Station wagon effect

[GunfleetSand]

Have a Hardy Fisher 20 wouldn’t mind a diesel heater on it am weighing up options to self fit or pay for install, if the latter probably Planar and through an approved fitter.

Assumed use would be at anchor, ideally underway, maybe the odd over night. Being at anchor doesn’t always mean that boat is pointing to wind.

How much of a concern is back draught or Station Wagon Effect. Does an approved fitter consider this in their plan, have others who have self fitted?

There is a maib report in this forum about this but couldn’t find much more.

Might be overthinking it but pretty high risk device so worth covering bases. I have seen numerous sail boats with port or s’board side outlets close to cockpits. Co2 is heavy and sinks. Cockpit>companionway>cabin is that possible if wind blowing against exhaust gas?

 

[fredrussell]

The key to using any kind of fossil fuel powered device on your boat is to fit a carbon monoxide detector or two. These heaters are easy to DIY fit and as long as common sense is used they are safe. However, given that you have concerns perhaps a pro install would offer some valuable peace of mind.

 

[Sandy]

Most installations I have seen have an exhaust on the transom and the unit in a cockpit locker with hot air ducting into the living areas.

As @fredrussell states a carbon monoxide monitor should be fitted.

 

[Sea Change]

Our exhaust is on the side of the coaming and I've never noticed any recirculation- it's a narrow gap between the coaming and the dodger which acts as a bit of a wind tunnel.
A properly functioning diesel heater won't be producing much if any CO.
£20 for a CO alarm is good peace of mind though.

 

[GunfleetSand]

Our exhaust is on the side of the coaming and I've never noticed any recirculation- it's a narrow gap between the coaming and the dodger which acts as a bit of a wind tunnel.
A properly functioning diesel heater won't be producing much if any CO.
£20 for a CO alarm is good peace of mind though.

I understood that the exhaust gases contained co2 ipso facto. It does appear that a functional correctly installed heater should not produce much if any co2, internal leaks from the combustion chambers is another matter but this would fall under faulty equipment.

The odd blown back exhaust fumes are not welcomed and several alarms would be essential

 

[Pye_End]

CO is not the same as CO2. CO comes from a poor burn eg not enough oxygen. CO is the big killer. Whilst CO should not be present in exhaust, you still need to guard against it.

 

[Stemar]

There was a small motor boat that used a genny, IIRC, and someone died from CO poisoning a few years back, but it was a petrol engine.

Is my impression that CO from a diesel is far less likely? What about a diesel heater?

 

[PaulRainbow]

CO from diesel/diesel heaters is minimal, if it's all working correctly.

What about that petrol engined boat that moored up behind in the night and left his engine running, with the fumes coming into your boat whilst you were asleep, never to awaken.

All boats should be fitted with CO detectors.

 

[Pye_End]

Is my impression that CO from a diesel is far less likely? What about a diesel heater?

All it takes, with any combustion, is a poor burn, eg from a poor fresh air supply, or a leak (or design!) in the exhaust giving exhaust gas buildup in the air intake area, or even the situation in the OP where the exhaust gas come back into the boat.

 

[ylop]

All good information for the OP - but little actually answering his rather complicated fluid dynamics problem of how to prevent exhaust air (whether CO or CO2) from ending up back in the cabin and if "professional installers" consider that. I think the first is that common sense can help with a lot - don't fit it immediately next to a window, think about where clean air is coming from for the burner etc. you'd hope the pro installers get that right. But weird effects where turbulence around square objects might blow back inside is less intuitive. If you consider that can essentially point at any angle to the wind it becomes an even harder question. BUT - where does the engine exhaust come out? That's probably a pretty sensible option as presumably it doesn't normally cause a problem.

I'd add that as well as a CO detector, you probably want a discipline of not sealing the boat too tightly when heating - which is tempting to get rid of draughts and maximise efficiency.

 

[Refueler]

Backdraft at the transom is a common effect - how many times have you had engine running and you get that whiff of exhaust in the cockpit ...

One post mentions see where engine exhaust is ...

That's not good indicator for position for diesel heater exhaust - why ? Engine exhaust is blowing out exhaust gas and water from cooling system ... so when transom outlet dips below waterline - it still exits ..
A diesel heater does not have such 'force' in the exhaust and you could have problems putting it so low. So the outlet is a lot higher ... often centre transom to avoid going underwater when boat heels under sail etc.

I'm in process of deciding where to fit a diesel heater ... and all these points are uppermost in mind.

As regards CO and CO2 - both in sufficient volumes instead of normal air are killers.

 

[PeterBoater]

Station wagon effect technically only applies when the boat (or station wagon) is moving forwards, caused by back draught circulation. I read that the opening ventilators below Land Rover windscreens (proper Land Rovers, that is) partly were to alleviate this effect. Motorboats, that don't heel, generally seem to have the heater exhaust on one of the quarters. With or without CO, exhaust fumes are unpleasant and harmful if they get into areas where people or animals are, and therefore should be avoided. A good article on station wagon effect and other CO problems is at: Carbon monoxide and boats (Maritime Safety Queensland) . Although CO gas is heavier than air (I think), initially it will be hot and will rise, which presumably is why detectors should be mounted high up.

 

[Refueler]

Station wagon effect technically only applies when the boat (or station wagon) is moving forwards, caused by back draught circulation. I read that the opening ventilators below Land Rover windscreens (proper Land Rovers, that is) partly were to alleviate this effect. Motorboats, that don't heel, generally seem to have the heater exhaust on one of the quarters. With or without CO, exhaust fumes are unpleasant and harmful if they get into areas where people or animals are, and therefore should be avoided. A good article on station wagon effect and other CO problems is at: Carbon monoxide and boats (Maritime Safety Queensland) . Although CO gas is heavier than air (I think), initially it will be hot and will rise, which presumably is why detectors should be mounted high up.

Whether boat is moving fwds or boat stopped and wind is blowing to aft - the effect is same ..... air doesn't care what is moving.

 

[ylop]

Although CO gas is heavier than air (I think), initially it will be hot and will rise, which presumably is why detectors should be mounted high up.

Its actually very slightly less dense than air. To be honest you need so little for toxicity that air currents in the room mean you probably can't hope to avoid it through natural separation. CO detectors commonly say install at "head height" because that's where you are breathing it in.

 

[AntarcticPilot]

Its actually very slightly less dense than air. To be honest you need so little for toxicity that air currents in the room mean you probably can't hope to avoid it through natural separation. CO detectors commonly say install at "head height" because that's where you are breathing it in.

Also because sources of CO are hot, and hot gases rise. As noted, CO has exactly the same density as N2, the main constituent of air.

 

[thinwater]

At least in the US, ABYC says that all heaters must be sealed; the combustion air is not drawn from the cabin. Some use a coaxial stack. This pressure imbalance is part of the reason; if the cabin pressure is lower than the exhaust stack location, they won't draw.

That said, even a coaxial stack may need some kind of baffle to avoid blowing out the flame. Unless the intake and exhaust are of the same design and in very similar locations, pressure differentials are possible in strong winds. My coaxial stack had problems above 40 knots (slight difference in height above the deck), so I added a wind break. Then it worked fine (the intake is below the exhaust).

CO generation depends entirely on the airflow, so the OP's concern is valid. Plenty of people are killed by CO from defective diesel home furnaces. I have also inspected many industrial heaters that were mechanically fine, but because of maladjustment or another factors affecting draft, were making loads of CO. Many of the broad states made here are not at all certain. There is no substitute for detectors.