Mast rake

[Fred Ficarra]

Dan, I'm surprised you didn't jump on Alans'

hurricane analogy. South Atlantic? But, then too, they might have had their FIRST ONE last week. P.S. Alan, one more question. As far as wind in one location having little effect on another location, haven't you heard of the 'Butterfly Effect'. It is the theorem that states projected or long term weather forecasting is impossible because the wings of a butterfly taking flight will generate air-movement that renders all long term forecasting nothing more than happen-chance. Just a theorem. I don't think it can be proven either way.

 

[Mike]

Earth's rotation also a factor

The earth's rotation has an effect on the wind's direction. The tilt of Earth also has some effect.

 

[O_Salt]

I'll try again

The wind speed just off the water is lower than the wind that is 40' off the water. Both of these winds are blowing in the same general direction. So their should be no build up of pressure. When you get wind of two different speed flowing the same direction the wind is going to bend.

 

[joe]

This is ridiculous...

Dan's comments are based on common sense and logic. If the air constanly flows downward at 4 percent, where does it go when it reaches the water? Does it just stop on the surface. I guess that's why the book's title has the word "bible" in it... because it's full of unsubstantiated statements that the believer would accept as truth. Dogma... in other words.Look for other reasons to explain the need to rake the mast... but spare me that one.

 

[Dan McGuire]

Thanks Joe

It is not a question of how such a thing could happen. The problem is that it can't happen everywhere all of the time or even most of the time. Did you ever see a piece of paper blow up into the air? Doesn't that make you suspicious that the air is not universally blowing downward?I did a rought calculation. It has to be rough because when you are calculating something which is impossible, you have to make some stupid assumptions. I calculated that with a universally 10k wind with a 5 degree down component, all of the air would be below 30 feet above sea/land level in four days and the total pressure would be over 30,000 psi.By the way I fully expect that someone will say my calculations are invalid because the wind speed is not universally 10 knots and the variable wind somehow accounts makes his theory correct. If the wind is universally flowing downward and not flowing out, the pressure will build up. This isn't rocket science. It isn't even bronze age science.

 

[james rohr]

so far every one has been off base!!!!!

mast rake or lack of it has absolutely nothing to do with wind angles etc.mast rake is a means of fine tuning the relationship between the center of lateral resistance of the keel,rudder and hull form to the center of effort of the sail plan. in short balancing the boat.there is a lead distance that the clr needs to be in front of the ce to give the boat a good balance for steering. i under stand most designers shoot for about 5deg of weather helm to give the boat a good feel, but not to much as to have to a lot of rudder angle to keep the boat on track going up wind. to much rudder angle and you are slowing the boat down, not enough and you don't have a good feel for what the boat is doing.idealy you would want a completely neutral helm. however with it it's very difficult to steer as you have little or no feedback from the rudder. every boat is differant. on my particular boat (an O'day30,1979 vintage) the designer called for 9" of rake. as set up this way the boat was slow as hell and a nightmare to sail in anything over about 6to8 knots of apparent wind. after a lot of experamenting by removing the mast rake 2" at a time and then sailing it in 10 to 15knots apparent i finally found that 2" of rake gave a very light helm to neutral helm. i have a tiller and now i can steer in that wind range with only a couple of fingers on the tiller. as originally set up it was both hands doing chin-ups. designer later corrected this flaw 1980 and later by moving the mast forward 3". what you need to do is find out what the designer called for and use that as a starting point to fine tune the balance. the boat must be sailed to do this and you may have to retune the rig several times to get it right. some of the one design boats have these numbers readily available because they have have been tinkering to find the optimum settings.the numbers usually given by the designers are usually just numbers that were calculated and not nesc numbers they derived after the boat is put in the water and sailed to get optimum settings.if you have excessive weather helm remove rake to move ce forward. lee helm, use more rake to move ce aft. hopefully designer wasn't so far off in his calc. that you can get a nicely balanced helm.reference; principals of yacht design, rolf larson:capn jim; Que Pasa?

 

[Andy]

I have to support Dan here

Sorry Ken, this has gone off your subject but it is surprising the stuff so-called "experts" state as "fact" sometimes. Dan is absolutely right. The wind can not be uniformly flowing "down" to any degree. It can only do that locally (thunderstorm downdraft for example) and then it has to go back up somewhere else. Call it fluid dynamics, logic or common sense. Perhaps we are taking this expert out of context though. Someone mentioned the wind gradient which is a well-known phenomenon. It occurs because of surface friction. Perhaps the aggregate effect of the gradient over the length of a sail "affects" the sail "as if" there were a 4 degree downward vector. If we are not taking this author out of context, then I submit he is a nut!

 

[Dan McGuire]

Agree with James Rohr

This discussion has absolutely nothing to do with mast rake. I sent an e-mail to Jim Murrant, author of Boating Bible. I asked him to either respond to the discussion in this forum or directly to me. We will see what happens.

 

[joe]

everyone is not off base, james....

... and most everyone understands what mast rake is and how it affects a sailboat's handling characteristics.. This subject has been brought up so many times these past few years but it's the first time I have seen the "downwind angle theory". I almost fell out of my chair laughing.... I keep picturing some guy raking his mast 5 degrees because some other guy said the wind flows downward at that angle, then he takes the boat out and flies a baggy mainsail while a couple of 250 pound relatives sit in those funny stern rail seats. He's probably got the traveller cleated down, or it's located on the cabin top so he can't reach it from the helm...and a big ass gust pushes the boat over to about 35 degrees.... oops there go the Margaritas...and how come she wants to round up so hard... better adust that mast rake again. Sorry you guys, I'm entertaining myself here.. no offense to anyone, please.

 

[Wanderer138]

If I may complicate matters even further...

I used to believe the whole mast rake/CLR vs CE weather helm theory until this one time I was out sailing in heavy air under partially furled headsail only. The boat kept rounding up violently in the gusts and I couldn't figure out why. I believed that with headsail alone, the CE would be forward of the CLR and the boat wouldn't have weather helm...but I had it big time.It finally dawned on me that excessive heel was the culprit. In a gust, the boat heeled and the forward component of the force on the sail (the driving force) also increased. The increased driving force tried to accelerate the boat but it was so far outboard of the centerline of the hull (because of the heel) and the hull couldn't respond as quickly as the rig (because of drag in the water) that all the force did was push the rig forward around the keel and cause the boat to round up. The fin keel didn't help, nor did the asymmetric shape of the hull under water.It's not that the CE-aft-of-CLR theory was wrong, it's just that I was blindly relying on it under the wrong conditions, and with the wrong hull type too.The 4-degree rake theory may be right, but I wonder if it's universally applicable, given the large number of other factors (and the high degree of variability of each one) that can affect boat trim/weather helm. I'm really curious as to how this will all turn out.PeterH23 "Raven"

 

[Dan McGuire]

No Response from Jim Murrant

This is old news. I sent an e-mail to Jim Murrant several days ago reference his statement that the wind blows downward. He did not respond.  I would like to know how he came up with this piece of information. He is apparently an outstanding sailor, who made a mistake in a book. I am disappointed that the did not respond.

 

[Bruce]

and another thing...

If you ever alter your sailplan, you probably need several alternate pre-dialed mast rake settings. I added a turnbuckle to my forestay, in addition to a lever, for just this reason. I get a somewhat different helm balance with a 150, 135, and 100 up front, and reefing the main introduces a whole new set of variables.

 

[Dan McGuire]

Question??

Bruce. What happens to the shrouds? Everytime I change the length of a stay, I have to change the length of all of the rest of the stays and shrouds. The sole exception is the back stay. Sometimes I can change its length to change the bend without changing the other lengths.

 

[Mike C22]

Friction

Perhaps what is really happening is that the friction of the wind at the surface is slowing it down. As the wind speed is measured higher up, it is faster. I read somewhere recently that the NWSB measures wind speed at a height of 30 ft. above the surface, for this very reason. So.. It may be that the wind is not constantly blowing at a 4-5 degree downward angle, but just moving slower at the surface and faster higher from the surface.

 

[patrick]

rake

mast rake improves windward performance but reduces off-wind performance as it makes it difficult for the sail to hold its shape when sheeted way out.

 

[Bruce]

Dan

I think you are right in saying that any change in forestay length should probably be accomapanied by a shroud adjustment (am I hearing you right?), although it seems to me that this is greatly affected by how backswept your shrouds (and spreaders, if present) are. Many lower shrouds are directly abeam of the mast, and I don't think rake alterations would change the tension a lot, unless the chainplate is way lower than the tabernacle. The uppers are another story, and if you have backswept spreaders and shrouds, you can really slacken (or tension) the shrouds a lot with minor rake adjustments. So maybe for most folks it isn't worth messing with. I play with adjustments, but I enjoy it more than most would.

 

[joe]

Shroud adjusters

...do yourself a favor and invest in a set of shroud adjusters if you change settings that often. Put one on the forestay too. Dinghy and catamaran sailors use them all the time. They have to because they can't change sails. If you can change sails to balance the boat for the conditions, you shouldn't have to change your rig settings too.

 

[Wanderer138]

Here's some rake and bend

I took this view from the mast base a few years back when I was fine-tuning the rig. The main halyard is shackled to the tack pin to measure the prebend (about four inches at this point). The rake, which would be measured with the halyard hanging straight down, is about 12 inches. Note the sweep of the spreaders. The mast is stepped on the cabin top and the chainplates are on the side decks well aft of the mast base.The lines in the picture from left to right are:1) backstay2) topping lift3) main halyard4) jib halyard5) forestayfrom top to bottom of the frame they are:1) lower shroud (attached to spreader base)2) upper shroud (same chainplate as lower)The most remarakble thing about this picture is that the backstay is absolutely slack. With the backstay on, I can easily double the mast bend.I'll try and post a photo of an H23 next to a "regular" rig so you can see the difference. Happy Easter.PeterH23 "Raven"

 

[Gerry Cooper]

Dan, a pity Jim didn't respond. His book is

recommended reading by at least one cruising magazine. The concern now is if the down blowing wind thing is wrong, what else is open to debate.After reading his resume I was impressed - who do you trust?Whatever the outcome this has been a stimulating topic.

 

[Ken Prichard]

I never did find the "formula" but I sure learned a lot about mast rake. It turned out to be a very interesting disscussion. Thanks.