Backstay on the M

[delevi]

What does an boat have that an boat lacks? I’ll leave out layout preferences, and this is in no way an M vs X thread. But a backstay is a vital piece of equipment which can really increase performance on any sailboat. Well, the guys at Arena Yachts came up with a solution. The backstay is set up just like on the X, attaching to an eye on the stern-starboard, below the stern rail seat for those of us who have them. It has a 6:1 block & tackle for adjustment and the mast rotation has been dealt with. Basically, there is a fitting at the masthead just like at the base of the mast for rotation. So, the mast rotates through, top and bottom, even with the backstay under load. I haven’t had a chance to sail with it yet, but did put some load on it and arched the mast back plenty. Tested rotation and the mast rotates under large backstay load. The forestay became very tight which is what this is all about. My only concern is that when rotated, the mast bends not only backwards but to the side. Actually, this is the case without a backstay, but with a loaded backstay, the sideways bend is more extreme. This was done without any sail up, so I’m hoping the mainsail will correct the issue. Arena yachts tested this mod and claim that they were able to point higher by 5 degrees and reduce heel by 5 degrees. Both improvements in performance are attributed to flattening the jib and eliminating all forestay sag (tested with CDI furler.) I'm sure the mainsail flattened as well, which is what a tight backstay will do. Looking forward to do some testing of my own. Assuming the mast holds up, which they assure me it will; the only compromises are: 1. backstay must be slacked off and clipped to the side when the bimini is in use (no big deal.) 2. There is a bit of interference with the use of the starboard stern rail seat, but it isn’t too bad. One can still sit there with reasonable comfort, and actually have a nice brace/support to grab on to. Probably more comfortable for a woman than a man

Fair winds,
LD TRIPPLE REEFED!

[beene]

Congrats Leon

We eagerly await your sea trials.

G

[Oskar 26M]

I have a backstay on my M, fitted by the PO (who also welded the rotating mast into a fixed position ).
I really haven't put the backstay to good use yet so I'll be interested to see what you think after your sea trials Leon.

[Currie]

Very nice Leon,

What did you end up using for the backstay? Did you say you went synthetic? Vectran? Looking forward to seeing the masthead. Very interesting.

~Bob

[bscott]

My with the rotator has two (2) back stays with a 4:1 Garhauer vang per side. There is a heavy duty Garhauer boom bail at the top of the mast to which each vang is attached. This bail attachment allows each vang to move independent of each other as the mast rotates.

Prior to each tack I have to ease both vangs reducing most of the vang force to allow the mast to rotate. Once the new tack is established, I harden up the appropriate vang and bend the mast.

A major flaw in the original single backstay is that it holds the mast to starboard. If you vang it, it bends the mast too much to stbd. This creates an umballanced rig so a second stay is necessary. There are lots of ways to centralize the rig for a non rotating mast but very few ways to do it on the without hindering the rotation. Leon, I'd sure like to see how your rig works.

bscott

[delevi]

The cable is a 1/8" SS 1x19 wire, just like a stock forestay. The blocks are Harken tripples with cam cleat which attach to the starboard eye on the stern and the cable shackles to that. The masthead has hardware just like the mast base. Imagine the mast base, flip it upside down and put it on the masthead. Bolted through with a 1/2" bolt going through the mast. The cable attaches to the aft section of the hardware. There is no need for stays port and starboard since the head swivels freely, the load doesn't pull the masthead to starboard, but straight aft (with mast in center position.) This not only eliminates the need for dual backstays but allows you to tack and have the mast rotate through with the backstay loaded.. no need to release. The compression loads are top and bottom and the mast rotates right through since the cable pulls on the swiveling hardware and not directly on the mast. bscott, your setup sounds like running backstays going to the masthead. Having had runners in the past, I was trying to eliminate having to release between tacks. This should do the job. As I've said, my one concern is with the mast rotated, there is sideways bend. It is not due to the stay being on one side. Just the nature of a rotated mast where the forces of all the stays are no longer pulling even. The mast twists out to windward whether it's rotated to port or starboard. I noticed this a while back with no backstay. The backstay just makes it bend more sideways which kind of sacres me. I mentioned in past thread that my theory on the 's rotating mast has a design flaw with respect to how much the mast rotates. I believe the angle of rotation is twice of what it should be. When I rotated mast half way, I noticed the sideways bend was minimal, even with loaded backstay, and with a mainsail hoisted, it will probably be eliminated alltogether. The "over-rotation" in my view has more detriment than just the sideways mast bend. The purpose of rotation is to get the mast out of the way of the wind on the leeward side of the sail. It does that, however, it rotates so far, it is no longer in line with the luff of the sail when sailing upwind. So you free up leeward wind flow but you block windward flow as the over-rotated mast now becomes an obstruction to the windward portion of the sail near the luff. I think I can solve this by attaching lines port and starboard to the vang bail near the mast base and up to a couple of fairleads which are mounted at the front of the jib tracks. I use these for a preventer. I'll measure the lines to allow for the mast to rotate up to a certain point (about half way) at which point the line should hold it in place from rotating further. Should work well, except the loads on the jib tracks may be high. I know they're very strong... worked great as attachement points for the preventer without problems, but these may be different loads alltogether. Would appreciate some input on that.

cheers,
LD TRIPPLE REEFED!

[Currie]

...The "over-rotation" in my view has more detriment than just the sideways mast bend. The purpose of rotation is to get the mast out of the way of the wind on the leeward side of the sail. It does that, however, it rotates so far, it is no longer in line with the luff of the sail when sailing upwind. So you free up leeward wind flow but you block windward flow as the over-rotated mast now becomes an obstruction to the windward portion of the sail near the luff....

Just a thought Leon - I have no personal experience of this, but I recall one (maybe more) of my design books saying that aerfoils are much more "pull" than "push". If I remember right, the pressure drop on the leeward side is 3-4 times the pressure increase on the windward side. I think leeward shape is far-favored because of this. Like I say, just a thought - you might want to do some investigating or trials. (I'll try to find the reference if you're interested).

Cheers,

~Bob

[delevi]

Bob,

Interesting.. Here's my theory and I have nothing factual to back it up other than my observations and a conversation I just had. Although the leeward pressure is critical for maximum lift, it is still lower than the windward pressure, thus the boat moving forward and sideways (if there was no fin/keel/board, etc.) If the leeward pressure was reduced i.e. an overtrimmed sail, lift is lost since you have nothing but that sideways force. On the other hand, the leeward pressure in and of itself does nothing. It needs the windward pressure and both create the lift. So when the luff of the sail flutters whilst the sail is in proper trim, I believe it is doing nothing at all.. so in theory any advantage of increasing leeward flow is lost if to do so, you block windward flow. I observed this in a number of scenarios close hauled. One may call this the headsail backwinding the mainsail, thus causing luffing-type bubles along the luff. If you relieve sheet tension on the headsail, the bubbles go away, but now your jib is undewrtrimmed. I believe this is the slot effect.. an increase in mainsail leeward pressure due to the flow off the headsail through the slot. With the mast rotated, the leeward pressure starts to exceed the windward pressure due to no blockage by the mast, combined with wind blowing off the jib through the slot and a reduction in windward pressure now created by the blockage caused from the rotated mast on the windward side. If the mast is rotated to center and pinned, the bubbles disapper. What that is telling me is there is sufficient leeward pressure without mast rotation since whatever is blocked is compensated by the headsail flow through the slot.

I just had a discussion with Jeff at Arena Yachts who agrees with this theory. What he told me is the benefit of the 's rotating mast is realized only on a beam reach, and a bit on a broad reach. Close hauled, close reach, and dead run, it does virtually nothing. Soooo.... rather than trying to limit the rotation, I will experiment with no rotation on all points of sail other than beam reach. Coincidently, this is the point of sail where the main is sheeted in such a way where the luff is in alignment with the mast groove when it is fully rotated. I guess the perfect mast rotating system would be one where the mast rotates in line with the boom... but I believe that would require a stayless wing mast. Why do I feel like I just opened a big can of worms Would love to hear what you find in your books.

Leon

[bscott]

Leon, I am aware that you do not like the idea of running backstays however I can limit the amount of rotation by controlling each backstay as the bail acts like a lever. From a rigging standpoint, the running backstay has less labor and deck clutter than installing another set of deck turning blocks and control lines to the base or vang.

I agree that the rotation should mimic the boom. I don't sail a genny so my slot is optimum for pointing. Since the does not have a traveller I triangulate my "end of boom" main sheeting with a second main sheet running to the gunnel which maximizes my main control and acts as a preventer on a run.

I initially thought the idea of the base plate turned upside down was a good idea but I believe you are correct that it is too easy to rotate and also places more weight on top of the mast. Hope it works for you,

bscott

[beene]

If a sail, like a mainsail, is to be compared to an airfoil like that of an airplanes wing, then I would have to say that a rotating mast would aid in close halled also.

The mast is still 30 deg off true wind direction at the very least, if not more like 40 off. So the airflow hitting the mast and bending around either side would make a smoother, less interrupted air flow onto the mainsail with rotated mast.

Without the rotation, I would think the airflow would be less efficient. Look at the leading edge of a wing. To create lift, as much as possible, the edge is rounded up onto the top of the wing as smoothly as possible, air travels across the top and bottom portions of the wing, but taking longer over the top then the bottom thus creating lift. Want more lift, increase the camber or the distance the air must travel over the top of the wing, uninterrupted. Want to kill lift, all you need to do is interrupt the air flow over the top of the wing. Which in my point of view is what a fixed mast would do. Kind of like icing on the leading edge of a wing, will reduce lift and if left unchecked, will cause the airplane to stall.

JMHO

G

[kmclemore]

!	kmclemore:
	Moved from "Modifications" to "Performance & Tuning" as it is more about performance and less about drilling holes.

[Hamin' X]

Just to play the devil's advocate, I would suggest that everyone needs to review their sailing theories. Most of what is published about how sails work and the interaction between sails is wrong. Do some online research and update yourselves.

I'm not saying that what Leon wants to do will not work, only that the reasons are wrong.

~Rich

[mikelinmon]

Actually, no.

The Sails/wings work only because of the leeward side of the foil being lower pressure than the windward side. Notice that this is the way it is always described in even the most elementary airplane books. Why? Because the pressure is unchanged (mostly) on the windward side! They measure that in wind tunnels. The pressure is reduced on the leeward side, that is what drives or causes a boat/bird/plane to work. The rotating mast on our boat )M26) needs max rotation for upwind work by test. Notice the rotation control on the back edge of the mast. We put it there so we could control the mast rotation, does not need to be reduced from max. We just use it for a vang because it is there, no need for extra parts.
I used the rotation control on my race boat, Mac 26, for a while, it needed max rotation even upwind, then took off the control as not needed. Good cheap place to enjoy experimentation though. I had fun with it. You don't even need blocks, just rope flipped around the LL post and run to the windward sheet cleats!
Mike Inmon

[Currie]

I agree with Mike,

FWIW - So does Lars Larsson in "Principles of Yacht Design". Although he doesn't go into much detail about the rotating masts, he talks extensively about the importance of leeward attached flow and reduction of mast drag on the leeward side. Also, his graph of pressure difference across a mainsail shows not a 3-4X difference, but more like 6X (near the mast). IOW, the leeward pressure-drop is 6 times greater in magnitude than the pressure-gain on the windward side. Don't mess with the leeward flow is the message.

So if the mast was in direct line with the sail's luff entry when pointing (the luff being rotated well beyond the boom angle because of the sail draft) - that still leaves a slight notch on the leeward side - an equal one on the windward side too. The mast should still be rotated even further to eliminate the leeward notch. This makes a beefy notch on the windard side but we care less about that.

Rotating all the way sounds good to me Would be fun to test it though.

Cheers,
~Bob

[bscott]

main sail is cut flatter (less draft) on the rotated mast. Pinning the mast will probably reduce the close hauled light wind drive, thus more rotation to increase total draft is required.

bscott