Sea cocks - open or shut?

SnaxMuppet

SnaxMuppet

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I dry berth my boat which of course means that it is out of the water most of the time. So should I leave my heads sea cocks open or closed when it is ashore?

I do "exercise" them each time I use the boat so they shouldn't sieze shut but I am unsure whether to open them when ashore so any water in the pipe work can drain.
 
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I guess my take on this is that your insurance company may not look generously on an incident following a sea cock left open.

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I think this is the point of my question I suppose... what kind of incident could I have with the boat ashore because my sea cocks are open?

I thought that perhaps rodents might get in or perhaps birds but is that really likely?
 
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I guess my take on this is that your insurance company may not look generously on an incident following a sea cock left open.

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I think this is the point of my question I suppose... what kind of incident could I have with the boat ashore because my sea cocks are open?


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Ah yes! I'd assumed you had a berth that dries out (with the tide) and that had periods of flotation.
 
I think it is a good idea to have a routine/checklist. I always close all seacocks when I leave the boat. If you stick to that routine, whether in or out of the water, then less likely to make an error.
 
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what kind of incident could I have with the boat ashore because my sea cocks are open?

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I've seen boats dropped by the crane when launching. Even if you are always there (are you?) could be enough damage to cause flooding.
I would always close when leaving for home.
 
On larger boats, like mine, it is actually advisable to close the seacocks just before you lift the boat out so that the water DOESN'T drain out of the system.
Once water has drained out it is quite difficult to re-prime the system.

I spoke with someone recently that often takes deliveries from boats that have been delivered by sea. The boats have been completely stripped and drained of water and fuel etc so when they put them back into the water its quite difficult to re-prime them. My engine room has a tap leading from the main water tanks so repriming can be done through this tap/hosepipe. If all the water has been drained from the boats fresh water systems, the only option is a bucket and often the boats are new so no bucket either.

So the advice for the larger boats is close the seacocks just before you lift to keep the engine water in. Of course you must drain if there's a risk of frost damage.
 
You were covering fresh water, fuel and engine raw water all in one go there mike? Just on the engine raw water, it isn't necessary to shut the seacocks before lifting becuase the engine raw water pumps are on/below the waterline anyway
 
It is if they are MTU's as they have centrifugal seawater pumps and they just cavitate if they get air in them, which can happen if the water drains out then you have to open the system and prime them.
 
yeah but you still don't do any of that priming if they're below the waterline, do you? So before worrying about this you gotta see if they'e below/above waterline
 
Pardon?

Do you mean that with MTU engines, if you're cruising some really rough stuff and get some air into the seawater circuit (which isn't as unlikely as it might sound), the pump could cavitate and not be able to self prime, requiring someone to get to the e/r and prime it?
 
spannerman is right

And yes - it an MTU thing.

If seawater is drained from the system it cant prime itself like the old Volvo systems. Yes, there is a clear path to the exgine but an air lock will exist in the top of the strainers. As spannerman says the pumbs are "all metal" centrifugal so cant lift any water if there is only air in the pump.

Small price to pay for not having to change that rubber impeller - especially the port engine on a D12 installation.
 
Re: Pardon?

Sure, that's obviously the most common case.
But if you think about it, if an intake under the hull gets out of the water - even if for just a second or two - while the engines are spinning at cruising speed, that circuit is bound to ingest some air.
Maybe it's a matter of how much air actually gets in, and in such situation possibly it's just too little, nevertheless I was surprised to hear that those pumps could cavitate, 'cause a few air bubbles should be sufficient to risk cavitation.
 
Ah sorry mike/spannerman, I was being thick, doh! I completely forgot about the strainers which are of course above the w/line!

Hmmm, I wonder about MapisM's concern. You'd think that in rough sea a gulp of air could defeat the pumping. My guess however is that (a) this is very rare as MTUs only go in big boats and if the intakes are 1/3 LOA from the transom an airborne leap is a very rare event, and (b) as the boat lands and with its forward motion enough water will be pushed into the pipe to reprime the pumps instantly. There are lots of MTU installations on 40kt mangustas et al and they seem to be ok!

Ah yes, D12 port engine. Depends on the boat. If the fuel tanks are athwartships and not jammed at the sides to make a central owners cabin the pump is very easily serviced! :-)
 
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There are lots of MTU installations on 40kt mangustas et al and they seem to be ok!

[/ QUOTE ]Oh yes, that was also my point actually.
And I'm pretty sure that Buzzi wouldn't have sticked four 12 cyl MTUs into his 80' if he couldn't trust them. Besides, I'm also sure that he went airborne more than once, even with such beast of a boat, when he made the round Britain at an average speed of 44kts, as I recall...
My point is, I accept that centrifugal pumps aren't completely self priming, but I'd also expect them to cope with some air, rather than "just cavitate" as spannerman suggested.
Otoh, if the circuit is completely drained, there's no cavitation surely?
 
Well, i wouldn't assume Buzzi had centrifugal pumps. He might well have modified those engines for that particular voyage! Though he might not..l. I dont know

"Cavitation" is being used a bit loosely here. This isn't cavitation, it's just an inability to prime/ventilation. The amount of air they can cope with depends on the pumps of course: although some centrifs are very non self priming, some do self prime a metre or so, plus all shades of grey in between.
 
Agreed, it's not really cavitation we're talking about here; I was just sticking to the term initially used.
I'm not sure if FB modified the engines in his 80', but I wouldn't be surprised if he didn't.
At that time, he clearly stated that those engines were the only obvious choice of the whole project, and I don't think he declared that just to please the builder...
 
Re. what I said about FB view on MTUs, I googled around a bit, 'cause I wasn't sure if I saw it somewhere on the web, but I probably read it on a magazine, back in those years.
Anyway, I found a nice video with an impressive view of the four staggered engines, if anyone's interested.
 
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