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

  1. #21
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    Default Re: Thin hulls have less wave drag than fat ones. Are you sure?

    Quote Originally Posted by Southern Star View Post
    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
    The best waterlinebeam to hull depth ratio is 2,5 to 1. Generally waterline length to beam ratio is not relevant for resistance. Length to displacement ratio is important. Wide hulls do not hobby-horse as much as narrow hulls, so they dont "shake the Wind out of the sails".

    Lars

  2. #22

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

    Quote Originally Posted by django View Post
    ....... Wide hulls do not hobby-horse as much as narrow hulls, so they dont "shake the Wind out of the sails".

    Lars
    ??? Experience with very fine hulled racing boats would suggest otherwise. Hulls with a higher prismatic coefficient will hobby horse less than a lower. Given that the bow has to be narrow, that would suggest that a narrower hull will have a higher prismatic coefficient.

  3. #23
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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
    ??? Experience with very fine hulled racing boats would suggest otherwise. Hulls with a higher prismatic coefficient will hobby horse less than a lower. Given that the bow has to be narrow, that would suggest that a narrower hull will have a higher prismatic coefficient.
    I wish I understood prismatic coefficients.

    :-(

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

  4. #24

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

    Quote Originally Posted by dmmbruce View Post
    I wish I understood prismatic coefficients.

    :-(

    Mike
    I guess it could be described as how close the hull form is to she shape of a rectangular prism. Basically it's about how much buoyancy there is in the bows and sterns, relative to the centre of the hull. The more the higher the coefficient.

    The super long, very skinny racing tri's and cat's have very high prismatic coefficients, and they don't pitch or "hobby horse" much at all. Whereas boats like old Catalacs, with very rounded hullforms (viewed from side on) have low coefficients, and pitch plenty.

  5. #25
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    Default Re: Thin hulls have less wave drag than fat ones. Are you sure?

    High Cp (prismatic) means more drag at low Froude numbers (below "hull speed") and less drag and higher speeds and vice versa with low Cp. Since almost any design with super long skinny hulls is probably a race boat or high performance cruiser it stands to reason that by design these hulls will have a high Cp since fast boats are expected to run at higher Froude numbers at which high Cp is a benefit.

    I'm pretty sure if get an existing hull shape and stretch it to double its length with equal station spacing the Cp will remain the same. Likewise if I add a long and skinny/shallow bow or stern to an existing boat the overall Cp will be reduced since the length (prism by which Cp is measured) went up a lot but buoyancy in the ends did not. But if I cut the hull in half in the middle and add the length there Cp will go up as there is a lot more buoyancy added, but the cross section remains the same (well it will float higher so not exactly the same).

    Still my boat needs a major haul out/full paint job etc and I may as well tack on a bit on the front and back. I will use some software to see how my proposed changes will change Cp. I want the bows to be fuller and longer (higher Cp) so it will a pretty big job. But I am not happy with the boats bow down trim, plus I like a challenge

    The current shape makes pretty much no bow wave. I think it has low Cp bows and High Cp stern. The bows are very fine and the sterns are wide.
    Last edited by dennisail; 9th January 2015 at 05:18 AM.

  6. #26

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

    Quote Originally Posted by dennisail View Post
    Still my boat needs a major haul out/full paint job etc and I may as well tack on a bit on the front and back. I will use some software to see how my proposed changes will change Cp. I want the bows to be fuller and longer (higher Cp) so it will a pretty big job. But I am not happy with the boats bow down trim, plus I like a challenge

    The current shape makes pretty much no bow wave. I think it has low Cp bows and High Cp stern. The bows are very fine and the sterns are wide.
    Very low Cp bows and High Cp stern will have a tendency to make the boat surf too easily and become unstable especially if the rudders are too small.

    Interested to know where the max hull beam is on your boat. Is the bow down aspect reflected by the countertops inside or is it just a bad external paintjob of the waterlines?

    Hulls on proas are equal end to end and can have a high Cp and are good to windward and do not tend towards hobby horsing because they have different windward and leeward hull shapes, also they don't tack they shunt. Catamaran configurations with very high Cp will tend to tramline and would be difficult to tack they need a certain amount of rocker to turn.

  7. #27
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    Default Re: Thin hulls have less wave drag than fat ones. Are you sure?

    I took all the weight out of the boat, removed the spare anchor from the front etc and it still floated down a bit in the bows compared to the DWL. Also the tanks are forward of the LCB (center of buoyancy) so the effect is greatly enhanced with full tanks of water and fuel. The waterline has been raised at the bows already!

    Remeber Cp can be made higher by going deeper, wider, or both. I think if you increase it by going deeper (immersed forefoot) you can still have fine bows and create smaller bow waves. That would probably tend to cause the tramline issue you speak of it is too extreme. She does surf easily, but its fun and so far controllable, but this is only in small waves.

    My bows are so fine at the waterline that they sides are virtually flat for something like 1/3 of the boat with no convex curve when viewed from above. I will have to show pics and vid of it at speed with pretty much no bow waves. There is a price to pay for this though. Its very sensitive to weight forward, and slices through waves in the up and down motion with little resistance (low pitch dampening at the bows) and is slow to lift with waves. The combination of that with bow down trim means more bridge deck slamming than I would like at the front.

    If you increase Cp by going wider at the bows the entry wont be as fine when viewed from above but it will be shallower. (same Cp with more rocker compared to the immersed forefoot version) this will make a bigger bow wave. But the advantage is less "tramlining" and easier to tack. But since the waterplane will be bigger at the front it will be less sensitive to loading, it will have a higher pitch dampening effect, and it will be able to rise with waves better. All leading to less slamming.

    My plan was to extend the bows for about 2 or 3 feet, so that the added buoyancy can be achieved while still retaining a fine entry. It would be a reverse bow shape (not for fashion) but because that will get the most buoyancy where it is needed with the least weight, windage cost and effort. This will make the boat heavier, but not by much (it can be built very light as it will be sacrificial because the original bows will still be under there) however the overall DLR will get a decent boost meaning more speed over all unless its very light.

    Next time I am out there I will try to estimate where the max beam is.
    Last edited by dennisail; 9th January 2015 at 02:58 PM.

  8. #28
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    Default Re: Thin hulls have less wave drag than fat ones. Are you sure?

    Pics taken with go pro, so it has fisheye which distorts the shape a bit.
    Attached Images Attached Images

  9. #29
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    Default Re: Thin hulls have less wave drag than fat ones. Are you sure?

    Not sure on all the speeds these pics were taken. Somewhere between 7-10K. This was going to windward so one hull is carrying more displacement than the other (pic shows 9.7K at about 37 deg apparent wind. The wind speed stopped working. I guess about 12-15K true. The pics from stern show the windward hull with very clean wake with a more disturbed wake on the other. Above about 13-14K the wake looks pretty good. Note almost no bow waves.
    Attached Images Attached Images
    Last edited by dennisail; 9th January 2015 at 03:40 PM.

  10. #30

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

    Quote Originally Posted by Peter View Post
    Catamaran configurations with very high Cp will tend to tramline and would be difficult to tack they need a certain amount of rocker to turn.
    Difficult to tack may be putting it too strongly. They will more likely tack more slowly. The racing cat's with very high Cp's can certainly still tack ok.

    IMO the trade off of tacking a bit slower vs going much faster between tacks is worth it.

  11. #31
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    Default Re: Thin hulls have less wave drag than fat ones. Are you sure?

    I think the trade off is is worth it also, especially for a cruising boat which usually does many miles before tacking. The extra 10 seconds when tacking is going to be nothing compared to even going 0.1K faster over any decent passage. As long as it does not want to be caught in irons, slower tacking would be good enough for me for an extra speed trade off. I am not beer can racing. I believe Richard W said Cp changes makes a difference in speed of up to 10%, (that's not including the reduced effect of hobby horsing of higher Cp)

    I do wonder though if bow steer could become apparent if the forefoot is too deep when running in big waves?
    Last edited by dennisail; 10th January 2015 at 02:52 AM.

  12. #32

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

    Don't know. We have a pretty deep forefoot, probably about as deep as any I've seen, but we track straight as an arrow surfing. The boat still steers fine too.

    I used to think we bow steered a bit, but that was just the crappy Coursemaster at work. Or NOT at work.

  13. #33
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    Default Re: Thin hulls have less wave drag than fat ones. Are you sure?

    Glad my boat did not come with a Cursemaster, even though it does have a HyDrive steering system. Your old unit seemed to work fine on Shane's mono though. I guess it just could not handle a faster more responsive boat.

    How deep do you think the forefoot is below the waters surface on Somewhere?

  14. #34
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    Default Re: Thin hulls have less wave drag than fat ones. Are you sure?

    dmmbruce, Prismatic coefficient is a measure of how much boat there is underwater towards the ends, as opposed to how much there is at the point of greatest underwater area, which is usually a bit aft of the center of the waterline. A barge that is has the same area in the middle, near the bows, and near the stern has a PC of 1. The more you 'whittle away' towards the bow and stern, making the ends finer than (just aft of the middle,) the smaller the prismatic coefficient. Monohull sailboats are traditionally .52 to .56. Catamarans are usually .60 to .65. Proas are higher, some between .7 and .8. The 'fatter ended' ie. higher the PC, the more pitching resistance you have. (Other factors influence pitching, as well.) If all else is equal, the boat with the higher PC has its greatest wave making resistance at higher speeds. At lower speeds, wetted surface (skin area) causes most of a boat's resistance.
    Currently concentrating on http://earthnurture.com .

  15. #35
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    Default Re: Thin hulls have less wave drag than fat ones. Are you sure?

    I agree with most of that, but I am not sure about this.

    the boat with the higher PC has its greatest wave making resistance at higher speeds
    Do you have any source? Since we know higher Cp shapes have less drag at higher speeds, and wavemaking drag is the predominant drag factor at high speed it follows that high Cp shapes have less wavemaking drag at high speed. Which seems to make sense to me as a higher Cp hull will have a narrower waterline and mid section, and a higher length to beam ratio for the same displacement.

  16. #36
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    Default Re: Thin hulls have less wave drag than fat ones. Are you sure?

    Quote Originally Posted by dennisail View Post
    I agree with most of that, but I am not sure about this.

    Do you have any source? Since we know higher Cp shapes have less drag at higher speeds, and wavemaking drag is the predominant drag factor at high speed it follows that high Cp shapes have less wavemaking drag at high speed. Which seems to make sense to me as a higher Cp hull will have a narrower waterline and mid section, and a higher length to beam ratio for the same displacement.
    "If all else is equal," is what I prefaced that remark with. The source is Skene's Elements of Yacht Design, but it isn't some controversial proposal. It is yacht design 101. You keep making argumentative, absolutist, dogmatic, simplistic statements about a complex subject about which you know very little. Watching you try to figure out yacht design is like the parable of the group of blind men trying to understand an elephant, and then one pronouncing that it is like a wall, another that it is like a hose, a third saying it's like a tree trunk, the fourth that it is like a snake, and so on, depending on which part of the elephant that they have felt.

    You are trying to imagine how things vary when you design a hull. Read Skene's Elements of Yacht Design, and then get a yacht design program. Then reflect that Skene's is a primer written before the popularity of the catamaran yacht, so take it with a grain of salt, when trying to apply his comments to catamarans. Go to http://www.newavesys.com/products.htm and download a free trial version. Play with it. PC is a relationship of the areas of the hulls sections. You can vary the area of a section in various different ways, not only by changing the beam.

    Another issue is your faith in tank testing. If you knew more about the history of tank testing, your faith in it would be reduced dramatically.
    Last edited by BigCat; 13th January 2015 at 01:01 AM.
    Currently concentrating on http://earthnurture.com .

  17. #37
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    Default Re: Thin hulls have less wave drag than fat ones. Are you sure?

    Quote Originally Posted by BigCat View Post
    "If all else is equal," is what I prefaced that remark with. The source is Skene's Elements of Yacht Design, but it isn't some controversial proposal. It is yacht design 101. You keep making argumentative, absolutist, dogmatic, simplistic statements about a complex subject about which you know very little. Watching you try to figure out yacht design is like the parable of the group of blind men trying to understand an elephant, and then one pronouncing that it is like a wall, another that it is like a hose, a third saying it's like a tree trunk, the fourth that it is like a snake, and so on, depending on which part of the elephant that they have felt.

    You are trying to imagine how things vary when you design a hull. Read Skene's Elements of Yacht Design, and then get a yacht design program. Then reflect that Skene's is a primer written before the popularity of the catamaran yacht, so take it with a grain of salt, when trying to apply his comments to catamarans. Go to http://www.newavesys.com/products.htm and download a free trial version. Play with it. PC is a relationship of the areas of the hulls sections. You can vary the area of a section in various different ways, not only by changing the beam.

    Another issue is your faith in tank testing. If you knew more about the history of tank testing, your faith in it would be reduced dramatically.
    So you don't have a source then. "All else is equal". That is a vague statement, and also impossible to meet as you cant have "all else equal". If you keep the hull to beam the same, and make deepest part of the immersed hull the same on the high Cp hull it will have greater displacement as it carries more weight in the ends even though the center part is the same. All that you are really showing then is a hull with greater displacement has more wave making drag as you have not kept the most relevant factor the same. Since DLR is shown everywhere to be the most important factor in resistance. "all else equal" should use displacement as its yard stick. Seems Richard Woods agrees below (post is about Cp)

    Would it be easier if you you looked at it the other way round. I think you can agree that long thin hulls are faster than fat ones? So for a given volume if you make the ends fuller then that volume has to come from somewhere. Thus the middle gets smaller and narrower? Does that make sense?
    To see the full context look here http://www.multihulls4us.com/forums/...2774#post12774

    Lets put it another way. Why do YOU think high Cp hulls have less drag at high speeds, if they have GREATER wavemaking resistance?
    Last edited by dennisail; 13th January 2015 at 01:36 AM. Reason: to quote insults edited into new post

  18. #38
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    Default Re: Thin hulls have less wave drag than fat ones. Are you sure?

    dennisail, you're just too arrogant to teach. Go post on http://boatdesign.net/ for a while, post your absurdities, and take your lumps.
    Currently concentrating on http://earthnurture.com .

  19. #39
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    Default Re: Thin hulls have less wave drag than fat ones. Are you sure?

    Quote Originally Posted by BigCat View Post
    dennisail, you're just too arrogant to teach. Go post on http://boatdesign.net/ for a while, post your absurdities, and take your lumps.

    I just noticed your post was edited to include more personal attacks. My post was again not inflammatory, rather asked for an explanation, which you are not able to give. In fact I have never seen so many logical fallacy's used in so little words. You refuse to address the content of my posts. You rather use:

    https://yourlogicalfallacyis.com/ad-hominem To attack me, rather than the content.
    https://yourlogicalfallacyis.com/appeal-to-authority To avoid backing up you assertions by palming them off onto respected Authorities, even though argument from Authority is a logical fallacy anyway. When one reads conflicting evidence from 2 authorities do you believe one must use Orwellian double think so as to beleive both at the same time?
    https://yourlogicalfallacyis.com/burden-of-proof you refused to back up your claim, leaving me to prove it wrong.
    https://yourlogicalfallacyis.com/ambiguity you used "all else equal" with out defining what equal was.
    https://yourlogicalfallacyis.com/genetic you automatically discredited what I said as you appear to have a strong dislike for me.
    https://yourlogicalfallacyis.com/strawman you made out that I am trying to undermine respected designers by being superior, then attacked that instead of the argument
    https://yourlogicalfallacyis.com/appeal-to-emotion you are trying to incite hatred toward me by using personal attacks on my character and intelligence.

    All I asked for was some evidence. But you have none. I am not even worried if you are correct or not. I would like to know if I was wrong about the way I though of it. Perhaps I am? You have been unable to show anything though.

    Thanks for the tip re Boatdesign.net. I have spent hundreds of hours reading there and over 800 posts since my join date in 2004. All without a single negative rep point. I have never been insulted there either.
    Last edited by dennisail; 13th January 2015 at 01:45 AM. Reason: to quote insults

  20. #40

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

    Quote Originally Posted by BigCat View Post
    dennisail, you're just too arrogant to teach. Go post on http://boatdesign.net/ for a while, post your absurdities, and take your lumps.
    How about you post a gem like this : http://www.multihulls4us.com/forums/...5599#post65599

    over there?

    Anyway, good to see you back, a good laugh is always welcome!

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