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How to trim a gennaker for downwind performance
Question: Hi Mike. I was sailing my W17 ‘Merlin’ the other day and the wind picked up as I turned to return home downwind. As I was concerned about nosediving, I reduced sail by furling the Jib and came home under main alone. Is this the best approach? JimV.N. Mass, USA
Answer: Actually not. To lower the forces pushing the bow down, you can reef the main to lower its leverage, but keep the foresail up. Actually you may even do better by setting your genny from the tip of the bowsprit, but deep reef the main if needed.
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Take a look at this photo of a modern Wayfarer and you will see I have added a small sketch to show the summary of forces. You will note there is quite a lift component due to the tack being attached well forward, and the longer the sprit, the higher this lift will be.
Australian Skiffs are an extreme example of this as they carry sprits of over half their boat length, and as they ‘fly’ downwind with their monstrous sail, it’s very clear what lift it is giving the bow!
(Please do not ask me how that sprit can take this, even if there is a little striker post half way along!)
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As Andre Baetz (now my associate to guarantee a healthy future for SmallTriDesign.com) has very good downwind gennaker experience from successfully racing his 28ft trimaran Flaneur, I asked him to write up his advice on how he uses it so effectively. Here is his reply.......
Hi guys ….
I have an asymmetric gennaker and I handle it like this:
You are trimming it right when the clew is close to the shrouds and the apparent wind comes from the side between 80 and 100 degrees, (at least for my boat). You start sailing fairly deep downwind, but then carefully luff up by steering more upwind until you really feel an acceleration. As soon as the boat accelerates you need to steer back down a bit to check the speed, otherwise the boat can go faster than the wind and then the gennaker will be luffing.
When you slow, steer up and accelerate again, but be ready to steer down again after accelerating. It’s always a kind of "snaking" course. Mike suggests a similar ‘wiggly” course when trying to work to windward in gusts with too much sail up..... see here.
Regarding a nose dive, you have to always watch the leeward bow and test how things look & feel with your own particular boat. If it seems to get dangerous*, turn downwind quickly to turn all 3 bows into the wave. Their combined buoyancy will lift the bow if need be.
* Mike suggests a dangeroius situation is eminent when you are seeing less that 1% of the boat length as 'mean-average freeboard' at the leeward ama bow. That is 2" (50) for a W17. 'Mean-average-freeboard' can still allow a short wave-top to pass over the ama bow.
But NEVER luff up! .. as that will heel the boat and is more likely to plunge the leeward ama under. You can also release the gennaker sheet if you have a spare hand. In case of an imminent nose dive you have to blow (release) the sheet completely! So make sure that the gennaker sheets are always clear to run out. Start practising all this in lower winds (say 7-10k) and then work yourself up to higher wind speeds. You will (and need to) get accustomed to the excitement to really enjoy this ‘fast & furious’ way of sailing ;-)
I personally find this ‘apparent wind sailing’ while flying the gennaker downwind, to be the most exciting form of sailing on my own trimaran (a Farrier F82R). 
Here is an example of a well-trimmed gennaker, making a downwind reach both fast and fun ---->
So tune your reflexes step-by-step, and you can one day enjoy the same excitement. Andre Baetz, Aug 2024
PS: Andre wants to share what an adrenaline rush it is to do this off-wind gennaker sailing, but also how demanding it can be also. So we combined notes to hopefully better 'share the feeling'.
It's a great stimulation to drive a boat at speed downwind, with water just shooting off the transom and the leeward float slicing through the water with often a sheet of spray (depending on the ama design). But you can't steer this intense 'knife-on-edge snaking' up and down, longer than about 40 minutes. It's just too mentally exhausting\. You must periodically fall off a bit and engage the autopilot to calm the adrenaline rush, but that's always accompanied by a sad feeling you just lost 2 or 3 kts of boatspeed, because 'the dumb autopilot just doesn't know how to do it right'. But after perhaps a quick snack and tea, you again disengage the autopilot, take the tiller again and brace a foot against the cockpit seat while you take off for another thrilling ride ;-)
Mike's Footnote
While this 'charge and release' sport-sailing may not be for everyone, it's really no different from other sports like car racing or even fast driving, where you go from gas pedal to brake pedal many times over .... and with a significantly lower chance of personal or fatal injury too.
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Copyright: mjw/ab - Sept 2024