Introduction
When estimating a visual surface target’s angle on the bow (AOB), the periscope operator relies upon experience. The more quantitative method described in this article is based upon the ranging methods described in NWP 71-1-1 and NWP 77, and may provide a quick sanity check for the periscope operator. All information taken from these NWPs is unclassified.
Method
The range (Rb) of a visually observed surface contact is determined using the formula:
Rh = K x MHH/#DIVv—– (1)
where MHH is the estimated masthead height, #DIVv is the number of vertical division, and K is a constant. However, this equation is also valid for the horizontal divisions:
Rh = K x MHH/#DIVv = K x Len/#DIVh—– (2)
where the relationship between estimated length (Le) and effective length (Le,,) is shown in Figure 1. Mathematically,
sin (AOB) = Lem/Lem—– (3)
Equation (2) can be rearranged and solved for Leff as follows:
Leff = MHH x #DIVh/#DIVv—– (4)
Equation (4) is solved to determine Len-, which is then used in equation (3) to find the sine of AOB. The sine thumb rules of NWP 71-1-1 are then used to find AOB.
Estimated Target Length
You will need to know the masthead heights and lengths of surface ships you may encounter in your OP AREA. Based on intel, you should know what to expect, and the ship dimensions are available in your handy-dandy copy of Jane’s Fighting Ships. In the absence of this information, use the estimated combatant dimensions in Table 1. These dimensions are based on a quick perusal of Jane’s Fighting Ships, so do not treat them as gospel.
Over-estimating target length leads to a smaller estimated AOB, which is conservative, as shown in Figure 3.
| Surface Combatant Type | Lemt |
|---|---|
| Cruiser | -600′ |
| Cruiser | -600′ |
| Destroyer | -500′ |
| Frigate | -400′ |
| Coastal Patrol | -150′ |
