Itís first necessary to clarify the difference between steam and white smoke.
White smoke will linger behind the boat
for several boat lengths, while steam will dissipate quickly - normally within 5 to 10 feet.
White smoke is almost always caused by oil
entering the combustion chambers and burning along with the fuel mixture.
Steam will normally be caused by one of the following conditions:
1) In the cool part of the season, it's rather common within our northern operation locations for people to report a "vapour" coming out with the exhaust on their boats. This vapor is simply a matter of warm moist air cooling
and condensing behind the boat. The problem goes away as soon as the weather
warms up a bit.
2) Early model cooling
systems will create a small amount of steam until the engine gets fully warmed up. While an early model engine is still cool, most of the cooling water is being recirculated, leaving an insufficient amount of engine cooling water
to be discharged into the exhaust system to cool the hot section, and what water does enter the exhaust system turns to steam. As soon as the engine warms up, more water will be discharged to cool the exhaust system, and the steam should dissipate.
3) In both early and late model engines, cooling water flow may have reduced due to a defective water pump
or a restriction in one of the cooling hoses, so that there is no longer sufficient water flowing through the engine to keep the hot section of the exhaust system cool, and a small amount of steam may be developing in the part of the hot section where the engine cooling water
enters the system.
4) Steam will sometimes be created as the hot section slowly clogs with precipitate, where the engine cooling water enters the hot section, usually just above the inlet to the water lift muffler
NOTE: In both (2) and (3) above, the creation of steam may be somewhat exacerbated if the hot section was recently wrapped with an insulating material, since the insulating material will result in a hotter temperature within the hot section.
5) The most serious scenario would be steam caused by a small amount of water leaking into the exhaust manifold through a crack in the block, head
or manifold. Steam created by a small crack in one of these cooling water jackets will usually elevate exhaust back pressure within the manifold or the rest of the exhaust system, which will, in turn, cause a very thick caramelized brown goo to form on the stems of intake valves. We have several cases on record
where intake valves have stuck open in only 5 to 10 hours of operation.
Sorry, I don't recall
from where I 'stole' this explanation.