Ray,
All I can think is it is all the beer I carry on board. That and the bucket you tie on back of La Reina. Not only does the beer weigh a lot but it attracts a lot of passengers.
Mark Powers
La Reina 26C
Vancouver, B.C.
I think there is also an error that is caused by the position of the sender,at least on our boat. Our knot meter is located just ahead of and slightly to one side of the keel. Any time you have any leeway, the flow around the leading edge of the keel is going to be accelerated somewhat. This should cause the knot meter to slightly over read.
Peter Moodie
Nonsuch 30U
Catalyst #366
That’s correct, Peter and it is normally addressed in the installation manual.
Joe Valinoti
S/V IL Gatto NS30U #221
Sea Harbour YC
Oriental, NC USA
Joe
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Thanks Joe, I did not install ours but will have a look in the installation manual.
Peter Moodie
Nonsuch 30U
Catalyst #366
Excellent response
Ray
Knot meters are mechanical things by enlarge and subject are a suggested speed not an actual speed. Good for measuring current and set against a gps and guesstimate of how fast you are or are not going. Keeps passengers amused.
At for speed in my 26… 9.6 falling off a big wave on a broad reach. I was told not to do that again.
On Mike’s 30U Bianka, we saw almost 20 knots SOG on the GPS one day going through Hell Gate with the tide, on the East River, in NYC. Not exactly apples to apples but was exciting and also illustrative of the difference between SOG and speed through the water.
David Young
Bay Cat, 30U #402
Suttons Bay, MI
USA
Thor:
This latest email from you, by and large, deserves a response.
You are quite correct that the older paddle-wheel knot meters read water flowing past the hull, if they are not slowed down by any marine growth or corrosion. The latest electronic versions from Garmin and others read water speed electronically from the flat-plate sonar thru-hull which should be more accurate.
However, as you imply, water speed past the hull bears only an approximate relationship to SOG (speed over the ground) unless you are lake sailing. When coastal sailing or sailing through major ocean currents like the Gulf Stream, the direction of (tidal) flow must be taken into account. Obviously, these currents are not necessarily directly ahead or astern, so they will affect SOG (plus CMG and magnetic heading) to a greater or lesser extent.
Fortunately, the latest GPS course plotters clear up most of this mystery, both for “real” speed and proper heading to maintain corrected course.
Your final statement deserves more discourse. Waves vary their velocity according to a formula based on amplitude and frequency, but they increase height and decrease speed in shallow water. Suffice it to simplify by saying that small waves travel about 4 knots while huge waves can travel at over 50 knots and tsunami waves can roar in at triple that speed.
When reaching or running downwind in a Nonsuch, it is delightful to “surf” down waves that are traveling faster than your hull speed…each wave adds a temporary boost of speed to whatever speed you are normally making good. The only potential danger would be in really huge and speedy waves
(think THE PERFECT STORM) where you could lose steerage and broach).
I hardly think that doing 9.6 knots would qualify for major concern as long as your sail isn’t blanked and you maintain some steerage.
Barry Stott
Chadds Ford, PA
ebstott@aol.com
Thor:
Your previous email (at bottom below) started off by referring to “knot meters that are mechanical things” so I naturally assumed you were referring to thru-hulls paddle wheels that can only read actual water flow speed from bow to stern along the hull. One of these could only read “river flow” or “tidal flow” speed if you are docked directly facing into the current.
I don’t know how the latest flat thru-hull sonar transducers measure hull speed through the water, but if they are sensing the movement of individual particles (i.e. dopplar shift of molecules), perhaps they can sense and display a lateral current while a boat is tied in a slip across the current.
Obviously, GPS speed across the ground (i.e. bottom) is the most important number in terms of navigating anywhere and reaching a mark or destination at a certain time, but hull speed through the water gives more immediate feed-back on proper sail trim than absolute GPS speed does.
Absolute hull speed for a displacement hull is computed as 1.34 x the square root of the waterline hull length. In your case for your 26, that would be just over 6.8 knots…anything faster on the GPS means that either you have a helpful push from tide/current or you are surfing downhill a following wave!
Wishing you a preponderance of these happy moments!
Barry Stott
Chadds Ford, PA
Hi Barry
I should have mentioned it was GSP speed not sonar speed I was referring to. I sail in an area where one town’s dock can have 3 plus knots flowing past the docks so I often use the paddle wheel to judge just how fast it is.
Sustained speeds over 6.5 knots are not possible but falling off waves and surfing along then can have an impact. Anything over 7 knots is transitory. I see at best 6.8 on a broad reach in 20 knots.
Cheers
Thor