Importance of mast rotation

[delevi]

The subject of mast rotation is of great interest to me. Although I have done some testing, I would like to get some input from Mike, since you’ve done quite a bit of testing. Of course anyone else with observations on the subject, please chime in. Clearly there is a benefit to a rotating mast, but my question is, just how much, and under what conditions. For starters, one may contend that if the benefit was so great, why doesn’t every sail boat in production use it? Truth is, a rotating mast is a very unusual item for monohulls. Apparently, they are more common on multi-hulls for reasons I don’t know. So the first question is why don’t race boats use this concept? Clearly a rotating mast has some complications which come with the turf. However, if these complications can be addressed in a Mac, a high-end race boat should be able to do the same. Secondly, and this one is difficult; is mast rotation an asset or a liability in heavy air. I wouldn’t place any specific wind velocity on this. Let’s just say you’re sailing close hauled in 15 knots of breeze or higher. Conditions warrant fully flattening the main and jib (when possible) or with higher winds, reefing the main and/or furling in some headsail. Let’s also assume the boat has an adjustable backstay. Now the skipper has two options in these conditions on this point of sail with boat so equipped. Flatten the main with all available controls and keep the mast rotated, or center & pin the mast and crank down the backstay. The conflicting theories are such that the rotated mainsail may reduce heel since the leeward front edge is using all its efforts to generate lift vs. just heel due to the front edge of the main being overtrimmed due to the blocked airflow on the windward side. In contrast, the centered mast eliminates the added draft of a rotated sail, and allows the backstay to be actuated, thus flattening the mainsail more, as well as flattening the headsail and tensioning the forestay to allow higher pointing. I would contend the latter is more beneficial for overall performance, and such has been my experience. In fact, the overall difference can be summarized as follows: tight backstay= 5 degrees heel reduction, 5 degrees higher pointing, and a few fractions to as high as 1/5 knot boat speed increase. If you’re still doubtful, consider this. How often does the luff of your headsail backwind when sailing close hauled? When it’s doing this (bubbling in and out) I would think that forward edge is doing nothing. I would theorize the backwinding is due to the slot effect on the leeward side of the mainsail, thus making mast rotation irrelevant in brisk winds, at least close hauled. Centering the mast reduces or eliminates the backwinding, and allows for decreased draft, as well as the opportunity to use the backstay if so equipped, further improving heavy air performance. So in summary, I believe mast rotation is beneficial in light air or sailing a close reach or lower. Higher than this (except for light air) a centered mast with flattening controls is more beneficial. I very much look forward to opinions on this issue.

Leon

[Currie]

Hey Leon,

A couple of thoughts - that are complete guesses. (FWIW - I'm one who thinks that rotating mast makes a lot of sense - but I haven't really put it to the test either)...

IMHO -

The more the mast cross-section is teardrop-shaped, the more it makes sense to let it rotate. IOW - round masts don't see anywhere near the advantage of one that is more aerfoil shaped. But, rotating aerfoil masts are hard to bend as they tend to fight it and snap one way or another. That means they are better suited for lighter rigs with less bend and less stay-tension. Mast tension is not only good for sail control, it's good for strength. The masts on monohull bluewater cruisers and racers are quite bent with a lot more rig-tension than a Mac - that's a super-strong configuration. I'm guessing that engineering rotating masts under those loads is a pretty good feat (the bearing being yet another issue). I also think that the multi-hulls are lighter displacement boats that like to use fathead mains with no backstay - perfect for a rotating mast. - plus they are just a different crowd altogether Anywho - just a guess.

~Bob

[bscott]

Leon, I believe that the RM has less positive performance effect at 15kts + because we try to flatten the sail and move the draft forward as much as possible as boat speed and apparent wind moves forward on a close reach. The KH2000 main is cut flatter to accomodate the RM's rotation thus inducing additional draft for off wind reaching. So pinning the mast on a close reach is probably better with less healing and more speed. Since I have dual backstays fixed to the outermost sections of the stern (original X mounting location) leading to an elongated bale at the top of the mast, I can keep the mast centered by hardening each backstay equally. I can get a fair amount of mast bend for a very shallow draft. Fully centered I have 8:1 bending effort. I can also flatten my 110 jib by hardening up my jib halyard. By hardening up both main and jib halyards and bending the mast I can get an extremely flat entry and point higher. So, mast rotation on a close reach at 15+ is not of any value to me.

In light winds, close hauled, I ease the backstays off with enough strength to stabilize the mast. The mast will rotate slightly and I harden up the appropriate stay to keep the mast where I want it--no mast bed as I want more draft both from the sail and the mast. I am not sure the RM does much, close hauled, light wind.

Off the wind I let the mast rotate as far as I want by controlling the appropriate stay. The problem I have is getting the mast to come back to neutral after a tack even though I ease both stays off to reduce any pinching of the mast base bushings. The RM does give a deeper draft off wind in all wind speeds = higher boat speeds.

I am thinking that unless I have a willing crew to deal with the RM, I am going to keep it centered and sail the Mac the same way I've sailed my other boats for 35 years. If, when attacked by another Mac, and my limited crew is willing, we will use every means to maximize boat speed by tweeking the RM including utilizing my ultimate secrete weapon

Bob

[John McDonough]

PS Whip me slap me I am starting to look at powerboats in the 21' to 26' range in which to be on the water in Lake Mead

Rick, Check your personel message.. I know of a 26` trojan at Lake Meade. Cheap.

[baldbaby2000]

Hi Leon,

I've always thought that the effect of the jib smoothed out the turbulent effects on boats where the mast didn't rotate. I've owned 3 boats with rotating masts: my 26M, my C-Scow and a Hobie 18 catemaran. The c-scow has no jib; only a mainsail. I also owned and raced an e-scow that had a jib but no rotating mast. Quite often the best sail trim on the e-scow resulted in the main being backwinded so in that particular case I suspect a rotating mast would have little advantage. When we raced the Hobie 18 the theory at the time was that the mast bend could be controlled by the amount of rotation; more rotation=more bend.

I do know that at least sometimes the mast rotation on the M helps because I've tried it both ways.

Dan

[Globalhobo]

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[sailboatmike]

Just a thought, bending the mast is just one of the numerous ways of reducing heel, flattening the main / depowering the rig in general.

A knowledgeable sailor should have many tricks up their sleeve to do the task.

I can think of at least 4 other ways to depower even before the first coffee of the day has settled in

[Russ]

BWY puts together a nice package sized correctly. Thrust bearings are not rocket science. Probably get them at your local hardware store.

Or ....of course Amazon
https://www.amazon.com/51102-Single-Dir ... t+bearings

[yukonbob]

Russ those bearings on amazon look way better than the bwy ones and a far better price.

[Ixneigh]

Hmmm. Well my 2 cents

I like the rotating mast on ThIS boat. Light narrow hull with marginal lateral resistance and high windage.
I think it provides more forward thrust if you have the sails set correctly. It also helps a lot if you just use the main, or get down to double reefs. I also sail the boat with little or no board. The rotating mast makes that possible. Even jammed up tight into the wind while crabbing the boat sideways across shallow areas.
If in deep water pointing high, the rotation is less important because the foil shape of the mast is more inline with the wind flow even if it does not rotate.
Most of the discussion on this board seems to be about deep water sailing. Try galloping across florida bay when the water is windblown and murky, with the board up, rudders down still making fair distance to weather at close to hull speed.
Youll see the mast in a new light.
Ix

[Russ]

Russ those bearings on amazon look way better than the bwy ones and a far better price.

I'm not even sure those are the right size. I just did a quick search on Amazon.
Thrust bearings are available at most hardware stores (and of course Amazon) and are not expensive at all. I guess they need to be sized about the same as the bronze washers.

I bought mine at BWY because I didn't know what they were. They've lasted now for years without a problem, others not so much. I think earlier BWY kits were undersized or something.
If they wear out, I'll probably just go down to the hardware store and match them as close as possible. For a no brainer solution, just order them from BWY.

--Russ