Mast Aft Wing Sails???

Skyak: thanks for your observations and tips. A thread devoted to this topic on the aero/hydro board would be interesting. I look forward to it.

 

I'm struggling to get my head around the effect of heeling on the aerodynamics of a sail (my apologies if this is obvious, but I haven't thought much about monohulls before). Is it correct to say that when a boat heels the angle that the wind passes over the sail changes and hence effectively increases the chord and consequently changes the aspect ratio of the sail? If this is the case is it therefore correct that in order to keep a constant AR when a boat heels that the mast would need to be canted forward?

 

If so I strongly suspect that its a very small effect in the midst of some very large ones. The entire flow of air across the rig must surely vary with heel.

As with much of this sort of high end stuff I would pay great attention to anything Mikko Brummer cared to offer on the topic: I'm very sure it is something he's paid detailed attention to.

 

With heel, the rig effectively becomes swept. Simple sweep theory says the component of the apparent wind in the spanwise direction doesn't contribute to the forces. So there will be an effective reduction in the apparent wind speed and a reduction in angle of attack.

For example if you imagine the boat being heeled 90 deg about its centerline, the sweep angle will be equal to the apparent wind angle and the angle of attack will be zero. The dynamic pressure will effectively be reduced by the square of the cosine of the apparent wind angle.

For other heel angles, you need to resolve the apparent wind into components in the axis system of the rig. Then you can work with the components parallel to the chord and perpendicular to the chord to get the effective angle of attack and apparent wind speed.

 

Thanks for the concise explanation. You have pointed me in the right direction, but I am still someway off being able to apply it to understand the effect of canting a wing compared to canting a normal sail.

In the meantime, I've got the canting mast aft wing working, but haven't had the chance to test it on the water yet:

https://www.youtube.com/watch?v=6H4heNvxhy4

 

Why do you think canting a wing would be any different to canting a 'normal' sail?

 

What I was thinking was that if you consider a conventional sail with a jib and a mainsail as a wing with a slot, there is a lot of control over the performance through control of the slot distance twist etc. For a typical wing there isn't quite so much control, in particular for the very small wings that I make.

 

The canting mast aft wing sail in action:

https://www.youtube.com/watch?v=Dqth16H9IrY

Regardless of how it compares to a canting conventional sail, it is a significant improvement on the non-canting mast aft wing sail. I have only tested in fairly light winds, but when canted to windward it does appear to point higher and go faster.

I was surprised to find that there is also a significant improvement in performance in very light winds. In these conditions canting the sail leeward helps to set the camber of the wing, which doesn't happen when not canted.

 

Jim, Food for thought

http://www.sailmagazine.com/best-boats/sails-best-boats-2014-atlantic-47
I like it.

 

Thanks for the link Jim. I particularly like the idea of making the mast into an effective wing.

I've been testing a canting mast aft wing on my 700 mm trimaran:

https://www.youtube.com/watch?v=TvUjFml6pnw

I think that it will take a few iterations to get this boat sailing well. The current configuration doesn't point as well as the same boat with a conventional wing sail. It is also going to take me a while to learn how to sail this boat properly and take advantage of the canting mast.

Something that worked quite nicely was a rotating symmetric airfoil ono the mast. It is a bit rough so I don't know that it reduced drag much, but the concept worked nicely and rotated to stay parallel to the wind.

 

Any thoughts as to what the advantage of this two mast design would be over a single mast with two jibs?

 

Lower CE for the same sail area = less heeling and more forward thrust.

 

It has taken me a while, but I think that I have an answer to this question but it has nothing to do with aerodynamics. A significant difference between a wing and a normal sail is the extra weigh of the wing which for a monohull exacerbates heeling. By canting the wing to be closer to vertical the disadvantage of having a heavier wing can be reduced.

 

That makes sense for an infinite length but what about end effects? Sail boats have one end plane termination (no loss?) and one end in open space making a vortex. My gut tells me that healing to leeward increases induced drag and that canting to windward can reduce it.

The other thing is that, as you pointed out, canting is like sweeping -healed is swept back, canted to windward is effectively swept forward. But what if you cant the wing to windward and sweep it back? The reason I ask is this was a surprisingly fast position reaching on an old windsurfer.

MIJ, canting the sail to windward is similar to canting the keel -but the hull goes with it so you get some form stability. It's not just the weight of the rig that would be hanging over to leeward. The wind force on the sail adds to displacement when it is heeled to leeward -it subtracts from displacement when it is canted to windward -significant on a dingy.

 

To get the 3D effects, you need to analyze the planform with at least a vortex lattice code, as lifting line theory doesn't handle sweep. Sweep will load the downstream end compared to the upstream end, because the downstream end experiences an increased angle of attack due to the trailing wake.

You can do vector addition to resolve the apparent wind into the wing coordinate system and then calculate the effective angle of attack and sweep. Canting forward and sweeping aft can be the same as having an unswept rig. It is basically an upright rig that is rotated about the apparent wind vector.