Displacement vs Planning

Hulls can be brought easily to plane or with difficulty, but here everybody is forgetting that the most important factor for planing is the power to weight ratio (in a sailboat this means the maximum push generated by sails over the weight). Even the best planing hulls won't plane if you do not apply enough power, by contrast heavier yachts can plane if you just throw in enough power.

Light boats will require little push. This is why some dinghies plane to windward. Medium weight yachts will require more power. This is why modern racing monohulls do not plane to windward but freely plane when off the wind. Heavier yachts can still go faster than displacement provided very strong wind and help from the waves. Going above a certain displacement, planing will however require unrealistic amounts of power, which would require either unsafe winds, too large sails, or simply cannot be handled by structures.

If a yacht has a lot of power for planing, every hull shape will plane. V-shaped hull plane (http://www.vsrfrance.com/images/gallery/VSR_5.8R_2011_2.jpg), albeit less easily than flat shapes. Some rocker will encourage early planing and reduce resistance at low speeds, but will limit max speed.

 

So I've heard a couple of times on this thread that rocker is NOT a detriment to planing. That's entirely different to what I had believed before this discussion.

Is there a rule of thumb for maximum speed per amount of rocker?

 

There are no simple rules of thumb: the subject is too complicated. Most of the the time the things we compare are approximations. And there's a whole world of difference between "Given enough power and low enough weight almost any hull shape will plane" which is really as far as we've got here and "this is the optimum hull shape for this sailboat, which includes the ability to plane on some/all points of sailing".

Rocker for example. If you make the hypothesis that the dynamic support necessary for planing is most efficient at a certain angle of incidence with the water then a hull with much surface that can be trimmed to that angle will produce more lift and "plane" earlier than one with less. Rocker is a definition of that angle on the centre line, but it doesn't tell you how much area of hull is actually producing that lift. If the buttock lines further out are doing very different things to the rocker line there may be low rocker and not much lift at any angle of trim.

All else being equal a hull with less rocker may tend to plane more effectively than one with more, but all else is not equal. If a lower rocker hull shape boat has so much drag at low speeds (maybe its low powered and heavy) that it never gets up to a speed where dynamic lift kicks in then it won't plane. A 70 foot inland waterway barge in the UK has a flat bottom and next to no rocker, but they aren't seen planing. Tow it behind the Nimitz and it will probably plane just fine.

Conversely a lighter thinner higher rocker hull may slip through the water more readily and get up to a speed where dynamic lift will kick in and plane.

So if you want a rule of thumb it has to relate not only to rocker, but also to buttock lines, prismatic coefficient, section shapes, available power, displacement, all the stuff that goes together, and it ain't a rule of thumb any more...

But if you restrict your problem to a small subset, maybe the class rules of a given class of racing dinghy where all the boats are fundamentally similar then sure you can generalise that a boat at the low rocker end of the design space will normally plane earlier and faster than one with more rocker, but it will pay a penalty at speeds where the disadvantages of greater wetted surface and form drag dominate the advantages from dynamic lift.

 

gggG, just to clarify: if a boat has 10 degrees of rocker (at centreline), then it would need to trim bow-up at 10+ degrees in order to plane? I've re-posted your previous sketch to illustrate this principle.

Am I understanding this correctly?

 

Nope, re-read the post. There is more to it than just trim angle or rocker angle at a specific point. As ggg has said (sorry for paraphrasing ggg) it involves Section(s) , Cp, waterline plan lines, buttock lines etc etc. and then the power input

You may need a 10 deg angle of attack on some craft but that would be a one off, specific most likely for that craft only. Forgive me but some craft almost no angle of attack on the bow to midship rocker ie a straight line will do and it will still plane - under sail power (no kite either). But there will be rocker behind this point to let it all behave correctly. The hull will still be generating lift to achieve planing mode with reasonable power.....

You will be aware of the curve of areas and the sort of distribution for decent displacement shapes, and even in planing mode one needs the CB, CG etc so that it (the hull) can be trimmed around these axes.

 

Not much discussion about weight Vs waterplane area, which has a lot of influence in the matter of how planing forces come into play.

 

'Tis true Mr E. Anyway although I've been referring more to sailing craft of relatively small size generally, here's a link to a classic motor craft from the C19th - Turbinia. One reason I have put her here is the large aft rocker (partly for the 9 props on three shafts) which SP seems to believe can inhibit planing....


https://en.wikipedia.org/wiki/Turbinia

 

SukiSolo, your examples are so random that I suspect you are digging deep to prove your case. I've lifted the pix from the Wikipedia site. Turbinia is definitely trimmed bow-up in the cruising photo. And, there is so much sucking and blowing with those 9 propellers that I'm not sure that the hull shape is to promote planing or to prevent starvation and cavitation at the props.

What else have you got?

Ok, you go first.

 

Curved buttocks (and rocker) can induce low pressure in the water flowing past (just like on the lee side of a sail) which sucks the hull down, inhibiting planing.

 

Turbinia was built to prove the turbine engine as a power unit, which she did. In fact her props which had to be redesigned, are so good that even modern CFD tools could only improve them by 2 to 3 %. She is also at a very early stage in the development of fast powered vessels, so bear that in mind.

All my examples do, is to demonstrate (I hope) that there is no single solution to planing for all craft. Primarily it is a power to weight exercise, but right on the margins, especially with sailing craft some shapes work better than others even when length, weight and power are equal. If I sat at the transom of a small dinghy, say less than 4.5 meters, to attain a 10 deg bow up attitude, I'd be last in every race.....

 

It seems me that the discussion is going out of topic. The initial question was: "I want to write a rule that prevents a local class of yachts to become planing. Which hull shape should I use?"
My answer was meant to underline that hull shape is not the only parameter to determine planing ability, but weight has almost the same importance.
All other comments further highlighted the fact that planing is a capability reached by a series of compromises between weight, shape, sail carrying power, etc... and that rough guidelines will not help much.
As has been said, it would be more helpful to know some more details about the class that ggguest is dealing with, or to know which boats are similar. It seems to me that by looking at boats similar in design, weight, and rig it will be easier to infer which parameters will be useful in writing the class rules, rather than by discussing very general concepts

 

PI, I appreciate the clear and concise answer. Of course there are exceptions and not every case will be the same. But you have provided an answer for the general case.

Thank-you very much.

Whenever someone responds with "it depends" or asks ambiguous questions, it leads me to conclude that they really don't know.

 

Sorry, I typed it on the phone, so my answer was brief!

 

More About Rocker

If a boat can be trimmed bow-UP or stern-DOWN to present a positive AoA for it's rocker, it may become less difficult to induce planing.

Or, if the face of a wave has a slope equal to or greater than the amount of rocker, the boat may start to surf.

I think everyone (other than SukiSolo) may agree with this general principle.

 

Yes, Super Piper, that what I meant with my poster about rocker. In my experience, I can compare 2 boats with similar performance but with very different designs: the 29er and the Laser4000. If you look at the polars (or race the two boats against each other) they have extremely similar performance, but their designs are quite different. On one side, the Laser 4000 is slightly longer (4.7 VS 4.2 m), heavier, designed for heavier crews, and with more sail area. On the other side, the 29er is shorter, more lightly built, designed for lighter crews, and with less sail area. The Laser4000 has more rocker and more volume overall: I think this is a consequence of having to accommodate heavier weights without penalizing too much the performance. Racing it, what can be felt is that the rocker helps the start of the planing with medium winds - the boat lifts at the rocker and feels "free". However,at higher speeds the boat starts moving too much water and does not accelerate that much anymore. I suppose it is a consequence of two facts: having more volume overall improves performance for a heavy boat in light winds, but penalizes it at higher speeds, and the "sucking" effect mentioned earlier. The 29er is a different beast, and much more efficient: is start planing smoothly and keeps on accelerating without too much hassle. However, it does not tolerate weight, because it has very little volumes. A heavy crew will have trouble making the boat plane in medium winds.
So, suppose that a 150 kg crew on Laser 4000 is racing a 120 kg crew on a 29er: in medium air the Laser4000 has an edge, because it can exploit its power and start planing a bit earlier. As the wind increases, the 29er keeps on gaining speed, until it comes a point in which one sails in survival mode and the Laser4000 again has an edge because of better stability.