# waterlines and trim

Discussion in 'Boat Design' started by breschau, Jan 16, 2011.

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### breschauJunior Member

I have a question if I may.
If the final COG-x (after adding the loads) is not the same as LCB the boat will trim right ? If one starts drawing a hull based on waterlines, how do you make sure the boat follows those waterlines when it is actually in the water ?

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### sorenfdkYacht Designer

Right

By ensuring that the CoG is directly above or below the LCB. It is almost always possible to move the CoG by moving weights. If not, you'll have to redo the lines.

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### gonzoSenior Member

That is right.

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### breschauJunior Member

I am beginning to understand the misgivings of designers over computer drawings. Anyway, what about calculating the trim angle resulting from an offset LCB. Whatever maths remaining in my brain says, it will involve the volume integral calculations of a shape cut by a plane of unknown angle on xyz coordinate axis.
Is there a practical way not involving 3d analytical geometry.

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### marshmatSenior Member

The shortcut way is to find the moment to trim one (cm or inch), which depends only on the waterplane shape and can be estimated by any of several rules of thumb, or by a fairly simple Riemann sum. Compare this to the trimming moment (boat weight times the longitudinal distance between LCB and CG) to get a rough, but close enough, estimate of how big a change in trim there will be.
Obviously, if you trim too far from the design waterline, the waterplane shape (and therefore the moment to trim) changes, but for most practical purposes it's a close enough approximation.
If you shell out for pro-level hydrostatics software, it might offer an option in which the nasty volume integrals and unknown planes are used for the calculation... but check the manual for the details of the algorithm used before blindly trusting any such result.

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### sorenfdkYacht Designer

Oh, no - most of us couldn't live without CAD!

Download Freeship (you can't beat the price - it's free!). I believe that you can input the x-coordinate of the CoG and let Freeship alter the lines so that everything fits (I'm not a Freeship user, so I might be wrong).

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### breschauJunior Member

Thanks to everyone for their input
Sorenfdk and Gomzo,
Keeping LCB and COG on the same x coordinate in the first place is a clever approach, which eliminates a lot of fancy maths.
Marshmat,
If I go over doing it the other way around, the approach you mentioned can be combined with some computer aid to calculate modified surface area and volume at each 1cm incline.

Best Regards,
Breschau

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### breschauJunior Member

Sorenfdk,
Freeship gives the hydrostatic data for specified trim but I dont know if it is possible to add artificial loads. Also it does not show trim according to hull shape, the draft line is perfectly horizontal with an offset LCB. Need to find out those externally and draw accordingly. Otherwise I will be in for a surprise later on.

Someone with more knowledge on the software correct me if there are some features I am missing.

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There are 2 pieces missing from your discussion above.

#1 The GA. Your LCG will come from your GA. Since placing an engine say at amidships, will yield a totally different LCG to that where the engine is place well aft. Sounds obvious. But without a GA (General Arrangement), you wont know if it is even indeed possible to move the engine fwd or aft. The whole layout of your boat and hence the LCG can only come for your GA

#2 Powering. The tank test report shows the variation of EHP with changes to LCG. This is one of the tests one must perform when doing tank testing. Because it is linked to the GA. If the tank test report shows that the best location for the LCG is say, 8% aft of amidships, then you need to arrange the GA so that your final LCG is 8% aft.

How you do that is the basic design spiral and has many other inputs too, which affect your GA. Good luck

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### breschauJunior Member

Do not worry I am not looking at anything with more load variables than a backpack and myself.

Sincerely,
Breschau

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It matters not whether it is just you and a backpack or 1000 passengers and fuel and stores or whatever. The methodology is the same, it is basic naval architecture.

You start with an SOR...which leads to your GA..whether a canoe or super tankers, doesn't matter, the procedure is the same. Failing to recognise this will lead you down the wrong path and many endless annoying iterations, which can be prevented by following the correct procedure for the design of a boat, whatever the type of boat.

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### gonzoSenior Member

If you graph the volume and the weights, the areas should be equal in average. However, the trim will vary a bit between fresh and salt water.

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### breschauJunior Member

You are right , of course. I am not disregarding the process, just stated the variables at hand. I am sorry if it sounded otherwise.

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### LandlubberSenior Member

Funny eh, in any trade or venture/design, we have to understand and use the basics before proceeding to the next stage. As a boatbuilder I sometimes read in amazement how some think they can build a boat, ignoring the most basic of facts /principles.

When I went to tech, I was a mature age student (40), I thought that I could sharpen a chisel, it took my teachers a week to make me realise that I did in fact , not know how to sharpen a chisel (but I do now)...a boat, like any other project starts at step one, miss that and you will eventually have to go back and do it again....do it once and do it right.....means a lot as we learn more.

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Exactly.
Ignore the basics and correct methodology at your peril....the desire and ambition of many, over runs this very simple procedure that must be adhered to, otherwise the inevitable will occur. Less haste less waste!

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