Rotating Wing Mast – theoretical discussion

This happens on any wand equipped multihull like the Hobie trifoiler, so its been done plenty of times. I think the lack of popularity of such compared to the Moths suggests that it may not be the right approach...

 

The current fastest Class B sailboat is Sailrocket at 47.36 kts (http://www.sailrocket.com/). From their web site:

Vestas SailRocket employs a wholly different concept (first documented by Bernard Smith in the 1960s) in which the sail and keel elements are positioned so that there is virtually no overturning moment and no net vertical lift. When used correctly this concept results in a boat which no longer has obvious stability limits and in which the only significant response to gusts is a change in speed!​

The Ketterman brothers were the first to use an active control system to pull down on the windward side. This culminated in the Longshot (current Class A record holder at 43.55 kts) and eventually turned into the Hobie trifoiler (http://www.hobiecat.com/sailboats/trifoiler/).

Given that the two examples above are the current world record holders for their respective classes, there is more than a little merit in these concepts.

The other example of nearly ballanced roll moment configuration is the Kite Boards. The overturning moment arm is very small such that overturning moment is not limiting at maximum speed. Rob Douglass holds the current overall speed record at 55.65 kts using this technology (http://www.youtube.com/watch?v=CtFxstw9Zjo)

Over in the "Monohull Speed: Speed Dream by Vlad Murnikov" thread, we have been discussing righting moment (portion of my post #172 copied below). Use of fully balanced systems (Sailrocket) or active control systems (Longshot / Trifoiler / Rave) are actually less efficient than a hybrid system. To the extent possible, all discretionary weight (crew for example) should be to the windward and combined with adjustable ballast (water tank filled via a scoop tube). This is because ballast to windward creates the required down force with no drag. In the post below, I discussed a hybrid where ballast is combined with float actuated active control surfaces (wings).

 

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According to James Grogono in "Icarus The boat that Flies", the first foiler design to use dual "feelers" similar to what Ketterman used(incidence controlled foils-whole foil moves) was Force 8 by the Pattison brothers in the UK about 1979. The first dual "wand" system used to control just a flap on the main foils(like the Rave, Osprey, Skat and F3 RC model) was developed by Philip Hansford on Philfly in the 80's. Both these systems have the "advantage" of developing 100% all the RM required (up to the structural limit of the boat) with the windward foil pulling down automatically when required and the lee foil increasing lift automatically as necessary. Both modern versions of these systems have exceeded the top recorded speed of a Moth in heavy air but the Moth is far superior in light to moderate air. Both the Rave and Trifoiler were build to the requirements of production boats that resulted in fairly heavy boats severely impacting their light air takeoff capability. The new all carbon Osprey by Dr. Sam Bradfield(who invented the modern planing wand) will be very interesting-and very light.

Click on image:

 

You are probably right.

Greg Ketterman (http://members.cox.net/gkettermanb/Home%20TriFoiler.shtml) indicates he was just at the model stage in 1981. The part I find neat is that they did their first full scale homebuilt prototype in 1987 and then progressed to a world record by 1991.

 

A couple of pages of fotos and video on Longshot coming out of mothballs for a
day of play in the SF Bay at the end of 2010

http://www.pressure-drop.us/forums/content.php?509-Longshot-Rides-Again

http://www.pressure-drop.us/forums/showthread.php?506-Long-Shot-back-in-the-game!


One of the captions reads for the "last time" making it sound like Namibia is
not in the cards... Rumour has it that Longshot was sold to the Makani Power Kiteboat folks http://project.kiteboat.com/page/project


Foils tend to cavitate at high speed
PBS-Scientific American Frontiers Date: 1994-02-16, Episode: Science and Sports: Speed Sailing: Windsurfer vs. Trofoiler vs. Planning Hull...Also shows foil cavitation in an MIT water tunnel
http://vsx.onstreammedia.com/vsx/pb...&ccstart=2470470&pt=0&vid=pbssaf405&entire=No

 

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Good stuff, thanks!

 

I know, this is supposed to be a theoretical thread, but one practical example could be enlightening. I have been trying to figure out the optimum camber for a square head full batten mainsail to be used in conjunction with approx. 4.5 x 9" tear drop wing mast section (Ballenger), extended about 3/4" by Tides Marine track. This is to be used on a fast tri. The dimensions of the sail are: luff 40.5', leach 42.25', foot 14.3', head 38", roach 34". I am more interested in close-hauled performance than, say, broad reach.
Would anybody know the answer or could point me the right direction to help me to find out for myself? Thank you.
Jan

 

Tom Speer:

Eric Sponberg:

It looks like you don't agree on this?

I'm planning for a free standing unarig. The worst wing shape I guess would be in the head, when the rig is fully depowered; low rotation at the bottom, flat sail with lots of twist. Then the top could become under-rotated.
Tom, can you share approximately how thick the USA17 mast 3 was, and the chord % of the total (at midspan or something)?

 

I think Tom and I are talking about two different things. Tom is talking about chord length of the wingmast in relation to the chord of the sail, and I am talking about section thickness of the wingmast. I am not sure that Tom takes into consideration that the chord length need not be constant. Perhaps he can clarify that.

In my wingmast designs, I taper the chord length along the length of the mast so that the top chord is roughly half the length of the bottom chord. I also use an entasis taper, not a straight taper, so that the middle parts of the mast are actually a little bit longer than what a straight taper would give. I also use a chord-to-thickness ratio of 2:1 which is relatively fat in the overall realm of soft sail wingmasts. Many people use a narrower section on the order of 3:1, and that is what I was refering to in the comment you quoted above. In my opinion, a chord/th ratio of 3:1 is too narrow and is less forgiving to the airflow. I think a chord/th ratio of 2:1 is better.

Eric

 

Hi Sigurd - All of the top sailmakers now have the capability to do CFD on mast wing combos. So pick your sail maker, say Norths and ask them the question and they will design the sail for you. You can give them a couple of mast shapes and they will tell you which one is best/worst. Cheers Peter s

 

So if I understand you correctly then, the most robust mast; 1) has big mast chord/sail chord, and 2) is thick. So, just big all over.

I need to do some work with xfoil now.

 

Ok petereng, that's good to know. About that, is it more difficult to control sails that are short chord / mast chord, because of the aspect ratio? Why does A-class have such short chord masts, but Gary Baigent has large chord masts?

 

Hi Sigurd - Lets take the mast/sail in general from history and from a technical aspect. 1) Being a free standing rig you will have the problem in that the top will not be "automatic" as the mast will want to bend at the bottom rather then at the hounds in a gust. This is the current problem with a free stander. 2) If you look at C-Class as the leading rig development class, as there are no rig rules the trend is as follows. Get rid of the jib, then to unarigs. Make the mast bigger and bigger until its about 50% the chord of the sail. Then graduate to a rigid wing, then to a 2 panel wing. But we still have rigging. 3) rigging at high speed has huge parasitic drag and will have to go, which you have taken the step towards 4) Make the aspect ratio as high as possible to improve aero efficiency 5) To answer the mast chord question - mast makers have economic limits and class rule limits. The trend with classes that have mast size limits is to reduce mast size. This is because we can do this if we use high modulus carbon fibre. So we answer the aero drag problem by minimising size. This also minimises cost as the sailor will not pay for the big mast (more material more labour more cost) 6) If there is no mast size rule in the class the trend will be for bigger masts. Bigger is lighter and aerodynamically better. But we have sails involved and we can't change masts size dramatically as we then have to develop new sails. When aluminium was used by skiffs we could cold set the mast shape and tune it to the sail. Now with CF we have to tune the sail to the mast. So its harder to get the sailmakers and mast makers to agree. ie there is a natural inertia to change once the sweet spot of the rig is identified. Mast builders and sail makers are trying to make money and development costs more money then sales so they want a steady sales stream not a development stream. ie The marketplace wants stability and the sailors want stability to lower costs. 7) I'm a mast/structures engineer so know the mast side well. The mould cost for a modern mast costs much more then the mast so you have to be certian you are going to get several masts from that mould, unless you commit to the mould for a one off or you build a cheap mould for a one off mast and do the labour yourself so the labour cost is zero etc etc. 8) So finally if you go down the free standing path - be prepared for a sail development program to get the sail doing what you want, if it is for racing. It may not perform automatically so you may have to add "running stays" to get some flick in the top if thats what you want. I have designed a hinge for a free standing mast that has controllable stiffness from the deck to acheive this but its never been built. 9) Also re free standing rig design the foundation so you can move the mast. In a usual rig we adjust the rake to get the boat into balance. But on a free standing rig if you miss the right spot you are in a bit of a corner. So if you can move the rig fwd/aft by design you have the opportunity to get it right. 10) engineering logic says free standing is best but this has not yet come out in practice for various reasons so be clear why you want to do this and don't try and over step the concept. Simple is best. Cheers Peter S

 

To add to this comment, the current AC72 have taken rig to a new level by end plating the jib and main. Perhaps you can consider this as well . What size boat or class are you talking about? Cheers Peter s

 

It is a 9m pacific proa. Has to be possible to take the rig down to go under a 3m bridge.
Telescopic mast. So, no taper and the top section is biggest. 11m luff, 21 m2. 2-3 ton-meter maximum righting moment.

End plate, do you mean pulling the sails all the way to the deck? I'm not sure it's a good idea. A good view is important.

Shouldn't it be possible to use other springs than the mast itself for aeroelastic washout (gust response)?

This comes from a UI 1720 as per tspeer's teardrop paper.