# Finding LCB and LCG

Discussion in 'Boat Design' started by TollyWally, Aug 28, 2008.

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### TollyWallySenior Member

Mostly as an intellectual exercise I would like to locate the longitudinal centers of buoyancy and gravity for my boat. This is my plan at the moment.

For LCB, with the boat at the dock with half full fuel tanks, run 2 vertical stripes of blue masking tape at the stern and bow. Set up my self leveling laser on the dock and scribe the laser line on the tape for and aft. Get 1 or 2 fat friends to get in the boat and go to the stern. Have them slowly and incremently move forward. The point where they are standing when the laser line is even again below the existing laser scribe marks would be the LCB.

For LCG, next time I have her hauled have the lift operator lower her repeatedly upon a single peice of blocking towards the center. The point where the boat balanced evenly, measured by how much effort was needed to push up or down on the boat and get it to move would be the LCG.

Suggestions, observations, and critisims would be appreciated.

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### rwatsonSenior Member

An interesting plan. It would be good to have a method to test newly launched boats for build quality.

I wonder what use the Centre of Gravity is for a boat ?, except for hauling out of the water I suppose.

You can get the COG of a ruler or piece of wood by placing a finger at each end and moving them together. Where they meet is always the COG. I wonder if there is a boat equivalent?

I will be wanting to test righting moments later this year.

The theoretical forces are Kilos per metre squared which means little to me. Is there an equivalent set of weights on the gunnel or a line from the top of the mast I wonder?

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### DMacPhersonSenior Member

Good thoughts, but there may be some practical problems actually implementing your idea. First, however, some good news - in a state of static equilibrium, LCG and LCB are at the same position. So, find one and you've found the other.

Two thoughts come to mind, however, that might make this impractical. First, assuming that you are interested in LCB in the loaded or design condition, you will need to have all weight - including your friends - on the boat when you make the initial marks. Adding weight changes draft and displacement (of course), which can change the LCB depending on the shape of the waterplane. For example, most transom stern boats will see LCB shift aft with increasing displacement. So, you need to set the boat in its loaded condition, then shift weights - and not add or remove weights. Unfortunately, shifting weight will not show you the position of LCB, but the position where all of the weights are back to their original position. Think of it this way - if your friends started at the transom when you set the initial waterline, then walked forward, your approach would suggest that the LCB is at the transom.

Finally, the precision that you might expect from visually seeing where the boat is back to "horizontal" may be difficult. The trimming will become very flat when the centroid of weight (i.e., the cumulative LCG) is back near LCB, and I would suspect that you might see a significant error just from inaccuracies in picking up the real point of "horizontal". Your idea is very similar to an inclining experiment, but with a different objective. The procedures that are required for an accurate inclining experiment can be quite rigorous (e.g., multiple angle indicators, precisely measured weights and positions). The imprecision in your approach just doesn't make for a reasonable accuracy in the solution.

Perhaps you can come up with a variant of this idea that would resolve these issues? (I don't have any obvious ideas.) Keep at it.

Regards,

Don MacPherson
HydroComp, Inc.

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### TollyWallySenior Member

Thanks to both of you for your input.

Don,
You've brought up a point that embarrassingly I had overlooked, the weight of the crew. So I guess I need to get 2 more people to sit in the wheelhouse for the test.

Your second point if I am understanding you correctly is that the weight of my initial 2 fat people has to be taken into consideration as the weight itself will affect the results.

A third point, my level of precision will be at best fuzzy as the trimming changes will be subtle as I approach my target and my method of measurement may also be too fuzzy.

Well perhaps I will have to settle for seeing if I can get it close. I hope I am understanding your points correctly.

Last night I was fooling around with the laser and scribed some marks with the boat empty of crew. I couldn't find anyone to help me so it was just a dry run. I will reset the laser and rescribe with people at the helm. I might as well record how much the boat settles with and without anyone on board. More data can’t hurt.

Then I will have the fatguys do the slow step by step march from stern to center and mark on the deck with tape to the limits of my home made system. Then I'll do the same thing coming from bow to center and mark the deck with tape. Then if need be I'll rescribe with my 2 fat people located at my initial point and start all over again and see what changes. I suppose I'll do that a few times, split the difference and call it good within the limits of my equipment and abilities. Considering the changing nature of my target under way with moving people and diminishing fuel I can really only hope to get close under the best of conditions.

I use a laser quite a bit in my work and have pretty good methodology. The boat is an older inboard 28’ express cruiser. I am guessing that the LCB will be somewhere around the 100 gallon saddle tanks and the engine. The tanks are just aft of the engine.

I have been to your site and read several of the pdfs in your library. Boy would I like to pick your brain! However I understand that is your stock in trade and am very much more than grateful for your help with my quest.

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### DMacPhersonSenior Member

Pick away, but I can't promise that on any given day you'll find anything worthwhile...

One point that you might have missed in my earlier reply is that I'm not convinced that you'll actually solve for the LCB using your approach. You have to set your weights (including friends) at some initial position, so your initial waterline - and corresponding LCB - reflects this initial location of the weights. By moving your friends to the transom then having them walk forward, you'll reach level trim again when they return to their own initial positions. So, their longitudinal position cannot be used to derived the LCB of the hull. All you'll find is where they were located when you took the initial reading.

Regards,

Don MacPherson
HydroComp, Inc.

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Location: Fox Island

### TollyWallySenior Member

Don,
I did get it in your first post about the flaw in my plan. And I appreciate it that you have again brought it to my attention. It is quite easy for the self taught to get off on a wrong headed tangent.

I am resigned to a lower level of accuracy. All I can think of is to try it from several different approaches and see how close it comes.

A couple of questions. How do the real guys do it? What is thier level of accuracy? How much does it move around with shifting and varying weights and diminished fuel levels in a small boat like mine?

If I can get anywhere close to the margin of error that inconsistant loading and changing fuel levels might cause I would consider it a success. I've been fooling around with some of the spreadsheets available and one of the inputs
is the lcb.

In any event I appreciate your help and guidance. Please don't hesitate to bring it to my attention if I head off in the wrong direction!

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