# finding the lcg of a boat

Discussion in 'Boat Design' started by northerncat, May 5, 2008.

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### northerncatSenior Member

i have just finished building a small 2.7m assymetric cat dinghy for my 40 ft catamran and im just wondering if theres an easy way for me to find its lcg given that i built it without plans from my head and yes a lot of guessgeneering went on as i couldnt find any plans to build from. thanks in advance for any assistance
sean

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### CTMDNaval Architect

If you've built it finding the LCG is easy (especially as its only 2.7m long) simply sit it on a single beam and move it backwards and forwards until it balances (I've seen yards do the with 40' cats too). Unfortunately I think your actually asking about the LCB. This will be much harder to calculate with drawings.

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### northerncatSenior Member

Does the motor need to be on for this to occur? if i make some drawings in autocad can i calculate the lcb then?
sean

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Put it in the water. The LCB and LCG will line up. Take a 10 or 20 pound weight and place it in the dinghy- find the place to put the weight which does not make the dinghy go bow down or stern down, but just sinks straight in. That is the LCB/LCG.

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### CTMDNaval Architect

I'm assuming you are trying to work out the LCG to set up a lifting system, is this correct?

1. Find the ballance point and measure it as a distance from the transom.
2. Weight the boat (a couple of sets of bathroom scales will do for this job).
3. Weight the engine.
4. Do the following calc.

Total LCG = (hull lcg) x (Hull weight)/(Hull weight + Engine weight).

This assumes the outboard LCG is right above the transom, which is probably close enough. If you have any other items to add they can be included in the calc as well.

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### northerncatSenior Member

yes i want to know so that i can place lifting points for hauling up out of the water
thanks for the info sean

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### Brent SwainMember

Plot the area of each underwater section on a cardboard graph. Cut out the graph and balance it on a pencil. That is your LCB, where your LCG should be, if you want it to float level..
Brent

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### northerncatSenior Member

9. Joined: Jan 2007
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Location: australia

### northerncatSenior Member

ok i have a few things that i would like to throw out there regarding my new dinghy it doesnt quite have the bouyancy that i would like so i am thinking of glueing 100mm of polystyrene on the bottom and sides of the hulls to increase bouyancy and then shaping it and glueing some doorsking ply over this and then glassing this back on to the original hulls, does this sound reasonable?
sean

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