beam windage

from John Shuttleworth:

The important facts to note from equations 8 and 9, are that the coefficient of parasitic drag (Cdp) is inversely proportional to sail area. In practice this means that if sail area is reduced, the effect of parasitic drag is increased, thereby increasing the aerodynamic drag angle and reducing pointing ability. Also if windage is increased the pointing ability is reduced. In practice an open bridgedeck cat of type 6 might have CdTOT = 0.33, while the addition of a bridgedeck cabin with flat sides would increase Cdp by 35%. This will give a CdTOT(including bridgedeck cabin) = 0.39.

If this increase is laid out on the polar diagram shown in fig 6, it will show an increase in EA of 3 degrees. This means that the boat will tack through 6 degrees more. If the Cp of the bridgedeck could be reduced from 1.2 (coefficient of a flat plate) to 0.3 by careful aerodynamic design, Cdp would only be increased by 8.7% resulting in a loss in tacking angle of only 1 degree. Therefore it is vitally important to pay very careful attention to parasitic drag, and to design clean aerodynamic shapes.

To stop a boat making any progress to windward the aerodynamic drag angle has to increase to around 60 degrees, and if the effect of the the waves knocking the boat to leeward is included, this could drop to say 55 degrees. In order for this to happen the total lift to drag ratio has to fall to 0.700. This is quite possible in slab sided bridgedeck saloon cat with a total coefficient of drag (Cdtot) of 0.392. In fact this will occur in such a multihull when the sail area is reduced to 27% of the full working sail plan. When you consider that the usual proportion of the storm jib and deep reefed mainsail is around 20% of the full working sail plan, it is obvious that serious consideration has to be given to boat aerodynamics, not only for good sailing performance, but for basic seaworthiness.

Worse still is that in an older type of multihull where the best tacking angle was only 100 degrees in the first place, (which implies an aerodynamic drag angle of 28 to 30 degrees), the sail area only has to be reduced to 36% before all windward ability is lost. And indeed this has proved to be the case in many of the old designs.



http://www.john-shuttleworth.com/Articles/NESTalk.html (about 2/3rds the way down)

 

The basic fact is that flat beam fronts represent a flat plate----the worst drag shape. Any fairing, at any angle, is going to be an improvement. The bull nosed shape is best. (read bullets and artillery shells.)
I would guess a simple wedge shape, flat on the top, is the most simple and light solution.
Contrary to some opinion a flat back is not a big problem. The trail of vortices behind the flat back forms its own "Streamline" shape and allows the boundary layer of faster air to flow smoothly away. (Also read bullets and shells.)
BTW bombs don't count. They don't rotate like shells and bullets do. They need tapered tails to allow for fins to keep the bomb pointing forward.

 

Interesting oldsailor7 - but years ago when I was a hopeless gun freak and determined South Island high country hunter, the fastest velocities in high powered calibres were gained with semi boattail bullets, meaning they tapered to the tail; Speer, Hornady and the like. Of course they were propelled by oversize magnum cartridge cases for power. But this is brute force stuff and doesn't apply to a yacht which is underpowered by comparison. Still call BS on all this fantasy non-drag, clean vortice stuff behind an abruptly cut short beam. You don't see any gliders with wings like that, would be disastrous for lift and performance. Cheers.

 

The kamm tail versus the tear drop? The longer tail will have less drag but at a certain point it is also a good idea to be able to control lift as Hedley Nicol found out. After the barrel roll those boats went to the Kamm tail. Really though low drag with a little down force to windward and low drag and up force to leeward makes sense. Now hitting those wave tops will slow anything down fast so better give them consideration. At a certain point enclosing the crew pays off though canopy sounds faster than pilot house...Air borne foils count too and deserve control surfaces.....

 

Yes this is what I thought at first but then when you think about it the wind is never coming from dead ahead when your sailing right? and at a certain heel angle upwind the beams are blocked completely by the hulls from the apparent wind, therefore would any type of fairings make any difference at all?

I agree fairings are a good idea for the water, but this isnt my concern at the moment, im only thinking about air drag...

 

I would think that a wind tunnel test would be in order on that one.
I wonder if the effect of the hull and float sides would be to deflect the airflow more axially when on the wind.
When racing I used to make my crew lay down, as a human body has about 10 sq.ft. of "negative sail area" when sailing to windward.

 

I was wondering if any of you guys could assist me regarding the actual structural requirements of the beams on a small tri. What are the loads and what do I design for?
I'm doing a little 17 foot microcruise/race boat.
I've been thinking along the lines of a triangular section as well, flat on top, slanted bottom and vertical rear panel. This will have to deal with bending moments and twist, whilst being fairly simple to construct. I'm very scared of completely overdoing the structure, and also of breaking up the first time the wind blows.

Is the bending moment at the root equal to the righting monent as the main hull leaves the water? (this is about the worst case scenario I can conjure up)

How about the added twist as the bows of the leeward ama buries into a wave and tries to lift?

What safety factors do I use, I've been thinking 100% ie each beam can handle the full max righting moment in bending.

Do I add the stresses caused by rig tension to the righting moment?

There are just too many questions today.

 

In a tri that small, simple thick wall alloy tubes would be Sufficient. IMO.

 

iv designed and built 2x sets of composite beams for 24ft and 28ft tris, making them strong enough is easy the loads arent huge, making them stiff and light is the challenge, in your case I would use carbon tube off the shelf 100mm or 125mm dia, not too expensive, very light and very stiff. The best thing you can do for this type of boat is check out what other similar boats have used as a starting point.

 

DriesLaas, why dont you start a new thread with your questions for your design? That way it will be easier to follow, and threads like this people can discuss the topic of this specefic thread, in this case Beam windage. Thank you
(Please dont get me wrong, because i wonder over many of the same questions as you are)

 

(It was GB4, not GB3. I sailed it extensively in 1980. Not sure if bullet design is relevant - aren't they supersonic??)

I wouldn't worry unduly about airdrag on trimaran beams. After all you've got a huge outrigger out there that blocks the wind hitting the beams much of the time.

But I would worry about the slamming from waves hitting the beams. Especially at the outer, outrigger, end.

Also be practical, the crew need to get on board and get to the outriggers when coming alongside, never mind when sailing. That is easiest to do by running along the beams. So make the tops flat, not curved. Then make the undersides curved if necessary

Drieslaas. 75mm OD aluminium tubes should be OK. 100mm dia max, unless you have a very wide or oddly proportioned boat. You can easily get the inertia for those sizes and then convert to the beam section/YM that you plan to use.

Yes, beware of the outriggers twisting. One stiff central beam won't work, nor will two light beams at bow and stern of the outriggers. When the Wolfson Unit designed the beams for GB3 they found the loads from the drag of the outrigger being immersed at speed was about 1/3rd the bending loads. So don't make the beams too thin, even if they are deep enough.

Waterstays can take the rig loads

Hope that helps

Richard Woods of Woods Designs

www.sailingcatamarans.com (also the designer of the Strike 16 and 18 trimarans)

 

No, many projectile bullets are subsonic in nature and are designed so for a purpose.

 

drag

I was involved with determining the difference in drag of a small wooden block on wheels compared to a streamlined shape. To make it short, at 8 mph, the shape drag was not much, at 12 mph, it became significant, and the plain block was drag limited at about 15 mph, while the streamlined shape could pass 20mph. A flat front beam looks the same as the un-improved block. I am sure the up-wind hull has a lot of drag as well as the beams, but it seems that any thing that can be done to lessen drag, should be. Rounding the outer deck edges of the floats and streamlining the beam fronts doesn't seem to weigh much and should result in some increase in speed- more as the boat size and speed increases. B

 

We put SS sheet bent into a bullnose on the front of the stilts on Kraken 40 number 1. (Ringo).
During the first night of the 1969 NY -Bermuda race, through the rough waters of the Gulf Stream, the wave impacts smashed them flat as pancakes.
Dear old Newton---F=M*A

 

Thanks Richard, this is pretty much my line of thought at the moment, its nice to see someone else agree's!

I may put a fairing on for the waves, but this would be a different design to one for the air, I would try and deflect water away from the helmsman's eyes if this is possible?

I would like to offer it as a project for a student to look into, does anyone know how to go about this?