Sail efficiency = speed gained ?

[Mikko Brummer]

Originally Posted by hprasmus

Is there not a sailmaker out there that could add wisdom to this hypothesis.

Thanks.

Sure, but first we need to know what you mean the 5% increase...

[daiquiri]

Originally Posted by hprasmus

Your answer suggests that not even the most advanced sailmakers have any idea of the performace gain of their various sails ? Very interesting. So all the hype of sails designed and manufactures to suit cruising, racing and AC are just unsubstatiated marketing gimmicks ?

Marketing has it's role, that's sure. Fact is, sailmakers can do their CFD and they can do their wind-tunnel tests, but at the end only the sea will tell the truth. Some "optimum" designs turn out to be the winning ones, and some others turn out to be... not so optimum.

An example: Luna Rossa (Prada Challenge) at Luis Vuitton's cup 2002. A huge amount of money (one of merits of AC event is that it attracts big funds for R&D) have been poured into towing-tank and wind-tunnel testing and optimization of both hull and sails during the design phase. At the end, an "optimum" boat has come out of the yard and was nearly declared by the press and some big names in the sailing world as a future winner of the LVC.
Too bad that during the races it became evident that the boat was not so optimum as it appeared to be on the computer screen. Alinghi has had it done in a different way, and Alinghi has started to win the races. So both hull and sailplan of Luna Rossa have been significantly modified on the go, making the boat somewhat faster and more similar to Alinghi. It didn't help, though - Alinghi eventually won both the LVC and AC.

So be careful when it comes to claims of 5% improvement in anything that concerns boats, if it is backed only by CFD or by theory.

Cheers

[hprasmus]

Even the AC designers have no idea of the performance effect 3DL sails have on their boats ! Only that they are lighter and more stable. So sorry for mocking you folks.

I can't quote my source obviously but it comes from a top AC designer - so all is (NOT) well.....

[hprasmus]

The 5% efficiency gain is based on the simulation across wind angles as the net effect of : side, lift, drag and forward forces

[Richard Woods]

Attached is a photo I took a couple of years ago in the San Blas islands of Panama. I doubt if these fishermen had ever seen an I-14

I agree with Daiquiri. A 5% performance gain is huge.

But to try to help. If your sail generates 5% more drive, then the boat might be 2% faster. Changing from a dacron (ie porous) sail to a laminate sail increases drive by about 5%, which is why no race boat, allowed the choice, has a dacron sail. 2% more speed for no extra effort, just extra money.

Rigs used in the past often had different roles. So the sprit rig was used on Thames barges because it was also used as a derrick in port. It had a boomless rig because many were used to take horse s**t out of London to the farms in Essex. A light high cargo with the sail reefed to clear the top of the pile.

Lug sails were often used by fishermen (eg the Looe Luggers based 10 miles from me). However they used them to sail 10 miles out to the fishing ground on starboard tack in the morning. Then they lowered the sail. Rehoisted it on the other side to sail home on port tack.

A friend rebuilt an original lugger as a live aboard cruising boat. He fitted a lug sail but after the first season replaced it with a gaff rig as he said the lug rig was far too hard a rig for ordinary sailing (and he is big, strong and a very experienced sailor)

Richard Woods of Woods Designs

www.sailingcatamarans.com

[hprasmus]

Thanks Richard Woods,

The most accurate reply I have had so far. And you are right about the complexity as well. My invention i a little bit more cumbersome than changing sailcloth. But nothing compared to changing to e.g. carbon spars. I do believe this idea has a future and I will tell the forum what it is all about when the patent is at safe harbour.

[daiquiri]

Just don't focus on a single aspect of sail operation, like aerodynamic efficiency. Consider things like materials choice, ease of production, cost, ease of handling, seaworthiness, weight and It's distribution (CoG, moment of inertia), and other aspects which will make a difference between success and failure - even more than few knots gained.
Awaiting to see the result.
Cheers!

[Cheesy]

It may have some value, a smaller diameter mast on a sport boat makes a rather large difference in the time it takes to get to the top mark...

[water addict]

hprasmus, I think after 4 pages there is a reason you have not received a definitive answer from some very qualified people on here. Because the query is not strictly answerable given the information. There is no cookbook formula that says for all boats if I have 5% increase in efficiency I get x% increase in speed. The physics is complex, and dependent upon the specifics of the boat.

Also, there are many ways to define efficiency. Do you have the manner in which your 5% number has been calculated? It makes a difference. Also if you get say a 5% better L/D ratio, that does not mean that translates directly to a 5% overall aero efficiency of the whole rig. Ex., you have drag from spreaders, shrouds, the mast that contribute to the total package. Also, you may be increasing weight aloft compared to a typical sail. All of these factors, and others contribute, and an answer cannot be given without knowing all of the required variables. And they will be different for every boat configuration.

I'm not saying you idea may not be a good one, just that there is a lot of input required to answer the question- input which is not readily available.

[tspeer]

Originally Posted by hprasmus

...

How much will a 5% increase in net sail efficiency contribute to a sailboats speed (below max. hull speed of course) ?...

I find the term "net sail efficiency" to be ambiguous. I think a better measure is aerodynamic lift/drag ratio, since it includes not only the sails but also the topsides. For example, if you reduce the windage of a boat you will increase the speed without any change in the sail whatsoever. And L/D has a direct connection to sailing performance.

The fundamental sailing performance relationship is

Vb/VT = sin(gamma - beta) / sin(beta)
beta = arctan(drag_aero/lift_aero) + arctan(drag_hydro/lift_hydro)
Vb=boat speed
VT=true wind speed
gamma=point of sail (gamma=0 => head to wind)
beta = apparent wind angle, measured between apparent wind vector and velocity through the water

You can improve the lift/drag ratio by increasing the lift, reducing the drag, or both. Changing the aerodynamics (especially the lift) also has an effect on the hydrodynamic lift and drag. So for the sake of argument, I'll assume you are improving performance by reducing the aerodynamic drag.

If you substitute in the relationship for the apparent wind angle and differentiate with regard to aerodynamic drag (and assume there is no combined effect on the hydrodynamics), you end up with

d(Vb/VT)/dDrag_aero = -sin(gamma)*L_aero*(L_hydro^2+D_hydro^2)/(L_aero*D_hydro+L_hydro*D_aero)^2

You can conclude that:
- an increase in aerodynamic drag hurts performance (minus sign)
- the change in performance depends on the point of sail (gamma), and the biggest change in speed is likely to be near a beam reach
- the change in boat speed is proportional to the true wind speed
- the sensitivity depends on the actual values of both the aerodynamic forces and the hydrodynamic forces, so it's not possible to generalize a specific amount for all boats and situations

If you have a velocity prediction program, you can plug in values for the lift and drag to see what the actual sensitivity is to a change in the aerodynamics.

You should also apply the chain rule, differentiating the performance equation by the hydrodynamic lift and drag and multiplying by the change in hydro force due to the change in aerodynamic force. For example, the hydrodynamic side force must match the aerodynamic load, and there will be a change in the hydrodynamic drag associated with that. So the hydro L/D will change significantly if there is a change in the aerodynamic lift. But you can get these sensitivities from the VPP model as well.