Here's something which is not as well known as it should be:
( ON EDIT :: provided there's a reasonable amount of wind :: )
A mainsail makes a good 'air rudder' if the boom is locked in position.
Say by setting up a preventer, and tightening the mainsheet against it.
(Or with a small sail or light wind: by a beefy crewmember or two pushing the boom forward against the mainsheet)
Particularly if engine power is available, such an 'air rudder' dodge can be a boat saver.
The more drive you have available in relation to wind strength, the closer in you can trim the boom, but on a beam reach it will work even with minimal engine power.
The thrust from the prop is beneficial in all phases, most obviously when you consider the beam reach scenario:
- when the sail is drawing properly, prop thrust reduces the imbalance from the fact that the sail drive is acting to leeward, trying to make the boat 'round up'.
- when the sail backwinds, the prop thrust is doubly beneficial:
Firstly because the turning moment (to correct the heading so the sail will draw again) is amplified.
This is because the sail thrust and the opposite propellor thrust combine to form a 'couple', which creates corrective yaw more quickly and positively than the sail alone
Secondly because it helps maintain boatspeed, or "way".
Loss of way is a problem if you are using the 'air rudder' dodge without engine power, during the phase in the cycle when the boat is heading too high, backwinding the main.
It's not strictly 'steerage way', because you don't have a rudder, but it's "directional stability way", because water
flow over the hull
underbody - and especially the keel
- creates directional stability.
If there is no engine power available, it will probably be necessary to "actively steer" using the air rudder.
This means that someone should continually trim the mainsheet, while someone else loosens or tightens the preventer against the new boom position (or pushes the boom as above, if realistic).
It IS essential that the boom is not free to move at all, otherwise much of the drive is wasted because the boat will have to head
up a long way before there is any steering
Ideally the person trimming the mainsheet should have had experience sailing a windsurfer. Windsurfers ONLY have an "air rudder". Preferably for at least a few hours, enough to get past the phase of steering purely by inclining the mast
fore and aft, to have acquired a good feel for steering by sheeting in and sheeting out.
(Or better still, experience of sailing a small dinghy without a rudder, simply using the sail)
ON EDIT :::
A headsail (with another savvy trimmer) will help if the boat is tending to round up too easily.
The following comments are not applicable to the OP-type sudden emergency
, but might be worth considering in circumstances which are less fraught:
If the boat tends to bear away too much (which is unlikely, unless the rudder is jammed rather than absent), drop any headsails (rather than just furling). I guess you could try easing the halyard
to let the draft
in the main go further aft. If this doesn't help, try towing a bucket or small drogue
off the windward chainplates on at least a boatlength of line. If you have more time, substitute something with less drag anchored further outboard
, say a long warp with nothing on the end, towed from a long, improvised outrigger (say a spinnaker
pole with inboard end clipped to the chainplate, with a lift
and two guys, and a block at the outboard end). Adjust the length of the warp in the water
to vary the drag.