A full cooler of root beer at the ready in the bow might be about all you'd need to offset it.
Remember, the distance times the weight gives the moment. So 100 pounds located 1 foot aft of the center of buoyancy would be exactly offset by a mere 10 pounds located 10 feet forward of the same point. Like the husky kid on the see-saw having to skootch forward in order to keep the skinny kid on the other end from launching across the playground.

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### Bal 66Junior Member

Thanks for the help guys, that has answered a number of questions.

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