Pointing ability: hull vs rig

Discussion in 'Sailboats' started by Seafarer24, Nov 13, 2008.

  1. Seafarer24
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    Seafarer24 Sunset Chaser

    Which has the greater effect on pointing ability: the hull or the rig?
    I've seen cruising hull shapes with tall, efficient rigs and wondered if they went to windward any better or if the hull shape has a certain limit after which no fancy rig will have an effect.

    Is there any way to determine the pointing ability of a hull shape before putting a rig on it and taking it sailing?

    I'm designing a 24' cruising sailboat, and want to use a free-standing cat-ketch rig with equal-length masts. I keep hearing comments about how the rig won't point very high and that I should go sloop or cutter, but I keep thinking "What's the difference if the hull won't do better than 45 degrees anyway?"
  2. marshmat
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    marshmat Senior Member

    I don't see any fundamental physical reason why a freestanding cat-ketch would necessarily not point very high. In theory it ought to be a fairly efficient, low-drag rig. Just ask Eric Sponberg.

    No doubt hull shape does affect pointing ability. But to what extent does the hull form affect the windward ability of the boat, as compared to that of the crew who must withstand the pounding and the spray? Fine bows are often considered desirable for this, but there's many a tubby little sloop that can claw her way upwind if prodded. And there's many a fine-lined craft whose hull looks plenty capable of beating at 35 degrees, but that goes way out of balance or just slides sideways if you try- the appendages just aren't in the right places.

    (I'm speaking more from observation (passing these guys in my own craft) than experience, though.... need to get out sailing more. Stupid Canadian winter.)
  3. Stumble
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    Stumble Senior Member

    My experience, though I don't have numbers to back it up is that the sail plan and the foils have more to do with how a boat goes upwind than the hull shape itself. I high efficience main with a large roach and a small overlapping jib in the 110-115 range with deep high aspect foils seems to be the best. As the foil gets shallower not only do you give up righting moment, you also give up the ability to have the keel help drive the boat to windward.

    That being said the few cat rigged boats I have been on seem to really miss the slot effect of the jib, even a small one, and either don't point very well or always seem a little sluggish.
  4. Hansen Aerosprt
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    Hansen Aerosprt Junior Member

    Not true in a contemporary design. Check out the Wyliecat 30 vs a Schumaker Custom 30 or an F27 tri...


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  5. PI Design
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    PI Design Senior Member

    Pointing ability is the sum of the aero and hydro drag angles - google Lancaster's (Lanchaster's? Lancester's?) Course Theorem.
  6. Seafarer24
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    Seafarer24 Sunset Chaser

    I suppose I wasn't clear enough: when I said "hull design" I was including the keel. No more than 4.5' draft and with more intent on course-keeping ability than maneuverability. Likely a separate skeg or maybe even spade rudder, depending on what I can get away with. There's always a chance I end up with a transom/keel hung rudder.
  7. messabout
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    messabout Senior Member

    If the boat will not "hold on" it cannot keep a good course and will not be weatherly. However, no amount of lateral area will do much unless the boat is moving. The next variable is how fast the boat is moving. In very light air, leeway is probable no matter how good the foils are. As velocity increases, foil lift is increased such that less area is needed. Use of the word foils refers to keel, fin, centerboard, rudder, etc not the hot doggy things that lift the boat out of the water.

    These realities constitute a convoluted set of requirements. The sails must be capable of propelling the boat and hopefully well enough cut and laid out to make good pointing ability a possibility. At the same time the boat must have enough "grip" to allow the sails to do their thing.

    So what is the answer to the original question? I doubt that there is a definitive answer. Too many variables. Step back a ways and consider whether your boat is going to be a speeder or a slogger. If it is going to be a speeder then you can get away with less fin, board, or whatever. If a slower boat then plenty of keel is in order. Tampa Bay has all sorts of weather. In mid day summers there may be almost no useful air. At other times, there is more air than you can use and the water can get pretty lumpy. So part of the consideration is when and where you plan to sail.

    How is that for a non answer?
  8. JesperW
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    JesperW http://journeyman.se

    The rig generates side force plus forward force. The better the sails (and sailor...) the more forward force per side force.

    The hull (mostly keel+rudder) generates side force (usually called lift) plus drag resistance. The better the foils are (high lift to drag ratio) the less drag you get.

    Symmetry is the key. The side forces of the rig must balance the side forces of the underwater body. Otherwise the boat moves sideways. And then your speed will be determined by the balance of the rigs forward force and the hulls drag force.

    So the original question is not really possible to answer. It is the balance between the two that gives the boat its properties. You can't look at one separated from the other.
  9. Eric Sponberg
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    Eric Sponberg Senior Member

    In my opinion, the rig and the hull have a symbiotic relationship that is a little one-sided. A hull can live without a rig, but a rig cannot live without a hull. Also, all movement of a sailboat comes from that which creates the power--the rig. In all things related to power in aero and hydrodynamics, there is lift (creates power) and there is drag (consumes power). A hull by itself can create a little bit of power, but it sure helps a lot if it has a keel and a rudder, so we include these with the hull. But the only way the hull and its appendages can create their own power is if they are already experiencing movement as generated by the rig. Therefore, in the hierarchy of sailing yacht design, the rig really is the primary factor that controls performance, and the hull with its appendages makes a smaller contribution. A bad hull design with a premier rig can completely kill the boat's performance. A bad rig with a good hull does not fare any better. That is why you need a good rig and a good hull--they have to go together. But the rig is the prime mover.

    Can we precisely say what portion (rig or hull) each contributes? That varies from boat to boat and we can debate it until the cows come home and not arrive at a definitive answer.

    I will say, and have always said, that a well-designed stayed rig is very efficient sailing to windward, and a well designed free-standing rig by comparison is not really different and not necessarily primero uno. The true benefits of the free-standing rig really show on other points of sail, in the ease of tacking, and in off-the-wind sailing. These features come into their own for cruising sailboats, particularly, because of the variety of wind and sea conditions that cruising sailboats confront, and because such boats are usually short-handed. In racing, courses typically favor the upwind legs in fair weather and sea conditions, and boats have enough crew to handle the tasks of tacking, gybing, and changing sail. Therefore, racing favors the Bermuda sloop rig.

    We have an unusual situation going on right now with three round-the-world races running concurrently: Volvo (multi-stop), Portimao (multistop), and Vendee Globe (non-stop). Of these three, the Volvo and Portimao necessarily have to sail in regions of both maritime weather and continental weather by virtue of their required stops on shore. This necessarily creates conditions that require excellent windward ability, therefore, Bermuda sloop rigs. Go back through the history of the multi-stop round-the-world races, study the tactics, and see how many times first place traded back and forth between boats in the final 100-300 miles of the leg, in the realm of continental weather--the instances are legion.

    The Vendee Globe, on the other hand, sails only in maritime weather throughout the race except for the very beginning and ending. This necessarily favors off-the-wind sailing and downwind running that would favor the free-standing rig. I have maintained for a long time that if one would really like to win the Vendee Globe, the free-standing rig should be the rig of choice because of its advantages off the wind. Likewise, for the Volvo, Portimao, and the Five Oceans (ex BOC and Around Alone) races, free-standing rigs are comparable in performance to a stayed rig, and off the wind they could exercise their advantages. However, overall, the free-standing rig's performance superiority would be less pronounced because of the nature of the routes with the regions of continental weather sailing where the stayed rig can excell.

    I would love to see more instances of free-standing rigs competing in round-the-world races. I think we have only scratched the surface in seeing what these rigs can really do.

  10. idkfa
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    idkfa Senior Member

    Seafarer24 IMHO the sail-area to displacement IS the limiting factor; power for largest drag component. Next, having high aspect ratio foils, BOTH above and below the water, not only lowers drag but also increases lift, adding a few degrees over “average” ratio foils.

    Being a cruising boat, the design probably has an average SA/Disp ratio and therefore average point to start with. The keel depth is average for hull length and probably average aspect ratio too. Don’t think the cat-ketch rig would hurt the angle at all or be any less than average.
  11. El Sea
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    El Sea Junior Member

    My experience, a fractional rig will point better and a beamy boat will not.

    El Sea,
    St Petersburg Fl
  12. idkfa
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    idkfa Senior Member

    Lets consider the ultimate windward machine an IACC boat. We lose the fathead and change the aspect ratio of the sails to average. We assume this increases total drag 5% and reduces lift 5% (fair assumption?). I’m suggesting (guessing) the windward angle falls around 3 degrees.

    Now lets consider a similar IACC made out of solid fiberglass and keel out of steel yet keeping total Disp the same. Now we have a hull/bulb ratio of 80/20 instead of 10/90. Also an Al mast instead of carbon. The righting moment goes way down (half?) and so SA would have to, but lets make the new sails high aspect ratio and fathead. I’m suggesting the angle falls 10+ degrees.
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    FAST FRED Senior Member

    Pointing really high is only of use to the racers , who happily trade speed for higher pointing to "beat" the other guy.

    For a cruiser , where HP is required to climb those little water hills that get bigger by the hour , a few degrees of pointing ability has no value as HP is required.

    For most cruisers with a long , groundable keel, the old Herrishoff rule of thumb of 1/7 LWL to Draft makes the most sense.40% ballast ratio for comfort.

    This will usually give a hull keel combo that sideslips 5deg , at reasonable angles of heel.

    The power required for hill climbing requires most boats to sail at least 5+ deg off the wind from what they might in still water.

    So 30deg for most rigs , 5+ deg for HP and 5 deg of hull slide is 40deg off the wind, to beat to windward.

    Admiral Nelson charts show his fleet tacking in 90deg , so a the years of NA work do make a difference , but not a lot for a cruiser.

  14. ancient kayaker
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    ancient kayaker aka Terry Haines

    It's not really the pointing ability that delivers the goods upwind, it's the speed made good over the course. Obviously PA helps.

    Also, it's not the sum of the efficiencies you are dealing with, it's what's left after you have lost out to the inefficiencies. If you could add the efficiencies you could just keep going faster by improving the rig on any boat, but you can't. If you improve the rig ad nauseum you will soon encounter diminishing returns for increasing investment. In other words, the world's best rig won't do much for a tub, even if the tub can hold it up. Generally there's not a lot you can do for the hull, but whatever you can do needs to be done.

    Hm, I don't understand that at all. Masters of obfuscation, you have met your match.

  15. timothy22
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    timothy22 Junior Member

    I have had it explained to me that the rig and wind determine how close to the wind you can point, and the hull and sea conditions determine your speed made good to windward. To cite an extreme example, an efficient rig may drive a normal hull at 40% of the true (moderate) wind speed in smoothish water, whereas a very "slippery" hull will allow the boat to become an apparent wind machine, adding enough of its forward speed to the true wind speed to generate an increased apparent wind, greatly increasing its speed through the water and to windward, but at the cost of having to bear off to keep the apparent wind from moving too far forward.
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