Knot meter curiousity

[CaptnNero]

another thought...

...
I'll just try to find how much the two measuring devices diverge by using both at a time when the winds are relatively calm & the tidal current in the bay at that point in time is not much of a factor and then look for any possible way to adjust the calibration on the knot meter...before posting the question, I did not know that adjusting the knot meter was possible...now I know that it is possible...just not certain if it with my particular instrument nor how to do so.

Thank you, Neal, for your contributions to the thread (and everyone else for that matter). The wealth of knowledge that can be gleaned from this site is one of the reasons that keeps me checking it over almost daily. David's (Emerald) observation that all of this would be interesting while spending a dull day at the office or dealing with a rainy Saturday implies something additional...this would have made for a really interesting discussion during the winter months...we'll just have to come up with something else about which to agree/disagree/embellish/and perfect through synergy then.

Regards,

Ed

Hi Ed,

Somewhere along the line I did mean to leave open the door about your knotlog possibly being out of whack and not necessarily dead-on-balls-accurate. So when you get around to whacking it upside the head with a formal calibration maybe you'll never see those kind of numbers again. Be careful what you wish for...

It has been delightful delving into these matters and I do appreciate those links that you have forwarded to us and to be archived on this list for the ages.

In the meantime you've got me thinking a bit, so I may just stop by the aquarium store to pick up something to convert to a on deck salinity gauge.

 

[Seth]

Neal is right

Ed,

You are right that there is a ROUGH limitation on speed through the water that is largely a function of LWL, however you are overstating the amount of power-be it engine power or wind power needed to the exceed hull speed.
15 knots of additional wind will DEFINITELY push you past hull speed if you were already in that ballpark. So will another 15-20 of hp. All that is required is that the bow be pushed up on top of the bow wave, and you will begin to "surf" on the wave-easily passing this limit.

Yes, it DOES take a substantial increase in "power" to pass this speed, but it can and certainly does often happen. It is not as dramatic as you have suggested. I have been on a 30+ in flat water averaging well over 7.5-8.0 knots many times (on the calbrated knotmeter). Without current.

Finally current has NO effect on boatspeed-only groundspeed. The GPS will show the combination of boatspeed and groundspeed (and leeway of course), but the knotmeter ONLY shows speed through the water. Since the boat will move at the speed and direction of any current (absent any other driving forces), if the boat is in neutral and there is no wind at all, the boat soeed will read zero while moving along in current. If, for example, 1800 RPM's gives you 6 knots in calm water, then regardless of whether you are going into or against the current, you will get 6 knots on the knotmeter at 1800 RPM's. The GPS will show you the net groundspeed (higher or lower than the knotmeter depending on whether you are going with or against the current).

Knotmeters, like any electronic device, can and must be calibrated to have proper results. Most harbors have a measured mile betwen 2 buoys or 2 markers on land, and these are intended for the purpose of calibrating your knotmeter. All manuals for knotmeters made in the last 20 or so years should have a section on calibrations. The ONLY time the calibrated knotmeter and GPS should match is when there are no other forces acting on the boat (basically current and/or leeway).

Hope this helps!

S

 

[ted_reshetiloff]

You may find that your knot knot meter is accurate at certain speeds and not at others. Seth is dead on with his explanation of SOG vs Boat Speed. You may try running a measured mile both ways at different speeds and see what you get. Your meter may be fine in the 4-5 kt range then gets way off and worsens the higher you go above 5kts.

My knot meterm, a Datamarine unit, has curved fins on the padddlewheel If I install it with the curves one way it reads a knot to 1.5kts lower than if I install it with the fins in a more aggressive fashion. Not sure if that made any sense. May need a picture.

 

[Tom Plummer]

Yes you can sail your boat faster than hull speed. There are two ways to figure out what your hull speed is one is theoretical via a mathematical calculation which has been discussed and the other is actual.

Actual hull speed is one bow wave length. As the bow of your vessel pushes through the water it creates a wave crest, trough and a second crest. The faster you move through the water the further aft the wave crest after the trough moves, when the apex of the crest of the wave is exactly even with your stern at the water line you are at your actual hull speed.

You can go faster but there is a cost because as you exceed your hull speed the wave crest moves aft of your stern dropping it into the trough forcing your vessel to in essence start to climb a hill. This has no penalty when under sail because your power source is the wind and essentially free as opposed to under power where the fuel penalty can get expensive. There is a down side to sailing to far past hull speed. It is theoretically possible to drive the stern and the whole boat under this was a real problem for square rigged ships when they encountered a wind squall at night. It takes a lot longer to shorten sail on a square rigged ship that a fore and aft rigged vessel and the problem occurred often enough to rate its’ own term it was know as “Sailing her under”.