Planing v semidisplacement

Discussion in 'Powerboats' started by Mat-C, May 27, 2007.

  1. Mat-C
    Joined: May 2007
    Posts: 255
    Likes: 12, Points: 0, Legacy Rep: 141
    Location: Australia

    Mat-C Senior Member

    I've never really understood the difference between a semi-displacement hull shape and a full planing one. I mean, obviously they operate at different speed/length ratios, but if one stands back and looks at a boat out of the water, how can you tell the difference?
    1 person likes this.
  2. SAQuestor
    Joined: Sep 2003
    Posts: 163
    Likes: 14, Points: 0, Legacy Rep: 91
    Location: San Antonio

    SAQuestor Senior Member


    A quick math addition suggests that there are (give or take a few) about 45,000 posts that have the potential to answer your question.

    At the top of the page is a search feature that actually works rather well. I suggest this because we have had a running discussion now for a year (give or take) about semi-displacement and full planing boat designs.

    There is so much information available here that a quick short answer would not do all of the several hundred individuals that contributed any justice. Someone would be short changed. I'm positive that you'll find the answer to this question - and dozens of others that will arise - when you you the search feature.

    [edit] I just realized that this could come off as being a flip answer - it is not meant to be so. It's just that there is a huge amount of information available on this site and a couple hours of reading will probably anwer your question - and more. Good luck!


    1 person likes this.
  3. Mat-C
    Joined: May 2007
    Posts: 255
    Likes: 12, Points: 0, Legacy Rep: 141
    Location: Australia

    Mat-C Senior Member

    Thanks for the suggestion (didn't take it the wrong way btw!) I've read much of the information in the threads you've talked about. I may have missed something, but I'm still unsure as to the actual differences beween the two.
    Ok - some semi-displacement boats have round chines, but not all. Similarly, some planing boats have (smallish) keels. Most semiplaning boats feature variable deadrise, with low deadrise aft - but then, so do plenty of planing hulls.
    So - is there a difference - or a semiplaning boats simply underpowered planing hulls?
    Joined: Oct 2002
    Posts: 4,519
    Likes: 111, Points: 63, Legacy Rep: 1009
    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    Semi plaining will usually have a running angle that keeps most all the hull in contact with the water, at speed.

    The plaining boat will need to lift the bow , to run on a small surface aft to decrease the resistance at higher speed.

  5. tom28571
    Joined: Dec 2001
    Posts: 2,474
    Likes: 117, Points: 63, Legacy Rep: 1728
    Location: Oriental, NC

    tom28571 Senior Member

    If you want one thing that separates planing from semi planing, it is that a semi planing boat will have some rocker in the aft sections and a planing boat will have none.

    There are exceptions to that rule also but if you think about it a bit, you will see the logic of it. You need to know a bit about hydrodynamics but that is necessary to understand how a boat works anyway. You don't need to know any math or even the details of the science. Just the old round side of a spoon stuck under the faucet stream trick will do. The spoon is the displacement boat, the flat of the knife is the planing boat and the semi is in between.
  6. Mat-C
    Joined: May 2007
    Posts: 255
    Likes: 12, Points: 0, Legacy Rep: 141
    Location: Australia

    Mat-C Senior Member

    Thanks Tom, Fred..
    Ok - I've seen quite a lot of semi-planing boats that have rocker - but then I've probably seen just as many with a straight run aft, or even hook in the aft sections.
    I understand the result of all these shapes, but what I don't understand is why any boat that travels beyond its hullspeed would still be better off with rocker in its sections- at least not from an efficiency point of view. From the point of view of comfort in a seaway, I can see some advantages....
  7. tom28571
    Joined: Dec 2001
    Posts: 2,474
    Likes: 117, Points: 63, Legacy Rep: 1728
    Location: Oriental, NC

    tom28571 Senior Member

    I had that same issue when I first looked at the aft bottom of a Carolina offshore sportfisherman. The boat was about 45' long and had a small but visible rocker in the bottom. The builder just said "that is the way the buyer wants it."

    These boats go out the the Gulf Stream in the inhospitable waters off the NC Capes and are expected to run through big waves going and coming at high speed. They have a sharp bow to ease entry into these waves which is understandable. So, I think the reason for the aft rocker is two fold. It keeps the bow high to avoid submarining into big waves and also to keep that sharp bow from taking over the steering when it hits an off angle wave.

    They power these boats (as big as 65+ feet) with huge engines to get the necessary speed to do their daily job of getting paying sport fishermen out to fishing grounds, spend time there and get them back no matter the weather. So, they can afford to live with reduced efficiency of the rocker at planing speed to meet their other objectives.

    As far as hook on a semi planing boat bottom, I'd guess that someone made a mistake.
  8. Willallison
    Joined: Oct 2001
    Posts: 3,590
    Likes: 130, Points: 0, Legacy Rep: 2369
    Location: Australia

    Willallison Senior Member

    Interesting comment Tom. The problem with hook - as opposed to using trim tabs - is that you can't 'dial it out'. In combination with a deep, high deadrise forefoot it can make for a very diificult boat in a following sea. And whilst most semi-displacement boats feature some sort of keel, they are usually quite small. This, along with smallish rudders, compounds the problem.

    But it's a valid question that Mat-C poses. I don't think I've ever read a text that describes the differences between semi-displacement and planing hulls. And these days it's the marketing department that choose what to call a boat, so that's certainly no guide!
  9. tom28571
    Joined: Dec 2001
    Posts: 2,474
    Likes: 117, Points: 63, Legacy Rep: 1728
    Location: Oriental, NC

    tom28571 Senior Member

    True enough Will. The reason that there is no solid definition between semi displacement and planing hulls is that there are an unlimited number of variations between the two. A hull with straight and parallel buttock lines, when powered adequately, is obviously a planing boat. As soon as ANY convexity is introduced to the aft bottom, it starts to move toward a semi planing hull. The case I gave is just a first move in that direction, although a small one and these sportfishermen are still clearly very fast planing boats. The upper end of speed range for a semi planing hull and a full planing hull will almost always overlap.

    I know that some propose thresholds of speed/length ratio as the breaking points between displacement, semi-displacement (semi-planing), and full planing hulls. Since I know of boats that totally ignore such distinctions, I don't think such a definition that is acceptable all the time makes any sense. Becoming locked in to any such dogmatic definitions is a sure way to stifle creativity. In my opinion, any one wanting to be a good boat designer should be able to accept the subtleties and compromises that exist in the process. For what it's worth of course, the opinion of an amateur designer.

    I do understand the value of speed/length ratio in describing these things to a layman (particularly a client), but not as a design tool.
  10. Guido
    Joined: Oct 2011
    Posts: 37
    Likes: 0, Points: 0, Legacy Rep: -12
    Location: Italy

    Guido Junior Member


    Dear forum users,
    I'd appreciate very much your opinion on a new semidisplacement/low planing speed boat project.
    I'd be very grateful in you could spend few seconds in filling in the questionnaire I prepared at following link:

    As soon as the poll will be finished, I'll share with you the main overall results.
    Thanks for your collaboration.

    P.S. Sorry if in the questionnaire you'll find some italian words, it's a limit of Google tool I've used to prepare the poll.
  11. gonzo
    Joined: Aug 2002
    Posts: 16,397
    Likes: 1,432, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    Mat-C: there is no consensus on when a boat starts planing. The debate will probably never be settled. It is usually agreed on that up to 1.4 times the square root the waterline in knots is "displacement" speed. At 3 times is definitely planing. The boundaries between one and the other are the source of controversie.
  12. Mr Efficiency
    Joined: Oct 2010
    Posts: 10,401
    Likes: 1,036, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    A semi-displacement boat is typically a narrow-gutted affair with the maximum waterline beam further aft than a typical displacement or planing hull. Unless someone can point to a beamy semi-displacement boat with full waterlines forward.
  13. Tad
    Joined: Mar 2002
    Posts: 2,316
    Likes: 200, Points: 73, Legacy Rep: 2281
    Location: Flattop Islands

    Tad Boat Designer

    Do you guys realize this thread is 5 years old? Our favorite pollster is dredging up anything that refers to semi-displacement hulls......oh and his link does not work for me
  14. tom28571
    Joined: Dec 2001
    Posts: 2,474
    Likes: 117, Points: 63, Legacy Rep: 1728
    Location: Oriental, NC

    tom28571 Senior Member

    Good fortune does happen to us sometimes.

  15. Easy Rider
    Joined: Oct 2009
    Posts: 920
    Likes: 46, Points: 0, Legacy Rep: 732
    Location: NW Washington State USA

    Easy Rider Senior Member

    My 2 bits worth is that it's all in the Quarter Beam Buttock Line. That is a line halfway between the keel and the chine at the aft end of the boat. The QBBL is straight on planing hulls and on SD hulls the line rises to some degree in it's run aft from about amidships or a bit aft. A lobster boat hull w a straight QBBL (and I think they all are straight) is a planing hull despite the swoopy sheer generally salty looks and very soft chines.
    So to the original poster one can see very clearly if a boat is out of the water if it's planing hull. If the QBBL runs all the way up to the surface (or very close to it) it's a full disp hull and those hulls that unquestionably neither are semi-disp. I realize this definition isn't without grey areas but it's as close as I can come. The only downside is that one needs to see the boat out of the water.
    There are some that assign numerical values to the QBBL angle to make determinations but most all QBBLs aren't straight. When they get really curved like on my own Willard it's hard to measure the angle but one can see clearly that it's steep and the hull is unquestionably a FD type hull.

    Attached Files:

Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.