# how to find waterline for desired displacement?

Discussion in 'Software' started by pulsar, Jan 2, 2008.

1. Joined: Aug 2006
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Location: Poland

### pulsarMechanical engineer

Hi all,

I’m trying to design a small hull. I have been using Freeship (Delftship). I have problem with finding proper(real) waterline for desired displacement. Waterline is theoretical know in those programs, but I feel that wouldn’t the same in reality.
Is it possible to check real waterline for design displacement with those programs? Or what kind of software could I try to check that?

Best regards
Pulsar

2. Joined: Apr 2005
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Location: Netherlands

### Martijn_vEMarine software developer

You can only calculate the real waterline if the exact weight and location of the center of gravity is known.

The simplest way however to calculate the draft is by assuming a level trim, so the center of gravity can be left out of the equation. In order to do this you'll need to calculate the displacement for a number of drafts. The exact draft can be calculated by drawing a curve trough the calculated points and read the corresponding draft for your displacement from the graph. This is how they did things in the old days....

3. Joined: Aug 2006
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### pulsarMechanical engineer

Sounds complicated ...
I have model in Freeship where wateline plane is parraler to top deck. I know where exactly is COG and mass of boat.
Is it posible that "old method tell" how will change angle between deck and real waterline?

4. Joined: Apr 2005
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Location: Netherlands

### Martijn_vEMarine software developer

Again, you can't do anything if you don't know the exact weight of your boat. In order to calculate the heeling angle you'll need the center of gravity too.

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### redcoopersMember

This is why programs such as GHS exist. All they do is iteratively compare displacement by cg. If the moments and forces don't match, it "moves" the hull and integrates again. If you are motivated, you can even write a recursive excel spreadsheet to do this, but GHS is easier for importing CAD files. I haven't seen newer versions of GHS, but even in older versions, you can import a fastship file.

6. ### Guest625101138Previous Member

Have you used the "Hydrostatics" function. It gives you displacement for a range of drafts that you nominate. It will not work if the model "leaks". If you want, post your model fbm file here and I will check it.

Freeship will also give the coordinates of the CoG if you set the mass for all the components of the boat. You can ensure the correct trim by locating the longitudinal CofG directly below the pitch axis.

Rick W.

7. Joined: Aug 2006
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Location: Poland

### pulsarMechanical engineer

Hi Rick,

I enclose file which I use in Freeship. Lenght of hull 3,6m, beam 0,8, draft about 0,15m (could be lees or more - it's desired value). Total mass with sailor is 130kg. COG for yacht with sailor according to Freesihip coordinates X=1160, Y=0, Z=1070.
Simplification of my question is enclose in dxf drawing.

I checked "Hydrostatic" with range of draft. In my opinion central point of gravity and central point of displacement are not even similar X-coord. So hull in static position will be in tilt (looking on longitunidal direction).

"You can ensure the correct trim by locating the longitudinal CofG directly below the pitch axis." I do not know that program so well to do this at that moment.

Iwill be appreciate if you could take a look at that model.

Best regards

Pulsar

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8. ### Guest625101138Previous Member

Pulsar
I have attached a pdf file of the hydrostatic data directly from your fbm file. The displacement of 130kg will be achieved with a draft of 146mm. Your current draft setting of 150mm gives a displacement of 140kg.

You need to locate your CoG at the LCB for this draft, 1.687m, to get level trim. As I posted earlier you can use Freeship to do this but you must include ALL the components; sailor, mast, rudder, keel, rigging, sails etc. and adjust their position until the X coordinate of the CoG is at the LCB. So far you only have the hull. Also note that the trim will only be level if there is no pressure on the sails. (I have assumed it is a sailing boat as you refer to a sailor not a pilot.)

As your sailor will be the heaviest object you could just make provision for the person to move position. The boat will roll and pitch easily so you need to be able to counterbalance the sail pressure with the movement of the sailor's weight.

Rick W.

#### Attached Files:

• ###### boat2_hydrostatics.pdf
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9. Joined: Aug 2006
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Location: Poland

### pulsarMechanical engineer

Thank you. It's more clear now

Best regards
Pulsar

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