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Old 16-03-2008, 16:17   #46
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Prismatic coeffecient

My target prismatic for catamarans is .65. I would choose a lower PC for a cat only if it were meant to be used locally in light weather, rather than offshore. My hull draft for a 65' cat is only 15" at 45,000 pounds of displacement, as I prefer to make cats light by giving them less accommodations than possible. So, my 65' cat has the accommodations of the typical 50' cat--maybe even your typical 47' cat. This is how you can get a cat with very little rocker, and yet have a typical hull form--by lengthening it.
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Old 16-03-2008, 18:26   #47
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Your target prismatic of 0.64 is way too high, unless it is a racing cat meant to be kept at speeds in the high teens a lot of the time. 0.64 is optimum for a vessel speed of around 12 knots. Are you really expecting to be sailing in that range most of the time?

What is it about boat design that makes so many people think no education or experience is required for them to design a boat <sigh>. I mean, you don't hear people say "I can do brain surgery, I'm sure. How hard can it be..."
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Old 16-03-2008, 19:41   #48
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Thanks for your sneering condescendtion

Quote:
Originally Posted by Evan View Post
Your target prismatic of 0.64 is way too high, unless it is a racing cat meant to be kept at speeds in the high teens a lot of the time. 0.64 is optimum for a vessel speed of around 12 knots. Are you really expecting to be sailing in that range most of the time?

What is it about boat design that makes so many people think no education or experience is required for them to design a boat <sigh>. I mean, you don't hear people say "I can do brain surgery, I'm sure. How hard can it be..."
***** ***** ***** ***** ***** ***** *****
11.2 knots, actually, according to Skene's Elements of Yacht Design. Thanks for accompanying your disagreement with a dose of sneering condescendtion. A high prismatic does less harm at low speeds than a low prismatic does at high speeds, to quote the naval architect Douglas Phillips Birts. Further, as has been cited above, there are more issues than speed when looking at multihulls offshore-such as comfortable motion and avoiding the stuffing of the bows leading to pitchpoling. I keep my designs narrow and light-with 12 to 1 hull beam to waterline ratios, and high sail areas-sail area to displacement ratios of 30. FYI, lots of yacht designers use high PCs for multihulls. Shuttleworth, for example has used a PC of .60 for a cruising catamaran with a hull length/beam ratio of 11.2. My designs have higher "K" factors. If you know what that is.
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Old 16-03-2008, 22:36   #49
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http://www.grandprixsailor.com/sailboats/boat-reviews/the-gunboat-62-comfort-at-speed

I would describe the Gunboat 62 as having moderate rocker, and no immersion at the bow. See Sailing Magazine : Perry on Design
for a profile drawing. It was designed by: Morrelli & Melvin Inc. They are built by: Multihull Catamaran Sailing Yachts, Luxury Catamarans - Gunboat
Tribe, I think the first of these, can be seen under sail at: UnCut Video - Now Playing "The Fabulous GunBoat 62 Sailboat" More can be read at: Sailing World - The Gunboat 62, Comfort at Speed . So, who volunteers to e-mail Morrelli and Melvin to let them know that they have no clue what they are doing when it comes to catamaran design? Volunteers? Anybody?
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Old 17-03-2008, 01:17   #50
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Originally Posted by Nordic cat View Post
Robert, what prismatic coefficient would you consider as high for a 49 ft performance cruising cat?

The height/depth from the deepest part of the hulls to the dwl is 40 cms toward the bows. Draught is around 55 cms. Transoms are 20 cms clear of the water at rest. I know there are alot more parameters that need to be fitted into the final equation. I'm aiming at cp of around 0.58. Max hull beam at around 55% back. Tulip shaped bows and large hull flares (asymmetric). Can post a rendering.

any input is appreciated

Regards

Alan
Personally, for a cat I'd go for about .65-.68. It gets difficult to get much more and still tack. I am not particularly fond of too much flare above the wateror too high a bow as the windage can be a handful. IMHO tank testing for obtaining optimum prismatic coefficients for hull resistance do not take into account the dynamics of the interaction between sailing forces and wave motion. I haven't given as much thought to cats as I have to the generic concepts, which is why I appreciate the Harryproa.
By bringing the max waterline width further forward, it makes it easier to keep the coefficient higher. with a tulip bow you can go a fair bit further. forward. I know I am going to get some flack suggesting such a high coefficient and the subsequent extra wetted area, but I have seen boats do well as such and have greater comfort in a sea, which is a strong component of fast cruising.
Robert
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Old 17-03-2008, 02:46   #51
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Originally Posted by Robertcateran View Post
Personally, for a cat I'd go for about .65-.68. It gets difficult to get much more and still tack. I am not particularly fond of too much flare above the wateror too high a bow as the windage can be a handful. IMHO tank testing for obtaining optimum prismatic coefficients for hull resistance do not take into account the dynamics of the interaction between sailing forces and wave motion. I haven't given as much thought to cats as I have to the generic concepts, which is why I appreciate the Harryproa.
By bringing the max waterline width further forward, it makes it easier to keep the coefficient higher. with a tulip bow you can go a fair bit further. forward. I know I am going to get some flack suggesting such a high coefficient and the subsequent extra wetted area, but I have seen boats do well as such and have greater comfort in a sea, which is a strong component of fast cruising.
Robert
Thanks. 0.65 sounds very high! Bringing the max. waterline width forward brings the hull speed down and adds resistance according to some of the latest thinking as far as I understand.
I agree that tank tests and most CFD programs don't take wave and sail forces into consideration, this is why I think that long narrow bows will initially cut through the water, and thereby immerse a bit more, and in fact increase the cp above the theoretical value calculated at DWL.
This design being an unstayed bi-rig without a jib (normally) has me a bit worried about tacking. The Radical Bays have a big problem in heavy winds.

Any thoughts on this?

Alan
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Old 17-03-2008, 03:17   #52
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Put both of them in one hull as a schooner rig. If it is the same hull as the daggerboard, you can control the fore aft balance quite nicely. another possibility is a small canard rudder well forward. I have heard some people love them as is without any problems.I'd check out all the sites with these rigs and see what similarities and differences there are that may be contributing to the reported difficulties with Radical Bay.
Robert
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Old 17-03-2008, 04:05   #53
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Quote:
Originally Posted by Robertcateran View Post
Put both of them in one hull as a schooner rig. If it is the same hull as the daggerboard, you can control the fore aft balance quite nicely. another possibility is a small canard rudder well forward. I have heard some people love them as is without any problems.I'd check out all the sites with these rigs and see what similarities and differences there are that may be contributing to the reported difficulties with Radical Bay.
Robert

Maybe we can get Rob Denney to comment? He is probably one of the few people who has any experience with these rigs on multihulls.

Haven't been able to find anything on any other sites regarding the Radical Bay issues.

See here: New Bi-Rig Performance Cruiser
and a page or 2 before.

Alan
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Old 17-03-2008, 09:43   #54
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Quote:
Originally Posted by Nordic cat View Post
Maybe we can get Rob Denney to comment? He is probably one of the few people who has any experience with these rigs on multihulls.

Haven't been able to find anything on any other sites regarding the Radical Bay issues.

See here: New Bi-Rig Performance Cruiser
and a page or 2 before.

Alan
G'day,

Happy to add my sixpence worth.

On High prismatics: The harryproa in the video is doing 15 knots in 15 knots of breeze. It's sails need work, there is a lot of drag from the rudders, it is overweight and nobody on board is trying very hard, if at all to sail it fast. This proves to me the advantage of high prismatics (0.8 in this case). Seeing it in a chop shows the advantages for reducing pitching, although there are many other features which help so I am not sure how big this is.

The only experience I have had with a biplane was on a beachcat. It tacked about the same as with a conventional rig. Thinking about it, this is not much of a surprise. The sail loads (that apply to tacking) are the same for stayed and unstayed, single sail or sloop.

First thing I would do is enable the rudders to rotate 360 degrees. This takes 90% of the anguish out of sailing (and motoring) backwards. Getting in irons will be less of a drama and easier to get out of.

You might like to change the relationship between the sails and water foils. Cat boats and Wylie cats have the rig way forward. Maybe move both the rigs and the daggerboards forward. Move the anchors and other weights aft to compensate and replace the forebeam with a wire for the tramp, or remove the tramp alltogether, there is plenty of lounging space on the rest of the boat. Maybe increase the prismatic.

A more radical solution (and a better one, I think) would be to move the rudders. Harrys have no rocker. They are very hard to push sideways or turn when stationary. Yet, at only a few knots boatspeed, if you wiggle the helm, people who are not holding on will fall over. I have never tried to tack one in a seaway, but the response is so powerful that except in light air I use only one rudder for steering. The rudders are oversize (no other leeway resistance such as daggers or keels) and 25% in from the ends. I think this set up would work better for a boat which was hard to tack than the conventional daggers in the middle with the rudders down the back. From here it is a small step to save yourself some weight, space and money by switching to a single rig in the same hull. Could call it a harrycat!

regards,

Rob
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Old 17-03-2008, 10:16   #55
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Typical catamarans

If the question is what is typical of cruising catamarans, I'd say a P.C. of .6 to .62 is typical. When it comes to hull waterline length divided by hull waterline beam-8 for smaller cruising cats and "roomarans," (the kind of boat where you can walk around a double berth,) up to 10 for large catamarans, and up to 11 for high performance cruising catamarans. The typical cruising catamaran has a "bevel" where the hulls join the tween hulls deck, for the stairways down into the hulls, and "shelves" for the berths. The structure holding the typical catamaran hulls together is a monolithic bulkhead at the forward end of a deck house which spans the two hulls, with the roof of the deck house comprising the top of a box girder, and the deck comprising the bottom of a box girder. ***** ***** ***** ***** ***** ***** ***** Those who have seen my website know that I don't do things the typical way, for the most part. Sometimes, you just have to trust your eye. If you make a cat hull that is very narrow, the kinds of P.C.s seen in monohulls just looks too scanty forward, and your gut instinct will tell you to widen the waterline, no matter what the numbers say. The traditional guidelines for prismatic coeffecients were worked out with much wider hulls proportionally than catamarans have. I sailed across the Pacific in a boat whose length divided by beam ratio was 2.5. I don't know what its PC was, but it was probably about .55 or .56. If I were designing a cruising monohull of the traditional proportions, I would go with that PC. I would guess that Evan, who is a naval architect, has no experience of designing catamarans, and doesn't realize that catamarans are normally given higher PCs than monohulls. How narrow are catamarans? The typical ocean kayak has about the same length / beam ratio as a small cruising catamaran. At 12 waterline beams to waterline length, the design on my site has quite a bit narrower hulls than a tippy high - performance ocean kayak.
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Old 17-03-2008, 10:43   #56
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Quote:
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G'day,

Happy to add my sixpence worth.

First thing I would do is enable the rudders to rotate 360 degrees. This takes 90% of the anguish out of sailing (and motoring) backwards. Getting in irons will be less of a drama and easier to get out of.


You might like to change the relationship between the sails and water foils. Cat boats and Wylie cats have the rig way forward. Maybe move both the rigs and the daggerboards forward. Move the anchors and other weights aft to compensate and replace the forebeam with a wire for the tramp, or remove the tramp alltogether, there is plenty of lounging space on the rest of the boat. Maybe increase the prismatic.

A more radical solution (and a better one, I think) would be to move the rudders. Harrys have no rocker. They are very hard to push sideways or turn when stationary. Yet, at only a few knots boatspeed, if you wiggle the helm, people who are not holding on will fall over. I have never tried to tack one in a seaway, but the response is so powerful that except in light air I use only one rudder for steering. The rudders are oversize (no other leeway resistance such as daggers or keels) and 25% in from the ends. I think this set up would work better for a boat which was hard to tack than the conventional daggers in the middle with the rudders down the back. From here it is a small step to save yourself some weight, space and money by switching to a single rig in the same hull. Could call it a harrycat!

regards,

Rob

Thanks for the input Rob. Rotating the rudders 360 degrees is not viable with helms, and underslung rudders, but maybe changing the balance of the rudders will help if it goes into irons.

The masts are quite far forward, at only 30% from the bows, compared to around 40-45% on most other cats. Daggerboards are roughly at the centre of the sailplan, without jib.
Anchor, winch, rode etc is only about 50 cms forward of the masts.

There is no forebeam, just a wire for the tramp. It is difficult to move the rudders further forward, due to the requirement of a cross-link that will fit under cockpit seating

Would it be wrong to conclude that I should try and get the prismatic up to around 0,62-0,64, and use bigger than normal rudders? (I will use an Akermann configuration for the rudders to aid tacking.)

"From here it is a small step to save yourself some weight, space and money by switching to a single rig in the same hull. Could call it a harrycat!"

Good one! I thought that Harry was your trademark! Rob, you of all people know how difficult it is for people to accept something different!
I'm taking it only a couple of small steps at a time, a forcockpit and an unstayed bi-rig for starters......
When I've flooded the market with these, the next step can be implemented

Regards

Alan
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Old 17-03-2008, 10:51   #57
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If the question is what is typical of cruising catamarans, I'd say a P.C. of .6 to .62 is typical. When it comes to hull waterline length divided by hull waterline beam-8 for smaller cruising cats and "roomarans," (the kind of boat where you can walk around a double berth,) up to 10 for large catamarans, and up to 11 for high performance cruising catamarans.

BigCat, I have a hullbeam/length of a bit more than 13. My saloon is further aft than "normal" as I have a forcockpit, so the main forward bulkhead is a bit further forward than normal, and will support the 2 unstayed masts.

Where did you find that Shuttleworth used a cp of 0,6 on a cruising cat, or rather what model was it?

I don't know what you mean by "K" factor, here in Europe it normally denotes a constant of some kind. Would you explain please. Thanks

Regards

Alan
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Old 17-03-2008, 10:58   #58
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K Factor, Shuttleworth and PC

"Where did you find that Shuttleworth used a cp of 0,6 on a cruising cat, or rather what model was it? I don't know what you mean by "K" factor, here in Europe it normally denotes a constant of some kind."
Hi, Alan - Here are links to answer your questions:
Southwinds - January 1999
Design of a 52ft. Aerorig catamaran.
(These and many other relevant links are to be found on my website.)
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Old 17-03-2008, 11:53   #59
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"Where did you find that Shuttleworth used a cp of 0,6 on a cruising cat, or rather what model was it? I don't know what you mean by "K" factor, here in Europe it normally denotes a constant of some kind."
Hi, Alan - Here are links to answer your questions:
Southwinds - January 1999
Design of a 52ft. Aerorig catamaran.
(These and many other relevant links are to be found on my website.)

Thanks Tim,
I calculated the "K-factor" for my boat using linear interpolation, and it came to 3.66. So if I understand this correctly, then my "hull speed" will be 3.66 x SQRTLwl = 3.66 x sqrt 46 = 24,8 knots??

Alan
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Old 17-03-2008, 12:17   #60
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The "K factor" clarified

Hi, Alan - The "K factor" is multiplied by the square root of the waterline. If you told us what your waterline length is, I don't recall. This would be your maximum theoretical speed, equivalent to 1.4 x the square root of the waterline for a traditional displacement monohull. So, about 3.33 x the square root of your waterline would give you Roberts' number for your maximum speed-ie. the speed at which your resistance becomes too great to exceed due to the creation of a huge bow and stern wave. The example given, 27', has a square root of @ 5.2, and this multiplied by the K factors on the table equals the maximum speeds given in the table. Unfortunately, his table jumps from a beam of 4.5 to 1 to 10.2 to one. It would be interesting to know what he predicted for 8 to 1, which is commonly found it your typical 44' cruising cat. The design on my web page has a 12 to 1 ratio, which has a "K factor" of 3.3, so its maximum predicted speed is 26 knots, or a bit more, as my hulls are actually a bit narrower than 12 to 1, and as I pick up a lot of added waterline with a bit more weight on board.
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