Thread: GIS Flotation and drainage?

Hi Ralph,

if you don't like the looks of big round holes, you can look for the square PVC-tubes made for air ducts. Most suppliers for house building have them.
For myself I've done rectangular 40x100 mm ply tubes from leftover 2 mm birch ply, but I haven't decided yet if and where to fit them in. Size resulted partly from my son's wishes, who wanted to be able to use the transom holes as a reentry step.
But for that I've kept the bottom ply in the middle of the transom 40 mm longer and glued a second and third 6mm layer on top, so there is a step. It's very unobtrusive and hardly to see on the first look.
German designer Jüs Segger uses that on almost all of this boats, it looks like a good idea. We will see.

But with the height of the transom holes over the bottom - can anybody of the experienced GIS-sailors tell how much of the transom gets wetted in a normal sail, so indicating a sensible distance from the bottom to the lower border of the draintube?

Greetings - Joern

Hi Joern,

Smart idea to use existing ply bottum as a first step.

I have seen these square PVC tubes too, but decided to buy standard 50 PVC draining tube. These will be sufficient to drain the bulk. Nice feature is that you can buy PVC plugs to close these tubes in case you don't want them open ( perhaps launching from a steep slope, or heavy loaded with 4 people on board )

I am also curious about the minimum height of the tubes above the bottum ply at the transom?
I guess in normal conditions the transom will not be submerged...but I wonder how deep it will be submerged if you sit on the back-seat single-handed ? ( f.e. using the outboard )
I would think that the lowest point of the drain-holes could be about 4-5 cm above the bottum ply to be safe and above waterlevel in 99% of conditions? Thechnically this could be brought down to 2 cm, but I wonder if that's still okay?

MIK ? Anybody else who has a GIS ?

I am about to start drilling the holes ..........so any information that avoids a mistake is very welcome!

Hi Joern,

It does depend how you load up the boat of course! I think the flaps I suggested to Watermaat should be just about completely waterproof for all the immersion depths that are possible.

The idea is making hinged flaps by gluing plywood to neoprene and have a positive lock down by using shock cord tied to the flaps to pull them shut all the time and you release the single piece of shockcord when you want them to open.

Here are clear ones with the shockcord attached. A neoprene backing acts as both a hinge and a seal.



Here are some that hinge sideways for some reason ... I am suggesting a hinge across the top.



MIK

MIK

Hi MIK,

Thanks for the instructing pictures, the idea is clear.

But - if you mind - please run your database through your stability program with a 15 kg outboard fitted and a 120 kg guy sitting on top or near the transom on the rear tank. Where would the waterline be? This would be helpful to our decisions.

Thanks - Joern

Originally Posted by joern

Hi MIK,

Thanks for the instructing pictures, the idea is clear.

But - if you mind - please run your database through your stability program with a 15 kg outboard fitted and a 120 kg guy sitting on top or near the transom on the rear tank. Where would the waterline be? This would be helpful to our decisions.

Thanks - Joern

Indeed, that would be VERY helpfull !

Originally Posted by joern

Hi MIK,

Thanks for the instructing pictures, the idea is clear.

But - if you mind - please run your database through your stability program with a 15 kg outboard fitted and a 120 kg guy sitting on top or near the transom on the rear tank. Where would the waterline be? This would be helpful to our decisions.

Thanks - Joern

Blimey, you'd need a periscope to see past the bow

Richard

Originally Posted by Daddles

Blimey, you'd need a periscope to see past the bow

Richard

"Periscope" especially when it appears that your "drain-tubes" are far below water in that position

Howdy,

I tend to forget the outboarding function!

Going back to the neoprene and plywood covers tensioned with shock cord.

At least when new they are quite good at water sealing even with a bit of pressure particularly if there is a bit of margin around the outside. They probably will need to be replaced down the line.

I can't see any solution if you want absolute watertightness when outboarding mixed with easy opening when needed.

Though lots of raceboats just put clear self adhesive plastic sheet over the top and rip it off when you need to drain. Not quite what is being looked at here - but it would work.

MIK

Originally Posted by Boatmik

Howdy,

I tend to forget the outboarding function!

Going back to the neoprene and plywood covers tensioned with shock cord.

At least when new they are quite good at water sealing even with a bit of pressure particularly if there is a bit of margin around the outside. They probably will need to be replaced down the line.

I can't see any solution if you want absolute watertightness when outboarding mixed with easy opening when needed.

Though lots of raceboats just put clear self adhesive plastic sheet over the top and rip it off when you need to drain. Not quite what is being looked at here - but it would work.

MIK

Looking at possible draining tubes I prefer standard industrial PVC tubes, since there are all kinds of connections and closure options available. For my 50mm tubes I have also found plug-in PVC plugs and there are even screw-on plugs in the market.
I prefer a slightly down-sloping tube that could be closed from the reachable inside ( BH4) with a plug ( that's a really tight, waterproof plug ) Whatever water comes in ( launching, reverse outboarding ) will be stopped in the tube and will run out again in normal conditions. When you are capsized you simply pull out these plugs ( I will try fix them with thin ropes to avoid loosing them ) .
According to me these down-sloping tubes ( 1cm down from horizontal should be sufficient, to keep them dry and empty in most conditions ) could be placed as close as possible to the bottum-ply in the transom?

NB I do agree that bigger drainholes with flaps in lots of race-dingies are very efficient................but when you are cruising a GIS, capsizing will not be a regular event? I am looking for additional comfort ( and perhaps some extra safety ) in case capsizing will happen. I personally think that a boxed central seat plus 2 smaller drain-tubes ( starting about 10cm above bottumply in BH4 ) will give this extra comfort without too much impact on the design and KISS priciple? My goal is to get rid of the bulk of water after capsizing and have a chance to sail to a more quiet place or even ashore to bail out the remaining water.
MIK, would that be possible?
I defenitely do not want a drain-system that leaks and makes my boat wet inside under most calm conditions.

The plugs on the inside end of the tubes is a good idea, well done. That way, you're only using the tubes when you need to and you have no danger of unwanted water creeping in, only unwanted water creeping out.

Richard

Today I have placed both tubes as mentioned. They still have an oversize, but after the epoxy has cured I will cut and sand them flat to BH4 and transom surface.
As you see they are sloping down ( about 12mm ) towards the transom and placed as low as possible ( cut halfway through the massive wood at the bottum of the transom )
Lowest point at BH4 about 9.5 cm above the bottum, at the transom about 2.5 cm above bottum.

I do not think it looks that bad and there is hardly any chance that it drains water in reverse under normal conditions. I can easy put plugs in from the inside to have a secure closure and perhaps I will make MIK's ply-neoprene flaps in case water drains in in more severe conditions.
I hope it works

Which end is oversized?

It still leaves a bit of bailing doesn't it but I suppose it gets the worst of the water out of the boat.

Richard

I left the tubes longer as finally needed, at both ends. Perhaps "oversized" is not the right English expression

Originally Posted by Watermaat

I left the tubes longer as finally needed, at both ends. Perhaps "oversized" is not the right English expression

Nah, it's fine mate. 'Over length' would have been more accurate but us mug aussies wouldn't even have bothered to comment.

There's a lot of talk and worry about gluing PVC pipe. Redback's mast step is a lump of PVC pipe and it's still sound - mind you, I haven't done a lot of sailing and it's pretty well braced into a corner but so far, no sign of problems. My feeling is that it'll be fine for your purposes (and it won't rot).

Richard