How to make a 19 foot planing hull efficient at displacement speeds?

Discussion in 'Boat Design' started by johnnythefish, Jan 3, 2018.

  1. johnnythefish
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    johnnythefish Junior Member

    In theory, what would one have to do to make a 19 foot constant deadrise, moderate V planning hull Center Console more efficient at speeds of around 8 knots? (i.e. above hull speed)

    The parameters allowed are a hull extension of up to 4 feet, and modifying the rear 3 feet of the existing hull. No modifications in the forward 16 feet other than weight distribution/ reduction.

    This is me trying to better understand what the key factors are in why a pure planning hull is so inefficient at these "semi-displacement" speeds and what the critical factors are in solving such issues?
     
  2. Barry
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    Barry Senior Member

    What power? Inboard straight shaft, v-drive, outboard, IO?
    Are you still wanting it to plane or is your only goal that the boat will the boat only become an 8 knot boat giving you more miles per gallon, km per litre,
     
  3. johnnythefish
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    johnnythefish Junior Member

    Outboard power - between 90 hp and 150 hp. Still want it to plane, but doesn't have to plane fast. Want it to be as efficient as possible at 8 knots. Basically, want to do everything possible to achieve a smooth transition between displacement and planing - with as little "hump" as possible.
     
  4. Barry
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    Barry Senior Member

    Why are you focused on "with as little hump as possible"?
    For planing hulls, the hump is a result of the center of lift moving rearward as the boats speed increases, bow up, poor angle of attack, by which I mean the horizontal resistance is high and lift is not optimized. It usually is in a small speed range. Your comment "little hump as possible" leads me to believe that PERHAPS you think that this small speed range of bow up attitude means a bad design? Depending on the hull shape, this speed range could be as little as 3 - 4 miles per hour.
    Possibly designing a hull that minimizes this bow up/hump situation as a focal point will not give you the most fuel efficient, ( efficient= miles per gallon/ kms per litre) at the most common
    utilized speed.
     
  5. johnnythefish
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    johnnythefish Junior Member

    Ok let me rephrase - as the hump really isn't a priority one way or another - other than the fact that for this particular hull it seems to be in exactly the speed range of about 8 kts that I wish to be efficient at. At this speed the boat seems to sit at speed too fast for displacement and too slow for planing.
     
  6. gonzo
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    gonzo Senior Member

    Your target speed is about 1.4 times the "hull speed". That is about the worst range for the boat. An extension would help, but you would need to get the waterline to be almost twice what it is now (31 feet).
     
  7. messabout
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    messabout Senior Member

    What you wish to have is a hermaphrodite boat. To do both things, operate in either displacement or planing mode, is possible but in neither case optimum. This is most often seen as a set of design features of sailboats More often, but not always, small sail boats.

    A displacement boat will need to have its bottom at the transom rise a bit above the waterline. That implies that the bottom will have some curve or rising bottom aft configuration in the elevation view of the boat. More about that in a moment. A planing boat does not have that configuration. The planing boat needs a bottom that is straight from the lowest point of immersion all the way to the transom. In many cases the keel is a straight line all the way from the foremost entry point back to the transom.

    A flat bottom will plane more quickly or with less power applied than a hull with section deadrise. A displacement hull will usually be somewhat better with some deadrise or at least with smoothly rounded after parts. Those sections must rise above the waterline at the transom. Surely there could be some sort of compromise.....yes there is but it is strictly a compromise.

    In order for the afterplane to have sufficient length and afford a livable bow up angle of attack at speed, the after parts must be as long and as straight and have the least angle with respect to the water surface. The quarter beam buttock angle needs to be less than perhaps 4 degrees. Less is better. The quarter beam is what it says that it is. It is half way between the keel and the chine. The buttock is the side view of the structure at some particular point in the fore and aft bread slice of the hull.

    Take a look at some of the planing sailboats. You can see that they are almost always "deep chested". That is to say that the deepest point is somewhat forward of the midships location. The reason for doing that is so that the quarter beam buttock angle is smaller than it would be if the deepest section were farther aft. That is easy trigonometry. ..... Thus the potential for planing is improved because of the smaller angle.

    The hull must be designed in such way as to support the designed displacement while the transom is clear of the water surface. That means that total weight is a major factor. The " aft rocker" cannot be a large number or the run angle will be too large for potential planing performance. All that and a lot more stuff is part of the initial design process of a boat that might be capable of two kinds of halfway decent performance.

    The bottom line here is that you cannot have it all. The flimsy explanation above is only elementary but the general idea is there. I hope this helps.
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    Define...efficient.

    And what compromises are you willing to take, to achieve your definition of ...efficient?
     
  9. TANSL
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    TANSL Senior Member

    Just an idea to unclog the issue. Why not proceed to the contrary ?. One says what an efficient ship is and tells the OP how to achieve that efficiency.
     
  10. PAR
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    PAR Yacht Designer/Builder

    The simple answer is nope, nadda, zip, etc., without making serious compromises to the plane hull's abilities.

    In a nut shell, to perform with any level of efficiency (relative term), you'll need to pick one performance regiment; full displacement or full plane. Now there is an in between area, called semi displacement, but what this usually results in, is an under powered full plane hull shape. In reality, the semi plane hull has some volume aft removed, so it's transom isn't quite as immersed, the buttock angles are a little higher and possibly a touch of rocker too. This would be a possability on your hull, but not an easy thing to effectively pull off, without a lot of effort.

    Given you speed range, you're looking at a full displacement mode craft that's over powered to get past a 1.35 S/L ratio, which isn't much faster than displacement speeds. For example, assuming you have a 17' LWL, full displacement speed will be about 6 1/2 MPH (1.35 S/L), which is about the best you'll do, without having to toss a bunch more power at it. At this speed, you'd need very little power, maybe 20 HP, depending on weight and hull shape, though your bow will be starting to rise a little. If you used a slightly smaller HP engine (10 HP), you'll half your fuel use and only lose a half a MPH in speed. At S/L 1.5, you'd be going about a 1/2 a MPH faster, your bow would be pointing skyward, without some trim modification and your fuel use would start to jump off a cliff, plus you'd likely need twice the HP too. This hypothetical 17' LWL boat will begin the transition to full plane mode around a S/L ratio of 2.25, maybe as much as 2.5, which means speeds of 10.5 - 11.5 MPH. The bow will start to settle, but not much, because you really need to be going a little faster (S/L 3.0+), but maybe some tabs would help in this regard. The fuel use would be double maybe more, than in the semi plane mode, handling would suck, but a fair bit better than below this speed and your power needs will double, possibly more, just to get to this speed (maybe as much as 60 - 75 HP). Once you toss some more power at it, the fuel use curve flattens out quite a bit (S/L ratios over 3.5) and your speed jumps to maybe as much as 17 - 20 MPH. The bow will have dropped down, so you're running level again and the hull will be as efficient as it can be, given the much high HP you're chewing on and huge the drag, of bashing your way through the water at these speeds and higher.

    Can you make these modifications, yep, you sure can, but the real question you need to ask is for you; "what do I truly need". If answered intelligently, you've come to the conclusion you need a different boat. If you want more speed, find a particularly narrow boat a little longer than you have at the moment. These are easier to propel above the usual LWL limited displacements speeds, without the usual difficulties associated with "climbing the hump". Additionally, you can motivate this type of hull with a smaller engine, which eases fuel use requirements. Yeah, you'll lose a little "elbow room", but you'll have more length to play with.
     
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  11. tom28571
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    tom28571 Senior Member

    johnnythefish Junior Member
    New In theory, what would one have to do to make a 19 foot constant deadrise, moderate V planning hull Center Console more efficient at speeds of around 8 knots? (i.e. above hull speed)

    Lots of useful and pertinent information passed on here. The best answer is likely to be to get rid of you boat and design or build one to meet your specific desires. It is not impossible, but they are very rare, even if some manufacturers may claim otherwise. Buying another boat is not a good option since at least 95% of V hull center console boats perform just like yours at or near 8kts. The inescapable truth is that, in order to go from displacement to planing mode, the hull must make a smooth transfer of lift from static buoyancy lift to dynamic lift.

    The only way for a boat like yours to develop sufficient dynamic lift at the "hump" speeds is to have a high trim angle because lift is a direct function of speed and the cosine of trim angle. Thus the boat sticks its nose in the air for more trim, power is applied to give more speed and the resultant lift is finally high enough to get up on plane and the trim required is lessened. The overlying factor here is hull weight since a lighter boat needs less lift. I used the term weight, but the real term is bottom loading in weight sq/ft of the water contact area. You can get past the hump by making the bottom area greater or by reducing initial weight as both reduce bottom loading. My boats are all designed to minimize bottom loading while making the inevitable compromises of satisfying human desires in other areas. They all have aft buttock angles of zero on monohedron aft bottoms forward to station six. The sequence of photos below show that the "hump" can be virtually eliminated with careful design and strict adherence to other important characteristics. Note that the transom has zero sinkage at any speed and that the hull trim angle is mimimum at the usual "hump" and increases to maximum of about 3 degrees at top speed.

    Bluejacket Boats – at mildred's cove boatshop http://bluejacketboats.com/
     
  12. Barry
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    Barry Senior Member

    So to further the discussion consider this option without a lot of cost
    Your original post said that you would be willing to extend the hull 3 - 4 feet

    So keeping the existing hull as it is.
    Build two 3 foot long (longitudinal) trim tabs from chine to chine, leaving just enough room for the outboard to turn. Ie max width, transverse length and 3 feet long front to back for each tab
    The attachment point of the tabs will be say 1 inch above the bottom of the boat to let the transom completely ventilate, a step so to speak so that the trim tab is not an extension
    of the hull but a new planing surface that will create an independent high pressure stagnation line. ie higher pressure area with low wetted surface

    Due to the long tab, which will increase the ability of the tab to lower the attitude of the bow at 8 knots, the increased moment arm of same, the new stagnation area,
    perhaps you can turn the bow down enough so that you are in essence planing with a ventilated transom with a low increase in drag.

    Feasible? In theory this could work, in reality, I think that you would have to try it.

    Assuming that these tabs will need 2 hinges per tab, another variation to this would be to have the hinge angle different than the deadrise angle. Perhaps horizontal or nearly so.
    To create more lift than a tab at say 20 degrees of deadrise
    I am aware that then the inner area of the tab would not contact the water early at low angles of trim down, due to the deadrise but perhaps this might not impact the performance.

    An issue with trim tabs are that if they are too narrow, you often need quite a bit of down angle of the tabs to create the lift to drop the bow. With a large down angle, the horizontal drag
    vector is high with a fuel cost associated with it. A wide tab is better if you have the room
    With a longer wider pre ventilated tab, you get a larger lift component and less drag than a non preventilated narrow shorter tab

    Then at planing speeds of say 15 knots and above, the tabs could be retracted or adjusted to optimize trim angle for higher speeds


    Several years ago a friend of mine had a jet sled, 60 hp outboard jet, about a 4 foot chine wide 16 feet long. The jet leg of course is wider than a prop leg, and 4 feet of chine is not a lot to work with
    I had been with him running some rivers and the bow was always at a high attitude, even at say 15 knots or so. This sled also had a tunnel ahead of the pump inlet to reduce chances
    of hitting rocks when underway which with the loss of lift near the back, helps to create the bow up attitude. The sled was flat bottomed

    I built him a set of tabs that were 18 inches long, front to back and I am thinking that maybe 16 to 18 inches wide, just enough room between the tabs for the engine to turn.
    The result was a much lower bow up angle.
     
    Last edited: Jan 4, 2018
  13. johnnythefish
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    johnnythefish Junior Member

    Thanks for the many replies.

    When I talk about efficient - I mean in relative terms - I know that a planing hull with a large engine will never be efficient in the way a long skinny displacement boat with a 10 hp diesel will be. But I do know that the boat spends a lot of time at 8 kts (fishing speeds) and this is certainly very inefficient - compared to other boats I have been on. Probably the most suitable boats for my purposes are Pangas - and I find it truly remarkable how someone can increase the power and they just seemingly start going a little faster - so you can essentially pick any speed you like and not feel like you are now pouring gas into the ocean.

    I was always under the impression that this was achieved mainly by having:

    1) A Variable Deadrise/ Warped plane hull (which I believe means that the angles of all the frames change from steeper to flatter as one moves from bow to transom - and that this transition goes all the way to the transom (as opposed to a constant deadrise where the rear portion of the hull is at a constant angle).
    2) Longer and Narrower
    3) Lighter

    Addressing point 1) - am I right in saying that Tom28571 addresses this issue in a different way as he uses a Constant Deadrise angle?

    2) Longer and narrower - is this or point 1 more significant/ important. In other words could one have a relatively short wide hull, with a warped plane that achieves what I am looking for. Again I feel like the Bluejacket Boats come someway to achieving this.

    3) Weight - I can get my head around this one on my own!

    In practical terms, I can extend the hull say 2 feet - keeping the constant deadrise angle; I therefore achieve a slightly longer hull. If I then add a transom bracket/ pod - with a step up - do I then create a situation where with water breaking from the back of the transom - I can make the back of the hull more "warped"/ displacement like?

    And are performance improvements from transom bracket makers e.g.. Armstrong substantiated or marketing hype?

    One day - I will build a boat specifically for my criteria - now that I understand my boating needs better.

    At the moment, I have a boat that is undergoing major repairs anyways, and if there was a practical way to improve it, now would be the time to do it.
     
  14. baeckmo
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    baeckmo Hydrodynamics

    The absolutely dominant factor when it comes to reduction of resistance in the "hump" speed region is slenderness ratio; Lwl/Displacement^(1/3). No other factor comes close. If the slenderness ratio (in metric dimensions) can be brought above ~5.5, the hump is almost gone, leaving a slight "plateau". With SR >6.5 the resistance shows an almost linear increase with speed.

    There are a number of basic V-shapes tested, the original series 62 and 65 hard chine hulls (see NSRDC report no 4307/april 1974) and for soft chines, the series 64 is a benchmark. Later there are results from the Netherlands available as well.

    That said, don't waste your time on reshaping the bottom. Go for addition of length (as with a prism) as far as possibly practical, and remove weight by getting rid of non-weight carrying material. And, of course, do the simple math on slenderness ratio you can achieve!

    If my memory is correct, I believe that the forum member AdHoc has published diagrams showing the resistance/weight ratio as function of speed-displacement coefficient for a number of SR's in a thread on this forum; go look for it!
     

  15. tom28571
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    tom28571 Senior Member

    Johnny,

    Since you want to have some improvement from a practical modification to your your existing hull, I would suggest that baeckmo has the most practical and workable solution in just extending the existing hull and paring/distributing weight forward. This will reduce bottom loading, increase aft buoyancy, increase slenderness ratio, increase waterline length (hull speed), depress the bow which increases the former factors in a positive way.

    Warped plane hulls do have advantages in lifting the stern at lower speed but also can have handling stability issues at high speed. Since my boats are not high speed hulls, I might have use a warped hull, but model towing tests showed high speed stability to be bad and even dangerous. Besides, I wanted to try out my own chine design so went with that solution which has been very satisfactory for the performance envelope intended for the Bluejackets.

    There are surely some boats that can benefit from transom brackets but I consider them to mostly be a fad much like jacked up pickup trucks and the like. My preference for a bracket would be a hull extension which provides a swim and loading platform plus the advantages of a longer hull. A transom bracket above the waterline is a cantilever with the heavy motor far aft which increases the trim angle and depresses the stern, all of which are the opposite of what you want. Ventilation of the prop in short waves, requiring throttle shut down to regain control is a problem I have experienced with a bracket that has never been experienced with a transom mounted motor. Trailering a boat with a transom bracket and mounted motor also puts large shock loading on the transom, requiring good engineering to avoid damage.
     
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