Main sail vs jib powered?
Posted by taintedchocolate@reddit | sailing | View on Reddit | 12 comments
Why is it that some boats are designed to have most of their power come from the main sail while others come from the jib? I’ve seen lot of people talk about how they like to only use the main to get around since their jib does not provide much drive anyway. However, on other boats like mine the main doesn’t really move you much at all until you get the jibs up, and you can run fine with just the jib if needed. Is one of these designs superior to the other?
johnbro27@reddit
Since no one has yet addressed the center of effort--the boat is to be balanced with both the main and jib set and drawing. This puts the center of effort at the right spot so that you have mild weather helm typically. People sailing with just one or the other sail set and drawing (not motoring with the main up, that's a different situation) are going to experience an unbalanced helm, so either a tendency to fall off heavily or head up strongly. Exception is dead downwind or almost broad reach with just a big gennaker or 130 set, in order to avoid blanketing the foresail.
ChazR@reddit
With conventional two-sail plans it turns out that the best overall performance upwind comes from a high-aspect main with a fractional jib. It won't go downwind happily, so you add an asymmetric kite.
For a compromise cruiser you go with a moderate aspect main, a fractional jib, and maybe a lightweight gennaker if you're keen.
Huge masthead genoas are an artefact of the International Offshore Rule in the late 1970s. IOR did not measure any overlap. Your Genoa size was calculated as being the size of the foretriangle.
This incentivised absolutely bonkers rigs with 170%+ overlaps. The Genoa went to the masthead, and dwarfed the main. It's a bad rig for a lot of reasons, but it influenced sailplane design for 25 years.
Modern Computational Fluid Dynamics allowed us to imagine, design, and test much better rigs. That's where the modern fractional rig emerged. It was hard to sell. People who didn't understand the fluid dynamics thought 'Small jib = less sail area = slower boat" which is completely wrong, but persuasive.
In a modern well-sailed fractional rig the power doesn't come form the jib or the main. It comes from the interaction of the sails with the accelerated airflow between them.
Boats like the F50s and AC boats take this further with windsails and tiny jibs that give them more power than they can deliver to the water.
The ideal sailplane is the one that works for you. If you're doing gentle cruising and round-the-cans racing a modern fractional is your best compromise. If you're doing a downing ocean crossing a solent rig might be the way. If you're doing bleeding-edge flat-water racing, wing and micro jib is the only option.
But it's the interaction of the airflow between the sails that drives performance.
pheitkemper@reddit
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Neptune7924@reddit
That was a great read, thank you.
waitnatara@reddit
Fantastic answer. Thank you for the education
lucidguppy@reddit
Amazing commentary thank you! I also feel like boats are tending to rigs that require less people to sail - and these self tacking fractional jibs are a big seller to people with reluctant to sail spouses.
TxTransplant72@reddit
As a former foredeck guy on a weekend racer, the big overlap takes more people to manage and better timing to tack the big thing around the mast. As I’m an older dude now, I’ll be looking a for a fractional rig, even though I’ll miss the feeling of a big Genoa as it pulls the boat along. Good to hear that I should get good performance from a modern rig as all my experience was on 30 year old designs.
roger_cw@reddit
While this video isn't directly answering your question it does get covered. I found it very informative https://youtu.be/01br26aoehc?si=ATKMYJXX-8DSgSNs
Wise_Focus_9865@reddit
I have a masthead rig and my big 150% Genoa is the boats main powerhouse. A practical consideration is that this takes longer to fully power up than a smaller jib when coming out of a tack, which isnt a great characteristic for racing.
redaction_figure@reddit
It all depends on your point of sail, type of boat, and AWA. On a reach, the mainsail will provide a great deal of lift. It is your main engine on a sailboat. Hoisting the jib increases airflow over the main & increases its efficiency. Downwind (unless you are wing on wing) the foresail will probably provide adequate sailing force without the main. On my cat, I find that I can point closer into the wind with just the genoa. YMMV
Lussypicker1969@reddit
I think it depends on the sail plan. Older boats tend to have bigger jib ratio to main and the newer racing yachts tend to have a bigger main to jib ratio. Also depending on the rig there is difference in what powers more.
ozamia@reddit
It depends a lot on the sail plan. On my boat, which has a 3/4 fractional rig, the foresail will always be relatively small compared to the mainsail. In my case, it's a self-tacking jib so it's even smaller. My jib has around 60 % the area of my main. So main alone for me means I lose just some of the power, but I can point higher. With jib alone, I lose a lot of the power and can't point well at all. Let's say that for a certain wind condition I can sail at 5 kts with full sails, then main alone would get me to around 4 kts, but jib alone just 2 kts.
However, for a masthead rig, the main is often quite small compared to the foresail. My boat has a "twin" model with a masthead rig, and there the foresail (usually a large genoa) is typically 170-200 % the size of the main. It sails very poorly with just the main up, but sails nicely with genoa alone, since the center of effort of the sail is much closer to the hydrodynamic center of the hull.