Professional BoatBuilder - December-January 2018

BEST PRACTICES: Exhaust

 e variety of riser designs include

those directly above the exhaust mani-

fold/turbo, those directly above the

mu er, and those before the overboard

discharge, typically adjacent to the

transom.  ese are dependent on vessel

design and engine/genset height above

or below the LWL.  e primary di er-

ence is that some are self-draining and

some are not. In self-draining designs,

mixing elbows are just a er the highest

portion of the system, so that no matter

how much water is pumped into them,

it runs overboard, even if the engine

isn’t running. In designs that aren’t self-

draining, water that accumulates in

them can  ow back to the engine and

into its cylinders.

The Jacketed Elbow or Riser

More commonly found on power

and larger sailing vessels, the jacketed

riser or elbow is a slightly di erent

approach.  is system surrounds the

hot exhaust gases with a double-walled

pipe through which cooling seawater

flows. Some risers simply consist of

a vertical inverted U loop traveling

For good reason, engine and generator

manufacturers provide clear, detailed

guidance for exhaust system design and

installation. They never give a warranty for

water ingress via the exhaust system.

be installed in the exhaust system between the raw-water

pump outlet and the injection elbow (see “Antisiphon

Valves” on ProBoat.com). According to ABYC, it must be

located “at the top of a loop which shall rise high enough to

assure that the high point where the siphon break is installed

will always be above the water level surrounding the boat.”

 is means the antisiphon device should be well above the

LWL regardless of the boat’s attitude, o en a minimum of

12 "–18" (305mm–457mm). On a sailing vessel it should be

located as close as possible to the centerline to ensure that

it doesn’t end up below the waterline when the boat is

heeled.

Most manufacturers of these devices have very speci c

installation requirements, particularly the vertical distance

between the waterline and the siphon break. Some specify

as much as 24" (635mm) to ensure proper operation. Check

the installation requirements even if you’ve used the same

unit previously or if one aboard a vessel you are working on

appears to be operating normally. It may be inoperative and

you’d have no way of knowing it.

In many installations, all that prevents a siphon is the

condition and resting position of the raw-water-pump

impeller. A single rubber impeller blade will o en prevent

a siphon, but if that blade is damaged, water can  ow into

the exhaust system while the engine is at rest.

Siphon breaks require periodic maintenance or replace-

ment. Most designs incorporate a small rubber “duckbill”

valve, which can become encrusted with salt or other debris.

 is can cause the valve to either stick open, resulting in

seawater dribbling or spraying out of it, or stick closed,

which invites siphoning. Given enough time, a malfunction-

ing siphon break stuck in the closed position can cause a

DECEMBER/JANUARY 2018 63

COURTESY NORTHERN LIGHTS

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