Displacement vs Planning

Excuse me, are you saying that each of these boats has planed?

 

No, because not all boats can. However a lot of boats can 'surf' or at least get above displacement speed. I'd guess that even meter rule boats can surf a bit too.

I personally know of a bilge keel Westerley Centaur that hit nearly 11 knots. Well if you google that old design you would not believe you could do it, but it did...

The Sigma is a sweet displacement type, not so far removed from some of the old IOR boats. As indicated before, it depends on what you are aiming at, whether you wish to aim at planing or wish to stay displacement. The former does not mean to diminish the latter but still needs decent displacement speed upwind where the sail carrying power will not generally allow planing upwind without extra moveable ballast (well off the rail - a sliding arm and/or canting keel.

Plenty of keel boats plane they just have to be designed for it, equally you can mire it to displacement (and surf) speed only. The best ones which do plane can be surprisingly good sea boats, with nice behaviour on waves.

 

All depends what you call planing... hull speed isn't a hard limit anyway and you can push well beyond it into the transitional region (what Bethwaite calls forced mode) with sufficient power and yet no significant dynamic lift. Ask any destroyer captain. Lead, gravity, and a steep enough wave provides significant additional power.

Planing, to my mind, is when a significant percentage of the displacement of the boat is supported by dynamic lift from the hull skin. There's no especial reason why this should have an intimate relationship with the Froude number, being a function of hull shape more than length.

 

Exactly ggg, that what was I was attempting to convey. The real trick is the seamless transition and the shorter the hull length the harder that is....

 

I totally agree with this. Far be it for me to criticise the work of Prof Dan Savitsky, but his papers all intimate a link between Froude number (or speed length ratio) which causes much confusion in my experience. Any link between planing and Fn is indirect at best, but so many people quote planing beginning at Fn = 1 or similar, and a rumour repeated often enough becomes the truth.

 

This is a hottly debated topic, but it can be generally agreed that displacement is speeds below 1.4 S/L, while the transition to plane is 1.4 to 2.5 S/L. Some suggest you need to be traveling faster than 2.5, maybe as much as 3 S/L, but it's an arbitrary point really, as the boat is actively driving over it's wave train with considerably velocity. There's a number of ways to get "shapes" to do this, though power to weight (assuming capable shapes are employed) is the key. A Nimitz class aircraft carrier is technically able to transition to plane speeds. It's theoretical displacement speed is 37 MPH and published speeds are 35 MPH, but I personally paced the Nimitz herself, doing well over this after her refit in Philadelphia, so . . . Her shape would suggest she's not remotely capable, but toss the reactors into safeties off, 110% output and she flips up her skirt and scoots.



Yeah, they actually get together and race these puppies.

 

It's generally agreed that before Columbus people thought the world was flat, but it's wrong.

A destroyer or a fast multihull is driven way beyond so called hull speed, but if the term planing is to have any value as a description of hull behaviour then they ain't planing. Nor is the Nimitz.

And conversely a sinker sailboard is supported primarily by dynamic lift way below hull speed, so rationally you have to say it's planing.

If you don't make this distinction of supported by dynamic lift from Hull speed/froude number then there's no such thing as a planing hull shape and you're going to need to coin a new word.

Pi design is exactly right about the confusion, and I've been as guilty as anyone in perpetuating it in the past, but as light structures and ultra thin multihull types increase in numbers planing behaviour and hull speed will become less and less linked, and we need to get the distinction right or create even more confusion.

One of the least heralded things in Bethwaites later work is that he cracked the reason for his so called humpless hull, and it's related to this point. The modern designs like 49er et Al have just as big a drag increase as anything else around Froude regions, but because of very different hull behaviour at different trim angles with dynamic lift and other factors, these boats need never be sailed in a high drag increase region. With different trim the hump is at significantly different speed, and in Bethwaites full size tow tests with a world class sailor on board the sailor was unconsciously optimising trim and evading high drag increase.

 

This hull is capable of planing. What are the features that identify it as a planing hull?

Has it ever planed under sail power?

 

PAR, I do disagree - at least with the inference that planing and S/l are related. Planing occuring at S/L 0f 3 is just a rule of thumb for stereotypical boats. There is no physical relationship between the onset of planing and wave formation, as implied by this type of statement.

Sticking (for now) with GGG's definition that planing is happening when the boat is predominantly supported by dynamic forces (vague, but a good starting point), then planing speed is a function boat weight and boat shape - not length. Light, flat boats will plane earlier (at lower S/L) than heavy, round boats.

The literature implies that planing can only occur once displacement or "hull" speeds have been exceeded, or more specifically when the S/L > 1.4. That is not true. Traditionally boats have only planed once at S/L ~ 3, but there is no physical reason (beyond boat building practicalities) that it can't occur at S/L 0.5, because any link between planing and S/L is just correlation, not causation.

Looking at your photo of the sailing tables, if you could magically extend the length of the table in the top photo by a factor of 10 from the bow, but with no added weight, then the S/L would be very low, but it would still be planing. Indeed, such a long boat would have such a high 'hull speed' that it would be planing before hull speed was reached, and so would have a 'humpless' drag curve (with some body movement to get the trim optimised).

 

I'm not sure where this obsession with planing or not planing comes from, there is no prize for pinning it down, surely.

 

You're right in that it doesn't matter if a boat is planing or not (although the Racing Rules of Sailing allow pumping only to promote surfing or planing, so being able to recognise it could be useful). But as planing is a (but not the only) route to speed, it is useful to be able to design a boat to do it efficiently.

 

PI, that is an incredibly elegant and concise explanation.

Do rowing sculls reach hull speed? Are they capable of planing?

 

Thanks.

I don't know how long or fast sculls are but I am certain that they don't plane are are not capable of planing whilst being rowed.

They are too heavy and narrow and anyway are not trimmed to an angle that will produce planing. Wide boats are more efficient for planing becasue there is more surface area for the stagnation point to act over.

However, if the crew were removed, the scull trimmed bow up by 2-3 degrees and towed at, say, 10kts, then I am sure it would be planing!

 

I wonder if some of the confusion around needing to exceed hull speed to plane is because the early studies were on large, untrimmable, vessels. The only way these could get a bow up trim to start planing would be to "ride your bow wave" with a S/L >1.4 (or use a scow bow).

 

Single sculls are roughly around 8.2 to 8.6 meters (27-28') in length for serious racing. They have been made to foil though....

Experiments were done in the 50s' with a 'planing' eight, but the nature of the pulsing propulsion stroke, prevented it from staying in a sufficiently steady state plane, as I understand it.