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Old 14-03-2008, 20:45   #31
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So you don't think there would be ANY difficulty with ANY modern boat in keeping the bow pointed high in rough seas and strong winds, with a partially furled heasail tacked off the bow?

Well, you're entitled to your opinion of course, just like the one about how the underwater shape of a boat has no impact on it's resistance to pitching.

This thread has taught me one thing anyway. I know which designers boat I personally would stay well clear of. If one ever gets launched.
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Old 14-03-2008, 21:05   #32
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CruisingCat goes on the attack

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So you don't think there would be ANY difficulty with ANY modern boat in keeping the bow pointed high in rough seas and strong winds, with a partially furled heasail tacked off the bow?

Well, you're entitled to your opinion of course, just like the one about how the underwater shape of a boat has no impact on it's resistance to pitching.

This thread has taught me one thing anyway. I know which designers boat I personally would stay well clear of. If one ever gets launched.
CruisingCat--Your comments are beginning to take on an ugly personal tone, and I feel that you are in no position to critique my comments, as you obviously haven't read them. I denied saying that the underwater lines have no effect on pitching, as you would know if you had actually read my remarks. If you dislike give and take so much, why post? Were you hoping for a fawning chorus of admiration? Do you actually know anything about yacht design? Have you ever made a lines drawing? Can you, for example, define the difference between the LCB and the LCF, and explain what the ramifications of each number are?
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Old 15-03-2008, 07:00   #33
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This debate has presented some strongly-held positions, in a fairly civil manner.
Please; let's avoid strong language, and keep personal comments and inferences out of, what should remain, a technical discussion.
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Old 15-03-2008, 07:44   #34
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I***** Many posters speak of rocker as though it could simply be dispensed with, instead of being a function of displacement with a given waterline and beam. This is just silly. For every 64 pounds that your boat and its contents weigh, you must put a cubic foot of boat under the water. Once you have chosen your waterline length, beam, and distribution forward and aft (the PC,) where are you going to put these cubic feet if not into rocker?
How do you think large ships are designed? They generally have no rocker.
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Old 15-03-2008, 10:48   #35
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Sailing yachts versus cargo ships

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How do you think large ships are designed? They generally have no rocker.
The remark should be read in context, which is a discussion of sailing vessels, in which it has already been remarked that deep forefoots (forefeet?) present a danger of "bow steering" and that transoms should be lifted clear of the water. There are a number of different requirements between sailing yachts and powered ships. Powered ships normally operate at a constant speed, often around the square root of the waterline length in feet expressed as knots, that is, a 144' waterline ship might operate at 12 knots. Further, a (cargo) ship will normally operate at widely varying, but very high, displacement, and its motive power comes from the propellers, that is to say, low and aft. A sailing yacht varies very little in displacement (as a percentage of its weight,) at greatly varying speeds, and its motive power comes from high up and farther forward. These differences govern almost all of the differences in hull form which can be readily observed.
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Old 15-03-2008, 20:12   #36
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BigCat,

Sorry it didn't seem contextual when you said rocker is a function of displacement, waterline and beam. Although "large ships" are generally considered to be power vessels, not all are - large sailing ships (ie tall ships) often had/have no rocker. I can't say whether they "bow-steer" or not. I'm interested to hear your opinion on why rocker is important in sailing vessels, but not power vessels. Not all large ships are cargo ships, that vary displacement greatly, and generally operate in a narrow speed-range. Warships and cruiseships for example have nearly fixed displacements and operate over a large range of speeds. I concede your point that the centre of effort differs between sail and motor boats, but don't understand how that makes a difference, since the thrust vector acts longitudinally along the hull. Looking at some of the newest big racing boats - the maxi-multis, VO70s and the like, you can admit they have very little rocker and it surely doesn't affect their manoeuvrability or cause them to bow steer. Is it an assumption that deep forefeet cause "bowsteering", or has this effect been observed in a particular boat? Rocker is not required to get the transom out of the water; cut-away does this, and canoe-sterns get rid of the transom altogether. Not trying to get in a pissing contest, just wanting to learn all I can.

Kevin
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Old 16-03-2008, 00:48   #37
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Hi, Kevin - I don't mind your comments and questions. I have answered a lot of them in past posts, however. If you read my posts on the link below you can see my and other posters comments on the subject of rocker: http://www.cruisersforum.com/forums/f48/characteristics-circumnavigating-cat-7846.htmlhttp://www.cruisersforum.com/forums/...-cat-7846.html

When you look at the profile of a sailing ship hull, you are seeing the keel, not just the hull. I think you will find that sailing ships have rocker, which is a description of the hull shape, and that the part of the profile that makes them look as though they don't are actually mostly keel. For a double ender to not have rocker, you would have to have the sterns plunge into the water down to the bottom of the hull, down to what used to be called the garboard strakes in the days of wooden ships. I have never seen a catamaran (which is what we are discussing, btw, this is the "multihull" section of the board,) with such a hull form. I don't actually know how valid concerns about bow steering might be, but I assure you that many have expressed such concerns. The effect the sails' center location has is on vessel trim. When running, it lifts the stern(s), depresses the bow(s), and on monohulls, swings the CE outboard when heeled, causing weather helm. This effect would cause deep bows to become deeper, exactly when you are most concerned about pitchpoling. This is why sailboats are much more likely to pitchpole than powerboats. If you want to read about bow steering, google 'bow steering forefoot' and you will find lots of references--don't forget the forefoot in the search term, though or you will find many irrelevant links. I have often said that rocker is a function of displacement, and that is certainly true of the latest generations of around the world monohulls. They have a very low displacement, a wide beam, and a very wide stern, and so don't need rocker to dispose of the volume required by higher displacements. If you have more questions or disagreements after doing the recommended reading, I will be happy to read them on this forum.
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Old 16-03-2008, 05:40   #38
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G'day,

I design and build harryproas, which are not cats, but do have zero rocker. The only times bow steering has been evident is in light air with one rudder, when by sitting forward the rudder works better, both for luffing and bearing away.

I have a 25' test boat which regularly sails in 25+ knots of breeze in reasonable waves for it's size. I have pitchpoled it once, and until the rudder lifted out,(say 30 degrees of pitch. although I was a little too busy to measure it) it was perfectly controllable. I also had a 12m/40' prototype a few years ago which I sailed in 20 knots of breeze against a 1-2 knot tide. The waves were large and steep. Again, no bow steering tendencies.

Rocker is a handy thing if you have two hulls a long way apart and need to turn through large angles (tacking) with small rudders. On boats which don't have this problem (harryproas, trimaran floats) it is counterproductive, except in very light airs where wetted surface is lower for a given displacement. Harry's get round this by having 50% the displacement of cats and tris get round it by floating on the middle hull in the light.

I have found that no rocker gives higher speeds, less pitching and with adequate rudders and low(ish) draft, good control.

Regards,

Rob
www.haryproa.com
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Old 16-03-2008, 06:26   #39
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I design and build harryproas, which are not cats ...
Rob
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Seems an unnecessary distinction to me.
If a Catamaran is a boat with 2 Hulls, and a Proa is a boat with 2 Hulls, of unequal length - then a Proa might merely be a particular type of Catamaran (a sub-set).

I don't mean to deny that Proas, and Harryproas in particular, might have very distinct performance (& other) characteristics from the more familiar (dare I say 'conventional' ?) Catamarans.
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Old 16-03-2008, 06:48   #40
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I come back to my reasoning for having a high prismatic coefficient with little or no rocker.
It is pretty clear that it does reduce hobbyhorsing from accounts of people with Oram cats and the experience of the Crowther designs and the Harry proas.
I would like you to refer to the two tris that recently had a go at the round the world solo record.
Contrast the bow shape of the boat that did it with ease, with the one that fell apart.

Taking it that a pitchpole is caused by the lw bow digging in and slowing the boat down, resulting in an increase in apparent wind at the same time as an increase in resistance to forward motion
A boat with a higher prismatic coefficient and less rocker, has more of its bouyancy forward, thus providing a greater torque towards keeping the boat sailing flat. If the bows do go under and there is not a flare on top, there is actually a slight decrease in wave making resistance.
If you have a lower prismatic coefficient and a more pronounced rocker, as you load up the sails, the transoms start to lift out of the water increasing the load on the bows (consider the changes in the vector components of the force on the rig), and decreasing your waterline length, and when the flare starts to come into action, the waterline length to width decreases drastically, increasing wavemaking resistance. If wide enough to plane, fair enough but otherwise you are drastically increasing your resistance, decreasing your speed, the apparent wind increases,and the transom is already pretty high. If the flared bow actually goes under with its flat wide top, then there is high resistance to popping up through the wave.
So my ideal is low down early hydrostatic lift with little resistance from the top of the bow to coming back up. Ideally, I'd like the lw hull to be a torpedo, stretched vertically, that just immerses the bow as the ww hull lifts, set a few meters forward of the ww hull. Most of this is where Sodebo is and more of it with a Harryproa.
Robert
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Old 16-03-2008, 07:17   #41
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I have experienced bow steering on a 22'tri I built. It had a slight bow bulb which I must admit did track, but was still nicely controllable and allowed me some ability to go up wind with less boards down. It handled Torres Strait very nicely (6 fathoms of water with 15-30 knots tradewinds on the port bow and currents and overfalls) I was very happy to make 6KNots over the ground travelling SSW against a SE with seven on board with supplies using an old Evonne rig. The hobbyhorsing was minimal for the conditions.
I have also had problems with bows falling away in a breeze. Once on a power cat while I was fishing in Bass Strait when a 45-50 Knot SW came through a bit earlier than expected. The boat had a hard time coming up into the wind to get back home, until all the abs and gear were loaded forward putting as much weight as possible on the bow to get some traction. this was a boat with a shallow forefoot.The 20 mile ride back was interesting and very wet with the props screaming as the transom bounced out of the water. I haven't experienced it in boats with a deeper forefoot.
The other time is on my small outrigger canoe. I ended up jumping overboard and towing it home , swimming.
Robert
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Old 16-03-2008, 08:13   #42
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Seems an unnecessary distinction to me.
If a Catamaran is a boat with 2 Hulls, and a Proa is a boat with 2 Hulls, of unequal length - then a Proa might merely be a particular type of Catamaran (a sub-set).

I don't mean to deny that Proas, and Harryproas in particular, might have very distinct performance (& other) characteristics from the more familiar (dare I say 'conventional' ?) Catamarans.
G'day,
Doesn't bother me how they are categorised.

I can assure you it will upset a lot of catamaran and proa owners, though. The latter all know that catamarans are a (slow, expensive, very poor sailing) subset of the proa genre. The former all think "Perish the thought!" ;-)


Regards,

Rob
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Old 16-03-2008, 10:54   #43
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I come back to my reasoning for having a high prismatic coefficient with little or no rocker.
It is pretty clear that it does reduce hobbyhorsing from accounts of people with Oram cats and the experience of the Crowther designs and the Harry proas.
Robert

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
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Old 16-03-2008, 13:50   #44
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... I can assure you it will upset a lot of catamaran and proa owners, though. The latter all know that catamarans are a (slow, expensive, very poor sailing) subset of the proa genre. The former all think "Perish the thought!" ;-) Rob
Touché ...
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Old 16-03-2008, 16:16   #45
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I have also had problems with bows falling away in a breeze. Once on a power cat while I was fishing in Bass Strait when a 45-50 Knot SW came through a bit earlier than expected. The boat had a hard time coming up into the wind to get back home, until all the abs and gear were loaded forward putting as much weight as possible on the bow to get some traction. this was a boat with a shallow forefoot.The 20 mile ride back was interesting and very wet with the props screaming as the transom bounced out of the water. I haven't experienced it in boats with a deeper forefoot.
Robert
Presumably this boat was an "1890's Cornish fisherman"?
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