Ok then given two hull forms with the same  wetted resitance and the same power and weight we know that the longer of the two will have the highest displacement speed.  My question is will the shorter plane earlier if so what are the sort of differences per foot of length.  Obviously they will need to be pretty close , within a couple of feet for the wetted resistance to about equal.

Further to that how big an effect is a reduction in weight going to make.  I'm really just trying to get a rough handle on the comparative effects of each.

Originally posted by Chris 249 Which sort of ignores the other point, which is that some of the best designers say they can't really tell whether a boat is planing or not; or rather they could tell only if someone threw a lot of money and a test tank at them......

THat is a good point ... I have often wondered HOW you tell if you are planing up wind ...

I think most people sense the sensation of surfing downwind but how do you know when your are plaining - what are the signs?

You know if your're going well but I don't ever really notice a step change in performance of any planing class I have sailed.

Rick

does anyone know if 300's plane upwind?

One of my targets for this year . Sounds like its a bit tricky though!

Col

I agree its a subtle thing in modern boats - they don't have that sudden increase of resistance when they reach a certain speed like some of the older designs and similarly they don't have that sudden release as they overcome it.

This is what so impressed me when I went from Scorpions to Lasers.  In a Scorpion you can feel it overcome the wavemaking resistance and start planing, in the Laser you can't be quite so sure.  The Laser4000 is similar but because it has more power you can do it upwind and you're pretty much always planing off the wind.

When you're planing you are leaving the stern wave behind and the boat is partly supported by the bow wave.  So in my book a sure fire indication is the forefoot of the bow rarely in the water but another indication is how far back is the stern wave.  Dinghies with their truncated lines at the stern ie cut off at transom, normally leave their stern wave about 10-30cm behind them, but when you start planing that increases and the faster you go the further behind you leave it.

Fundamentals rather than a philosophical discussion on the merits of planing...

1) Yes the shorter boat will plane at a lower speed if you define planing as exceeding hull speed.  It has to because it is shorter.

2) At what point is the wetted resistance equal?  All the way from rest to terminal velocity? in which case it doesn't matter which planes first, they both have the same power and the same resistance so they will go at the same speed... 

3) 20% reduction in weight in a 12ft dinghy seems to give about 20% reduction in drag in theory...how good the theory is is up for grabs.

According to whose theory? It isn't nearly so simple. Weight makes much less difference in displacement mode than in planing mode. At lower speeds induced drag from foils is important, at higher speeds it is less important. 

This one by Kevin Ellway...

Thanks. A logarithmic Y-scale would have shown more detail in the bottom left-hand corner but it does indeed appear that, as I suggested, the weight difference is much more proportionally significant at planing speeds than at displacement speeds.

Who is Kevin Ellway and any idea how he produced this data? 

I never said it was a reasonable definition.  It is however the only definition of planing to which you can provide a simple answer to the question.

As you know better than I do Jim there are far more factors than just length that affect the onset of planing (rocker profile, rise of floor, beam, trim, heel just to name a few).  Apart from that there is a continuum from 100% of the lift required to stop the hull from sinking coming from displacement to 100% coming from dynamic lift (if you can ever actually achieve this). 

Where on this continuum do you define planing as starting?  The answer is that you can't define it without an arbitrary or subjective imposition of a border between the two.

Depends on the class I think. Merlins, for example, have been seeking earlier planing for the last decade and will now, in very favourable conditions, plane upwind.

AFAIK the definition of planing refers to the hull being raised above its wavetrain by hydrodynamic lift. Planing is, as you pointed out, not the only way hull speed can be exceeded. Very long narrow hulls such as multihulls and some warships can also exceed hull speed without planing.

The first time I took my 14 out I got someone to video us from a RIB behind the boat. It was single wire occasional marginal twin wire conditions, upwind you can see as soon as the crew hits the wire the wake behind the boat change as it starts to plane. Interesting to watch.

Despite my 14 being a old design the transition from displacement to planing upwind is very subtle, but after sailing the boat a few times you can feel it happen.

Lift is defined by the equation:

L=0.5CLñsv2

NOTE: ñ should be Rho the greek letter P but the symbol won't work on here!

where CL is the coefficient of lift, ñ is the density of water, s is the surface area and v is the velocity squared

Lift will be close to the displacement weight at the point of planning, i.e is the boat and crew weighs 300kg then 300kg of lift would take it out the water but something less will start to lift it out.

Rearranging for speed gives

v = square root (L/0.5CLñs)

The density of water is fixed and the coefficient for lift ahould be assumed relatively constant so the only thing that affects the speed is surface area.

This is why in all RYA learn to sail bits it tell you to lean back when its windy on a reach.  This pulls the bow up and effectively reduces surface area and hence the potential to plane.

If you had a boat with flat sections all the way down the hull it is hard to reduce surface area even when hanging of the back.  Clever designers produce hull shapes that allow the surface area to be reduced the faster the boat goes.  Think of a jet plane wing.  Next time you take off look at the wing.  They extend the trailing edge to increase surface area at lower speeds to increase lift.  Once in the air and speed increase they retract the trailing edge to give a smaller surface area and therefore the ability to fly faster.  If only it was this simple in a dinghy

Look at RIBS and you will see that most have a 'shallow' V hull i.e. relatively flat, which is good for getting onto the plane but then difficult to reduce wetted area easily.  This is why it has spray rails to allow the boat to climb progressively up the rails and hence reduce wetted area.  Offshore RIBS tend to have 'deep' V hulls so that the wetted area can quickly be reduced to give higher speed potentials.

So our friend Froude quite rightly says that a long boat will go faster than a shorter one but a shorter one can plane faster if the hull design allows wetted area to be reduced and has the available power to maintain the increases in speed required.

Actually no. The coefficient of lift is proportional to the angle of attack (roughly speaking and up to to the point where seperation begins). See, for example http://www.allstar.fiu.edu/aero/lift_drag.htm - http://www.allstar.fiu.edu/aero/lift_drag.htm

So one reason to move back in the boat is to increase the angle of attack, and hence Cl and hence lift. Unfortunately this also increases drag.

Unfortunately it is also wrong. Not "detail" wrong, or "second order effect" wrong but fundamentally wrong.

Thanks Stefan for that well balance and informative argument. 

Having looked at the link you posted I realise that my head must have been up my a$%e when I wrote my orginal post.  I can't belive I got so much of it fundementally wrong.  I really must look at my uni notes on hydrodynamics again and write a stern letter to my old lecturer who obviously has been fooling everyone over the years

Could you please publish the dates of your naval architect whisperer tour as I'd look to come and hear you speak

Agreed JimC but from an understanding point of view you have to start somewhere.  Sailing is a complex sport to understand and to succeed in knowledge transfer, you have to start somewhere.  From a coaching point of view, its all about building up the jigsaw piece by piece without getting to 'complex' at the start.  As a qualified naval architect, I am lucky to have been taught about the complexities of this subject over a number of years.  The majority of dinghy sailors haven't had this luxury and hence it is better to introduce a starting block from which to build on.

As for the rant, it just annoys me when someone just says 'your wrong at all levels' with no justification.

My posting on this subject is at an end, thank you and goodnight

i thought planing was the boat overtaking its bow wave or is this another "oversimplification" i dont no much on this topic, all i do is sail. i dont understand the hydrodynamic whatsits.