Impact of Asymmetric Spinnakers on Performance

Sort of a noob here, but interested in understanding the impact of mounting an asymmetric spinnaker on a single sailed (upwind) sailboat like the gennaker like the RS100.

When the gennaker is hoisted, I assume it must move the centre of lift forward rather substantially. I figure if only experiencing drag downwind, this wouldn't be too much of a problem as it would just dig the bow in. However, under aerodynamic stress, wouldn't this create a lot of pressure on the helm as it tries to rotate the boat?

Thoughts on whether mounting these sails is ever worth it and how it would best work would be greatly appreciated.

 

Welcome to the forum.

I highly recommend visiting your local sailing yacht club. Pre-covid most YCs had held low key races often called beer can races. Boat owners usually need additional crew so newbies are welcome. It will be a great way to learn spinnaker handling.

Yes CE is moved forward and off of center line. The rudder may have to counter great force. Check out round up videos to see spectacular spinnaker mistakes.

Spinnaker actually use lift to propell the boat and raise the bow.

 

Thank you! I do! I'm generally on bow working with a symmetric spinnaker on an older boat with a large genoa, and thus even when reaching with a spinnaker, i'm not sure the CE actually moves much. The skippers response my questions seeing a RS100 flit about was essentially "modern composite and design magic". The pressure on the rudder must be dramatic since it seems the RS100's spin is at least as big as the main. Might need to get across the river to where they launch to ask about it specifically...

 

Realistically; a large Genoa (160-180), a gennaker, and an asymmetric spinnaker all fill the same niche; as much rag up as possible in light winds. Where they differ is how their draft is cut based upon the true wind angle they are used for. Genoas are predominately used for on the wind and generally cut fairly flat; not that you can't put belly into them with foot and leech lines and/or car position for downwind work. Gennakers or "Code" sails (i.e. Code Zero, Code 1, etc.) are for more beam or off-the wind work and therefore have more draft cut in. Finally, asymmetric spinnakers are the fullest cut and excel at simplifying the sail handling for downwind reaching. Unlike a symmetric 'chute, none of these sails are particularly suited for DDW work. Additionally, because of the full, deep draft in the sails, neither the Gennaker or asymmetric is suitable for work much before the beam and may quickly overpower the vessel. "Code" sails are made in a range of drafts, with a Code Zero almost flat and a Code 4 effectively an asymmetric spinnaker. Unlike a Genoa, "Code" sails are set flying with a Code Zero being effectively a large flying genoa.

It must be remembered that any of these sails, mishandled, can overpower the vessel; and rounding up is more a function of hull form and heel angle.

Ask SAIL: Types of Asymmetrical Spinnakers https://www.sailmagazine.com/diy/reaching-sail-nomenclature#:~:text=A%20Code%200%2C%20for%20example,a%20specific%20wind%20angle%20range.

 

Yes, it is "worth it" in terms of speed, compared to not having a spinnaker or assy. However, the all-round increase in speed is a lot smaller than the overall increase in cost and rigging and handling complication; many boats only gain about 2-4% in overall speed by adding an assy. Whether it's worth it for the individual is another issue; most sailors prefer simpler boats.

If you do a search here, you'll see a long thread about whether assys lifted the bow. I think the outcome was that they provided lift (partly through the effect of increased speed on the hull) but also a rotating moment. I'm not sure if the study applied to the extreme end of assy design, as in the Australia 12 Foot Skiff that have a vast kite and an assy pole as long or longer tha the hull and which have an extreme bow-up angle and sometimes "float" in the air. In many fast boats, the assy does seem to provide vital lift to the bow downwind in strong winds.

How it best works is very complicated. Basically, the faster the boat the better an assy works (in terms of increasing speed) up to the speeds of fast foilers. To get an assy to work as it is meant to, you have to generate flow from luff to leach, so you generally go downwind at 45 degrees above dead downwind (on average).

In the slowest assys (like the RS200) it's hard to generate sufficient boatspeed to bring the apparent wind forward enough to tack downwind at high angles (too hard to further describe at the moment) so a conventional spinnaker boat can be faster downwind in light airs. The spinnaker can't add enough boatspeed to make up for the fact that to get it to work properly, you have to head up to about 45 degrees from dead downwind.

The assy doesn't create lee helm because it affects the wind flow over the mainsail. The mainsail therefore has to be trimmed in tighter than if you did not have an assy. The fact that the mainsail is trimmed tighter compensates for the fact that the assy is pushing the bow down.

 

Thank you! That was very helpful, will take a read!

Interesting, especially around the balance of the assy and the mainsail, I guess balancing of the two at a variety of points is what the nautical engineers go to get their complicated degrees for. Or does it only need a single point of sail as you sail the same apparent angle, with boats just pointing further down (relative to true) as they accelerate and shift their apparent wind angle?

 

The original discussion on the topic was on rec.boats.racing over 25 years ago. We're getting old mate!
This article covers the end result. What Happens when you put the Kite up https://www.devboats.co.uk/sailing/cherubweb/liftbow.htm
Basically any sail with a sufficiently angled luff provides an upward lift force on the boat as a whole, but because the sail force is so far above the hull drag they nevertheless provide a downward pressure on the bow. Roughly speaking it pushes down the bow and lifts the stern.