# Mass / Densities of components of a Multihull

Discussion in 'Boat Design' started by MSANOK, Jun 28, 2012.

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### MSANOKNew Member

How would one go about estimating the mass of the vessel in the image provided?
I need a rough order of magnitude estimate for mass, densities, centre of gravity, and mass moments of inertia of the design, all of which are intrinsically linked.
It is an Offshore Wind Farm Service Vessel. Usually these vessels are aluminium catamarans. However, as a postgraduate research project I am examining what the advantages and disadvantages of this design.
The total mass is to be 60 tonnes (judged by the mass of existing vessels for this purpose)
The length is 24m
The Beam is 8m
The vessel must carry a cargo of 20 tonnes
There is a frame (not detailed) required to hold the buoyant tubes together
The heave plate shown is not to be included and there are to be 7 horizontally and 4 tubes vertically
The waterline is intended to be close to the diameter of the second highest row of tubes

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### MSANOKNew Member

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### masalaimasalai

- - What the heck is it?
- - Will be very inefficient and difficult to push through the water?
- - Why not just build a barge, far more effective and use-able... - - - - Have a careful think...

Seas get quite rough in those waters what you propose is madness... as the bindings will make the multiple cylinder vessel no better than a ??? (pile of bricks - - words fail me)
- - - and you are a post graduate? - of what - kindergarten? - - - Where is your common sense and basic boat knowledge - lost in the consumption of copious quantities of finest Irish whisky?

They are currently using a catamaran as you say -
- - - What is its maximum speed and current service speed...
- - - What will the vessel need to carry...
- - - How will it deliver that to a wind-turbine tower...
- - - Will it need to visit the towers in any weather...

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I agree with Masali's assesment. I suggest that you calculate the displacement of the tubes versus the displacement and potential stability of a simple barge structure. The difference in material cost alone will convince you that the barge is superior, to say nothing of the debilitating drag of the nest of tubes with its support mechanism..

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### upchurchmrSenior Member

Don't calculate anything on that picture, just go look at a real catamaran. You are not going to save money by repeating the same cylinder 40 times. You will be laughed out of the boat yard.

Why did you want a multihull?

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### MSANOKNew Member

Thank you for your prompt responses.

Yes it will have a high amount of drag, but it is not intended to travel very far. It is intended to travel around the wind farm but not to it as the current vessels do. This will be achieved with mother ships and the like. I hope that that drag will also damp the vessel’s motions.

Thanks for making it clear that it is a little mad, a reality check is always good. But that is not my primary concern, as I did not invent the concept. I am examining it for the inventor and to do that, I must put realistic numbers on the masses of the different components (as I explained in the original post).

To answer the question “what the heck is it?” maybe I need to explain the motivation of such a design. In Europe there are a lot of offshore wind farms, but currently the majority of them are in quite sheltered waters and the current vessels that carry technicians out to the wind turbines to repair them are fast catamarans. But they are limited to transferring personnel from the vessel to the wind turbine at a significant wave height of 1.5m. due to the excessive motions of the vessel above this limit. This design is intended for the next generation of offshore wind turbines that are further offshore. The intention is that they stay at the windfarm and the crew are lodged in purpose built accommodation onsite. But as they are in less sheltered waters then the significant wave height limit that personnel transfers can be carried out needs to be raised. Ideally to about 3m. So when I am fully grown up and am of legal age to consume Irish whiskey a vessel under 24m in length that carries out this task to DNV’s standards of an offshore service provider category 1 will be in operation.

It could be a long time before this reaches the boat yard (possibly never). I need to build a model of the design to test it to explain to the inventor what are the merits and deficits of the design. The result does not affect me I just need to do the work! To do that I need I need a rough order of magnitude estimate for mass, densities, centre of gravity, and mass moments of inertia of the design.

I was wondering does anybody here know how I should go about sizing the vessel?
- What weight of a structure do I need to support a cargo of 20 tonne?
- What thickness do the tubes need to be, and an assumption of the size of frame required to hold the tubes together? etc.

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### upchurchmrSenior Member

This is completely unique, there won't be a design guide.
You need to start with basic concepts, like how much volume would be required to float the payload, then size a tube to not break during bending in a wave with a factor of safety of ???? 3 or 5 or whatever the DNv standards are. Have you seen the standards?

You might just start sizing the flotation with standard pipe and standard I beams to see what it weights, before checking to see if it breaks. Then add ??? double?? for the superstructure. It might show that the size is ridiculous but you would have something to start from.

Hopefully someone who knows ships standards will comment, I'm just an Aero engineer.

Good luck, but start doing something, even if it becomes obvious it is wrong.

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### gonzoSenior Member

That work should be done by a Naval Architect or Engineer. You are way over your head. Also, if you do that kind of tests, there are legal matters that will affect you. If there are any property or life losses due to your claims, you will be responsible.

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