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Old 22-02-2008, 07:41   #301
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44'cruisingcat, Bob Perry does not design catamarans, but as catamarans are also displacement hulls, it strikes me that the same prinicples should apply - certainly at lower speeds. Big Cat, I agree with respect to the importance of keeping weight out of the ends to avoid hobby-horsing ( although I was criticized for suggesting the same a few posts back). As to the quote from Perry - I don't have the book here, but as I recall it was with reference to his design for an offshore monohull - a 'go everywhere boat', or 'go anywhere boat'. There is also a drawing of a mid 50 foot monohull in the section I am referring to. Hope that helps. If not, I'll dig the book up over the weekend and find the precise page.

Perry certainly suggests including rocker as a design objective for an offshore boat, and not merely as a byproduct of other design features (although in the end result, I'm not sure how much it matters).

And as to a bi-plane rig, I can see the advantages that you extol. My only concerns are:
1. increased weight/expense with 2 rigs.
2. the increased work involved in trimming two rigs (albeit, as in a ketch, the two sails wouled be smaller in area than one large cat rigged main and hence much easier to handle).
3. the performance to windward. I know from some experience sailing a Nonsuch (with an unstayed cat rig) that, while it performed better to windward than many expect, it nevertheless was not able to point as high as a comparable marconi rig. On the other hand, I suspect that the slot between the two sails would go a long way to improving that.

Are you actually sailing a bi-plane rig right now, or are you still in the design stage?

Brad
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Old 22-02-2008, 11:54   #302
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Biplane rig, rocker

Hi, Brad I'm still in the design stage, so my comments are based on the observations of those who have used the biplane rig. Biplane rigs are self-tacking, and so less effort to trim than marconi rigs. The weight may be more, but the weights are lower, as the rig is lower. Windward ability is not compromised, as the wind from each sail is not disturbed close hauled by the other. It may not be as fast on a beam reach, as you have to have to let out the windward sail and trim in the windward sail to compensate for interference of one sail with another on that point of sail. IMHO, when you have two sails, you have interference on some point of sail, so the question is just where you will have it. The rig I have designed will be much cheaper than a marconi rig, because it won't have all the expensive goodies required for a marconi. See my website for a full description.*************************** I have indeed said that weights forward can either increase or lessen pitching. That is because pitching depends on the wave period, which varies. All of this talk about rocker, as though it were an optional variable, is written by people who have never made a lines drawing. The only way to increase rocker on a design, once you have designed a reasonable bow and stern, and work with a given displacement, is to narrow and deepen the hull past the optimum width and depth, and any slowness that results is the result of a deeper hull than optimum. Such a design would also have higher wetted surface than necessary. John Shuttleworth established this optimum depth / beam ratio in tank testing.
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Old 22-02-2008, 12:11   #303
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Actually Perry has and will design a cat for clients.
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Old 22-02-2008, 12:38   #304
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I found the Perry quote. As I suspected, it was in the context of his typical sailing monohull, and it was in defense of heavy boats-boats with a displacement / length ratio of 250 to 300. Such a boat will necessarily have a lot of rocker unless it is very, very beamy. As I have been saying, once you have made you main choices about beam, length, bow and stern shape, etc., rocker is a function of displacement. By the standards Perry is describing, no multihull has much rocker.
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Old 22-02-2008, 13:57   #305
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Big Cat, I agree totally - no multi would ever need the rocker in Perry's design (and indeed, by splitting displacement between two hulls, even a relatively heavy displacement cat would not need equivalent rocker). Nevertheless, his design has more rocker than would be required even with the displacement/length ratio of that boat. What interested me were his comments about the importance of rocker in avoiding pounding and in creating a comfortable motion in heavy seas. It strikes me that the same would be applicable to catamaran hulls in like conditions.

I'm going to check out your site right now. Any thoughts as to when you may get construction underway? I must say that I have always dreamed of designing (or at least, of working with a naval architect in designing) my own boat. Clearly the only way to get what YOU want, as opposed to what others think that buyers that meet your profile want.

Two self-tacking sails would be a huge advantage for shorthanded sailing and it strikes me that when running, or even on broad reaches it would be leave both sails with pretty clean air.

Brad
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Old 22-02-2008, 14:16   #306
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Rocker, BiPlane rigs

Hi, Brad I am building this boat under USCG supervision, so I can't start until their plan review is complete. So far, they have been reviewing it for 3 1/2 months! **** **** **** **** **** Perry's rocker is a given, once he has chosen his beam, the beam at the waterline at the stern, his waterline length, his prismatic coeffecient, and his displacement. Water weighs 64 pounds per cubic foot, and you must displace your weight in water volume. This means that your other choices govern your rocker, as you must put those cubic feet somewhere, and the only place left is in rocker. **** **** **** **** **** **** You can read Schionning's remarks about sailing on his Radical Bay design with a biplane, aka twin rig, at smartyachting .
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Old 22-02-2008, 16:09   #307
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Thanks BigCat, I will check it out. Very interesting design - and I certainly agree with the notion of eliminating any under bridgedeck protrusions in order to increase accomodation as they will inevitably slam. In any case, more than enough room without.

How far can the wing-sails be reefed down? Any concerns about being overpowered in heavy air (or at anchor)?

Brad
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Old 22-02-2008, 16:44   #308
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Wingsail rig, and "little' big boats

Hi, Brad The masts are round, and taper to a small size at the top, so no concerns about being overwhelmed when reefed or furled. I sailed a junk schooner, which has low, round masts, and found that it had less windage than a taller sloop rig. Each batten is a reef point, and is a reef point which requires no no struggle and no ties to accomplish. Slack the halyard, and down it comes. Pull in the surplus sheet, and off you sail. (Surplus because the sheet goes up the leach, and slacks as you reef or furl the sail.) This rig is an evolution of the junk rig, with 2 changes to the old junk rig to make it better to windward-1) a sleeve that is wing shaped that covers the mast when the sail is up, and 2) a hinge that allows about 14 degrees of freedom, 7 degrees to port and the same to starboard, to give camber to the sail. I sailed a junk rigged boat across the Pacific, and can tell you that it has no match for reefing, furling, or self-tacking. I did this after sailing 5000 miles on a standard fin keel sloop, and the improvement in handling when going to the junk rig was amazing.**** **** **** **** **** **** The no shelf thing requires a longer boat, to get the same accommodation, but that gives you a faster and more seaworthy boat for the same dollar figure. The only downside here is if you want to keep it in a marina. This is the kind of trade off that always appeals to yacht designers, but which they usually have trouble selling to people seeking more conventional ideas.
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Old 22-02-2008, 18:10   #309
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Robert Perry reviewing his own design "Perry Cat" in Sailing Magazine's feature: Perry on Design. He details the effects of design parameters on hull volume and rocker.

"The biggest challenge with this design was the deck layout. Paul wanted to be able to walk across the stern of the cat while fishing. This required a solid deck aft spanning both hulls rather than the typical deck cutaway between the hulls where you can tuck away a dinghy. From a design standpoint the biggest problem with this type of deck is that it moves the center of gravity aft and big cats already have a tendency to trim by the stern. I added hull volume and flattened the hull rocker aft to help deal with this. Transom rake was reduced to expand available deck space and help with the fishing requirements. With the mainsheet and boom removed, the entire aft end of the boat can be clear for fly fishing. The entire cockpit is on main deck level."
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Old 22-02-2008, 18:35   #310
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The LCB and Perry's catamaran

When you move the center of the weights aft, you must move the center of buoyancy (LCB) aft so that the boat will sit on its lines when loaded. This would have the effect of moving the center of the rocker farther aft, making the run of the buttocks steeper, and making the run from the bow shallower. This is the kind of hull shape that Simonis was describing in my earlier post on this thread. Perry's way of describing hull shapes is pretty distinctive, and not one that would naturally occur to me. Traditionally, rocker is a term that has been used to describe differences in small, traditional open boats from the 19th. century on the East coast. It is little used in yacht design, and in fact it is not found in the indexes of any of my yacht design text books. Hull shape is much more exactly defined with other information that is in common use, such as the PC and LCB.
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Old 22-02-2008, 20:47   #311
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When I was looking at a boat, I read similar threads and in the end, I got so confused that I put it all aside for a while to clear my thoughts. I am not a multi hull sailor, but the arguments that have been made in the forum are similar in any situation for choosing a boat and asking for people's opinion. Looking at your earliest post, you want a combination of characteristics that seem to exist only in boats that are out of your budget. You want to spend up to 400K, preferably it seems on a new or newer type boat and so the options are more limited. Statistically, you would have more chance finding the boat that is closest to your requirements amongst the extensive fleet of production boats that come out of charter. Like cars, they have depreciated and you will get a good value right now, as the market seems to be down and therefore prices will be depressed. Go for something that meets your must-have characteristics and accept some of the disadvantages that the preferred boat may have. You always will have to make some compromise in some areas, unless you have so much money to spend that you can ask a designer to draw you a custom design and have it built by the best in the industry.

I don't know how much time you have on your hands, some people say circumnavigation but what they really want is sail in a number of nice places. In that case, you could consider something like Moorings where you can sail in many places in the world, up to 6 or 8 weeks I believe, depending on the type of program. Even including air travel to the various places, you probably will spend less money than having yr own boat. And you don't have to worry about running into something on the high seas, or fast performance for that matter. A colleague of mine told me that according to some research done in France, the majority of the boats owners spend less than 2 weeks on their boat during a season.

All the best!
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Old 23-02-2008, 22:55   #312
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Rocker is not necessary in a cruising boat

[quote=BigCat;136718]If you draw a lines drawing of a sailing hull form that is not a dingy, give it a counter that is reasonably lifted out of the water to reduce drag at speed, and give it normal proportions (prismatic coeffecient and beam,) and normal displacement, you end up with rocker. A boat with no rocker is either an ultralight planing design with a very wide stern, or a planing powerboat. If you look at a number of lines drawings, you will see that this is so. If you immerse the bow profile in an effort to avoid rocker, you end up with a danger of bow steering when running fast. If you immerse the stern to avoid rocker, you end up with a fine stern that is prone to pitching and which is slow when sailing at speed, or you end up with an immersed transom which is a benefit only when motoring at planing speeds, but which has a huge resistance and wake at lower speeds. **************************************
The Harryproas can sail very nicely with no rocker. Check out the You tube video of Visionarry doing 15 in 15. There is a small compromise in making the ends fine enough for minimising drag at speed, but the no rocker give optimum carrying capacity for the draught. It is also an extremely comfortable boat in heavy chop, with no signs of pounding or hobbyhorsing. There is no sign of difficulty in steering, and, as they do not need to tack, there is no problem of getting caught in irons. The boats go though breaking waves over bars very well, but prudence is required coming in over bars as in any boat.
Robert
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Old 23-02-2008, 23:14   #313
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Brad.[ I still fail to see how, with front overhang, the bows meet the chop 'all at once'. Rather, it strikes me that as the bows are depressed, there is a gradual increase in bouyancy in at least two dimensions (three if there is also some outward flare/knuckles at the bows).

>>Rob Consider the shape of a wave moving towards a bow. With overhang, the bow angle is closer to the angle of the front of the wave and therefore the time of impulse is short and the direction of reaction is to throw the bow into the air, especially if there is siginificant flare. If you have plumb or even reversed bows, the bottom of the bow meets the wave first, giving a slow and gentle rise.

Brad. And while I agree that rocker contributes to hobbyhorsing, we are not talking about a dramatic amount in a catamaran (although 44' cruising cat underestimates the rocker on the Gunboat as it must, in all but static states, include the rise of the transoms above the waterline).

>> Rob. Agree for most fast cats

Brad. In any case, lets face it the designs of all boats are a compromise. I suspect that the existence of rocker on most offshore cruising cats was specified as a compromise between reduced pounding/reduced drag through an elevated transom, and an increased tendancy to hobbyhorse.

>>Rob. I feel the pounding question has been answered quite well regarding the Oram cat being a bit noisier but probably only because it was going faster to windward than the equivalent cat with more rocker and greater tendency to hobbyhorse
If you get round the need for tacking but needing a double ended boat, then one looks at the boat quite differently.
If I was to go for a cat, I would have some rocker with a sugarscoop transom for ease of getting out of the water, sitting just at the waterline or a shade below. As the boat drives forward there is a tendency to depress the bows, leaving a little to play with before dragging, and allows one to sleep at night without the slop slapping under the transom when at anchor,
Cheers,
Robert
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Old 24-02-2008, 00:22   #314
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Robert, the problem with a sugarscoop transom a shade below the water line (apart from increased turbulence with moving forward), is the tendancy to bury the transoms when moving in reverse. No, you aren't going to actually 'bury' them under normal powering maneuvers, but you can generate a ton of spray!
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Old 26-02-2008, 06:36   #315
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Characteristics of a circumnavigating cat

Gentlemen please help and give an opinion of this cat for world cruising. I have searched far and wide and i believe it is a great design. This thread has truly been a huge help. I am able to build this cat in South Africa at approx. 120 000 US dollar. It means to me a new boat (built to designers specs) with new fittings and that i dont have to worry about the unknown aspects of a used / abused boat.

The idea is not to have a floating palace - but a reasonable boat - well finished - that will be safe in moderate climates / weather and that could handle the unforseen storm.
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