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Thread: Prismatic Coefficient and getting wrapped around a wheel

  1. #41
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    Default Re: Skene's Graph of Optimum Prismatic Coefficient

    Quote Originally Posted by 44C View Post
    The theoretical hull speed of 1.34 x sq root waterline length has been found only to apply to hulls with a LWL/BWL ratio of around 8:1 or less.

    Most multi's are better than that.

    http://www.multihulldynamics.com/new...?articleID=174

    "Hull Beam and Hull Length to Beam Ratio
    A very significant factor in boat performance is the ratio of waterline length
    (Lwl) to hull beam (Bh) (not overall beam), Lwl/Bh vs. Length, plotted in Fig. 1
    below. In Bruce and Morris work Design for Fast Sailing, it was shown that a
    good performing cruising catamaran or trimaran (main hull) should have a ratio
    of at least 8. In displacement hulls, the bow wave includes a trailing trough
    that a boat stern tends to set down into, limiting the speed of the boat. This
    is called hull speed. Slender hulls, those with Lwl/Bh ratios greater than 8, do
    not create a large enough bow wave for this to be an appreciable effect[3]."
    All very true, but not relevant to the above discussion, which is not about hull speed as related to hull beam, but optimum bluntness versus fineness, if you will, at various speeds. At the bottom of my web page there is some quantification of the effect you are describing, under the footnote about the K factor.
    Currently concentrating on http://earthnurture.com .

  2. #42

    Default Re: Skene's Graph of Optimum Prismatic Coefficient

    Quote Originally Posted by 44C View Post
    The theoretical hull speed of 1.34 x sq root waterline length has been found only to apply to hulls with a LWL/BWL ratio of around 8:1 or less.

    Most multi's are better than that.

    http://www.multihulldynamics.com/new...?articleID=174

    "Hull Beam and Hull Length to Beam Ratio
    A very significant factor in boat performance is the ratio of waterline length
    (Lwl) to hull beam (Bh) (not overall beam), Lwl/Bh vs. Length, plotted in Fig. 1
    below. In Bruce and Morris work Design for Fast Sailing, it was shown that a
    good performing cruising catamaran or trimaran (main hull) should have a ratio
    of at least 8. In displacement hulls, the bow wave includes a trailing trough
    that a boat stern tends to set down into, limiting the speed of the boat. This
    is called hull speed. Slender hulls, those with Lwl/Bh ratios greater than 8, do
    not create a large enough bow wave for this to be an appreciable effect[3]."
    Results of Delft Series and other known test results clearly show that primary factor effecting the resistance is DLR (displacement-length ratio), i.e waterline length and weight of boat. Beam is one of secondary factors (such is CP, LCB, etc.). It is quite common to overestimate the importante of beam

  3. #43
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    Default Re: Skene's Graph of Optimum Prismatic Coefficient

    Quote Originally Posted by Albatross View Post
    Results of Delft Series and other known test results clearly show that primary factor effecting the resistance is DLR (displacement-length ratio), i.e waterline length and weight of boat. Beam is one of secondary factors (such is CP, LCB, etc.). It is quite common to overestimate the importante of beam
    I don't think anyone will argue against that. In the real world, there is a tendency for a lot of these factors to correlate, of course. That is, longer, lighter multihulls will usually have less hull beam.

    Otherwise, the sections would be so flat that there would be an awful lot of pounding, which would also slow the boat on the wind, or the hull shape would have to abnormal in some negative way, such as having the center of displacement forward of the optimum location.
    Currently concentrating on http://earthnurture.com .

  4. #44
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    Default Re: Skene's Graph of Optimum Prismatic Coefficient

    Hi, I'd like to join this very interesting discussion - 2 points are causing me to head scratch. I'm designing the main hull for a trimaran. It's overall Cp seems about right, at 0.615. However, at the moment fwd Cp is 0.627 and aft is 0.603, even though the bow still has a fairly fine entry and the stern sections are broader and flatter, no real similarity of shape between bow and stern sections. Can the Cp itself give an indication of resistance to pitching?
    Second point is - why LCB at 54 - 56% aft? It's probably a doh question once you know the reason, at the moment I don't quite get it. My tri hull has LCB at 50%.

    Hopefully,
    Langdon

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    Default Re: Skene's Graph of Optimum Prismatic Coefficient

    Quote Originally Posted by langdon2 View Post
    Hi, I'd like to join this very interesting discussion - 2 points are causing me to head scratch. I'm designing the main hull for a trimaran. It's overall Cp seems about right, at 0.615. However, at the moment fwd Cp is 0.627 and aft is 0.603, even though the bow still has a fairly fine entry and the stern sections are broader and flatter, no real similarity of shape between bow and stern sections. Can the Cp itself give an indication of resistance to pitching?
    Second point is - why LCB at 54 - 56% aft? It's probably a doh question once you know the reason, at the moment I don't quite get it. My tri hull has LCB at 50%.

    Hopefully,
    Langdon
    A and B are connected - Move the LCB aft and A should be more reasonable. The LCB at @55% has been found to be faster- Based on experience, not theory. It also will get your buoyancy aft, where it can help support your engine. Separating the LCB and CLF a percent (of the DWL) or two is said to reduce pitching.
    Currently concentrating on http://earthnurture.com .

  6. #46
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    Default Re: Skene's Graph of Optimum Prismatic Coefficient

    Okay, BigCat, thanks. I'll go back to the lines and work the changes. Thanks also for a good deal of interesting and useful material on your site.

    Langdon

  7. #47
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    Default Re: Skene's Graph of Optimum Prismatic Coefficient

    Hi, y'all and a Happy New Year.
    Okay, so I've got my Cp's up in the 0.63 's, even came up with one hull re-draw up into 0.64. BUT, having probs with LCB...I've got a reasonable seeming hull shape with LCB at 53.4% and LCF at 55.8%. But going further to try and get LCB at 55% the rear end just looks like a great big fat girl sitting on a bar stool. OOps, sorry PC tribe...I mean the slope of the aft sections in profile looks too steep and the whole stern too bouncy and just asking for a wave to pick it up and throw the whole shebang buttocks over eyebrows.
    So, do I settle for my 53.4% LCB?
    And another question - how well do Froude number based calculations work with hulls with greater than 8:1 Bwl to Lwl ratios?
    One source I've looked at relates Froude No and LCB thus : LCB @ 51% good for Fr 0.3 and as much as 56% @ Fr 0.4. Fr 0.4 for my current design would be 7.9 knots and the hull Lwl to Bwl is 10:1.

  8. #48
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    Default Re: Skene's Graph of Optimum Prismatic Coefficient

    Quote Originally Posted by langdon2 View Post
    Hi, y'all and a Happy New Year.
    Okay, so I've got my Cp's up in the 0.63 's, even came up with one hull re-draw up into 0.64. BUT, having probs with LCB...I've got a reasonable seeming hull shape with LCB at 53.4% and LCF at 55.8%. But going further to try and get LCB at 55% the rear end just looks like a great big fat girl sitting on a bar stool. OOps, sorry PC tribe...I mean the slope of the aft sections in profile looks too steep and the whole stern too bouncy and just asking for a wave to pick it up and throw the whole shebang buttocks over eyebrows.
    So, do I settle for my 53.4% LCB?
    And another question - how well do Froude number based calculations work with hulls with greater than 8:1 Bwl to Lwl ratios?
    One source I've looked at relates Froude No and LCB thus : LCB @ 51% good for Fr 0.3 and as much as 56% @ Fr 0.4. Fr 0.4 for my current design would be 7.9 knots and the hull Lwl to Bwl is 10:1.
    How wide is your stern at the waterline? It sounds as though it might be quite wide, if you are getting steep buttocks at an LCB of .55. Simonis likes steep buttocks, by the way. Steep buttocks are contrary to traditional yacht design, however, as your thrust is somewhat up rather than aft, and for every action there is a reaction.

    I don't think it's a big deal whether your LCB is .534 or .55, but you may well need to have your LCB aft when it comes time to make the weights balance out, if you have inboard diesels, especially if you use saildrives or v drives to get them farther aft.
    Currently concentrating on http://earthnurture.com .

  9. #49
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    Default Re: Skene's Graph of Optimum Prismatic Coefficient

    Hi, BigCat, thanks for your reply. My trimaran main hull is double chine for plywood and epoxy, flat bottom, flared u shape. The transom is right on/at the waterline and is 1' 3" wide at the waterline. The design is for a centre cockpit and I'm aiming to put the engine very close to LCB with a conventional shaft drive.

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    Default Re: Skene's Graph of Optimum Prismatic Coefficient

    Quote Originally Posted by langdon2 View Post
    Hi, BigCat, thanks for your reply. My trimaran main hull is double chine for plywood and epoxy, flat bottom, flared u shape. The transom is right on/at the waterline and is 1' 3" wide at the waterline. The design is for a centre cockpit and I'm aiming to put the engine very close to LCB with a conventional shaft drive.
    Hi, Langdon

    I think we need more context. If you gave it previously, it was quite a while ago. What is your DWL, BWL, displacment, etc.?
    Currently concentrating on http://earthnurture.com .

  11. #51
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    Default Re: Prismatic Coefficient and getting wrapped around a wheel

    So far the discussion has focussed on the Cp and LCB of the hulls when on their DWL. Since we rarely sail directly down wind I suggest that it is also important to look at the LCB when a hull is immersed 400mm or 500mm below the DWL.

    A case in point is the cat I currently own, Crowther Design #462 www.bloomfieldinnovation .com When driving hard on a spinnaker reach at speeds of 16 to 18 knots (this is a heavy cruising cat displacing 14.5 tonnes loaded so it is not as fast as lighter models) the leeward bow actually lifts out of the water 100 mmm or so. Stuart Bloomfield has explained that while a bit of this is due to the rocker in the hulls it is mostly due to the change in the hull shape just above the waterline. This change is subtle and not noticeable to the eye but it moves the LCB a long way forward when the hull is submerged.

    When the boat begins to surf at speeds in the 22 to 25 knot range it shows no tendency to bury its nose as it slides down into the tough and hits the wave ahead. It throws a heck of a lot of spray, which we should have anticipated and built in a deflection strake, but it keeps its stern in the water, slows gently and with no really noticeable deceleration to speeds in the range of about 10 to 12 knots and then accelerates away again. After 8 years, 40,000 miles and dozens of fast rides I am pretty sure this design would not expereince the problems the Norseman mentioned in an earlier post ran into.

    I believe the changing LCB and also, possibly, the resulting change in Cp make a significant contribution to safety and the ability to sail the boat hard with confidence.

    I would be very interested in any comments or rebuttals.

    Steve
    Last edited by steve sharp; 7th January 2010 at 02:32 AM.

  12. #52
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    Default Re: Prismatic Coefficient and getting wrapped around a wheel

    Quote Originally Posted by steve sharp View Post
    So far the discussion has focussed on the Cp and LCB of the hulls when on their DWL. Since we rarely sail directly down wind I suggest that it is also important to look at the LCB when a hull is immersed 400mm or 500mm below the DWL.

    A case in point is the cat I currently own, Crowther Design #462 www.bloomfieldinnivation .com When driving hard on a spinnaker reach at speeds of 16 to 18 knots (this is a heavy cruising cat displacing 14.5 tonnes loaded so it is not as fast as lighter models) the leeward bow actually lifts out of the water 100 mmm or so. Stuart Bloomfield has explained that while a bit of this is due to the rocker in the hulls it is mostly due to the change in the hull shape just above the waterline. This change is subtle and not noticeable to the eye but it moves the LCB a long way forward when the hull is submerged.

    When the boat begins to surf at speeds in the 22 to 25 knot range it shows no tendency to bury its nose as it slides down into the tough and hits the wave ahead. It throws a heck of a lot of spray, which we should have anticipated and built in a deflection strake, but it keeps its stern in the water, slows gently and with no really noticeable deceleration to speeds in the range of about 10 to 12 knots and then accelerates away again. After 8 years, 40,000 miles and dozens of fast rides I am pretty sure this design would not expereince the problems the Norseman mentioned in an earlier post ran into.

    I believe the changing LCB and also, possibly, the resulting change in Cp make a significant contribution to safety and the ability to sail the boat hard with confidence.

    I would be very interested in any comments or rebuttals.

    Steve
    I believe that Shuttleworth made some such comment in one of his articles. This would be a bit difficult to accomplish, however, given that hulls usually are narrower forward than aft. More flam forward and less aft would be how you would do this, as well as long, low overhanging bows.

    Simonis has suggested that having steeper buttocks aft due to having the LCB aft rather than centered causes the lee stern to squat, giving the effect you describe. This is counterintuitive from a speed standpoint, as avoiding squatting has always been a goal of yacht design.

    The lower the center of effort (shorter mast, lower aspect ratio sails,) the less leverage towards pitchpoling. A lot of flam in the topsides forward also should help resist depressing the bows. The last two approaches appeal to me the most, as you can see in my designs. Higher PCs also have more bouyancy resisting bow burying forces.
    Currently concentrating on http://earthnurture.com .

  13. #53
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    Default Re: Prismatic Coefficient and getting wrapped around a wheel

    I have very little overhang bow or stern and the flare in the hulls appears very gradual with no noticeable exaggeration above the DWL. The hulls are quite fat with a fineness ratio of only 12:1 and the sections below the waterline at all but the forward 3 stations and the transom are very nearly semicircular.

    My rig is very tall in order to provide the power to move a sturdy and heavily loaded cruising boat. We carry 133.2 Sq M of working sail and the spinnaker is 140 Sq M.

    My uneducated eyeballing of the lines indicates that the positioning of the buoyancy aft when the boat is on her DWL is achieved through carrying the draft a bit further aft (stretching the semi-circle down a bit) and that, if you measured a hull section with its bottom at the DWL and the new waterline several hundred mm above the DWL the LCB would be very much further forward. When I add 2 tonnes of fuel and water to the light ship the bows sink very little, if at all, but the hulls amidships go down 30 or 40 mm. When the ship has been lightened of fuel, water and gear the transoms lift out of the water quite noticeably, the bows much less so. This suggests that squatting at speed due to Mr. Bernouli working on the rocker is a relatively small part of the total equation and that the LCB moving forward and the Cp rising rapidly as the hull is immersed account for most of the effect.

    All the above being said, in practice the boat is not terribly fast having a top speed under sail in flat water of 18 knots with approximately 90% of the buoyancy provided by the leeward hull. We seldom sail her beyond a constant speed of 12 knots as we value a generous safety factor. We average about 200 miles a day on our passages.

    Steve

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    Default Re: Prismatic Coefficient and getting wrapped around a wheel

    Hello Friend.

    May I put my two bits in as a new member and suggest two books for the better understanding of what a low Cp or a high Cp does for a multihull.

    The first book is what every person who wants to buy or build a multihull should read, is from Chris White, titled the Cruising Multihull.

    This book will give you a general basic understanding of what goes on with multihulls vs. monohulls.

    The second is: Multihulls for Cruising and Racing by Derek harvey.

    They both are available for $ 20 or so from Amazon.com.

    Don't dwell on too much on monohull design questions and answers.

    Multihulls are totally different animals from monohulls. Depending on what you want to achieve with your multi, a very low Cp will give you good speed at low wind conditions. Very fine bow entry will slice through water... but it might submarine on you at racing in high wind conditions.
    Broader and flatter aft section will encourage early planing.
    Placing the Cb of the amas slightly forward of the main hull's Cb will help the submarining problems. ...Fine bow entry with heavy flair above the water line etc.. also helps.
    There are as many solution as there are questions...

    The bow fineness or the aft section shape is a matter of a compromise depending on what you want to achieve with your boat.

    For years now I have been searching for answers on this subject, (and a lot more for a cruising power tri.) but most of the secret is just that, a somewhat guarded secret!

    Winning body shapes are big money makers for the manufacturers.

    However, I have ascertained through my search that there are way too many variables to define what is best, because what people want is just as varied.

    Furthermore, an absolute ideal boat hull design is partly science, partly imagination and partly an art, like most other engineered marvels.

    Read the books I suggested, especially the second one. You will get a better understanding of the importance of the Cp.

    They will also give you simplified versions of many design calculations. Equations simple enough for a reasonably educated or trained person to understand, yet good enough to end up with a safe boat.

    Both of these books deal mostly with sailing multis...
    Not just the manufacturing part, but the sailing and safety aspect also.

    Regards,

    Stephen I. M.
    Last edited by aranda1984; 7th January 2010 at 12:11 AM.

  15. #55
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    Default Re: Prismatic Coefficient and getting wrapped around a wheel

    Quote Originally Posted by steve sharp View Post
    I have very little overhang bow or stern and the flare in the hulls appears very gradual with no noticeable exaggeration above the DWL. The hulls are quite fat with a fineness ratio of only 12:1 and the sections below the waterline at all but the forward 3 stations and the transom are very nearly semicircular.

    My rig is very tall in order to provide the power to move a sturdy and heavily loaded cruising boat. We carry 133.2 Sq M of working sail and the spinnaker is 140 Sq M.

    My uneducated eyeballing of the lines indicates that the positioning of the buoyancy aft when the boat is on her DWL is achieved through carrying the draft a bit further aft (stretching the semi-circle down a bit) and that, if you measured a hull section with its bottom at the DWL and the new waterline several hundred mm above the DWL the LCB would be very much further forward. When I add 2 tonnes of fuel and water to the light ship the bows sink very little, if at all, but the hulls amidships go down 30 or 40 mm. When the ship has been lightened of fuel, water and gear the transoms lift out of the water quite noticeably, the bows much less so. This suggests that squatting at speed due to Mr. Bernouli working on the rocker is a relatively small part of the total equation and that the LCB moving forward and the Cp rising rapidly as the hull is immersed account for most of the effect.

    All the above being said, in practice the boat is not terribly fast having a top speed under sail in flat water of 18 knots with approximately 90% of the buoyancy provided by the leeward hull. We seldom sail her beyond a constant speed of 12 knots as we value a generous safety factor. We average about 200 miles a day on our passages.

    Steve
    Hi, Steve

    I have a very educated eye, but I wouldn't hazard a guess about the PC or LCB by eyeballing a boat. It sounds as though your tankage was placed rather too far aft. You wouldn't change trim if it had been centered over the LCB. The LCB is the center of the hull's intended underwater volume, and it doesn't sound as though that changes. The term deserves a little explanation. The LCB is where the designer intends for the underwater volume center to be. If it isn't there, then the boat is out of trim, which is what you describe happening when you load liquids and supplies. The LCF is the center of the designed waterline, and when you put your boat below its marks, this is the center you must balance weights around to keep the boat in trim.

    A fineness of 12 is quite fine for a cruising catamaran, and your other figures are also very respectable for a cruising catamaran. More fineness and higher speeds to me would indicate a stripped out racing boat with very little accommodations. One almost never sees a true semicircle section, as they are usually somewhat deeper than that, except in a dory like flat bottomed shape used sometimes by Kelsall.

    The LCF is normally aft of the LCB, and so as you load a boat, the LCB usually moves aft rather than forward. I agree, however, that the deeper you load a boat, the higher its PC will typically become.
    Currently concentrating on http://earthnurture.com .

  16. #56
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    Default Re: Prismatic Coefficient and getting wrapped around a wheel

    Quote Originally Posted by steve sharp View Post
    ...
    A case in point is the cat I currently own, Crowther Design #462 www.bloomfieldinnivation .com


    Steve
    Took me a bit, but here is the correct link for this design.

    Crowther Design #462

    innovation, mate

    Fair Winds,
    Mike

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    Default Re: Prismatic Coefficient and getting wrapped around a wheel

    Hey! On my keyboard the "i" and the "o' are right next to each other. Don't I get points for being close?

    I do aplogise. I try to proofread my hunt and peck typing but some things do get by me - Outremer cats for example. I have corrected the original post.

    Steve
    Last edited by steve sharp; 7th January 2010 at 02:42 AM.

  18. #58
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    Default Re: Prismatic Coefficient and getting wrapped around a wheel

    The tankage is actually not too badly located. At half-load are actual waterline is parallel to the DWL. When fully loaded we are a bit stern down but we seldom fill all 1350 litres of fuel tankage and only fill the 800 litre water tank with rainwater or when going into a dirty harbour where we will not be running the watermaker for a couple of weeks. Considering all the junk that we have stowed any old place it all works pretty much as intended, especially the bit where the leeward bow rises as the hull is depressed as we steam along on a good trade-wind reach. We have watched other cats do nose plants and are glad not to have that happen to us.

    Steve

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    Default Re: Prismatic Coefficient and getting wrapped around a wheel

    Quote Originally Posted by steve sharp View Post
    The tankage is actually not too badly located. At half-load are actual waterline is parallel to the DWL. When fully loaded we are a bit stern down but we seldom fill all 1350 litres of fuel tankage and only fill the 800 litre water tank with rainwater or when going into a dirty harbour where we will not be running the watermaker for a couple of weeks. Considering all the junk that we have stowed any old place it all works pretty much as intended, especially the bit where the leeward bow rises as the hull is depressed as we steam along on a good trade-wind reach. We have watched other cats do nose plants and are glad not to have that happen to us.

    Steve
    Judging by the drawing on your link, you have a long flat run aft, quite different than what Simonis described. A long, flat run is traditionally considered fast, and your boat sounds quite fast for the loads you carry. I can't explain why you lee bow lifts from what I've heard so far, but it's certainly a great trait to have.
    Currently concentrating on http://earthnurture.com .

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    Default Re: Skene's Graph of Optimum Prismatic Coefficient

    Hi, BigCat, here's the latest form of my design, slight changes from last time around, but still basically the same. Cruising trimaran for UK waters to Spain area, not initially intended for ocean crossing, will usually have man and wife crew and mainly short passages no more than 24 hours. Construction ply/epoxy, no exotic materials, simple, basic outfit level. Small inboard diesel 20 hp. Centreboard, rudder on skeg

    LOA 36.5' LWL 35.25' Displ. 7500 lbs BOA 24.3' Draught 2' / 5.5' board down
    BWL Main Hull 3.5' Freeboard 4'
    Main Hull LCB 53% LCF 56%
    Cm 0.74 Cp 0.64 Fwd Cp 0.62 Aft Cp 0.67 Cb 0.47
    Wetted surface inc amas 190 sq ft
    Under beam clearance to waterline 2.75'
    Ama displ @ 2' immersion 4244 lbs / 56% of total displ.
    Ama total displ 196%

    SA 660 sq ft Bruce No 1.3 D/L 77
    Last edited by langdon2; 12th January 2010 at 11:56 AM. Reason: mistake

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