Tuesday, 18 March 2008

Moth Mast Raking System.

This is old news but I have been getting hassle again to publish my diagrams of this system and I found the diagrams during the move to the new flat...

Back in the mists of time (~1994) Adam May and I build 2 Moths. I was in my second year of University and Adam was finishing his A-levels. The boats were Aussie Axemen and mine "Tomahawk" has done the last 2 Worlds and Europeans. It is the oldest or one of the oldest Moths to foil.

One of the ideas we copied was Klaus Hofer's Moth Mast raking system This system was amazing and is detailed in the 1994 yearbook, which can be downloaded here: http://www.int-moth.org.uk/Archive/94Yearbook.pdf

We reworked his system and built in the kicker so that it was automatically tensioned. Here is the description of the system from my Moth Paper (http://www.culnane.net/dc/sailing/moth/wshpaper/appen2.htm):

This system is a development of Klaus Hofer's Moth mast raking system. It allows the mast to be raked by 12° without losing rig tension and on one control line.

The shrouds and forestay are in effect tied together. Their geometric positions and their relative purchases are such that as the mast is raked the rig tension stays constant.

The shrouds are shackled onto 3:1 levers. The non-linear purchase of these levers (due to their operation over a 90° arc) means that the system can be turned to either increase or decrease rig tension with rake, if so desired.

The movement of the mast is stopped by the jammer which cleats the zig-zagging movement of the forestay in both directions. The jammer is uncleated by tension in the line connected to its trigger. There is tension in the line when the grabber is engaged by a pull on the control line. Then, once the sailor releases this line, the grabber flies back and the trigger engages the jammer. The system is then ready for another pump. There are two pump systems which move the mast backwards and forwards.

The system is designed so that, if there is a failure in the grabber mechanism, the rig will remain cleated by the jammer and there will be no mast movement.

The cascade kicker is linked into the system to keep the leach tension reasonably constant as you rake. Although it is set up to ease slightly as the mast goes back. This twists and de-powers the sail. Tuning is required to achieve the best effect.

The raked kingpost sets up the kicker so that it is eased as the boom is sheeted away from the boats centre line, if the mast is upright.

When the boat is on a dead run (and the mast is forward) the kicker can be applied to hold the boom hard against the shroud. This makes the boat easier to sail down-wind. However, the kicker has to be played during a gibe as the boom will not sheet in if the kicker is not eased.

In strong winds the raked mast is parallel to the kingpost and therefore kicker tension is constant with sheeting angle. This is important as a very twisted leach can be extremely uncontrollable down-wind in strong winds.

There is as boom lifter ( flattener ) control on the boom so that it can be lifted making tacking and jibing a lot easier when raked.

Here are the 2 missing diagrams (pictured with my mobile phone because I do not know how to use the scanner at work).

The system did work surprisingly well on the two boats but it took a lot of tuning and maintenance to keep it working well. The ropes and controls were hidden under the foredeck and lead through wing tubes...

When I got my boat delivered to the Euros by Adam in 2004 one of the first things I did way disable the system. I think it is too fiddly and heavy to be worth doing on an other Moth. However it was a very cool gimmick and you could freak people out in the dinghy park as they could not really get their head round it at first.

No comments: