Adjusting the knot meter vs SOG of GPS

[Craig Weis]

I have a Raymarine ST-40 instrument on my CP-19 and a Raymarine 435i chartplotter mounted on the boat.

The chartplotter will give and indicate SOG [speed over ground].

The ST-40 can be set: factory default, or adjusted to read the boat speed, faster or slower in 0.1 tenth's of a knot.

So the other day I was traveling on the 'iron wind' [my 5 hp O/B] for 17 or so miles back home from an overnighter in a dead calm bay of Green Bay with zero current and my boat putting along at a very steady speed with a arrow straight wake with a bare pole.

Being bored I was using both instruments and set the ST-40 to read about 0.1 [knots] less then the chartplotter's SOG. So the ST-40 is no longer in a factory default mode.

Question...which is more correct?

1~The ST-40 in factory default mode?
2~The ST-40 in an adjusted setting?
3~The Chartplotter's SOG?
4~The ST-40 set to the SOG? Plus or minus 0.1 knots.

Any of these change the nautical miles traveled. And will change my S-T-D, [speed-time-distance] for dead reckoning on charts.

Insteresting side note: I can not set the ST-40 deadnuts to the SOG. It is either a tad high or a tad low. This has to be because the time between sampling speed using the ST-40 vs the sampling of speed from the chartplotter.

So who has expirence about these matters?

Thanx for the thoughts in advance. skip.

[mrb]

Without knowing what is giving information to ST-40 (wind, compas or depth) hard to comment on that however chartploter is probably getting info from compass and as far as speed over ground the plotter is usually most accurate.  understd that now days they have some sonar devices that will give sog but most small boat speed indicators give water speed and to count out all current which you would have to do to turn speed through water into sog would always be asking a lot.  over the short run though wouldn't make a lot of diff.

Interesting question and will be interested to see what other more experienced members have to say.

iron wind is much better than no wind
mrb

[Craig Weis]

As per the last line...errr never mind. It was 'dirty'...I knew a girl who said that her brother would say, "incest was better than no cest" and the cadence of those words were the same cadence I heard in my mind when I read the 'Iron wind line'. And that reminded me of her words.

The Raymarine ST-40 displays...depth, speed, water temp, alarms for shallow, and drifting say away from an anchorage area, may be into deeper or less water depths.
I agree I can nearly set the ST-40 to the chartplotter's SOG while in zero current and zero wind and forget it. 

[mrb]

last line meant it was better to be on the water under most conditions than not to be on the water. I haven't been on for two months and that's getting old.

Thinking over your question I think I understand that not having current and or wind then taking sog from chart plotter you should be able to set  through water to (as read on ST-40) match sog.  Then until conditions change both units should read same.  Sounds right to me. I agree with your assessment of sample times ST-40 versus chart plotter. 

As to your ultimate question of which is more correct I have to go with sog of chartplotter.  Sailing in the pacific north west many of times (most of time) I have seen speed through water readings of 5 and more knots with a ground speed of 0 to 1 knot or even boat loosing ground.  I've also seen the opposite, 1 or 2 knots through the water but 7 and higher sog.  Thats why I like the gps reading for sog but in those conditions knowing  water speed is also vital.

working to get back on water this week end.
gs
mrb 

[curtisv]

Skip,

The GPS chartplotter and the knotlog (ST-40) measure different things.  The GPS measures speed over the bottom.  The knotlog measures your speed relative to the surface water (boat speed).

If there truly is no current then SOG and boat speed are the same.  It is very hard to detect a few tenths of a knot of current by just looking around.  If you really want to check your knotlog, then motor in one direction and then in the other at the same speed according to the GPS (SOG).  If you get the same reading from the knotlog in both directions then you really do have no current.  Otherwise you have to average the two numbers and adjust until the average is the same as the GPS reading.

My knotlog was quite far off when I got it.  I never went to this trouble but adjusted until the GPS and knotlog seemed to agree and was closer.  I sail in such variable currents that trying to get and accurate DR is fairly futile but still fun trying.  When safety is at stake I cheat and check the GPS.

Curtis

[Craig Weis]

Ah haaaaaa! Good point in coming about for a second run at a different pass. Like the salt flat racing course, adjusted for any windage. Thanks I have to check into this over the weekend. Sounds like fun. skip.