affect of mast on offset sail

I am exploring various rigs with sails offset from the mast.When a mast is off set from the sail what affect,if any, does it have on the sail and if the affect is detrimental can it be mitigated, or can it indeed be induced to actually help lift the sail as in a jib main configuration (circulation theory?) ? Any input would be appreciated.

 

Hi Timothy,
I'm also interested in this type of rig configuration.
I haven't heard of anyone managing to get a net increase in lift from the interaction.
My research (untested sadly) suggests that the mast to leeward of the sail can lead to flow delamination problems, but anecdotal evidence from Junk sailors is that the difference is undetectable on less high performance rigs.

The obvious mitigation is to streamline the mast (like several of you example diagrams). A highly streamlined rotating (or sleeved I guess) mast has negligible drag compared to conventional rigging and is unlikely to cause serious disturbance to flow over the sail.
Some (dipping) junk and lug rigs drop the sail and swap it to the other side of the mast on each tack to keep the flow optimised, but if a streamlined mast was used, and a reasonable clearance allowed between mast and sail I think this would be found to be unnecessary (no proof again). If a square rig (a la dynarig) is used this redundant obviously, as the mast remains always on the same side relative to the wind.

The advantages I see are
-A single sail (efficient) balanced rig can be made, reducing sheeting loads, and reducing sail twist.
-Sail shape is not dictated by the structural requirements of a mast. Similar to mast-aft rigs a clean leading edge can be made, and without the high staying loads required for useful mast aft rigs.
-Mast bend has little effect on sail shape.

I'll be interested to see any more info you get with this thread.

 

In the diagram options 5 and 6 are not good , the rest are.

Streamlined mast to windward is a large improvement to a mast obstructing the leading edge.

If the mast is to leeward it must be a very generous distance away from the surface so as not to disturb the flow. This is not an evident way of engineering the setup. Therefore mast to windward is always the correct choice.

The dipping lug was always a rig of choice on traditional boats where speed was of major import.

Whether the mast itself is streamlined when it is not interfering with the sail is of secondary importance.

 

Cube,

Do you have any reference material to which you can point, a link, whatever, which can be used for study?

 

Although the modeling used by JavaFoil is inaccurate for largely turbulent flows it is still close enough to give a rough idea of such gross effects as i illustrate now.

Two pairs of images ; one with an obstruction to leeward and the other with the same obstruction to windward. On the polars it can be seen that the L/D is almost halved when the obstruction is to leeward.

Further reading is found in Marchaj's Aero/Hydrodynamics of sailing pages 328-338 & 540

 

Further test reveals that to return to the L/D of the obstruction to windward as shown with the obstruction to leeward requires about one chord of separation.

This follows basic aerodynamic understanding that low pressure side flow is more critical to the L/D characteristics than high pressure side flow.

It also means that streamlining of the mast if it must be to leeward is of much greater importance than if it is place to windward.

 

To expand a little more;

Consider first drag-
When the mast is to leeward it is in a higher local airspeed zone than far field speed and when it is to windward it is in a zone of less than far field airspeed. As drag force is a function of airspeed squared this makes a considerable difference. Further drag effects are invoked by the interference between the mast and sail, so if the mast creates premature flow separation on the sail then this is an extra drag that must be added.

Secondly lift-
Here it is somewhat more complex , but looking purely at circulation strength (Kutta-Joukowski theorem) it seems to me that the mast to leeward creates a greater hindrance to circulation than when the mast is to windward.

 

This is all interesting and I thank you for the time you took to assemble your thoughts.

In light of your conclusions; How, then, do we square with the regular reports of sailors using balanced lug rigs.... making comments that they are experiencing no discernable difference between tacks, even though one of the tacks clearly has a mast bisecting the sail, totally disrupting the flow.

If all of your comments above are counter to that argument, how can it be that so many make the opposing observation?

 

Is it possible to do the same analysis in the case of the dipping lug with the rotating mast , with a suitably shaped mast rotated to an appropriate angle of attack and treated as a second foil (extremely thick high aspect main?) inside a radically over lapping Genoa. I would be interested to see if the mast would lift the sail as the main is purported to do a fore sail . Chris. I have also read that as reported by sailors there is little decernable difference between tacks but that there is a difference. That begs the question is the benifit of dipping the lug worth tHe effort?

 

That is an interesting and valid observation , but it is actually distinct in that in the case of the standing lug the mast is not some distance away from the sail and not affecting its shape. Rather , when on the 'wrong' tack it 'creases' the sail and hides within a separation bubble. This is being very general as it may not actually work out that way in a given case.

Essentially the standing lug does perform worse on the 'wrong' tack but all things considered the difference is too small to readily be noticed.

I think in the case described in the thread the difference would be greater than in the typical standing lug scenario, perhaps to the point of being actually noticeable.

One must also remember that when talking about the relative efficiencies of different systems the differences are not usually apparent until you sail alongside another similar boat for reference.

 

Timothy, regarding whether dipping lugs are worth the effort; it depends on your priorities. Obviously many generations of working sailors past found that in their particular situation yes it was worth the effort. The normal dipping lug setup offers a remarkably clean airfoil with a planform that is not too bad an approximation of a hemielliptical planform and with the mast in an area of lesser airspeed, all beneficial factors for increasing performance.

Before a wing mast becomes effective as an airfoil in and of itself it must be made a thickness to chord ratio that is far lower than any of the usual wingmasts, and this is where engineering concerns would be a limiter. Also these kinds of relative positions do not offer any kind of 'slot' advantages.

 

Accepting that an offset mast cannot benefit the sail and that to reduce drag it would be beneficial if marginally to present a foil shaped mast to the apparent wind what section shape would be best? It just occurred to me that another example of a sail on an offset mast is the windsurfer rig,really a kind of dipping lug is it not? The presence of a very non aerodynamic mast to wind ward doesn't seem to have prevented it from until recently holding the sailing (in water) speed record. Perhaps at the speeds capable of a displacement boat in the case of the dipping lug the drag of a circular sectioned mast is negligible and not worth adressing.

 

Hoyt's Offset Rig

Have you guys seen Garry Hoyt's lastest...

http://www.garryhoyt.com/id19.html

I am always amazed when designs like this evolve along independent tracks.

 

A bit saucy of Garry Hoyt to patent that, as balanced rigs with offset sails been around for years. However, the streamlined sock around the mast is something I had not seen before.


I built a similar rig for a sailing canoe which also featured roller reefing and furling, some time back when I knew even less about sails than I do now. In anything more than a gentle breeze, to keep a decent sail shape required so much tension that extra rigging was needed to prevent mast damage, since I was not using carbon elements.