Induced Drag?

[Gary Wyngarden]

I just read an article about mainsail shape written by Steve Dashew and am confused (he often confuses me). Steve extols the virtues of the full roach main in comparison to a straight triangular or a hollow leech main. He says the full roach main delivers more power and also enables the boat to sail with less heel and less weather helm.This is the part that confuses me. It seems to me that a full roach main will move the center of effort of the main aft and up. The latter would seem to add to the amount of heel and consequently add weather helm. The former, by moving the center of effort aft with no change in the center of lateral resistance would also seem to increase weather helm.Dashew admits this is counterintuitive and says the explanation relates to a reduction in "induced drag" apparently resulting from a more elliptical tip (head?). He didn't do a very good job explaining induced drag.Anybody out there got a simple explanation for this phenomenon?Thanks for your help.Gary WyngardenS/V Wanderlust H375

 

[Fred Ficarra]

Without an aeronautical engineering background

it might be the same drag that we reduce with better keel shapes. The whale tail comes to mind. It has a roach too.

 

[Jeff M21319]

Okay, I'm confused too

Hopefully someone can explain it 'cause it seems backwards to me! Wouldn't more roach INCREASE the drag? Same old story, the more I learn, the dumber I feel!

 

[Fred Ficarra]

Another example.

Look at all of the effort expended on America's Cup roach area. They try to make it as big as possible.

 

[MArk]

More confusion

Hey Gary,I always thought the big roach sails were a result of race rules, not optimal for cruising. I can see where an increased sail head cord length might reduce tip vortices and their associated drag in light air. The extra construction expense and decreased sail life would only be acceptable to someone who mostly races.On the other hand, sail shape is very important to the position and direction of the lift created. As wind speed (actual or apparent) increases, the center of lift moves farther back on the sail. (Maybe more than 2 feet!) This increases weather helm and heeling force while decreasing forward thrust. The only way to bring the lift back forward is to decrease draft in the sail or better yet, change to a flatter, heavier sail.Speaking of weather helm, too much heel is a major cause. When you heel, your sail (your motor) is out over the side of the boat. Imagine what would happen if you put your outboard on a plank sticking out the side of your boat.One of my favorite links on sail adjusting below. Specifically check out "Quest for the Perfect Shape" and "Telling Tales...".Happy sails *_/),MArk

 

[Fred Ficarra]

One thing I absolutely guarantee, no one has ever

learned it all in this sport. cocktail hour.

 

[R.W.Landau]

Cockrtails were three hours ago...

nm

 

[Dave]

Vortices and Separation

Drag can occur from separation of the wind stream from laminar flow across the surface. A longer surface (e.g. a full roach) might help to reduce turbulent flow across the top of the sail? As the two air stream come together at the aft end of the sail they invariably mix together, note they are also at different velocities and different states or turbulence so drag is a result of the mixing of these streams. We did some water tunnel work in engineering school to measure the effects of changing the downstream vortices coming off control surfaces. We tried various corrugates shapes etc. The science is mostly empirical but CFD (computational fluid dynamics) might provide some insight.Dave

 

[Tim]

"Winglets" on Jet Aircraft

I'm not an aero engineer (nor do I or anyone else play one on TV), but, I believe that many commercial and BizJets are using winglets - small upturned tips at end of their wings - to take advantage of the drag reducing properties Dave mentions below. Intuititively you'd think that adding wing structure would increase drag, but apparently the added structure and its shape somehow "manages" the airflow to reduce drag overall. For a B737, the winglets improve fuel burn something like 8% - which turned out to be a much greater improvement than most of the engineers expected.Tim BroganApril IV C350 #68Seattle

 

[Jack Tyler]

Winglets don't reduce drag but rather...

...end tip vortices, as I recall, which in turn induce drag. It may sound like quibbling but there's a difference.Gary, while the full roach may move the CE aft and up a bit, it isn't much. Instead, what you have is a much more efficient wing, generating a lot more lift. If you have the chance and haven't already done so, try to sail boats of similar sail plans with and without full roaches; experimenting with sail performance and simply watching the wing work in a given wind pattern should be noticeably different.Essentially, I think what Dashew (who also confuses me...) is trying to say is that the benefits overcome whatever small liabilities exist in the larger roach.Jack

 

[MArk]

You do the math

Equations on link below...Definition: Racing roaches - Something to do on a hot Louisiana Saturday night. Usually started with a light switch rather than a starter pistol.Happy sails _/),MArk(formerly of New Orleans)