Help from Archimedes

Hello,

I'm new to this forum, so bare with me if I'm posting in the wrong area.

I've got a "simple" design problem and need some help. I have a rectangular section of a floating dock (6' x 20'). I know its mass, so I can easily calculate how it will set in the water (18" above and 18" below surface). The section is uniform, so the CG will be in the middle of the block. Now I want to suspend a mass from the dock. Knowing the suspended mass and its buoyancy, I can calculate how much lower the dock will set as long as the suspended mass is acting through the CG. But that is not my case. I actually want to suspend the mass from one end. I know that the CG is going to shift toward the suspended mass end. I can use a "sum of moments" approach to calculate the shift in CG (1.55'). But here is where I need help. My intuition says that the dock is going to want to tip; the suspended mass end deeper into the water, and the opposite end out of the water. How do I calculate how much the dock section is going to tip.

Thanks in advance.

Friar

 

Hi Mate, your problem has simple solution.First of all you have to know the pointf of LCF (longitudinal centre of flotation) Because thats the point where the trim is generated.If you mount that crane or whatever the extra mass on the vertical line of LCF you will end up with parallel sinkage.But if you add the extra mass on the fore or aft ends,you will have trims.The way to calculate the trim is explained in a brief way here,you'd better have a look at it.

http://fas.org/man/dod-101/navy/docs/swos/dca/stg4-04.html

Tumer Berk

 

Merhaba Tumer,

The explaination you referenced is OK, except that it indicates that I need to know the "Moment to Trim 1 Inch (MT1")". But it does not go into how to calculate it. Everything else seems straight forward. Any help on the MT1" for a rectangular object?

Friar

 

Hi Friar,
an useful approximation to calculate MT1, that works with exactitude for a rectangular floatation, is to take the square of the waterplane area, multiply that by 0.35 and divide the result by the waterline beam. Measurements in feet, final result in ft-lb/in.
In your case: MT1 = 840 ft-lb/in
Cheers.

 

Hi Guillermo,
I have had a look at your website and I saw the Jet-Pack system.Could you please give me some info about that water-jet system,technical specs and price etc.Cause I am gonna need it soon in the project that I have started to deal with.Cheers

Tumer Berk

Naval Architect (MSc)

Elegan Motoryachts

Tel:00905323510132
00447891834132

email: alpamis34@hotmail.co.uk

 

Hi, Tumer
You may contact Sword Marine, LLC, the manufacturers, through the contact data at their site:
http://www.swordmarine.com/Company/contactus.htm
Person in charge: Mike Moses
Cheers.

 

Have a look in "Ship Hydrostatics and Stability" by Adrian Biran. In his book he discusses Initial (small angle) stability and the effect of hanging masses from cranes etc. It's all in the book, you can solve the problem, and understand it too.

This is the Link to Amazon -- http://www.amazon.com/Ship-Hydrosta...f=sr_1_11/002-7580032-0356805?ie=UTF8&s=books

Tim B.