Rotating Wing Mast – theoretical discussion

[BOATMIK]

Good one Tom,

Was aware of the tufts but not aware of the stagnation pointer thingy.

I wonder how such systems would relate to a boat continually changing pitch roll and yaw. I guess it would be try it to see how it works. Maybe someone with an A-cat. And to see whether it led to a real performance difference.

Is anyone using such systems now on As or the square metre classes? If not - wonder why not?

Also you gave one example of a top level iceboat person not using them at all.

So wonder if the advantage of accurately lining up stagnation point with optimised areas of the mast might be a minor effect and not really worth worrying about.

Like in the way that fat conventional masts when compared to slim conventional masts "never seem to be as bad as we expect" as Milgram once put it.

If I can feel my head snap back when I get it right on a sailboat - I'll be very happy!

:-)

MIK

[national]

Decided to do a bit of work on this.

I have rotated various mast sections and have found really incredible results. Reductions in drag of 60% and an increase in lift of 25%. It is amazing how much there is to gain. As soon as i find somewhere to present/publish it i will let you know. Still struggling to find somewhere suitable though. Answers on a postcard please, travelling not an issue.

[PI Design]

If its ground breaking stuff, I would recommend RINA (as you are in England). Either submit a paper for peer review prior to publication in the Transactions, or offer to present a lecture to a local branch meeting, which are held monthly in most areas. Alternatively, why not just put your findings on here?

[gggGuest]

You probably should take a look at work from the Amateur Yacht Research Society in the UK. You migvht well find others have trodden the path before you...

[CT 249]

Originally Posted by national

Decided to do a bit of work on this.

I have rotated various mast sections and have found really incredible results. Reductions in drag of 60% and an increase in lift of 25%. It is amazing how much there is to gain. As soon as i find somewhere to present/publish it i will let you know. Still struggling to find somewhere suitable though. Answers on a postcard please, travelling not an issue.

What sort of boatspeed increase would you get in a dinghy with a drag reduction of 60% and an increase in lift of 25%?

Emperical experience indicates that the wingmast is very rarely worth it.

It has been tried by Herreshoff (Suicide class, '30s), Curry (Renjollen, '30s), International Canoes ('30s by the US champ, top the '80s by a NA), R Class ('50s by a Gold Medallist; '70-'80s by a multiple class champ; 90s by an ID champ); 12 Foot Skiffs ('90s or '00s by a repeat Interdominion champ); 18 Foot skiffs ('50s and '80s); Moths; Merlin Rockets; windsurfers; even Frostbiters.

They work in NS14s, although even there for all the writing about the superiority of the wingmast, the non-wing poles were very competitive for many many years.

While the results are interesting (as they were when other people got them) isn't it also fascinating that the test results never seem to be borne out in sailing? Is the application in sailing, by gold medallists etc, that is at fault or is the testing missing something and therefore creating incorrect results?

[national]

Thanks for the AYRS, it seems like a good place to start.

[Steve Adolph]

Gents,
There appear to be some talented people on this thread. Maybe I could get some help?

I'd like to be able to compare the following, based on a proctor D mast section.

Case 1) Nominal. Non-rotating mast.

Case 2) Rotating mast, angle of rotation set to be such that the sail exits the mast on centerline of the mast. A symmetrical fairing exists on both sides of the sail, with the purpose being to improve the aerodynamics of the sail/mast. shape TBD

Case 3) as per case 2, but with no fairing

Would Case (2) offer significant improvement?

Any opinions?

Since Proctor (Selden) no longer offers the D, the Cumulus section could be substituted. That sectiion profile is available at the proctor selden site.

thanks in advace.

Steve

[Steve Adolph]

Thomas-Interesting paper!
Could an implementation of a teardrop wingmast be accomplished with a standard dinghy tube, like a Proctor Cumulus, and a foam fairing attached to the sail?

Originally Posted by tspeer

See Aerodynamics Of Teardrop Wingmasts

Bethwaite sent me some tracings of Tasar mast sections, but I've been unable to analyze them because the backward-facing step is too extreme for a panel code like XFOIL. Maybe some day I'll be able to tackle these types of mast sections with a Navier Stokes code like NS2D.

But the principles should be similar. There's going to be a separation bubble behind each step, like the case of the under-rotated teardrop mast. However, the separation point will be further aft. So the design objective would be to maximize the leading edge suction while minimizing the size of the windward and leeward separation bubbles. The pressure recovery after the leading edge suction peak would have its steepest portion in the attached flow on the mast.

The windward side separation point can be moved aft significantly, which may reduce the size of the windward separation bubble despite the aft facing step. But the separation bubble on the lee side will inevitably be larger than for the corresponding teardrop section. This may be why the Bethwaite sections have evolved from a rounded wedge to more of a boat-tail trailing edge that has a reduced base area.

[national]

If you find me the geometries in a CAD format, I can probably run them for you.
The rotation would definately help the flow, both in increasing lift and reducing drag. From the results I have had any rotation on pretty much any mast going upwind is beneficial.
Jon

[MalSmith]

I have used over rotating round masts on both a moth and an NS14 in the ealy 80's. While I have no measurements to support the percieved benefits, there is a noticable jump in pressure when moving the mast from the under-rotated to the over-rotated position. This effect was often commented on by otherwise skeptical people who sailed the boats.

I don't know why round masts are not over-rotated more often. Its not that difficult to do. I would guess that the benefits of over-rotating a round mast are almost as good as the benefits of rotating a more streamlined mast section, with regard to minimising the separation bubble, but there seems to be a general perception that if it's a wing mast, it rotates, if it's a round mast, its fixed. Even if the benefits are only a few percent or less, every bit helps. I'm currently planning to rotate the round mast on my IC.

Mal.

[national]

It's probably because the classes that have the flexibilty to rotate masts probably also have the freedom to change the mast shape, and an elongated mast section will produce more lift. So if you are going to go half way why not go the whole way?!

[MalSmith]

Originally Posted by national

It's probably because the classes that have the flexibilty to rotate masts probably also have the freedom to change the mast shape, and an elongated mast section will produce more lift. So if you are going to go half way why not go the whole way?!

By over-rotating a round mast, you are going more than half way. In practical terms, the lee side airflow (and hence lift) is improved as much as for the elongated mast, only the drag will not be improved as much (if at all).

Round masts sections are freely available. They have known bend charactoristics and the tuning methods are the same as for fixed round masts. An existing fixed round mast can be adapted to over-rotating for minimal cost. Elongated mast sections are not so freely available (admittedly due to low demand) and are therfore more expensive or have to be custom made. Elongated mast sections have bend charactoristics that vary with the degree of rotation. Tuning and using an elogated mast section requires learning a new set of skills. The degree of rotation of the mast strongly effects how the sail behaves and therfore adds another level of complication to the operation of the rig. When rotating a round mast section, usually you will just rotate it as far as possible, and that's it.

Yes, over-rotating a round mast is a compromise, but it delivers much of the benefits of using an elongated mast for less cost and complication.

Mal.

[national]

I disagree on your comment upon lift and drag. Round sections do not create as much lift and create more drag than elongated. The actual flow over the sail is pretty similar but the increased area of the mast actually contributes significantly to the lift and drag reduction.

[PI Design]

Originally Posted by national

I disagree on your comment upon lift and drag. Round sections do not create as much lift and create more drag than elongated. The actual flow over the sail is pretty similar but the increased area of the mast actually contributes significantly to the lift and drag reduction.

Can you explain that a bit more please?

[national]

If you take the mast and the sail as seperate entities they both have a drag and lift force assosiated to them. Together they create a total drag a lift force which is the important factor for a design. When a mast is rotated the force on a mast can actually contribute significantly to the Total lift. Thi sis especially true when an elongated mast is used as it creates significant lift which can actually act to drive the mast forward rather than backwards.

With a rotated round section the force on the sail is similar to the elngated section, the main difference is the force on the mast. This change is significant.

I have spent so long thinking about this I have forgotten what I was writing about! But i think this makes things clearer!