Hull speed differential between a traditional full keel boat and a modern hull

Discussion in 'Boat Design' started by Matthew777, Jun 12, 2021.

  1. Matthew777
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    Matthew777 Junior Member

    I have a question for the community.

    As I understand it a displacement hull is limited by the length of its hull. On a sailboat I would assume that a modern fin keel would come close to this speed limit whereas a traditional full keel sail boat would fall significantly short of this maximum speed.

    What is the differential on say a 10 meter sail boat? Would a traditional hull of 11 meters reach the same speed as a 10 meter high tech boat? Or, would the difference in length need to be significantly more?

    Where am I going? I guess I really want to know how much longer I would need to build a traditional Tom Colvin looking sail boat to match a smaller fin keel race boat.
     
  2. bajansailor
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    bajansailor Marine Surveyor

    They both have displacement hulls, which means that their maximum hull speed is governed by waterline length.
    However the fin keel race boat will have a better ability to surf (when the conditions are suitable) and hence go faster (sometimes).

    Saying this, I remember surfing at 14 knots on a 16 metre traditional long keel yawl with everything up, including a spinnaker and mizzen staysail - and I had one hand on the tiller, and she was very happy.
    I had a big grin on my face as well :)
     
  3. wet feet
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    wet feet Senior Member

    The relevant part is the first question.What destination do you have in mind ?For local sailing you might want to get there an hour or so earlier and for an ocean crossing you might be looking to save a week or two.How eager are you to match the performance of the modern boat ? Because a longer boat will cost you more for materials and take more time to complete.Not to mention needing hardware that is a size up on the smaller boat to handle the larger sails that go along with the bigger boat.At haulout time,you will have to buy an extra can or two of bottom paint too.The other significant consideration is whether the object is to build a boat or to go sailing.If the objective is to go sailing,you can buy a good used boat for less than it will cost to build one and then sail away in a couple of weeks.You will also have saved a year or several of your life that can be devoted to sailing.Alternatively you can devote a lot of time to building a boat that you really want to own.Occasionally such projects are advertised because the builder is too old or decrepit to finish them-they had a dream and enjoyed the dream without actually going anywhere.
     
    Last edited: Jun 14, 2021
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  4. TeddyDiver
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    TeddyDiver Gollywobbler

    Excluding all the lightest wedges the speed is merely about the wind conditions. Light winds modern wins by SA/D ratio, upwind the same with higher aspect ratio sails and keel & rudder. Westward with trade winds pretty much equal. Reaching in strong wind or gale conditions some long keels are unbeatable.. This is only a generalization so there are many exceptions..
     
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  5. DCockey
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    DCockey Senior Member

    That is an over-simplification. Tow any boat behind an aircraft carrier going 30 knots and the boat will go 30 knots.

    The heavier a boat is for its waterline length the more rapidly the resistance increase as the speed approaches "hull speed". Conversely a boat which is very light for its waterline length will not have a rapid increase of resistance as the speed increase. The weight/displacement of the boat for it's waterline length is much more important for this increase in resistance than the keel arrangement. Now it is true that there are very few full keel boats which are light for their waterline length. But there are fin keel boats which are heavy for their waterline length.
    The boat speed versus wind speed relationship of a heavy boat compared to a light boat will be very different. Changing the length will not make the relationship the same.
     
  6. BlueBell
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    BlueBell "Whatever..."

    Matthew777

    Weight, length, shape (including wetted surface area) are the predominant drag/speed determining variables.
    A full keel boat is likely to be heavier than a fin keel and has more wetted surface area.

    Cheers, BB
     
  7. Matthew777
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    Matthew777 Junior Member

    Thank you very much for the clarification. This also explains why there is such an emphasis on boat weight.

    It's important you realize that these types of posts end up on google searches and it is not just I you have helped but countless others seeking to better understand the relationship between hull speed and weight.
     
  8. Will Gilmore
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    Will Gilmore Senior Member

    Let me clarify the definition of Hull Speed. This is a concept that is founded in the power to speed ratio. Hull Speed is defined by a formula: 1.34 times the square root of the waterline length.

    This is what it is in feet of length and knots of speed. This number represents the point considered to be that moment when the application of more power begins to translate less and less into speed. Any boat (including sailboats new or old) should be able to exceed Hull Speed. Hull Speed is not a limitation. It's also not theoretical, it's a definition.

    The reason the formula is based upon the waterline length is because, as a boat starts to move through the water, displacing the water ahead and leaving a hole in the water to be filled behind, waves begin to form along the length of the hull, the wake. The bow piles the water high, which then races aft and forms a trough after which it then builds to another wave peak. At low speed, multiple wakes ripple along the hull. Increase the speed and the wave length of these wakes increase so there are less waves along the waterline. Eventually, the boat can go fast enough that the wave length of the wake is equal to the waterline length. This is Hull Speed. Different Hull shapes and weights achieve this more or less easily.

    The boat can still be pushed beyond this point and they do so all the time. However, once the wavelength starts to get longer than the waterline, the stern begins to drop into the trough of the wake. This creates a condition where the boat is not only trying to overcome friction, but also gravity because the boat is essentially trying to drive up hill. You have felt it on a planing boat just before climbing to the top of the wave and up onto a plane.

    A full keeled boat can and usually does reach the same Hull Speed as a fin keeled boat. Each has other advantages to consider. Under sail, Hull Speed can be changed through design. The old 12 meter race boats were built with lots of over-hang that goes into the water under sail, effectively increasing the Hull Speed by increasing the waterline length as she heels. They did this to meet a racing boat measurement limitation while trying to take advantage of this concept.

    Hull Speed is often used by sailors as a standard to sail to. If they can't achieve Hull Speed, they consider their trim and sailing conditions inefficient. The other useful application is when calculating what auxiliary power to use. Hull Speed is the maximum efficiency at the calculated speed, so it makes sense not to over power your boat as it often just wastes energy with less and less return.

    -Will
     
  9. CT249
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    CT249 Senior Member

    Which long keels are unbeatable in strong wind reaches? Even the J Class only reached at about 14 knots, which is a speed many smaller boats can easily exceed.

    Modern boats also win because of lower wetted surface; more efficient foil sections; finer bows and wider sterns, more efficient sectional shapes, and other reasons. Look at development or "box rule" class and we see the newer boats winning despite having the same SA/D ratios.

    I did a national championship on one of the world's most successful "traditional" 1960s designs. At no time was it ever equal in speed to (then) modern production racer/cruisers of the same LOA, nor did the extremely experienced owners claim it to be.
     
  10. CT249
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    CT249 Senior Member

    Well, the famous traditional-looking Herreshoff 73' ketch Ticeronderoga rates 54 PHRF on long races, where there's lots of reaching. That's about the same as a Farr 30 and much slower than a J/111.

    Now,aTi was one of the world's fastest boats in her day, whereas a Colvin was never designed for speed. So let's say, being very generous to the Colvin, that the hull is similar in speed potential for length to say a Hans Christian 38, rating 195 PHRF or about the same as a modern 18 footer. But even the Junk RIg Association's fastest sailor says the JR is 10% slower than a bermudan, and he uses a carbon mast. So let's say the JR is 12% slower than a Bermudan sloop. A ketch is normally 5% slower than a sloop, roughly, so now we have a boat that is 15% slower than a sloop. All things being equal, to go 15% faster you normally have to go about 15 ft longer.

    So even assuming that the Hans Christian is as slow in basic shape as a Colvin, by the time you put on the typical junk ketch rig you'd need a 53 footer to keep up with a fin keel 18 footer. From what I've seen of Colvins, that's extremely generous. After all, even the famous Ticeronderoga is slower than a 2000-vintage boat of 41% her length.

    But, being VERY generous, you'd have to have a Colvin of say 90ft to keep up with a modern fin keel 30. Actually that seems pretty ludicrous since fitting a JR to Ti would mean she'd have to be expanded to 80ft (at least) to maintain the same speed, and I can't see a Colvin being remotely as fast as a Herreshoff 10ft shorter.

    PS when I was googling Colvin boats I found the account of a Colvin schooner that was lost in a storm. Funny how when a lightweight boat gets lost people say that it proves they are unsafe, but when a heavy or traditional boat is lost
     
  11. TeddyDiver
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    TeddyDiver Gollywobbler

    What I do say is modern boat becoming slower when you reef and waves turn steeper and higher while the longkeeler is just becoming happy and getting a steady heel and reach their max speed. Of course if you have on a modern boat a set of (hank on) sails for these conditions you do much better but how many have nothing but furling head sails for light and moderate conditions.. What comes to hull forms some older IOR's (not extreme) do well in those conditions IMHO.
     
  12. tlouth7
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    tlouth7 Senior Member

    Traditional boats often have a much sorter LWL for a given LOA. Of course the actual waterline length increases under sail due to heeling and the presence of the bow and stern wave, but this may not reach 100% of LOA (which is what the LWL of lots of modern boats is). As such the theoretical hull speed would be lower.

    The point about hull speed is that it (roughly) represents the point where increased power doesn't much result in increased speed. If you turn that round, it means that increased drag (from an inefficient keel or rig) does not much reduce speed either. So if anything it is in strong wind conditions that you are likely to see the least difference in performance between traditional and modern designs (ignoring planing).

    A lighter modern boat might reach hull speed in less wind, or a narrower, heavier traditional boat might be able to maintain close to hull speed in choppier conditions. A bigger difference is likely to be noticed in light winds where the excess wetted surface of a full keel and wineglass hull will hurt the traditional boat disproportionately. This is of course not particularly related to hull speed though.
     
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  13. sharpii2
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    sharpii2 Senior Member

    I don't think there is any controversy on whether light, modern designs are considerably faster than heavier, more traditional types--especially if they're roughly the same Length and Beam.

    The lighter boats are much faster.

    They are capable of what I call "super displacement speed". This would be an S/L well north of 1.5. I've seen J24s do this routinely, sailing at better than 10 kts while on a good reach.

    To get a heavier boat to do that is nearly impossible.

    But the main caveat here is they can do this when driven hard, with human ballast out to weather.

    They may also be able to do this single handed, with a skilled, attentative sailor at the helm.

    But on a longer passage, with more casual sailing, the odds tend to even out.

    For one thing, the sailor is likely to bring more stores and gear along, making the light boat considerably less light.

    It will almost certainly have its performance more greatly effected with the extra weight than the slower, heavier boat.

    Loaded down, the lighter boat may still be faster, but likely much less so.

    And the lighter boat is likely to be less comfortable, as it responds quicker to wave action. This may discourage the crew from sailing it harder, so the performance potential may never be reached.

    But, if your sailing is going to be mostly local, and you and your buddies want to just open her up and see what you can do, get the light, modern boat. The heavier, tradional boat can't compete.
     
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  14. Will Gilmore
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    Will Gilmore Senior Member

    Matthew, as you can see the answer to your question is not so straight forward as the question would imply.

    While you can build two different style boats that are equal in speed under some conditions, you can't build two different style boats that match each other on every point. You will have to decide which compromises you will want to make for what you want your boat to do.
     

  15. bajansailor
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    bajansailor Marine Surveyor

    Everything in boat design is a compromise.

    Arthur Beiser wrote a seminal work in the 1960's called 'The Proper Yacht' - and after the success of the first volume, he added a second volume in the 70's (or maybe it was an edited second edition with some additional boats added as examples).
    Here is a link to the first edition -
    https://www.amazon.com/Proper-Yacht-Arthur-Beiser-1970/dp/B08JVFBBR6/ref=sr_1_2

    And the second edition -
    https://www.amazon.com/proper-yacht-Arthur-Beiser/dp/0877420963/ref=sr_1_1

    In this book he attempted to determine / define what a 'proper' yacht was / is - and I think in the end he concluded that there was no perfect boat as such for him.
    But the boat he had then (re the first edition - I have a copy of this) was a steel ketch called Minots Light which he was very happy with, and she came the closest to his ideals at the time.

    I think that his next boat was a Swan 47 called Quicksilver - I think that she is mentioned in the 2nd edition, and she is very different in every way imaginable to Minots Light!

    And I am sure that if he was writing that book today, his definition of 'the proper yacht' would again be very different from his thoughts in the 70's and 80's.
     
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