Dick, Rotating mast?

Figured I’d better start a new post here from were we strayed on the IOM question post.


Rotating mast? I’m wondering how you will be attaching the mainsail to this mast. Then I have some other questions.

Dick- Does your rotating mast work like a fulsize Tornado or F18 mast, where the mast is allowed to rotate seperate to the boom, so it works as a camber inducer, or does it work that the boom and mast act as one unit, a bit like a swing rig but only for the mainsail?

If its not blowing it sucks!

This was going to be my next question. I’m not a fan of the 'rotating mast myself due to just this fact. When the wind picks up, the mast will rotate, doing just what you don’t want to do and that’s to increases the draft in the mainsail. When the wind picks up, you would want to do the exact opposite and flatten the sails.
If your luff is in line with the pivot point of the gooseneck, then rotating mast will be OK. If you could put the luff aft of the pivot point then this might be the real trick.

Added this;
No aft of the pivot wouldn’t work either. That would keep boom from easily swinging out on reaches and runs in light air. Not a good idea.

Dean DeRusha who started the mini Harken business a few years ago claims he is back in it now…

Doug Lord
High Technology Sailing/Racing

Dick- I dont think having a boomless rig is the way to go, if you use a traveller, you need a fairly powerfull (and therefore heavy) servo/winch to move the car under load. The backwards kicker that you describe with the boom attached to the deck would surely eliminate the need for an extra servo, unless you want to adjust the kicker on the water- for that you could use a small servo and use a powerfull perchase or lever system as not much travel is needed.

Im also wondering if it would be posible to link the mast rotation to the boom in a way that as the boom is let out the mast rotates in the same direction but to a greater degree. This would mean the more you sheet out the more camber is induced, but as the boat speeds up the the apperent wind comes forward and you sheet in, the camber is reduced as less is needed.

If its not blowing it sucks!

One other thing you could try with the extended boom is to put a counterbalance on there so that the boom would not excessively tension the leach of the sail in really light winds. This would be a great alternative to a topping lift.

As far as the bend charcteistics of your wing mast - ask some of the DN sailors in your area about lateral bend… They have some really radical ideas and for the most part they work really well… The bend they are dealing with is scary but it works…

For the traveler system, take a look at model railroad track. Finding a traveler car to ride on the track may be tricky, but the track itself is easy to come by and should be well shaped for the purpose. If you do use this system, you should consider adjusting the height of the track above the plane of rotation of the mast to set the leach tension of the sail for different points of sail. You would want the sail to twist quite a bit on the reach, but on the run, you would want to tension the leach. By raising the track in areas where the car will be for reaching you can relieve the leach tension and add twist. Then by loweing the track out where the car would be on the run, you can increase the tension again to reduce the twist. If you do it right, you would not need a mainsheet at all. Rather you would have a fixed length attachment to the boom and then you would just move the car around using the servo. If you wanted to have some on-shore adjustments for the boom attachment, you could attach the boom to the car with a turnbuckle.

If you wanted, you could have the traveller car on a closed loop sheeting system with to a winch so that you could drive the car to any position you wanted - including all the way out. This would allow you to sail backward if you did happen to snag some weeds… The car would be near the middle of the travel when the Transmitter stick was in the middle. So you could leave the self centering spring in place and control the closehauled sheeting with the fine tune. you could also force the sail to gybe without having to oversteer until the sail backwinds.

There are a lot of merits to this idea…

  • Will

Will Gorgen

Here are some photos of a main sheet traveler that I had made for my big J boat. Basicly all SailEct parts with a bit of work . This system worked quite well. The travler was conected to a Whirlwind drum winch.

Rather than trying to reinvent the wheel, you might take a look at the Walicki Boats website. He makes a rotating carbon fiber aero mast that sits on a mast jack/mainboom deck fitting. All of his parts are of extraordinary quality.


Seems to be a lot of “aerodynamic” theory floating around that doesn’t quite add up. For instance, the “gap” between mast and leading edge of the main that is supposed to be good on IOM’s doesn’t add up if a wing mast(on a model) is a good idea–at least I don’t see it. Does anyone have a theory on the subject?
Why wouldn’t a “leading edge slat” instead of a jib be a good idea particularly on model multi’s at least up to a certain windspeed? Will ?

Doug Lord
High Technology Sailing/Racing

Hey Doug,

There are a couple of competing aerodynamic principles at play here…

Here is my understanding of the gap issue that has been proposed for the IOM. A round mast produces a lot of turbulence in its wake. By placing a gap between the mast and the sail, this turbulence can be shed off the suction side of the sail (leeward side) by fresh flow coming through the gap. This would allow cleaner flow to establish on the sail resulting in potentially lower drag. More importantly, the sail will be less likely to stall which should allow you to sail a little higher or trim a little harder and get more power out of your sail. The results to date have not been conclusive but there is some merit to the idea.

Another advantage of a gap is that the leading edge of the sail is free to take on a better incidence angle to the incoming flow. If you attach your sail to your mast with a boltrope, the leading edge of the sail will be forced to extend straight aft from the trailing edge of the mast. If the wind is not strong enough to boss your sail into a nice cambered shape, you will end up with an “S” shaped draft shape. Not good. The more flexible the connection between the mast and the sail, the more the leading edge of the sail is free to take on a nice clean incidence angle.

Now in the case of Dick’s wing mast there are 2 major difference from the concepts above. One is that the wing mast should have very low turbulence behind it. The wing mast is aerodynamically faired so it will have a nice smooth boundary layer. The second difference is that the wing mast will establish the incidence angle to the incoming flow. Thus, if the sail extends straight off the trailing edge of the mast, that should be aligned with the flow and should form a nice cambered airfoil shape.

So with a wing mast, there is no need to have a gap.

  • Will

Aerodynamicist in residence…

Will Gorgen


I?ve been using the ?gap? system for the last season on three classes of boats, the IOM, US1M and Star 45. I swear by it and will continue to experiment with gap size. I use about a 10mm gap at this time on the IOM.

In watching the AC this last year, I noticed how disturb the leading edge of their mains were on close-hauled conditions. This is where I got the idea to do this gap. I see that the infamous Lester Gilbert was thinking the same thing.

I find a much nicer sail shape, but you must pay much closer attention to mast bend with this system or it can be a disaster.

Hey Greg,

I changed from a boltrope to hanks on my Fairwind as well (I think I posted a picture here sometime back). The leading edge entry is much nicer - especially in light wind. I run with a very small gap of ~1-2mm (1/32" to 1/16"). I have not tried a larger gap than that, but I can see that there might be some advantages to doing so.

I think the disturbance you saw on the AC mains was really backwinding from the genoa. I noticed this as well. The AC boats had a great variety of conditions to deal with on the race course. At one end of the course the water would be relatively flat and at the other end, it was choppy. For the choppy conditions, you would need to foot a little harder to punch through the waves. So you would select a sail set that gave you enough power for this portion of the course. With these sails up, when they got to the flatter water, they would shift into pointing mode. It was in this pointing mode where the jib slot closed up substantially (they pulled their genoa cars inboard) and the genoa would begin to backwind the main. This was especially true when the boats were a bit overpowered in the puffs. The main timmers would drop the traveller down a bit to relieve the windward helm and this would cause the main to get backwinded even more.

But getting back to our little boats. A round mast will result in a lot of turbulence especially at our sizes. With our non-overlapping jibs, it is difficult to re-energize the flow on the suction side of the mainsail. So if you can shed that turbulence and then re-attach a new, undisturbed airflow pattern on the sail you can probably postpone stall a bit. This should allow you to trim a bit tighter and/or sail a bit higher. But the trick is to get rid of the turbulence. If the sail ends up diectly downwind (aparent wind) from the mast, then the turbulence would end up hitting the leading edge of the sail and you would be right back to where you started. In fact, it might be worse in that your wake could be oscillating with a Karman vortex sheet. That might make for some really nasty airflow patterns on the leading edge of your sail.

For me, the biggest benefit of switching to hanks was sail shape. I may try opening this gap some to see if I can notice any improvement in speed. I utilized 10 evenly spaced hanks for my sail which allows the sail to respond quite nicely to mast bend when I want it to. With a larger gap, I’m not sure what the result would be as far as my ability to get the sail shape I want.

Perhaps I will see if Lester wants to pop his boat in the wind tunnel with and without the gap and use some flow visualization wands to see where the turbulence goes…

  • Will

Will Gorgen

You are right, Dick.

As Fairwind class secretary, I may have to propose a rule amendment for my class to outlaw that…

The biggest problem with a luff sleeve like that is getting it to rotate cleanly when the sail tacks (assuming a non rotating mast). In heavy air this would not be a problem but in light air, I doubt if it would rotate well at which point you would be left with a pretty crappy sail shape…

  • Will

Will Gorgen

The “sleeved” or “double luff” sail has been around forever in r/c sailing. It was largely abandoned by most development classes because superior shapes could be obtained from other attachment systems.

Hey Dick,

Actually, the minimum amount of wind required to move the boom to leeward produces very little heel. So it is not the heel that is causing the boom to move to leeward, it is the wind.

My experience with the Fairwind was that at this minimum amount of wind, the stiffness of the sail material is enough to dictate the sai shape. Thus, if your sail is attached to a bolt rope which creates a lot of friction inside the mast groove, the luff of the sail will not curve into a nice incedence angle. it is at these extremely light wind conditions that a fully sleeved luff would, I think, have a hard time tacking due to friction between the sleeve and the mast.

Perhaps using Sailkote would help… But that is another topic…

If you can get a luff sleeved system to work well, it will produce a favorable sail shape. A rotating wing mast is still going to be better (the circular cross section of most masts are not aerodynamically ideal). Many full sized use an elongated mast section that strikes a fair comprimise between leading edge bluntness, stiffness (fore/aft and lateral) and weight aloft. Such shaped masts are rare in RC sailing simply because they are really hard to make in small sizes. Tubes, and especially carbon tubes are pentiful from other sports (kites, etc) so most folks figure out how to use such readily available “sections” rather than figure out how to form some sort of specially shaped mast on a mandrel, in a mold or around a core.

At our very low reynolds numbers, the drag of the mast is quite low and the potential gains of shaped masts would be very small…

  • Will

  • Will

Will Gorgen

Alright, so here is a question:

The US1M class prohibits “rotating masts” and “wing sails”. so does a double luff sail qualify as a wing sail? If you took the concept shown in figure 3 (from Dick’s previous post)and created a rotating, hard, double luff sail that wrapped around a non rotating mast could you in fact argue that you had neither a wing sail nor a rotating mast?

The equipment rules of sailing define a soft sail as any sail where “the body of the sail can be folded flat in any direction without damaging any ply other than by creasing”. The “body of the sail” is the sail with the exclusion of “sail reinforcements, batten pockets, windows, stiffening, tabling and attachments”. Stiffening is limited to corner boards and battens.

So as long as the sail is comprised of soft material and the system enclosed by the double luff is not considered part of the sail, but part of the mast, you might be in business. If you designed the system internal to the luff to flex or flop from side to side to form the leading edge shape, does that count as mast rotation? Remember, the US1M rules do not prohibit wing masts - only wing sails - so if the mechanism looked like a wing or a portion of a wing, that would not necessarily make it illegal.

Perhaps I should send a note off the Jim Linville and see what he has to say. I think he is on vacation until mid january so I might not be able to get ahold of him…

  • Will

Will Gorgen

Read the US1M rules again. There is a mast diameter restriction.

Roy, you are right. So, I suppose that any “mast mechanism” I come up with would have to be counted in the max diameter of 3/4". The rule does not specify that the mast need to be round (although something that is not round really does not have a diameter, does it). If I built a bullet nose mechanism that would fit inside a 3/4" ID tube and fit a double luff mainsail sleeve around that, would it be legal?

The mast on my Fairwind is aluminum and 9.7mm (3/8")in diameter and 48" long and it is plenty stiff (the way I have it rigged). I could see making a 65" mast 3/8" diameter out of carbon fiber rod and have it be plenty stiff with the right rigging. That would give me quite a bit of room for a bullet nose mechanism that extended 1/4" in front of the mast and was less than 3/4" wide and it would still have room to flop from side to side the way it is shown in Dick’s diagrams. There is really no reason the “leading edge shell” needs to be as big as it is shown in Dick’s diagram in order to form an effective double luff. In fact there is a point further forward in the diagram where the same mast diameter gives the same restriction on mechanism travel…

I’m not sure what the mast diameters are of any of the US1Ms at our pond, but they are certainly not 3/4".

  • Will

Will Gorgen

Ruling years ago in USOM was that the sleeve luff was legal as long as the mast did not rotate, but when the area of the sail was measured the sleeve was included. That’s about 82.5 sqin deduction from the area behind the seam of the sleeve on a 55" rig. Ooooouuuucccchhh!