I'm planning to build a boat with a jacking system, similar to SeaJack/Fred Olsen use for building wind farms.

I'm stuck on how to pull the vessel out of the water / put it back in a controlled way.

As the vessels are heavy lift, they use pneumatic rams, but is this the best way to do on a scale model, or is there an easier way?

Each leg will be 7x7cm and housing 15x10cm. Total weight of the structure is expected to be 13kg, so 2.2kg per leg

Any help is appreciated.

So this is a model boat? The solution rather depends on the shape of your model. After all you wouldn't want some non authentic mechanism showing would you?

Do you have a link to the design that you have in mind?

Greensprings: Electric motor to hydraulic pump to hydraulic ram?

Would this be a tad large? Are hydraulic pumps on this scale easy to source?

http://www.boschrexroth.com/en/xc/trends-and-topics/technology/the-great-ascend This shows a graphic of how MPI have built theirs.

The aim is to understand how it works.

Theres quite a shallow draft on the vessel of 3cm of the 10cm depth with the 13kg limit, but the vessel will be 138x40cm. The scale is 1:100.

If using a pump then i will use 1 for all 6 legs.

Ah right. Well it all depends how authentic you want the model to be. One method would be to have a cable attached to the top of the leg, running down to a block (pulley) at the bottom of the leg where it turns to run horizontally, just above the deck. The end of it would be attached to a nut on a long screw. A motor turning the screw would gradually pull the cable. This would tug the top of the leg downwards forcing the bottom of the leg out through the opening in the bottom of the vessel.

I doubt if that's the way the real vessel works. I suspect they use a combination of hydraulic rams and prawls acting like a ratchet.

That could work. Will try and draft something / google, and see what comes out.

What sort of force do you need ? Oil boilers used to use gear pumps for te atomisers which make a handy small pump.

Syringes can be used as rams , dont use mineral oil though it makes the rubber bungs swell.

13kg across 6 legs (assume 2 break they need a 50% risk).

I need to do the full maths, but the vessels run on 300bar leg pressure ( i doubt it will need this value or be able to produce it safely).