I/O Siphon

[edcourter]

Given that there is engine plumbing below the waterline, is it possible for water to siphion from the out drive into the boat, while at rest, due to leaks or mechanical faults?

 

[Scott Danforth]

what do you have going on? generally, yes, so if you have a hole in your y-pipe, or you have a leak at the transom due to rotten transom or a bad bellows you can get a leak.

so back to the question, what do you have going on?

 

[thumpar]

It doesn't siphon in it just leaks in. Those spots that Scott mention would be the places to check first. If you don't know when or its been many years since the bellows were changed I would look there first.

 

[Bondo]

Ayuh,.... Short answer is,...... Yes,....

 

[edcourter]

Transom and bellows leaks I understand. But can water siphon through the water pump up the leg and down to a leak in the cooling system, or up the leg from the exhaust and do the same, and only through a high/low siphon action, boat at rest engine off. I do see where the risers protect the valves. Does a pump impeller close off the pump when stopped. I don't have an actual leak, just trying to learn if this is another known issue that a cautious boater should be aware of. Can any engine hose be pulled when the boat is in the water?

 

[Scott Danforth]

the way your cooling system works, the water comes in from the transom shield, goes thru the motor, then out the manifolds and elbows. the high-point is the elbows which are a minimum of 14" above the water line.

so if you have a breach in the system below the water line, you have a leak.

No, generally the hoses cant be pulled when the boat is in the water without water coming in.

 

[GA_Boater]

If you have water coming in while the motor is off, you have a leak. A leak or removing a hose below the waterline will allow water to flow, not siphon.

 

[tpenfield]

The short answer is . . . Water does not siphon uphill . . . I'd worry about something else, unless you have a leak to attend to.

 

[Lou C]

On most I/Os the water intake hose is at or below the static water line of the boat, so if you had the boat in the water and you disconnected the raw water hose that goes from the transom mount to the thermostat housing (Alpha or Cobra) or the raw water pump (Volvo or Bravo) and held it up, water would not siphon in, but if you dropped the hose down below the static water line then yes water could for sure siphon in. When I have had to change a thermostat with the boat on the mooring, I would hold that hose up and put a wood plug in it, with a clamp to be on the safe side.

 

[muc]

The short answer is . . . Water does not siphon uphill . . . I'd worry about something else, unless you have a leak to attend to.

si·phon
ˈsīfən/
noun

1. 1.
   a tube used to convey liquid upwards from a reservoir and then down to a lower level of its own accord. Once the liquid has been forced into the tube, typically by suction or immersion, flow continues unaided.

 

[muc]

Given that there is engine plumbing below the waterline, is it possible for water to siphion from the out drive into the boat, while at rest, due to leaks or mechanical faults?

Yes it is possible. No the impeller usually won't block it.
This is why a MerCruiser single point drain system has a blue drain plug on the thermostat housing, It's not a drain, it's to break the siphon if your using the drain system when the boat is still in the water.

 

[Bondo]

or up the leg from the exhaust and do the same, and only through a high/low siphon action, boat at rest engine off. I do see where the risers protect the valves.

Ayuh,.... At That point, the water level is right about at the top of the Y-pipe, or down-pipe on an in-line motor,.....

I've changed shutters in a floatin' boat, 'n every ripple outside the boat dribbled water over the down-pipe into the bilge,...

 

[edcourter]

Thanks all, I now understand.