Lots of modern power boats now, both pleasure and commercial
, have no paddle wheel
log but rely entirely on GPS
for speed. Of course, many of them are much faster than sail boats so drift is of less importance to them. Also, in my experience they are not so hung up on tradition especially as real time electronic nav on a chart plotter or PC adds so much to safety
when cruising at high speeds so has been strongly adopted.
For sail boats the importance of a paddlewheel log for determining set, drift and leeway is to me very over rated. That because of the errors involved.
To determine actual sum of set, drift and leeway using a log we rely on a vector diagram dependant on fixes, course steered, speed through water
(or distance through water) and time. If we assume that the error in time and the fixes is too small to worry about we are left with any errors in the log and in the course.
First the log - they are notoriously inaccurate (as another has pointed out) and in my experience they seem to be often calibrated high, if calibrated at all
(look, for example, at the speeds some report for their boats which one knows are very unlikely given the boat type and WL length). Most are intended to operate in the laminar zone against the hull
and that is difficult to achieve on a small sail boat due to the changing profile of the turbulent zones from wave action and heel and due to the proximity of the appendages. They may also not be linear (due to axle friction?) so accuracy changes according to actual speed and they are also subject to fouling.
Then the course steered - most small sail boat compasses are only graduated in 5 degree increments for a start but in the end helmsmen will not be able to sail an accurate course. Wave action, pinching into and falling off the wind
according to the point of sail on (I know I tend to pinch up in lifts if sailing a close hauled course - I think a habit from growing up racing
dinghies needing to gain to windward), distraction, boredom, etc will all contribute to inaccuracy. I would suggest that in other than calm conditions maintaining a course within 5 degrees on a small sail boat is maybe all one can hope for - an autopilot
may or may not do better.
So, in the end the calculation of set, drift and leeway will not be very accurate at all.
Then we need to consider that the result is a historic one based over, say, an hour of sailing. When coastal, and especially in pilotage, there may be no reason whatsoever that the set, drift and leeway are going to be the same over the next hour. In fact in my home waters they can vary enormously (like reverse
) minute by minute even 6 miles out from land - but that is not what we all face, of course.
So back to the GPS
- in the end when calculating set, drift and leeway we are wanting to know what course to steer in order to end up where we want to go. If one uses a chart plotter or Electronic Charting System on a notebook there is no problem doing this with no log. One just extends the look ahead vector (now officially known as the tennis raquet thingie
) and adjust the course steered so the extended look ahead vector lies over where you want to end up. That automatically allows for tide and leeway and that in real time, not based on some calculation based on the previous hour.
If one doesn't have a chart plotter or ECS but a GPS instrument that allows entry of waypoints (I think pretty much they all do) one just has to create a waypoint at the place you want to end up or somewhere along the route
from your current
position to it, and then adjust the course steered as one goes so that bearing to waypoint remains constant as you proceed (or so XTE is maintained close to zero). That is you in effect sail up the bearing line from your start point to the destination
and required corrections in course due to changing set, drift and leeway you can correct in real time. The closer you place the waypoint to your current
position along the desired route
the faster the bearing to waypoint will change should you stray to one side.
So in the end I think a paddle type log (or towed one) is pretty much irrelevant to safe navigation
on a sail boat with a GPS - it certainly is on small power boats because in the countries I am familiar with the statutory requirement for speed measurement on commercial
vessels is met by having a GPS alone.