Mondays 03/01/2010 Question and boating safety tip. ANSWERED

CaptainScott · Post by **CaptainScott** » Mon Mar 01, 2010 11:32 am

Safety Tip:
Knowing how to calculate how long it will take you to get from point A to point B can be incredibly helpfull! Getting back before dark, or befor a front hits can mean the difference between a happy crew and an angry mob! Sometimes you must make a decision as to go or not to go. Maybe your lights do not work properly and you want to get somewhere before dark. Using very basic math techinics and knowing a simple formula can provide to you some very important answers!!!

Around here in the San Juan Islands and Gulf Islands moorage can get very hard to come by in some of the popular harbors. Since most require you to vacate your transient moorage around 11:00 AM it is advisable for new arrivals to show up around noonish. This greatly inhances your chance of actually getting a slip!

Question:

You are currently anchored in a quiet cove with your bride. Tomorrow you must motor to Friday Harbor 12.5 miles away so you can recharge your batteries on shore power, restock supplies, use the showers, empty the holding tanks, and get more rum. Since you wish to get an overnight slip you want to arrive at 11:30 in hopes of getting a recently vacated spot. Tomorrow will be warm and sunny with no wind or currents so you will need to motor the entire way. Your new environmentally friendly 8HP 4 stroke motor will push you comfortably along at 5.5 kts SOG

At what time do you need to leave your anchorage tomorrow morning so you can arrive at the designated time?

lecker68 · Post by **lecker68** » Mon Mar 01, 2010 12:42 pm

I would say after converting miles to kts the trip if you did not have any current and it would take 2hr and 40 min so you leave at 9:20 unless you are close and crossing a time zone.

tgentry · Post by **tgentry** » Mon Mar 01, 2010 1:21 pm

I missed the nautical mile to statute mile conversion. Very tricky, Cap'n Scott.

Alanhod · Post by **Alanhod** » Mon Mar 01, 2010 6:35 pm

CaptainScott · Post by **CaptainScott** » Thu Mar 04, 2010 7:10 am

ANSWER:
I must leave my anchorage at 9:31 AM!!!
Here is why. . . . . .

As noted, I provided statute miles for distance and nautical miles per hour of speed. We must work in all statute miles or all nautical miles. Either way works. Since I do all speed calculations and displays in nautical miles that is what I prefer to use.

First step is to convert my distance from statute to nautical miles.

1.15 statute miles(miles) = 1 nautical mile (nm)

so . . . .

miles/ 1.15 = nm
12.5 / 1.15 = nm
12.5 / 1.15 = 10.87 nm rounded to 2 decimal points.

Next step is you need to caculate the time it will take to travel 10.87nm
the formual used is :

T= 60D/S

where:
T= Time
D= Distance
S=Speed
and the 60 is to convert all time values to minutes. This makes for easier math later.

So now, just plug in the data and run the math!

T= 60 x 10.87 / S -----Swap the D for the Distance calculated from above
T= 60 X 10.87 / 5.5 ---Swap the S for Speed in the question
T= 652.2 / 5.5 ---------multiply 60 times the Distance
T= 118.58 --------------Time now equals 118.58 minutes!
T= 119 Minutes! I rounded out to remove partial minutes. No need for that!

TIME to travel is 119 minutes or 1 hr and 59 minutes

OK then since I did not ask HOW LONG it will take to get there I asked when do I have to leave by to get there.

Since I wish to arrive at 11:30 and it will take 119 minutes to get there I must subtract 119 minutes from 11:30.

Which tells me I must leave my anchorage at 9:31 AM!!!

Scott

PS: Yes Alan, that was a great overnighter! Our slip was not ready for us so we took a mooring bouy. Poor Alan was stuck listening to me yammer all evening! Heh heh heh!

lecker68 · Post by **lecker68** » Thu Mar 04, 2010 8:01 pm

Oh wow I would have gotten you there 10 min early "better early than late".