Bottom Loading

Discussion in 'Boat Design' started by TollyWally, Jul 18, 2009.

  1. Willallison
    Joined: Oct 2001
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    Willallison Senior Member

    Tom - I think what Yellowjacket was trying to say is that fast boats are generally more comfortable if you slow down a bit...:p Though whilst it's true that slamming loads are generally reduced by slowing down, as you say... it's not always the case

    I don't think it's a case of a lack of understanding about the importance of bottom loading. Simply a matter of other, more important (read, more marketable) attributes taking precedence. In fact, as I eluded to earlier, for the bulk of the boat-buying public, the pursuit of ultimate efficiency would be somewhat of a false economy: it really depends on how (&/or how much) you actually use your boat. For Jo-Blo who pays 50K a year for his marina berth (by length), and who rarely travels more than a couple of miles from it, the 4-storey, short, fat floating condo is a design success. Of course to then whack a couple of thousand horsepower in it and to sell it to him as a sports boat.... well... you know what I think of that.....
  2. solofast
    Joined: Oct 2008
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    solofast New Member


    It is really pretty simple, lift is a function of speed squared. An increase of 20% in speed (three knots above 15 kts)is actually an increase in lift of 44% (1.2 squared is equal to 1.44). So as you can see, a relatively small increase in speed provides a lot more lift. That is why, as you accelerate over the hump the boat gathers speed pretty quickly.

    The actual equations take into account area, trim angle, lift coefficient (hull bottom shape) and the density of water, but for a given geometry, the biggest factor is the speed squared term. Dingo Tweedie published a spreadsheet that, if you plug in the parameters of your hull, you can get a very good idea as to how you hull will respond to the changes your are talking about.

    Here is a link to the spreadsheet, it is about halfway down the page, download the file planing.xls and play with it for a while, it is most enlightening.

    On the spreadsheet you can add more length to the hull and move the center of gravity forward a corresponding amount and see what the afterplane will do for you in terms of power at a given speed. Dingo also had a note of caution in that his calculations aren't perfect near hump speed, but you should be able to get a good idea of what the changes will do, just don't assume that this is gospel, but it should point you in the right direction.

    If you are satisfied with that cruise speed, and the boat isn't nicely up on a plane, then increasing the area with an afterplane will actually do a couple of things to make the hull more efficient. First you will add lift (in this case goodness), and that lift will be at the back of the hull. This will bring down the trim angle and that will put more of the forward part of the hull into the water, which will also increase the wetted area, reduce the bottom loading further, and will make the hull more efficient at this lower speed.

    Also, think about weight and where it is in the boat. At these low speeds you want the CG forward to keep the trim angle down and get more surface into the water. Move the heavy stuff forward, and look at what you can do to reduce weight in the boat. You might be surprised how a series of seemingly small modifications add up to big results.

    Since you have a fuel flow meter, make one change at a time, and take data with each change. That way you can quantify the results.
  3. tom28571
    Joined: Dec 2001
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    tom28571 Senior Member


    I was not trying to be negative or argumentative. While it appears you were wanting to be general in your statements, I find that you have a great many points put forward. There is no way to have a coherent discussion of so many points at one time. For instance, while it is true that lift increases as the square of speed on a planing hull, that will not normally happen. This is because lift is a function of trim angle and increasing speed will normally reduce the trim angle, resulting in less lift resulting from the trim. In smooth water and at higher speed where almost all lift is dynamic, the lift must be constant and equal to the weight of the boat for all speeds so the trim angle must go down at the higher speed. Otherwise the dang boat would soon take off and fly.

    I can see how you are looking at the various accelerations the hull experiences but, there is too much in your reply to get a good grip on in a forum venue. For instance and relating to the trim angle mentioned above. The vertical accelerations do not always increase with speed. This is because the trim angle will be reduced as speed increases and vertical accelerations are also a function of trim angle. That is one reason why I think I often get a smoother ride by going a little faster. There are also other factors at work here that affect the vertical accelerations at different speeds but, one thing at a time.

    I, and I'm sure the others here, appreciate your joining the forum and your analytical inputs. I am a bit late in offering my welcome to the boat design forum but Welcome it is:) . Where did you go to school? GA Tech?

  4. Yellowjacket
    Joined: May 2009
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    Yellowjacket Senior Member

    No offense taken, just trying to explain the physics of what we all know happens if you go too fast into or off of too big a wave...

    No, actually I went to Florida. I grew up in south Fla. lived by a lake and grew up sking, running hydroplanes and deep sea fishing with my dad. Was fortunate that a good engineering school was available in state, otherwise I never would have been able to afford it.....

    Your points on actual trim angle are certainly valid. And, as you noted in some chop situations lowering the trim angle (by going faster), maintaining contact with the tops and riding on the chop can actually make the ride a lot better. That has to do primarily with the height and frequency of the chop, and the length of the boat, and its abilty to bridge across the top of the surface peaks.

    Obviously the trim angle (and with it planing area also) has to vary to maintain the summation of forces to balance with the vertical load and the summation of moments too maintain proper trim. But, as speed increases the potential lift from any given trim angle goes up as a speed squared function. Remember too, that as you enter a rising sea (not small chop here but a swell of a foot or two or more) the bow lifts first, and the trim angle (relative to the horizontal, and that is what counts here) rises. At the same time the wetted surface area actually increases too. They combine to provide the potential to create a huge amount of lift. With all of this potential lift available, the hull balances its trim angle to what it takes to stay on the surface and, provided you have sufficient speed, it rides up the face of the wave. If you don't have sufficient speed to generate that much lift (and the g's to go with it), you will pass through the top of the wave and any additional lift will be generated by displacement, and will be much less than the dynamic lift. My point being that as you go faster, this will happen, if you are going slower, the lift isn't there to create that much vertical acceleraton and you won't get launched.

    So, if you are mushing along close to planing speed, the increase in dynamic lift isn't going to be as big , since you are already at a high trim angle and the hull is producing about as much dynamic lift as it can. The only additional dynamic lift comes from the unwetted area forward, and if this part has higher deadrise, it doesn't generate that much additional lift at these low speeds.

    It all conspires to result in what we have all seen and felt, if you go to fast up the face of a wave you get can get pounded when you hit it or when you go off the top.
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