Longitude

Posted by Director of Education on September 18, 2014 under About NauticEd, Bareboat Charter, Celestial Navigation, Coastal Navigation, Skipper | Comments are off for this article

The Mystery of Determining Longitude

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Have you ever been to Greenwich? If you haven’t, make sure it is on your bucket list. Not only is it a delightful place with a spectacular view of London and the Thames River, but it is the real birth place of “Longitude” and home to some of the most amazing master mathematicians of the burgeoning modern times of the 17 and 18oo’s.

Greenwich

The zero degree line of longitude was selected and placed in Greenwich in 1751 and then later in 1851 it was moved a few feet to align with a new telescope placed at the now zero deg.  zero minute, zero second longitude position. This designated line placed on earth is a humankind determined and positioned line. It stretches from the true north pole to the true south pole.

Meridian Marker

The zero mark could have been placed anywhere on earth. It was placed in Greenwich because of the incredible work being done by the Royal Observatory mathematicians and astronomers. They needed to set a zero point and what better than exactly underneath their telescope  The axis of spin of the earth also passes through the north and south poles. This is unlike the zero degree latitude line of the equator. This line is a universally given line and can not by randomly placed by humankind.

George Airy

In the 1700’s in bars and cafe’s all over the topic of the times was that determining Longitude on the ocean was akin to perpetual motion – it was seemingly impossible. Yet the challenge was there and £20,000 had been put up as cash incentives to inventors to help solve the issue. £10,000 of which was ordered by the King as prize money to the person who could solve it. Longitude was that illusive!

The Longitude Act

If you understand that in those days because of ships not knowing exactly where they were, there were many shipwrecks costing lives but also huge amounts of money. Thus global positioning was imperative. Latitude had been pretty easily solved much earlier. If you do a noon shot of the sun to determine its angle above the horizon then compare this with tables of the suns angle on a specific day of the year you get your Latitude. But longitude had no such luxury of determination.

I had the luxury of recently visiting Greenwich and was delighted to see and learn about one of my favorite topics. Longitude. Here I am with one foot in the eastern hemisphere and one in the western hemisphere.

The zero degree Meridian Line in Greenwich

The zero degree Meridian Line in Greenwich

What was particularly awesome about my trip was that the Maritime Museum in Greenwich was displaying a whole history tour of “Longitude” and I was actually able to see the time pieces that John Harrison invented to allow ships to keep time at sea and thus solve the Longitude problem and collect the Kings prize.

I highly recommend this short read of the book Longitude.

Variation aka Declination

We discuss Variation/Declination heavily in our NauticEd Coastal Navigation Course. This is the difference between the charted true north and what will read on your compass due to the earths magnetic poles not aligning with the axis poles. Interesting enough, one means used to determining rough longitude int he old days was to take a measurement of the variation. True North can easily be measured by the north star in the Northern hemisphere and magnetic north by a compass. This difference combined with a variation map of the world allowed approximate longitude determination. The method was rudimentary at best and provided no accuracy.

On that same topic and quite interesting also is that a very accurate global map of variation exists today and is programmed into every magnetic field chip used to determine compass directions. Your smart phone can tell you magnetic North and this is measured by the electronics measuring the earth’s magnetic field. To go from magnetic north to true north, the phone needs to know where on the planet it is. Once this is known (usually via GPS measurement) it applies the known table of variations and can then show also true north.

Here is also a great website resource to see your position on earth but also find your variation.

http://www.ngdc.noaa.gov/geomag-web/?id=ushistoricFormId#declination

It’s astounding to know that these guys back in the 1700’s knew all this and were the discoverers of this knowledge. Another part of this whole story is bought to life by Cook’s first voyage in 1769 to Tahiti. He was sent there by the Royal Astronomical society to measure the times that Venus was to transit the sun. This was predicted by Haley more than 40 years earlier. By comparing the measurement of times of the transit from many different locations on the planet, the mathematicians of that time were able to calculate the distance to the sun. AND the nailed it within hundredths of a percent.

Here is the picture I took of Cook’s Monument in Tahiti at the place where he measured the transit of Venus across the sun in 1769.

Cooks Monument in Tahiti

Cook himself used John Harrisons Chronometer (Watch) on his second voyage around the world. Cook’s logbook notes about 6 months into the trip, said that he believed more and more that Harrisons time piece was to be the way of the future.

Harrisons H5 Longitude

I really hoped you enjoyed this little trip through the discovery of knowledge that we all use today virtually in our everyday lives. Whether we are aware of it or not, Latitude and Longitude is touching us manytimes every day in our every way.

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Grant Headifen
Global Director of Education

 

P.S – If you are interested in Navigation, take our Coastal Navigation Course

Coastal Navigation Course

Coastal Navigation Course

PRESS RELEASE: New Electronic Navigation Course Released

Posted by Director of Education on December 11, 2012 under Skipper | Comments are off for this article

Fresh off the ePress – Electronic Navigation Course
A few weeks ago we announced our coming Electronic Navigation Course AND NOW it’s here and available online AND you can get it for FREE.
Electronic Navigation Course

Electronic Navigation Course

If you’re considering chartering a bareboat, do local/long-distance cruising or racing then this is the course for you.

ABOUT THE COURSE:

The Course is Laid out in Eight Modules

Module 1 emphasizes that electronic navigation is an aid to the good sailor’s senses but can not replace them.

Module 2 introduces and brings back some of the basics that you should already be familiar with in regards to navigation.

Module 3 is an in-depth discussion of wind. In particular it delves into the calculation of true wind and shows how important true wind direction is when navigating.

Module 4 is all about boat speed. How to navigate using optimum speeds and how to find your best course to achieve your destination in the fastest time. We define velocity made good on course and velocity made good upwind.

Module 5 prepares you for the shotgun of jargon that will be delivered in module 7.

Module 6 introduces technologies such as AIS, RADAR and Weather GRIBS and electronic chart overlays.

Module 7 is a step by step walk through of a real GPS chart plotter unit. You’ll gain the confidence, working knowledge and user experience, through various animations, to fully work a chart plotter device and apply this to your sailing navigation.

Module 8 will round everything out so that you’re confident in your ability to navigate using electronic instruments.

This course is one of our best and is a must take for anyone. It’s loaded with interactive animations of using an actual B&G GPS device. No matter if you are a beginner or a master, we guarantee you’ll learn some really cool hi-tech stuff and some essential stuff. AND we’ll make you look really smart in front of your boating friends.

Here’s how to get this $29 course for FREE

This course is a great value at just $29 and now it is on an introductory offer for just $25 … but when you complete your investment in the Bareboat Charter Master bundle of courses you get this interactive Electronic Navigation course for FREE. This means the Bareboat Charter Master bundle has now over $50 in savings. If you’ve already invested in some of the courses, that’s ok you get $ credit for those courses already in your curriculum. Same goes for the Captain’s bundle. When you login and click on register for the Bareboat Charter Master of Captain Bundle you’ll see the credit issued to you if you’ve already invested in some courses previously – Login Now!

More About This Course

This last week I was interviewed by Smithsonian Magazine on the topic of “Should sailors rely on gps units”. My response was two fold:

  • It’s inevitable that people will use Electronic Navigation and it provides some amazing tools to make our oceans safer and more fun.  And, so we as educators, must take the responsibility to properly teach what students will seek to use, regardless.
  • Educators have to ensure that students still understand the fundamentals and so a course that demands both is paramount.

About 2 months ago we embarked to write this course and we received tremendous support from a proponent B&G who are one of the premier manufacturers of chartplotters. B&G provided a lot of content and images for the course. They lent us one of their Zeus Chart Plotters from which we created dozens of animations to really lead someone into the depths of a chart plotter set up and how to gain the best practical and applicable data from it.

We felt that such a course was paramount to modern sailing. So much so that we logically had to make this information part of the Bareboat Charter Master list of courses. Almost every charter boat in the world has a GPS unit and a very large percentage of private boats have some sophisticated electronics installed. Why then, we asked, was every other sailing education body in the world ignoring this topic? We don’t know the answer to that. Perhaps that they’re old and stuck in their ways … nah couldn’t be!

You’re really going to enjoy the course  – it’s the only one of its kind in the world and I guarantee you’ll learn some really big gems out of the course whether you’re a racer or cruiser (or crew).

Register for the Bareboat Charter Bundle of Courses with a $50 savings

Register for the Electronic Navigation Course

 

Sailing in Tahiti: Navigation Tips

Posted by Director of Education on June 30, 2012 under Skipper | Comments are off for this article

June 2012: This last week, the NauticEd crew headed out on our annual Bareboat Charter Master Graduation Sailing Trip. This year we chartered catamarans from Dream Yacht Charters in Raiatea,  about 200 km North West from the main island of Tahiti in French Polynesia. To get there you need to fly to Papetee on the island of Tahiti then take a puddle jumper for about 45 minutes to Raiatea. The Dream Yacht Charter Base is about 5 minutes from the Raiatea Airport but we stayed our first day and overnight at the Hawaiki Nui Hotel another 5 minutes away. I highly recommended the Hawaiki Nui on trip advisor – the huts were awesome and the food was great as with the staff. We did a night scuba dive on a wreck just off the dock at the Hawaiki Nui in about 60ft of water. It was a pretty cool dive and incredibly convenient – the dive shop is right at the hotel. By the time we finished the dive, the hotel was in full swing of a very lively and traditional Tahiti Dance show.

Hawaiki Nui Hotel - Raiatea

Jerome from Dream Yacht Charters showed up earlier that day and gave us an excellent chart briefing of the sailing area in preparation for us to pick up our catamarans the next day. Jerome was incredibly friendly and gave us a PDF of the chart briefing which we were able to put onto our iPads.

Saturday morning we split up into 2 groups – 1 to get squared away with the boats and 2 to attain provisions. Then by about noonish we were ready and we all set off.

Sailing in the area was very different than typical Caribbean sailing. First off, the place is sooooo beautiful that I ended up taking over 1000 photos with my Nikon D3100. Each island is surrounded by an outer ring reef and so the sailing that you do is either inside the lagoon or in serious open water between islands.

Bora Bora Lagoon

In the photo above you can safely sail in the blue water shown inside the lagoon. Although as you will discover safely is a relative term.

Sailing in the Lagoons

Above, one of the Dream Yachts Catana Catamarans in our NauticEd flotilla fleet.

Below, waves crashing on the outer reef. You’ve got to enter the lagoon only through the marked passages.

Waves on the reef at Bora Bora

Sailing inside the lagoons present a challenge – it’s not necessarily a hard challenge but certainly an awareness challenge as shown by the following photo  which we snapped late one afternoon. The powerboat is pulling on a halyard to heel the sailboat over whilst the sailboat is trying to kedge out off the reef with the anchor and windlass. NOT GOOD.

On the Reef

In this photo below you can see extreme shallow water right next to the darker deep blue water.The photo a very typical scene – a well marked reef/shallow area next to an indescribably beautiful turquoise color with a deep rich dark  blue in the foreground. The turquoise represents shallow water while the dark blue runs deep. You can notice a distinctive transition line between the colors. This is because the depth changes almost instantaneously. IE you get no warning from your depth meter. The front of the boat can be in 6 feet of water whilst the aft can be in 50 ft.

Turquoise and dark blue water

This photo below was a depth transition from 20 ft of water to 3 ft.

Depth transition highly visible by color change

Navigation markers however are reasonably abundant and so the navigation sailing lesson begins here.

Green Navigation Marker - Green to Sea

The rule to follow in French Polynesia when sailing around the island is to alway keep the green triangular navigation markers to the seaward side of the vessel and keep the red square markers to the island side of the vessel. For example if you are sailing so that the island is on your starboard then you keep red markers on starboard and green on port. If the island is on port then keep red to port and green to starboard. This rule will keep you in the navigable waterways.

Keep red to the island side

An exception happens when you approach an entrance or exit channel. These are marked by channel red and green markers and exhibit no difference to the island waterway markers. The Pacific (except Japan) use the IALA-A system of markers and so when entering a channel you must keep red on your port and green on starboard. IE it’s NOT red right returning. You must not confuse the green and red  entrance/exit channel markers with the island waterway red and green markers. (In the USA intercoastal read and green markers have a yellow square on the marker to separate them from the red and green channel markers. Most other places in the world don’t do this).

Entrance channel to Bora Bora

When entering a channel there is usually a set of transit markers with a compass heading marked on the chart. This entrance into the Bora Bora lagoon is marked as 112 deg. So you just line up the transit markers on shore and follow the compas heading in. To exit you just take the reciprocal heading.

When the Transit Markers align you're in the center of the entrance channel

In French Polynesia they also use the handy Cardinal marker system.

How to read Cardinal Marks

These also aided our navigation. This one below told us to stay west of this mark. You can see the shallow water on the other side of the mark.

West Cardinal Mark

But from a distance this one looked like an isolated danger mark because we could not see the 2 upward triangles especially with the huts in the background. This mistake got us into some shallow water and we had to turn back. Additionally this was not placed on the chart.

A cardinal mark but difficult to see its marking

As we sailed around in the lagoons we soon learned that a more than keen lookout was required to spot the marks and look for water color change. What was interesting was the Jerome from Dream Yachts completely discouraged us from using the GPS. He said he’d rather us rely on keeping our heads away from a screen and looking at the water and navigation marks. We took note and sailing the whole week with out a GPS. Even when returning from Bora Bora to Raiatea we used line of sight 3 position fixes via compass to plot our position on the chart. It was good practice and we nailed the entrance. Cook would have been proud.

Crew Spotting the Navigation Marks

Another indication of shallow water is a bit more rudimentary but work none the less is that the locals have gone out and stuck sticks into the reef.

Sticks work as Navigation Markers

The shallow water is pretty obvious above in good light conditions but less obvious as the light changes and so the sticks below were a good help.

Sticks work as Navigation Markers

Dream Yachts provided us with some very good and accurate charts.  So fortunately the whole navigation exercise was not that difficult but certainly we’ve tried to instill  above:

  • you really need to know the markers,
  • know where you are at all times
  • keep a constant watch out 100% of the time
  • use the overhead light to identify shallow areas
  • use other signs and indicators

We highly encourage you to get on down to Tahiti for an incredible sailing adventure overall we had a real blast and so did the kids that we took along.

Vivian, Alexandra and Evan Sailing in the Bora Bora Lagoon

A few highlights of the trip was a visit to the Anapaperles Pearl farm.

Pearl Farm Hut (complete with convenient rainbow)

Inside looked something quite different.

Inside the Pearl Hut

And right underneath we went for a 30 minute snorkel through the oyster nets where the oysters grow and produce the pearls.

Oyster Nets

Another highlight was one of the best snorkelling experiences I’ve ever done (and I’ve done a lot). We snorkelled between two Motos (Islands) where the current runs at about 1 knot in a depth of about 5 feet. The area is called the coral river and it is loaded with fish and coral heads. Here’s a short video with a fun end.

Thanks to the Mai Kai Marina and Yacht Club in Bora Bora. They were incredibly hospitable. Filled us up with water and some very nice Mai Tai drinks and excellent food. We well worth while stop. They are on Bora Bora Island on the left just after you enter the main entrance into the Bora Bora Lagoon.

Mai Kai Marina and Yacht Club

 

The final coup de grace of the trip was to visit the place on Tahiti Island where Captain James Cook witnessed the transit of Venus across the sun in 1769. He’d been sent to the south pacific to witness the event and take time measurements so that the Royal Astrological Society could use the data to calculate the size of the solar system. Ironically this event reoccured the week prior to our visit to Tahiti.

Monument to Cooks Effort in Observing the Venus Transit

Monument to Cooks Effort in Observing the Venus Transit in 1769

 What next?

Take the NauticEd Coastal Navigation Course – it is definitely what is needed to instill the confidence to do a trip like this.

Contact us if you’re interested in sailing vacations in Tahiti or beyond – we can arrange the trip for you,  give you some good advice on getting there and places not to miss.

How to Get to a Waypoint

Posted by Grant Headifen on September 11, 2010 under Bareboat Charter, Celestial Navigation, Coastal Navigation, Crew, Skipper | Be the First to Comment

Recently on our NauticEd flotilla with the Moorings to the Kingdom of Tonga we wanted to pass through the Fanua Tapu Pass which is a gap in the reef to get to the eastern islands of the Vava’u archipelago. Normally the gap is marked by a series of buoys, however the latest storm ate them.

The pass is well documented by two waypoints. Traversing using a GPS map, however, was out of the question because Tonga is one of the last places on earth to be accurately placed on the world coordinates. Yes you’re reading that right – the islands are not actually where the maps say they are and especially reefs and rocks are not where they are positioned on the maps. That’s pretty absurd for this day and age but it’s true. Call the Moorings base in Tonga for your self.

Tonga-fanua-tapu-pass

Tonga-fanua-tapu-pass

Navigation is performed using good old eyesight (some of our eyesights are older than others) coupled with map reading skills, a depth sounder and a keen watch out on the foredeck.

So anyway we had to get the first waypoint dead on to pass through the reef. The first waypoint was 18 deg 43.914 min South and 173 deg 59.12 min West. Our position was 18 deg 44.902 South and 173 deg 62.014 West.

So it’s a bit funny trying to hit a point like this because you’ve got to be able to work with a few obvious things but understanding the principles makes it much easier. First you’ve got to know which directions you need to be heading based on the hemispheres you’re in.

In the northern hemisphere to increase the latitude you’ve got to head north but in the southern hemisphere to increase latitude you’ve got to head south.

Similarly,  in the eastern hemisphere to increase longitude you’ve got to head east where as in the western hemisphere to increase longitude you’ve got to head west.

OMG how do you remember that? Especially in the heat of the moment with waves and rocks all around you and your life depending on it.

I’m sure there is a memonic for it but it’s best to understand the principle first and below is the way where I can best understand it. For me, I find that principles are better than memonics.

Imagine you’re standing on the intersection of the prime meridian (below grenwich) and the equator. You’re at 0 deg Latitude and 0 deg Longitude. Move in any distance to the North and the latitude increases North. Move any distance to the South and the Latitude increases South. Now place yourself at about 17 degrees south latitude. Move North and you’re moving towards the equator and towards 0 deg Latitude.

So in principle then, if you understand this; move towards the equator  you’re decreasing the Latitude no mater which north or south hemisphere you’re in. So in our example above our latitude was greater than the waypoint so we needed to head towards the equator. We were in the southern hemisphere so we needed to head north.

IE when dealing with latitude – just figure out if you need to head towards the equator or not. That should take care of that from an understanding principles point of view.

Longitude. Back to our Prime Meridian/Equator intersection. Looking towards the North pole, everything towards your left is West Longitude right? And everything towards the right is East Longitude. So anything from England, past the Americas and all the way around to Hawaii is West longitude. Any everything from England, past Asia and all the way to Australia and New Zealand is East Longitude. This is why the USA is known as western society and Asia is known as eastern society.

So now you just got to know where you are East or West. In Tonga we were on West Longitudes. So anything back towards the Americas or England from that point was decreasing the Longitude numbers towards the zero prime meridian in Grenwich. Which meant to get to our waypoint we had to head East to the America’s.

So overall we needed to head to the North and to the East. Next we looked at what was the relative differences between desired and present positions for latitude and longitude. The longitude difference was about 3 times that of the latitude difference. This means we needed to head more east than north.

In the old days (I mean the old old days) before longitude could accurately be determined, traders would head north from Africa and purposefully miss England far far to the west of England. Then once on the latitude (easily discovered by the angle the north star makes with the horizon) they would then travel East. This ensured they would miss all the potential dangers. Hundreds of ships were being lost due to the difficulty in accurately determining the longitude. In early 1700’s the King of England offered a 10,000 pound reward to figure out how to accurately determine Longitude. For those of you interested, watch the history channel show on this or read the book “Longitude”. Both are excellent!

 

So lets go back to the principle. Where ever you are you should establish this before any issues come up. IE if you’re on a bareboat charter – answer these questions before you leave the base.

Am I in the southern or northern hemishere? Then based on that, embed into your head which way do you go to increase/decrease latitudes. Should you head towards the equator or away.

Am I in Eastern Longitudes or Western Longtiudes? Then decide which continent you should head to decrease or increase longitudes.

Here’s a little test then. You’re in the Aegean Sea at:

36 deg 56 min North Latitude, 27 deg 19 min East  Longitude

you want to get to:

36 deg 57.897 min North Latitude,  27 deg 17.295 Min East Longitude.

Which way should you be heading?

Simple enough – we’re in the North Eastern hemisphere. We want to increase the latitude so we need to move away from the equator and thus head north. We also want to decrease the longitude and head towards Grenwich England which means head west.

Both are almost 2 minutes in difference and so the VERY APPROXIMATE direction should be North East. We say VERY APPROXIMATE because the latitude lines and longitude lines are not the same distance apart and vary according to latitude. The closer to the poles the closer are the longitude lines. Therefore the heading would be more north of northeast.

In this blog we’re placing quite an importance on this concept. The reason being is a funny (potentially not so funny) story attached. On the Tonga trip one of the crew was an ex Airforce Navigator. He got turned around for a second in the reef because we were heading east to reach the waypoint but his brain was telling him to head west. The reason is that he was used to Navigating around New Zealand which is in the Eastern Longitudes. Tonga is just on the otherside of the 180th Meridian in the Western Longitudes. Whoops being turned around in the middle of reefs is NOT good. There were rocks all around us and correct decisions had to be made fast.

OK and here’s a real scenario to scare you into taking this blog and the NauticEd sailing simulator serious. A family member falls overboard at night and you hit the MOB button on your hand held GPS. You’ve got the lat and long where they went over. By the time you get turned around and the sails down with all the confusion – all you’ve got is their lat and long and yours and a compass. How do you save your family member’s life?

MOB is at 16 deg 33.250 min N and 62 deg 11.501 W

You are at 16 deg 33.200 min N and 62 deg 11.595 W

Which way do you head? Quickly now the current is drifting them away from that position.

Leeway and a Bottle of Rum

Posted by Director of Education on June 22, 2010 under Bareboat Charter, Coastal Navigation, Crew, Skipper | Be the First to Comment

Leeway is just one of those things that is a law of the universe that we have to put up with. It’s just like gravity. Still with gravity – the advantage is that it’s highly predictable. And so then is leeway.

Leeway is the sideways slip motion of our sailboat down wind from the pressure of wind against our boat and sails. It results in a course that is less than desirable.

Leeway Slips Your Boat Side Ways Down Wind

Leeway Slips Your Boat Side Ways Down Wind

Airplanes suffer from the same issue. When flying in a cross wind, the plane crabs (slide slips) downwind. The course becomes different from the heading.

Not accounting for leeway will have you sailing (or flying) in a fairly unnoticeable arc to get to the mark. To represent an example with a mark to the north and a westerly crosswind, here’s what happens; you aim for the mark at 000, your boat slips sideways to the west. Now your mark is at 359 but you don’t really notice it. After a few minutes your mark is at 358 still in noticed. Minutes later your heading is 355 then 350 etc. All because you keep aiming at the mark but you’re being pushed to the east by the wind. Your course over ground becomes an arc and is the long way around.

The prudent sailor will account for the leeway and sail a constant heading depending on their known leeway of say 350 for the example above. The sideways slip motion will deliver them to the mark in a straight and shortest line.

Now that we’re in the electronic age, navigators will plug in the destination to the gps. The autopilot which is cross talking to the gps takes care of the rest. The gps analyses the cross track (the boat’s distance away from a straight line to the mark) and feeds back to the autopilot the proper heading to minimize this in real time. Thus resulting in a straight course to the mark.

I’m doubt that during the Wednesday/Friday night beer can race such electronic methods are utilized. So I’m suggesting that to take line honors and win the bottle of rum at your club race by taking account of leeway.

Leeway is particularly more prevalent when you are sailing on a close haul or close reach and can be as much as 20 degrees depending on the wind conditions, water conditions, your sailboat design, your apparent angle with the wind and how your sails are set.

However, other than buying a new boat, the only thing that you have control over is the trim of your boat and sails.

Here’s a couple of general rules to follow:

  • Over sheeted sails cause more sideways force and thus sideways slip (leeway). Fly the telltales diligently.
  • Aim for a position to windward of the mark you’re trying to go around. The more you are sailing on an upwind course, the more the degrees upwind you should aim.
  • The higher the wind speed, the higher above the mark you should aim.
  • In general, on a close haul, allow 10-15 degrees. Adjust this less if the wind is light, more if the wind is strong. Reduce this amount linearly as you bare away from the wind.
  • Make sure your boat is trimmed with slight weather helm.

When doing serious navigation we absolutely must account for leeway and an excellent understanding of how your boat performs leeway wise is essential and how to solve for it once you know it. NauticEd developed an educational navigation video solving a leeway and current exercise at http://www.youtube.com/watch?v=1LQcFOGSJQs

Using a gps and a nice steady windy day, you can do a simple determination of your sailboat’s typical leeway.

(1)   Begin sailing on an angle slightly off a close haul direction and with a recognizable land marker dead ahead.

(2)   Measure your speed

(3)   Douse the sails and begin motoring at the same speed in exactly the same direction.

(4)   Take note of your gps course.

(5)   Deploy the sails and turn off the engine.

(6)   Continue to aim for the same point on land.

(7)   Now read out your gps course.

(8)   The difference in course angles will be your leeway

(9)   Repeat for different points of sail

(10)    Repeat for the opposite tack.

(11)     Repeat on different days with different wind strengths

Note that:

(a) this method is relatively immune from current because you have normalized it out by performing the motoring task.

(b) this method will not account for the leeway due to the hull of your boat presented to the wind.

We hope you enjoy your bottle of Rum!