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No Water Coming Out Of The Exhaust
Your engine is raw-water cooled. Seawater is drawn in through the hull, pumped through the engine, and discharged with the exhaust. If water has stopped coming out of the transom, the engine has stopped being cooled — and a marine diesel with no cooling water destroys itself in minutes, not hours.
Shut down now
Stop the engine. This is not a fault to investigate while running.
Beyond the engine damage, the exhaust hose on most installations is rubber, kept alive only by the water flowing through it. Without that water it can melt or burn through, which puts hot exhaust gas into the engine bay and opens a hole below or near the waterline. Overheating here is a fire and flooding risk, not only a mechanical one.
If you are under way and this is your only engine, anchor if you can before you begin diagnosing.
Likely causes, ranked
| Cause | How common | Confirms it | Typical fix |
|---|---|---|---|
| Blocked raw water strainer | Very common | Clear bowl full of weed, plastic or growth | Clear it — minutes |
| Seacock closed | Common | Handle not aligned with the hose | Open it, then inspect the impeller |
| Failed impeller | Very common | Blades missing or set hard | Replace; recover every fragment |
| Blocked intake or scoop | Common | Strainer clear but no flow when hose cracked open | Clear the skin fitting |
| Collapsed suction hose | Occasional | Soft hose, flow fine at idle only | Replace hose |
| Blocked heat exchanger | Occasional | Flow drops progressively over weeks | Service the stack |
| Pump shaft or drive failure | Less common | Pump turns but no flow; salt weep at the seal | Rebuild or replace pump |
| Blocked exhaust elbow | Less common | Back pressure, soot, power loss beforehand | Replace elbow |
Start at the top of that list. The first two are free and account for a large share of cases. The most expensive possibilities are also the least likely.
What you can check yourself
With the engine off:
- The seacock. Is it open? A closed seacock is the single most common cause after a trip on the hard, and running even briefly against it destroys the impeller.
- The strainer. Close the seacock first, open the strainer, clear it, close it, reopen the seacock. Weed, plastic bags and shell are the usual finds.
- The impeller. If you carry a spare and know the engine, this is the most worthwhile repair to be able to do yourself. Account for every blade — a missing one is downstream in the heat exchanger and will cause the same problem again within hours.
Do not run the engine to “see if it clears”. Every second without water is damage.
Impeller replacement · Raw water system repair
When to call a mechanic
If the strainer, seacock and impeller are all good and there is still no flow, stop. The remaining causes — a blocked exchanger, a failed pump, a collapsed hose, a blocked elbow — need someone who can pressure-test the circuit rather than guess at it.
Call immediately, rather than after another attempt, if the engine reached operating temperature or the alarm sounded while running dry.
What diagnosis costs
Usually charged as attendance plus time, and a raw water fault is normally found within the first hour because the diagnostic order is short and cheap.
What marine engine work costs in Australia
Common on these engines
Any raw-water-cooled marine diesel can do this, but impellers and strainers cause it disproportionately on engines that sit between uses — yacht auxiliaries and seasonal boats especially.
→ Yanmar · Volvo Penta · Beta Marine · Marine engine brands
Frequently asked questions
Can I motor a short distance without cooling water? No. Damage begins within a minute or two, and the exhaust hose can melt. Anchor or get a tow.
How long can a marine diesel run dry? Impellers are damaged almost immediately. Serious engine damage follows within minutes. Treat it as zero.
Water was coming out and then stopped — what changed? Most often the strainer blocked with something the boat passed through, or the impeller finally shed its blades. Both are found in the first ten minutes.
There is water but much less than usual — same problem? Yes, earlier in its progression. Reduced flow is the warning; act on it now rather than after it stops entirely.
Should I carry a spare impeller? Yes, with the gaskets and the tools. It converts the most likely breakdown you will have into an inconvenience.
Practical summary
Use this fault guide safely
Stop now
Shut down for oil-pressure alarms, overheating, fuel spray, smoke in the engine space or loss of cooling-water flow.
Check safely
Start with visible leaks, strainers, fluid levels, alarms and loose connections—only when the machinery is safe and isolated.
Record evidence
Note rpm, temperature, smoke colour, alarm text, sea state and exactly when the symptom began.
Escalate
Give the evidence and engine model to a qualified marine mechanic; avoid repeated restart attempts.
Sources and verification
This page provides general Australian guidance. The current manual for the exact engine, gearbox or drive remains the authority for procedures, specifications and service intervals.
- 1Dry exhaust
- 2Mixing elbow
- 3Waterlock
- 4Hose rise
- 5Hull outlet