# Single Car Ferry Project

Discussion in 'Boat Design' started by greg.dillow, Dec 14, 2010.

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### greg.dillowNew Member

Hello I hope someone can help. I am currently studying mechanical engineering at college and as part of the course I have to complete a project. I have decided to design a temporary means of crossing a river for a vehicle up to 4.5m long, 2m wide and up to 3.5 metric tonnes. The way I have chosen is by boat. Could someone please give me some basic calculations that I need to consider. I understand that I will need to make sure the Centre of Buoyancy is below the Centre of gravity to prevent it capsizing and that the upthrust is equal to the weight of the displaced water. Other than that I have little or no lnowledge on it, please help.

Greg

2. ### Submarine TomPrevious Member

Great project Greg, you have much research to do.

Buoyancy = 62.5 pounds per cubic foot.

Will it be propelled or on a tow line across the river?

-Tom

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### greg.dillowNew Member

Hi Tom
No I'm not sure about that statement, that is what I have been told.

Greg

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### indianbayjoeSenior Member

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### hoytedowWood Butcher

Munson Packman looks like a great boat. You need to scale it to your metric needs.

1 litre of water weighs 1 kilogram, yes?

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### hoytedowWood Butcher

3500 litres volume displaces cargo. Must add enough litres volume to account for weight of boat, crew, power, fuel and gear plus safety margin. Extra freeboard to allow for errant rogue waves and other factors.

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### hoytedowWood Butcher

Centre of buoyancy will vary with wave action listing of hull happens in rough water. Assume only use will be on flat water?

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### hoytedowWood Butcher

What is that little river to the west of Guildford?

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### rambatMember at large

Amphib

Greg, someone is either misinformed or pulling your leg, the lower the CG, the better. If you can get the CG below the COB it will be more stable. Although allot of amphibians have a Cg above the COB try to keep as much weight as low in the arrangement as possiable, consider how a sailboats weighted keel helps to keep it righted.
I am not sure any scenario would make economic sense for a single vehicle barge. One expense is hiring a ferry operator or do you get out the car and drive the barge? how do you affix the barge to a fixed pier to allow for tidal action? it would need all the complexity of a car ferry including the floating ramps. You could design, test, re-build test again, retrieve, re-build and create a (safe?) roll-on/roll-off barge platform with tracks or wheels powered by the car tires on deck rollers like a dyno. Pop up chocks needed to stop the car from driving off until reaching the other bank. The barge would need to climb up onto the bank to prevent any deckhands to tie-up or susceptibility to current. Steering would be a whole nother challenge unless it was cable guided. Que the better answer..... You would do better to buy an amphibious vehicle (shameless design pitch here) buy a 829 Amphibious Rescue Vehicle 4x4! http://www.industrialobject.com/sth.html

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### greg.dillowNew Member

Indianbayjoe
Thanks for that, it will prove very usefull.

Hoytedow
Yes we are assuming that the river will be almost still. Yes 1 litre is one kg. Thanks

Rambat
Thanks for your help but Unfortunately that won't fit the criteria. The whole idea is to proved a temporary means for other vehicles to cross a river when a bridge has been washed away or in a military environment.

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Hmm..if the bridge washes away....probably global warming. How big is your river ? If you have some elbow room its Best to build one of these, keep the fuel tanks full, then get outta town and over to the Continent fast. If the continents on fire and you expect a long cruise , Perhaps one of these surplus turf rigs would be appropriate.

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### hoytedowWood Butcher

During our pioneer days we just built a large wooden raft and rolled the Conestogas or whatever onto it and pulled it back and forth by ropes. Simple technology, sturdy and dependable.

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### indianbayjoeSenior Member

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### indianbayjoeSenior Member

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### DCockeySenior Member

You need to read the chapters on hydrostatics and stablity in a basic naval architecture book such as "Basic Ship Theory" by Rawson & Tupper. In particular you need to understand the concept of the metacenter.

Center of Buoyancy is the centroid of the displaced water volume so it's always below the surface of the water. The buoyancy force acts vertically through the CoB.

For submarines CoG needs to be below CoB or the submarine will be unstable and roll over until CoG is below CoB. But your ferry will not be a submarine.

For surface vessels the vertical height of CoB is relatively unimportant. Many if not most surface vessels have the CoG above the CoB, an exception may be a sailboat with a deep, heavy ballast keel.

What is important for stability is how the CoB moves horizontally when the vessel heels or pitches. As a vessel heels the CoB will move towards the low side. (If it moves towards the high side then the vessel will generally be unstable) This means the buoyancy force also moves to the low side. The important question for stability is the relation between the buoyancy force vector and the CoG. If the buoyancy force vector is outboard of the CoG then the vessel will roll upright and be stable. If it's inboard then the vessel will roll further and be unstable.

The Metacenter is a location above the CoB that the buoyancy vectors pass through for small heel angles, and stability requires the metacenter to be above the CoG.

Now, get a book which covers this and study.

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