posted 08-13-2012 07:28 AM ET (US)        

First of all, all the boats in our fleet had digital selective calling (DSC) radios, and the DSC radios were integrated with GPS receivers and chart plotters. We could all request a position report from the other boats, or we could send a position report to the other boats. This proved to be very handy. For example, one evening one of the boats split from our group and headed to a different overnight stop. When they arrived, they sent a position report. We knew they had made it to their destination. And, of course, we could selectively call the other boats to initiate a conversation.

In addition, two of the boats had AIS receivers. When we were in the St. Mary's River, a very busy commercial shipping channel, the AIS receivers let us know when freighters would be passing our anchorage. We even found another recreational boat with a Class-B AIS transponder.

posted 08-16-2012 08:12 AM ET (US)            

All the boats had modern SONAR units.

Two of the six boats had electronically operated engine throttle and shift controls.

I believe most of the boats had a NMEA-2000 network. One boat had a SmartCraft network.

That is a lot of small boat electronics in the fleet of six boats.

The outboard engines were also quite electronic and modern. Five of the six boats had low-emission engines. Two of the boats had four-cycle outboard engines with sophisticated electronic fuel injection. The other four boats had two-cycle outboard engines, but three of those four were modern, direct-injection, low-emission, computer controlled two-cycle engines. There was only one boat with conventional carburetor two-cycle engines.

posted 08-20-2012 01:35 AM ET (US)            

In the past they would let you add XM radio for $10 per month if you had the weather package. I recently installed a XM satellite compatible stereo so someday I might add that.

http://www.xmwxweather.com/marine/data-service-pricing.php

One of the electronics Jim forgot is the heading sensor that is connected to my GPS/Radar. It allows the radar and chart plotter to work much better together and MARPA is now functional. I will need more time to play with this feature before I really have a handle on what it can do.

posted 08-21-2012 08:41 AM ET (US)            

XM Marine Weather service via satellite is available with GARMIN's GXM 51 receiver with MSRP $700. See

http://www.xmwxweather.com/marine/
https://buy.garmin.com/shop/shop.do?cID=394&pID=28710

Sirius Marine Weather service via satellite is available with a LOWRANCE LWX-1 receiver with MRSP $99. See

http://www.siriusxm.com/marineweather
http://store.navico.com/ProductDetail.aspx?ProductName=000-0022-46

posted 08-21-2012 08:54 AM ET (US)            

http://www.kvh.com/Leisure/Marine-Systems/Compasses/All-Compass-Systems/ Azimuth-1000.aspx

This also provides a heading output on NMEA-0183, and this tells the auto-pilot the boat's current heading.

posted 08-21-2012 09:46 AM ET (US)            

Unfortunately the Simrad AP 11 Auto pilot's flux gate compass will not share data with the rest of the electronics on board.

The information shared between the Garmin GPS and the Simrad Auto pilot only goes one way, location information is sent to the Auto Pilot from the Garmin.

I have the Garmin GDL 30A weather data receiver. This unit can receive XM audio as well as weather. It has a black box and an antenna while the new receivers have eliminated the back box.

posted 08-22-2012 08:10 AM ET (US)            

There are GPS heading sensors, but they are based on having two GPS receivers. The GPS receivers are usually located several feet apart and oriented so they lay on a line that is 90-degrees to the keel. The two GPS receivers independently compute their position. Then the heading is deduced by knowing the location of the GPS receivers relative to the bow. This is the only way you can deduce heading from GPS--you need two receivers and a sophisticated algorithm. Such a sensor can also calculate roll angle by comparing the elevation of the two receivers.

A GPS dual receiver system that can deduce heading is rather expensive, about $2,500 for the least expensive one I can locate. A magnetic compass based heading sensor is less expensive, about $500 to $600.

I don't know why marine heading sensors based on magnetic compasses are still so expensive. There is a nice magnetic compass sensor in an Apple iPhone. An iPhone also has a nice inclinometer and accelerometer, too.

posted 08-22-2012 08:20 AM ET (US)            

"Yeah, it calculates the heading from movement. The only time it's wrong is when the boat is stopped, at which point it reads some random heading which has no relationship to the way the boat is pointing."

No, this is wrong. The heading is often quite different from the direction the boat is moving. I already gave you two examples of this:

--the boat is making sternway. The heading is roughly 180-degrees from the direction of movement;

--the boat is drifting in current, wind, or waves and laying abeam; the heading is 90-degrees from the direction of movement.

Even when making forward propulsion the heading is often different from the direction of movement due to the influence of current or wind.

posted 08-22-2012 11:56 AM ET (US)            

M-W.COM says:

heading

posted 08-22-2012 12:50 PM ET (US)            

Jim responds "A GPS receiver cannot measure heading."

posted 08-22-2012 03:05 PM ET (US)            

In aircraft the radio-frequency transmitters in iPads--if they have that option--are required to be switched off by regulations. They're not likely to be a source of interference when off. Let's see if they could interfere when switched on.

The L1 carrier power on a GPS satellite is about 25-watts. The antenna is specified to have 13-dB gain, so the effective transmitter power is 500-watts. This is a power level in dBm of +57dBm. A GPS satellite is roughly 21,000-KM above the earth. The path loss is about 182-dB. The signal level on the ground should be about 57 -182 = -125dBm.

The transmitter in an iPad is specified with an output power of +15dBm. The transmitter in an iPad is not on the GPS frequency, but lets assume it might be malfunctioning and can interfere. An aircraft at 25,000 feet is 7.6-KM above the earth. The path loss is about 114-dB. The signal level on the ground, assuming no loss for passing through the metal fuselage of the airplane, would then be about 15 - 114 = -99 dBm.

Although the GPS satellite transmitter is more powerful than the iPad transmitter by 42-dB, or by a factor of about 15,000-times, the GPS transmitter is farther way from the Earth than an iPad in an airplane might be. The result is that the signal from an iPad at 25,000-feet might be stronger than the signal from a GPS satellite at 21,000-KM in orbit. The difference is about 25-dB in favor of the iPad, but, again, we are ignoring the effect of the iPad being inside an aircraft fuselage. I suspect that the attenuation of being inside a metal tube of the fuselage will significantly reduce the effective radiated power of an iPad toward the earth. However, you can see why you are required to shut off radio transmitters on airplanes. They could produce signal levels much greater than the GPS signals.

posted 08-22-2012 07:58 PM ET (US)            

If you travel slow over long distances or you are swinging on an anchor your compass and GPS will disagree. If they do not something is wrong with your compass.

The heading sensor allows the chart plotter and the radar to line up. In other words the radar is always aiming at the bow, but the chart plotter aims at the direction of travel, not necessarily the bow.

If all you do is travel at high speeds in a little boat your compass and GPS will read close to the same when you are running.

The correct terms:

Heading is the direction you bow is aimed. (compass)

Course over Ground (or Track) is the direction the boat is traveling in relation to the bottom. (GPS)

If there is any wind or current it is doubtful that the heading and the Course over ground will be the same.

I did not spend $600 on a heading sensor but please do not feel sorry for me because I own one.

posted 08-23-2012 09:48 AM ET (US)            

Also, the Ritchie compass which comes with a 170 Montauk moves around a lot when the boat is in motion over anything but 100% flat seas, and thus is pretty hard to read with one or two degree accuracy.

And I also will admit that it is good to be aware of those situations, that you list, in which the two devices could be far enough apart, that only the magnetic compass heading shild be trusted (and one should also be aware of situations/disturbances that affect the compass reading).

I am also sure that there is probably still something wrong with my corrected statement.

posted 08-23-2012 11:09 AM ET (US)            

I understand how it works and choose to add a heading sensor to my boat to make the system more accurate. I do not think it was a wast of my money.

I do not have a heading sensor on my 16 Sakonnet, I also understand that my GPS really does not know my heading. But in this boat I don't care if the GPS is a little confused.

I can say with complete honesty that my 1966 Richie compass, often agrees with my GPS receiver. All this tells me is that I have little deviation on the Sakonnet and I usually run it in calm water at high speed.

I can also say that my 2011 Richie compass on my 23 Walkaround hardly ever agrees with my GPS. This tells me that the 23 has more deviation, runs in rougher seas, travels more miles at slower speeds, and of course the compass might need adjustment.

I find it kind of funny that I am justifying my electronics in a post about small boat electronics.