Monday, April 18, 2011

Shaft Zinc Installation Tip # 27

Apply a pinch of handitack on the zinc hardware nut to secure and prevent loss during the underwater installation. Keep available extra hardware in the event nuts or bolts are dropped to the bottom.

Friday, April 15, 2011

Dutchman's Log

When the knot log fouls and the GPS goes on the blink then there is always the Dutchman's Log for estimating the speed through the water. Just count the number of seconds a bit of floating material takes to travel from bow to stern to get knots.

Use the formula:
Boat Length in nautical miles / drift seconds from bow to stern = Knots / 3600 seconds in an hour

Example for 33 ft. boat given 6076 ft. in a nautical mile and 3600 seconds in an hour:

Formula: .00543 NM boat length / drift seconds = knots / 3600 sec.

Knots = .00543 x 3600 / drift sec.

Knots = 19.6 / drift sec.

Table of Speed Though the Water for 33ft. Boat

9s = 2.2 kts.
8s = 2.5 kts.
7s = 2.8 kts.
6s = 3.3 kts.
5s = 3.9 kts.
4s = 4.9 kts.
3s = 6.5 kts.

Saturday, April 2, 2011

Sextant Scope Filter

Index error (I.E.) can be determined more precisely during the day by using a custom fitted solar mylar film filter (the type used to observe a solar eclipse) over the objective lens on the sextant scope. The result will be a pair of sharp uniform orange Sun images. Overlap the Sun images for direct index error readings or take half of the difference between the two limb to limb readings.

-When off the arc the reading is 60' minus the micrometer value.

-Shading the sextant scope directly allows the horizon mirror shades to be pulled aside which opens up better access for the adjustment tool.

Thousand Oaks Optical (928)692-8903