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YachtVALHALLA 02-02-2006 11:21 AM

What would Archidemes say?
My friend wants to leave his hard dinghy in salt water for several months while away from his yacht and doesn't want the dinghy to sink from rain water. He wants to know where he can have a hole in the boat to keep it from sinking. What would Archidemes say?

Nausikaa 02-02-2006 01:52 PM

Hmmmm! Interesting one, but not unsolveable.

Given that the relative density of fresh water (rain) is 1.0 and the relative density of sea water (on average) is 1.025 then, if the boat is made of material laee dense than sea water, it should float. Freeboard, which should in this case be measured in microns, might be a bit on the low side though.

Oh - forget the hole. That will allow the differing types of water to mix and thus sink the craft!

One other slight problem may well occur. The boat will loose a lot of stability due to the free-surface effect of the rain water. To counteract this, a cellular structure will need to be placed in the boat. Use some light, non absorbant material for this.

The cellular division of the hull will lead to another problem of course - how to find the place to sit in the boat. But that wasn't the problem was it?

Probably the best solution however would be to leave the dingy in shallow water after filling it himself with sea water. Having already sunk from the salt water your friend filled the boat with, it can't thereafter sink from accumulated rain water!

Maybe your friend should cionsider giving up sailing and learn to play the violin instead?




rod hodgson 02-02-2006 04:51 PM

Is putting a cover over the dingy with a box in the dingy and under the tarp to provide a a higher surface than the dingy that the water can run off not a possible solution?


Nausikaa 02-02-2006 05:36 PM

If your friend is serious - then I agree with Rod about the tarp.

If this is not serious - then see my sugestion!

Cheers //



YachtVALHALLA 02-03-2006 06:46 PM

Actually this is a serious question.

The dinghy has some floatation in both the bow and stern provided by air chambers. My thinking is if the dinghy is filled with fresh water to the point where the water level inside is the same as the water level outside then a hole could be drilled at that water level which would let any additional fresh water run out and stability would be restored.

A good point was raised by Nausikaa about the hole letting the different densities mix which would make the water inside heavier, causing the dinghy to sink further.

Perhaps filling the dinhy with sea water to the equal water level point and putting the hole there would work better? Then adding fresh water into the dinghy would cause it to rise slightly and any rocking motion would let it reach equilibrium.

Come on folks ... any help here?


JeanneP 02-03-2006 09:09 PM

Nah, you don't want a hole in the dinghy if it isn't plugged up. If the dinghy has flotation, can you put a one-way valve in? Most "self-bailing" dinghies have them.

"Automatic drain plug" is what it's called in the Defender Industry catalogue.

Water can go out, but can't come in. You drill a hole in the stern of the dinghy and install this drain. This isn't going to keep all the water out while it is tied up - but the higher level of water in the dinghy will flow out the drain until the level in the dinghy is the same as what it is floating on. - It's not very clear an explanation, is it? Sort-of like a one-way siphon.

Nausikaa 02-06-2006 03:54 PM

There is a type of bilge pump I have seen advertised in Scandinavia which uses wave motion to pump out a small boat. I don't have the details but I will try and track it down and post the info here. I think that might do the trick.



Nausikaa 02-06-2006 05:47 PM

Got it!!!

It is a Norwegian device but I managed to trace an importer in the U.S. so check this English language site out It may just solve the problem.

Cheers //



YachtVALHALLA 02-07-2006 12:02 PM


Good show! Many thanks. Have passed this to my friend.


Gallivanters 02-08-2006 04:24 AM


About 20 years ago, I installed a small bilge pump and float switch inside a common plastic battery box, fitted with a small maintenance free car battery and a small (15 watt, I seem to recall) solar panel attached to the lid. I drilled a series of 1/4 inch holes around the base to strain water as it entered the box plus a hole to accomidate the pump discharge hose which was then secured over the side. The entire unit was self contained and weighed less than 20 pounds. It kept my little sailing dinghy afloat and relatively dry while moored in a seawater pond during 6 months of absence from Woods Hole. The trick is to make sure the pump can move water a little faster than the inlet holes permit. Local weather conditions, of course, will dictate sizing of all components and I do remember the drenching rains you can get in Langkawi. Happy Hunting,

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