Dead Reckoning

Dead reckoning is the process of plotting your position on a chart based on your course and speed from a known point. Of course it's never exact, because you have to estimate leeway and the effect of the tides. But for hundreds of years better sailors than me have used it to cross oceans and I wanted to get better at this skill. I realized how necessary it was on my only attempted long trip on my small boat. I had over estimated my speed and it wasn't until I got a hard fix from points on land using my hand-bearing compass that I realized how necessary getting a better estmate of the vessels speed was. Of course, as my wife informed me, I could just get a GPS. She didn't have any problem finding her way around Florida with her Garmin. The problem is that I'm kind of a nut and want to develop skills that don't rely on electronic devices.

In the age of the Tall Ships, a device called a "chip log" was used to measure the speed of a vessel under sail. This device consisted of a piece of wood, a long length of line and a 28 second glass. The first description of this device is from a book written by William Bourne in 1574 called, "A Regiment for the Sea". Fans of Patrick Obrien can see the device being used in the movie, "Master and Commander".

The whole concept of its use is based on the measurement of a nautical mile. Until 1954 the US considered 6080 feet to be a nautical mile, but the international standard 1.852km is now used. (Most of us use the chart to measure 1 minute of latitude, rather than the scale to figure out a nautical mile). In any event, using a chip log, a vessel travelling at 1 nautical mile per hour would take 28 seconds to travel 47' 3". A knot in the line would mark the first 47' 3". A vessel travelling at 2 nautical miles per hour would travel 94' 1/2" in 28 seconds. 2 knots would be placed at this second 47' 3" and so on down the log line. This is why we use the term knots, because literally sailors used a knot to measure distance and speed.

So how does this help me? I made a chip log that's how. In the JUL/AUG 2009 issue of Small Craft Advisor there are directions for making a chip log. I made mine out of 50' of tarred seine twine and a triangular piece of plywood I found in my workshop. Basically, rather than having several hundred feet of log line reeling off the stern of my 20' sloop, the article recommends using just 50' of line and a stop watch. It works the same way as previously described, except that we use 50' as the distance measured. Using this method a vessel travelling 1 nautical mile per hour will go 50' in 30 seconds. A vessel travelling 2 nautical miles per hour will go 50' in 15 seconds. I wrote down the distance/speed calculations on the wooden wedge, because I knew I'd forget them anyway.

Using the methods of sailors for the last three centuries, I'll check my speed using this device once every 30 minutes on a long trip, noting the compass course at the same time, then plot this on my chart. I'll report back how this goes during my next foray to Florida.

Davis Island 27 54'N 082 27'W

22 DEC 2009:

The weather was supposed to be overcast, so I decided to do a short sail in Hillsborough Bay rather than a longer sail in Tampa Bay out to Edgemont Channel. There was also the indication that the wind would be a steady 10 knots from the east, which meant that I could sail north up Hillsborough Bay on a long beam reach to Davis Island and back. Two goals would be accomplished; a) I would get a chance to look at the approach to the Davis Island boat ramp from seaward and b) I would get a chance to try a self-steering idea that I'd read about.

Anyone who has sailed a small boat single-handed will understand the need for some way to let the boat sail it's self. Whether its to fix something or just sit back and enjoy the sail without constant adjustment of the helm, the ability to just let go of the tiller without bad things happening is really nice. During the summer I had tried lashing the tiller with some success. In steady winds the boat holds course. But the slightest gust will have her turn up into the wind. Lashing the helm can only give you a short amounbt of time to deal with problems forward or to provide your arm some relief during a long sail.

I decided an auto-helm or a wind vane would be overkill on a small 20' sloop, so I looked into sheet to helm steering. Essencially, this means attaching the main sheet to the tiller and off-setting the pull with elastic of some sort. I learned about how to do this mainly from John Letcher's book, "Self-Steering for Sailing Craft". It's a great book and goes more in depth than I really needed, but he actually sailed across the Pacific using this meathod, so I thought he'd be a good guy to listen to.

The set-up is different based on the wind, but basically the elastic is tied off on the leeward side of the boat opposite the helm and then lashed to the tiller. A 1/4" line is then made off to the main sheet with a rolling hitch (or using a snatch block), then through a block opposite the helm on the windward side, then lashed to the tiller. The helm is balanced through trial and error until in a steady breeze, the boat will hold her course. If a gust comes up, the force of the wind on the sail will tighten the main sheet, pulling the helm opposite to and counteracting the force of the wind. If the wind lessons, the elastic will pull the helm back. The boat stays on course because of the two forces acting against each other.

John Letcher suggests surgical tubing makes the best elastic for the purposes of self-steering. He indicated that surgical tubing, though it degraded in the sun, did have the right amount of elasticity. I didn't have any surgical tubing. However, I did have a bungee cord and tarred seine twine. I figured if you have a bungee cord and seine twine you can do just about anything. I was wrong in this instance, but it usually is true.

On the way north toward Tampa, I didn't try to use the self-steering, I just enjoyed the sail. The sun was out, the wind was a steady ten knots and I sat back and enjoyed holding the tiller and getting used to the feel of the boat again. However, on the way back I decided to try the system.

I let out the mainsheet, so the main sail leuffed a little. Then I hooked the bungee cord to the leeward aft cleat and looped it around the tiller. Then, while trying to hold the boat more or less on course, I tied a rolling hitch with the 1/4" nylon line around the standing mainsheet, put it through a block attached to the windward aft cleat, pulled in the main sheet until it filled and then wrapped the tail line around the tiller. Unfortunately, it didn't work, though I saw why it should and probably will with different materials.

I tried for the better part of an hour to balance the pull of the elastic against the pull of the wind on the main sheet. The bungee cord was too strong though. Maybe in high winds 20 knots or better, the bungee would have worked, but I need something just as elastic, but not as strong.

Surgical tubing ....

Hillsboro Bay 27°50'N 082°25'W

20DEC2009:


I checked out the boat on Saturday (19 DEC 09) and found that I had to replace the jack on the trailer. After failing to find what I needed at the local trailer supply place, the local farm supply place, the local Auto Shop and the local WalMart, I finally found what I was looking for at the local Home Depot. Though the WalMart turned out to be a good stop because I scored #36 tarred seine twine for $7.36 for a 1lb roll. Apparently they use it for sport fishing down here. Who knew?


I checked NOAA and got the following for Tampa Bay; "SUNDAY NORTHWEST WINDS 10 TO 15 KNOTS. BAY AND INLAND WATERS A MODERATE CHOP." NOAA also indicated the following tides at Edgemont Channel for the 20th; High: 12:28AM and 03:21PM, Low 07:17AM LST 06:12PM with a tidal rise and fall of around 1.5 feet. The tides hit the Alafia River, my intended ramp, about 2.5 hours after the entrance to Tampa Bay at Edgemont Channel.


The tide tables always used to confuse me until a captain on the schooner Pioneer asked me what the tide was at Hell Gate. As mate I'd never checked Hell Gate before, I mean who cared we only looked at what the tide was doing at the Battery since that was where we were going. However, the tide at the entrance to the bay - or the two entrances to New York Harbor in this case - determines the tides for the bay. It made sense to me only when I thought about it as an incoming wave or as water being drawn out to the sea, rather than as a rise or fall in the Bay itself. Since then I've always checked the tide at Edgemont Channel rather than at the ramp in Tampa Bay I might use. It makes me more aware of and observant to what the tide is doing.


As I indicated I put into the Alafia River boat ramp around 1200 on Sunday the 20th. There were few other boats at the dock and the air temperature was only 55 degrees when I put in, so I donned foulies. For some reason, I did not put a reef in the mainsail. Because Marjorie is so tender in a fresh breeze, I usually tuck a reef in the main if there were projected gusts to 20 knots, but I was somewhat overconfident. The wind appeared out of the north and I thought I'd try sailing off the dock, so I ran the docklines over the cleats and back to the boat, then raised the main and jib. Unfortunately, by the time I got the engine started, the wind had veered to the west and had pinned me to the dock. I slipped the bow line, backed down on the stern line until the wind caught the jib, then cast off, put the engine in neutral and sailed down the river and out the channel.


Once clear of the Alafia, I realized almost immidiately that I didn't have a good feel for the boat. I wasn't anticipating the wind and I didn't have a good feel for the tiller. I hadn't sailed at all in two months and I hadn't sailed Marjorie since the summer. I was rusty and decided to do a couple easy tacks across Hillsborough Bay just to try and get back the muscle memory. The winds were gusty and the scattered white caps did seem to indicate that the wind was around 15 knots. But the wind was out of the northeast, while the waves were larger than they should have been and were out of the north (toward Tampa and the head of the Bay). I was expecting around 2 - 3 foot waves - and I certainly had them here - but every ten waves or so was a 4 - 5 foot wave. There was no traffic in the bay, so it wasn't wakes. I still haven't figured it out.


About half way across the Bay, heading toward Ballast Point, I fell off from a close reach to a beam reach, toward MacDill AFB. The waves were largely on the starboard quarter, when I suddenly had a gust push the boat over to about 80 degrees. Water poured in over the coaming and I had two thoughts; I can't believe I didn't anticipate that and I should have covered over those old speakers in the cockpit. Yes, the previous owner had put speakers in the sides of the cockpit, which now allowed about twenty gallons of seawater to rush into the main bilge. Note to self: get rid of the speakers.


I immediately released the main sheet and pushed over the tiller and came up into the wind. Then when she righted, I continued with the tack, passed the jib and sheeted in the main. We were now closehauled and pounding into the waves heading toward Pendolla Point. It was exhilerating and exhausting. I was carrying way too much sail, but I was still having fun, despite my best attempts to sink the boat.


I still kept up the full main and stayed out for about three hours all told. But I recognized that I need more tiller time to get back my feel for the boat. It was really more of a struggle than it should have been.