Raked Masts

[Pernicious Atavist]

Okay, somewhere along the line I missed the reason masts are raked.

-Why are they raked?

-Are they always raked aft?

-How do they affect the handling?

-Should I have asked this question under (yet) another psuedonym?

Ed (no! not Ed, uhhh...Chuck, that's it--Chuck)
:rolleyes:

[Venchka]

A classic example of it depends...

http://www.parker-marine.com/east16.jpg

Don't look to me for answers!

Wayne
In the Swamp. :D

[Karl A. Hilbert]

Is it to keep the center of effort low without sacrificing sail area?

[Keith Wilson]

When the mast is raked aft, the end of the boom lifts as it moves outboard. This has the significant advantage that the boom is less likely to catch a wave when rolling downwind. Also, the boom tends to come back to the centerline by its own weight. although you'll only notice this when there's absolutely no wind, or when the sail is furled and the boom is supported by the topping lift - it's still handy, though. Masts raked aft also look better.

[George.]

Dalia has masts well raked aft. Based on our experience, this is because:

1) With raked masts and aft shrouds placed well aft, you don't need no stinkin' running backstays.

2) As Keith said, the boom rises as it goes out, and you can roll to your heart's content.

3) When you jibe in light airs you can do it all standing, and the rake slows down the boom as it swings past the centerline.

4) It looks good! :cool:

[MAGIC's Craig]

I confess to have liked the look of the raked masted pilot schooners when designing our schooner. Another advantage is that with spreaders raked also, we have found that the need for running backstays or preventers is reduced to only those conditions when conditions were sloppy going to weather (or we thought that we might go a billionth of a knot faster with a slightly tighter headstay ;)

http://www.imagestation.com/picture/sraid109/p13852107e80d8f52d2f52c44d3c0723e/f93adfa2.jpg

[Nicholas Carey]

Originally posted by MAGIC's Craig:
I confess to have liked the look of the raked masted pilot schooners when designing our schooner.[/IMG]Exactly :D

Der Hamburger Lotsenschoner N° 5 ELBE ex WANDER BIRD (http://www.lotsenschoner-no5-elbe.org/Bilder/index.html):

http://homepage.mac.com/stevenmeans/.Pictures/wb%20sail%20june%202000%20/wbMVC-874X.jpg

[Ian McColgin]

The solid spars of traditional gaff rigs were - are still - raked exactly to avoid running back stays. Between the weight of the spar and the shrouds landing abaft the partners, the mast was not likely to carry away when off the wind.

Secondary advantages to a raked rig include:

The boom natuarally lifts the further off the wind you sail, a pain on light air days but in a breeze it's good to get that boom up away from the water; and

When you use a hallyard to hoist things, the go up nicely clear of the mast.

[Paul Pless]

So with all the advantages, what are the disadvantages ?

[Wooden Boat Fittings]

.
No-one's yet mentioned the fact that adjusting the rake moves the CE of the sail(plan) and thus affects the helm. It's worth trying this if you want to, say, reduce too much weather helm.

But there's no doubt that raked masts somehow just seem to look "better" as well, anyhow. Bombigher's Miss Simplette --

http://www.woodenboatfittings.com.au/public/miss-simplette.jpg

(But god help them if they lose the bobstay...)

Mike

[John B]

The disadvantage is the tendency for the boom to keep swinging inboard on light air days.

[johnw]

Sometimes masts are raked forward, usually for reasons of helm balance. However, the fishing ketches of England' West country had their masts raked forward, and I don't know why.

[Aramas]

The masts of junks are commonly raked forward, presumably to keep the sail boomed out when running in light air without having to resort to preventers. The English ketches that John mentioned may have been so for the same reason. A forward rake also allows the sail to be backwinded and the deck cleared when the sheet is let go, which might be a useful trait with some fishing methods.

Any rake at all is less efficient than a vertical luff.

[Gary Bergman]

I rather like the rake in this shot we took of the Lynx this year........

http://www.privateerinc.org/images/Lynx1.gif

[John B]

wow.

[JimD]

Having the wind back on a square rigger with pronounced rake to the masts was a good way to lose a mast, I have read. The forward angled stays are at quite a disadvantage.

[Gary Bergman]

Well, here's a disclosure, folks....87 m.p.h. winds in-slip this week; tore the gaskets off my tops'l, wind astern due south...tops'l set,full and by..shredded the tops'l, but..........my raked masts are just fine..now to dig out the Pfaff and start repairin',eh?

[JimD]

Gary, yikes! Glad to hear your masts are ok. Really sorry about the rest.

[Gary Bergman]

Oh well, eh wot??..Usually this is the time of year that I sway down the 'yards, and run fore and aft 'till spring and the return of my neighboring square-riggers, who by commercial operation, head to SoCal.I had just removed my main course before the typhoon remnants came thru; just hadn't gotten to the tops'l yet..I said I was gonna head south with the others this year, but haven't left yet, so..mebbee next year!...For reference and rig definitions, I do have runnin' backstays on both masts, and the yardarm braces act well as extra two part 'stays' when not flyin' the squares..

[John Kohnen]

Originally posted by John B:
wow.http://www.boat-links.com/PT/PT2003/Lynx-02.jpg

[John Kohnen]

Oops! Wrong one! Try this:

http://www.boat-links.com/PT/PT2003/LynxRigging-02.jpg

[Meerkat]

Hmmm... with a raked mast you could fly a sail off each tine! ;)

[Gary Bergman]

Great shots!...Altho' I try to keep the rubber dingies outta my web postings!

[John B]

well, double wow thanks John.
you can see why they were such spectacular performers when you see how far they can brace those yards around.
I remember talking to one of our own square rig/ general wooden boat fanatics/ characters as he tut tutted over a later square rigged training vessel. By design the yards couldn't brace around far enough to get any windward performance to speak of.

[Gavin Atkin]

If you like raked masts, you might feast your eyes on these fabulous lug-rigged craft:

http://www.lacancalaise.org/

http://www.lagranvillaise.org/galphoto/index.htm

They're even more impressive in real life.

Gavin

[Pernicious Atavist]

absolutely a peach...but, i still haven't learned why a boat is DESIGNED to have raked masts. maybe there is no reason other than esthetics?
designers?

[Dave Hadfield]

That's a great picture of Lynx under the bridge.

Studdingsails! That's what I need for Drake! I've got all winter, right? I should be able to make a hollow yard with extensions that extrude out to take the extra panels....

Too bad about your topsail. I've never sat out winds of that strength at anchor, but several times I've struck my yardarm to reduce windage during 30-40kts.

Glad I wasn't operating a flight into SFO that day. Those kind of winds over the hills south of the runways 19 make for horrendous turbulence....

[Gavin Atkin]

Perni:

I think I disagree. I think you've been given lots of reasons including easier gybing and sail handling, eliminating backstays, tradition, appearance and safety when rolling. To that list , I think I'd add that raking a mast might make more space in the middle of a boat because it allows the mast foot to be placed a little further forward.

What you haven't been given (and I can't offer either) is anything very convincing to justify the rigs we've seen where the masts are spraddled fore and aft. It's quite well known that longer rigs tend to give directional stability just as long keels do, but I'm not convinced... Perhaps those old fellas on the Chesapeake and the old timers of Brittany just didn't believe in the slot effect and liked to keep their sails as far apart as possible.


Gavin

[Gary Bergman]

Dave H....Yep,it was a fine day 'under the 'bridge'!...I liked this one especially with the corner of our 'Kings Colors' and the fireboat; comp is everything, eh??.. I do want to ask the crew why all we ever see is one stuns'l..Royaliste next event with her is in April '05, I'll ask then..The blow really 'sucked'...I had the main course down, but my tops'l requires removin' a yoke, and it didn't seem prudent to be at the top 'o the topmast at 2 a.m. in the middle of the gale.....swayin' it down today, if the rain stays away......Also a plank to replace this week behind my chainplates.

[John Kohnen]

Originally posted by Gary Bergman:
Great shots!...Altho' I try to keep the rubber dingies outta my web postings!Yeah, the rubber dinghy really spoils the effect! :( If they had the money to build a ship like Lynx why couldn't they find a bit more to build a tender to match? <harumph> At any rate, they should have left the rubber boat ashore while they were out showing off in the bay. Isn't there a law against displays of bad taste like that? :D

John

[John Kohnen]

http://www.boat-links.com/Sucia/Feather-08.jpg

Not a very good photo, but doesn't she look piratical? Isn't that reason enough for a raked mast? :D

John

[Meerkat]

Originally posted by Dave Hadfield:

Glad I wasn't operating a flight into SFO that day. Those kind of winds over the hills south of the runways 19 make for horrendous turbulence....I'm glad you weren't either! the hills are to the west of the airport! :eek:

The nearest hills to the south are about 40-50 miles away, south of San Jose!

[JimD]

John, looks like a Bolger Chebacco (the red hull) in the background. Not quite as much rake but nice lines all the same.

[Gary Bergman]

I'd hang either off my snailback davits.......I'm in the market again....

[Ken Hutchins]

According to an article by B.F. Butler in Harpers Magazine in 1885 America's masts rake was reduced from 2-1/2 inches per foot to 1-1/2 inches per foot to improve performance. ;)

[Dave Hadfield]

Meerkat, we have different definitions of "hills". The land immediately adjacent SFO (just beyond the highway) to the south rises steeply. This is quite a factor if you're lining up to take off on either 19, or if you're on approach and are considering the plan for a go-around. The weather there that produces strong southerly winds and a 19 operation is typically very wet, low cloud, and very rough on approach. Nasty.

As for rake, as Ian said, if you had a mast that leaned over the cargo hold, it was a lot easier to rig a tackle to hoist things out of it.

[bainbridgeisland]

Originally posted by Pernicious Atavist:
absolutely a peach...but, i still haven't learned why a boat is DESIGNED to have raked masts. maybe there is no reason other than esthetics?
designers?Boats generally sail upwind faster with the mast raked aft. Offwind they tend to be faster with the mast raked forward. You will find that some high performance sailing dinghy's adjust mast rake for this very reason. It is significant enough to be worth the trouble, at least on high performance boats. Even 'sportboats' adjust the mast rake for the conditions.

So, one reason to rake the mast aft is to improve upwind performance.

[davef]

I am fairly certain that Lars and Elliason's Principles of Yacht design has a section on why masts are raked. I'll try to look it up. The reasons are partly aesthetic and partly performance and are actually related.

Having recently designed a sail plan (follows) I can say that raking the mast (both forward and aft from nominal design position) provides a way to move the lateral center of effort (CE) to achieve a design "lead" relative to the laterial center of resitance (LCR) which ensures proper helm response. Since setting lead during the design process is an inexact science, once the boat is actually built and sailed it may be necessary to adjust the lead. Some boats adjust the lead depending on weather conditions and some race boats adjust the lead depending on the point of sail. If you can't move the mast fore and aft, your only way to adjust the lead is to adjust the rake. If your mast had no rake and you found the boat had lee helm you would have no choice but to rake the mast forward which would look quite goofy. Hence, I beleive that during the design process you start with a rearward rake to ensure you'll never have a positive rake after tuning the boat. Over time this has become an accepted aesthetic.

Therefore, on my boat at least (and on a lot of other boats I think) you start with a rearward rake so that you will always have some rearward rake after the rig is tuned. As you can see from what follows I've designed 8" of rake which give me around 7.5" of lead (around 3% of LWL) by design. In practice I expect to rake the mast forward by a few inches but we'll see.

I think this is right but welcome corrections if I'm spouting BS.

Dave

http://www.imagestation.com/picture/sraid149/pe9566e0739be62a3728ecb0781c062f4/f5f44804.jpg

[Spissgatter W-9]

So, how would one go about determining how much to rake a mast?

[John B]

From a sailing perspective? Its how the helm feels.From a design perspective .. its about setting the lead based on empirical experience with that type of hull.
In our boat.....when we were sailing a bermudan rig in an originally gaff design I raked the mast back a metre to get the helm I wanted. Now that we're gaff again, the mast is plumb. Actually we will pull a bit of forward prebend into it with the jib before we set the main to get a nice tight forestay but the net result is still near enough to plumb.

[bainbridgeisland]

As John B points out, sail changes alter the balance of a boat.

This is true for different sail plans, like converting gaff to jib headed, adding roach or adding overlap to a jib. These methods all adjust the location of sail area.

The balance of the boat is also changed by sail shape. As draft moves forward and aft, the center of lift moves with it. So two sails with the same perimeter may feel noticeably different because draft is not in the same position. Another example is that old sails tend to have their draft further aft than new sails producing more weather helm.

Sail tune can also alter the location of the center of lift. For example a tight leech on the mainsail will move the center of lift aft producing more weather helm.

Sails set differently, for example most gaff mains have more twist than jib headed mains. So they would commonly have a different position of lift with respect to their center of area.

DAVEF; 3 1/2% lead may not be enough, depending on hull shape and sail shape. At the beginning of my carrier, 25 years ago, I designed a jib headed catboat with 3% lead. I was very unhappy with the performance until I put less area in the centerboard and more in the rudder producing a 6% lead. This seemed to be enough for this particular sailing dinghy.

As pointed out by others, hull shape affects the true center of lateral plane (CLP). Here are a few examples: The sharper and finer the hull shape forward, the further forward the true lateral plane will be with respect to the center of immersed area. The larger the rudder and centerboard or keel with respect to the hull, the less significant is the hull contribution to the position of the true CLP.

[davef]

Originally posted by Spissgatter W-9:
So, how would one go about determining how much to rake a mast?For me it was not so much how much to rake the mast but how much to set the lead at. The book I used (principles of Yacht design) specifies how to figure out the CE. Essentially you determine the CE of the main (as the geometric center of the main sail, neglecting the luff) and the CE of the jib (as the geometric center of the fore-triangle neglecting any part of the jib that overlaps the main).

There is then a formula for determining where the entire rig CE lies in relationship to the CE of the main and jib - Assuming you have that type of rig.

POYD also has charts on what to set the lead to depending on your type of boat and, more importantly, style of keel (fin, full, etc.)

As John mentioned, these relationships are all empirical, based on years of experience with lots of different hulls. I don't believe the actual physics bear much relationship to the empirical formulas since the lead always ends up with the CE forward of the LCR in the charts which would imply a lee helm. In reality, the charts seem to yield proper response... apparently. We'll see.

On my rig, I needed to rake the mast simply to get the CE aft far enough. I could have moved the keel forward but was reluctant to do that for a couple of reasons that may now prove to be a mistake!

So, I basically set the sail area, planted the mast and then raked it back until I got the CE roughly where I wanted it. I used a CAD program so various itterations were pretty quick.

bainbridgeisland... re your comment on my Lead being insufficient, I recently hired Buzz Ballenger to complete my rig design. Does anyone on here know Buzz? He's a West Coast based rig manufacturer and apparently comes very well recommended.

I originally had the lead set at around 7% which was closer to what was recommeneded by POYD. Buzz looked at my initial design and expressed a concern that my lead was too large based on his experience. I think POYD is basically intended for people who are building much larger boats than mine (20 ft loa) so I differed to his judgement.

I set it back to 3% based on his advice. If I end up having to move it foward I can slide the mast forward on the deck. I can't take it any further back because the mast is supported on an isthmus that protrudes into the cockpit.

Thanks for the heads up. I'm going to re-lay out the rig with a 6% lead and see where it puts the mast or how close to vertical I'd need to run the rig.

I may end up with a forward raked mast yet!!!1

Thanks and wish me luck.

-dave

[bainbridgeisland]

davef, I know Buzz Ballenger. His company has built a number of rigs for boats I have designed. It will be real interesting to see how your boat balances. Be sure and let us know.

[Wooden Boat Fittings]

.
I've just come across a treatise on the raking of masts, dating from the early 19th Century. It provides a good rationalisation for mast rake in square-riggers, and it seems at least possible that the justification carried over into hermaphrodite rigs and them into fore-and-afters.

Anyway, here it is as a lengthy copy-and-paste. The reference is Papers on Naval Architecture and other Subjects Connected with Naval Science. Volume I. Whittaker, Treacher, and Arnot, London, 1826. pp 45-47

Mike
____________

Remarks on the Raking of Ships' Masts, by Mr. W. Henwood, Naval Architect.

One of the many instances of the want of fixed principles in naval architecture, is that of determining the positions of masts. The practice and experience of a long series of years have established rules, according to which the masts of ships are generally placed; yet the frequent recurrence of ships requiring the position of their masts to be altered, shows that these rules are not of general and certain application. The different forms of ships, and particularly the relative degrees of fineness of the fore and after bodies, are necessary elements to be included in all rules for determining the position of the masts. To obtain such rules as will always determine their position in relation to the properties of ships, will require a combination of experimental and theoretical knowledge, greater, perhaps, than the present state of the knowledge of naval architecture admits.

Among other difficulties of this subject, the rake given to masts of ships is frequently a question concerning which there is considerable difference of opinion, and the propriety of which is often doubted.

In some instances the sailing of ships whose masts are too far forward is improved by the rake of the masts being increased, which carries the surface of sail further aft; but the desired effect would be better gained, as far as regards this consideration, by carrying the masts wholly further aft. The advantage of raking masts must be considered on other principles.

The French, and the Americans especially, have carried the practice of raking masts to a greater extent than the English; and as some of their vessels, in which the masts have been much raked, have been remarkable for their superior sailing, it naturally becomes a question, whether or not the peculiar manner of placing the masts has, in any degree, contributed to produce this excellency?

In examining this question, it will be necessary to consider in what way the surface of the sails is subjected to the action of the wind, both when the masts are upright and when they are inclined.

Suppose the masts of a ship to be placed perpendicularly to the surface of the water; then, if the yards were braced up, the tacks and sheets close-hauled, and the ship in an upright position, the planes of the sails would be vertical; and if the ship, when sailing by the wind, could be prevented from heeling, the planes of the sails would receive the impulse of the wind in the most advantageous manner possible. But as it is impossible to prevent the heeling, the planes of the sails cannot remain vertical when they receive the action of the wind.

If the masts, instead of being upright, were to be inclined towards the stern, the planes of the sails, when they are close-hauled, and the ship is upright, would be inclined to the horizon; and as the ship is heeled by the force of the wind, this inclination of the planes of the sails is gradually diminished, and it may be reduced to nothing, if the inclination of the masts was to be in a given proportion to the angle of heeling. Thus, if the angle to which the ship may generally be permitted to heel is 10°, if the rake of the masts was such that the inclination of the common section of the sails, when close-hauled, with an athwartship vertical plane, was also 10° when the ship became inclined, the sails would become vertical.

Again, when the masts are placed perpendicularly, the area of sail presented to the wind, when the ship is upright, is greater than when the ship is heeled, in the proportion of radius to the cosine of the angle of heeling; but when the masts are raked, the area increases as the ship is heeled, in the proportion of the cosine of the inclination of the ship to the radius.

It appears, therefore, in order to make a ship sail by the wind with the greatest degree of velocity, the masts should be placed at a certain angle of inclination towards the stern, and not in a vertical position.

In the above reasoning, the sails of a ship have been considered as plane surfaces; it has also been supposed that the wind acts uniformly all over their surfaces. Both of these hypotheses, however, are inaccurate. It must be considered in what manner this circumstance affects the above conclusion.

When a ship is sailing by the wind, the curvature of the sails is very small on the weather side, although it is very considerable near the leech on the lee-side; also, as the particles of air impinge very obliquely on the surface of the sails, and as each particle, in gliding off after impact, takes off a part of the action of some of the more leewardly particles, the effective action of the wind on the sails must be gradually diminished from the weather-side to the lee-side. The rush of air across the ship may also probably produce a diminution of pressure on the fore-side of the sail near the weather-leech, which is not the case in the vicinity of the lee-leech.

From these considerations, it appears that the pressure of the wind, on a sail that is trimmed sharp, is greater on the weather-side, where the surface is nearly coincident with a plane, than it is on the lee-side, where the surface is much curved, and that the diminution of pressure is gradual, from the one side to the other. This is confirmed by the well-known fact, that when a ship is on a wind, the tension on the weather braces is always greater than that on the lee ones; and also, by the general proposition, that a plane surface, which is acted on by a fluid in an oblique direction, always endeavours to assume a position perpendicular to the line of action.

It appears, then, from the above observations, that the inaccuracy of the hypotheses admitted respecting the surface of the sails, and the manner in which they are acted on, cannot materially affect the justness of the former conclusion; for the part of the sail which receives the greatest part of the force of the wind does not differ greatly from the circumstance supposed in the reasoning, and the part least agreeable to the supposition does not require any reasoning which opposes the principle of the argument.

The writer is aware that there are other considerations relating to the raking of ships' masts, and that objections are sometimes made to the practice, particularly by the increased strain brought on the materials by it; he is, however, the more desirous to offer arguments in its favour, as it has received the sanction of experience, and considers that the above reasoning, as far as it goes, is correct in its conclusion; that masts, to produce the best effect on ships' sailing, should be placed at a certain inclination towards the stern.
.

[Pernicious Atavist]

NOW we have some great discussion and input! i LOVE this site! mike, that was a great piece you posted.
thanks, y'all!
ed

[Ian McColgin]

Except that Henwood's analysis is irrelevant to fore and aft rigs and, while a good try, not correct for square rigs anyway.

Look at details such as his pressure differential on the sides of the sail being confirmed by tension on the weather brace. The observation is correct but because the venturi effect was not yet understood he's wrong both about the cause and about the meaning of tension on the braces. regarding the braces, whether a yard has a sail set, furled or on it at all, the yard's axis of rotation about the mast ensures that it must, if left to its own devices, swing till it is normal to the wind. Thus to hold it when sailing on the wind, it's the weather brace that's in play. You want a bit of steadying strain on the leebrace but you could actually trim the sail without it. I've sailed a topsail schooner with the lee brace adrift.

Some of the confusion stems from designed trim of the mast, whether raked or not, versus adjusting the rake, usually within narrow parameters, to get the boat to balance correctly.

As was pointed out in the first few posts, the noticable designed rake was initially an engineering solution to how to keep the mast up without cumbersome back stays. On a square rig one does not need to do this very much as the braces and sheets will carry the forward thrust back to deck, but even there there was some at least static convenience to having the masts lean back, as it were, against the foreward stays.

Moving to foils far more esoteric than the reason masts are sometimes raked: One can look at more advanced areodynamics and find a reason for a swept back (raked) foil.

The F-104 has its foils about normal to the airflow. Other high speed designs have swept wings for speed but can extend to less swept for greater lift at low speed. Many moderate speed commercial designs have swept wings to gain lift and strength from a more compact structure. Each of these choises demands a different approach to the foil's cord design.

Similarly among fin keels, the cords of verticle fins v. swept fins are quite different. So, you can design fast/powerful foils for pretty near any angle of attack. One aspect of keel design is simpler - compared to an aircraft wing it operates in a far narrower speed band and nothing really bad happens if the foil stalls.

The sweep or rake in contemporary ice boats is more an engineering feature to simplify staying the rig than it is an airfoil feature. In a strange circularity, iceboating design problems recapitulate the development of sail power.

In terms of pure airfoil, closer to normal is more efficient and sweeping the foil back is really the result of other compromises and design requirements.

[Pernicious Atavist]

thanks ian! i think.....you just completely exceeded my knowledge base..... :eek:

[Ian McColgin]

Heck, I probably exceeded my knowledge, what with over-simplifing from an broad but not especially advanced technical background. We've enough true experts around here that if I'm too far off, someone will rein me in right quick, eh?

I did not mean to denigrate the early attempts to explain, by the way.

About the same time the first scientific efforts to understand hull design were underway. I forgot who it was, Michael may recall his name for us, who towed lumps of ice about to see how the natural melt worked, on the theory that the shape would be naturally most efficient. This led to the poeticly named and way over-loved "mackrel head and cod tail" theory of design which was a scientific advance despite being wrong about the dynamics and leading to poorer designs in the short run - an advance because it was corrigible inquiry that was attempting to marry theory and experience rather than a merely empiracle evolution of narrow experience.

[Mike Field]

.
Hell, Ian. Here I post this wonderful theory, nearly two centuries old (okay, it wasn't right, but still....) and you come along and demolish the whole thing with one well-aimed kick from your canvas deck-shoe.

Have you no soul, man? smile.gif
.

[rbgarr]

Ian-

I can't refute your comments entirely, but if your theory of how sails work depends on a venturi effect, then I'm confident that has been proven false. The best description of how sails actually create thrust, interact, etc. that I know of is in Tom Whidden's "The Art and Science of Sails". Tom, of North Sails and past America's Cup leadership (and a Wianno sailor, to boot!) has a couple of chapters on it, complete with experiments you can do yourself.

I think this conflict over how sails work has been argued here before, but I can't find the thread. Does anyone know where it is? I didn't intend to derail this one.

[Wild Wassa]

Setting the rake on a dinghy (if you are able to) is the art of dinghy racing.

There is one thing that I know about rake on a dinghy, move it one inch incorrectly (between races on the same day, in an attempt to improve the tune) ... and I may have gone from 1st to last because of one badly calculated inch.

Rake determines pointing in relationship to wind speed. Depending on the wind speed so the rake is appropriately adjusted either for' or aft.

A few of the dinghy class web sites (thinking of a 505 in particular, where every possible parameter is calibrated, even minute changes to the swing or depth of a CB should be calibrated) will list advised changes to the rake (particular to the class) depending on wind speed and conditions.

Warren.

[ 12-18-2004, 03:02 PM: Message edited by: Wild Wassa ]

[Ian McColgin]

On sail dynamics:

It's been a while since I read Whidden's book but my memory is not that he disproved the venturi theory so much as added to it.

Here's the problem of oversimplification.

The grade school science model where the air molecules part company at the leading edge of the wind and rejoin at the trailing edge, the molecules moving over the convex side going so much further and thus the venturi bit is a falsly oversimplified view of a wing and utterly irrelevant to a sail anyway.

Further, that representation ignores the reality of angle of attack.

Quick aside, I'm not prepared to look into the subtlties of venturis in general and the greater specificities of Bernoulli's law, which applies more to liquids anyway. Please allow me to use the word venturi to cover all that stuff about dynamic pressure differences between fluids.

Venturis are complex phenomena in shich the role of turbulence is lost if we stick to the ninth grade wing model.

Whidden brought down to sea level the debate about the role of the angle of attack. A very bad analogy is that angle of attack is a bit like a boat planing on the water - the deflective force of the air hitting the angled weather side of the sail and pushing it forward.

Old fashioned flat blade windmills such as are still common on the plaines for pumping work more by angle of attack, probably, than by venturi.

However, and I thought Whidden bore this out, both effects are important.

Take a bit of paper and hold one edge horizontally just above your mouth with the free edge of the paper naturally curling down. Blow. Your breath will strike the bottom of the paper and lift it up. Angle of attack.

Now move the paper to just below your mouth and blow. The paper will again rise. Venturi.

Both these forces are important to a sail. The angle of attack becomes more important as you get to running but you'll still have a venturi effect, perhaps a bit stalled on a main sail but very significant in a high shouldered spinnaker.

The invention of the modern hang glider and the refinement of the para-sails really devolve from a sophisticated analysis of venturi and angle of attack.

Depending on wind angle and strength you can play with venturis and turbulence. For example, if you have tell-tales properly back from the luff of a jib, you may find that you've more power if you can sail right at the edge where the leeward tells break a little but the sail's not yet luffing. What you have here is a boundry where the angle of attack thrust on the concave side of the sail can be maximized. But you'll also be gaining clean flow further aft along the sail and picking up a stronger venturi there.

But not always.

Add to complexity the practical problem of seeing what's up and correctly controlling it.

Which is why equally good sailmakers can have such entertaining arguments at the club bar after the race.

[rbgarr]

The problem of how to trim sails is not well understood generally, because of received wisdom about venturis, angles of attack (especially what causes them to change) and so-called slot effects.

I'll only (re)recommend Whidden's chapters on sail dynamics, and then the subsequent ones on sail trim implications, which are too much to reprint here. The descriptions of the forces at work are very closely argued, explaining important things most sailors have never known about, much less understood (the Kutta condition, for example). The book would be an excellent Christmas present and rewarding winter reading.

In any case, sail dynamics do not lend themselves to brief or oversimplified explanations at all. Attempts to do so have lead to 'wives tale' trimming practices and traditions used by most sailors... which are also taught by instructors for the most part.

I'll offer my own weak analogy: New automobile drivers learn how to use the accelerator and brake pedals, and they can get around well enough. But if they think that the accelerator (itself) is something that, say, directly makes the flywheel spin faster, or changes gears, and thus make the car go faster, then they don't really understand what's going on. Do they have to? No, not really, but if someone tells them that IS what's going on then it may be useful to point them in the direction of more accurate and complete information.

None of what I write here is directed at Ian, please be assured.

[ 12-19-2004, 11:46 AM: Message edited by: rbgarr ]

[bainbridgeisland]

Originally posted by Ian McColgin:
One can look at more advanced areodynamics and find a reason for a swept back (raked) foil.

The F-104 has its foils about normal to the airflow. Other high speed designs have swept wings for speed but can extend to less sweep for greater lift at low speed. Many moderate speed commercial designs have swept wings to gain lift and strength from a more compact structure. Each of these choises demands a different approach to the foil's cord design.
I don't think looking at airplane wings clearly shows when and why rake of a sail plan is correct. There are a few problems:

Airplane wings operate at far higher speeds than sails. High speed aircraft are especially apt to mislead you.

One objective of a wing is elliptical span wise lift distribution to optimize lift versus drag. This is accomplished with a rigid wing by balancing taper ratio, sweep and thickness to approximate the optimum lift distribution. Sails are not as rigid. Changes in draft (in effect thickness), mast bend as well as twist profoundly influence the span wise lift distribution. Thus, the optimum rake (sweep in aeronautical terms) for a sail would be different than a rigid wing.

The sail plan; gaff, jib-top, high roach and so forth also affect the optimal rake. This is because different geometrical shapes would need different sweep angles (i.e. rake) to approximate the optimal lift distribution. For example, a square headed rig would likely perform better with less rake than a triangular profile. However, as mentioned in the previous paragraph changes in draft and twist could alter this as well.

In real life, we adjust sail twist and draft to get the best performance from the mast rake established. Some of these adjustments mimic the effect of rake. For example increasing draft aloft is the same as increasing the thickness of the tip of a rigid airfoil, which has a similar effect to reducing sweep or rake.

[peter north]

I inadverttanly discovered a disadvantage to mast rake. In adapting a used wood rig to my project ketch, I opted for 2 degrees of aft rake, mainly because I think rake looks salty. 2 degrees is a lot more than you might think visually, but the problem occurs when you try to buy used sails. It's enough rake to make the boom droop unappealingly and hazardously. It works ok if I use the "flattening reef" for the clew. I guess I'll have to recut the foot someday.

[George.]

Originally posted by Ian McColgin:
...the poeticly named and way over-loved "mackrel head and cod tail" theory of design This is an interesting point. Ships had such a shape way before any scientific theory was applied to shipbuilding - one could say it evolved, rather than was created. It is tempting to dismiss what our ancestors did as "primitive" or "wrong," but sometimes it bears a closer look.

My hypothesis (please feel free to jab at it) - ships and boats of old were meant to carry a big load. If the bulk of the load is up forward, and the run is narrow and long, you get good leverage for the rudder and the mizzen, which was mostly meant to aid in steering anyway. Thus the old naus and galleons had a broad rounded entry and a narrow run - otherwise they would not turn.

Only when construction techniques evolved to allow long and narrow ships, like clippers, did changing that design become feasible - and even then, clippers carried high-value light payloads like tea.

Also, ships of old mostly ran - avoided beating like the plague. A vessel will run more stably if it is bulkier up front, and thus has most of its sail area there.

[Lucky Luke]

Originally posted by Wooden Boat Fittings:
.
But there's no doubt that raked masts somehow just seem to look "better" as well, anyhow. Bombigher's Miss Simplette --

....(But god help them if they lose the bobstay...)
MikeThanks Mike for citing my deceased friend, Daniel Bombigher.

When I asked him, someday, why he designed boats with so much rake aft, the only "sensible" answer he gave me was that it was much easier to spot where is your boat on a crowded anchorage when you want to go back aboard after some nicely "alcohoolised" dinner ashore.... :D

And, you know, this remains the one and only good answer to this question I ever heard! smile.gif

(don't worry too much about them loosing the bobstay, Mike: Daniel's masts could stand up anything even without any rigging :rolleyes: )

Since there are so many factors that influence the general layout ("architecturing") of any boat, its intended use being one of the major ones, and although most of the exposed reasons hereabove are perfectely valid in a certain context (even Aramas' saying that the always-best is zero rake: not valid for catboats for example), there is no absolute answer to this questions but that:"it depends..."

Same applies for every single design element of almost everything.... :cool:

[ 12-25-2004, 02:59 AM: Message edited by: Lucky Luke ]

[Hwyl]

Originally posted by Lucky Luke:
[QBmy deceased friend, Daniel Bombigher.

When I asked him, someday, why he designed boats with so much rake aft, the only "sensible" answer he gave me was that it was much easier to spot where is your boat on a crowded anchorage when you want to go back aboard after some nicely "alcohoolised" dinner ashore....

[/QB]Thanks for making me smile Luke. Been there and done that, the IOR days were the worst--those boats looked all the same.

[bainbridgeisland]

[/QB][/QUOTE]Thanks for making me smile Luke. Been there and done that, the IOR days were the worst--those boats looked all the same.[/QB][/QUOTE]

Ahh - don't you remember? The paint jobs; it was the psychedelic colors that allowed one to find his ride.

[Nick C]

Real simply put a raked mast on a sloop will increase your ability to point into the wind (go to weather). Conversely you will loose some downwind ability.

Some boats, usually racing sailboats (the Star being a classic example) allow you to adjusts the mast rake while you are under way. So you can rake it more for going to weather and stand it up going downwind.

If you have a mast that is stationary then the trick is to either figure out whether your boat is faster downwind or upwind and rig to your advantage or take advantge of the type of conditions and directions you like to sail in.

Bottom line is most people don't sail well enought to make a difference.

[Gary Bergman]

I have lost my bobstay with raked masts, and...it was really 'interesting' on each ensueing square wave with the jibboom 'lofting' about 10 feet at the tip, but no masts came down!..I made an 'emergency' docking in front of the bow of Larry Elisons megayacht, jumped ashore, tied one dockline, cut the link of chain with a boltcutter, added a new shackle, untied the dockline, set my tops'l, and tacked away, with his crew rather stupified!...Sail ho!