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Coolant


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That is rather odd; there should be nothing in the expansion bottles if the header tanks are empty as they should drain back, make sure the tube twixt header and expansion bottle are clear. Probably time for a drain down and replacement of the coolant anyway as it should be done every two or three years as the anti corrosion properties degrade, unless you are sure it has been done recently. The header tank should be full and the expansion bottle should be filled up to the “cold†mark if it has one, otherwise I would fill it about 1/3rd full as a starting point.

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Odd is what I thought - maybe the previous owner has been filling up at the wrong place??

Wouldn't be the first time and I doubt the last either, depending on the relative height of the expansion tank it may well not drain back into the header but still be 1/3rd full.

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:oops: Ok! stupid question time here, what is the header tank actualy for?? as some of you may know I now have a diesel boat, my first, as far as I'm aware it's raw water cooled as is the prop shaft bearing, but it has a header tank, unfortunatly I think I may have a small leak as it drains down after a week or two if it's left, I carry a 5ltr bottle of 50/50 anti freeze to top it up, which I was told is the right mix, but what purpose does the header tank actualy serve as I never had one on any of my petrol boats??? :oops:

Regards Frank,,

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Frank, if you have a header tank and you are putting coolant mix in then your boat is not raw water cooled, except to say that the raw water cools the coolant via a heat exchanger rather than being pumped directly around the engine as with most petrol boats. The header tank is simply the highest point on the closed cooling side, usually directly atop the heat exchanger, it acts as a swirl chamber and somewhat as a reservoir, however if what you are talking about is a separate bottle with a tube leading to it from said header tank then that is the expansion bottle. But.. regardless of where you’re putting it the need to constantly top up can only mean one thing, you are losing it somewhere and that should be addressed unless very minor and you are happy to go on as you are. Bear in mind it could be anything from a simple loose hose clip to the onset of something more serious.

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:grin: Thanks Antares! I think get it now, The engine is a sealed system as such and raw water cools the coolant a bit like the rad on a car, I pressume because of the anti freeze this is better for the engine as you won't get the corrosion and other problems you get with a raw water cooled engine, I just have the header tank and no expansoin bottle, I'm pretty certain that it's a leak as I had one or two loose Jubilee clips that I had to tighten up, I'm going to change all the hoses as I think they are the same ones that were fitted to the boat 30 years ago, there's no white sludge in the oil and it dosn't overheat, it's just that after a week or two standing unused I have to add about 1/2 litre of coolant to top it up, the odd thing is it dosn't seem to loose very much when the boat is in use, and being under pressure I would have thought it would loose a lot more,,

Regards Frank,,,

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Frank, further to what David says, most boats are both fresh water and raw water cooled in a manner of speaking. The cooling system on a car works by circulating fresh water around the engine and then passing it through the radiator where the heat is carried away by the air flow that passes through the radiator as the car moves. The cooled water then passing back into the system and circulates again with a single water pump, normally driven off the timing belt, doing the pumping. The constant supply of cool water is what keeps the engine temperature level.

Obviously the same setup isn't practical on a boat yet the cooling system still works in much the same way. Normally there is a fresh water side (which will contain anti-freeze, is sealed and will circulate around and around) and which does its job in much the same way as on a road engine. But the radiator is instead replaced by a heat exchanger and that is used to conduct away the heat instead. The heat exchanger is where the raw water system comes in. The raw water system pumps water up from the river / sea and circulates that through the heat exchanger. The two don't mix but the cold river water significantly cools the fresh water in the same way the airflow would through a radiator and the cooled fresh water is then circulated back into the engine as in the car whilst the warmed up raw water then exits the system back into the river.

Apologies for any egg sucking lessons I may have just given but there does seem a bit of confusion here about anti-freeze and raw water systems. If the coolant level in the fresh water system is dropping then it can only be that there is a leak somewhere. If it's very minor it can be hard to find but if it is losing quite a lot then simply looking at the engine after it has run can generally help you spot the wet bit unless it's being lost internally due to a blowing gasket in which case it'll probably find its way into the oil way and you'll have emulsified oil showing up on the dipstick. I once had a car where it actually leaked onto the cylinder head and that gave itself away as steam coming out of the exhaust (bit harder to see on a boat that one).

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Well the set up gets stranger.

The tank I assumed is a header tank is actually a little lower than the filler tank on the engine. The is also a pipe that goes from the bottom of the plastic "expansion" tank to the engine as well as a pipe from just under the filler cap to the top of the plastic tank.

As the tank is lower that the filler it will always be full???? very confused :?

Wayne

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:grin: Thanks Plesbit! it's all very clear now, and sorry about hijacking your thread Wayne, as you can tell I know bugger all about header tanks and the set up on boats but I do know expansion tanks are usualy higher than the engine on cars, so how about this for an idea as it seems to have the experts baffled, is it possible that it's an add on that a previous owner has fitted for some reason? like just having to visualy check the expansion bottle to see if the header tank needs toping up, instead of having to check the actual header tank,

Regards Frank,,,

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Well the set up gets stranger.

The tank I assumed is a header tank is actually a little lower than the filler tank on the engine. The is also a pipe that goes from the bottom of the plastic "expansion" tank to the engine as well as a pipe from just under the filler cap to the top of the plastic tank.

As the tank is lower that the filler it will always be full???? very confused :?

Wayne

Wayne, if there is a pipe coming from below the "radiator" cap, then the tank you describe is definately acting as a reservoir, to collect coolant from the overflow pipe to prevent pressure build up and thus venting from the radiator type cap (which will have a psi rating). It does seem very odd that it is set below the level of the engine cap, but that may be due to poor installation or lack of practical alternative. You may find if you top up the engine filler to the top, that after running for a while, you lose coolant from the filler cap as it has nowhere else to go, and therefore the expansion tank is just like a "buffer".

Frank, the only exception to the rule that Simon (plesbit) has descibed so well is if your boat is keel cooled. A lot of earlier boats did not have a raw water side as such, but simply pumped the anti-freeze coolant via a skin fitting through some copper pipes mounted on the outside of the hull beside the keel to shed heat directly to the river. This was popular some time ago, but not so much now, as the vulnerability of the pipes made them a bit of a liability. Just have a look at the coolant pipes on your engine and see if they pass to a skin fitting, which would indicate a keel cooled set-up. If you do have this system, it could be a leak outside the boat and you would never see a wet patch.

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:grin: Thanks Mark! it's definatly not keel cooled it's just got a raw water inlet/strainer that goes to the water pump, some of the hoses are a bit iffy and are due for a change, on thinking about it at the end of last season I changed a hose that feeds the calorifier and had to refill everything using a 50/50 mix of anti-freeze, there was no anti-freeze in it before, and I know from cars that as soon as you add anti-freeze if you have a leak the anti-freeze will find it, I'm not too bothered by the leak as I'm going to change all the hoses, everything else seems Ok engine wise, and once I dry my bilges out if I do have any leaks i'll be able to find them, the main thing was! I couldn't work out why the new boat had a header tank and what it actualy did, as I've always had petrol engines in the past so the header tank is a first for me as is the diesel engine, seems I have to re-learn everything again,

Regards Frank,,,

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