On a recent sail, the GPS showed 6.8 knots but the depth sounder ( with the little wheel under the hull ) read 7.6 knots. Which is more accurate or should I just average the two readings? By the way, the wind was from the shore, with very little waves and we were just bootin' really fast ! Russ
gps & depth sounder
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Russ, I think you meant the knotmeter. Either of two things: current or your knotmeter is not calibrated.
Russ, also.... your knotmeter reading is suspiciously close to your GPS reading if you are using MPH. ;-) Are you sure your knot meter is set to knots?
I have an older knot meter and I did have to calibrate it after I did a bunch of work to my electrical system and left the boat without power for a couple of weeks. Thankfully the PO left the documentation to the knot meter on the boat when he sold it to me.
DanM.
I have an older knot meter and I did have to calibrate it after I did a bunch of work to my electrical system and left the boat without power for a couple of weeks. Thankfully the PO left the documentation to the knot meter on the boat when he sold it to me.
DanM.
Russ, Your GPS is giving you your speed over the ground. You can set the GPS to give you either Knots or MPH. Your "depth sounder" will give you the depth of the water you are in so I think you mean your speed meter "the little wheel under the boat" is giving you the speed the water is passing under your boat. It you are in a current it is going to give you a faster reading, than the GPS, if you are going against the current and a slower one if you are with the current. If they are the same and most likely the will never be, as the knot meter under the boat isn't totally accurate if you have some growth build up that may slow it down and the GPS is using a average over a short period of time, which some GPS's can be set, like over a 1 second to a few minutes interval.
Thanks guys. I get it now. The instrument which I called the depth sounder is indeed the main display but in smaller font, it also shows speed. Both gps and knotmeter are calculated in knots BUT I think you are absolutely correct as the gps is over land speed and the knotmeter does not subtract or add for the current. This makes sense to me. Thanks again for straightening me out. Russ
You have kind of got it right..... but it is the "difference" between the speed log and the gps that helps you with current... neither of the devices measure current directly.... because current doesn't always flow the same direction as the boat.... sometimes it's sideways or diagonal.
What the speed log, or knot meter, measures is the boat's speed through the water.... (current is not considered).... and this information helps you determine how well the boat is performing....i.e. how well you have the sails trimmed, etc. In your example the boat was travelling at pretty much its maximum speed through the water.... but since the water was also moving (current or waves or both) the boat' speed over ground was even higher. In another direction, the gps number would have been lower than the speedo which most likely would have remained constant.
The idea is to trim the boat to its maximun speed through the water using the speedo as your guide.......... but ...... incorporate the gps information (speed over ground) to determine the best strategy for reaching your destination... Most sailors use the velocity made good (VMG) feature on GPS to help in this area.
and the earth is flat. 1.38 times the square root of the waterline in feet, unless you've rediscovred phlogistion, or can get your boat to plane. sorry pal.
just to make this clear, the velocity of a hull through the water does not include the water already moving, either way. capisci?
Mortyd....... with enough horsepower the Catalina 30 could certainly exceed hull speed ......... The theoretical hull speed model is not an absolute limit. It was a formula used to determine the most efficient steam engine size for a full displacment hull vessel. Now it's often used in a similar manner for sizing auxillary diesels. With enough power, any boat can get on plane. (the power required, of course, is considerable.... exponential even... )
Remember, it's THEORETICAL Hull Speed.
if you have the time and inclination, study fluid dynamics 101. once hull speed is achieved all you do by adding force - not power - is push the bow wave further ahead. just watch a tug boat. i'm curious, where did you get the idea that the titanic could have planed with enough steam? could she have towed skiers? how many?
We have been over this "ground" literally so many times. The GPS charting over ground will always be more accurate then a speed sensor reading from a rotary wheel. There are far too many variables in marine growth & water slipping over the wheel from different directions, etc. to make the speed reading inaccurate on a wheel sensor. The least bit of growth on that wheel restructs its movement, & then it dosesn't work properly. I, & many others on this forum have also proved that you can indeed sail a boat faster then its theoretical hull speed! If you are sailing dead downwind, with the tide helping you move your boat along with full sail, any sailboat can move faster then then its theoretical length of its hull, waterline length, etc. I guess Mort just hasn't hit the right wave yet or sail combination & proper trim. Too bad. It's a rush when you get it right. I even have digital photos that I've taken of my GPS & speed readouts to prove that I've gotten well over 7.2 knots on my C 30, & I wasn't even trying that hard. I hope there are other sailors of C 30's & 27's who can back me up on this one. Cheers.
ps. Mort, If you've even watched "MythBusters" on cable, they were able to get their fat host to waterski by pulling him behind a punting scow with a half dozen rowers. That was a boat that was less then 20 feet long, under power of rowing alone. I'm sure that there were naysayers that said it couldn't be done, but there you are.
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Take the time to read this short comment: http://en.wikipedia.org/wiki/Hull_speed
Also, I invite you to take the time to study the history of steam and diesel powered vessels...... if we were restricted by dogmatic thinking we would never have broken the sound barrier or gone to the moon. http://www.lesliefield.com/other_history/speed_boat_developments_from_the_past_into_the_future.htm
Your Titanic waterskiing analogy is ridiculous. The theoretical hull speed of an 882ft vessel would be somewhere around 40 kts.... However, the technology of the day provided her with only 46000 hp in steam power.... limiting her cruising speed to 21kts to 24kts max.. However... that's certainly fast enough to pull a few skiers.
http://en.wikipedia.org/wiki/Waterskiing Just imagine how she would perform with modern nuclear powered turbine propulsion.
it's not dogma, it's science. i have a masters degree in aerodymamics and had to study fluid dynamics first, but may the earth is really flat. but, unfortuately on this planet, hull speed is hull speed, and the titanic analogy is a joke. but you did metion planing, remember?
Sorry professor.... I only have a business degree, from a state university, at that.. but it's enough to allow me to understand the light technical reading I provided.
If you're unwilling to explore the information available, you could at least cite some kind of evidence for your point of view. Saying " I went to college" doesn't count.
Is there some absolute scientific law regarding displacement speed that you might care to explain or provide reference to? Perhaps an explanation of how the study of fluid dynamics relates to displacement hull speed theory....
Is there a theoretical hull speed for an airplane.... since it has to push its way through the air....a la displacement theory.... but, uh keep it simple for me, please.
I had my hunter 31 to over 7knts. Full sails broad reach and surfing down the front side of 10' rollers of NJ coast. WOW!
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