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Thread: Thin hulls have less wave drag than fat ones. Are you sure?

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    Default Thin hulls have less wave drag than fat ones. Are you sure?

    http://eprints.soton.ac.uk/46442/1/071.pdf

    I have been reading the above paper by the University of Southampton regarding DLR (displacement to length ratio) and B/T (beam to depth) and this quote below is sure to make people think twice about B/T.

    As beam to depth gets bigger, the hull is fatter so the Length to beam ratio is reduced. The tests had the same displacement and WSA (wetted surface area). So the factor they isolated was the B/T and therefore also L/B, or skinny hulls VS fat hulls.

    For the highest Length : Displacement ratio (Models 6a 6c), Model 6a with the smallest B/T tends to have the largest resistance coefficient. For the low Length : Displacement ratio (Models 4a .4c) the trend has been reversed and Model 4a (with the smallest B/T) tends to have the lower resistance coefficient over much of the Froude Number range beyond the resistance hump speed.
    So according to my interpretation of the above quote, which is that over the ranges tested, the trend for light displacement hulls is that, WIDER hulls actually have LESS resistance than narrow hulls? And heavier displacement hulls have less resistance with narrow hulls? Where does this leave us with a performance light weight cruising cat vs heavy cats? Why do we see the opposite in actual designs?

  2. #2

    Default Re: Thin hulls have less wave drag than fat ones. Are you sure?

    What do they mean by "resistance hump speed"? are they talking about planing or semi-planing type hulls?

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    Default Re: Thin hulls have less wave drag than fat ones. Are you sure?

    Quote Originally Posted by 44C View Post
    What do they mean by "resistance hump speed"? are they talking about planing or semi-planing type hulls?
    When you look at the drag curve for pretty much any hull, including narrow ones you see a increase in drag around the froude number of 0.4, commonly known as hull speed. On fat hulls the hump is massive, as the hulls get thinner the hump gets smaller until some point where its hardly even there.

    Unfortunately I don't have the L:B for this hull, but its a displacement cat hull. But you can see the hump at hull speed. The paper I linked also tests B:T against DLR where interestingly it shows the light hull tested had less drag with the fatter hull.

    Seems to me the obvious single factor determining drag is DLR (displacement vs length). Given the same DLR the width of the hull is only a small consideration and can actually work the opposite way you might think in certain situations.


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    Default Re: Thin hulls have less wave drag than fat ones. Are you sure?

    Here is an example of a hull with the same wetted surface and displacement but one is twice as fat as the other. Common thinking would have you believe that the fat one would have much more drag. That is just not the case. It might have a few % more drag at worst and according to the tank test I linked it might even have less drag in some cases.



    Thread on BD.net, but I think this forum bans linking.

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    Default Re: Thin hulls have less wave drag than fat ones. Are you sure?

    If the wetted surface is the same then wouldn't resitance be the same?

    Other factors must have an influence such as how fine the bows are and how the hulls work in real waves ie pitching, rolling and hobby horsing.

    Also the thinner hull would be less resistance to loading.

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    Default Re: Thin hulls have less wave drag than fat ones. Are you sure?

    WSA is only one factor in drag. Its the predominant factor slow speeds (light wind sailing). As speeds go up wavemaking drag is the predominant factor. Most sources will tell you that wide hulls have more wave making resistance and that this is a big deal.

    Multihulls can go over "hull speed" because they have hulls narrow enough to not have a prohibitive resistance hump at hull speed. This is even though multihulls generally have more WSA than monohulls. That usually means cats need more power to go at slow speeds, but they require a lot less power at higher speeds.

  7. #7

    Default Re: Thin hulls have less wave drag than fat ones. Are you sure?

    So what I think they're saying is that a wider, shallower hull will have less wave induced drag at planing speed, but it's "hump" will be bigger than a narrower deeper hull, which if long and narrow enough may have virtually no "hump" at all.

    That seems consistent with what you see on the water - short fat boats like planing dinghies or runabouts can make pretty big waves at non-planing speeds, but once planing make very little wake.

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    Default Re: Thin hulls have less wave drag than fat ones. Are you sure?

    I have never seen a big cruising cat planing. What speed would that occur at, 25kt or 30kt? It doesn't seem likely. So are they always stuck on the speed hump?

    Mike
    Nothing works on an old boat, except the skipper.

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    Default Re: Thin hulls have less wave drag than fat ones. Are you sure?

    Quote Originally Posted by dmmbruce View Post
    I have never seen a big cruising cat planing. What speed would that occur at, 25kt or 30kt? It doesn't seem likely. So are they always stuck on the speed hump?

    Mike
    Wouldn't anything consistently over hull speed be considered partially planing? On our old Catfisher 32 you could feel the stern rise up once it consistently got over hull speed. And trust me, if the Catfisher can plane any catamaran can plane!

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    Default Re: Thin hulls have less wave drag than fat ones. Are you sure?

    The hull tested was not shorter. Just wider, but has the same WSA and weight. They are rounded displacement hulls. No planning. They even monitored hull sinkage at speed. Planing hulls get over hump speed by planning over the bow wave. Skinny hulls push through the bow wave.

    The "fat" hull was something like 12:1 and the "skinniest" one 15:1. In fact the DLR is about the same as what I calculated for an Oram 44, and it also had a similar length to beam of 12 which I believe is 12.5 for an oram 44. So its a good representation of that boat actually! It was the one with a good result I am talking about.

    The hump speed for cat hulls comes and goes. Its false to say it does not exist. Right around hull speed there is an increase in wave resistance, but its not large enough to probit further speed unlike a fat and or heavy hull. Once hump speed is passed wave resistance goes down.

    I guess this thread is about when you look at almost every cat designers website they go on and on about length to beam, and how important it is. When the real important thing is weight (DLR), which is barely mentioned.

    I am going to say this test I mentioned goes against the usual trend of more drag as the hulls get wider. But the fact is even the trend for wider hulls to have more drag is small. As long as weight stays the same. But a wider hull will be heavier to build anyway, and will have more space to put stuff, so will get heavier again if you are not careful.

    Look at Schionnings nice new boat the Arrow 1200 http://www.schionningdesigns.com.au/arrow-1200 I like it, but I do wonder why on only a 12m (39") crusing boat the hulls need to have a 14.5:1 ratio? If it were something like 12 or even 11:1 I am sure it would hardly be heavier, but feel more spacious and not bog down as much with the inevitable cruising loads, and according to the data have negligibly more drag.

    I watched the bris to glad race last year and the very nice Schionning WL 48? That was in the race. I love the boat and know its fast, but damn was that thing sticky in the light wind! Really skinny hulls have a lot of WSA and are slower in light wind.

    Here is something common on most designers websites.

    A good cruising cat would have a Waterline beam to length ratio of 11.5 to 12.5:1. A performance cruising cat 12.5 to 14:1 and a racing cat 14 to 20:1.
    The 44c is probably one of the fastest genuine cruising cats I can think of yet it's barely even performance cruiser according to this. The list would be more realistic if they swapped the L:B list with a DLR list. IE, heavy boats are slow and light ones are fast.
    Last edited by dennisail; 25th June 2014 at 05:26 AM.

  11. #11

    Default Re: Thin hulls have less wave drag than fat ones. Are you sure?

    Quote Originally Posted by dmmbruce View Post
    I have never seen a big cruising cat planing. What speed would that occur at, 25kt or 30kt? It doesn't seem likely. So are they always stuck on the speed hump?

    Mike
    Some boats - ie those with monohull type length-beam ratios of around 3 or 4 to 1, basically HAVE to plane to get much above hull speed.

    Most cats are around 10 : 1, and can be driven well above hull speed without planing.

  12. #12

    Default Re: Thin hulls have less wave drag than fat ones. Are you sure?

    Quote Originally Posted by smj View Post
    Wouldn't anything consistently over hull speed be considered partially planing? On our old Catfisher 32 you could feel the stern rise up once it consistently got over hull speed. And trust me, if the Catfisher can plane any catamaran can plane!
    No. Cat type LWL:BWL ratios can go well above hull speed without planing.

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    Default Re: Thin hulls have less wave drag than fat ones. Are you sure?

    Yeah, just look at the data. Skinny hulls that go over "hull speed" usually SINK into the water rather than lift out. This is measured in my link.

  14. #14

    Default Re: Thin hulls have less wave drag than fat ones. Are you sure?

    Quote Originally Posted by 44C View Post
    No. Cat type LWL:BWL ratios can go well above hull speed without planing.
    From my experience, when cats reach around their theoretical hull speed the sterns seem to squat. To get past that speed it takes a good gust then the sterns rise and the speed increases. I'm not sure if that's planing or not.

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    Default Re: Thin hulls have less wave drag than fat ones. Are you sure?

    It is not planing. You need that gust because even thin hulls still have "hull speed" but its actually possible to get past it. Most people would say the thinner the hull, the smaller the hump, but its actually more accurate to say the lighter the hull, the smaller the hump. Weight matters MUCH more than the width.

    I know the paper I linked is a hard read, but the graphs show a hump in wavemaking resistance at "hull speed" which then reduces as speed increases. The charts also show that in most cases, the hulls sink further into the water with speed, which clearly shows they are not planning, but rather doing the opposite.

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    Default Re: Thin hulls have less wave drag than fat ones. Are you sure?

    "Hull speed" is roughly calculated as being a constant times the square root of the LWL. That constant is generally thought to be 1.34, but that is just a general number, and mostly related to monohulls (and fat deep ones at that).

    The multiplying constant is actually a function of the hull design, and on catamarans is generally a larger number than on monos.

    So catamarans exceeding 1.34*sqrt(LWL) are not exceeding their "hull speed" - they are just experiencing the hull speed for their particular boat.

    There are very few catamarans that plane.

    Mark
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    Default Re: Thin hulls have less wave drag than fat ones. Are you sure?

    Quote Originally Posted by colemj View Post
    "Hull speed" is roughly calculated as being a constant times the square root of the LWL. That constant is generally thought to be 1.34, but that is just a general number, and mostly related to monohulls (and fat deep ones at that).

    The multiplying constant is actually a function of the hull design, and on catamarans is generally a larger number than on monos.

    So catamarans exceeding 1.34*sqrt(LWL) are not exceeding their "hull speed" - they are just experiencing the hull speed for their particular boat.

    There are very few catamarans that plane.

    Mark
    If you actually look at the data, which has been presented by the way You will see that at around the froude number of 0.4, which corresponds to the generalisation of "hull speed" or SL 1.34 there is the beginning of a ramping peak in wavemaking resistance in the drag curve right at that point, the fact this hump occurs in the drag curve has little to do with hull design. Its related to speed of which waves travel. When the hull sits between 2 wave crests it will be at this point of higher resistance. The shape of the hull has little to do with this phenomenon. It will happen around the same speed for all hulls. (hulls that make smaller waves have a smaller hump but its still at approximately the same speed)

    Hull design can only make a relativity minor difference to where this hump is. For example it might be at Froude number of 0.5 instead of 0.4. But the cat hull will still pass 0.5 without planing anyway if it has enough power. That is the difference. Also a light cat hull will have a much smaller hump in the curve than a heavy mono but it is still there.

    Hull speed is not talked about in naval architecture. They use speed length ratio or Froude number. The only reason I used the phrase hull speed is because most people would not know what I was on about if I just said Fr 0.4. I don't like the term really as its confusing and leads to incorrect assumptions such as it is an impassible barrier.
    Last edited by dennisail; 26th June 2014 at 03:20 PM.

  18. #18

    Default Re: Thin hulls have less wave drag than fat ones. Are you sure?

    Quote Originally Posted by dennisail View Post
    It is not planing. You need that gust because even thin hulls still have "hull speed" but its actually possible to get past it. Most people would say the thinner the hull, the smaller the hump, but its actually more accurate to say the lighter the hull, the smaller the hump. Weight matters MUCH more than the width.

    I know the paper I linked is a hard read, but the graphs show a hump in wavemaking resistance at "hull speed" which then reduces as speed increases. The charts also show that in most cases, the hulls sink further into the water with speed, which clearly shows they are not planning, but rather doing the opposite.
    It is a very specific technical paper it ignores certain real life factors and interesting as the results are, it would be unwise to make general conclusions on face value alone.

    For example bow entry and exit and the profile variations along the hull are much more important in fixing the characteristics of a particular hull shape in it's intended operating conditions than is it's thinness ratio.

    A hull with a narrow forefoot and fine entry and max beam 2/3rds to 3/4s of the way from the bow will have totally different characteristics than a hull with parallel water plane, a triangular bow and round stern.

    The idea that the hull sinks further on a thin hull is more likely down to laminar flow effects on the emersion / displacement rather than just thinness.

    It seems to ignore the fact that a semi circular hull has a minimum WSA when compared to a deep or fat hull shape and it would be easy to use this as an optimum design feature if the boat always maintained the same weight and always operated in flat water.

    Your statement that weight is more important than width is certainly true but this is down to the Power to Weight Ratio and therefore applies to any hull shape and in real life is also dependant upon where the weight is. Very apparent on small vessels where people placement will have dramatic effects.

    Where the power is applied is also important, very different on a sail boat where the CE is high up or a power boat where it is low down and even this is affected by whether the shafts are horizontal or inclined.

    Wonder how the conclusions would apply to SWATH vessels, very, very, thin BL ratio but relatively large WSA DL

    Interesting stuff!!!

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    Default Re: Thin hulls have less wave drag than fat ones. Are you sure?

    Once again, if you actually look at the tests and read the paper you will see that prismatic ratio and WSA were kept the same. Sure its going to be impossible to have exactly the same shape but all efforts were kept to keep things as comparable as possible.

    But to keep WSA the same a non optimum WSA shape was used throughout. In reality the displacement of the boat determines the hull to beam ratio automatically when you use a semi circular minimum WSA shape for any given length. But that works at rest, what happens when the wind picks up and say 75% of the displacement is on the leeward hull and only 25% is on the windward? I guess it all goes out the window anyway?

    Certainly interesting.

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    Default Re: Thin hulls have less wave drag than fat ones. Are you sure?

    I'm with Peter on this one. While the a lighter hull that is beamier will likely be faster in flat water, what happens when waves are added to the equation? Yes, the wetted surface is not much greater on a beamier hull of the same weight in static conditions: while wider, the narrower hull will be deeper for the same displacement. However, if the hulls of both boats enter a wave that is three feet high, the wave will make contact with a much larger surface area on the beamier hull creating additional resistance.

    Brad

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