MacGregorSailors.com

Only if you're moving at the speed of light!

I don't think 'boat' can go that fast.

It's easy to not notice current effects on speed. Current can have a huge impact on speed-over-ground as measured by GPS. In the east river in NY with ballast in and a full load of cargo and crew, I easily did 18 knots north, but then turned around and could only do 12 knots out.

So my speed in the water, as Brian (seahouse) points out, was really 15 knots as measured by reciprocal course, which matches what I see for speed under similar load conditions. Cheaper to do the math than buy the sensor, but keep in mind that the math is only really accurate if your course aligns with the direction of the current (as it would in a river).

And yes, GPS alone does not provide real heading. Trying to use GPS to steer to a course in a heavy fog makes this very apparent--you wind up doing a snaking S course because the "heading" is calculated from track and is always about five seconds old, so you can't find the true course except by hunting or using a real compass.

Matt

Hey mastreb,

Do they have a compass built into the chartplotters we have on our boats like an Iphone or is it all just GPS?

Of course, using a compass in the fog would not result in a straight track to your destination either, but a curve, and one that would be unlikely to have you end up at the “aimed for” destination point.

As Dave points out, you would only be facing the direction the compass indicates (“heading”), but one could consider oneself to be extremely lucky (unlikely) to be actually moving in that direction too (“track”).

Like 20/20 hindsight, the GPS is dead accurate at indicating your heading (and SOG) after you have done it.

What I've done is to pick a heading that allows for some leeway, and monitor course made good. Make adjustments as needed, but I can usually find a heading (for those conditions, which can change of course) by making small adjustments and holding that while monitoring actual progress. Just an electronically assisted method of dead reckoning, I suppose.

I use the nav page that shows my route like a highway on the water

Set the width to 100 feet either side of center

I let the auto fall off a bit either side, then tweak it back 3 deg back to center every 10 min or so

Results in long S over several miles

Keeping an eye on eta, always make it to dest within 1 min of orig calc

All that over 30 mile route

I'm happy with that method

G

BOAT wrote:Hey mastreb,

Do they have a compass built into the chartplotters we have on our boats like an Iphone or is it all just GPS?

Does not have a compass. You can add an NMEA 2000 fluxgate compass.

I'm using a Garmin 76CX hand-held GPS integrated (USB) with a MapTech software program attached to my netbook. I bring up the chart, place a 6 inch ruler to the screen, and plot my course by holding the ruler on that spot while moving the chart's compass rose toward the ruler to establish a heading. That's it. The chart has a small red boat which tracks my progress as we move along. With multiple waypoints, I'll write down all of the new headings like a flight plan and take up those new headings when my red boat indicates we've arrived at the next waypoint. I also have the full size MapTech charts and the navigation tools on board for reference. This adds more fun and tradition to boating.

Work is killing me - I was supposed to be off today but can't get away - I really wanted to sail today and tomorrow because it's the first two days of all day sunshine we are going to have on the coast over here in weeks. Crap - I want to test the speed runs on 'boat'.

I am so busy with work I can't even blog you guys - crap.

It's okay - I'll go sailing today for BOTH of us.

BOAT wrote:Work is killing me - I was supposed to be off today but can't get away - I really wanted to sail today and tomorrow because it's the first two days of all day sunshine we are going to have on the coast over here in weeks. Crap - I want to test the speed runs on 'boat'.

I am so busy with work I can't even blog you guys - crap.