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Catalina/Capri 25/250 Sailor's Forums

General Sailing Forum

Boat speed tip

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jsummerfield
1st Mate

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USA
96 Posts

Initially Posted - 12/25/2003 : 21:52:56

Instead of posting on the VMG topic, I think that other navigation issues are worth discussion. Simple speed measurement is an issue on "Texas Tango". Perhaps the VMG post raises simple boat speed issues for other sailors. I will describe a few things tried on my boat. Perhaps others can post a few tips too.

I sometimes measure velocity made good based upon timing between two fixes. Lacking a knot meter and not using a GPS lately, I sometimes practice traditional sailing techniques to estimate boat speed. Most of these tricks are easier with two people and some hand signals.

One technique uses something biodegradable like an apple that doesn't require retrieval. Throw the object overboard, and start timing from the point of the bow to the point of the stern. Calculate the speed knowing the fixed boat length and time. Another technique forms a simple speed log by tying a floating object to a length of line. The object can be thrown overboard and recovered. With the tied object, see how long it takes the line to go taut. Use the speed, distance and time arithmetic to obtain speed. The measurement technique provides feet or meters per second. The conversion from meters per second to kilometers per hour simply requires multiplying the meters per second by 3600 seconds to get meters per hour then divide by 1000 to get kilometers. One nautical mile is 1.852 km. Thus, there is 0.5399568 or about 0.54 nautical mile per km. From km/hour, if knots are desired, multiply by 0.54. Convert the English units to calculate knots (nautical miles per hour) as follows:

There is 5280 foot per statute mile and a nautical mile is 6076 foot. There are 60 seconds per minute and 60 minutes per hour; multiplied to obtain 3600 second per hour.

( L ) ft 3600 sec/hour
V nm/h = ----------------------------
6076 ft ( t ) sec

Note that the ratio of 3600/6076 = 0.592 or about 0.6

0.6 ( L ) ft
V nm/h = ---------
(t ) sec

V is your speed in knots, L is the length of the line or boat in feet, and t is the time in seconds. In calm weather, it is possible to spit off the bow of the boat and time the bubbles to reach the stern - in which case L is the length of the boat or 25 foot LOA. A displacement hull boat speed is generally limited by the length of the bow wave along the water line. The formula for this speed is hull speed in knots equals 1.34 times the square root of the length at the water line, LWL. “Texas Tango, at 22.1667 foot LWL, has a theoretical hull speed of 6.3 knots. I would estimate well below hull speed for navigation assumptions.

What other navigation tricks might be interesting to try?

John
C25 3973 FK/SR
"Texas Tango"

Edited by - on

coach
Navigator

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USA
231 Posts

Response Posted - 12/26/2003 : 00:15:50

Waaaaaay too much thinking going on in that post!

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triley
Captain

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USA
251 Posts

Response Posted - 12/26/2003 : 09:27:50

Coach

Can you fix your JB quote to "TOO late". Just being pedantic

tom

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Derek Crawford
Master Marine Consultant

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USA
3324 Posts

Response Posted - 12/26/2003 : 14:35:46

Another useful number to remember is that 1 knot is equivalent to 1.7 feet/second. At 5 knots boat speed you travel 1 boatlength in a fraction under 3 seconds!
Derek

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JimB517
Past Commodore

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USA
3285 Posts

Response Posted - 12/29/2003 : 13:02:46

In my limited experience, the C25 starts to make a stern wake and the tiller/boat feels like it's coming alive at 3.5 knots.

At 5 knots there is a noticable stern wake with 2 waves perpendicular to the stern of the boat about 10 to 20 feet off the stern. And the boat feels like it's moving fast. There is also a noticable "roaring" sound from the stern

At 6 knots you are throwing spray from the bow, noticable bow wave, feeling weather helm, sheeting out in the gusts, I'm thinking about reducing sail.

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JimB517
Past Commodore

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USA
3285 Posts

Response Posted - 12/29/2003 : 13:12:01

Here's another useful coastal navigation tip, it's called doubling the angle.

If you measure the angle off your bow to an object on land (say a lighthouse or flagpole), measure the length of time it takes to double that angle. For example, if the angle off the bow at first is 10 degrees, time how long it takes for the angle to double, to 20 degrees.

The distance you sailed during this time is equal to your distance to the object.

This is usually used to measure how far away you are standing off from a headland or bouy (assuming you know your speed).

However, if you know the distance to the object (from a chart or by local knowledge) you also know how far you sailed during that time. You can then estimate your speed using speed = distance / time.

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The Officers, Staff and members of this site only provide information based upon the concept that anyone utilizing this information does so at their own risk and holds harmless all contributors to this site.