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  #21  
Old 10-02-2012
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Re: Any Ideas

JohnB, we were in a 5-mile wide stretch of Chesapeake Bay - not a narrow river or channel. The SOG on the Lowrance GPS is extremely accurate, and I checked the tidal currents during the same time frame and the maximum tidal current was less than 1 knot, so that wouldn't account for it either. I knew there would be skeptics - that's why I photographed the GPS.

The Lowrance HDS7 GPS/Plotter is a pretty amazing device, and over the years I've found the Lowrance systems to be extremely accurate, much more so than the Garmin and Raymarine systems many of my friends own. Part of this may have something to do with the number of satellites being tracked at any given time. Obviously, the more satellites tracked, the greater the degree of accuracy. The few times I've looked at the satellite tracking information on the display I've noted that there seemed to be at least 5 to 7 in range that were identified. As for the sampling rate, I've never had a reason to look at it, but I know it is adjustable with just a few button pushes and menus.

I talked with the local, Lowrance rep about boat movement affecting speed accuracy and he agreed that if the boat was rocking in any direction it would have some affect, and the violently the boat's fore to aft movement, the greater the degree of inaccuracy. The boat speed at one point ranged from 9.9 to 10.1 for a period of about 20 minutes. During that period the wind was fairly consistent, very few puffs, and at the end of about 3 hours, like almost every day during the summer, it died off to 5 to 10, at which point the black flies moved in for a feast and the sun began to set.

While I realize those wonderful hull speed formulas were created by some fairly smart folks, for some unexplained reason, they just never seem to be all that accurate. To unequivocally state that the Morgan OI will sail at a certain speed because the math doesn't allow it, well, like I said at the outset - that's why I took the photos. And the guy that was with me on the boat, who was at the helm at the time, is a 25-year veteran sailing instructor, was amazed as well. He verified the speed on his hand-held GPS - just to confirm the Lowrance readings. His handheld GPS was reading 10.4, which I guess is within the margin of error. Maybe that ice cold water that is more dense has a bearing on your sailing speeds in Canadian waters.

Cheers,

Gary
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  #22  
Old 10-03-2012
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Re: Any Ideas

Gary, sorry, it just doesn't work that way. The fact that two GPS units displayed similar errors doesn't make them less in error. If you stand a 1/4 mile due West of a large iron ore deposit, you could have a hundred compasses, and they will all be in error. The factors that make any GPS unit report errors will make just about all such units in those circumstances report similar errors. For example, it's very common for a stationary GPS to report that the vehicle or vessel in which the GPS is located is either moving or turning, when if fact it's doing neither. Yes, there are well known exceptions to displacement hull speed calcs, but your OI 33 certainly does NOT fall in any of the excepted categories (e.g. multi-hull or ULDB). I understand you took photographs, but they can only prove the possible degree of error of your instruments given the circumstances. Try turning on your GPS while at the dock and watch it for 10 minutes. More than likely it will show your boat changing heading, picking up speed, etc.

BTW, 5 satellites is barely above the minimum number for accurately determining location (4). There are many authorative documents available online to explain why a GPS errors on speed. To pare it down the gist is:
Quote:
The most inaccurate is the method that tries to
estimate an average speed over some
“accumulated distance” between trackpoints.
Due to trackpoint inaccuracies, the line
connecting all track points is a zig-zag, even
if the real path of a speed competitor is a
smooth or straight line. Since the length of
this zig-zag is always longer than a
smooth/straight line, the “average speed”
determined with the “accumulated distance”
method always overestimates the real speed.
The less accurate are trackpoints (the less
accurate is a GPS unit) – the larger the
estimated “average” speed and the more
impressive is the “achievement”…
Think about it. The GPS reports position with an accuracy of 16 meters (you can't even get that accurate with 5 sats in view). That means the actual position is anywhere within a 16 meter RADIUS of reported postion. That's a circle with a diameter of about 100 feet. That makes it obvioust that what George said is absolutely on point: the sampling rate MUST affect the accuracy of the reported speed.
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Old 10-03-2012
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Re: Any Ideas

Hate to spoil the fun, here, but it's probably about time to note that Gary's plotter is set to display MPH, instead of KNOTS...

Even so, sustained speeds of 8.5 knots on a Morgan 33 is a bit difficult to imagine without a bit of assist from the tide... Without knowing the date in question, I'd still have to guess that the tide was likely on the ebb during much of that ride...

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  #24  
Old 10-03-2012
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Re: Any Ideas

I know it was set on MPH, and I never mentioned knots, but a lot of others sure did. As for the accuracy of the speed, I guess that's up in the air (lol) at this time. One thing is certain though. I zoomed past a lot of larger sailboats, probably a couple hundred that day, and made the trip down the bay faster than I, personally, have ever traveled on a monohull boat.

This debate could go on forever, but not for me. I'm headed out this morning for sunny climes and beautiful waters 1,150 miles south of Havre de Grace, MD - the lower, Florida Keys.

All the best,

Gary
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Old 10-03-2012
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Re: Any Ideas

Using the time displayed and the date of September 22 which I beleive was the Saturday mentioned the assist would have been .55 knots of current at Howell Point current reading which is just north of Still Pond 1.5 miles from Worton Creek entrance.

We were out sailing that day headed about the same course 15 miles away going to Annapolis, and there was 15-20 wind with flat conditions. Our C&C MKIII ( racer cruiser) which is not know for being a slow boat was humming along at 7.9 knots with a .2 current assist.

I know Gary and I am sure the MPH was an honest mistake. Since he was comparing to previous data he used on his same boat he was really moving compared to what he had done before. I dont think he was comparing to other boats who obviously have a lower prf rating( Morgan OI 33- 177 vs C&C MKIII 127).

Dont think he would want to challenge us racing. First he would need to stow all his liveaboard stuff now and lighten his load ( smile).

Hope you are having a good time heading south.

Great day for sailing
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Last edited by chef2sail; 10-03-2012 at 09:57 AM.
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Old 10-03-2012
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Re: Any Ideas

Quote:
Originally Posted by travlineasy View Post
I know it was set on MPH, and I never mentioned knots, but a lot of others sure did.
Gary, I had not clicked on the link to your thread, so I didn't see the pic of your trip page until Jon posted it in this thread. But you really DID mention knots:
Quote:
Prior to my purchasing the 33 everyone, including the owner, said on a good day, with 15 knots of steady wind, on a beam or broad reach, you might get 4 or 5 out of the boat. With a clean bottom, 15 knots, beam or better yet, a broad reach, 7.5 to 8.2 is not out of the question.
No mention of mph. You can't switch units of measurement in a statement like that. Would you do it with Fahrenheit and Celsius? The calculated hull speed for your boat, converted to mph, is 8.1. So it looks like you (inadvertently) switched units from knots to mph, hence the reaction to your speed claims.
Quote:
This debate could go on forever, but not for me. I'm headed out this morning for sunny climes and beautiful waters 1,150 miles south of Havre de Grace, MD - the lower, Florida Keys.

All the best,

Gary
Wow, envy you that! Hope you have a great trip, fair winds and following seas!
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