# CRESCENT BEAM

A beam bounded by arcs having different centers of curvature, with the central section the largest.

## Related Terms

### BEAM

The width of a ship. Also called breadth.

The width of a ship.

### HALF-POWER POINTS

Power ratios used to define the angular width of a radar beam. One convention defines beam width as the angular width between points at which the field strength is 71 percent of its maximum value. Expressed in terms of power ratio, this convention defines beam width as the angular width between half-power points. A second convention defines beam width as the angular width between points at which the field strength is 50 percent of its maximum value. Expressed in terms of power ratio, the latter convention defines beam width as the angular width between quarter- power points.

### INDIRECT ECHO

A radar echo which is caused by the electromagnetic energy being transmitted to the target by an indirect path and returned as an echo along the same path. An indirect echo may appear on the radar display when the main lobe of the radar beam is reflected off part of the structure of the ship (the stack for example) from which it is reflected to the target. Returning to own ship by the same indirect path, the echo appears on the PPI at the bearing of the reflecting surface. Assuming that the additional distance by the indirect path is negligible, the indirect echo appears on the PPI at the same range as the direct echo received. Also called FALSE ECHO.

A transverse beam placed over the transom with fairleads for mooring warps.

### SCANTLINGS

Dimensions of ships structural members, e.g., frame, beam, girder, etc.

### BEAM END

The side of a ship. 'On her beam ends' may mean the vessel is literally on her side and possibly about to capsize; more often, the phrase means the vessel is listing 45 degrees or more.

### FORE HORSE

A transverse wooden or iron beam afore the main mast to which the foresail sheet is attached.

### KEELSON

A lengthwise wooden or steel beam in ship for bearing stress.

### LOBE

The portion of the overall radiation pattern of a directional antenna which is contained within a region bounded by adjacent minima. The main beam is the beam in the lobe containing the direction of maximum radiation (main lobe) lying within specified values of field strength relative to the maximum field strength.