GRP tube for cockpit drain

contessaman

contessaman

Well-Known Member
Joined
29 Oct 2009
Messages
823
Visit site
I'm seriously thinking about removing my skin fitting / seaock / hose arrangement and glassing in some pre-made straight through GRP tube for my cockpit drains.

My previous boat had this arrangement and I much prefer it with two less seacocks to fail or service. Since they are left open all the time anyway, I dont see what a closable seacock offers over hammering in a wooden bung into the hole in the cockpit floor should I need to close it in an emergency.

Ive found a place online that sells the ready laminated tube and I'm comfortable glassing this into the hull. Question is, what do I do with the inside of the tube to protect it?

As supplied the tube doesn't even have a gelcoat finish. I feel it would need gelcoat at least if not epoxy giving its planned continuous immersion.

What do all the major boatyards that fit such arrangements from new do?

I've also seen around boatyards numerous times marine engineers glassing grp tube in through the transoms of big motorboats for exhausts underwater exits. Again, how are these protected?

Thanks
 
Whilst it sounds like a good idea, success will depend on how much the cockpit floor and hull move in relation to each other. Failed joint at bottom end could be catastrophic and the advantage of hose is that it will allow for any flexing in hull or cockpit.
 
Not sure I would glass in tubes for the reason Graham says. Seacocks are very reliable if of the right material and periodically exercised. Comparisons with exhaust tubes are not valid because they are short solid tubes, but connected to the engines by flexible hose. In fact some cockpit drains use a similar system with a short glass tube at either end and hose in between, with no seacock.
 
GrahamM376 said:
Whilst it sounds like a good idea, success will depend on how much the cockpit floor and hull move in relation to each other. Failed joint at bottom end could be catastrophic and the advantage of hose is that it will allow for any flexing in hull or cockpit.
Click to expand...

You could try a mix of the two. Rigid tube taken up well above the water line and then flexi to the cockpit sole.
 
GrahamM376 said:
Whilst it sounds like a good idea, success will depend on how much the cockpit floor and hull move in relation to each other. Failed joint at bottom end could be catastrophic and the advantage of hose is that it will allow for any flexing in hull or cockpit.
Click to expand...

+1 for this. it's surprising how much movement there can be in the hull of a boat, even old-overbuilt 1970's tanks can and do flex when sailing, particularly in rougher weather. This movement could lead to cracking of a GRP tube bonded in unless the tube was very thick indeed. I'd personally stick with the tried and tested sea-cock and flexi hose solution.

GRP stern tubes are commonly used but are only attached to the hull so far less stress on them when the hull is flexing.
 
While it is true that there may be some movement of one end of the tube in relation to the other it is also true that, on many boats, the rudder stock passes through a rudder trunk that is nothing more than a tube that passes from the hull bottom right up to the cockpit floor or even the aft deck.
I don't see any problem with glassing a tube in place provided that you incorporate substantial webs around the tube.

p.s. If you are familiar with laminating FRP you could even make up your own tubes using plastic drain pipes of a suitable diameter as a former on which to wind the fibre reinfocement and resin. I used to make such tubes as a matter of course to use up any left-over resin mix instead of throwing it away. (Leave the plastic tube.in place). The tubes were usually incorporated later as stern tubes where the prop shaft went to a P-bracket.
 
Hmm interesting reaction...was wondering about protecting the tube but seems whole idea is controversial.

My previous boat, a beneteau had solid tubes from cockpit to hull and also a solid rudder tube which went from hull below waterline and glassed into deck in a similar manner.
I went through lots of weather in that 30+ year old boat with no such cracks.

I have a bowthruster tube glassed in across the bow of my current boat well beneath the waterline and I have seen small sailboats with outboard wells cut into them again without failure.

Looked on a Bavaria 38 once had solid GRP tube cockpit drains too.

Maybe I'm missing something here?
 
Did you expect anything different? There are clearly different views among designers and builders - and i expect ease/difficulty of construction and cost both have a part to play in the decision.

However, that is not your dilemma. The only reason for going to the trouble and expense of changing your arrangement is if it has failed or is prone to failure and you new system is stronger and or more reliable. Difficult to see how the latter is possible as you have a well proven and widely used system. Corrosion is not a problem if the valves are DZR, bronze or composite. Hose can be easily and cheaply replaced and is durable.

So perhaps better to stick with what you have and put effort into other things where there are clear advantages.
 
Tranona said:
Did you expect anything different? There are clearly different views among designers and builders - and i expect ease/difficulty of construction and cost both have a part to play in the decision.

However, that is not your dilemma. The only reason for going to the trouble and expense of changing your arrangement is if it has failed or is prone to failure and you new system is stronger and or more reliable. Difficult to see how the latter is possible as you have a well proven and widely used system. Corrosion is not a problem if the valves are DZR, bronze or composite. Hose can be easily and cheaply replaced and is durable.

So perhaps better to stick with what you have and put effort into other things where there are clear advantages.
Click to expand...

My boat has an alarming number of through hulls. 23 in total. I've removed a pointless sink in my aft cabin. I'm manifolding the two sinks in the galley. Engine, toilet, water maker all can't be helped so I'm removing and servicing / replacing those. My many scuppers also exit below the waterline with seacocks too. The hose on my cockpit drains is old, stiff and brittle and the seacocks don't look great either so I need to replace them anyway hence wanting to try and think outside the box a bit with the solid tube option. Also for some reason when the boat was built the cockpit drains don't go symettrically downward so one of the seacocks is really quite inacessable on the far side of the engine.

It was all these factors that had me thinking a pair of solid tubes going straight down through the hull like my beneteau had could be a good idea.

However, the only ready made tube available does not seem to be even gelcoated on the inside let alone could it be protected with epoxy. I wonder how it was done on the old boat. They seemed to be gelcoated all the way inside.
 
Expect they were made by winding glass cloth round a gel coated mandrel. However, GRP tubes are used for stern tubes which are full of seawater so should be suitable for this application. Presumably you are looking at 40mm or even larger internal diameter. Lake Engineering do 38mm ID epoxy glass tubes at around £170 per 4' length.
 
I find it awful hard to understand the point in cockpit drain seacocks, since they introduce additional points of failure and add very little protection. If the hull fitting fails they don't help (because the water can still come in) and if the cockpit fitting fails they don't help (because the water still can't come in). My boat doesn't have them and if she did I'd remove them.
 
JumbleDuck said:
I find it awful hard to understand the point in cockpit drain seacocks, since they introduce additional points of failure and add very little protection. If the hull fitting fails they don't help (because the water can still come in) and if the cockpit fitting fails they don't help (because the water still can't come in). My boat doesn't have them and if she did I'd remove them.
Click to expand...

Mine has two drains crossed in the traditional manner, and both have massive seacocks.
One is permanently open while I keep the other one permanently closed. This is because the prop is offset to port and is directly below the outlet of the Stbd drain. I found out the hard way that while in operation the prop was actually drawing AIR down the drain leading to sudden loss of thrust whenever the stern lifted on a passing wave!
 
Graham_Wright said:
You could try a mix of the two. Rigid tube taken up well above the water line and then flexi to the cockpit sole.
Click to expand...

This is how the cockpit drains on my boat are made from factory (CC Bavaria 40 Ocean). The tubes are stainless steel with welded on skin fittings (no seacocks) and led well above the waterline, then exhaust hose is hoseclamped to them from there up to the cockpit drains to allow for flexing. Since the tubes are metal, I treated them with metal AF, same as prop and saildrive (Velox plus). If they were GRP, I would just coppercoat the insides, which will last longer. I've done the latter with my plastic seacocks elsewhere and that worked fine.
 
Tranona said:
Expect they were made by winding glass cloth round a gel coated mandrel. However, GRP tubes are used for stern tubes which are full of seawater so should be suitable for this application. Presumably you are looking at 40mm or even larger internal diameter. Lake Engineering do 38mm ID epoxy glass tubes at around £170 per 4' length.
Click to expand...

Yep 50-60mm diameter. Interesting point about tubes to above the waterline then hose. Maybe that's what in saw on the Bavaria ocean 38.

I guess hoisting on board some of the points here.. Make a much sturdier job of the webbing in at the lower end of the pipe sonic it did flex it would crack at the cockpit above then waterline...
 
ghostlymoron said:
Perhaps the reason cockpit drains are fitted with seacocks is so that you can then change the tubing whilst in the water.
Click to expand...

Think the origin is the belief that every hole under the waterline should have a seacock - but of course that was in the days of wood build. Things have changed and you can design boats that have integral drains (like yngmar's) or don't have any conventional drains below the waterline as in may boats with open(ing) transoms or a cockpit going right back to the transom with just a slot to drain, like the Marcon Sabre. Difficult with centre cockpits and the moulded in tube above the waterline with a hose to the drain in the cockpit floor makes sense.
 
Tranona said:
Think the origin is the belief that every hole under the waterline should have a seacock - but of course that was in the days of wood build.
Click to expand...

Does the hull material matter here? Surely the only reason to have a seacock on a hull opening is because you think the skin fitting or hose might fail - it won't help if the surrounding material springs a leak. Or am I missing something?
 
JumbleDuck said:
Does the hull material matter here? Surely the only reason to have a seacock on a hull opening is because you think the skin fitting or hose might fail - it won't help if the surrounding material springs a leak. Or am I missing something?
Click to expand...

No, it should not matter but think back to the good ole days when all fittings had to be attached, sealed with tarred hemp or some such thing, pipes were lead and boats leaked all over the place.

Fast forward and materials and fittings have changed and it is possible now to build wooden boats that don't leak. so it is all in the mind, but based on the views of the past.

BTW a "proper" wooden boat like mine does not have a self draining cockpit so the issue never arises.
 
Last edited:
You must log in or register to reply here.

Share this page

Top