GPS Receiver Below Decks

[Tim Godber]

Only slightly relevant, BUT, years back in dense fog in the Chenal de Four, we were counting off the bouys and marks as we were swept through at about 6 knots. We were all out watching, wife me and sons, every time my wife came out to scour the 25 yards visibility the GPS signal went, it was son no 1 that spotted that she always sat on the coaming, it was me that spotted that the GPS antennae was under that coaming, and son number2 that was unwise enough to point out that the GPS signal could get through 1/2 inch of fibreglass but not get through his mothers butt!

[Bill_E]

I may hijack this thread a bit. I'm looking for a good, inexpensive USB receiver to run with Open CPN on my laptop. Can anyone suggest the most sensitive receiver? I'd like to use it (mostly) below deck.

Bill

[a64pilot]

Originally Posted by Tim Godber

Only slightly relevant, BUT, years back in dense fog in the Chenal de Four, we were counting off the bouys and marks as we were swept through at about 6 knots. We were all out watching, wife me and sons, every time my wife came out to scour the 25 yards visibility the GPS signal went, it was son no 1 that spotted that she always sat on the coaming, it was me that spotted that the GPS antennae was under that coaming, and son number2 that was unwise enough to point out that the GPS signal could get through 1/2 inch of fibreglass but not get through his mothers butt!

That is indeed hilarious, surprised you guy's figured it out

[glenn.225]

Originally Posted by Bill_E

I may hijack this thread a bit. I'm looking for a good, inexpensive USB receiver to run with Open CPN on my laptop. Can anyone suggest the most sensitive receiver? I'd like to use it (mostly) below deck.

Bill

Just got a GlobalSat BU-353-S4 USB GPS Receiver from Amazon. Works very well, seems like best specs for this purpose.

[Ziggy]

Originally Posted by Robin3

Will it not display apparent wind data since the two are the same, the precise display label is meaningless?

Robin,
I don't mean to be pedantic, but they are not the same. Apparent wind angle is measured relative to the boat's heading; apparent wind speed is measured relative to the boat. The true wind angle and speed are relative to the North and speed, respectively. The Maretron display still wants to have the heading and speed to display the true wind, even when the speed is zero. So you need to provide it with data from a heading sensor and a knotmeter or gps. While the GPS will not provide the heading for the wind display, it does provide speed over the ground.

[Robin3]

Originally Posted by Ziggy

Robin,
I don't mean to be pedantic, but they are not the same. Apparent wind angle is measured relative to the boat's heading; apparent wind speed is measured relative to the boat. The true wind angle and speed are relative to the North and speed, respectively. The Maretron display still wants to have the heading and speed to display the true wind, even when the speed is zero. So you need to provide it with data from a heading sensor and a knotmeter or gps. While the GPS will not provide the heading for the wind display, it does provide speed over the ground.

I don't mean to be pedantic but if there is no boatspeed or heading data received then apparent windspeed and true windspeed displays will look just the same if the boat is stationary as in in a slip or anchored if the boat starts to move then apparent and true windspeeds will differ but if you are talking about is the wind direction NE and then changes to maybe ENE then I agree you need more information to display it. What I said/mean is for all practical purposes the two sets of displayed info are the same IF the boat is stationary but yes that is relative to the boat not to true north or magntic north or to Fred Bloggs boat on the next dock....

[noelex 77]

Originally Posted by Ziggy

Robin,
but So you need to provide it with data from a heading sensor and a knotmeter or gps. While the GPS will not provide the heading for the wind display, it does provide speed over the ground.

I do mean to be pedantic

It is helpful to understand that "true" wind derived from STW (from the log) and SOG (from the GPS) are different.
The nomenclature is confusing, but the convention seems to be to call the former "true wind" and the latter "ground wind". These are not great terms, but we are stuck with them. (Many argue ground wind is the real "true" wind).

The situation is further complicated by many instrument referring to "true", or ground wind relative to the bow, others insisting that the terms are only valid when referenced to compass heading. Throw in difficulties such as leeway and it is a complicated subject and does need a pedantic approach to understand.

The most important factor is to understand what your instruments are displaying. If setting up your instrument system consider if "true" or "ground" wind is better. (If the instruments offer this option)

[Robin3]

Originally Posted by noelex 77

I do mean to be pedantic

It is helpful to understand that "true" wind derived from STW (from the log) and SOG (from the GPS) are different.
The nomenclature is confusing, but the convention seems to be to call the former "true wind" and the latter "ground wind". These are not great terms, but we are stuck with them. (Many argue ground wind is the real "true" wind).

The situation is further complicated by many instrument referring to "true", or ground wind relative to the bow, others insisting that the terms are only valid when referenced to compass heading. Throw in difficulties such as leeway and it is a complicated subject.

The most important factor is to understand what your instruments are displaying. If setting up your instrument system consider if "true" or "ground" wind is better. (If the instruments offer this option)

exactly so, but I fail to see what in real terms the point is of inputting the extra data streams makes and brings to the party to a stationary boat, to a moving one sure but not to one tied up in a dock, unless of course the whole marina has broken adrift and is hurtling downstream...

AND to think today I spent 2 hours with my electronics wizard trying to explain to him why I wanted boatspeed data as well as SOG/groundspeed data, he could not see why anything other than GPS speed, (SOG) was useful unless racing and even then said he didn't use it

maybe it is me that is out of step here, but I just spent an extra $400 or so being able to provide boatspeed from a paddlewheel transducer to my new Garmin N2K set up and that would have bought some other kind of valentine's present for the boat or even for the boss lady.

[noelex 77]

Originally Posted by Robin3

exactly so, but I fail to see what in real terms the point is of inputting the extra data streams makes and brings to the party to a stationary boat, to a moving one sure but not to one tied up in a dock

Nobody buys and installs a sophisticated wind instrument display to give them information when tied to a dock.
In this situation Apparent wind strength will equal ground wind strength. Ground wind and apparent wind direction relative to the bow will be the same, but ground wind may also display the ground wind compass direction which can be useful although it is an easy mental calculation.

True wind will be different to the apparent wind (if there is current). True wind is not really giving you any meaningful data tied to a dock and generally best ignored.

Originally Posted by Robin3

maybe it is me that is out of step here, but I just spent an extra $400 or so being able to provide boatspeed from a paddlewheel transducer to my new Garmin N2K set up and that would have bought some other kind of valentine's present for the boat or even for the boss lady.

This is an easy one. Spend the money on the lady

Don't forget Valentines day.

[StuM]

Originally Posted by Robin3

AND to think today I spent 2 hours with my electronics wizard trying to explain to him why I wanted boatspeed data as well as SOG/groundspeed data, he could not see why anything other than GPS speed, (SOG) was useful unless racing and even then said he didn't use it

maybe it is me that is out of step here, but I just spent an extra $400 or so being able to provide boatspeed from a paddlewheel transducer to my new Garmin N2K set up and that would have bought some other kind of valentine's present for the boat or even for the boss lady.

It's frequently useful to compare ground speed and water speed to determine what sort of current you are in.

[goboatingnow]

Just to deal with the source select. This is an area that NMEA neglected and there no easy answer. Unless specifically supported in the mfd and in every other device that wishes to receive GPS., there's no easy answer to auto switcher

Dave

[Robin3]

Originally Posted by StuM

It's frequently useful to compare ground speed and water speed to determine what sort of current you are in.

which is one reason why I wanted to have it. speed checks by the marine police et al too are related to waterspeed as read from a radar gun too not SOG I believe and in a 2kt contrary flow you could be doing 8kts in a 6kt limit whilst the GPs display is showing just 6kts SOG

I also spent most of my cruising years in areas with high tidal currents and navigation as in dead reckoning required knowledge of distance and direction travelled plus tidal current predictions and estimated leeway. In determining a useful CTS across changing streams, an idea of boat speed through the water is necessary. Boat speed at a known engine rpm in calm waters can also gives an indication of when fouling of the hull is occurring and may probably be spoiling performance. On my last 41ft sailboat a slimy bottom woudl knock 0.5kts off the usual 6.5kt cruise speed under engine at 2000rpm or require more rpm (and fuel) to maintain that 6.5Kts, time for baby's bottom to be wiped...

[Robin3]

Originally Posted by noelex 77

Nobody buys and installs a sophisticated wind instrument display to give them information when tied to a dock.
In this situation Apparent wind strength will equal ground wind strength. Ground wind and apparent wind direction relative to the bow will be the same, but ground wind may also display the ground wind compass direction which can be useful although it is an easy mental calculation.

True wind will be different to the apparent wind (if there is current). True wind is not really giving you any meaningful data tied to a dock and generally best ignored.


This is an easy one. Spend the money on the lady

Don't forget Valentines day.

too late she wrote the cheque to the elctronics man herself this morning and we agreed to forego the gifts and cards in favour of a decent meal out in good company (us)

[Dockhead]

Originally Posted by Ziggy

Robin,
I don't mean to be pedantic, but they are not the same. Apparent wind angle is measured relative to the boat's heading; apparent wind speed is measured relative to the boat. The true wind angle and speed are relative to the North and speed, respectively. The Maretron display still wants to have the heading and speed to display the true wind, even when the speed is zero. So you need to provide it with data from a heading sensor and a knotmeter or gps. While the GPS will not provide the heading for the wind display, it does provide speed over the ground.

Actually True Wind is also displayed relative to your bow. True Wind is derived from Apparent Wind by applying Speed Through Water vector. No Heading or COG is needed, because True Wind is displayed relative to the bow.

Ground Wind is derived from Apparent by applying COG and SOG vectors - it needs COG so that it can be displayed relative to North. The instrument should use Heading if there is 0 SOG and no COG, but it doesn't. Nor will it give True Wind relative to North.

[Paul Elliott]

Originally Posted by Dockhead

Actually True Wind is also displayed relative to your bow. True Wind is derived from Apparent Wind by applying Speed Through Water vector. No Heading or COG is needed, because True Wind is displayed relative to the bow.

Ground Wind is derived from Apparent by applying COG and SOG vectors - it needs COG so that it can be displayed relative to North. The instrument should use Heading if there is 0 SOG and no COG, but it doesn't. Nor will it give True Wind relative to North.

The Ground Wind calculations should also include Heading because, due to leeway, the boat isn't usually pointed in the exact same direction as COG. AWA is relative to the bow, so this will make a difference when calculating Ground Wind. I don't know how many instruments do this, but, for example, NavMonPc (my free program) does this.

For selection among multiple GPS sources, some NMEA multiplexers (ShipModul, for one) will let you select a primary source and a backup. NavMonPc also does this.

I've installed GPS antennas / receivers under the fiberglass deck, and they have worked well. Given the option, I would prefer to have the GPS outside in the clear.