Siphion break for engine exaust

JonnyBoats · Post by **JonnyBoats** » Sun Mar 18, 2007 2:53 pm

Hal Roth in his book "How to Sail Around the World" on page 338 describes a unique method of preventing cooling water from siphoning back into the engine when it is shut off.

I bleed off a small percentage of the cooling water via a small hose that goes to a back corner of the cockpit. When the engine is running it is easy to glance down and see whether the cooling water is flowing. When the engine is off, air runs back down the hose and prevents siphoning.

Does anyone have experience with this system and what do you think of it?

Post by **Tim** » Mon Mar 19, 2007 6:07 am

I tried that in my original diesel installation. Most of the exhaust water ended up in the cockpit; it was miserable, and I removed it.

JonnyBoats · Post by **JonnyBoats** » Mon Mar 19, 2007 9:23 am

I tried that in my original diesel installation. Most of the exhaust water ended up in the cockpit; it was miserable, and I removed it.

Did you provide some other form of siphon break or did you dispense with a siphon break entirely?

Post by **Tim** » Mon Mar 19, 2007 12:53 pm

I don't have a siphon break at all. I feel comfortable without it.

In my "Hal Roth" setup, it's possible my siphon hose may have been too large--so I won't discount the idea entirely--but it sure made a mess and wasn't successful the way I rigged it.

JonnyBoats · Post by **JonnyBoats** » Mon Mar 19, 2007 9:53 pm

Beth Leonard in ?The Voyager?s Handbook?, second edition on page 107 describes a variant on the ?Hal Roth? setup.

Replace the siphon break with a hose fitted with a large one-way valve that ends as high above the waterline as possible ideally in a coaming above the cockpit.

I guess one could install a normal valve on the hose and leave it open when the engine is not running. When starting the engine the valve would be left open untill cooling water started to flow out of the hose. One could then close the valve entirely or close it almost all the way and allow a small amount of water to continue to flow, thus allowing one to occasionaly glance at the hose to verify cooling water flow rather than looking over the side.

I think this topic is rapidly approaching "boat nerdery"....

bcooke · Post by **bcooke** » Tue Mar 20, 2007 2:40 pm

What about a standard vented loop?

Or would that tend to clog up? Of course, when they fail, which I hear they do with regularity, then you would no longer be protected and probably wouldn't notice until your floorboards were floating. Maybe just plan on replacing the vented loop every couple of years as preventative maintenance.

If you have a freshwater cooling system then you get a pretty good warning of low flow when the coolant temps start rising. With raw water cooling I guess you wouldn't know until it was too late. Maybe an oil temp guage would provide a secondary indication that you had a cooling problem.

I would be curious about an exhaust water flowmeter though.

-Britton

Post by **Tim** » Tue Mar 20, 2007 5:01 pm

Standard vented loops vent saltwater regularly (unless one is a lucky soul who sails in fresh water only). Not only does this crud up the vent, rendering it useless, but since these loops are often squeezed into tight engine rooms, the runoff typically ends up dripping all over some part of the engine, thereby ruining it. Or, worse, above the batteries or electrical panel. I've seen all of these things be ruined by dripping green saltwater corrosion from the thoughtlessly installed vented loop that everyone thinks is so important for some reason.

So the moral of the story with vented loops is to either locate them somewhere where they can cause no harm during normal use, or, better, to run a hose off the vent outlet down to some safe discharge location.

Every installation is different, but I don't have a vented loop at all now. I think the chances of backflow from siphon are minimal at best, and there's adequate protection afforded by things like the exhaust loop at the outlet and the depth of the waterlift below the engine riser. Your installation may vary, of course.

I agree that a good temperature gauge and alarm (and paying attention to said gauge) is better defense against overheating than a cooling water telltale spewing hot smoky water into the cockpit.

keelbolts · Post by **keelbolts** » Tue Mar 20, 2007 5:26 pm

Favona had a vent in her cooling system. It vented thru a small hole in her topsides. At higher RPMs a little stream of water peed out the side. No problems ever.

bcooke · Post by **bcooke** » Tue Mar 20, 2007 5:37 pm

By a chance coincidence (is that an oxymoron?) I received the book today that Rachel talked me into last week, Offshore Sailing by Seifert and Spurr.

The book put me onto an exhaust flow alarm (www.aqualarm.net) that costs about $75. Not a bad upgrade to my mind.

Thinking hard about it I agree that a siphon is probably not that important. I try to make a habit of closing the RW seacock along with the fuel shutoff valve when the engine is shut down. Plus, my closed loop is over a foot above the level waterline and only drops about an inch before being "injected" into the exhaust so there isn't much pull unless my boat was really well heeled putting the loop below or at the waterline.

I don't suppose there is ever enough flow to fill the exhaust outlet hose with its own loop and cause a siphon from that end. A ball valve at the outlet might be in my boat's future which would eliminate that scenario as well.

-Britton

keelbolts · Post by **keelbolts** » Wed Mar 21, 2007 8:12 am

Many boats for offshore use have a seacock at the exhaust. It's not unheard of to fill your engine with salt water thru the exhaust system. Imagine several days of following seas in a boat with the exhaust thru the transom. It's easy to see how the problem might arise.

Rachel · Post by **Rachel** » Wed Mar 21, 2007 11:28 am

I do hope you enjoy the book I "talked you into." <snort>

Next time I'll make sure to endorse something more expensive.

R.