forum project: Develop Low Drag Device for Standing Rigging

[Nordic Cat]

The 20 m/s is the relative wind and not totally crazy, 12 m/s (24 knots true) and maybe 15 knots boat speed will get you up around that speed.

The hull and saloon windage is more difficult to estimate , but using a cw of around 0.5 is what is reccomended for calculating e.g. anchor loads...

½ x 1.29 x ws2 x cw x Area = drag in N

Will of course be greater than the standing rigging.

For the mast the reccomendation is for a cw of around 1.1. (If. it is not a well profiled wing shape pointing directly into the wind)

Regards


Alan

[Guest625101138]

Originally Posted by Nordic Cat

The 20 m/s is the relative wind and not totally crazy, 12 m/s (24 knots true) and maybe 15 knots boat speed will get you up around that speed.

The hull and saloon windage is more difficult to estimate , but using a cw of around 0.5 is what is reccomended for calculating e.g. anchor loads...

½ x 1.29 x ws2 x cw x Area = drag in N

Will of course be greater than the standing rigging.

For the mast the reccomendation is for a cw of around 1.1. (If. it is not a well profiled wing shape pointing directly into the wind)

Regards


Alan

Alan
You need to consider the vectors not the numerical sum. Only wind turbine driven boats will sail directly to windward. To get 20m/s on the rigging vectored directly against the direction of travel the wind will be of strength to be creating survival conditions for most craft.

I am not saying getting rid of rigging is a bad thing I am making the point that it is only one factor.

Also at 20m/s against the rigging your concerns would likely be elsewhere - like maybe running before it, finding shelter or dropping all sails.

Rick W.

[Nordic Cat]

Originally Posted by Rick Willoughby

Alan
You need to consider the vectors not the numerical sum. Only wind turbine driven boats will sail directly to windward. To get 20m/s on the rigging vectored directly against the direction of travel the wind will be of strength to be creating survival conditions for most craft.

I am not saying getting rid of rigging is a bad thing I am making the point that it is only one factor.

Also at 20m/s against the rigging your concerns would likely be elsewhere - like maybe running before it, finding shelter or dropping all sails.

Rick W.

I agree that this is at the high end of wind speeds, if we drop the relative wind to just 10 m/s, we will still have 210 N of drag, probably like towing a couple of dingies behind the boat.

Here is an example from Larsen and Eliasson, a great book that i reccomend. This is the windage calculation for a 40 ft. monohull, just to put things in perspective.

Regards

Alan

[CT 249]

I seem to recall the J Class having lenticular (streamlined) rigging. Sopwith and Fairey, J Class owners and manufacturers of military biplanes, were probably fairly aware of the issue. I have a vague memory of it popping up every now and then and being banned due to cost issues.

If pure speed under sail was all that mattered for racers, everyone would be sailing things like A Class cats. The vast majority of people are not stupid or ignorant, yet they don't choose to sail things like A Class cats; in fact there seems to be a swing away from "fast" sailboats. And actually, in quick small boats streamlined rigging could possibly cause severe injury in a nosedive; I've hit the rigging on our A-Class like cat, an 18 Foot Skiff and a Tornado and it is not a pleasant way to spend a weekend afternoon even when the wire is round.

In cruising boats, upwind speed in strong winds (rough water) is not all that comfortable (and that's something confirmed by a brother who was so into performance that he lived and cruised for years in an 1800kg racing tri with a wife and baby). Okay, better speed in normal conditions is a plus, but given the normal restriction on sailors (the size of their wallet) is it better to worry about streamlined rigging, or to concentrate on creating cheaper boats that allow us to build bigger and therefore faster craft?

[brian eiland]

Originally Posted by brian eiland

We know that 'no rigging' certainly cuts the aero-drag. But there are THOUSANDS of sailing craft out there now that might benefit from a simple plastic extrusion design that would cut down on the aero drag...even a little bit.

I mentioned 'plastic extrusion' here possibly thinking of the familiar slit plastic shroud covers that were very popular on beach cats. I only meant this as a concept.

What I really had in mind was paper sheet thin material (likely a modern plastic or cloth) that would wrap around a variety of rigging diameters and adhere to itself at the trailing edge while still allowing itself to rotate around the enclosed rigging element. This 'adhesion' unto itself at the trailing edge might be attained via one of the superior chemical adhesives we have today, or a 'clip' type mechanical arrangement in an extruded design.

I was hoping NOT to contribute any significant extra diameter to the rigging element, and to make it very inexpensive such that replacement was a ready option upon deterioration in sunlight.

[tspeer]

Originally Posted by Rick Willoughby

...Have you done the numbers on the wind drag on the hull/cabin?...

John Shuttleworth has a great article where he goes to considerable length to reduce the windage of the hull in a 50' cruising catamaran design. Here is his breakdown of the major drag components:

WIND
Sail lift - lbs 3610
Sail and rig drag 361
Air drag of hull 652
Total air drag 1013
WATER
Keel lift 3610
Keel only drag 176
Hull drag (from tank testing) 511
Total drag 687

Note that the air drag of the hull is greater than the water drag of the hull itself and almost as much as the total hydrodynamic drag! And this is for a boat that has an exceptionally clean topsides.

Correction: The numbers are for his Tektron 50 design, which has more typical (but still fairly clean) topsides, with hard chine gunwales, etc.

[brian eiland]

Just happened across this old subject thread of mine and thought I might bring up the subject once again....

Originally Posted by brian eiland

I mentioned 'plastic extrusion' here possibly thinking of the familiar slit plastic shroud covers that were very popular on beach cats. I only meant this as a concept.

What I really had in mind was paper sheet thin material (likely a modern plastic or cloth) that would wrap around a variety of rigging diameters and adhere to itself at the trailing edge while still allowing itself to rotate around the enclosed rigging element. This 'adhesion' unto itself at the trailing edge might be attained via one of the superior chemical adhesives we have today, or a 'clip' type mechanical arrangement in an extruded design.

I was hoping NOT to contribute any significant extra diameter to the rigging element, and to make it very inexpensive such that replacement was a ready option upon deterioration in sunlight.

I remember some test like these 'ribbons'

Originally Posted by tom28571

Bruce Woods comments reminds me of devices used on instrument tow lines in deep water long ago. These were simply plastic ribbons that disrupted the formation of vortices. I think that the resulting reduction in drag was not due to a reduction in the absolute drag of the wire though. I think the idea is to break up harmonic resonance and thereby reduce the effective cross section compared to a vibrating wire. I tried this on the lifting cable on a swing keel boat and it did stop the vibration and I assumed that it also reduced the drag although I had no way to measure it.

[Squidly-Diddly]

http://www.sponbergyachtdesign.com/StateoftheArt.htm

Given advances in materials and engineering, I'd say it stands some looking into.

[Petros]

The free standing mast has very large advantage here, but will add considerablly to the weight (and cost) of the boat, however consider that all the early aircraft had wire braced wings but by WW2, all aircraft went to cantalivered wings.

Of course aircraft fly at much higher speeds, so drag reduction is worth the extra weight, but one thing than can be done as a design compromise, a strut braced mast. Using stream line ridgid struts, the mast can be braced in a tri-pod configuration. IT means the mast step has to be good for both tension and compression (the mast base has to be designed agaist up lift). And with cleaver design where the struts meet the main mast, you can still have a rotating mast.

A few years ago I did a drag and weight trade off study, and it appears that the excess weight will be minimal, and drag reduction of the rigging would be considerable. Also consider that the air going over the sail (and shaped mast) would be cleaner, so there will also be gains in the total amount of thrust generated by the sail.

Stowage should not take any more room since the struts should will not be any longer than the mast, so they can be stowed with the mast. And no cables to mess with. All the halyards should be run inside the mast of course.

I have been considering making a small version of this rig to put on a cheap used Hobie cat to try it out, or on my dingy.

[tspeer]

Originally Posted by Petros

....Using stream line ridgid struts, the mast can be braced in a tri-pod configuration. ....

What is the orientation of those streamline struts?

[rob denney]

Originally Posted by Petros

The free standing mast has very large advantage here, but will add considerablly to the weight (and cost) of the boat

We are currently building 6 unstayed masts for 3 x 12m biplane rigged cruising cats. The cost for the round masts is $14,000 each.

The masts are pretty much the same as the one in the video at http://www.youtube.com/watch?v=8chR6DAFjGA, which weighs 120 kgs/264 lbs. Two thirds of this is labour, and anyone with basic skills and a vacuum pump could build it.

The cats have the same righting moment as the proa, but are considerably heavier. Hence they have 2 mainsail rigs vs the ballestron sloop rig on the proa.

When you consider that a stayed rig needs chainplates, turnbuckles, additional bulkheads, dolphin striker, fore beam etc, the costs and weights are very similar. Add in the need to remove the stayed rig annually for inspection and that some insurers apparently want the standing rigging replaced every 5 years and the unstayed rig is probably cheaper.

The unstayed rig may also be lighter, but a few kgs either way is not really relevant on a cruiser. What is relevant is the height of the centre of gravity, which is much lower on the unstayed rig.

rob