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Old 14-03-2008, 09:07   #16
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I believe one of the advantages of having transoms clear of the water is a cleaner exit of water flow. Transoms dragging in the water induce turbulence back there that is most definitely 'slow'. Additiionally, when a cat is pressed the leeward hull is deeper and carries more weight. Having that clean run aft and the leeward transom still clean though depressed is an advantage.

My own observations and questions have more to do with the interaction of wave trains coming off the hulls. Many designs seem to be quite prolific in wave making which must be speed robbing in some way. (i.e. any energy used making waves will have come from the potential energy imparting speed to the boat). So as much as a clean run and minimal turbulence aft is desirable, one may need also to look into that wave interaction.

BTW, the exception to the 'transom in the water' issue that you see in some of the big racing cats, is due to the fact that at speeds generally higher than 12-15 kts, there's enough separation from the transom that the hulls act more like a planing powerboat hull and that turbulence doesn't exist in the same way anymore. Unfortunately for us cruisers, while we may say speeds over 12 kts in some brief conditions, they don't seem to be the norm on average.
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Old 14-03-2008, 09:12   #17
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Wave interaction is definitely part of the resistance equation, and once I'm just about ready, will be playing around with Michlet to see the different resistance levels caused by wavemaking.
I don't think that this is influenced by the transoms being free of the water or not, but maybe when hard pressed...

Alan
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Old 14-03-2008, 12:07   #18
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Catamaran hull forms

By "completely flat," I assume you mean flat but not straight, ie. rockered fore and aft, but straight from side to side. This hull form is used only as a compromise between the optimum hull form and ease of building when using sheet materials, which will bend only in one direction. Oram uses it as part of a multi-chine form for ease of building and to get a reasonably wide cabin sole. Oram's hull designs approximate a round section, which minimizes wetted surface to the extent possible when using sheet materials. Flat bottom sectins will pound forward if sailed against a chop, and have higher wetted surface than the typical "v" forward-round amidships-flattened aft hull form. ***** ***** ***** ***** ***** ***** *****When designing for sheet materials, I prefer a shallow "v" form, with one chine only, and I radius the chine to soften the look and reduce wetted surface. I like the shallow "v" form because it doesn't pound forward or slap aft. ***** ***** ***** ***** ***** ***** *****Regarding prismatic coefficients, there are different optimum prismatics depending on the speed of the hull. .63 to .65 is usually used in catamarans because they are rather fast. This is also the prismatic usually used in displacement power boats. ***** ***** ***** ***** ***** ***** ***** Immersed transoms are used only in powerboat hulls, as they have a lot of resistance at slower speeds--anything less than completely flat out. Sailboat speeds vary with the wind strength, so immersed transoms are never used in sailboat hulls.
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Old 14-03-2008, 12:21   #19
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immersed transoms are never used in sailboat hulls.
Not true. You forget the sawed off transom that Chance designed for one of Turners 12 meters.

Although the sawed off transom was not successfull it did lead to one of the funniest lines to ever be uttered by Turner.

"Brit, what the hell were you thinking? Even a turd is tapered on both ends."
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Old 14-03-2008, 13:16   #20
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pirate Tres Joli!

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Not true. You forget the sawed off transom that Chance designed for one of Turners 12 meters.

Although the sawed off transom was not successfull it did lead to one of the funniest lines to ever be uttered by Turner.

"Brit, what the hell were you thinking? Even a turd is tapered on both ends."
Tres Joli! I should have said, immersed transoms are never used in sail boat hulls by anyone with a clue about what they are doing-
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Old 14-03-2008, 15:25   #21
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Originally Posted by cchesley View Post
I believe one of the advantages of having transoms clear of the water is a cleaner exit of water flow. Transoms dragging in the water induce turbulence back there that is most definitely 'slow'. Additiionally, when a cat is pressed the leeward hull is deeper and carries more weight. Having that clean run aft and the leeward transom still clean though depressed is an advantage.

My own observations and questions have more to do with the interaction of wave trains coming off the hulls. Many designs seem to be quite prolific in wave making which must be speed robbing in some way. (i.e. any energy used making waves will have come from the potential energy imparting speed to the boat). So as much as a clean run and minimal turbulence aft is desirable, one may need also to look into that wave interaction.

BTW, the exception to the 'transom in the water' issue that you see in some of the big racing cats, is due to the fact that at speeds generally higher than 12-15 kts, there's enough separation from the transom that the hulls act more like a planing powerboat hull and that turbulence doesn't exist in the same way anymore. Unfortunately for us cruisers, while we may say speeds over 12 kts in some brief conditions, they don't seem to be the norm on average.
There are transoms that are clear of the water, mine are around 50mm clear of the DWL, and then there is 200mm clear of the water - on my boat, which has a very shallow release, you would sacrifice about 1 metre of waterline length for every 100 mm you raise the transom. IMHO any gain would be outweighed by the loss of waterline. Bearing in mind that we are talking about a very small transom in this case - only around 250mm wide at the bottom, and 500 wide at deck level.

While it is a cruising boat, by the time the leeward hull is pressed such that the transom would be immersed to any meaningful extent, you would certainly be sailing at speeds in excess of 12 knots.
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Old 14-03-2008, 15:37   #22
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Originally Posted by BigCat View Post
By "completely flat," I assume you mean flat but not straight, ie. rockered fore and aft, but straight from side to side. This hull form is used only as a compromise between the optimum hull form and ease of building when using sheet materials, which will bend only in one direction. Oram uses it as part of a multi-chine form for ease of building and to get a reasonably wide cabin sole. Oram's hull designs approximate a round section, which minimizes wetted surface to the extent possible when using sheet materials. Flat bottom sectins will pound forward if sailed against a chop, and have higher wetted surface than the typical "v" forward-round amidships-flattened aft hull form.
Interesting how often this is said by people who have never sailed on an Oram boat. Bob's boats generally have quite a deep forefoot, which keeps the bow immersed in a moderate chop, and bouyant hull ends (ie. little rocker) which greatly reduce pitching, to the extent that although I have done hundreds of miles on his boats, and some in very choppy (2 + metre windwaves) windward conditions, I haven't experienced this "pounding" you speak of yet.
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Old 14-03-2008, 16:02   #23
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Wavemaking, Flat bottoms

I agree that Oram's designs have a deep forefoot, and I would say that prevention of pounding is why. I didn't say that Oram's designs pound, I said that flat bottoms pound. A narrow bow and deep forefoot are Oram's countermeasures against this. His deep forefoot has the danger, some would say, of inducing "bow steering" if heavily pressed while running. A narrow bow, especially if carried up to the deck, provides little defense against stuffing the lee bow and pitchpoling if overpowered. And yes, you haven't had these problems. I hope you never do, either. ***** ***** ***** ***** ***** ***** ***** ***** ***** As for less pitching, how would you know? I have posted about this previously, and I don't believe that less rocker = less pitching. t is not like a rocking chair, as the curvature of the hull is immersed. This rocking effect would only occur if the boat were floating so high that most of it were out of the water. My designs have very little rocker, because they are long and light, and it is the need to find a place to put a lot of displacement that leads to rocker. I attain this without a deep forefoot, however, as I am concerned about the possibility of bow steering. ***** ***** ***** ***** ***** ***** ***** ***** ***** Wavemaking from a catamaran's hulls at a give speed will be proportional to the beam, (more beam = bigger waves,) and will be greatly increased if berth "shelves" are interacting with the water. See Charles Kanter's article The Elusive Cruising Catamaran Performance ***** ***** ***** ***** ***** ***** ***** ***** *****Bigger wavemaking = slower speeds, which is why I am so keen on long, light hulls with no "shelves."
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Old 14-03-2008, 17:12   #24
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Ah, big cat you sound like a considerate and knowledgeable person.

Speaking of bowsteering, I'd be interested to see how one of the minimally rockered proa hulls perform down wind in ugly conditions. With the rudder positioned so far in from the stern, I'm not sure I'd want to be on it. I notice we keep getting quotes of high prismatic coefficient as being the most important element in hull design, And the cure of all evils, but without the topsides (which dont affect quoted figure) your going to need a pretty short rig to stop it pitching or nose diving even with a high prismatic coe.

Yer I suspect the topside design is also pretty important.
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Old 14-03-2008, 17:41   #25
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My experience with fuller hulls forward, is that these boat are pretty uncomfortable in short waves. The diagonal stability is the main factor, as pitching occurs when going to windward mostly. Fuller bows give alot of lift on the windward hull, that depresses the leeward transom, followed by a lift of the leeward bow and depression of the windward transom. This can give a jerky ride. I don't think that the amount of rocker plays much of a role in increasing or decreasing this.

As in all design, it is a compromise. You can make the sterns fuller, but when the wave passes the sterns, then you get too much lift there.
I have extended the length of the boat to enable long relatively thin bows, so that the lift kicks in slowly and further aft, in an attempt to smoothen the ride. The transoms are about 60% of hull beam when totally immersed, but at rest they are 200 mms or 8" above DWL.
Fuller transoms make the boat "stickier in light winds I believe.

I have made the last 12 ft (of a 49 footer) off the hulls relatively flat at the bottom, to minimise "Squatting" at higher speeds, not due to construction reasons, as the boat will be moulded.

Alan
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Old 14-03-2008, 18:00   #26
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Who said anything about "fuller hulls forward"?
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Old 14-03-2008, 18:16   #27
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I agree that Oram's designs have a deep forefoot, and I would say that prevention of pounding is why. I didn't say that Oram's designs pound, I said that flat bottoms pound. A narrow bow and deep forefoot are Oram's countermeasures against this.
You might say that, but you would be wrong. Bob has other reasons for having a deep forefoot - bouyancy foreward, for reduced pitching, and to reduce the tendency for the bows to fall off the wind when sailing to windward in a chop.

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His deep forefoot has the danger, some would say, of inducing "bow steering" if heavily pressed while running. A narrow bow, especially if carried up to the deck, provides little defense against stuffing the lee bow and pitchpoling if overpowered. And yes, you haven't had these problems. I hope you never do, either.
The narrow bow isn't carried all the way up to the deck - Bob extends the chamfer panels all the way to the bow, which gives considerable flare higher up. Actually at higher speeds Bob's boats tend to LIFT their bows, not bury them.

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Originally Posted by ;BigCat143172
As for less pitching, how would you know? I have posted about this previously, and I don't believe that less rocker = less pitching. t is not like a rocking chair, as the curvature of the hull is immersed. This rocking effect would only occur if the boat were floating so high that most of it were out of the water. My designs have very little rocker, because they are long and light, and it is the need to find a place to put a lot of displacement that leads to rocker. I attain this without a deep forefoot, however, as I am concerned about the possibility of bow steering.
Again you seem convinced that the underwater shape of a boat has no influence on control of pitching movement. I find that amazing. That's like saying the underwater form of a monohull has no influence on it's resistance to roll.

Certainly the above water shape matters, but so does the underwater shape.
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Old 14-03-2008, 18:59   #28
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I have no problem with Bob Oram's work

I am not criticizing Mr. Oram's work. His work looks quite professional to me. When I look at the choices he has made, I can follow his reasoning, and I can see excellent reasons for his choices. Yacht design is an endless series of compromises, and while I might sometimes make other choices, that doesn't mean that I think his choices are unreasonable. I agree that a deep forefoot increases volume forward, though I have my doubts about the idea that it keeps a bow from "blowing off." Many boats that lack this feature have no problems with bows "blowing off." As for the bows lifting, I'll take your word for it. Perhaps his flat forward sections develop some dynamic lift when moving at speed. You will grant, I would hope, that if your bows lift, they can hardly be keeping your bows from "blowing off," as they are no longer under water, or at least are no longer particularly deeply immersed. Really, these forefoot issues were all settled decades ago, as yacht designers progressively whittled away the traditional deep forefoot, and yet boats kept getting more weatherly. ***** ***** ***** ***** ***** ***** I never stated that the underwater shape of a boat has no influence on pitching. Rocker is just one feature of underwater shape. The width of the waterline well forward and well aft will influence pitching, as will the volume fore and aft (the PC.) In addition, as I have stated elsewhere, I believe that Simonis is right to state that separating the LCB from the LCF reduces pitching. I think that today's higher volume sterns reduce pitching, compared to the very fine double ended shapes that once were common. If I recall correctly, this is a view expressed by Shuttleworth. Such a shape necessarily has a matching underbody, and you can't really say that it is just a matter of the hull above water or under water. If you want a fair hull, the shape of one affects the shape of the other. ***** ***** ***** ***** ***** ***** My approach to sterns is to give moderate deadrise aft so that the resting waterline is fine, and then to increase the section's beam quickly higher up. I do this so that the waterline is reasonably fine aft when sailing slowly, and yet picks up beam swiftly at the stern by rapidly increasing beam as the stern gets sucked down from sailing or motoring fast. This deadrise aft is compatible with speed under sail if the buttocks in the quarters are flat and exit at a shallow angle, as they will if the displacement is light for the length.***** ***** ***** ***** ***** ***** 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? Oram's designs don't really fit the term, in that they are straight forward due to the deep forefoot, but rise aft. As the rocker concept was coined with the assumption of relative symmetry fore and aft, and so the term just isn't a good fit for Oram's characteristic profile.
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Old 14-03-2008, 19:26   #29
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I am not criticizing Mr. Oram's work. His work looks quite professional to me. When I look at the choices he has made, I can follow his reasoning, and I can see excellent reasons for his choices. Yacht design is an endless series of compromises, and while I might sometimes make other choices, that doesn't mean that I think his choices are unreasonable. I agree that a deep forefoot increases volume forward, though I have my doubts about the idea that it keeps a bow from "blowing off." Many boats that lack this feature have no problems with bows "blowing off." As for the bows lifting, I'll take your word for it. Perhaps his flat forward sections develop some dynamic lift when moving at speed. You will grant, I would hope, that if your bows lift, they can hardly be keeping your bows from "blowing off," as they are no longer under water, or at least are no longer particularly deeply immersed.

This is getting tiresome. Are you being deliberately obtuse? The bows lift AT HIGH SPEEDS. While Bob's boats go to windward very well, like just about any other boat, it's not their fastest point of sail.
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Old 14-03-2008, 19:49   #30
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The blowing off concept has long been obsolete

Yes, that's what I said, "at speed." Dynamic lift only occurs at high speed. The bow blowing off idea sounds like something a Cornish fisherman would have said in, say, 1890. Sorry, Capt. Nat "exploded" the notion about 90 years ago.
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