dory catamaran hull

Discussion in 'Multihulls' started by frank smith, Feb 2, 2010.

  1. masalai
    Joined: Oct 2007
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    Location: cruising, Australia

    masalai masalai

    Try this, cheap and easy That is in Australian dollars add a second hand mast and sails and you are away... for fun sailing in coastal (sheltered waters around Keelakarai, to Keechankuppam or so on the E coast of Tamil Nadu or the inland waterways of Kerela with a mast that is light and easy to lower for the bridges or road transport on your trailer... BUILD it LIGHT...

    If you are aftere something around 39 ft x 21 ft have a look at my build,2.0.html and earlier build images here,29.0.html Bob Oram can be contacted here but for a build in India I doubt there would be much by the way of personal inspection and actual assistance in India... The kit can be pre cut and put in a container for india (I would not recommend a shared container except if there are four people building together as kits for 4 will fit in a standard container... Check with ATL here
  2. Guest625101138

    Guest625101138 Previous Member

    I expect a lot of this is to do with fads than actual performance comparison.

    The comparison of the drag is usually based only on wetted surface anf the round chine has a slight advantage. However once wave drag is accounted for they are close to equal.

    I have done comparison on light hulls using Godzilla and built examples of both styles. The square chine ended up the better because I was able to build it a bit lighter than the round chine.

    If you have an example I can do the comparison.

  3. guzzis3
    Joined: Nov 2009
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    guzzis3 Senior Member

    Perhaps technically saying "does not matter at all" was wrong, but you have to remember the power available in a sail boat is tiny compared to what is usually deployed in a power boat, power boats go less sideways than sail boats and as Rick Willoughby has pointed out water is not still and flat in real world open water boating. This was my point.

    If fuel consumption is big issue for you there are other ways to reduce it more easily than the added complexity of a rounded hull, moreover while they may be measureable I expect they will be small differences due to hull shape alone (assuming your using a sensible hull). If your building 100 boats off a glass mould you'd think about it long and hard, but the payoff for a one off probably isn't there. If the hull is generally ok it'll probably do.
  4. DarthCluin
    Joined: Mar 2009
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    DarthCluin Senior Member

  5. Guest625101138

    Guest625101138 Previous Member

    If you take a 4t cat limited to 11m long and aim to get the lowest drag hulls for 12kts then technically the lowest drag hull will be nearer hard chine than fully round section.

    In this case a hard chine will have a drag of 1.88kN and the rounded section will have a drag of 1.96kN.

    So the hard chine will have lower drag and the best choice if efficiency is the prime objective. The fact that it is easier for one-off construction makes it hard to go past.

    I also like the better pitch damping that the flat sections give on the slender hulls. The extra dynamic lift from the flat section in the bow resist diving when running as well.

    I expect most think that the round chine is harder to build one-off so it must be better.

    The quick calculation to compare a round perimeter over a square perimeter to enclose a given area does not mean much once the complexity of wave drag comes into play.

    Rick W
  6. ancient kayaker
    Joined: Aug 2006
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    ancient kayaker aka Terry Haines

    I wouldn't think a dory style hull could plane very well if at all; if this is indeed the case then the question of hard vs rounded chines is moot. For a cat without the heavy rocker of a dory, and a long and narrow hull, the hull speed is much greater than the usual value, as is the case for most canoes and kayaks. Surely planing would be less of a concern?

    For a single hull, if the hull shape minimizes flow across the chine then the hard chine has a great deal of merit but if this is not done the resulting turbulence should be a significant factor in creating drag. I assume the software tools the professionals use have the ability to predict this. Perhaps the lack of that capability explains why designers used rounded hull forms for such a long time?

    Since I don't have much cat experience I am looking for information rather than trying to promote a point of view.
  7. Guest625101138

    Guest625101138 Previous Member

    The dynamic lift at 12kts on a slender 2t hull will not be a large proportion of the total displacement but it will be enough to alter trim and get the bow up a bit. On a flat hull the lift will be greater than a round hull but I have assumed negligible lift in the results above.

    In the case of my pedal boats, at 12kts the round chine hull has slight net sinkage while the hard chine hull has slight net lift. I expect you will see similar results for heavier displacement hulls. It is only 1 to 2% of total displacement but it is another advantage to the hard chine.

    Rick W
  8. Guest625101138

    Guest625101138 Previous Member

    The benefit of the hard chine has nothing to do with planing. It is simply that the hull has a larger cross sectional area for any given beam if the section is rectangular rather than round. This means that for a given displacement the hull will be narrower and have lower wave resistance. This becomes important for any hull operated above hull speed.

    The hull speed for 11m hull is 8kts so at 12kts the hull max WL beam will be a big factor in the overall drag.

    Rick W
  9. garydierking
    Joined: Sep 2004
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    garydierking Senior Member

    On my own sailing canoe designs, I've noticed an advantage at low speed in the round hulls, but at higher speeds the flat bottom dory hulls come into their own and definitely develop a lift component in the forward sections.
  10. Chris Ostlind

    Chris Ostlind Previous Member

    Thanks for the comments, both Rick and Gary. Your observations are a valuable source of info on the potential.

    One thing, though, consistently hits home for me when I look around at a wide sampling of the power cats that are being shown in various stages of construction on the Internet. Virtually none of the production and homebuild style of power cats use a flat bottomed, dory-esque hull form. The only power cats that do have flat bottoms are decidedly rocker free as they move aft. Very clearly, these boats are built more for planing and semi-planing conditions and nothing at all to do with the 12 knot speed regime mentioned in the early part of this thread.

    So, this observation of what is actually going on out there in cat designville, would strongly suggest that designers are actively choosing radius chined, traditional u-form style hull shapes and not hard chined flat bottomed hulls. These guys have every opportunity to create their hulls in any way that will be advantageous for their potential customers and they clearly vote with their pencils away from the dory style hull form. This observation applies to all the boats I could find, be they production examples built from female tools, as well as home builder designs from either kits or purchased raw materials.

    I looked at Shuttleworth, Oram, Grainger, Hughes, Tennant, Morrelli-Melvin and a whole collection of other design shops. These guys are no lightweights when it comes to sophisticated design studies and have typically decent R&D budgets when creating new boats that are the best handling and most efficient. They use high end software packages that are tuned to produce precise design solutions for a given design brief.

    That kind of design agreement speaks to me very clearly.
  11. Guest625101138

    Guest625101138 Previous Member

    We are talking about 12kts here on a 10 to 11m boat. How many of the boats that you are referring to are designed for this speed to length. Most power cats are aiming for 20kts minimum.

    Have you got any information on the actual R&D budgets and if any of it is spent on hydrodynamics.

    I had a lot of views on hull design based on common wisdom that does not stand up when you start with something like Godzilla - it takes me to some interesting insights. I do not know if there is any equivalent being used by designers. (I have had requests from three universities on how to apply Godzilla) I think most serious designers do tank testing based on variants of series hulls or slight tweaking of current design. This limits the scope of their developments to small steps.

    One example I have of a relatively flat bottom is the main hull on the LCS Independence. This is designed for high speed to length ratio but is the sort of shape Godzilla comes out with for this speed range. The chines are slightly rounded but the sections are far from "U" or semi-circular.

    The shape depends on the design speed but in some case a flat bottom, or close to it, can actually result in the lowest drag hull.

    I doubt that many have done side-by-side comparisons of the two forms to the same degree I have. Most automatically dismiss flat bottoms because the common wisdom is that they are inferior. The data I have does not show a flat bottom is inferior. There are cases when flat is higher drag but not for 12kts, 4t, 11m catamaran.

    Rick W

    Attached Files:

  12. Chris Ostlind

    Chris Ostlind Previous Member

    Nobody is saying that your Godzilla stuff is out the window. The Michlet package is a very nicely put together piece of work.

    What it is saying, though, is that a whole bunch of really smart guys with tons of experience in these matters, are going in another direction and there has to be a reason. The boats I looked at were from 21' to 40' LOA. Some of them were sporty and some were very much cruiser style. All the designs were aimed at efficient fuel use compared to other, non-cats and balls-out, planing machine products on the market, mono or multi.

    This isn't a scientific study by any means. I don't have time to list all the boats and present a spread sheet for comparison. It's more like a very compelling impression after looking at what is out there. I'm telling you what I saw at web site after website. It didn't take long for a very strong pattern to emerge and it was global in its reach. I'll further support the web search observation with direct observations I made when I was in the Caribbean last summer when I visited the boat building facilities of Gold Coast Yachts on St. Croix in the Virgin Islands.

    So, the question is... if hard chine hulls are the best in comparison, then why are so many top designers around the world going in a different direction?
  13. Guest625101138

    Guest625101138 Previous Member

    The performance advantage of a flat hull is not universal. You need to look at individual cases. The case in question here; 12kts, 4t and 10-11m the lowest drag hull is close to flat. You may not want to build the hull that way for a whole range of reasons - often driven by the need for usable space.

    There are other reasons, such as plate rigidity, with hulls why you might want some curvature. A dead flat plate is undesirable anyhow.

    I dare say though that most things you are looking at are based on design fads. Very few designers work from first principles or even understand them. There are only a couple who participate on this forum who work from first principles and they are not hard core boat designers. They work in relatively narrow specialties.

    I have been fortunate enough to be given large sums of corporate funds to undertake research in fields much more lucrative than recreational boat design. I have a fair understanding of what research costs and how it is done. I can tell you that fundamental research in mature fields like boat design is limited. You end up taking pragmatic decisions, typically the lower risk option, because you cannot get the fundamental science completed in time or the designer is not even interested in understanding how it actually works.

    Even with military funded design for multi-billion dollar builds there is often limited scientific knowledge. Take a bit of time to read through the linked paper: PR.pdf
    The first line of the second paragraph in the Exec Summary should be an eye opener if you think the science is understood. Similarly reading the report and looking at the conclusions on p52 you can see how poor some of the design aspects of the Collins Class submarines are.

    I trust what I have measured myself then checked and am very skeptical about claims until I validate them. I make no assumptions about the knowledge of people doing design work - that way I am not disappointed if I find out they do not know much.

  14. Willallison
    Joined: Oct 2001
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    Willallison Senior Member

    Wow... some pretty bold claims there...
    I don't want to get into another shouting match here, but I do have a couple of questions...
    How is it that there's nary a single intelligent mind in the established design industry - how it is that collectively those who have dedicated their proffessional lives to the study and understanding of naval architecture are unable to understand the fundamentals of the craft, yet you (Rick) alone are?
    And how it is that those same designers require the massive R&D budgets to which you ellude in order to undertake meaningful research - yet you are able to do so with nothing more than a freeware computer program?

    I fail to understand why you choose to 'immerse' yourself in a comunity that you obviously have little respect for. Or at the very least, why you find it necessary to be so insulting in your every remark regarding them. Is it not just possible that all those designers that Chris listed are actually onto something?... That they may actually be right...?

    There is little doubt that a great deal of design is the result of incremental improvement, rather than radical rethink. And you are quite correct when you suggest that in a great number of cases, designers - or more correctly their clients - will opt for a more proven form over one that may or may nor work, but is it really necessary for you to say as much in such a derogatroy fashion?

  15. daiquiri
    Joined: May 2004
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    daiquiri Engineering and Design

    Rick, I am impressed by the huge amount of contempt towards other people's work and knowledge transpiring from these words. Don't you see what you have written above?

    Regarding hard vs. round chines - please remember that Godzilla/Michlet (a great tool when used with proper knowledge, precautions and validations) doesn't account neither for turbulence and vorticity produced by hard chines, nor for the relative dynamic lift produced. Hard-chines' lift and induced drag are invisible to it.

    So I think it would be better if you tried to avoid making rushed conclusions about the incompetency of the general scientific and design community based just on the output numbers coming out of your laptop.
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