Help best VMG Downwind this time

[Dsanduril]

Originally Posted by 44'cruisingcat

To reef when going downwind or deep reaching, I'd first make sure the gybe preventer is off, then move the traveller up toward the centre then sheet on a bit, but still leaving some mainsheet out.

This allows the boom to lift.....

Exactly the same for us, I couldn't say it any better, so +1.

And our observation is that tacking downwind on our boat the gybe is usually 60-80 degrees; 30 to 40 degrees either side of DDW to keep the sails happy, which means 15-25% increase in speed required. Sometimes that happens, sometimes not.

[vorpal blade]

I use both methods and it all depends on whether I want go get to the "finish" as lazily as possible by soaking flat off or enjoy the sailing of the boat to its maximum with either the Zero or gennaker and doing a few gybes along the way. Other considerations in this formula are the windspeed and the availability of other crew.

[Dave_S]

Gybe preventor = a rope from boom to stay on my boat. Is that the same hi-tech state of the art equipment everyone has or is it a real thing?

[Dsanduril]

Originally Posted by Dave_S

Gybe preventor = a rope from boom to stay on my boat. Is that the same hi-tech state of the art equipment everyone has or is it a real thing?

Ours is a rope to a block mounted on a strong point at the aft quarter then to a winch in the cockpit that allows us to adjust it easily. But basically its just a rope from the boom to something strong outboard.

[fxykty]

Originally Posted by Dsanduril

Ours is a rope to a block mounted on a strong point at the aft quarter then to a winch in the cockpit that allows us to adjust it easily. But basically its just a rope from the boom to something strong outboard.

We mount our preventer from the end of the boom to a snatch block soft shackled to the midship cleat (which is adjacent to just aft of the mast) then back to a cockpit winch. It needs to be quite tight to take the stretch out of it and prevent the boom from working. In preparation for a gybe we take it forward to the gooseneck and make it fast there. Gybe, then ready to go on the other side.

Stretch or no stretch for a preventer?

[44'cruisingcat]

Originally Posted by Dave_S

Gybe preventor = a rope from boom to stay on my boat. Is that the same hi-tech state of the art equipment everyone has or is it a real thing?

I mounted stand up blocks about 1.5 metres aft of the chainplates.

A line from the end of the boom through one of these to a secondary winch is my preventer arrangement. It also pulls down on the boom which holds the sail clear of the shrouds and spreaders.

[Dsanduril]

Originally Posted by fxykty

Stretch or no stretch for a preventer?

My thinking is that if you keep it made up tight then no stretch, stretch if it is used loose and subject to shock loading because the boom can start moving.

Originally Posted by 44'cruisingcat

.... It also pulls down on the boom which holds the sail clear of the shrouds and spreaders.

+1. Once the end of the boom gets past the end of the traveler this helps with sail shape and also reduces chafe (as the mainsheet being out allows the boom to go up and down with gusts). On monohulls this would be the job of a vang, but with our width we prefer to use the preventer for both duties when running deep.

[Just Another Sa]

Originally Posted by Dave_S

I enjoy upwind more than downwind, maybe I'm doing it wrong (in fact as I write/process this I am realising that I am doing it wrong) I haven't had to do much/any that I can think of direct downwind sailing. My targets have always been reachable downwind on a single run so I just point it at the target and make the best of it, but that is probably why I haven't found it exciting with flapping sails and no apparent.

I spoke to a guy on a similar boat who gibes at 145* TWD and sails at 80* apparent. So he is actually sailing upwind anyway. That's about wind speed so it sounds close to do-able for me ???

Just looking for some other people's experiences to give me an idea of what to aim for and some short cuts on the best VMG.

I worry about not reefing and what is the best approach if I miss my reefing point and find I have too much sail up. Can I turn direct down wind, let out more Genoa and align the main backwards directly into the wind and bring it down a couple of reef points ? Or is it just going to ballon out.

Depends on true wind speed and sea state.

In heavier air you might concentrate less on VMG, and increase safety by using smaller sails and pointing higher to keep apparent and boat speed up and VMG still reasonable for a cruising boat. When a gust hits just bear away to DDW course and apparent reduces enough not requiring immediate further reefing. Even better if that happens automatically if rudders lift of or ventilate or stall, meaning boat balance tend to make it bear away, not autopilot or you. If the boat tends to jump off wavecrests it's easy to momentarily change course a little to avoid that, as high flowspeed around rudders make them work great. In such conditions don't forget to reef down mainsail enough, as it's far easier to sheet a headsail or assy completely out to reduce power by flapping like a flag than reef the main quickly while sailing downwind if not done early enough.

In less than 4...6 knots true DDW is typically also faster, as low Reynolds number prevents normal lift coefficients when sailing higher, but has hardly any effect on drag coefficients. Bearing away won't reduce wetted area of the Cat either in such light winds, as even when hotting it up the boat is not heeling much at all.

In between you are most likely to find out conditions when bearing away more and hotting it up improves VMG and imposes no risk due to too much sail up even with full sails. Works best with a large assy, not with a small and flat reacher. If you do not have one it's less likely to create better VMG, but still possible if your cat is fast enough.

VMG downwind is always affected by sail selection your boat has, you ability to trim them, and sea state has a lot to do with how safely VMG can be optimized.

[Dave_S]

I like the idea of the preventor running down to the deck to pull down on the boom. My traveller is only short so to let the boom right out it needs a lot of slack on the mainsheet and gives the boom too much freedom.

I should be able to fit a shackle on the end of the boom and use one preventor swapping sides with a quick release. At the moment I have a rope that drags across the roof where the solar panels are and occasionally the rope pulls a panel corner up.

[tomfl]

Originally Posted by Just Another Sa

Depends on true wind speed and sea state.

SNIP

I have to wonder why there is a lot more discussion about sea state. I have sailed dead down wind in what I will call sizeable rollers where broaching is an issue and sailing maybe 30 degrees off (even if the speed increase would not make it worth it) greatly reduces the possibility of a broach or gybe you were not expecting. Even one broach or unexpected gybe can slow you down; not to mention the possibility of damage to the rig/boat which would really slow you down.

There is also the consideration of getting in the troughs between waves where the wind is blocked which can result in the sails starting to luff and then slamming when you go up the face of a forward wave.


Comfort can also be an issue. Just as falling off a few degrees going up wind can reduce bridge deck the same is true going down wind. Going dead down wind in some sea states an result in burying the bows falling down the face of a wave which will slow the boat; not a good thing since once you go up the face of the next wave you may get a sudden acceleration when more of the sails are exposed to the wind.


Since there are so many possible combinations of wind conditions and wave conditions it is not possible to make a general rule about going dead down wind or sailing off the wind a little.

[Zzmeyer]

Most boats have “polars” available. They are graphs for a number of wind speeds and for all the wind angles. You can see the best downwind angle from the chart. Generally, the lighter the wind, the higher performance the boat the higher you will sail. The logic is that in these states the longer distance sailed is made up by the percent speed increase. I sail dead downwind about 1500 miles a year and have found I always get there whether I head up or down. As several have pointed out if you elect to sail DDW rig a preventer.

[stormalong]

I am surprised that no one mentioned sailing DDW wing and wing with the jib poled out. This is a very common way to make long trade wind passages. I have probably done 50 thousand or more miles that way. Flying light air sails like spinnakers with short handed crew in heavy trade winds is a recipe for disaster.



With a roller furling jib and a little thought the pole can be set up so the jib can be furled just as easily with the pole as you would without it. Sail area can be cut in half very quickly by one person.

[Catmandu]

My experience is that with wind/currents a destination is rarely DDW but will favor one side or the other of a broad reach or downwind tack.

I favor the downwind tacking on the side I with the least extra distance to pull the apparent wind forward if for nothing else to get more fresh air flowing through the boat on a hot day.

That being said, as tomfl mentioned, sea state and swell direction is typically the number one consideration of whether to choose one tack over the other or to sail DDW. For me, if there is a good swell heading directly towards our destination and its not bashing the boat (e.g. burying bows, trying to round us up ... etc) then DDW course is probably the best bet ... YMMV.