Siphon break on 175xr2 SportJet

Hey, all -

According to the manual, the SJ has a "bilge siphon" feature in which the big pump is also used to evacuate the water out of the bilge at speed. This sounds pretty cool, but there are all sorts of warnings about keeping the siphon break open and locating it above the waterline - which all makes sense. However, the PO of this boat did a... let's just call it a "custom" engine installation... in which nothing seems to have been done by the manual. As a result, the siphon hose comes up from its fitting, goes above the water line, and then simply splits into two hoses (which drop down on both sides of the bilge) via a tee. No sign of a vacuum break anywhere - and I do get water in the bilge after a run at speed, probably about a gallon in 15 minutes. Nothing coming in while I just sit there; only during, or after a fast run.

(He had also routed the bilge pump hose without raising it above the waterline; I found out about this when water fountained up through the cockpit drain while we were fishing in the middle of the bay after the waves had been slapping the hull for a while. That's now fixed, the SJ got a good rinse out of the deal, and the bilge pump float switch is the very next project I'm doing on this boat.)

So my question is, should I just drill an .020 hole - which is supposed to be what was in the original siphon break - in the tee fitting and call it done? The manual doesn't list a part number for a vacuum break (unsurprising, since it's not really a part of the jet or the engine, just a necessary installation feature) and the breaks I'm familiar with are a good bit bigger than that. The tee is easily accessible via the round panel in the back of the boat, so I could easily check it as part of regular maintenance.

You lost me on that one ,most jets have a small siphon tube inside the forcing cone creating a constant siphon created by the jet water exiting the pump drawing from the tube in the hull in back where the water would collect when the bow is up, this is separate from a bilge pump. The siphon tubes have either a valve like a duck bill rubber type that stop reverse siphoning or a small air tube that stops reverse siphoning so I don't know exactly what one you are having an issue with. On a bilge pump some people just run the hose with a loop in the hose secured above the water line, this usually works to prevent reverse siphoning as it allows air in the tube to stop any siphoning effect.

Cobra Jet Steering LLC said:

You lost me on that one ,most jets have a small siphon tube inside the forcing cone creating a constant siphon created by the jet water exiting the pump drawing from the tube in the hull in back where the water would collect when the bow is up, this is separate from a bilge pump.

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Yep; that part is in place - it's just a tube tapped into the jet body with a hose running forward from it. Took that off and blew through it; it's clear, at least, so the suction part should be working.

The siphon tubes have either a valve like a duck bill rubber type that stop reverse siphoning or a small air tube that stops reverse siphoning so I don't know exactly what one you are having an issue with.

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That's the exact gadget I'm talking about - it's missing! The PO just brought the tube up above the water level, then teed it off to both sides of the bilge (I don't really get that, any more than I get why I have two drain plugs - either one of them drains the bilge completely...)

Anyway, that's the gadget I need to add back into the system - I don't have one right now. Any recommendations? I've thought about running a small air tube to the deck above the engine, too, maybe bent over like a standpipe... but that seems a bit fiddly.

On a bilge pump some people just run the hose with a loop in the hose secured above the water line, this usually works to prevent reverse siphoning as it allows air in the tube to stop any siphoning effect.

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Yep; that one is done already. Rerouted it so there would be enough slack, then secured it to one of the screws that holds the blower intake hose to its vent. Done deal.

Well the suction from the pump needs a tiny air line in it to prevent a reverse siphon effect so tap in a smaller line get a plastic fitting with a smaller diameter line coming off it or use a tap and tap in a threaded fitting for a smaller line and run that line up high inside the boat so when the jet pump shuts off there can't be a reverse siphoning effect, obviously you want a small tube for this so your pump still pulls off water through it's larger line in the hull , the smaller bleed off line just needs to be large enough to add air to the main line when the siphoning of the pump stops since only air is running in this line it won't need to go outside of the hull. As long as that small line is above the water line inside the hull it won't allow any water to back feed into the boat either. The main siphon line to the jet pump also needs to rise up above the water line, with the siphon line mounted closer to the pump

Right, the manual mentioned an .020" diameter hole, so - tiny hose, or maybe a slightly larger one with a plug in the end and a hole drilled to size. Heck, I think I'll just go with that: I should be able to get a small enough thread-to-barb fitting to just screw it into the hose from the side, then run a line up to the deck with that plug. Rain, or other water won't matter, especially if I mount the end of the line horizontally (or just below the deck inside the engine compartment.)

Thanks, Jeff!

if it is just the jet pump siphon you really do not even need the small vent hose to exit the boat , one for the bilge pump would need to exit the hull as water would also flow out of it but on a suction side from the jet siphon hose it isn't necessary since it is just for air.

Heh, yeah - I caught that while thinking about your last response. "Hey, why am I bringing it out, anyway?" Under the deck in the engine compartment is where it'll be.

If the siphon is pulling water and full when the engine is shut down the small tune will siphon water into the bilge if the level is lower than the water level outside.
a siphon break would be a good idea, even if it is not likely to happen.
There is a service bulletin on the small siphon breakers delivered by Mercury that have the hole covered over.

RedBarron55 said:

If the siphon is pulling water and full when the engine is shut down the small tune will siphon water into the bilge if the level is lower than the water level outside.
a siphon break would be a good idea, even if it is not likely to happen.

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Yeah. I've pretty much lost count of boats that I've seen sunk by siphoning (including plugged vacuum breaks) and single (or cheap) hose clamps on thru-hulls. A lot less chance of it happening on this one, simply because I'm always on board and on watch whenever she's in the water, but the fact that I missed it - simply through not knowing about it until recently - still gravels me.

Mine is done at this point, but it's been too cold to put in that automatic float switch. Tomorrow is supposed to be relatively warm, but still too chilly to go boating, so that's project #1 for 8 a.m. I've also wired a buzzer to the pump switch, so that if the pump ever comes on by itself I'll know about it right away so I can look for the reason.

On the 240SJ I have the break is drilled in the plastic fitting that joins the upper and lower sections of the hose [ I only have one ]. It's near impossible to see but it is there. Never had any trouble with it but if it were to become a problem it could be blocked off as it really isn't needed , my bilge pump is automatic. Don't know how well it really works, the pick up is so fine it takes hardly any thing to clog

Thanks, @Bilmon! That's what I was going to do originally, but decided that I liked Jeff's idea better - specifically because I realized that a tiny drilled hole would be impossible to see. In fact, I combined it with another project: there was a 4" round hole above the stern, which must have had some kind of a cover on it that the PO had lost, so I repurposed it by screwing down some rigid white plastic over it in which I mounted a female hose fitting that I connected to my engine flush hose (before, I had to open the engine compartment and climb up on the trailer to get to it.) I also mounted the end of the anti-siphon hose right behind that plastic cover - so any time I take it off in the future, the little white plug I put in the end of the hose will be staring right at me, begging to be inspected.

I'm also thinking really hard about that suction being split to two hoses - and I'm not liking it one bit. It seems to me that if the boat is heeled over at all, the jet is going to suck air from the "uncovered" hose end - it's a lot easier than picking up water and lifting it a couple of feet! - and that's going to happen as long as water sloshes in the bilge. I do believe I'll be removing one of the hoses that goes down and replacing the splitter with a coupler... so my setup will be more like yours.