Preventer for the Main

Yesterday I rigged a preventer to use when running downwind. It consist of about 30 meters of 8mm polyester braid line(stretchy) that runs all the way to the stem (outside everything), through a snatchblock and back to a jammer on the cabin top.

I can control the preventer from the cockpit and put it on a winch to set and (most importantly) ease off gradually when necessary. Sunny Spells has the main sheet attached to the end of the boom, and the preventer is attached at the same point, using a snap-shackle. This prevents bending moments being applied to the boom by the preventer and main sheet working in opposite direction, resulting in a broken boom… Fortunately this arrangement is also good at preserving the boom intact should it get dragged in the water!

At 25 meters, the preventer is long, almost double the required length. I did this on purpose so that I can just throw off the jammer when changing tack without worrying that the line will pull through the clutch. I just leave it shackled to the boom and re-run it after the gybe. Also, the snap-shackle stops the line from pulling through the snatch-block on the bow. When I need to re-set (or stow) the preventer, I just pull it through in the cockpit, flaking it on the cockipt floor, until the snap-shackle stops in the snatch-block on the bow. Now I go forward, swap the snatchblock to the opposite toe-rail, take the snap-shackle (outside the lifelines) and walk back to the cockpit, pulling it through as I go. I just snap-shackle it to the boom end, close the jammer and pull it tight – too easy.

Rigging a Boom Brake

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

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

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