What I have learned about planing hulls so far

Discussion in 'Powerboats' started by magwas, Sep 7, 2010.

  1. magwas
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    magwas Senior Member

    First of all, all criticism are welcome, as long as at least you tell me what is the error, and you stick to the subject.
    Please do not tell me that I am clueless, or that my design is just plain wrong. These facts are dutifully noted, reiterating them won't add anything to the discussion.

    Chine flats: http://www.bowdidgemarinedesigns.com/Bowdidge_Marine_Designs_1/Chine_flats.html
    Porpoising: http://www.walleyecentral.com/forums/showthread.php?t=77446
    In FreeShip, when calculating resistance with Savitsky method, I have found warnings about porpoising in the "results" tab.
    After much experimentation of different hull forms I have finally noticed the notion of deadrise. It is the angle of V in the bottom. As soon as I have filled it in the Savitsky form, the warnings about porpoising went away. As I understand the V added some drag such that the hull didn't trim up so easily.
    I have also learned what chine walking is. There are articles about it on the net, but all treats it from the boat handling perspective with the assumption that the hull form is already given. The main point is that if buttock curvature is too convex near the chine, a suction force may develop. It sucks down the side of the boat, but easily drops due to a disturbance. When this happens, the heeling moment and inertia turns the boat to the other side, process repeats. It is a dangerous dynamic instability, but as I understand it occurs at much higher speeds than the designed cruising speed of 20 knots.
    To be on the safe side - I am not sure about the speed range where chine walking occurs, and haven't a clue about how one estimates it - most of the boat has straight buttocks, and at the bow I did not design a chine. I have also added lots of buttock lines to the drawing (is 10cm counts as lots?) to help estimate this phenomenon.
    There are also a heap of useful information in the thread http://www.boatdesign.net/forums/powerboats/chine-strake-design-15072.html You can find information about strakes/spray rails there.
    I have also read about stepped hull design, but it seemed too advanced to even think about trying it: http://rib.net/forum/showpost.php?p=115166&postcount=8
    There are the trim tabs, with them you can trim the trim angle:) , so the boat gets to planing mode sooner. They are also useable for some roll control: http://boatsafe.com/nauticalknowhow/captpat1.htm

    Summary:
    Planing hulls should have submerged transom and hard chines with a chine flat. The bottom should have some V in it to facilitate boat handling. Deeper V means more resistance but easier handling. Beam and weight are two important factors in resistance. There are several effects which should be considered, like porpoising (Savitsky method predicts that) and chine walking.

    Now let's see how I have applied it so far:
    The boat is specified as a "similar to Norman 20" LOA=6.1m, beam=2.1m powerboat used for casual fishing tours and cruising (Dubrovnik - Crotona) at 20 knots. The weight given has been some 750kg while draft was given as 0.4-0.7m (which just does not add up for me). Specified headroom is 1.7m. Motor is a Mercruiser 3.0, available building material is plywood (in 8, 10, 12 mm) and white oak.

    With some experimentation I concluded that midship deadrise could be as low as 4 degrees and there is still no porpoising within the 30 knot range. Does it mean that I can safely redraw the hull to have a radically lower deadrise, or I should stay safe and stick to this form at the expense of some 3 kW more power requirement?
     

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  2. magwas
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    magwas Senior Member

    Oops, I have not yet calculated the Froude number for the boat at 20 knots. There is no spray rails designed yet, and I hope the number will be under 4.0, so I don't have to worry about them. Because I don't know how to estimate spray root and water flow direction which - as I understand - would determine -among other factors I don't even know - the placement of them.
     
  3. mydauphin
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    mydauphin Senior Member

    and....
     
  4. powerabout
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    powerabout Senior Member

    you dont need to drop the chine in the water to have chine walk its more complicated than that.
    Many race boats will do this somewhere mid range a good driver knows how to steer it out.
    On a stock boat the small amout of play in the steering is usually what causes it
    I'm talking outboards and sterndrives here.
    Prop design, gearbox shape, engine height, engine set back all can invent it and remove it

    Porposing is just a lift drag COG thing
     
  5. mydauphin
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    mydauphin Senior Member

    Here is one for you. Get too identical boats. One will always be faster than the other. Tiny difference in the hull do make a difference. Even how smooth or rough a hull is.
     
  6. powerabout
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    powerabout Senior Member

    and also the tiny difference in hull CoG, stiffness, engine, set up and prop
     
  7. PAR
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    PAR Yacht Designer/Builder

    That deadrise will require more power to hold it up on plane and it will take longer to get up on plane, then a flatter one. It will be fairly comfortable in a chop, but I really can't see enough of the lines to get the full picture. You could improve it's "climb up" ability with wide chine flats, which would also help her roll characteristics.

    Much depends on what you want to do. Given your target speed, you don't "need" this much deadrise and a flatter run would help in plane efficiency too. Personally, I'd increase the deadrise well forward and decrease the depth of the forefoot, so she doesn't try to bow steer when it gets rough. The finer entry will help part chop. Also what is the logic behind the knuckle in the forefoot? In reshaping for the shallower forefoot, I'd smooth this knuckle out into a nice sweeping curve.

    I'd move the deck structures forward a little and I'd strongly attempt to reduce the "weight" (visual bulk) of the forward most deck structures. Visually, these structures need considerable refinement. The resulting larger cockpit will be welcome, particularly if a I/O box lives at the aft end.

    With all the colors and shading, I can't tell what your buttocks are doing, but they should be straight, assuming you haven't put any rocker in the after portions of the centerline.
     
  8. tunnels

    tunnels Previous Member

    wow i am glad i dont have a cad program !with all that info its no wonder theres some terrible boats out there that do all kinds of strange and not so wonderful things ,
    Go find a good performance boat and find out why it is good !! Go find a not so good boat and see why it is the way it is , then pour all the info in the program and see what you come up with Computers have no common sense or a brain . If you dont put in the right info how will you get the right answer ?? and is it really the right answer anyway ?????:confused: :p :D
     
  9. PAR
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    PAR Yacht Designer/Builder

    Really, it's all about study. After you've absorbed enough understanding, studied enough hull forms and gained enough experience, you can look at a shape and tell what it's target speed is (roughly), how comfortable it'll be under certain conditions, etc.

    The above shape isn't the one I'd use for your target speed, but it'll do. I think it could be improved a great deal, but satisfying my ideas and needs doesn't do a darn thing for the design brief or the client.

    Establish what you want from the hull, then develop a shape that has the volume for your needs and the shapes to suit your preformance goals.
     
  10. tunnels

    tunnels Previous Member

    Maybe i have been living amongst fast boats, good boats and professional boat designers for to long but its possible to simply look at a hull and see what the pit falls are and see the potentual problems and have a pretty good ideal what a particular boat will perform like in certion conditions .You have to have a feeling for what you are trying to design , if you do this then that is what you will get . If the problem is ---- than this is the cause , and this is what you can do to rectify the problem .I worked for company and we made tunnel boats some were family type boats but others were f1 type .Each type had its differances but they also had the simularities , and things that were the same .
    We also made off shore racers with 3 outboards and speeds well over the ton , all the way through were the unwritten about tricks that were used to tweek that little extra .
    After a while all this knowledge rubs off and its possible to predict how a boat will perform before it even hits the water .
    Like i said before ,find the good and understand why they are good and the way they are and then find the bad and understand the differance between the two . Big or small makes little differance the same things cause the same problems in all baots . There is no one design that fits every ones needs . Understand all about all the differant designs what they can do and what they will do . understand how the differant parts of a hull work, then marry all the part together . There are a million thoughts to just a few questions ! get some first hand exsperiance , go work for a boat shop of some sort . learn as much as you can and move on because the next place will have a completely differant set of ideas about the same problems .Some good some bad but its up to you to learn what works and what dosent .:)
     
  11. magwas
    Joined: Oct 2009
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    magwas Senior Member

    This is just a quick update and thanks. I am doing my homework now.

    Thank you for your suggestions. My problem with looking at a hull and predicting what it does that I don't have the experience. I am trying to gain it right now, by fiddling about with shapes and carefully listening to your good advice. But I am so clueless that for some expressions - which might even be trivial for a native English speaker even if doesn't know about boat design - I have to search a lot to understand.
    If there are good examples where hull shapes are shown, and their strengths and weaknesses are analyzed, I am definitely interested in them. The ones I have found so far regarding planing hulls:
    http://www.boatdesign.net/forums/powerboats/express-cruiser-1004.html
    http://www.boatdesign.net/forums/projects-proposals/playing-around-10-m-trailer-cruiser-17597.html
    http://www.boatdesign.net/forums/boat-design/calculating-planing-velocity-14289.html
    http://www.boatdesign.net/forums/powerboats/hull-design-questions-14053.html

    Should I post the lines without color to make it more easy to review?


    I have learned another term. Forefoot: The heel of the stem where it connects to the keel.
    I have problems with the terms knuckle and rocker. I just don't know what do they mean.
     
  12. PAR
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    PAR Yacht Designer/Builder

    Magwas, no disrespect intended, but if you don't know the terminology, then understanding there relationships with each other is well beyond your current grasp. You need to stop worrying about discussion forums and start reading several books on yacht design, such as the current edition of "Elements of Yacht Design". Without a basic understanding of the fundamentals, you will be absolutely helpless in designing your own boat. This is a terrible place to attempt to develop your design understanding and skill set. This is a good place to discuss ideas, but you have to understand what we're talking about for it to be especially useful.
     
  13. apex1

    apex1 Guest

    That was the nice way, which several of us tried already some month´s ago (while you were on vacation). By so far the result is another thread with the same attempt. He thinks, the Forum replies, he rethinks, the Forum denies to reply, he thinks he has to rethink...........
     
  14. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder


  15. magwas
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    magwas Senior Member

    Thank you for the warning but to know the terminology I need to know at least something about the stuff involved. I also think that when I done my homework, and come back here with a definition of words I have learned, it will make easier for those who haven't done it yet to find it without asking (you cannot imagine how hard to find the meaning of "knuckle" in the sense used here in the 'net). This would make this forum a less terrible place to develop design understanding for real n00bs like me.
    Also I am also told when I was a kid that there is only one type of wrong question: which you do not ask.
     
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