Mysteries of hull dynamics

[TollyWally]

I had high hopes.

[SamSam]

What? Your connection never came through?

[Danny White]

Sorry chaps i am interested in your thread so i subscribed to it, i am new to forums and i cant seem to get a handle on veiwing the complete thread can you help

[SamSam]

What you see is all there is. I don't understand it either.

[rwatson]

The thing about mysterious hull dynamics is that you have to be there

[TollyWally]

The original post. I was discouraged by the lack of response and withdrew it.

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.

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.

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.

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.

5. As a boat moves through the water the water is displaces is generally speaking being pushed down and to the side.

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.

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.

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

[TeddyDiver]

Maybe there wasn't much wrong with your thinking...

[TollyWally]

Perhaps, but feedback, positive or negative would be greatly helpful. I have been around boats all of my life but am a layman completely selftaught. I have read many and various things but it is hard to quench my thirst for knowledge.

I posted my understanding of such things, what I think not what I know. I seek clarification. Thank you for your kind words.

[Lt. Holden]

Your thoughts all seem to make good sense to me.

[TeddyDiver]

Actually your "thesis" contain the essential principles of hull planning in a nut shell so it could be used as a reference always when some basics understanding of the issue is needed..

[terhohalme]

Yes, accepted.

Just coudln't resist to add this:

http://www.hydrocompinc.com/knowledg...aningSpeed.pdf

[Riverrat1969]

TallyWally, it looks to me like you have the theoretical, number based ideas, but are still lacking "something"? There is nothing like real world experience, and "gut feeling".

To get what it is you are looking for, I suggest getting behind a boat with a set of water skis on, and closely observing what is happening at your feet, and the amount of pull in your arms. Especially, talk to the boat operator, and have a set of signals worked out, for speed. Pay close attention to your drag/lift, as the boat VERY SLOWLY accelerates, dragging you through the water, from pure displacement, to semi-planning, to planning speed. Once up on the surface, experiment with slower and slower speeds, noticing differences in manuverability and drag. Pump your legs and notice how much the skis sink at low speeds.

Second, make models of your ideas, and try them out beside a real boat, traveling at model speeds. Rig a 2 X 4 boom, to get the model in clean water. Watch it, maybe even take video of it, to watch again later.

Third, try to make kayak sized models big enough to ride in, and feel what they will do in various wave sizes/angles, and at various speeds..........similar to ship models 30-35 feet long,, with 3-5 HP engines, designed to help potential skippers understand the great time lag in handling larger vessels. These experiments don't need to be made from expensive wood, but hardware store stuff that will last just long enough to prove/disprove an idea or design.

Math is good, but intuitive design from actually observing/feeling the responses of a model is the only real standard to determine if your idea has merit, or not. Think about it; theoritically, a Bumblebee cannot fly.

Our ancestors that made a living designing boats used intuitive thought, experience, and the seat of their pants, and made good boats, and some not so good.
TRUST YOURSELF, and take a shot at it, using what you have right now. Get a "moaning chair", because you will be needing one..............all builders eventually use them to sort out road blocks. I hope this helps. Sometimes, just taking the first step is hard to do. As long as your experiments are not too expensive to create, take the plunge! Yes, you WILL make mistakes, but all designers do. Just learn from your mistakes, persevere, and never give up! Every successful design originated from a series of mistakes, until the "bugs" were worked out.

[Leo Lazauskas]

Originally Posted by Riverrat1969

Think about it; theoritically, a Bumblebee cannot fly.

That's true if the theory used in the analysis is too simple. E.g. flat plates used to model the wings etc.

It's an old urban myth.

Leo.