I'm having a hard time determining how my 5.2 is responding to adjustments in mast rake. My previous boat was a P19mx that ran a fair amount of rake. I down sized as it got to be a hand full single handed .. especially with a spinnaker. I'm currently set up with a square top main and spin/ snuffer. Not much "pre-bend" .. and the way I've measured existing rake is like this ... with the deck of the hulls level, and with a 15" crescent wrench hanging from the halyard, it dangles about 6" aft of the mast.

My limited experience with nacra's leaves me wondering if this is to concervative or not. I've raked it back further, and raked it forward more ... and am having a hard time determining the difference in performance.



any suggestions ???



mark c

Dear Mark,

It's like that story about those four guys, Anybody, Somebody, Everybody, and Nobody. I saw your post and thought, "Anybody knows more about mast rake with a spinnaker than me, Everybody can see that post, so Somebody will answer it." Now I feel guilty because Nobody answered you.

That's not typical of this forum.

Last time I opined on mast rake, Mark Michaelson was kind enough to draw my attention to several facts his experience tells him are very important. Turns out, I'd forgotten reading those same things in Rick White and Mary Wells' book, "Catamaran Racing: For the Nineties" That's a book I recommend very much - and not just for racers, by a long shot. It makes sense, is approachable, gives equal time to other points of view, cites empirical research, teaches fundamentals of sailing, and will make anybody a better sailor.

As I mentioned above, my experience and the opinions I've gotten from much better sailors is limited to non-spinnaker boats, so I'd recommend you post a "Hey Spi sailors, how does a spi change your Rake?"

Asymmetrical hulled boats like the H- and P-16 need lots of aft rake, for the reasons below and others. Symmetrical hulled boats like your Nacra 5.2, in fact all boats with boards, can use the following game plan for rake, with some success:

Here's the short story, the supporting theory follows:

Rick's guideline for mast rake on your boat calls for you to use it to induce a desirable amount of weather helm. A "desirable amount" is that your tiller crossbar should be a quarter inch to windward of the center of your crossbeam, when traveling in a straight line going to weather.

To adjust this, you simply rake your mast aft to make the tiller crossbar come to windward; moving your mast forward will cause you to sail with your tiller crossbar more to leeward.

Sounds simple, and it is, provided you understand why it works. All this was aimed to get the rudders slightly "in the game" in providing some lift to windward, but not so much that they were creating damaging amounts of drag.

Now for the theory:

According to Rick and Mark, and borne out by my own experience, the most significant impact of rake is on the true tendency of the boat to turn either into or away from the wind. (Just for a moment, don't think about the feeling of "pull" in the hiking stick - we'll handle that later, and it's only partly influenced by mast rake. More below but for now, take my word for it.)

The boat's tendency to turn upwind (or down-) is determined by the positions of two things: One, the center of effort (CE) of the sails - the net point at which all the lift those bad boys make is centered. Two, the center of lateral resistance (CLR). Think of the CLR as the center point of all of the resistance to leeway made by all the wet parts of your boat, boards, hulls, and yes, even rudders resist leeway, under the right conditions - more below on that.

A sailboat only goes straight when the opposing forces pushing it sideways exactly align against each other.

Push downwind on the stern and upwind on the bows, she's gonna turn upwind, until you lift the sterns with the rudders and then she'll go straight. In other words:

When your CLR is forward of your CE, it's like you're pushing the sterns to leeward, and resisting that push by holding the bows on course. The boat tries to turn to windward, and you turn the rudders so the trailing edges are pointing to leeward, "lifting" the sterns, moving the CLR aft, and so balancing the boat and it goes straight. This is called "Weather Helm"

The reverse is true when the CE is forward of the CLR, this is called "Lee helm"

Keep on not thinking about the pull in the stick! We'll get to that later, it's a different thing!

You can move the CE around while under way with the various sail controls and you can even steer your boat that way, but when you set your sails to go upwind fast, you don't get a lot of choice about where the CE is, except by where you rake your mast. This is done, on the 5.2, like most boats, 'on the beach'

You can move the CLR around on the water by shifting your weight forward or back, raising or lowering your boards (if you have 'em) but again, when you're trying to go to windward as fast as possible, your hulls need to be just right, and there's an optimum setting for your boards, too.

The biggest thing you can do to affect CE, and hence the boat's Helm, is to rake your mast.

So here's the point:

You want to get maximum lift to windward while setting all your running controls for best boat speed.

That means you want your rudders to generate some lift to windward, too. The way to do that is to induce some Weather Helm with the rig, so you can balance it with the rudders, and they will be generating some lift. "Some" means just enough that you aren't creating so much drag with the turned rudders that you lose the benefit.

Okay, we're still ignoring the feeling of pull in the hiking stick, for just a little while longer...

"Cat Racing..." says to set up for your best course to windward, the one that gets you the most progress to windward, not the highest you can point. While holding that course, you want to see how straight your rudders are (not how hard the stick is pulling). You can check this easily by looking at your tiller crossbar and seeing how far to windward you are pulling it. Compare the bracket where the stick attaches to the center of your tiller crossbar to the center of your aft crossbeam. Use your traveler jam cleat for a guide. Rick's guideline for "some" weather helm is that your tiller crossbar should be a quarter inch to windward of the center of your crossbeam, while traveling in a straight line, going to weather.

To adjust this, you simply rake your mast aft to make the tiller crossbar come to windward; moving your mast forward will cause you to sail with your tiller crossbar more to leeward. (All this assumes you've got no significant 'play' in your rudder system - the blades are shimmed into the castings, and the rudder pins aren't worn.)

Sounds simple, and it is, provided you understand why it works. All this was aimed to get the rudders slightly "in the game" in providing some lift to windward, but not so much that they were creating damaging amounts of drag.

Finally, let's talk about that pull in the stick.

Turning a rudder away from wherever it 'wants' to point is hard work. It's only manageable at all because a properly tuned rudder is angled so a small fraction of the rudder is actually forward of the hinge or "pintle." That forward part has tremendous leverage, and balances the forces acting on the trailing part, allowing mere human muscles to generate enough deflection in the rudders to shove around the back half of an 18 foot boat under tremendous loads from it's sails, hulls, etc. That's why when your rudders aren't all the way down, they're hard to pull to steer - it's not just that they're further aft; it's that they no longer have enough of them forward of the pintle.

That's why mast rake seems to affect the way your hiking stick pulls: the more you rake the mast aft, the more you need to steer so as to deflect the rudders underway to generate lift to windward, and the more you will "feel" any imbalance in the rudders! That's why most folks confuse rudder rake with helm - if your rudders aren't tuned, you feel it more with aft mast rake. That's why you need to ignore the 'pull' in the stick and concentrate on rudder deflection when measuring your mast rake.

If your rudders pull too hard, it's not necessarily caused by a 'wrong' mast rake. Sure raking the mast forward will seem to reduce the effort you use to hold the stick, but only because it reduces the forces acting on the rudders, and those forces can be quite beneficial, used the right way!!

So, first check rudder deflection as above, and rake accordingly. If the stick is still hard to hold, then rake the rudders forward until the stick is easy to hold. Don't go too far, or you'll experience a tendency for the boat to turn off wind when you let go of the stick, which can be disastrous.

Again, all this is up for modification by the spinnaker skippers out there, applies primarily to symmetrical - hulled boats, and there are other points of view, some held by very successful skippers. I'm slow to argue with success on the course - racing is a pretty good gauge of where the rubber hits the road in the "theory vs. opinion" dept.

I do sometimes think that even really good sailors can have the occasional misperception about tuning a rig and overcome it with sheer brilliance out on the course, all the while citing their success as proof of logic of the tuning of their rig. So I do like to understand why a tuning tip works, too.

Hope this helps!

Ed, excellent reply. The insight on sailors doing brilliant things on the race course and crediting it to the wrong reason is great. Finally, due to hard work like yours and Wouter in fighting deeply ingrained misconceptions with facts and logic (which has been very painful for many), many are sailing better and enjoying it more. Although I believe your reply addresses Mark's situation, with an understanding that will help him adjust the info for his needs, his question makes me think about 2 factors in spin (or broad reaching wo/spin) sailing. First: doesn't the small lift off both rudders and boards work against us going downwing? If I understand lift correctly (my mind is boggled that a symmetrical foil can create lift, when I came to understand this at a landsailing event, I felt as though I had a religious experience) the lift from the underwater foils is pulling us windward and we want to go leeward. Yet my experience and that of the Tornado sailors indicates you are at least no worse off to leave everything down and probably better off. (Seems like individual boat design should make a large difference on whether to pull up boards. My P-19 doesn't like to slip sideways even with all boards up, but a N5.8 slips very easily with boards up, yet a P-19 is a little faster downwind than a N5.8. I'm puzzled because both boats have very similar length/sail area but have very different hull shapes. I had imagined that the N5.8 could slip sideways at least half as fast as its forward speed allowing it point a little higher giving it killer apparent wind and very deep sailing angles)

Second: The mast raking to get the rudders working harder should cause the rig to have less power downwind, which should be more important than getting the rudders working and upwind performance. All of this makes me think I may not have the proper understanding of the forces at work, UNLESS the answer(s) lies in raking the mast less than x degrees makes minute changes to the sail area/ lift factors and makes good improvement in balance and drag and possibly lift in another plane.

Ed,



nice dissertation on mast rake and rudder rake.



You wrote:

To adjust this, you simply rake your mast aft to make the tiller crossbar come to windward; moving your mast forward will cause you to sail with your tiller crossbar more to leeward.



Raking your mast aft, like steering a windsurfer, will cause the tiller crossbar to shift more to leeward, thus 'heading up' the boat to windward. And the opposite by moving the mast fore, as in falling off on a windsurfer, would tend to make your crossbar go to windward, and the boat would fall off to leeward.



As I said, nice dissertation, just wanted to clear up that one, small point about crossbar and boat direction.



I enjoyed your post.



P.S. re-read for the fourth time and now I see what you meant, that to balance the CE, you would MOVE the crossbar in the direction you stated to balance the forces acting on the boat. Oh, never mind.

I debated getting into this aspect of the problem when writing the original post. Two things stopped me: 1. My post was getting kinda long (didja notice?) and 2. my memory of Rick's remarks on this point are slightly hazy.



I believe the trials he cited showed that the gains upwind outweighed the losses (if any) downwind.



Speaking of possible losses downwind - I know you're talking about the 'lift to windward' that deflected rudders make, but here's a related point. On a downwind course, boards up/down is a debated question, or was... indicating it's a near thing if the extra leeway (which is distance "made good" on a DW course) is all that much more usefull than the extra boat speed you get when you resist leeway hence increasing the apparant wind. I guess it's like pointing vs footing - the different bonuses trade off, inexactly, yielding a sweet spot somewhere on the curve. Has the case been proven either way? I'd really like to know.



Finally, we're not talking about raking a typical boarded boat "all the way" back like a H16. A comparitively very moderate rake is all that is required in many cases to set up the right amount of ruddertrack.

Hi Todd,

Yes, your "PS" is a correct description of my opinion.

If you had to read it four times to make sense of it, then I did a bad job writing it



Perhaps that paragraph should read:



To adjust this, you simply rake your mast aft to make the boat sail straight with the tiller crossbar further to windward; moving your mast forward will cause you to sail with your tiller crossbar more to leeward.



Thanks for your help, I'd hate to confuse anybody after all that typing intended to pass along usefull stuff.

A belated addition or two - -



The CE/CLR balance to windward is vastly different than the CE/CLR balance downwind. The sails are eased considerably, the "E" vector is way forward of the beam, and the CE itself must move forward too; and to leeward, no? As a matter of fact, I wonder if rig tension, and consequent variation in how far the mast falls off to leeward, does as much for helm downwind as Mast rake does upwind.



I'd sure love an expert opinion on that one.



Speculating here, and inviting debate:



You can make distance "good" two ways on a DW course; by making headway, and by making leeway.



Within certain limits, headway seems a more effective use of the wind's energy than allowing it to push your boat sideways, turbulating and swirling etc. But clearly, impeding your forward progress to eliminate all traces of leeway isn't likely to be too good either. Also, in heavy air, do we care about squandering some of the wind's energy on leeway, since we've got so much to spare? Conversely, in light air, don't we need to resist leeway as much as is reasonable (ie, w/out creating more drag than tolerable), to create some apparant wind in order to go deep at all?



(Under *really* light force, my cat will move sideways almost as easily as forwards. Next time you're waiting in a dead calm for the wind, try pushing gently with fingertip pressure only on the side of your cat, then on the rear crossbeam. I infer that the slower the wind, worse the leeway/headway ratio gets.)



So I infer a continuum, where wind strength, wave conditions, individual boat config, crew weight etc all play a role in deciding how much board (if any) to put down, how desirable lift to windward is. Whether or not you're wild-thinging it probably is more important than most of these factors. A click or two of mast rake, just enough to cause you to deflect your rudders a hair on the upwind leg (a quarter inch on the crossbar is much less on the blade), probably isn't an enormous impact on downwind, especially given the first paragraph's speculations.



Or is it? I'd enjoy hearing from the experts out there.

Great discussion....I have always been puzzled by one aspect of the rudder rake issue. I have my rudders adjusted such that they are almost balanced, ie very little pressure on my hand. I would expect therefore that about 50% of the rudder area would be in front of the axis of rotation and 50% behind it. I suspect the actual condition is about 20% in front and 80% behind the axis of rotation. Can anyone explain why...Thanks Dan

Dan,



The answer to your question is, of course, very complicated. In an attempt to assist in your understanding, I would suggest that you consider the following conditions:



The rudder's center of lift or center of force is not in the middle of the rudder, but farther forward.



The center of mass of the rudder is far ahead of the center of the length of the chord. The center of the length of the chord may be what you call the center of the rudder, but it is not the working center of it.



The angle of attack of the leading portion of the rudder is much sharper than the portion of the rudder that lies behind the thickest portion of the rudder. The trailing portion (of the attacking side) of the foil is somewhat parallel to the angle of attack.



Do not forget to consider the angle of attack is not parallel to the rudder's center of thickness. In fact, the angle of attack is somewhat inline with the trailing end of high pressure side of the rudder.



I hope that this can help. I am not a teacher, nor a hydrodayamic/aerodynamic scientist. I only hope that I can lend some insight and help you to question any ideas that you, I, or we can come up with.



GARY

Thanks Gary....Your explanation does help to clarify what is actually going on down under...Dan

I never really thought about *why* a forward pointing control surface is so much more powerful, inch for inch, than a trailing surface.



I'm not sure the foil shape has the dominant impact on the leverage equation here, and here's my (slightly goofy) reason:



Mid century airplanes, the ones controlled by wires affixed to the steering column, with no hydraulic power assistance, used forward surfaces to balance their elevators - and if memory serves, those surfaces weren't foils! The larger horizontal stabilizers often were foils, but the hinged-on elevators weren't, I believe. And yet, visualizing it now, I do think the ratio of forward surface to aft was about 1 to 4-or-5; certainly nowhere's near 50-50!



So I'm liking Gary's point about where the highest pressure is located on the side of a deflecting surface, foil or no. If the highest pressure piles up pretty near the leading edge of the attack side, then you have to keep that awfull close to the fulcrum, or pintle-axis.



I can visualize it being about getting the same (leverage adjusted) amount of pressure on both sides of the pintle-axis.



I hope somebody who knows the actual answer jumps on this....

The center of lift for a wing (i.e. the balance of lift) is usually very near the chord (the thickest portion of the foil).

This is in response to Steve Blevins reply: He said: "sailors doing brilliant things on the race course and crediting them to the wrong reasons"



I heard this on the Louis Vitton Cup racing last night, and thought how true it is.

"The good sailors seem to have all the good Luck"

They were refering to a wind shift that benefited the boat out in front.

My theory is that is probably somewhat true, but dont you think these "good sailors" put themselves in the position to have all the good luck?



David Mosley

www.seacats.org

An other word says:



"A good speed makes you smarter".

Mark,



If you are using a stock jib and have drastically increased sail area of your main (with your new square toped sail), then you can expect your boats weather helm to have increased from its stock form. You have added sail area to the main, and by not adding the same percentile gain to the jib you have moved your CE aft on the rig.



I would find out what the mast rake recommendation is for a stock 5.2 and start moving it forward from there.



As far as noticing performance differences, a gps with a VMG function could come in handy. This is especially true if you get a steady wind (direction) condition.



Fire up the gps and turn the track log on. Point as high as you can without leaving the sweet spot on both tacks. Note the headings. Average the headings and project a way point 1000 miles up the center of the two tacks.



Now use the go to function to sail toward that way point and read your headings and VMG speeds. Then change the rake of the rig and do it all over again, including the establishment of the place to project the way point (to account for wind shifts between each test). Notice your speeds and and headings. Are they faster, closer to the wind? Watch your VMG as you tune the boat.



Do a few more tests, each time moving the rig farther forward or aftward. Make particular note of the point at which the boat begins to exhibit lee helm. You won't ever want to set the rig farther forward unless off course you were going to do a one way trip, downwind.



Now go home and download the track to your computer (BEFORE you save it in the gps, which will take away the speed readings). Analyze the data and pay particular attention to your tacking angles. This is where you will easily see how well you are pointing.



I sailed with a buddy once (Hi Steve!) who was having lee helm on one tack and neutral on the other. We were curious as to what the problem was. It turned out that a shroud was fouled with a trap line, causing said shroud to be short. The rig was farther aft on when the short wire was to weather than on the other tack. So on the tack were the rig was farther aft, we had less lee helm.



When I analyzed our track, I could see that we were pointing higher on one tack than on the other and our overall taking angle was not very good. After fixing the fouled wire and raking the mast back a bit, the boat sailed higher with more balance between tacks.



As an example of track analysis, I am attaching a picture of a track I made in August. (to view it, go to upper left of the screen and click on the attachment link under my name)



The yellow portion is an upwind beat from leaving the harbor, hardening up at the FLR6S marker light where I go from south bound to more south west. The wind is out of the west and I am on Starboard tack from the marina to a point where I make a right turn and tack to Port. That leg was 1.5 nMiles and I averaged about 8.5 knots.



Then I take off, about 110º away from the course I was on and eventually am able to bring her up to 100º from the Starboard tack. I hold this course for about 20 mins. and cover 3.1 nMiles.



Then I tack about 85º but must have been on a lift because I end up falling to about 100º. Then you see me drop off about 70º as I begin to reach back home.



In this picture you can see that the waves have an effect on my courses. The waves out of the Southwest are on my bows when I am on Starboard tack and they slow me. When tack to Port, my bows are pushed away from the wind by the waves, but my speed increses.



On the Starboard tack leg I went about 1.5 miles in 10 mins. On the Port tack I went about 3.1 mi. in 20 minutes.



GARY

DANG! that's why I lost my speed and time data from RTI in my GPS...Thanks Gary! I probably would have saved the data a hundred times before I figured that out.

No prob. Jake,



Take a look at this informattion in regards to the eTrex Legend:



When I saved a track once, I lost the data that was most important to me, my speed readings. Garmin told me that to conserve memory, the gps automatically tosses out certain information when saving a track and that I could not get it back nor could I choose what information would be saved or tossed.



So now I always download the data without saving the track. When you download an active track, you get all the data available for that track. I download it to the mapsourse program, then save that file as a *.mps and then I have all of the data I want for later study.



That led me to the next problem, I was filling up the track memory before I finished my whole trip. Filling up the track log before I could get to my computer to take out the information that I wanted to save on the computer. I noticed that my log, when the interval setting was on "auto" was logging about every half second. I chose "time" and "every 5 seconds" and the log lasted a LOT longer. Once I set it for "distance" intervals and chose "every .5 miles." This resulted in a track log that could have lasted even longer.



The big bonus was that I instituted this, along with "battery saver," and "no WAAS" and battery life went from about 6 hours to about 20 or more hours!



GARY

Steve,



Frank Bethwaite says that the least resistant (to forward boat movement) angle of the rudder is when it is at around .75 to 1.0 degrees above or to windward of center.



On a boat with gybingn centerboards, then it should be .75 to 1.0 degrees higher or to windward of the angle of the centerboard.



He adds that this happens to be almost exactly the same degree of deflection that the water stream coming off of the side slipping center board is presenting upon the rudders.



GARY

Hi, Gary,

If I'm doing the trig right, (#$% excel works in radians, needed to convert) using a guestimate of 2 foot long rods connecting your tiller crossbar to your rudders, I'm getting like 0.4" above dead center on the crossbar to produce 1 degree rudder deflection. Sounds awfull close to Rick's advice to pull the crossbar a quarter inch to windward... the difference, 0.15", is well within the allowable error of the system, with loose pintles, wave action, slop in the casting/rudder connection, imperfectly alligned rudders etc. ANd if the rods are shorter than 2 feet, Frank and Rick are even closer. Cool.

Hi, Gary,

I never in a million years would have thought of such a carefully controlled methodical experiment to measure performance to windward. You're doing great empirical work to help us understand our boats; thank you, for instance, for publishing a benchmark for tacking. 100 deg on a boarded boat. Now I don't have to scratch my head wondering if 105-110 on a boardless is good or if I should be looking for serious impovement. And the observation about wave vector explaining differrential tack performance - I guess out on the water I've made allowance for a heading sea vs a beaming one, sure, but to see it in B&W like that really makes me think.



I have some reservations about the CE moving much as a direct consequence of changing to a square top. I agree, the main's total contribution to the combined CE goes up some (5%? 10%?? 2%?), but since that combined CE is so heavily influenced by the main already, further incremental increases in the *power* expressed at the main's CE doesn't seem (to my intuition) likely to move the combined CE terribly much. (I'm too lazy to do the math, so I'll cheerfully accept correction (and 40 lashes) if you care to run a sample ratio and comparison - say 4:1 vs 4.2:1, how much does the combined CE move on a 5% increase in main's effort, if 5% is proper allowance for the squaretop, etc etc. yikes, all the guestimating...)



Small changes in the *Location* of the main's CE should provide relatively larger changes in the combined CE. However, the CE of the main doesn't seam likely to move especially aft on the change; particularly in the case of the "same sail area" squaretop, where I believe the foot is typically shortened to match the extra material up top, right behind the mast. Even the squaretops with more sail area have that extra area added right behind the mast, pretty much on top of the pinhead's CE. Heck, maybe a sailmaker will opine on whether the main's CE moves at all? I'm guessing not very much, or even "not even a little" in light of your post about chord thickness and effort center.



Finally, Marc added a spi too. Shouldn't that balance any impact of the Square top? Of course, only when it's actually up the pole! So my main question for the real experts out there is, "Do you still use the same process of raking to produce optimum lift off the rudders with a spinaker on the boat, or is there a different strategy? For instance, is the offwind performance of the rudders more significant, with a spin, and in which direction does that modify the larger process? Do you want more headway or more leeway, as a specificic strategic goal consequent on using the spi?"