hollow waterlines, rowboat

Discussion in 'Boat Design' started by wolle, Jan 30, 2012.

  1. Eric Sponberg
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    Eric Sponberg Senior Member

    As an example, I offer some pictures of two of my designs. The gray boat with the yellow bottom (upside down), pictures 1 and 3, is Bagatelle, which has fairly hollow waterlines forward. This is more than I normally like, but the owner really likes the way the boat performs. Bagatelle actually naturally trims a little aft, so we had to put about 400 lbs of ballast up forward to level her out. That weight, coupled with the very knife-like waterlines makes Bagatelle really slice through the waves.

    The yellow boat with the black boottop stripe, pictures 2 and 4, is Saint Barbara. Here you can see the waterlines forward are very different--no hollow. This owner really likes his boat also. He says it is one of the sweetest, slickest boats he has ever sailed. This hullform is more to my liking, and I was a little more proficient in hull shaping on the computer when I designed her. I was maybe a little less proficient with Bagatelle, I think, new software, new design control tools, etc., etc. and that is why it ended up with a little more hollow than I like. But, both designs work well for their respective owners.

    I see a lot of hollow in sailboat waterlines, and I don't think that in all cases it is necessarily a good thing. I have discussed this publicly before, I think a lot of hollowness in waterlines comes from the software constraints. I have had other designers comment to me on a number of occasions that yes, it is really hard to get the hollow out of hollow waterlines--they are not necessarily desirable. So you have to know how to manipulate your software really well to make sure you get the sorts of waterlines that you really want.

    That said, sometimes a bit of hollow in the waterlines is desirable. So the next question is, can you detect any performance differences based solely on waterline shape using current performance software such as Michlet (which I don't have, by the way, I don't use it). But I am interested in the results. I'd like to know how effective it is. The only alternative is to do model tank testing, and for that you need really sensitive testing equipment, large models, and a talented model testing team. It would also cost lots of money, and I don't know of any clients outside the America's Cup or the Volvo and Open Class venues who would have the money or interest to do such testing.

    Eric
     

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  2. river runner
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    river runner baker

    Ok, yeah, I think I see what you mean and that is what I thought. Looking at a bunch of Robert Perry designs, I do see that most of the newer ones don't have any hollow. But it is pretty standard for canoes to have hollow, expecially one's for touring, rather than racing. I think one possiblility is that hollow waterlines might actually be more efficient at speeds that aren't approaching hull speed, but less efficient when you really want to push things. So someone designing a row boat for fishing might want some hollow, but someone designing one for racing might want to elliminate hollow in the waterline. Does this sound reasonable?
     
  3. Eric Sponberg
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    Eric Sponberg Senior Member

    I think you have that right.

    Eric
     
  4. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Racing kayaks operate at Froude numbers between about 0.6 and 0.8.
    Touring kayaks operate below about 0.4 where there are humps and hollows
    in the wave resistance curve. I would say that it is easier to travel at a Fn
    where there is a hollow rather than a hump. :)

    I hope you won't be offended by my correcting your English usage.
    It is a "plumb" bow, not "plump". Plumb means vertical, plump means fattish.
     
  5. river runner
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    river runner baker

    Who said plump? Not I.
     
  6. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Sorry, I was referring to our German friend's posts.
     
  7. wolle
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    wolle Junior Member

    i will try to improve my spelling, i mean vertical not fattish.

    I decided, not to get off the holloeness of both hulls, 1 and 2. Just forget hull 2. Now here is hull 3, where the following things are equal to hull 1:
    - LWL
    - beam
    - displacement
    - Cp
    - wetted surface
    I cant see any hollow waterlines any more.
    The hull is not very nice, especialy not above the waterline, but its only made to make an answer to the question above possible.
    But now, i have a problem with the .mlt file. There are SRANGE ENDS at some waterlines displayed in the michlet "horizontal cuts" menue. I did a further subdivision in delftship and set the resolution to highest, but they are still there. I dont want to make a diagram of the resistancecomparison hull1 vs hull 3 if this is not solved. Any idea? .mlt and .fbm are attached.


    wolle
     

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  8. Leo Lazauskas
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    Leo Lazauskas Senior Member

    The "strange ends" in the Michlet graphic are apparent because I use
    straight-line segments to produce the curves for display purposes.

    Remember too, Michlet is not a hull design program - it is a workbench
    that can be used to quickly estimate the resistance of thin ships.

    Your hull is pushing the limits of thin-ship theory which requires the
    longitudinal slope of the hull to be small. What "small" is can be debated,
    but estimates should be reasonable for L/B > 6. The thinner the better!

    Leo.
     
  9. NoEyeDeer
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    NoEyeDeer Senior Member

    My 2c: don't worry about the hollow waterlines as long as everything else is good. You may find this interesting: Series 7 vs Series 1 hulls


    A few general points.

    I would start by deciding how much stability you want and how fast you want to go. Once you have decided on these factors, run some hulls through Godzilla and see what comes out. This will give you good targets for length, beam and prismatic, as well as for the actual distribution of volume (since it is possible to have quite different distributions of volume without varying the prismatic).

    If the "optional small sail" is just for fun in fairly light winds, or downwind in a breeze, you should be able to get away with BMT of around 0.5 metres. When rowing, that will still feel fairly stable to the average, clumsy beginner, so you can throw any friend in it without them going swimming.

    Just looking at the lines you have posted, they seem good on the whole, but it has a lot of wetted surface. This means a lot of drag, and that in turn means that there is a limit to how fast you will be able to push it in practice. You may be better having a shorter boat if you really need that much BWL. As a rough guideline, when I was doing fixed seat rowing every day I could push a boat to a speed where its resistance was around 40 kN over some distance, and around 50 kN or a bit higher in a sprint. Easy cruising speed would be around 30 kN or lower.

    I would also suggest flaring the sheerline forward, so that you can have adequate beam where you will want a second rowing station.
     
  10. mcollins07
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    mcollins07 Senior Member

    About a year ago, I ran several hundred variations of the Sharpie hull through michlet and Godzilla for my education. I have not reviewed that data for this post, but will share with you the trends as I remember it off-hand.

    As Leo has already said, the hollow waterlines hulls with slight hollows have lower resistance but only for a very narrow range of Froude numbers.

    I’d add that I you will only find those cases in relatively low DLR hulls.

    More generally, for a given displacement, reducing Bwl or Tc in a hull with hollow waterlines, and thus removing the hollow waterlines, should result in lower resistance for the same displacement.

    On sailboat hulls there are cases where hollow waterlines have their place, because you are designing to gain high performance when heeled. Again, with low DLR. In these cases, the hollowness is small and pushed very far forward. I’m not sure how a row boat could benefit from the hollow waterlines.

    ~ Michael
     
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  11. wolle
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    wolle Junior Member

    So if these "strange ends" are not a sign, that something is wrong, here is the diagram comparing hull 1 and 3.
    Edit: hull 1 is in pale color, 3 in full :Endedit
    It tells (me) not a lot about if hollow is bad or not, because the difference in resistance is so small, so Eric is right in his opinion.

    This statement is also helpful in making further decisions:
    For a preliminary conclusion about hollow waterlines (rowboat, sheltered water) i want to promote this:
    - dont worry to much about (slight) hollownes, if you dont want to go to the limit of performance (speed)
    - its not easy to avoid them "per se" using some hull design software
    - there is a (more or less) positive effect in tracking/courskeeping (other threads and my thoughts)
    - there is a (more or less) negativ effect in pitching/hobbyhorsing characteristic
    - wetted surface and so Rf is increased (if else stays same)
    - might be more difficult to fair during finishing process (not talked about this here yet)
    - this hull has a lot more potential for getting optimised for general performance, reduce beam and wetted surface, wolle!


    This is nice, numbers (are you sure "kN" not "N"?). But how can i imagine, what kind of a format you guy have? Maybe, you are athletic, young, trained, 1,95 (meters) and the same as tough as nvc is in practice: "row hard, no excuse", are you? (i am [sadly] not)

    For me, it would be great to know, that 30 N is possible for cruising and continue the design spiral with that.

    wo
     

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  12. wolle
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    wolle Junior Member

    As i am sailing a 470, i want to say, that unballasted craft like every dinghy has to be sailed allways upright with no heel, else you are not sailing well. You have to trim the boat with your bodyweight, if the wind gets to strong, reef or trim the sail more flat. This (the first) is is not possible with bigger boats (so they have these heavy keels), thus make the demands to the hullform design still more complicated. This should not be necessary in this case.

    wo
     
  13. NoEyeDeer
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    NoEyeDeer Senior Member

    Honestly, I'm not sure about this last one. When I was running Series 7 hulls in comparison to Series 1 hulls it was pretty obvious what was happening. The Series 7 hulls (which had slightly hollow waterlines) were trading off a lower prismatic against wetted surface in the ends of the hull.

    In other words, by using a lower prismatic you save some wetted surface since the girths of the end sections are less. Obviously, this has to be offset by increasing the area of the central sections, but doing this still ends up with less wetted surface overall.

    This means the length can be increased without increasing the wetted surface. This is important, because not only is the residuary resistance roughly proportional to the cube of the length (at speed/length ratios of interest) but the coefficient of friction decreases with increasing Reynolds number, with the result that the longer hull will have less frictional resistance even if it has the same wetted surface.

    Now you can say that by using straight waterlines towards the ends you could still get the lower prismatic with the corresponding lower girths towards the ends. This is true, but without running detailed comparisons it's hard to tell if the straight waterlines will be better or not.

    Given that recreational rowboats are usually of comparatively low displacement for their length, and given that you'll usually be looking at Froude numbers around 0.4-ish unless you're talking about sliding seat craft, and give that the low length/beam ratios mean any hollows in the waterlines should be slight if the craft is sensibly designed, my wild and totally unsupportable guess is that there wont be a lot in it.


    Yes, sorry about my mixing up the units. I meant 30 N. 30 kN is a bit optimistic. :D

    At the period I was talking about I was in my early to mid 30's, and fairly fit but no triathlete. 1.73 metres and about 70 kg. I would think that most fairly fit and average sized people could easily sustain driving a boat that had 30 N resistance.
     
  14. JosephT
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    JosephT Senior Member

    Wolle, your 3rd hull is looking better in my opinion, but there are some attributes of a classic rowboat that some would consider missing as some have pointed out (e.g. curved bow).

    My approach to designing a hull is to find a hull some place on the planet that is:

    a) a proven hull
    b) close to your performance criteria (e.g. resistance, initial & secondary stability)

    The rowboat below looks pretty nice.

    http://www.newfound.com/liz.htm

    You could order the plans and be on your way, or design a similar hull that is more suited to your needs. Unless you're very familiar with some key features of a fast rowboat starting completely from scratch may take some time.

    I do prefer strip boats since they are light & strong...fun to build too.
     

  15. daiquiri
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    daiquiri Engineering and Design

    Question:
    Keeping the same displacement and wetted surface, how do hollow waterlines affect the resistance? What is the physical reason for that increase? You might say, a pair of shoulder waves are created where the waterline changes the direction, hence increasing the wave resistance of the ship. But why are these waves created? I already have an idea, but don't have time to write about it now.
    Once there is a clear answer to the above question, less shaky rules can be made about cases in which waterlines can be hollow, straight or fat.
     
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