Why CalMac make cars drive through water?

[gloomyandy]

I've just got back from visiting the Small Isles (Muck, Rum, Canna and Eigg) off the West coast of Scotland. I traveled on CalMac ferries to each one. One thing I noticed was that on each island (well at least three of them) the ferry would arrive lower the ramp in such a way as it was under water. Any vehicles (not many as you need a special permit to take one to the islands) would drive off through the water. The ferry would then lift the ramp and edge forward so that after being re-lowered the ramp was now clear of the water for foot passengers (and cyclist!) to walk off the ferry.

Does anyone know why they do this? I intended to ask, but didn't get round to it! I wonder if it is some form of pest/infection control. Several of the islands have some sort of nature reserve on them and have put a lot of effort in to eradicating rats (I think this was to protect ground nesting birds). That's my guess, but I'd love to get a more definitive answer!

[Elizabethsdad]

Posibly with the cars on the ferry sits to low in the water to approach further, Once the weight of the cars has been removed, the ferry rides higher in the water and can then get in a bit closer

[gloomyandy]

I don't think that can be the case, there are typically only one or two cars on the ferry, so not very heavy. Also on one occasion I saw them use the articulated ramp to form a dip to place the ramp in the water. Once the van had driven off, they adjusted the ramp to remove the dip. So I'm pretty sure they are deliberately making cars drive through the water when exiting the ferry. They don't do this for cars/vans going on to the ferry, only coming off.

[Grarea]

Sounds interesting.
Could it be a 'luck' thing.
Or perhaps prevention of spreading disease on the wheels of the cars ?

[Elizabethsdad]

OK this has sufficiently got my curious that I have just e-mailed Cal Mac to ask. I'll post their reply in due course.

[Elizabethsdad]

Good Afternoon,




I have spoken to the vessel and basically the reason is when fully loaded there could be up to 40 tonnes extra weight so when the vehicles
are unloaded this allows the vessel to move up further.

Thanks
Julie




Cal Mac Ferries Ltd

[mattsccm]

Well that was simple.
I suppose that those islands use smaller boats.

[PDQ Mobile]

Well I think the ferry company has shares in the motor industry and by immersing vehicles in salt water it makes them rust away more quickly.

My local council is the same.

[gloomyandy]

Thanks for getting the reply from CalMac. Interesting that they still seem to do it even when the ferry is very lightly loaded. Only a single car on one of them! Also I wonder why they don't need to pull back when loading?

[Cyril Haearn]

One can not change the laws of physics

A heavier boat sits lower in the water, and the tides come in and out, then there is the bathtub effect, that is why the ramp is sloping

When vehicles drive off the boat rises and might lose contact with the ramp

Seems simple when one thinks about it

But what happened to Boaty McBoatface? I thought his name was democratically chosen by a great majority

[Cyril Haearn]

What about the channel ferries with hundreds of vehicles driving on and off, do they rise and fall, or is the change in weight insignificant?

[foxyrider]

What about the channel ferries with hundreds of vehicles driving on and off, do they rise and fall, or is the change in weight insignificant?

They cope with the effect by having moving ramps on shore which are adjusted as needed - that's what all that lifting gear at Dover is for.

River ferries have a very shallow draft so they can get further up the ramp - this wouldn't work on the CalMac boats as it wouldn't be stable enough on the open sea.

[gloomyandy]

Interesting, have been doing a little more digging, it would seem that this procedure is a result of the design of this ship and the shore facilities in use:
http://www.shipsofcalmac.co.uk/h_lochnevis.asp
The Lochnevis is stern loading and the process is designed to avoid any chance of the props hitting the shore side ramp. They use the articulated ramp to form a V shape that has the dip in the V underwater and this ensures that it makes contact with the shore ramp well before the props will hit anything. I think they handle any chance of the ramp losing contact with the shore by having the ferry "push" against the shore during the off loading, you can clearly hear that the engines are in use.

As to tides etc. CalMac seem to use two different types of shore side landing setups. On the Small Isles and other small destinations they have a simple concrete ramp that slopes into the water. But on larger destinations they have a much more complex setup that makes use of a sort of movable (at one end) ramp, this is known as a linkspan. The height of the end of the ramp is controllable from the shore and is adjusted for sea state and the type of ship in use. I assume this also handles the change in displacement of the ferry.