What defines a hull designed for planing

Discussion in 'Boat Design' started by Mik the stick, Feb 6, 2013.

  1. Mik the stick
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    Mik the stick Senior Member

    The figures published on the net for the diesel duck 38ft
    SL Speed HP
    1 6.05 5.2
    1.1 6.36 6.6
    1.15 6.96 8.1
    1.2 7.27 12.2
    1.25 7.57 17.2
    1.3 7.87 23.3
    1.35 8.17 31.7

    Voyaging under power calculations require 80hp for 9 knots. My calculations for 8.17 knots is 32.7hp. Published power requirements are 50-80hp. I think 80hp is way over powered.

    The diesel duck is a single chine hull not round bilge (as I thought). There is no published data for deadrise but I expect it would plane with enough power. 80hp would make almost 11Kts (~SL 1.8) semi planing speed.

    The diesel duck is I think never intended to plane, so what is it that defines a hull designed for semi planing or planing speeds.
     
  2. gonzo
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    gonzo Senior Member

    Check out the thread about the definition of planing. There is no consensus about what planing is exactly or at what speed it starts. Can you define your question?
     
  3. Tad
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    Tad Boat Designer

    The DD is not even close to a planing hull. And those figures are completely wrong, if they came from Buehler's site they seem to be EHP, not what's actually required IRL. To EHP you need to add the power lost in the transmission and bearings, that needed to turn alternators and pumps, and the efficiency of the prop. As a rough guide EHP is about 50% of required. So for 8 knots the DD will require about 60HP, and for 5.5 knots about 13HP.

    What defines a planing boat is power to weight. More than 50 pounds per HP is probably not going to plane, less than 25 lbs per HP is almost certainly a planing boat.

    But what defines a planing hull? Probably the best guide is the bottom shape aft. If the buttock lines are straight, it may be a hull intended for planing or semi-planing speeds....If the buttock's are curved, it's probably not intended to plane. The DD, with it's rounded buttocks, will stand on it's tail at any speed above S/L 1.34.....Straight buttock lines that rise aft are usually indicative of a semi-planing boat, buttocks parallel to the waterline or dropping aft usually indicate a planing hull.

    As always, boats are complex systems and no one feature will necessarily define anything.......
     
  4. Petros
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    Petros Senior Member

    hulls designed for planning tend to have sharp corners had harsh lines, the water breaks clean off these sharp corners when planing and it would have less drag at planning speed. The flatter surfaces will lift the boat up out of the water and reduce the amount of contact with water the hull will have, so at speed it will have less drag. Also, the lighter the boat the faster it will come up on plane since it will take less speed to lift the hull out of the water.

    displacement hulls tend to have round edges and round aft shapes to allow the water to flow smoothing around the hull. The hull must push it way through the water so as smoothly as it can be done the less drag it will have.

    There are very different design approaches with different goals trying to be achieved by the designer. Efficient displacement hulls do not make good planning hulls, and efficient planing hulls will have much higher drag in displacement mode.

    You can make anything plan with a large enough power supply, but that means large fuel consumption unless it was designed specifically for planning. And a planning hull will operate in displacement mode when too slow to generate lift and/or underpowered or too overloaded to achieve planning speed. But it will also consume more fuel than if a hull designed for displacement operation was designed for that size, weight and hp.

    It is very difficult to get a boat that will operate efficiently in both displacement mode and planing mode since the properties required are very different. You would compromise one or the other, or both to get modest performance in all mods.
     
  5. gonzo
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    gonzo Senior Member

    Power to weight does not define planing at all. They are completely different things. A harbor tug has a much higher power to weight ratio than a speed boat.
     
  6. Easy Rider
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    Easy Rider Senior Member

    As far as I know the Quarter Beam Buttock Line tells the tale. Some time ago here on BD someone specified what angle was the dividing line between Full Displacement hull type and Semidisplacement type. But of course it's not as simple as it sounds as the Quarter Buttock Lines aren't even straight so where does one measure the angle? But it's as close as anything I've seen.

    Lots and lots of disp type craft have hard chines so that's not applicable.

    Some say when the bow starts to rise hull speed is reached and I now know that's not applicable as the bow starts to rise well before hull speed is reached. I personally thought that for years.

    And Petros Lobster boats have nice curvy chines but the've got straight Buttock Lines and are planing hulls.

    A rule of thumb is that a FD hull will require about 3 to 4 hp per ton. So if a boat has more than that it's either a planing hull, semi-displacement hull or over powered FD hull. And there are a lot of over powered hulls of all types.

    But as TAD says regarding the defining element of hull types "Probably the best guide is the bottom shape aft." and I will add that the shape aft is probably the best way to judge the overall hull performance of any boat.
     
  7. Mik the stick
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    Mik the stick Senior Member

    So buttock lines point towards the type of hull, I can understand that. The published figures are for BHP. The formulas for the calcs are available on this site and elsewhere, but I didn't want to start discussing formulas here though.

    There is much I have to learn the unanswered question in my mind is, if a round bilge hull is the best shape for a boat at displacement speed why build a single chine hull shape limited to around SL 1.34.
     
  8. gonzo
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    gonzo Senior Member

    It is a mistake to talk about a "best shape" in general. Each application will have a "best shape" which includes the building budget.
     
  9. PAR
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    PAR Yacht Designer/Builder

    You have to take the HP figures George publishes for his Ducks, with a grain of salt or the whole salt shaker, depending on how you look at it. They are notoriously low balled for some reason, usually by quite a bit.
     
  10. Petros
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    Petros Senior Member


    yes, both of these is true, but neither of these would be optimized shape either. There are many round bilge sailboats that also come up on plane as well, and their power to weight ratio is much smaller than what you listed. I pointed out you can plane almost any shape hull with enough power, but it is not as efficient as it would be if it was optimized for planning. You can bring up lots of examples of where my generalizations are not true, but that does not change the fact that an efficient planning hull design is not very efficient in displacement mode.

    It is a complex interaction of shape and speed. Perhaps "hull loading" in terms of lbs per sq ft of projected wetted area might come in to it too. On aircraft the equivalent would be wing loading, total weight divided by the wing area, this gives an idea of how much power is required to get it in the air. Planning a boat hull will have similar forces involved, but the dynamic lift is not required to support the full weight of the boat as it would on an aircraft.

    Needless to say, the best shape for a planning hull will not be very efficient when in displacement mode, and visa versa.

    with all the variable it would be difficult to define a clear line on what exactly is a planning hull and what is not.
     
  11. Easy Rider
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    Easy Rider Senior Member

    Petros wrote;

    "with all the variable it would be difficult to define a clear line on what exactly is a planning hull and what is not."

    There is no clear line of course but one was offered in the form of a buttock line angle. Knowing this angle could (if it's accurate (and I use the word accurate loosely)) we could be closer to the fuzzy truth.

    By the way neighbor I live in Concrete Wash now.

    Mik the stick,
    80hp max is not (in my opinion) over powered for a DD. They are FD and heavy w a hull speed of 8 knots so 80hp may be needed to achieve 9 knots. Also you've got to take into consideration that marketing has a great deal to do w how much power boats have .. especially FD types. They actually require so little most people don't believe it's nearly enough. My 30' FD Willard only needs 20hp to cruise and people are used to lots of power in cars and can't imagine a boat weighing tons only having a 36hp engine. I could do fine w 28 to 32hp but I could sell my boat faster w the 40hp that it has.
     
  12. tom kane
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    tom kane Senior Member

    A flat board is the best for quick and easy to get on the plane but that would not be a ideal for many other reasons.
     
  13. FAST FRED
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    FAST FRED Senior Member

    I would guess 80 Hp is used for the DD as the John Deere engine (rated 80hp) is common and nice ,smooth and quiet (for a diesel).

    It also suffers less from underloading than many similar sized engines , a concern when long range cruising may be at 1GPH , or about 18-20HP , 1/5 of the engines rating.

    9K on a DD might be a goal to take a broken leg crew member ashore ,
    but few will power the boat with such a large engine for a once in a lifetime event.
     
  14. Mik the stick
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    Mik the stick Senior Member

    Well Parr I thought Georges calcs were too low especially as he recommends 80hp and claims 31hp will do SL 1.34. Now I don't think he made a mistake but the answer is in the phrase "calm conditions". One formula I found says total resistance is 55lbs/ton @ SL 1.34.

    (55 x 32600/2240 x 0.003)/0.5 =38.9hp This should be multiplied by 1.33 for adverse conditions giving 51.8hp. George recomends 50-80hp. Defining adverse conditions my guess is gale force 4+. 80hp begins to look like a lot less like to much power. I would bet 80hp could power the DD faster than most crew would care to travel in bad weather.

    The formula I used to get 32.7hp was Froude's formula for WSA +10% for appendage drag (6% might be better).
    Then Froude's formula for skin friction +5% for eddies.
    Then wave friction 12.5 x Cb x (V^4/L^2) x Displacement in long tons.
    Total resistance convert to HP and divide by 0.5 to get brake horsepower".
     

  15. Easy Rider
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    Easy Rider Senior Member

    When things get nasty and I'm buck'in 7 or 8' seas in 40 knots of wind I ALWAYS reduce throttle about 300rpm. NEVER have I increased power to buck winds. My Willy does not, however have a forest of spars and rigging to push through the wind. But if I did I probably would get along fine at normal cruising rpm and as things got worse reducing throttle would probably be more likely.

    Mik,
    My 30' 8 ton Willard is said to require about 18hp for it's 6 knot cruise and I have 40. I'd consider it just a wee tad overpowered and in 7 years and 1000hrs I've never needed even close to full power. I think I'd be just as well off w 28 to 32hp total but I suffer so little I can truthfully say I think I'm powered just about right .... perhaps even perfect. That puts me at 5hp per ton and I know quite a number of FD boats operate at less that 5hp per ton but the Willard is not as efficient as a sailboat or some other FD boats so regarding hull efficiency the DD 38 and my Willard 30 are probably close.

    So is the Duck 16 tons?
     
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