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Old 22 November 2007, 15:30   #1
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Elephants Trunk Again

Sorry to bring it up again but it has been awhile and it hasn't answered my query. What does the trunk do is it for the bilge pump to bilge through or is it just another form of one way valve. I see many ribs with two is this just to get rid of alot of water quicker or do they serve different purposes???
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Old 22 November 2007, 15:45   #2
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Yes you have answered your own question.

Elephant trunks are used to clear the deck of large amounts of water very quickly. A good example would be if you swamped the rib by stuffing the bow into a large wave, water would be ejected through the trunks helped by the forward motion of the rib.. There are some previous interesting threads on the subject with diagrams etc if you do a search.
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Old 22 November 2007, 15:45   #3
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The trunk is a way of clearing the boat of a large amount of water fast or just emptying the small amount that gets in. They only work at speed usually planing speeds. They are used aswell as or instead of a bilge pums.

Hope that helps

edit: yoyo has answered your question. Posted at the same time

James
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Old 22 November 2007, 15:52   #4
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hi, this is how I worked it out;
assuming that there was no trunk, just the hole, then think of the boat going forward and through a wave, water gets in the boat, then the hole will help get the water out the back- (or over the transom depending on how big the wave was!!!.)
next assume stopping the rib, what the hole will do now. so it has a trunk on it which acts as a means of stopping the water coming back in-when its up and above the water level

i used a boat that had a single engine, and it had two trunks off centre away from the engine,
then we had it changed to twin engines and had to get the holes/trunks moved to one central one.
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Old 22 November 2007, 15:52   #5
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So where is the bilge pump pipe supposed to go please, not where the sun don't shine i hope.
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Old 22 November 2007, 15:56   #6
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There are simple one way valves so why trail a tube?
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Old 22 November 2007, 16:17   #7
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So where is the bilge pump pipe supposed to go please, not where the sun don't shine i hope.
Either over the top of the transom, or through the transom, via a skin fitting

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Originally Posted by shirehorse View Post
There are simple one way valves so why trail a tube?
Some RIBs have the one way valves, but the majority use trunks as they're simpler, less likely to leak water back into the boat and when moving forward, they produce a draw on the water, ejecting it quicker.

Did you manage to find a decent BWM RIB?
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Old 22 November 2007, 16:23   #8
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Elephant trunks are typically 75-100mm diameter, this gets water out fast if required. The one way flap valves that you mention are usually found on smaller inflatable tenders etc.

I have two trunks and a bilge pump on deck. Often a bilge pump is found in a sump situated on the aft deck which acts as a water collector. any collected water can then be pumped over the transom.
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Old 22 November 2007, 17:09   #9
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Thanks for your replies i think i am getting it now.As to your question Downhilldai i have bought a deepsea 21 but have no idea if it is a good one as you see i am very green and at the moment i am stripping off alot of antifoul to see if i have a good hull or not.This site has gone a long way to helping me with my problems but did confuse me over my choice of rib.
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Old 22 November 2007, 17:10   #10
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There are simple one way valves so why trail a tube?
The one way valves are to be blunt, crap
They get gummed up or stick open very easily and as Dai said they can't shift as much water.


Better to put a pipe over the transom for the bilge pump if a skin fitting would have to be close to the waterline. Nothing stops water coming back in through a bilge pump.
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