From classic bow to scow bow

Discussion in 'Sailboats' started by Dolfiman, Feb 17, 2018.

  1. Dolfiman
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    Dolfiman Senior Member

    Some news of the Class 40 Carac, she was 4/12, at 50mn behind the first, at the "1000 Milles des Sables", a single handed race qualifying for the La route du Rhum.
    These 2 videos explaining the boat concept with this rounded bow but within the Class rule (in French but images speak by themselves) and the teaser showing the bow in various conditions :

     
  2. Dolfiman
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    Dolfiman Senior Member

    Carac was at only 6 seconds of its first victory !!!, after 6 days 50mn for this 1100 NM Normandy race inc. 27 Class 40 competitors, "with an abundance of different winds sea states and currents", confirming that she can be highly competitive with this bow design.
    Ref : http://www.normandy-race.com/index/newsdetail/idnews/391
     
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  3. OzFred
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    OzFred Senior Member

    Sorry for late response. I don't know whether scow moths were any worse for nose diving than other boats with minimal bow volume. I think skinny skiffs were much more prone to going down the mine, as are modern skiffs at 12', 13', 14', etc. Skiff Moths addressed the issue with a horizontal foil on the rudder, though none of the scows I know of use one (and they don't seem to have a problem with nose diving, though maybe techniques to avoid it are better known now).

    A horizontal foil on the rudder seems to be the best way to avoid nose diving, but they're banned by many classes (I don't know why, they're a great asset for both downwind and upwind performance).

    A wide "semi–scow bow" seems to have some great benefits for cruisers, providing greater width for the cabin in the bow. It does look a bit bulky though.
     
  4. CT249
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    CT249 Senior Member

    The scow nosediving info came from people like Wardii, as well as my own experience as a kid. I think Shorty McKay even wrote about the problem back around 1970, when he was racing his scow Imperium against Europe-style fat skiffs. Designers like Morrison, Bethwaite, Patterson, Moore, Nash and Claridge and lots of world champs in high performance dinghies and cats have also spoken about the fact that a full bow has higher resistance when pressed down and therefore causes the boat to trip. As Dolfiman says, this is dinghy experience - but downwind in decent waves lots of yachts also stick the bow under so the tripping issue may be a real problem.

    Designers like Finot have been consciously trying to make the bow fuller to create more room in cruiser/racers for eons now, but they reach a point where the boat slams and slows in head seas. After all, there's a reason that design in both dinghies and yachts has been moving towards a finer bow for many decades.

    Sure, a scow bow may give more room for a certain LOA - but what's the inherent advantage in that? Why not just make the boat longer? If the bow has to be shortened to keep the boat in some sort of LOA limit then the scow bow could perhaps be seen as a rule-induced feature. That also goes with Dolfiman's point about the fact that the scow bow may only work in classes that have a very big rig for their LOA, which again can be seen to a rather artificial feature.

    It's interesting, but perhaps not really of relevance to most craft.
     
  5. Doug Halsey
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    Doug Halsey Senior Member

    It's no surprise that the Australian Scow Moths would have a nosediving problem. They didn't have much more volume forward than the fat skiffs, and in addition the low freeboard at the bow could generate a lot of force when aimed downward. The photo on the right shows Shorty McKay & Imperium in 1969.

    But the scows in this thread are much more like the higher freeboard Moth scows that also existed at that time (in the US, at least). The scow on the far left belonged to my sister & I assure you it had much less tendency to nosedive in the choppy waters of Tampa Bay than my fat skiff (to the right in that photo). As CT says though, slamming into the head seas was another matter.


    ContrastingScowMoths.jpg
     
    Last edited: Jun 10, 2018
  6. Skyak
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    Skyak Senior Member

    If we assume that "the scow bow may only work in classes that have a very big rig for their LOA" I think we can prove things rather than just argue.

    1) From Dolfiman's original analysis, we see a linear relationship between righting moment based on form and the geometric scow variable he identified (call it S). My only correction would be to use diagonal righting, but the length is fixed so diagonal righting is proportional and thus also proven linear.

    2) The scow hull will weigh more, but as long as that weight add can be taken out of the keel and still result in positive righting change, we have a linear net increase in righting per S. This takes some support but it is safe to say that for the weight of a little lead, you get lots of carbon fiber arch and dome strength.

    3) In righting limited conditions thrust is proportional to righting (by definition of 'righting limited conditions'). The only limit to this relationship would be sail area limitations, but again, this box rule has very generous sail limits.

    4) In flat water, drag as a function of S is a constant -a little higher at low speed/length, a little lower at high speed/length. This is based on all studies of block coefficient.

    The maximum (optimum) of any system proven linear is at the limit. In this case the S variable increases the limit of boat size in the box rule. The only loose ends are;
    - 'wind limited' conditions, but with these crazy high SAD ratios, conditions are almost never wind limited.
    - waves -this takes more work, but it is sufficient to say that these race boats are optimized for the waves going their way. We should turn the conversation to characterize how S affects behavior in waves -it's the key to seeing if large S hulls have something to offer broader sailing. The other question is how lower SAD ratios cap the S value. It could well be that the optimal S is small for the SAD found in common cruisers. The SAD ratios of the box rule are super high because they have canting keels. Conventional keels may be too limiting which would be a shame because of the improved heel angle they get. Dolfiman, this would indicate that the articulating wing keel would be extra beneficial on a production scow cruiser by providing variable righting and displacement.
     
  7. CT249
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    CT249 Senior Member

    Good points and great pics, Doug. Thanks. It will be interesting to see the effects of the increased bow height of these boats.
     
  8. Dolfiman
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    Dolfiman Senior Member

    Dear all, many thanks for your interest and comments about this issue.

    As regard the behaviour when sailing upwind in choppy seas, and the risk of too much occurrences of slamming, an expected disadvantage which could turn into a really unpleasant and crippling flaw : not easy to judge without returns of experience but we can intuited that a rounded scow bow in both horizontal and vertical dimensions (as designed by David Raison and followers for their Mini650's), so always presenting roundness to the incident waves, should less suffer from slamming than a "2D" scow bow presenting a flat bottom. Not only because of less high peak of pressure but also on a rounded surface more resistant to flexure, so a supposed double advantage : less drag, less structural sollicitation. Take care of the roundness, avoid flattness, seems the line to follow.

    The issue of the optimum for a cruising boat, as comprehensively adressed by Skyak :
    From the initial study, I forgot to give you the evolution of the hull surface and of the deck one (and being in travel with just an Ipad , I have not my files with me). If I remember well, at constant LOA, the % of increase of the hull surface is very low (cause the more you put a dose of scow the less the Lwl / plumb bow one), it is more the one of the deck surface which occurs. But yes of course, to introduce a scow bow leads to a bit more mass more forward. The study shows also the evolution of the wetted surface of the hull, at heel angles 0° and 20° heel : respectively + 5,9% and + 2,5% for S = 0 to 0,95 , while RM increases of 30,5 %. So considering this 5,9% combined with the % of extra displacement should lead to the new sail surface for an equivalent performance in light winds, likely in the range of 15% (the extra mass including the strengthening of the rig due to more RM).
    So about 15% more sail on one side, about 30% more RM (at 20° heel) on the other side, the boat is stiffer and the performance is equivalent in light winds and should increase with winds > force 4 as long as no slamming breaks the speed. At least, a delayed reefing is expected.
    To add more sail surface, e.g. 30% instead of 15%, is another option, shifting the speed curve towards light winds, so a better speed by force 2 to 4, but no more top speed beyond (RM unchanged). I would privilege this option to be more powerful by force 3 -4 and corresponding short choppy waves.
    Then the real question : with the budget of a boat with 5 to 10% more displacement and 15 % to 30% more sail surface, can we design a bit longer boat offering more (volume, performance) ? Again not easy to answer without a detailed comparison of 2 designs, inc. with reliable VPP.
    To add a canting keel leads to another step in the performance, with pro's and con's as currently debated in another thread. If the question is : is it a necessity when designing a cruising boat with a scow bow, I would say no, the above option with more sail surface in proportion with more RM seems sufficient.
     
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  9. Skyak
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    Skyak Senior Member

    I can't help thinking that this is relevant to about half the posts I am seeing.

    Two Meleges 20' C Scows For Sale for $1200

    Two Melges 20' C Scows For Sale https://chicago.craigslist.org/nch/boa/d/two-melges-20-scows-for-sale/6606595198.html

    From what I can see these are serviceable, ready to match race today (and for a decade) boats on trailers -$600 a piece.
    The class is still active and relevant -locally for sure.

    Speed doesn't sell?
    Public bored with one design?
    Surely it crushes the thought that sailing costs too much.
     
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  10. CT249
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    CT249 Senior Member

    Do the public know that sailing doesn't cost too much? Down here in Australia, where sailing probably costs less than in the USA, most people think it's very expensive and therefore they don't think of even looking to get into it.
     
  11. valery gaulin
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    valery gaulin Senior Member

    Interesting discussing, I am currently starting the design of a modern interpretation of the catboat with a spherical bow. The idea is for a coastal cruising sailboat with a modern interpretation of the catboat.

    This is the basic hull shape that I am starting to study.

    LOA: 26 foot
    BEAM: 13 foot
    DRAFT: 2 foot
     

    Attached Files:

    Last edited: Jun 30, 2018
  12. Dolfiman
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    Dolfiman Senior Member

  13. Dolfiman
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    Dolfiman Senior Member

  14. Dolfiman
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    Dolfiman Senior Member

    The bow design is in line with the ones mentioned in this thread, but I don't understand the purpose of the concave line part of your sections, at first glance that leads to more wetted surface, and a bit more hull surface for a bit less volume contributing to the righting moment.
     

  15. valery gaulin
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    valery gaulin Senior Member

    The purpose of my design is intended for cruising and not racing. Coastal cruising and shallow bay mooring. I am giving a modern twist to the traditional Catboat hull shape! The concave part in the bottom of the hull is just a way to give some standing headroom in a shallow draft boat. The concave section is around 3 foot wide, it will be the floor of the cabin, no bilge!
     
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