Dave Gerr - Boat Mechanical Systems Handbook-How to Design, Install, and Recognize Proper Systems in Boats

total length from the lift outlet to the through-

hull. If the run must be longer, you should

make the hose diameter larger. For lift runs up

to 60 times exhaust-line diameter, increase the

line size by 20 percent. With such a long run, a

21 / 2 - inch (65 mm) exhaust inlet muffler should

be fitted with a 3-inch (80 mm) outlet. Still

longer runs are possible, but you might have to

increase diameter even more, and check with

the engine manufacturer about the maximum

acceptable back pressure. As with any exhaust

system, you should use as few bends as possi-

ble with the largest radii possible; tight bends

also increase back pressure.

NOTE: On waterlift installations, it is

common and accepted to have a fairly tight

180-degree U at the top of the lift line. You

can use this whenever needed; but it’s still op-

timal not to, if possible.

Gasoline and Waterlifts

Should Not Mix

Although waterlift mufflers have been used

for years with both gasoline and diesel

engines, I recommend against employing

them for gas. There may be no problem for

years, but just the right (or wrong) set of cir-

cumstances can fill the top half of the muffler

canister with a mixture of gasoline vapor and

air—an explosive combination. Though usu-

ally minor as such things go, there have been

a few cases of explosions. Even if—taken as

a whole—the odds against mishap are well

in your favor, and even though such blasts

haven’t been catastrophic, I personally would

sleep better on a gas-engine or gas-generator

boat not equipped with a waterlift system.

The “Other” Exhaust Systems

So far, we’ve examined standard powerboat

exhaust installations and waterlift mufflers.

There are, though, other exhaust systems

that are not only useful, but also date back

to the first installation of internal-combustion

engines in boats. These systems still can solve

specific engine and interior-arrangement

problems practically. One of the most inter-

esting of these is the North Sea exhaust sys-

tem. Strangely, this setup is little known and

largely forgotten today. Indeed, I’ve employed

it on several motor cruisers, yet some engine

manufacturers’ technical staffs had never seen

it before. Being a suspicious lot—in spite of

my assurances—they insisted on running their

computer back-pressure analysis before giv-

ing their official go-ahead. They seemed gen-

uinely surprised, when—after much number

crunching—the old and tested North Sea

arrangement proved to be as efficient.

A North Sea Problem Solver

The fact is the North Sea exhaust system

solves one of the most common problems on

many inboard cruisers: with engines roughly

amidships, how do you get the exhaust out to

the transom? Those long exhaust runs and

their mufflers take up valuable space and are

expensive to fit. With the North Sea exhaust

system, the answer to the question is you don’t

take the exhaust to the transom. Instead, you

run it out the sides of the hull—out bothsides.

You can see how the system is arranged

in Figure 7-14. This is a modern version us-

ing a waterlift muffler. The original North Sea

configuration simply had the exhaust riser

from the manifold make a large loop up and

then drop down to the transverse pipe.

(Sometimes a standpipe was used; more on

this later.) Both methods work, but the water-

lift muffler is quieter and makes installation

even easier. Once the water exits up out of

the waterlift canister, it doesn’t care which

direction you lead it, as long as there aren’t

sharp bends or numerous curves (back-

pressure no-no’s). On one boat, I ran the

exhaust up from the canister and forward,

then down to the transverse pipe.

The beauty of the North Sea exhaust sys-

tem is that not only does it eliminate the long

run to the transom, it also has twice the

outlet-opening area. You can’t ever get back-

pressure problems (once the system is set up

properly, that is). Naturally, when the boat

rolls or when the sea slaps against one side,

the other side is clear for full exhaust flow

and for instant drainage. Because the outlets

don’t face aft, you’re also free from worry

about flooding when backing down hard.

In normal operation, the exhaust fumes

exit the hull low enough so that they trail aft

before they can rise to deck level, where they

would create unpleasant smells. The North

Sea exhaust system is so named because it

PART THREE: EXHAUST SYSTEMS