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Old 21-02-2008, 13:01   #286
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No rocker is not an option in a cruising boat

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. ************************************** Vessels like the Gunboat have rocker, but less than heavier vessels, because they don't need to find a place to put as much displacement. To have a cruising vessel of that hull form means either having very little accommodation, equipment, and "cargo" aboard, or spending huge amounts of money on high-tech construction involving epoxy, carbon fiber, and kevlar. ******************************I don't agree with the comment about bow overhangs-bow overhang is optional, and you will find many successful sailing boats with and without it. It has been eliminated in many designs in the quest for maximum waterline, so as to attain maximum speed. Boats with no bow overhang are usually drawn with narrow decks forward, as well, in order to reduce weight. The combination makes burying the lee bow a greater danger, as there is little volume forward to resist forces tending to depress the lee bow.***************************Sailing catamarans do have a hull speed, but it is higher than that of the typical monohull-if you don't make the hulls too deep in proportion to width, the narrower the waterline, the faster the hull speed. This has been called the "K Factor."****************************As has been mentioned, a boat with rocker is easier to turn, and one without it may fail to come about. However, other design considerations described above eliminate "no rocker" as an option, so the issue doesn't come up in vessels which have to carry accommodations, equipment, and passenger effects. *************************** As far as pounding when sailing to windward goes, a vessel with flatter forward sections will pound more than a vessel with v shaped forward sections. Contrary to what is often stated, distributing weights more forward and aft of the center of the boat may actually reduce pounding, by increasing the period of the pendulum effect and thus slowing pitching.
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Old 21-02-2008, 14:15   #287
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Even those who advocate 'scudding' in extreme conditions concede the increased risk of a pitchpole at high speeds, and the need to pay particular attention to the helm so that you are not heading directly into each oncoming sea. Increased velocity will increase the boats momentum and the force which the bows will exert down into the face of an oncoming wave. This will increase the risk of burying the bows, and because of the increased forward momentum of the boat, thereby increase the risk of pitchpoling over the bows into the oncoming wave.

The hull speed reference was originally raised by Robert in response to my query as to whether he believed that Robert Perry was wrong in his comments about the importance of rocker in coping with heavy seas. It should be noted that performance monohulls, even those designed by Perry, will also often exceed hull-speed - and by rather large margins if they are surfing down large waves. In the end result, I am still of the view that Perry's comments about the importance of rocker (when dealing with large seas and extreme conditions) is equally applicable to the hull designs of both monos and cats.

Indeed, I suspect the fact that most cats designed for offshore use (including your Catana and Atlantics and Outremers etc.) have distinct rocker rather than flat underbodies for and aft, is precisely because the hulls will have less likelihood of pounding in large seas.

Brad
Hang on, are you saying you think cats can pitchpole travelling INTO the waves (and wind)?

Pitchpoling happens when travelling in the same direction as the seas, when you surf down a wave fast enough to overtake it, and run into the back of the next wave.

A large amount of rocker won't help prevent this. Bob Oram's approach is to have the centre of bouyancy well forward - around the location of the mast beam, so when the hulls are planing the bows actually tend to LIFT rather than bury as a lot of cats tend to. It's noticable on his boats that the accomodation is more central, (where the bouyancy is) rather than being well aft as in most designs.

Getting back to rocker - more rocker = less resistance to pitching movement. This could actually result in MORE tendency to pound, because there will be a greater tendency for the bows to lift higher and more often, and return with more force. Again Bob Oram's approach is to have little rocker, and a deep forefoot, so there is less tendency for the bows to lift clear of the water. The deep forefoot and plumb bows provide greater bouyancy in the bows also.
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Old 21-02-2008, 14:39   #288
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Rocker is curvature in the profile of the boat. This is not the same as the distribution of volumes fore and aft, which is the prismatic coeffecient. PC is normally chosen to be optimum for the speed at which the boat is to be least resistant--small PCs are good for light airs, but not good for driving the boat at speed. There is not usually a numerical description of the topsides, but a lot of flam (usually miscalled flare) gives you more interior room and more reserve buoyancy in the area that has it. In a monohull, it also gives you more resistance to heeling. You get that in a multihull, as well, but to a much lesser extent. Some designers keep the waterline narrow but increase room and reserve buoyancy by making the transition from a bigger topsides to a narrower underwater hull with a chine or knuckle, but you can get a similar effect with less abrupt checking of pitching and increase in form resistance by doing the same without the dramatic sudden increase of volume of a chine.
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Old 21-02-2008, 14:46   #289
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[quote=44'cruisingcat;136750]

A large amount of rocker won't help prevent this. Bob Oram's approach is to have the centre of bouyancy well forward - around the location of the mast beam, so when the hulls are planing the bows actually tend to LIFT rather than bury as a lot of cats tend to. It's noticable on his boats that the accomodation is more central, (where the bouyancy is) rather than being well aft as in most designs.


Cats do not plane They are always in displacement mode! Whether the bows lift or not depends on many factors, such as wind direction, mast rake, sailcenter position, buoyancy centre and distribution.

Having alot buoyancy far forward (or aft) makes for a very uncomfortable ride in choppy conditions.

I prefer a solution where there is plenty of buoyancy forward when the bows are depressed, but the compromise here is that this also adds drag, which can be good when you need to slow down.

Regards

Alan
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Old 21-02-2008, 14:49   #290
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Simonis has his own plan re: bow burying

In an interview in Multihull magazine, Simonis indicated that he felt that moving the lcb aft, that is moving the immersed volume center of the hull aft, decreased the tendency to bury the bows. Essentially, he feels that the increased steepness of the buttocks aft which this causes leads to squatting when surfing down waves, and thus to the lifting of the bows. Traditionally, steepness of buttocks is associated with slower hull speeds, but he is discussing a specific situation, which is running at speed in a cat, in a situation in which you actually want slower speeds to avoid bow-burying. ******************************In the same interview, he also indicated that he felt that separating the lcb from the lcf reduces pitching. The lcf is the center of the volume of the waterline, whereas the lcb is the center of the boat's displacement. By decoupling them, you in effect cause a pitching boat to shift its center of buoyancy as it pitches, which would indeed dampen the pitching.
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Old 21-02-2008, 14:49   #291
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Rocker is curvature in the profile of the boat. This is not the same as the distribution of volumes fore and aft, which is the prismatic coeffecient.
I never said it was the same. Although a boat with less rocker is likely to have it's volume distributed more fore and aft, and less centrally isn't it?
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Old 21-02-2008, 15:13   #292
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"Although a boat with less rocker is likely to have it's (sic) volume distributed more fore and aft, and less centrally isn't it?" No, this is why I clarified the difference between rocker and the PC. It is probably hard to get a handle on this without actually making lines drawings and looking at the resulting statistics. I have made lines drawings, and then increased and decreased the rocker without affecting the PC. What changes is the displacement. Software makes it possible to look at these kinds of changes in a few moments, whereas it used to take a day's labor to check something like this. Software doesn't do anything to improve the designer's understanding of the ramifications of these changes, however. That still takes a lot of study and thought. It is actually possible to combine some of the Simonis approach and the Oram approach described above, as one is focused primarily on the area below the DWL, and the other is focused, I would think, primarily on the area above the DWL which gets immersed as the boat pitches. I prefer to keep the lcb in its traditional zone, which is at .52 to .56 aft of station 0, and increase beam in the topsides forward with flam. I do like to keep the LCB in the forward part its traditional zone, to maximize volume forward. Keeping the LCB in its traditonal zone maximizes speed potential, however, so I prefer not to vary from the usual in this regard.
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Old 21-02-2008, 15:14   #293
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I never referred to pitchpoling while sailing to windward, but rather in the context of 'scudding' or 'surfing down large waves'. I mistakenly thought that this would denote sailing downwind to most people. I am prepared, however, to accept your criticism of my careless use of terminology. Indeed, as I look at it now I can see that those who do not know the term 'scudding' (as used by Moitessier and others) might somehow think that it refers to sailing into the wind/seas in storm conditions.

In the context of pitchpoling, when I referred to burying the bow into an 'oncoming sea' or wave, I was referring to the situation which occurs when you scud (or surf) down one wave into the trough and then the backside of the next wave. It is 'oncoming ' only in the sense that you are travelling towards it - and I apologize for the less than clinical use of the term oncoming.

As to whether rocker reduces pounding, I gather you are of the view that itactually increases pounding and that Bob Perry's opinion is diametrically opposed to realilty. You are entitled to that opinion, but doesn't it make you wonder why rocker is employed in the hull designs of virtually all offshore sailboats? Surely it would be more expensive to build (or in the case of frp, to make molds) than for a relatively flat bottom. Since it is all negative, in your opinion, in terms of both performance and motion it seems odd indeed.

In any case, while I can understand the benefits in performance of no (or virtually no) rocker in relatively calm conditions, I would still prefer some rocker in the hulls of any boat I purchase that can be expected to experience heavy seas. I suspect that as Perry suggests, the fore/aft curvature will keep more of the hull in contact with a wave than a flat shape (precisely because the bow is already raised somewhat at the entry in relation to oncoming seas). Regardless, I believe that Perry and varioius other naval architects who employ rocker do so because they believe it works. If they are wrong, then at least I am in good company.

Brad
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Old 21-02-2008, 17:10   #294
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Thanks Big Cat for the imput. It strikes me that we are left with something like this:

1. Rocker improves the ability of the boat to tack, and reduces the risk of getting put into irons. No small thing in a catamaran.

2. Rocker aft (leading to a raised transom) reduces turbulence and therewith drag, in the average cruising cat.

3. Rocker permits the prop/propshaft to be elevated above the maximum draught of the hulls, increasing protection.

4. Rocker may, if Bob Perry is correct, provide for a more sea-kindly motion (less pounding) in heavy seas by use of a shape that orchestrates contact between the waves and the hull.

5. Rocker increases the volume of the hulls towards the center, precisely where it is most needed for accomodation and weight carrying ability.

6. Rocker increases the tendancy of a cat to hobbyhorse. However, it is far from the being the only (or even a determinative) factor. Indeed, hobbyhorsing tends to be more of a problem in cats than monohulls, even though the latter tend to have more depth to their hulls below the waterline and hence more rocker (if they are to have elevated transoms and a relatively fine entry).

7. Factors such as lcb and lcf will also effect the tendancy to hobbyhorse.

8. Question: Would not the location of the mast also have an impact? It strikes me that a mast well forward with a large main and relatively small jib would tend to move the center of effort in most boats forward of the center of bouyancy. Isn't this at least part of what Bob Oram is attempting to counteract in his designs? It strikes me that, rather than moving the center of bouyancy forward, the same could be accomplished with a more balanced rig - with the mast further aft.

In any case, it still strikes me that in the context of a cruising cat designed for a circumnavigation (the topic of this thread), some rocker is still a very good idea. And thanks again to Big Cat for providing some additional fuel on the fire that I originally (and rather carelessly) lit.

Brad
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Old 21-02-2008, 18:08   #295
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Catamarans and rocker, again

Hi, Brad Hi, Brad-I'd like to read Perry's exact words. I have a copy of "Sailor's Secrets," but don't find either "Perry" or "rocker" in the index. Where is the quote? ****** ***************** ************** *************** ************ As far a rocker goes, I'd say that, as far as I know, nobody designs around the concept of rocker. You input other things, and rocker is the result of displacement, which is in turn a function of construction and payload. I, too have managed to keep rocker to 2' in a 65' boat, (it was mentioned that this was the rocker of a 50 odd foot Gunboat,) using slightly above average materials (vinylester vs. polyester, and quadra-axial unidirectional e-glass vs. mat and/or rovings, and using balsa for the topsides and deck core,) by virtue of keeping the accommodations lean for the boat length. This was not the result of seeking a particular rocker, however, but rather was the result of the accumulation of other, perfectly conventional, decisions. I see no reason why rocker should have any effect on the tendency to hobby-horse. A low PC combined with narrow topsides forward might, but this is not to be confused with rocker. I'd say a lack of buoyancy forward would probably be the biggest culprit, and having your weights distributed in such a way as to have a pendulum period consonant with the wave period you are experiencing would be a problem, too. Keeping your weights as centered as possible would tend to help with this, as it would keep your pendulum period shorter than most wave lengths. On a cruising boat, however, most weights location are chosen by convenience. Dingy and dingy motor location, engine location, and anchoring gear location tend to be determined by convenience. If you are willing to devote more of your boat to engine room than some production boats, you can move your (inboard diesel) engines somewhat forward and reap the reward of having a conventional drive train rather than a saildrive, as well as getting your engine weights closer to the center of your boat.******** ******** ******** ******** ********The problem with keeping the mast close to the center of a catamaran is that this is not usually the strongest or most convenient place for it. I doubt if mast location has a big influence on pitching, though I think it has a bigger influence on the tendency to stuff the bows under. Most people assume that a mast will have jibs in front of it, and as such, varying mast placement will not necessarily change the location of the sail centers very much. Keeping the center of the sail area as far aft as possible should help reduce bow stuffing, as will keeping the masts as low as possible. Selecting the biplane rig configuration helps with both of these goals, but such unconventional thinking is simply too intimidating for most people.********* ******** ******** ******** ******** Likewise, most voyaging design goals are furthered by making boats long and light, but once again, production boats are not designed around this philosophy, and economics, fear of the unconventional, and the desire to avoid getting involved in custom/ home boat building keeps most away from the kind of boat that almost any yacht designer will espouse if given half a chance. If you are planning on making a custom or home built boat, only lack of imagination stands between the owner and a superior voyaging boat, as long and light need cost no more and take no longer to build than short and massive to build in a one-off situation.
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Old 21-02-2008, 18:13   #296
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Fossil-Fuel-Free

I'm a brand new member of the group and have enjoyed reading some of the discussions. What a great asset this forum is!

Within the next year I'll be purchasing a cat, with a goal of making it fossil-fuel-free. Just wondering if anyone else has gone this route and what the experience has been.

Thanks in advance!

Tom
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Old 21-02-2008, 22:26   #297
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I never referred to pitchpoling while sailing to windward, but rather in the context of 'scudding' or 'surfing down large waves'. I mistakenly thought that this would denote sailing downwind to most people. I am prepared, however, to accept your criticism of my careless use of terminology. Indeed, as I look at it now I can see that those who do not know the term 'scudding' (as used by Moitessier and others) might somehow think that it refers to sailing into the wind/seas in storm conditions.

In the context of pitchpoling, when I referred to burying the bow into an 'oncoming sea' or wave, I was referring to the situation which occurs when you scud (or surf) down one wave into the trough and then the backside of the next wave. It is 'oncoming ' only in the sense that you are travelling towards it - and I apologize for the less than clinical use of the term oncoming.

As to whether rocker reduces pounding, I gather you are of the view that itactually increases pounding and that Bob Perry's opinion is diametrically opposed to realilty. You are entitled to that opinion, but doesn't it make you wonder why rocker is employed in the hull designs of virtually all offshore sailboats? Surely it would be more expensive to build (or in the case of frp, to make molds) than for a relatively flat bottom. Since it is all negative, in your opinion, in terms of both performance and motion it seems odd indeed.

In any case, while I can understand the benefits in performance of no (or virtually no) rocker in relatively calm conditions, I would still prefer some rocker in the hulls of any boat I purchase that can be expected to experience heavy seas. I suspect that as Perry suggests, the fore/aft curvature will keep more of the hull in contact with a wave than a flat shape (precisely because the bow is already raised somewhat at the entry in relation to oncoming seas). Regardless, I believe that Perry and varioius other naval architects who employ rocker do so because they believe it works. If they are wrong, then at least I am in good company.

Brad
Which cats does Bob Perry design again?
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Old 21-02-2008, 23:06   #298
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Bob Perry

I suppose that inasmuch as rocker is the opposite of flatness of the sections, it would reduce pounding. Perry is noted for designing medium displacement cruising monohulls, and in the absence of more knowledge, I would guess that this was probably in the context of defending that type of boat in contrast to racing types, which tend to be flatter in sections. As far as I know, he has never designed a stock catamaran. This catamaran was designed by Perry: Sailing Magazine : Perry on Design . There are Perry brand catamarans made in Australia, not to be confused with boats designed by Robert (Bob) Perry of Seattle, whose reviews are at Sailing Magazine
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Old 21-02-2008, 23:13   #299
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Actually Perry is probably right - more rocker will very likely result in less pounding, going to windward. The increased hobbyhorsing would reduce the efficiency of the sails, and so the boatspeed, resulting in reduced pounding.

I have actually experienced more vs less rocker in boats that are otherwise fairly similar - similar weights (both quite light boats) similar lengths - a 40' Chamberlin design (more rocker) and a 38' Oram (less rocker).

Certainly the Oram was noisier in similar conditions (windward in 15 - 25 kts ) but also quite a bit faster. (even though it had a smaller rig and sailplan) I feel if we had slowed the Oram to about the same speed it would have been similarly quiet.

IMHO the noise on the Oram was less troubling than the pitching and occasional resultant sail slamming of the Chamberlin. And as I said, we did have the option of slowing the Oram down. We couldn't have sailed the Chamberlin much faster though.
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Old 21-02-2008, 23:45   #300
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I'm a brand new member of the group and have enjoyed reading some of the discussions. What a great asset this forum is!

Within the next year I'll be purchasing a cat, with a goal of making it fossil-fuel-free. Just wondering if anyone else has gone this route and what the experience has been.

Thanks in advance!

Tom
Hallo Tom

We Produce all our cats with the Green Motion system.
This system has 2 retractable electric motors/generators that propel the yacht and regenerate the batteries when sailing.
The system is not fossil free yet since we carry an emergency generator on board that needs to be used with long periods without wind, we also integrate up to 1600 watt of solar power and one masthead mounted wind generator. Cooking is done electrically with induction cooking.
We are at the end of the testing of the system in our testing basin in Durban South Africa and the first FastCat 455 is being built and will be launched in June this year.
If you want more info on the system go to the vendors forum @
http://www.cruisersforum.com/forums/...ion-10261.html or mail us at info@africancats.com
greetings
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