Category Archives: About Sunny Spells

Descriptive posts about Sunny Spells, her equipment, history and performance.

Rigging a Boom Brake

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Before I left Sydney for Hamilton Island in May, I rigged a boom brake to control the boom during gybing. I knew I was going to be short-handed some of the time and a recent fatality on the coast where a man was killed when struck on the head by the boom during an accidental gybe was fresh in my mind.

My boom brake is very simple – a Figure 8 “rescue descender” used for rock climbing, which I got off EBay for $45, and 20ft of 1/2in polyester braid line. The line is shackled to the starboard toe-rail and runs through the Figure 8 to a block shackled to the port toe-rail and then back up to the cabin top jammers via a free sheave in the line organiser. I use a winch to grind it on. Because of the set-up, it effectively has a 2:1 purchase.

When the line through the Figure 8 is slack, there is virtually no friction and thus no resistance. Wind the boom brake line on tight though, and quite a bit of braking effect is generated. The tighter the line, the higher the friction. I had to play around a bit with the attachment points on the toe-rail and boom to even out the friction through the boom’s swinging arc, but as the boom attachment lug can be moved anywhere between the vang attachment and the boom end, this was not an issue. I’ve opted for higher friction at the end-of-arc, with less friction on the centreline. This way I know the boom will be gradually slowed down as it reached the end of its travel.

A boom brake has a number advantages:

  • The forces on the boom are controlled by friction, so it won’t over-stress the boom when dragging the boom in the water.
  • The boom brake acts as a second vang, pulling down on the boom towards the toe-rail. This is beneficial side-effect is most noticable when running with the main let off. Under these conditions I’ve found the vang to be a bit under-powered; the boom brake significantly reduces the strain on the vang.
  • If you managed to break the traveller or otherwise stuff up the main sheet, the boom brake could be used as an emergency main-sheet.

Connecting NMEA input to Navman 8084 Chartplotter

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When I got around to installing the AIS receiver in Sunny Spells, I had a bit of a frustrating time working out how to connect to the NMEA input of the Navman 8084 (similar to Navman 8084, Northstar M84 and Northstar M120). The handbook was no help at all…

A quick call to Navman confirmed what I had found elsewhere on the internet:

  • The NMEA 0183 input is on the white “Fuel/Nav” input on the Navman/Northstar unit.
  • You need one of the Navman/Northstar extension cables with an LT8 plug on one end and bare wires on the other. AA002412 is one of the part numbers but there are several others that would work (different cable lengths have different part numbers…). Your friendly Navman/Northstar dealer can help you order the right cable.
  • The WHITE and BLACK wires on the extension cable are used for the NMEA 0183 input. Black (you guessed it…) is the NMEA negative (earth) and white is the NMEA positive.

Navman 8084/8120 and Northstar M84/M8120 rear connections

Remember that the chartplotter will look for some inputs on the NavBus input if that option has been activated. For example, if NavBus is turned on, the chartplotter will NOT look for wind data on the NMEA input even if there is no wind data on the NavBus input – very annoying! The NMEA input automatically selects the high speed option (38400 baud) if a high speed data stream (such as AIS or a multiplexer with a 38400 baud output) is connected.

Getting Navbus, Seatalk and NMEA to talk

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Wout at Brookhouse replied to my e-mail today. (Thanks Wout for your prompt reply and the USB driver). Of course, I remembered this morning that I had printed out the user manual for the MUX and had bound it nicely, with a pocket for the CD, in the back…

The MUX is now connected to the PC via USB. All operating at 38,400 baud as the AIS is also connected. Digiboat’s Software-on-Board shows all wind data present and correct (see screenshot). So where’s the problem?

Looks like I’ll have to go to Navman for this one…

Wind data displayed on SoB

Getting Seatalk, Navbus and NMEA to talk…

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Here’s a mystery…

The navigation electronics on Sunny Spells is a mix of Navman and Raymarine equipment. Navman has its own, proprietary communication protocol called ‘NAVBUS’. The Raymarine equipment uses ‘SeaTalk’. On top of this, there is the AIS receiver which transmits data in NMEA0183, but at 38,400bps (bits per second), while most NMEA equipment communicate at the slower data rate of 4,800bps. Sounds like a nightmare to get them all to talk?

Yes and no…

The core of the communications backbone is the Brookhouse multiplexer (MUX) with SeaTalk and AIS options and USB output. When I installed the MUX, I wasn’t expecting immediate success; imagine my delight when the whole system aapeared to work straight out of the box – no modification required!

However, I have started playing around on the chartplotter, and all is not as it seemed…

The data from the wind instrument is not available on the plotter or the Repeat 3100 (both on Navbus). I’m assuming it’s a Seatalk to NMEA conversion issue, but I haven’t been able to investigate as I can’t get the laptop connected. Unfortunately I seem to have mislaid the CD with the USB-Serial driver for the Mux, so I can’t get the laptop connection sorted – aaargh! I’d love to have the wind info on the plotter (which is at the helm) as the crew are forever blocking my view of the wind instrument!

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