# Definition of Planing

Discussion in 'Hydrodynamics and Aerodynamics' started by Leo Lazauskas, Nov 2, 2012.

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### tom28571Senior Member

This is the only definition that makes sense to me. However, not all of the various indicators put forward on this thread must be present. All are not "necessary and sufficient".

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### YellowjacketSenior Member

Agreed, there are too many "gozintas" to quantify it precisely. Too many differences that are specific to any one configuration to allow only one answer, and in some cases one or another may or may not be present.

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### Mikko BrummerSenior Member

Planing - so hard to define, and so hard to simulate (too large a time step/coarse resolution)

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### JoakimSenior Member

Define Savitsky calculations!

The basic Savitsky calculation (from the 60's) fails at rather low speed for racing and other light boats. It doesn't take into account aerodynamic drag, whisker spray drag and most importantly the reduction of effective beam (chine beam). I have all of those models in my Savitsky model and I think it is quite accurate at high speeds as well. You also need to model drive drag accurately, since that will be very important at high speeds.

E.g. the 4 m 250 kg (with driver) racing boat I used to have has chines totally of the water at above 20 kn. Thus you could say Savitsky calculation fails at that point. Still my current model predicts the 48 kn top speed with 40 hp quite accurately (efficient beam is 52% of the original according to my model). At that point the accuracy depends very much on how you model friction, propeller efficiency etc.

I don't really see any reason why Savitsky calculation would fail as long as you are allowed to include models needed. E.g. aerodynamic lift for tunnel boats. If you stick to the 60's model, you will really rule out all faster and lighter boats.

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### YellowjacketSenior Member

Jokim,

I don't think you are quiet there yet. If you look at the surface depth with that much weight, and at that speed, you are not into the "ripple effect" yet and I agree that Sativisky would predict your performance quiet well. Your Crouch's coefficient is around 180. I think you have to be higher in speed, and the predicted depth of plane has to be less than 1/2 the wave height to break out of Savitsky calc's.

There are boats like the C stock and mod runabouts here in the US that are running over 70 mph (APBA kilo records) or a D stock runabout (55 hp 530 pounds weight) and running almost 90 mph. Crouch's coefficients are up around 270! Satisvisky won't predict that, I've used Dingo's spreadsheet (modified to include aero lift and drag) and it seems to fall apart at those kinds of conditions. Not saying you couldn't mod his analysis to cover the condition and match it up with the real world, but I don't think it will work up there unless you make some modifications.

Would be an interesting thesis subject to match the data and come up with a correlation.

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### JoakimSenior Member

The world record of T400 class is 99.3 km/h.They use officially 25 hp engines, but in practice you'll get something like 36 hp from Selva 25. The minimum weight is 240 kg. My Savitsky calculator gets that spot on with perfectly smooth botton and 65% propeller efficiency. But it is already outside of the Cv limits given for the original lift model (1964).

What kind of boats are D stock runaboats? Are they prismatic? Is that 55 hp real SHP?

Using the parameters for T400 my model predicts 73 hp at 78 knots (90 mph) and 68 knots (78 mph) at 55 hp. T400 is not at all optimized for those speeds.

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### YellowjacketSenior Member

T400's are padded V hulls. The planing surface width is pretty narrow and they are also likely pressing through the ripple effect. I don't think a padded V hull will have a planing depth of between a half an inch to 1/4 of an inch unless it's going really fast. At what speed are you predicting the planing depth gets down to 10 mm or less?

D stock runabouts are an APBA class with a length of 13 ft, powered by either a Tohatsu 50 on a Bass (racing) lower unit or Mercury 44XS (Merc 500 powerhead on a racing lower unit) running a prop shaft centerline depth of about 3/4 of an inch below the planing surface. Either motor makes about 55 hp at the prop shaft. They have a min weight of 530 pounds (with driver). Planing surface width is about 24 to 30 inches. The hulls I'm talking about here are flat bottomed (by rule) so the're prismatic for sure. Typical speeds are in the low 80's for a boat set up for 1 mile ovals. The kilo straight line record for the class is 89+ mph.

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### daiquiriEngineering and Design

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### rxcompositeSenior Member

Interesting topic. From my experience, Savitsky do predict good results if used correctly.

We have mass manufactured over a dozen 7 meter prismatic monohull with an inclined shaft. It was made by two groups because of geographical location using the same mold, same jig, same specification. We tested our boats in a protected area, always in calm/smooth water and the results was always predictable, within 0.7 knots using GPS.

The other group data was scatterred. Some 1.7 knots faster, some equal to our performance. The data however showed that they were doing sea trials in widely different conditions from smooth water to rough seas. They were forced to because of time constraint. The data showed that that the boats were considerably faster when the seas were rough. It appears that rough seas provide a "carpet ride" or "roller bearing" effect on the boat, making it go faster.

The next project, different company, was a 30 meter patrol boat. Using Dingo's Savitsky, I predicted 35 knots. When the protype was finished. sea trial showed 37+ knots max. I first thought the spreadsheet was rather conservative but the seasoned NA who conducted tthe test said, "Don't worry, we tested in rough water. The boat is always much faster when the water is rough".

Savitsky can only predict EHP, not the efficiency of the prop and the condition of the sea. A fixed thrustline with a fixed propeller will not give good result if the prop efficiency is not factored in. That is where a good powering program comes to play.

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### JoakimSenior Member

I have modeled the T400 as pure V-hulls. Some of them may have a very narrow pad, but that is not taken into account. The draft is still 60 mm at 99.3 km/h and 45 mm at 89.5 mph.

You can get quite small drags for that D stock boat, if you just can get a reasonable trim angle (as in your picture). I think you need to include aerodynamic moment for that. By just setting LCG way back I got down to 43 hp (at 65% efficiency) at 89.5 mph. Then trim angle would be 1.05 degrees and according to porpoising model the boat should porpoise at any angle. Porpoising is likely stabilized by aerodynamic forces. The draft is only 4 mm. What is a typical LCG of D stock?

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### YellowjacketSenior Member

Joakim,

Obviously a 60 mm planing depth is going to be deep enough that the ripples in the surface won't matter any more, and the Savitsky analysis will be correct. So I believe that you can predict the perofrmance with Savitsky for yor T400.

Even with a narrow pad you should run the analysis as the pad only. I think you will be surprised (or maybe even amazed) how much even a narrow flat pad does to change the depth of planing. I was looking at designing a padded hull for the same class (they run marathon races here in the US and the flat bottom boats take a real beating) and even with a narrow (like 6 inch) pad the planing depth went to a very small number. Not down in the ripples like a flat bottom but it was a lot smaller than 60mm as I recall.

I have the LCG at home, I'll check it, but I'm thinking that it is somewhere between 17 and 19 inches forward of the trailing edge of the planing surface. The motor actually has a cg about 5 inches aft, so the sum of the moments gives a pretty aft cg. I'll take a look at it this evening but it's somewhere in that range. And yes, my analysis also shows very shallow planing depths at high speed for that kind of boat, which is how I came up with the theory that the depth of planing was less than the surface ripples. Something like 4mm sounds pretty typical, so I believe you are correct. If you had a 8mm ripple (easy to get) and a relative density of .5 (a sinusoidal surface) then at a planing depth of 4 mm or less you will no longer be entirely in the water, you would be riding on the wave tops so to speak. And, OBTW, in order to obtain the lift, once the planing depth gets to less than 1/2 the wave height, the trim angle has to increase to obtain lift, again an area where Savitsky breaks down.

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### JoakimSenior Member

Modeling T400 as a 0.15 m beam flat bottom boat yields drafts of 66 and 38 mm at the same speeds as before. Thus not much of a change there, but that really doesn't work at all. Having the pad that deep certainly means that you would get lift and drag also from V portion of the hull, since the angle of the "pad chine" is not that big. In my model trim starts to decrease at about 50 knots and minimum drag is around 60-70 knots, which clearly shows that pad only model doesn't work at those speeds.

I don't buy the ripple theory of Savitsky break down. Certainly small waves can effect drag and speed, but only by in the order of a knot. Are you saying that you get higher trim angle in ripple or that Savitsky would predict that? I would think that in waves/ripples you would have about the same draft to the mean level. I think the slight drag reduction comes from reduced wetted area or induced air.

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### YellowjacketSenior Member

Ripples have no effect and the depth of planing is measured from the mean level, so long as the depth of planing is greater than the wave height. What I'm saying is that as the planing depth becomes lower than the ripple height different things are happening and the analysis doesn't account for them. As the planing depth becomes smaller than on half the wave height, you need to take these things into consideration or the analysis will not accurately predict the results.

Three things happen when the planing depth gets smaller than 1/2 the wave height.

The first is that the effective density of the fluid starts to decrease. Or to put it another way you need more depth to obtain the same momentum change of the water to create the same lift, because less water is present. While the hull is rising up onto the wave tops, the trim angle has to increase since the effective density of the fluid (the mass of fluid that is being pushed down) is lower. Therefore, the tirm angle has to increase to provide the same lift once the planing depth calculated is equal to one half of the wave height. Savitsky has no mechanism to account for that change. You could obviously program it into the analysis, but right now I no of no one that has considered it. It wouldn't be a big deal, and the calculation is relatively straightforward, but I don't know that anybody has done it.

The second thing is that as the hull compresses the air trapped under the planing surface in the ripples the air tries to get out and there is a change in the amount of spray, so there needs to be perhaps a different correlation to account for that. Compressing air and releasing it takes work, and that has to come from somewhwere. If you have a sharp and deep entry it isn't an issue and whisker spray is pretty well understood. If you have a wide flat surface at a very low trim angle that's something different.

The third thing is that, if the depth of planing is less than 1/2 the wave height (trough to tip), the flattened wave tips can't fill the next trough, and now air can pass under the hull and there is reduction in wetted surface drag because now there MUST be air under the hull. There is no correction for that at higher planing speeds.

As I said, I've run these hulls through a modified Savitsky analysis and if you get to very hight speeds and shallow planing depths I'm not getting a good correlation. I've added much better aerodynamic drag analysis to Dingo Tweetdy's spreadsheet, and also added an aero lift component that seems to pretty accurately preditct when the hull blows over, so I think the lift is being added reasonably accurately, but the total power isn't there to match the speeds that are being run unless the prop is near 90% efficient (not likely).

I came up with what I'll loosely call the "ripple theory" to explain why a very simple hull can suddely go a heck of a lot faster than it should. These boats have been around for half a century running these kinds of speeds, so it isn't just a fluke. When the science doesn't predict the observed results, you have to figure the prediction isn't that good anymore. For all of your V or padded V hulls that aren't running that shallow, I'm confident that Savitsky would predict their performance very accurately. But I'm just saying that my Savitsky models don't give me a D runabout that can run 90 mph, and the record books say it's been done.

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### tom kaneSenior Member

It is a pity that explaining what the definition of a planing boat is ..is not as simple as when does an aircraft fly.
An observable every day experiment.
As your aircraft rushes down the runway it reaches critical speeds.
At V1 it is committed to take off.
At VR your craft rotates on it`s undercarriage because the tail plane angle of attack has changed which changes the angle of attack of the wing.
At V2 you should be flying..with a bit of luck.

15. ### tunnelsPrevious Member

finding the sweet spot !! and every boat has one !!

It is really that simple just others dont see it that way !!every boat has a sweet spot and it dosent take figures and calculations to find it ! just take feel and knowing and the breeze in you face !

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