Why does a cutter rig point higher & sail faster?

[mattotoole]

Originally Posted by IOR Fan

It would seem that Wallace agrees with my assessment. The outboard sheeting angle of a cutter is the limiting factor in pointing. If you put the lead farther outboard to accomodate a staysail you harm the pointing angle.

This is true, but there are other factors limiting pointing ability.

First, as I pointed out before, is the sail type. It's not fair to compare a racing genoa designed to go upwind and not really anything else, to a cruising sail designed to do everything plus roller-reef.

Second is the rest of the rig -- the mainsail, mast, rigging, etc. As an extreme example, an in-mast furling main with no roach, no shape, and a stout mast section, will limit pointing ability too. I know because I sail on a boat so equipped. It's otherwise a fast boat, but won't point because of the kind of rig ahd sails it has. Without at least a new main and some retuning it wouldn't any higher, and even then it wouldn't be much. But a staysail would make it faster without affecting its pointing ability. Most cruising boats are probably similar, including the O'Day being discussed.

Finally, there's the hull. I doubt this limits the O'Day, but so many cruising sailboats have inefficient hull shapes and keels with poor lift. Glomming on more sail makes the most of what they *can* do, because more speed generates more lift.

[brian eiland]

Originally Posted by CT 249

Terrorist's and Hawkeye's bilgeboards were only marginally faster, if at all.

Don't know about Hawkeye, but Terrorist was significantly faster and better VMG if I remember correctly.

Originally Posted by CT 249

Brian wrote "Anyone with a rotating mast (particularly on a small cat) has experienced this phenomenon…..rotate the mast too much (in a desire to flatten the main) and you severely ‘choke off’ the slot between the jib and the main….performance drops immediately".

Not so; some world-class guys in some high performance small cats classes over-rotate their mast dramatically to depower; others in the same class de-rotate to depower. They often end up side-by-side around the course. We're talking guys who are top 10 in the world's most popular cat classes, here. They are NOT stuffing up.

The problem with correctly discussing this subject is defining the different conditions that can be present.

In some cases the diamond stays on the rotating mast are left very loose, so upon rotation the mast bows out considerably in the middle thus dragging the mainsail flatter for either depowering in heavier conditions or maintaining connected flow in light conditions For moderate wind conditions the mast would be under-rotated to give a fuller main. Notice here we are using the mast rotation to effect mainsail shape more so than as a leading edge element.

On the other hand some folks operate with tight diamonds that effect the mainsail shaping in the opposite manner. Very likely if we know it is going to be all light air sailing (racing) that day, we would opt for tight diamonds, flat mainsail, with a better leading edge to the under-rotated mast.

Next consideration would be the rotation center for the mast...where the rotation 'ball' supports the mast. If this rotation center is too far forward (towards the leading edge of the mast), then as the mast is over-rotated the slot between the jib and main is sqeezed (choked) off by the trailing edge of the mast and the jib

Originally Posted by CT 249

I remain puzzled by "I would interpret Too-Sweet’s observation thusly; I believe he is of a performance vein, and thus would be dissatisfied with the ability to sheet the genoa in closer than the outer shrouds to effect pointing capability."

How could anyone expect to sheet a genoa inside the outer shrouds, unless in an extremely unusual boat?

My mast-aft configuration is one example of an unusual boat that allows wide (or very wide) spreaders with narrower sheeting.

[brian eiland]

Brian wrote:
I found this discussion today on the web that I took the liberty to add to this forum discussion of cutter rigs.

Raked Cutter Rig by WindSpeedYachts.com

History.
Ever since 1851 when the Schooner America with her 11.5 degree mast rake won the America's cup, mast rake has been used to improve the performance of sail power as a propulsion method. Every high speed craft including land yachts, ice boats, windsurfers and racing Multihulls use mast rake to improve performance.
Yachts that race to a handicap don't use mast rake because it is penalized by the handicap rules. In handicap racing the fastest yacht is not the winner, the yacht that sails closest to her handicap is the winner. Our yachts are not designed to any racing rules they are designed to be safe, efficient and comfortable.
The cutter rig has historically been the rig of choice for the monohull cruising sailor.

The Modern Raked cutter Rig.
In 1991 we proudly redeveloped and introduced the modern, efficient raked cutter rig into cruising Multihulls.

The Raked Cutter Rigs features:
+A 7.5 degree mast rake, the key to the whole modern cutter rig working so well:
+Parallel fore stays for the greatest cutter effect.
+The distance between the forestays is spaced for maximum sail efficiency.
+The Jib easily tacks between the stays.
+Low aspect ratio rig to increase stability, reduce rig loads and reduce pitching.
+Low centre of gravity, low wind resistance cruising rig.
+The stay sail is close to the deck which improves efficiency and pressurizes +the windward nets and lifts the leeward nets reducing on board spray and increases stability.
+With the roller furling Jib on the composite bowsprit, which is wide enough to walk on and the fully battened stay sail on the fore beam, the fore triangle is broken into two easily handled sails.
+The large fully battened mainsail is reduced in area by the mast rake.
+This long low sail plan increases stability, reduces the sail handling effort required by the crew and off the wind increases the power to propel the yacht.

Notes
This rig is the complete opposite approach to the trend for high aspect ratio fractional rigs being installed on cruising yachts.
We have observed the trend by designers and builders to increase the height of masts to "improve performance" however we are informed by boat owners that increased speeds are achieved by reefing the mainsail. This is not surprising, tall masts raise the centre of effort of the sailplan, this increases the heeling force which pushes the leeward hull further into the water increasing hull drag. If you look into the hull shapes, for example a 12 m catamaran with fat hulls 1.2 to 1.4 m wide on the water line and fixed keels, the drag on these hulls when the leeward hull is depressed increases considerably. This type of hullshape is happy with both hulls evenly floating while motoring or sailing down wind not being pressed hard to windward or reaching.
The high aspect ratio racing rig works exceptionally well on racing yachts however it does not make slow hull shapes fast hull shapes.
How does mast rake increase yacht speed?

The more horizontal the fore stay angle the greater the vertical lift generated by the jib. Effectively lifting the bows which reduces the yachts displacement which reduces wave drag and increases the yachts speed. Raking the mast aft has a similar effect to fore stay angle.

With a vertical mast the forces generated by the mainsail are forward and downwards but by raking the mast aft these forces are rotated to become forward and upward resulting in increased speed due to decreased displacement.

The effect of mast rake is apparent sailing to windward but far more obvious when reaching and running.

Because the bows are not driven down by sail pressure we can use finer bows which further reduce drag and pitching.

Another advantage of mast rake is reduced pitching caused by the lack of the pendulum effect.
With a vertical mast in a sea way the masts mass and momentum forces the bows and sterns up and down alternately greatly increasing the pitching.
With a raked mast in a sea the masts mass and momentum forces are rotated to force the bows forward and the sterns up and down alternately greatly reducing the pitching.

The combined effect of the lift generated from the raked cutter rig, the lift generated from the under wing, the extra buoyancy and dynamic lift generated by the inboard flair and the stabilizing effect of the rudder mounted foil combine to reduce heeling, under wing wave impacts, yacht motion and increases average speeds.

The Raked Cutter Rig of the WindSpeed 40 has proved to achieve higher average speeds than any other rig aboard a liveaboard cruising Multihull yacht yet encountered.

Test results
Test have been undertaken on many yachts over many years but the most convincing test was aboard a 2000 kg 10 metre racing Multihull with a 15 metre rotating mast. This yacht started life with a vertical mast, and over two years the mast was raked progressively aft with consistently improving performance. The lift generated by the raked mast allowed us to carry more sail area in stronger winds off the wind and when sailing to windward, the crew of four would sit forward of the mast to keep the bows down.

On a 1.5m long radio controlled test model trimaran the mast was raked so far aft that the model capsized bow over stern from the lift generated from mast rake!

Note: We were testing the feasibility of using surface effect wings for racing trimaran beams at the time and these foils may have contributed in the capsizes [tested in 1981]

Conclusion
Our theoretical calculations and practical testing indicate that the raked cutter rig on the WindSpeed 40 generates 240 kg of vertical lift in 18 knots of apparent wind and sailing to windward at 8 knots this reduction in displacement equates to approximately 0.1 knots extra yacht speed! The lift generated by the raked cutter rig increases as the sheets are eased which increases the improvements in downwind yacht speeds

(opinions of www.windspeedyachts.com.au)

Brian added a note: It doesn't appear from their posted drawings that the mast rakes back 7.5 degrees?

[Skippy]

Originally Posted by brian eiland quoting WindSpeed Yachts

The more horizontal the fore stay angle the greater the vertical lift generated by the jib. ...
With a vertical mast the forces generated by the mainsail are forward and downwards but by raking the mast aft these forces are rotated to become forward and upward resulting in increased speed due to decreased displacement.

Which as CT has pointed out, introduces stability issues, which are especially important in cruising.

And since the mast and stays are heeled leeward, the aft rake decreases sail efficiency by increasing sweep, at least for the headsails.

Originally Posted by brian eiland quoting WindSpeed Yachts

The effect of mast rake is apparent sailing to windward but far more obvious when reaching and running.

Should that be "effect" or "benefit"? It sounds like the design is marketed to customers who respond to the idea of zipping back & forth on a reach, but may not appreciate the importance of being able to point. Plus the rake looks cool, i.e. more marketing.

Originally Posted by brian eiland quoting WindSpeed Yachts

Ever since 1851 when the Schooner America with her 11.5 degree mast rake won the America's cup, ...
The Raked Cutter Rig of the WindSpeed 40 has proved to achieve higher average speeds than any other rig aboard a liveaboard cruising Multihull yacht yet encountered.

So do racers, but they're designed for racing.

[brian eiland]

Skippy,
I would like to just point out that those quotes you attributed to me should be directed to WindSpeedYachts from which came the discussion on their website

[Skippy]

Originally Posted by brian eiland

those quotes you attributed to me should be directed to WindSpeedYachts from which came the discussion on their website

ok, good point. btw, their cutter doesn't sound too bad from a marketing point of view (not that I'm in that field). There seems to be a trend these days toward "traditional values", and I can see that being associated with a traditional design like the cutter.

[CT 249]

Re "The Raked Cutter Rig of the WindSpeed 40 has proved to achieve higher average speeds than any other rig aboard a liveaboard cruising Multihull yacht yet encountered."

Where's the proof?

[Paul B]

You're asking for reality from the people who wrote that? Also from the text:

"Yachts that race to a handicap don't use mast rake because it is penalized by the handicap rules."


Wha?


The simple answer to the original question, "Why does a cutter rig point higher and sail faster?" is:

It doesn't (compared to a sloop rig).

[FAST FRED]

On older boats the mast was raked so the weight of the tree would tighten the forestay.

With great backstay adjusters avilable , and metal rigging that stretches less than hemp , why bothjer EXCEPT for marketing?

FAST FRED

Originally Posted by FAST FRED

On older boats the mast was raked so the weight of the tree would tighten the forestay.

With great backstay adjusters avilable , and metal rigging that stretches less than hemp , why bothjer EXCEPT for marketing?

FAST FRED

Kidding right?

[gybeset]

Quote: Originally Posted by FAST FRED
On older boats the mast was raked so the weight of the tree would tighten the forestay.

With great backstay adjusters avilable , and metal rigging that stretches less than hemp , why bothjer EXCEPT for marketing?

FAST FRED

Are U serious? says
Kidding right?

Maayyyteeee . we are serious, real question is Are U

No we are not kidding, Fast fred is on right track right, to keep headstay tight with timber mast , stretchy halyards , shrouds running stays , no powerful modern winches to tension jib halyards or runners ( Yo! 8" self Fabr. Highfield levers)
would be impossible on an Upright stick, looks at any old 18 footer, sandbagger type , bermuda dinghy or schooner of the day, rake is faster and still is in a few specific cases (Contender, Hobie 16,49er)

go do a real sail apprenticeship on overcanvassed antipodean skiffs ( that is including setting up and tuning) and find out , then U B serious

[dionysis]

Remember that aerodynamically, rake reduces forward thrust as the cosine of the angle. This leads to proportionately less sail drive in the most effective forward direction, which in turn reduces pointing.

This may be the major reason why racing boats tend to make their masts and forestays as vertical as possible.

[skinny boy]

Originally Posted by dionysis

Remember that aerodynamically, rake reduces forward thrust as the cosine of the angle. This leads to proportionately less sail drive in the most effective forward direction, which in turn reduces pointing.

This may be the major reason why racing boats tend to make their masts and forestays as vertical as possible.

Ah, most racing boats I know have rake and pre-bend making them anything but vertical. The fastest boats around these parts have more rake than the others like it. My rig carries a couple of degrees of rake and used to have more until I got my new sails. Pulling the mast back a bit increases my boat's pointing ability and every other boat I have been on. Standing the rig up even leaning it forward a bit helps off the wind to project more area.

[brian eiland]

I think you will find that mast rake is used quite effectively in a great number of vessels. On small cats such as Hobie, Prindle, Tornado, etc, it allowed for higher pointing upwind, and reduced bow burying upon fast tacking downwind.

I've not had any ice boat experience but I would venture to guess they incorporate mast rake as well..... land-sailers as well.

A visit to WB-Sails and click on "News" then "European Sail Trimguide" brings up some data on mast rake.

Then have a look at http://www.curtin.edu.au/curtin/cent.../papers/prc98/ ,"the aim of this research was to determine the interaction characteristics between the main sail and the jib, particularly the influence of raking the mast aft. The motivation for this investigation came after the World Titles in Kingston, Canada where the British fleet used rigs with masts raked aft by approximately 4. The Australian fleet, which up to that point had been dominating the international scene for a number of years, was significantly slower with upright masts, particularly in light airs."

Finally, have a look at this article by Paul Bogataj, "How Sails Work", and note particularly this passage about the cord line of the mainsail, "Isolated Sails: A mainsail by itself (cat rig) is tapered, but if the mast is close to vertical it is actually swept forward. Recall that sweep is measured relative to the 25% chord line, which in the case of a tapered sail on an upright mast is angled forward. In this case, the forward sweep would have somewhat of a canceling effect on the increased upwash due to taper. The actual degree of upwash depends on the magnitudes of taper, sweep, and aspect ratio (height/width) of the sail. The sail still operates in the twisted flowfield caused by the boat moving through the earth's boundary layer, so an amount of twist would be appropriate. Raking the mast back increases sweep and will cause additional upwash on the top of the sail, necessitating more twist to the sail. Genoas and jibs are very tapered and swept. Those two features, combined with the already twisted apparent wind, cause significant upwash toward the head of the sail."

[dreamer]

Originally Posted by SeaDrive

Dreamer, I had gotten the impression from somewhere that a boat rigged like yours was more likely to have good balance with full main and topsail (yankee) than with full main and staysail. Is there a good reason not to strike the staysail as a first step in reefing? (It would go up again with reefs in the main when the yankee was struck.) Second question: what sails do you fly off the wind?

SeaDrive - sorry for not replying sooner (like about a month ago) .

The yankee (headsail not a topsail) has a high clew and when we've tried to fly yankee / main in high winds, we heal farther than we'd like to. I'm no expert, but I suspect with the high clew the CE is higher. The staysail is obviously lower and closer to the center of the boat which seems to keep us pretty well balanced. We will (hopefully) be getting a lower clewed headsail this season. Our full keel/attached rudder is fairly forgiving in terms of balance. That just means I'm a poor resource when talking about balance.

Off the wind on a dead run in 15kts or up we just fly the main. If it's light, we won't try a dead run and will sail with all sails on a broad reach. This is where this setup really seems to make a difference. From about 2-3 kts up to about 15 kts on a broad reach I'm surprised at how well the sail combination works. I suppose it would be nice to have a 130 or possibly an asymmetrical for light air, but we're lazy and keeping a spinnaker full sounds like too much frustration in light air.

You can see our optimal sailing orientation in the link

http://www.sail2live.com/boats/sonad...a_sailing1.jpg

In this pic, we're heading due South. Wind's coming from slightly North of West if not straight form the West at around 15kts I believe and I'd say we're healing about 10 degrees. If I could choose the perfect way to sail around the world, this would be it!

Fair Winds,

Rick