Why Is 'My Hull' So Heavy?

Discussion in 'Multihulls' started by BlockHead, Apr 7, 2020.

  1. BlockHead
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    BlockHead Junior Member

    I drew a simple, fine, 50' hull bottom (pic (pdf) and specs (docx) attached).
    Light displacement (total for two hulls) is 35,697 lbs.
    Then I compared it to Fountine Pajot's 50' Saba at 34,600 lbs (light).
    'My hulls' must have a significantly smaller water plane area and yet they have greater volume.
    I think it is because contemporary cruising cats have don't have submerged transoms so all that volume is cut out.
    I don't get to see multi hull bottoms much let alone their detailed specs so I'm just wondering what I'm missing.
    Any comments?
    Thanks.
     

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  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Without seeing lines drawings of both boats underwater portion, no comparison can really be made.
     
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  3. bajansailor
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    bajansailor Marine Surveyor

    As per what Mr E. says.
    Also, the pdf does suggest that you have a lot of immersed transom area - this will create a lot more resistance compared to no immersion.
    And I think that you will find that most modern cruising cats (including the Fountaine Pajots) are designed to have little or no transom immersion - especially as they invariably have swim platforms on the transoms.
    Remember also that you are just estimating where your waterline will be - you will have to do an initial weight estimate to get a better idea as to where she will actually float.
    Your calculated displacement at your assumed load waterline is 1,097 lbs more than the Saba - working the other way, how much will your draft decrease by if you reduce your theoretical displacement by this amount?
     
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  4. BlockHead
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    BlockHead Junior Member

    I appreciate your comments bajansailor, especially with the limited info.
    I think I understand how and why 'they' 'flatten out the stern'.
    They can still have a length volume graph of a sine curve/wave.
    Instead of transitioning from a semicircular midship through smaller semicircles to a point at the bow,
    they transition through shallower ellipses to a flat stern.

    Although, I believe it introduces other problems.
    I reduces hydrodynamic efficiency somewhat.
    Also, it would 'suck down' the stern at higher speed,
    although I'm not sure this is a problem.
    Most importantly however, I expect it harm seakindliness.
    The stern would be thrown up by waves exacerbating pitching.

    Everyone seems to be doing it but is it really better?
    Everyone seems to be doing fat and full hull but they're not always better, just always cheaper.

    Also, said another way, does anything else seem 'weird' about the hull bottom I drew.
    BTW The LWL is in line with the flat upper surface.

    Thank you.
     
  5. bajansailor
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    bajansailor Marine Surveyor

    I see that your prismatic coefficient is 0.602; bearing in mind that your L/B ratio at the waterline is about 9 (ie very skinny relatively, compared to a monohull) do you think it might be better to increase the prismatic a bit?
    And your quoted effective hull speed is 7.6 knots, giving you a speed / length ratio of 1.07.
    I am thinking that with an L/B of 9, your effective hull speed should be higher than this?
    If your speed / length is 1.4, then your hull speed is about 10 knots - which is still rather modest for a 50' cat (?)
    In this link
    http://www.kastenmarine.com/_pdf/coefficients_of_form_equations.pdf
    they suggest an ideal prismatic of 0.62 for a speed / length of 1.35, so you could maybe even go up to 0.64?

    Have a look at the hull shapes featured on Richard Woods' page www.sailingcatamarans.com
    There is a huge amount of info here - you just have to trawl through all the different designs, open the (excerpts of) study plans, calculate ratios where info is given......
    And see what you come up with. Richard knows a lot about cat design. He might even say hello on this thread.
     
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  6. BlockHead
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    BlockHead Junior Member

    I really appreciate your thoughtful comments bajansailor, thank you.
    A 7.6 knots, the hull wave should go down and back from the bow,
    be at its lowest ~29' back,
    then rise up again to meet ~8' (oops only 8' not 58') behind the stern.
    Hence the effective waterline length and hull speeds.
    Agreed the hull speed should practically be higher and my numbers don't quite sync up.
    I think this is because the hull should easily exceed that speed.
    In such cases, I don't believe the numbers of any hull sync up anymore.
    Having shared those initial thoughts, I'm going to study the links you offered.
    Thank you.
     
    Last edited: Apr 7, 2020
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  7. oldmulti
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    oldmulti Senior Member

    Hi. Hull shapes are anything you want but history gives a hint. Most designs have reasonable PC's eg .58 to .65 bot some designers have gone to .75 (denny proa's). The length to beam is just a function of how shallow or deep you want the hull and the displacement you want. You appear to want a cruising cat hull of about 9:1 with semi circular hull shape. Minimum wetted surface. OK. A deep transom is OK if you don't mind the noise and slight drag increase at low speed. The real issues are where the centre of buoyancy and the centre of gravity of the boat is. If you have fine bows then your centre of buoyancy is likely to be further aft especially with a full transom, therefore your centre of gravity needs to be further aft. Shuttleworth initially designed his hulls with forward fullness above the waterline so when the hull pitched or went into a wave the hull buoyancy moved forward to help lift the bow. The latest thinking is to have wave piercing bows to minimize pitching which requires longer bow shapes to build up buoyancy to lift the bow out as required. Translation to all this: The shape above the waterline can be as important as below and the weight/centre of gravity position of the total boat can influence the hull shape you need. The French cats think 8:1 L:B is acceptable for there incredible weight carry capacity but Grainger Schoining think 11:1 or 12: 1 are good for fast cruisers. The difference is their payload carrying capacity. A few hull jpegs for you. The 226a Crowther is a very good version of a 42 foot fast cruiser. The Grainger wave piercing cat is a good example of the shape. The racing cats give some hints but some are foilers.
     

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  8. BlockHead
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    BlockHead Junior Member

    Hi oldmulti. I am sorry for my delay in responding. Partly I was reading the information on the Woods Design site recommended by bajansailor above. I appreciate your thoughtful comments and helpful lines drawings, thank you. How nice to feel understood. And glad to 'gather' that my hull doesn't seem 'bizarre'. Yes, I was looking for a fine, fair hull with minimum wetted surface. Also glad to hear the transom should only slightly increase drag. I had a Tornado and noticed the gurgling at the stern when I was sitting too far back, before I noticed any drag. I've been thinking about the centers of buoyancy and gravity. The center of buoyancy is ~28' back which seems reasonable. I wanted to stop and check my thoughts on the hull bottom here before finishing the whole multi and getting the approximate center of gravity.

    My idea here is to surround my accommodations with the minimum, fair (arcs fore and aft with no humps or hollows) water plane and them make it all semicircular below. I'm drawn to this kind of hydrodynamically efficient hull shape for seakindliness and pointing. I agree with you and bajansailor, that the hull should have had a higher Prismatic and Speed:Length of about like an Outremer(?). Perhaps Speed:Length around 2 and an effective waterline length of about 100'. That still would have only been an optimal speed of about 10knots. SO I thought I'd just keep it as 'lean' as possible and trust that the higher low speed drag would be easily compensated for by lower overall drag.

    Do any of my thoughts sound 'wrong' to anyone.
     
  9. oldmulti
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    oldmulti Senior Member

    Hi. The centre of buoyancy on most hulls I have seen is 51 to about 55% back from the bow. The shape at the ends can have more influence on a hulls performance than its L:B. Lock Crowther (australian multi designer) told me that fuller end hulls at L:B 10:1 hulls were faster than fine ended L:B 12:1 hulls with similar PC's for cruising boats due to reduced pitching and less problems with payload capacity. Leopard Cats found the same thing. Grainger does full ended 11 or 12:1 hulls which work well but have less payload capacity. Secondly the shape of the ends can reduce pitching without adding excess buoyancy which may shift the buoyancy from the ideal place. Think about a wide stern but above the waterline it being cut back to the deck line IE "sugar scoop sterns" with eg steps. As the boat pitches the stern does not pick up buoyancy that pushes the bow down into the next wave. Don't worry to much about speed length ratio's. If you study actual cruising data you will find a reasonable consistency. Modern mono's do about square root of the waterline, modern well designed NOT overweight cats do about 1.3 times the square root of the waterline. Both types of boats can double those speeds when driven hard but in cruising mode you will find higher speeds uncomfortable and you will want to slow the boat to about 8 - 9 knots for a 50 footer. If your L:B is around 10:1 with PC of 0.60 to 0.65 and fuller ends for a cruising boat you will end up with a relatively good performer. Racing or light weight flyers require differing things like 14:1 hulls, rocker further back, centre of buoyancy further back, slightly higher PC's, finer reverse bow wave piercing bows, shapes more orientated to reducing pitching etc. Have fun in the design process then get a few other opinions. The truth will be their somewhere as there are no absolutes in boat design.
    Once mono's believed fine bows and wide sterns would win races until 2011 when David Raison sailed a 6.5 meter mono in the mini transat that had a scow very full bow. (jpegs attached) Now you cannot win the mini transat unless you have a scow design and the 40 foot racing class is going the same way. Good luck.
     

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    Last edited: Apr 10, 2020
  10. gonzo
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    gonzo Senior Member

    Using race boat design as a comparison to cruising boats is not really accurate. Race boats are designed to beat a formula. A more accurate comparison would payload per dollar, or speed per dollar.
     
  11. waikikin
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    waikikin Senior Member

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  12. BlockHead
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    BlockHead Junior Member

    I appreciate your note of caution gonzo. I believe I am mindful of these differences.
    Having said that, I think there is significant confluence between a sea-kindly hull and a performance orientd hull as opposed to a comfort oriented one.
    And disconcerting how the market doesn't seem to know and/or care much about sea-kindliness.
     
  13. BlockHead
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    BlockHead Junior Member

    I appreciate your thoughts waikikin. These ideas are of interest. I feel I am just getting ready for this investigation and I hope to have an improve my understanding in the near future.
     

  14. BlockHead
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    BlockHead Junior Member

    Hi again oldmulti.
    I believe I understand your points about fuller hulls.
    Like stiffer auto suspension, they can improve speed and prevent 'bottoming' but they only work best in a particular speed range.
    Around town, comfort gets compromised.
    In the same way, I think multis will move away from full bows for pitch mitigation.
    With every increasing demands for comfort and ever greater acceptance and even reliance on complexities I expect we will eventually see widespread adoption of passive and even active stabilization for comfort and performance.
    For now, I prefer fine ends and expect I'll require differing things to reducing pitching etc. as you note.
    BTW Why isn't anyone putting Predator type bows (attached) on multis?

    These planing hulls are interesting to me and illustrate your point about assumptions.
    I tracked down some chapters of Lindsay Lord's book Naval Architecture of Planning Hulls.
    In case anyone is interested and has more money than me:
    https://www.amazon.ca/Naval-Archite...f Planing Hulls&qid=1586638554&s=books&sr=1-1
    I read enough to get a bit excited about it and wonder why it wasn't being used.
    This is the first I've heard of MAGNUM(?) or anything like it, thank you.
    Glad to hear for the second time that I might not be 'crazy'.
    I noted that the AC72 also had what I would call essentially square bottoms (hadn't seen that before either, thank you).

    Speaking of 'squared off hulls'...
    In the current project I've been asking about, beam has been critical, even with fine ends.
    Of course it's valuable for comfort but tends to carry a big penalty in weight, performance, and cost.
    I've been wondering about a hull still be relatively efficient, with relatively less wetted surface, and a length volume graph of a sine curve/wave but a shallower one (lighter and less costly).
    The midsection could be 'flattening out' by going from semicircles at the bow to shallower ellipses amidship and back to semicircles at the stern.
    In a way that should also make the ends relatively fuller.
    Does that sound reasonable?
    Have any professionally designed multis used this?
     
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