For Total Wally

For Total Wally - Planing theory

Wally,

I missed all of your posts and only just read your private message. Sorry, but I have had some stuff to deal with and have been under the radar for a bit.

Here is your most pertinent post on Mysteries of hull dynamics. My comments are the result of my personal thoughts and studies and should not be taken as scientific gospel. In general, I'd say that you have grasped the ideas very well and need only minor clearing of some concepts.

Your earlier post.
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I have followed the thread on lifting strakes with great interest. I have also studied Tom Lathrop’s essay on planning boat theory at his website for the BlueJacket. I am slowly getting a modest grasp of some of this but it is still a bit foggy. I am putting out a list of my assumptions and would appreciate any expansions, clarifications or corrections in my thinking from the collective brain trust. I very much want to point out that I am asking not telling here. LOL, this is what I think, not what I know! I have numbered my points to make it easier to respond.

1. The old 1.34 * the Sq. of the waterline is a mathematical description of the wave system generated by the hull moving through the water. At roughly what is generally considered hull speed the bow is trying to climb over the top of the first wave as the stern settles into the hole created by the second wave. I would only say that the "hull speed" here has the boat comfortably sitting in the hole between these waves. The 1.34 is only an average number for average fat boats and changes for other shapes. It only works for boats in water that is deep compared to the LOA also. Slower in shallower water since waves there are of shorter period than deep water waves. I prefer to ignore it since I don't work much with this kind of boat.

2. A boat is planing or semiplaning if it creates enough hydrodynamic lift to either partially or completely climb up out of the water enough to get the boat up and over and beyond the top of the top of the bow wave. Generally true except for the cases where the wave making is too little to create a significant bow wave. Here, the boat passes from buoyancy supported to dynamic supported condition without having a bow wave to climb over.

3. The resistance of the hull moving through the water once it is planing determines the horsepower requirements to acquire or maintain a certain speed. I am going to call this resistance drag. There are some other forms of drag but this is the principle one. Wave making is never zero and there is aero drag.

4. There are at least two correlations between power and weight. The lower the weight the lower the power required to achieve or maintain speed through the water, lower weight contributing to lower resistance. And the idea of bottom loading, which I take to mean weight of the boat divided by the area of the hull moving through the water. I am thinking that generally speaking the lower the weight per square foot of hull traveling through the water the easier it is for hydrodynamic lift to push the hull up out of the water thus creating less resistance as the hull moves through the water. Well yes, but the area I was speaking of is the projected planar footprint on the water making contact with the hull. Thus a round hull, V hull and flat bottom hull can obtain lift only from the projected area, not the whole area.

5. As a boat moves through the water the water is displaces is generally speaking being pushed down and to the side. Water is also pushed to the side, depending on the amount of V or angle and pushed forward depending on the trim angle of the hull bottom.

6. Hydrodynamic lift is pushing the hull up out of the water perpendicular to the angle of the bottom. Thus a flat bottomed hull is easier to plane because more of the lift is pushing the boat up out of the water compared to a v hull where some of the lift is pushing sideways. This is a little bit confused. The sideways part is not lift, only the part perpendicular (normal) to the hull bottom at any given point. Actually lift is only the part of force that is vertical. All else is drag of some form or other.....Edited to say...That second sentence is an example of how easy it is to say something other than what is meant. The force perpendicular to the hull at any given point is the total hydrodynamic force. Lift is only the vertical component opposing gravity.

7. In a very simplified way delta pads, lifting strakes etc. work by adding the extra lift available to a flat bottomed hull to a v hull. Combining if you will the easier planing attributes of a flat hull with the riding characteristics of a v hull. My thoughts exactly although some other factors like aeration and reduction of skin friction may be involved..

8. The shape and placement of these extra lifting areas can change the trim of the boat at speed for better or worse. You bet.

 

Thank you very much for the reply Tom. I am going to reflect upon your answers a bit. Most things seem clear.

On point 4,

"Well yes, but the area I was speaking of is the projected planar footprint on the water making contact with the hull. Thus a round hull, V hull and flat bottom hull can obtain lift only from the projected area, not the whole area."


I am understanding "projected planar footprint" to mean the area of the hull still in the water as the boat moves at speed through the water. That the hull only generates lift on this area and not obviously the whole bottom surface area, some of which is no longer in the water.

Again, thanks ever so much for the reply. I am fascinated by boats and am extremely curious about their workings. The article at your website was most illuminating. LOL, I wish you'd write a book! You are quite careful to point out that these are often your personal ideas based on physics and not the gospel. But your slightly unorthodox explanations not only make a lot of sense they make the more standard explanations easier to "see". I hope that what ever has been taking up your free time is under control and not unpleasant.

TollyWally

 

Sorry I got your name wrong.

 

Tom,
Now I get it. If I am correct in my understanding along with the hull shape being flatter or more v'ed in a side to side fashion the running angle fore and aft will also effect bottomloading. Not just how much of the hull is still in the water but how flat that portion of the hull is. The steeper the angle the less effective the remaining square footage of the hull is.

The mysteries of hull dynamics indeed.

 

Yes, almost,

That is nearly right except for the last sentence but there must be some trim angle or there will be no lift at all. The total force on the bottom is proportional to the sine of the trim angle. Most people who have investigated this say that a trim angle of 4 to 5 degrees provides an optimum balance between losses due to wave making and frictional drag. Still, some choose a lesser trim angle for other reasons like a smoother ride, a more level ride or better visibility forward.

The boat will automatically assume a trim angle that generates the lift necessary for equilibrium at whatever speed the boat is running. For most boats, this means a higher trim angle at lower speeds that decreases as speed increases.

Consider the water skier. The skis are at a very steep trim angle when starting off at low speed. There is tremendous drag but also high lift. The lift must be high even when starting off to get the skier up out of the water and we must accept the high drag of the high trim angle. It takes a lot more power from the tow boat to get the skier started than to keep him up there. This is much more exaggerated than the "planing hump" of the planing boat of average efficiency because the bottom loading on the skis is much higher than an average planing boat.