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GPS saying i was doing 8 knts???

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77Gypsy
Captain

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USA
356 Posts

Initially Posted - 07/06/2003 : 14:28:33

yesterday i was in the Chesapeake Bay sailing with main and spinnaker and my GPS was displaying 8 knts at times. How is that possible if the boat's hull speed is 6.3? i figured maybe the current but the bottom of gypsy is still infested by barnecles so even with the help of current i had a hard time beleiving i could do those speeds.
btw, the winds were 10mph.

Steve
78 C25 SR/FK - Gypsy
<img src="http://www.websdf.com/logo.jpg" border=0>

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timpky
1st Mate

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USA
41 Posts

Response Posted - 07/06/2003 : 17:07:28

I would bet on current. Hull speed is pretty much a reality. GPS is usually pretty accurate and will include the effect of current.

Tim Peoples
Sarasota, FL
78 Catalina 25 SK Hull #875

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ClamBeach
Master Marine Consultant

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3072 Posts

Response Posted - 07/06/2003 : 21:06:13

A GPS will have momentary variations as it's position jumps around..
(At least the older non-WAAS and non-differential ones will).

Tp get a more accurate speed you'll need an average over an extended time/distance. An average over 15 minutes or so should be pretty accurate.

Also, getting a boost of 2 to 3 kts from current isn't unheard of.

Currently maintaining two holes in the water...'77 Venture 23 and new to the family,
'78 Catalina 25

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frankr
Captain

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256 Posts

Response Posted - 07/06/2003 : 21:06:38

Are you sure the GPS was in Knot mode and not MPH?

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Dave B
Admiral

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Mali
863 Posts

Response Posted - 07/06/2003 : 21:31:42

With spinnaker, huh? Downwind with some fair-sized seas, you can often "surf" on the front side of waves, gaining speed well past hull speed for as long as you can sail "downhill." It feels like the waves are giving you a temporary push. (YEE-HAAAA!) The GPS calculates speed from periodic positions and the time between them, so the effect of your surfing may show up a few seconds later on the GPS.

Current, of course can help--if it's strong enough, I suppose a fouled bottom could work slightly in your favor (as well as decidedly against you when going against the current). Without either current or some good surfing, I seriously doubt your boat can exceed 8 knots in 10 knots of wind, downwind or up.

Evidence that you're moving through the water above hull speed is a substantial increase in your stern wake and some spray coming off the forward part of the hull, caused by the lifting of the bow as you exceed the magic number. It can be done!

Dave Bristle, 1985 C-25 #5032 "Passage" SR/FK/Dinette/Honda in SW CT

Edited by - dave b on 07/06/2003 21:34:04

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77Gypsy
Captain

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USA
356 Posts

Response Posted - 07/07/2003 : 01:53:58

As some of you suggested I agree with the current being the issue. However,
1. my bottom is FULL of barnacles
2. I’m not the best sailor in the world
3. my sails are pretty old and not in the best of shape.
With all of that said, I think the absolute best speed I could get without current is maaaaaybe 4 knts. That would mean the current would be another 4 knts??? That seems a bit powerful for a calm day on the Chesapeake bay.

Steve
78 C25 SR/FK - Gypsy
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77Gypsy
Captain

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USA
356 Posts

Response Posted - 07/07/2003 : 04:24:39

by the way, my GPS is the garmin GPSmap 76s with "WAAS" and all the other little bells and whistles.

Steve
78 C25 SR/FK - Gypsy
<img src="http://www.websdf.com/logo.jpg" border=0>

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Brooke Willson
Admiral

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USA
983 Posts

Response Posted - 07/07/2003 : 07:01:02

Steve:

You were sailing on the 5th in the Chesapeake? I was in the Rappahannock that day and it was, as they say on the Eastern Shore, "right airish." Where I was, the winds were better than 10 kts.: I had to reef my main, which usually means the breeze is above 15 kts.

I think it's entirely possible that you were sailing at near hull speed and getting a current boost of close to 2 kts. I've hit 6.7 on my GPS on a beam reach in the Rappahannock, probably also boosted by current.

Brooke

Edited by - brooke willson on 07/07/2003 07:03:12

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77Gypsy
Captain

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USA
356 Posts

Response Posted - 07/07/2003 : 13:00:59

Brooke,

I was on the chesapeake on the 5th. approx 20 miles north of you though, right off chesapeake beach. i probably didn't have the winds you had because i was sailing comfortably without a reef and i was flying my chute. it was a beautiful day out there though!

Steve
78 C25 SR/FK - Gypsy
<img src="http://www.websdf.com/logo.jpg" border=0>

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JimB517
Past Commodore

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USA
3285 Posts

Response Posted - 07/07/2003 : 13:07:49

I've noticed my handheld GPS giving some weird speeds and headings when I've been holding it and moving around in the cockpit. Usually only for a single 2 second reading.

<img src="http://www.indiscipline.org/cat25/pictures/icon.jpg" border=0>Indiscipline 1978 FK #398

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Champipple
Master Marine Consultant

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USA
6855 Posts

Response Posted - 07/07/2003 : 16:49:17

If you were in following seas, (as indicated by spinaker), I have no doubt you were doing 8+ kts. Or at least that is what you saw over a short segment of the trip.

As mentioned before, surfing off the back of waves can greatly increase your boat speed. And you don't have to be Dennis Connor to do it, All you need are wave hitting your transom. We've gotten the Wasp up to around 9 or 10 knots without even trying.

The Evelyn 32, which has a hull speed of around 8 (give or take) has pushed 19's on the knotmeter on a reach with following seas.

D. Wolff - "The Flying Wasp" #401 sr/sk
Chief Measurer C-25/250 National Assn.
<img src="http://www.flags.com/dreamimages/Flags/measurer.jpg" border=0>

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frankr
Captain

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256 Posts

Response Posted - 07/07/2003 : 20:24:15

Calculate the old way http://www.tallshipbounty.org/chip.html

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Heartbeat
Navigator

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USA
161 Posts

Response Posted - 07/09/2003 : 17:39:49

I would believe 8 knots with current. Catalina 25s love the spinnaker!

I had a reaching leg a couple of years ago that was just incredible. We were on a beam reach with apparent wind about 95-100 degrees. Spinnaker up, and a 5 person crew in winds that started at 10 and built to 15 knots. Myself on the helm, one person on a cabintop winch for the spinnaker sheet, one person standing at the mast trimming the kite, and the other two sitting on the windward side of the bow. Average speed for the leg was just over 7 knots according to the knotmeter. About half way through the leg, my crew put the cover on the knotmeter because I was spending too much time watching it. I did see 7.8 on the knotmeter before they put the cover on, though. (Since the knotmeter measure speed through the water, it's actual boat speed and is independent of current.)

-Matt

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IndyJim
Navigator

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USA
130 Posts

Response Posted - 07/09/2003 : 20:12:50

Steve,
I just returned from a week long voyage on Lake Michigan. Like you I got some unbelievable readings on my GPS. Just when I thought it was time to toss my Garmin in the lake and buy a new GPS, the other boat I was sailing alongside got similar readings. We had 4-6 foot waves, a 10-15 mph tail wind, and running wing & wing. My GPS had a maximum reading of 8.7 kts and my sailing buddy on a 25 foot ODay had a 9.1 knot reading. Like you, I've read all the info about the 6.3 hull speed of the C-25's and this really perplexed me. I know I was doing some major surfing and we only averaged 6 knots on our 26 mile leg of the trip, but some of those pushes were better than any amusement park ride I've ever been on.
Thanks to all of you for your logical explainations of why I should keep my GPS! Life is good!

Jim on "Itza Dew Sea"
'84 SK/TR #8525

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dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 07/09/2003 : 20:17:10

<BLOCKQUOTE id=quote><font size=1 face="Verdana, Arial, Helvetica" id=quote>quote:<hr height=1 noshade id=quote>Life is good! - Jim<hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face="Verdana, Arial, Helvetica" size=2 id=quote>

Indeed! <img src=icon_smile_approve.gif border=0 align=middle>

<img src="http://www.catalina25-250.org/c25sm.gif" border=0>Don Lucier<img src="http://www.catalina25-250.org/c25sm.gif" border=0>
<img src="http://im1.shutterfly.com/procserv/47b2d904b3127cce9f7cd9ffdf1d0000003010" border=0>
North Star SR/FK

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jsummerfield
1st Mate

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USA
96 Posts

Response Posted - 07/11/2003 : 20:04:33

In addition to the GPS, go with the Tall Ship Bounty concepts. It would have been easy to test the boat speed on the water separately from the GPS. The following is lifted from the manual that I prepared for my own boat. However, much of the origin likely came from Sailnet articles, etc. Here it is if interested.

Sailors need three velocities. These include the velocity over the water, the velocity between fixes or speed over the bottom, and the third is the velocity of the current. Currents cause a difference between these related speeds. Even lacking a knot meter one can practice traditional sailing techniques to estimate boat speed. One technique uses something biodegradable like an apple that doesn't require retrieval. Throw the object overboard, and start timing from the point of the bow to the point of the stern. Calculate the speed knowing the fixed boat length and time. Another technique forms a simple speed log by tying a floating object to a length of line. The object can be thrown overboard and recovered. With the tied object, see how long it takes the line to go taut. Use the speed, distance and time arithmetic to obtain speed. The measurement technique provides feet or meters per second.
The conversion from meters per second to kilometers per hour simply requires multiplying the meters per second by 3600 seconds to get meters per hour then divide by 1000 to get kilometers. One nautical mile is 1.852 km. Thus, there is 0.5399568 or about 0.54 nautical mile per km. From km/hour, if knots are desired, multiply by 0.54. Convert the English units to calculate knots (nautical miles per hour) as follows:

There is 5280 foot per statute mile and a nautical mile is 6076 foot. There are 60 seconds per minute and 60 minutes per hour; multiplied to obtain 3600 second per hour.

( L ) ft 3600 sec/hour
V nm/h = ----------------------------
6076 ft ( t ) sec

Note that the ratio of 3600/6076 = 0.592 or about 0.6

0.6 ( L ) ft
V nm/h = ---------
(t ) sec

V is your speed in knots, L is the length of the line or boat in feet, and t is the time in seconds. In calm weather, it could be possible to spit off the bow of the boat and time the bubbles to reach the stern - in which case L is the length of the boat or 25 foot LOA. A displacement hull boat speed is generally limited by the length of the bow wave along the water line. The formula for this speed is hull speed in knots equals 1.34 times the square root of the length at the water line, LWL. A Catalina 25 at 22.1667 foot LWL has a theoretical hull speed of 6.3 knots. Estimate well below hull speed for navigation assumptions.

Although these are not an original ideas, consider use ing them the next time that the GPS shows velocities over theoretical hull speed.

John
3973

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