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Old 16-02-2008, 05:27   #271
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Old 16-02-2008, 06:09   #272
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No, or minimal rocker is generally considered a very bad idea for a blue-water boat: it will increase the tendancy to pound in heavy seas. Robert Perry has this to say about designing a hull for heavy weather: " I have come to see that pounding is not so much a question of U-sections versus V-sections, as it is a question of rocker. Rocker orchestrates contact between the hull and the waves." (Robby Robertson and Michael Badham, Sailors' Secrets Camden Maine, International Marine, 1999 at p. 192).

Yes, Robert Perry is a designer of monohulls, but the principles of hydrodynamics remain the same. One can eliminate hobby-horsing by keeping the weight out of the ends, but still retain a hull that will not pound through the use of adequate rocker.

Brad
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Old 16-02-2008, 06:36   #273
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Rocker

I'm not a designer and I'm sure I don't know all the pros and cons to rocker/no rocker - but I have done a lot of surfing in small cats and based on my experience, ample rocker is vital in surfing conditions. I expect the same would apply to cruising cats surfing in high seas.

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Old 16-02-2008, 07:24   #274
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You're right Dave, same principal ( although it is also critical going to weather into heavy seas).

Brad
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Old 17-02-2008, 03:30   #275
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Hi Robert.

Do you have a particular design in mind with your list? How long are you forseeing it?
I've looked at many different cats, but can't think of one that would get a pass on your list.

I have a couple of questions for you:

---Quote (Originally by Robertcateran)---
Long lee bow with high prismatic coefficient and minimal or no rocker to avoid hobbyhorsing and pitchpoling.
---End Quote---
Can you please explain what you mean.
>>When the boat is powering up, the lee bow starts to depress. Therefore I would like the leebow to be well forward to prevent going over. Simply having a flare does not help a great deal as the sudden increase in width increases the resistance and exacerbates the torque leading to pitchpoling. Some of the early Crowther boats ended up putting on bow bulbs to reduce this effect after Twiggy pitchpoled in an around Britain event.
High prismatic coefficient and little or no rocker reduces the chance of a boat rocking backwards and forwards when going through large chop. If you have a skinny bow below the waterline and flare and overhang above, when you go into chop the bow rise quickly and then rocks back again. If you look at some of the latest Nacras, they have reverse of this. Wider at the bottom than the top to help slice through waves and make for a more stable ride. The idea is for the bows to slice through the waves, relying on hydrostatic lift to help them rise again, and not to wide at the top to alow them to shed water. Consider how often Hobie 16s have done some spectacular cartwheels compared with modern Nacras.


---Quote (Originally by Robertcateran)---
Able to stop and reverse under sail. I know this is a strange one but it is possible and very useful;
---End Quote---
Why do you think this would be very useful? I've sailed quite far without needing to sail backwards. Stopping is easy though: Just drop your sails.
The rudders wouldn't exactly be able to perform miracles for you if you're sailing backwards.
>>If you have ever had to pick up things from the water, such as a hat or a careless passenger, it is nice to be able to simply reverse back and pick them up, with fine control to adjust for leeway. When I was doing my commercial ticket and was doing my man overboard drill on a small trawler, I found it very difficult and would have killed the poor sod if hea was n’t a dummy. In my small tri, I could reverse and could easily pick up someone’s hat if blown overboard.. Crossing a bar under sail, it is nice to be able to organise the boat to be able to hang back and wait for the right moment, or going into a narrow are without enough room to spin around. Going into a beach or a rocky headland to drop someone off, just reverse straight back out again when finished, and you have less problem in going beam on to the waves. Stopping for most sailing boats usually requires spinning around and going into irons. I am a marine botanist and I would use such a facility in my work. Commercial boats rely too much on motors for my aesthetics, but reversing was very much needed in my years on fishing boats.
This brings the concept of a double ended boat, with reversible rudders, asymmetric with one long hull to be the lw hull and a shorter fatter hull for the ww, with the stores and accommodation in the ww hull, maximising righting ability. The unstayed rig is in the leeward hull and sheets lead back to the ww hull, thus in an accidental jib the rig weathercocks The fore and aft reversible rudders allow the boat to crab allowing for very fine adjustment in picking things up in a surface drift.
The configuration reduces many of the rigging loads on the vessel compared with a stayed rig on a well braced crossbeam, and allows the boats to be a fair bit lighter and cheaper, in spite of the judicious use of carbon.


---Quote (Originally by Robertcateran)---
Float sufficiently high upside down to have a dry area for sleeping inside. It assumes that the escape hatch is above both the upside down and the right way up waterline;
---End Quote---
I'd think very few (possibly no) cats would be habitable on the inside after a capsize, unless maybe if they were very big.
>>I am not saying comfortable but enough dry space to lie down. Put inflatable buoyancy in the roof lining and it is a bit easier, and you could set up some washboards for the upside down configuration and pump out. I am not saying it would be easy, but doable.
There is such a boat on the market and for the price. Unfortunately I don’t have the money to buy and will have to build when I finally get sufficiently clear financially.
Check out Harryproa .com and there are some video-clips on Utube, if you want to see some pictures of them sailing. Once you get your head around them, they make sense.
I don’t have any commercial interest in them except to want to get one.
Thanks for the interest
Robert
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Old 17-02-2008, 03:51   #276
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Old designs used to incorporate a lot of rocker to help them tack better. Modern boats tack better because they sail faster = more momentum to get them through the tack.

More rocker causes more hobbyhorseing, which reduces speed to windward. modern boats have little rocker. ie - The Gunboat 62 draws only 2 feet - so max 2 feet of rocker over a 60 foot waterline.

Hull pounding is less of an issue because of how fine cat hulls are, especially in the bows.
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Old 17-02-2008, 03:52   #277
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You're right Dave, same principal ( although it is also critical going to weather into heavy seas).

Brad
There is a world of difference between surfing down a beach wave and an open ocean wave. Look at how much rocker the racing round the world multihulls have.
The most comfortable work boat in my experience was a powercat that had almost no rocker and a 50cm bow bulb. It worked on the shark fisheries in Bass Strait and was generally out working in about 5-10knots more than anyone else. A boat designed to sail at monohull speeds is a very different concept than one slicing through its bow wave. How does the rocker match the shape of the waves when on top of the crest? Tacking boats need rocker to avoid dragging too much while coming about and going into irons, but a non-tacking boat doesn't. Get your hydrostaic lift early and smoothly and allow the bow to shed water and there is much smoother motion compare with bows that bounce off the wave
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Old 17-02-2008, 07:12   #278
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Thanks Robert for the comprehensive and lucid description of the theory behind your designs. My point about rocker (or more importantly Robert Perry's), was that it reduces pounding in heavy seas, not that it improves performance (or reduces drag). In your opinion, is Perry wrong about this?

I fully understand that current designs for offshore racing multihulls (and indeed for monos - witness the offshore 60's) have very flat bottoms with little or no rocker. That being said, they are notorious for pounding if forced to sail upwind into heavy seas. You may recall the Vendee Globe several years ago, where an Italian entrant sailed upwind for about 50 miles into heavy seas in the Southern Ocean in order to rescue a fellow entrant who had capsized. The race organizers worried whether the (relatively) flat bottom on his boat would even allow him to complete that short passage in one piece (albeit in terrible seas) and he confirmed that the pounding was sensational!

Hobby-horsing is undesireable for a number of reasons and I can certainly see that extreme rocker would exacerbate the situation. Nevertheless, I had always understood that weight in the ends was the principle enemy and that flare above the waterline (and/or knuckles) increase the bouyancy of the bows and therefore reduce the effect. Carrying your maximum beam below the waterline would, of course, ensure that there is less of an increase in drag when the bows are depressed - but surely it increases wetted surface and therefore drag when sailing with the bows on their lines.

This, of course, is to say nothing of the resulting shape of the hulls which tends to put maximum beam precisely where it is not needed in a cruising cat: the shape of the human body dictates that more space is needed higher rather than lower. Furthermore, as you have already fairly pointed out, rocker assists in tacking - no small thing in a cruising cat.

I can also see where overhangs would tend initially to inrease the tendancy of a boat to hobbyhorse - they put some additional weight forward (and aft) of the static waterline. On the other hand, in the bows they tend to make for a drier deck and, as depressed, operate so as to increase bouyancy. In the stern the case for rear overhang seems overwhelming - they keep the transom out of the water in most circumstances and thereby dramatically reduce turbulence. I haven't seen your designs, but if they have transom sterns, I assume they also have some rear overhang. If so, I suspect that you accepted this compromise in terms of hobbyhorsing because of the advantages inherent in having them.

I haven't sailed a Nacra, but a look at the photos certainly suggests that they also have significantly more freeboard than a Hobie - surely this too dramatically reduces the chances of burying a bow. While on this topic, modern cruising cats typically have a great deal of (some say excessive) freeboard. It strikes me that the greatest risk for a cruising cat is not in burying the leeward bow while sailing to windward, but rather in burying both bows when surfing down waves in extreme conditions. In this case the differential in drag between leeward and windward bows is not an issue, and flare (and yes overhang and knuckles) above the waterline would all tend to increase bouyancy as the bows are depressed into the oncoming seas. Or am I missing something?

In any case, an interesting debate. While I haven't seen your designs they sound incredibly fascinating and fast, if perhaps a little less comodious that most cruising cats. I am always impressed by somebody who is prepared to shake off conventional wisdom and push the envelope in terms of design, and it would seem that you have developed some ardent followers - and no doubt deservedly.

Cheers to you as well!

Brad
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Old 17-02-2008, 09:54   #279
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Quote:
Originally Posted by Robertcateran View Post
There is a world of difference between surfing down a beach wave and an open ocean wave.
Hi Robert - My sense is that there really isn't much difference. A wave is a wave. If it's steep enough to surf it seems the only difference may be size.

Brad said:
Quote:
I haven't sailed a Nacra, but a look at the photos certainly suggests that they also have significantly more freeboard than a Hobie - surely this too dramatically reduces the chances of burying a bow.
When it comes to pitch poling, the classic Hobie hulls (vintage 14s and 16s) with their extreme rocker are WAY more sensitive to pitching than other small cats like Nacras with very little rocker. But this is limited to what I'll call powered pitching - pitching because of the power generated in the rig. In this case, the extreme rocker of the Hobie design allows the boat to literally rock forward when the resultant force generated by the rig way up the mast acts like a long moment arm rotating the whole boat forward resulting in sticking the bows into the water. The bows stick and slow the boat and momentum and continuing rig power cartwheels the whole thing. Believe me, I've done it numerous times.

Where the extreme rocker pays off, however, is surfing. Surfing is gravity induced and speeds up the boat and effectively takes over as the motive force vs rig power (or drastically reduces it). Gone is the force acting on the long moment arm tending to rotate the boat forward. I sailed H16s in and out of the surf for many, many years and never once pitch poled while surfing. Never. The extreme rocker enables this. In comparison, Nacras are very poor surf boats and stuff their bows at the bottom of the waves with potentially disasterous effects. Two different designs with two different purposes in mind. Each performs very, very well in their intended environment.

Just my humble opinion.

Dave
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Old 18-02-2008, 01:27   #280
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Hi Robert - My sense is that there really isn't much difference. A wave is a wave. If it's steep enough to surf it seems the only difference may be size.

Brad said:


When it comes to pitch poling, the classic Hobie hulls (vintage 14s and 16s) with their extreme rocker are WAY more sensitive to pitching than other small cats like Nacras with very little rocker. But this is limited to what I'll call powered pitching - pitching because of the power generated in the rig. In this case, the extreme rocker of the Hobie design allows the boat to literally rock forward when the resultant force generated by the rig way up the mast acts like a long moment arm rotating the whole boat forward resulting in sticking the bows into the water. The bows stick and slow the boat and momentum and continuing rig power cartwheels the whole thing. Believe me, I've done it numerous times.

Where the extreme rocker pays off, however, is surfing. Surfing is gravity induced and speeds up the boat and effectively takes over as the motive force vs rig power (or drastically reduces it). Gone is the force acting on the long moment arm tending to rotate the boat forward. I sailed H16s in and out of the surf for many, many years and never once pitch poled while surfing. Never. The extreme rocker enables this. In comparison, Nacras are very poor surf boats and stuff their bows at the bottom of the waves with potentially disasterous effects. Two different designs with two different purposes in mind. Each performs very, very well in their intended environment.

Just my humble opinion.

Dave
It is certainly a buzz sufing small cats and Hobies certainly do that well and Nacras don't. I still hold that there is a difference between beach waves and breaking waves in deep water. A storm wave on the open sea is a very different type of wave compared with a wave steepening as it comes into shallows. The storm waves are not as steep and are breaking because of the wind pushing the tops over. They are more foam than hard waves. consider surfing in onshore conditions compared with the wind offshore. Out at sea the white water seldom gets far down the wave and the swell is still moving fast. The wind will be pretty strong in those conditions and so the wind lever arm is still there. Towing a drogue is needed if you start to get out of control.
There is, I must admit a case where there are convergences of currents that can steepen things a bi
Robert
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Old 18-02-2008, 02:26   #281
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Thanks Robert for the comprehensive and lucid description of the theory behind your designs. My point about rocker (or more importantly Robert Perry's), was that it reduces pounding in heavy seas, not that it improves performance (or reduces drag). In your opinion, is Perry wrong about this?

I fully understand that current designs for offshore racing multihulls (and indeed for monos - witness the offshore 60's) have very flat bottoms with little or no rocker. That being said, they are notorious for pounding if forced to sail upwind into heavy seas. You may recall the Vendee Globe several years ago, where an Italian entrant sailed upwind for about 50 miles into heavy seas in the Southern Ocean in order to rescue a fellow entrant who had capsized. The race organizers worried whether the (relatively) flat bottom on his boat would even allow him to complete that short passage in one piece (albeit in terrible seas) and he confirmed that the pounding was sensational!

Hobby-horsing is undesireable for a number of reasons and I can certainly see that extreme rocker would exacerbate the situation. Nevertheless, I had always understood that weight in the ends was the principle enemy and that flare above the waterline (and/or knuckles) increase the bouyancy of the bows and therefore reduce the effect. Carrying your maximum beam below the waterline would, of course, ensure that there is less of an increase in drag when the bows are depressed - but surely it increases wetted surface and therefore drag when sailing with the bows on their lines.

This, of course, is to say nothing of the resulting shape of the hulls which tends to put maximum beam precisely where it is not needed in a cruising cat: the shape of the human body dictates that more space is needed higher rather than lower. Furthermore, as you have already fairly pointed out, rocker assists in tacking - no small thing in a cruising cat.

I can also see where overhangs would tend initially to increase the tendency of a boat to hobbyhorse - they put some additional weight forward (and aft) of the static waterline. On the other hand, in the bows they tend to make for a drier deck and, as depressed, operate so as to increase bouyancy. In the stern the case for rear overhang seems overwhelming - they keep the transom out of the water in most circumstances and thereby dramatically reduce turbulence. I haven't seen your designs, but if they have transom sterns, I assume they also have some rear overhang. If so, I suspect that you accepted this compromise in terms of hobbyhorsing because of the advantages inherent in having them.

I haven't sailed a Nacra, but a look at the photos certainly suggests that they also have significantly more freeboard than a Hobie - surely this too dramatically reduces the chances of burying a bow. While on this topic, modern cruising cats typically have a great deal of (some say excessive) freeboard. It strikes me that the greatest risk for a cruising cat is not in burying the leeward bow while sailing to windward, but rather in burying both bows when surfing down waves in extreme conditions. In this case the differential in drag between leeward and windward bows is not an issue, and flare (and yes overhang and knuckles) above the waterline would all tend to increase bouyancy as the bows are depressed into the oncoming seas. Or am I missing something?

In any case, an interesting debate. While I haven't seen your designs they sound incredibly fascinating and fast, if perhaps a little less comodious that most cruising cats. I am always impressed by somebody who is prepared to shake off conventional wisdom and push the envelope in terms of design, and it would seem that you have developed some ardent followers - and no doubt deservedly.

Cheers to you as well!

Brad
Rocker is a strong component of hobbyhorsing. I am not sure how much weight in the ends is when condidering the moments of inertia of weights in the end making it harder to change the speed of rotation.
Consider the extremes, a log or a ball. With rocker, the boat can rotate further before increasing hydrostatic lift in the bow or encounter hyrodynamic damping. Also, overhang means the bows meet the chop all at once and bounce. With plumb or reversed, the bottom of the bow meets the chop first and the bows gently rise, Rather than have forward overhang, extend the bows at or below the waterline. I think this is also relevant for surfing down one wave into the back of the other so your hydrostatic lift starts earlier and the stop is not as sudden, though I would consider hydrodynamic lift back from the bow. Study the bows of the tri tdriven by Dame Ellen? that had the solo round the world record for a little while. That handled the Southern Ocean swells at speed without problems. There are good reasons for modern cats having plumb bows. A knuckle does a nice job of reducing spray and has better hydrodynamic damping.
About aft overhang, it certainly avoids dragging a transom. A double ended boat has a moderately fine exit as it has to act as a bow half the time. This is one of the compromises, between the fineness needed for a clean exit and the volume needed to keep the bows up. The bows are still narrower than amidships so the wavemaking drag is not significantly affected and there is only a small increase in wetted area
About pounding, certainly the big tris blasting to weather through the swells, encounter slamming as they land on the other side. Some of the racing tris have now made Vs at the bow to reduce this. A boat with significant rocker in the same circumstance would rise higher and almost come to a stop.
A boat that doesn't tack, doesn't need rocker to tack.
I guess I do disagree with Perry as far as fast vessels are concerned. I haven't given it much thought for vessels constrained by their waterline length.
The design is not mine, it is the brainchild of Rob Denney. If you want to explore the concept check it out on his web page, Harryproa.com
regards and thanks for lucid and civilised argument. (More so than mine at the moment)
Robert
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Old 18-02-2008, 09:37   #282
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Thanks Robert, you make some interesting points (and although not the designer, are one heck of a 'disciple' of the designs). 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).

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).

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.

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Old 18-02-2008, 10:51   #283
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PS Robert, I've been mulling over what you have said, and it still strikes me that while what you describe is correct on relatively flat water, it may not be applicable when sailing in near survival conditions. Firstly, in those conditions you almost assuredly do not want to be exceeding hull speed because of the risk of pitchpoling. Secondly, slamming occurs when the bows are lifted into the air by a large passing wave; in those circumstances, I still believe that Perry was right in terms of rocker reducing that tendancy and the resultant slam when gravity brings your bows back to earth (or in this case, the sea). Richard Woods describes an experience in huge seas with a sea-anchor deployed from his catamaran and fullly a third of the length of his boat being airborn, from time to time. In those circumstances, any boat is going to slam, but I would still prefer to have some rocker to soften the blow.

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Old 18-02-2008, 11:58   #284
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Firstly, in those conditions you almost assuredly do not want to be exceeding hull speed because of the risk of pitchpoling.
Hey Brad - What do you see as the relationship between hull speed and pitch poling? Granted - it takes speed to pitch pole.

To be honest, I have never understood hull speed with relation to multihulls, anyway. I suspect I'm missing something because I keep seeing it referred to and I don't understand why. I can routinely exceed theoretical "hull speed" on my boat by a wide margin - even upwind. And I haven't pitch poled yet.
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Old 21-02-2008, 10:27   #285
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2 hulls, I am not referring to hull speed per se as a design, or operation limit for a peformance cruising cat (or monohull), I am suggesting that in extreme conditions one would likely not wish to exceed (or at least significantly exceed) hull speed for reasons of safety. The risk of pitchpoling increases dramatically when surfing down very large waves at high speed. Most people would want, in those circumstances, to reduce their speed initially by reducing sail, then by streaming drogues and ultimately, even by fully deploying a series drogue (which is intended to reduce speed to approximately 1.5 Knots).

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
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