Plastic Seacocks.

[boomerangben]

I apologise if this one has been covered before but if it has, I have had no luck finding it on a search.

Modern mains water pipes in homes is plastic 15 or 22mm. Would this stuff (pipe and fittings) be suitable for the seawater inlet for a raw water cooled engine? Big advantage for us is that the local fisherman's co-op doesn't stock much in the way of seacocks, but there are plenty of plumbers merchants (ok there are two) and there would be a big saving in postage.

 

[simonfraser]

how worried are you about your boat taking on unwanted water ?

marelon seacocks are ok in my humble opinion, pbo articles seem to agree . . . .

 

[tugboat]

I'm not au fait with the domestic water pipes you mention, but would reckon that a proper flexible reinforced hose should be used for raw water, the domestic stuff could get work hardened/brittle as it is not intended to cope with vibration and flexing. Proper seacocks are available through mail order/ online chandlers who would I'm sure post to your location. The insurance implications of a leak resulting from using the wrong materials are worth considering too.

 

[Becky]

There have been some posts on the subject of de-zincification of bronze/brass skin fittings. We went into the subject when I changed a skin fitting on my previous boat.
However, on Cornish Maid, we are replacing a waste pipe outlet with plastic, because it will be perfectly OK. They are available in good swindleries, we used Alladins Cave.

 

[Talbot]

There are specific plastic fittings made for boats out of marelon(sp). These seem to be fine. However I would only use domestic mains water stuff below the waterline if I was really tired of the boat and didnt mind taking a bath! This is not an area to try to make silly savings.

 

[boatmike]

No!
Apart from the danger of failure due to the obvious, these pipes are made to take pressure not vacuum. On the engine intake side you have negative pressure and if you must use plastic pipes use pipe that has been tested for negative pressure. Also reflect that no classification society will approve plastic valves for these uses anyway due to the risk of immediate failure in the case of fire. Yes I know the hull is GRP too but its a bit difficult to burn BELOW THE WATERLINE with seawater only mm away. A plastic valve would not survive any time at all. At least with plastic pipe on a gunmetal valve you can still shut it off when you need to....

 

[PaulJ]

Having been through three sets of seacocks before I finally found some with which I am happy, perhaps I am not the guy to answer this..... but then again, maybe I am!

Seacocks are something on which you cannot afford to economise. There are some decent cocks made for the plumbing trade but there are also some which are definitely NOT suitable. Why do you want to use plastic? Do you have a metal boat? Many people feel that plastic seacocks are simply not strong enough and also there is a potential problem of them melting in a fire..... In my own case, I have a steel boat and I felt that the problem of corrosion between dissimilar metals outweighed the other considerations and I have done my best to protect them both from physical damage and from fire. All life is a compromise but in anything other than a metal boat I would certainly prefer to use Stainless or Bronze seacocks.

OK, so if you really want to use plastic, the main problem you will have is matching the threads to both your skin fitting and then to a hosetail which is the correct size for the hose to the engine. The seacock should screw directly onto the skin fitting so this is likely to decree which size of cock you buy, unless you also replace the skin fitting. Whatever type of pipe you use to connect it to the engine, it should be "fire resistant", cheap clear plastic hose is no longer "approved" and I am not sure how plastic plumbing pipe would stand up either. As said in one of the other replies, don't forget that it is operating under negative pressure and must be of a type that will not collapse. Personally I would not want to use copper pipe because it is rigid and I would be concerned that engine vibration could eventually cause it to fatigue.

The long and the short of it is that seacocks are something on which you should not try to economise. I have eventually ended up with those made of Marelon by "Forespar" in the States but BE WARNED...... They make two ranges. I started with the "cheaper" type (£30ish each) and all but one of them leaked. I ended up ordering their "OEM" type which were a bit pricey but in my opinion are the best you will find.

http://www.forespar.com/catalog/plumbing/integrated.htm

Sorry, but you did ask! /forums/images/graemlins/wink.gif

PaulJ

 

[boomerangben]

Thanks everyone. I am not convinced about the low pressure issue - after all it is not inconceivable that domestic pipes would be subjected to a low pressure (eg feed pipe to a loft tank being full and the water pressure being turned off and vented). I agree with the fire issue although any fire intense enought to melt a seacock would be largely acedemic - the flexi hose from seacock to engine would fail and the fire would be too big to fight using the poxy extinguishers found on most boats. So I guess it would be down to insurance, but it would be interesting to know from a technical point of view what is wrong with using domestic fittings.

 

[Birdseye]

The pressure on an inlet water seacock would be less than mains, but I doubt they would be re-inforced nylon like the boat ones. These are designed so that they will withstand something falling on them when the boat is bounced around in bad weather.

Wouldnt risk it myself. Doubt it would be acceptable to the insurance company and it smacks of risking the ship for a haporth of tar etc.