# Sea Seesaw?

Discussion in 'Boat Design' started by Saqa, Dec 20, 2014.

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### SaqaSenior Member

Hi fellas, I need some help with understanding a concept. I have tried searching online but either no one has been discussing this or I haven't been able to correctly phrase a query

Take two 6m narrow hulls of identical outside dimension and identical total dry weight

Hull 1 is 400kg + 80kg outboard on the back + 100kg of gear and fittings in the middle.. total dry weight = 580kg

Hull 2 is 200kg + 80kg outboard on the back + 300kg of gear and fittings in the middle = 580kg​

1 - Would I be correct that with 200kg of crew in the middle the draft or waterline be the same on both vessels?

2 - What I want to learn about is if that 200kg of crew moved to the bow extremity, would one bow of one vessel dip more then the other if its crew did the same?

Would really appreciate your help with some laymans terms types explains, thanks

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### TANSLSenior Member

The two vessels have a different trim and therefore, in general, hydrostatic values for every draft will vary from one ship to the other. Imo that means that, in general, the draft at Lbp / 2 for 580 kg will not be the same.

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### gonzoSenior Member

1) You would only be correct if the center of gravity of the boat and the load were always the same.
2) You can use calculus to figure that. However, more information is needed.

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### SaqaSenior Member

What does this mean? LBP/2?

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### SaqaSenior Member

Actually I cant use calculus, have no idea how to. What information can I add?

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### gonzoSenior Member

Basically, you would need to include the lines plan and weight distribution. For example, a hull with a lot of flare will have less trim change than one with vertical sides. What are you needing to accomplish?

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### TANSLSenior Member

Saqa, Lbp means Length between perpendiculars.
I see that Gonzo has taken the issue so I'll leave you in his hands. Surely he clearly explain you the concepts needed so you can make the calculation that he believes to be done. He will tell you what those calculations are, how to accomplish them and theoretical basis of the whole process.
It is not necessary that there are two technical explaining the same.
Good luck.

8. Joined: Oct 2013
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### SaqaSenior Member

It started with a discussion I had with someone about my light stich and glue 15' build vs his heavy ply on solid timber frame 15'er. His boat is almost twice as heavy as mine. Hulls have similar shape and vee. He reckons that his be easier to move around in, that mine would pitch and heel more when moving around the boat

I am interested in making a pontoon cat about 6m with a pair of slender hulls about 50cm hull beam and 3m separation. Slight cosmetic bow rake (not reversed). My two options are stitch and glue ply or solid 8lb PU foam core. Building strong for bad weather mooring. The solid foam core will be around 3 times heavier then the ply pontoon

If something like a Woods Scoota 20 hulls were foam filled vs regualr empty hulls

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### gonzoSenior Member

Do you have any photos?

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### SaqaSenior Member

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