Ship Launching Query !!!!

the yard am workin in is gonna launch a vessel in 2 weeks time....the plan is to flood drydock and pull out the vessel...however all the ballast tanks are not available and wen i did the trim calculation, the trim is comin to around 1.5 m....aft draft of 3.6m and forward draft of 2.1 m..so my question is this...once water starts filling into the drydock, at what level of water will the forward part start rising???...will the forward part start rising first and hence produce a hinge like mechanism by which there will be a huge force acting on aft dock blocks??...plz give some clarity on this matter

 

I am puzzled on how you can expect any meaningful calculations with such sparse details. You have provided no proper dimensions to calculate bouyancy or any other critical info.

In any case, if you think about it for a minute, how would having the bow or stern supported on water, put extra strain on the other end sitting on blocks ? If the boat is sitting happily on blocks at the moment, why would replacing the support with water at one end be any different that it being supported on blocks ? Assuming water stays level (not many waves in a drydock I would hope) it will just be a gentle transition from the hard stand to water support.

How did you get to "do" the trim calculations and yet not understand such a basic principle ?

 

@rwatson : so according to you even if the forward part does lift first, the bouyancy present will ensure that no additional force will act on the aft blocks?...the dock blocks has been placed so that the weight is distributed among all the blocks...the doubt i had in mind was wheather fwd part rising first would cause a load to act only on a few aft blocks...now ther is some clarity....

with regard to the question i asked, a few senior members in this forum told me '"never ever be affraid to ask questions and/or look dumb wen u start out on your first job ""...i was just following that..its better to ask a dump question and clear the doubt forever rather than being shy to ask question thinking that it may sound dump...

and yes..i'm capable enuf to do the trim calcs and more but ppl do hav some silly doubts at times which only needs a brief explantion by more experinced like urself to clear ...

 

Sorry I was a bit terse, but the question did worry me on several levels.

Yes, you are right about asking dumb questions - its much better to check. Heck, the number of times I had a simple answers staring me in the face ......

I suppose that having someone elses expensive boat to launch might keep me awake at night too.

Good luck with it all - but what could possibly go wrong ?

 

The fore and aft parts of the ship are connected. You can't look at them as independent. The pressure on the forward block will decrease until the water gets to 2.1 m over the lowest part of the keel and then it will be zero (floating)

 

Jose8747, I hope you have done well your ballast calculations, lest you find yourself in a situation like this one: http://www.youtube.com/watch?v=IPoAOXf8RIo&feature=related

Back to your question - there will be no additional force on the aft blocks, because the ship weight (G, a constant) will be partly supported by the increasing resultant buoyancy force (B) and partly by a decreasing resultant contact force from the dock blocks (D). See the attached drawing for clarification.
What will change is the position of the resultant contact force D, which will decrease by moving aft at the same time, contrary to the buoyancy which increases while moving fwd. The distance of the two vectors will be such that the total pitch moment around the ship CoG equals zero.
At some point the bow will start to slightly rise, while a certain number of the aft blocks will still be touching the hull, giving some residual contact support. This is possible because of the hull girder elasticity, which has to be taken into account.
Finally, when the buoyancy force becomes equal to ship's weight, and the resultant buoyancy vector gets back in line with the CoG, the boat will float freely and trimmed according to your calculations (hopefully ).

Cheers

 

@daiquiri : thanks a lot for the detailed info mate ...just what i was looking for...nd nice video btw...hopefully i wont be seeing that 'live' in 2 weeks time (( i'll double check my calcs ))...lol..

@rwatson : thanks...hopefully all goes well..

@ gonzo : thank you...