Exhaust Spacers

The installed exhaust height is crucial to the longevity of the marine engine. Without proper compliance water will quickly enter the marine engine through the marine exhaust system causing catastrophic engine damage. The purpose of the exhaust height specification is to ensure that an air break exists at some point in the marine exhaust system. You can think of it as the exact opposite of a P-Trap in your sink.

Regarding the proper height for the marine exhaust system this will vary by manufacturer. Mercruiser for example requires 13" minimum from the top of the exhaust elbow to the water line measured with the boat at rest in the water. Therefore if you have a deep V boat the engines will typically sit lower in the hull and therefore may require the use of spacers to get the exhaust elbow in compliance with this minimum height dimension. Select the following link to see how to verify if your exhaust height is correct. 

The following image illustrates both types of exhaust marine system; one without exhaust spacers while the other shows a 3" spacer. The most common sizes for the exhaust spacers range from 3" to 6". 

Exhaust System Requirements
IMPORTANT: It is the responsibility of the boat manufacturer or installing dealer to properly locate the engine and install the exhaust system. Improper installation may allow water to enter the exhaust manifolds and combustion chambers and severely damage the engine. Damage caused by water in the engine is NOT covered by most Manufacturers Limited Warranty, unless this damage is the result of defective part(s).

Determining If Exhaust Elbow Risers Are Required
To determine if exhaust elbow risers are required, take measurements (a) and (b), with boat at rest in the water and maximum load aboard. Subtract (b) from (a). If (a) minus (b) is less than specified in chart, select appropriate size exhaust elbow riser kit and exhaust extension kit that will correctly position exhaust elbow.

MCM (Sterndrive) Engines With Thru-Transom Exhaust
When designing and installing exhaust system, the following must be observed:

Minimum Hose Size Inside Diameter (I.D.)

1. Exhaust flanges must be of proper size to accommodate 4 in. (102 mm) I.D. exhaust hoses. They must also be equipped with internal water shutters, and use an exhaust flapper over each outlet.
2. The exhaust hoses and pipes must not be higher than exhaust elbows at any point.
3. The exhaust outlet must be located so that a minimum of 1/2 in. (13 mm) drop per foot(305 mm) downward pitch exists in the exhaust hose from the engine exhaust elbow tothe outlet. (This is an American Boat & Yacht Council recommendation.) The drop must be constant so that a low spot does not exist at any point in the exhaust hose.
4. Exhaust outlet must be slightly above the waterline with boat at rest in the water and full load aboard.
5. Back pressure must not exceed 4 in. (102 mm) of mercury when measured with a mercury manometer to exhaust elbow outlets.