Humpless Planing

Indeed, a number of boats tends to plane from that number, or go in semi gliding mode...

 

This thread sounds like it picks up on something I said back in April 2003. This concerned the transition from displacement to planing. The link below references the particular post....

http://forums.boatdesign.net/showthread.php?t=1437&page=11&highlight=Lark planing

The description given in the afore-mentioned post gives a solution to planing in marginal conditions. That is, that boat trim will affect the transition speed. The trick, therefore is to guess when the boat is about to enter it's transitional planing stage and "fly" it through it to get the boat planing.

There is little doubt that a racing dinghy can actually plane, and is usually devined as a significant portion of the bow being out of the water ( at a sensible (say 3 degree) angle of trim. One generally notices planing by the sudden reduction in helm force due to the CLR moving aft.

If we consider a plate at an angle to a fluid flow, we can work out the upward and rear-ward force on the plate, this relates directly to lift and drag on a yacht hull. Obviously, the drag is minimum when the plate is parallel to the flow (assuming the edge is not immersed), but the lift is also 0.

However, once the yacht is actually planing, the wetted surface area is reduced and the drag drops considerably. The hull can then afford to be inclined to the water and produce a little more drag to maintain the lift that is reducing the wetted surface area.

Complication comes in the sails producing a bow-down pitching moment, and this, if not corrected results in negative pitch, leading to the opposite scenario to planing. Correcting the pitch of the hull towards 0 degrees allows the drag (and negative lift) to reduce to the level where with just a little encouragement the hull will rise out of the water and plane. The bow-up trim required for this to happen is very small.

The extrordinary (and frightning at times) acceleration between displacement and planing regimes is usually observed when the boat is not trimmed properly. Even with the bow down a little the boat MAY produce a little dynamic lift, but there will be a considerable amount of drag. The reduction in wetted surface will be huge when the boat does finally plane, thus a big accelleration or "hump".

A well set-up boat with a good helmsman should never notice a "hump" between displacement and planing modes. The boat should almost be "flown" through the transition. It takes practice, but is possible.

Tim B.

 

Flying

Flying PAST the transition phase sounds good to me-like a foiler Moth.

 

49er Planning

I sail a 49er regularly and can say that it is a very differend form of planning than many other boats. It is hard to tell by looking at the wake if the boat is planning or not. I have sailled on small sports boats and a B14 and when you are planning you know it. The noise changes the feel of the boat throught the helm changes. With the 49er this does not happen. It just goes. The other thing is that the trim does not seem to change when the boat goes onto the plane. We try to keep the front knuckle in the bow just touching the occational wave. It feels the same weather it is doing 4 knots in light air or 10 knots fully powered up.

If you get the chance turn a 49er over and have a look at the hull. You will find a very fine bow with a chine that starts not far back. The chine widens out to be very wide with a slightly concave rear planning area. The chine has funny curve across the baot. It is hard to describe. Just go and have a look at one.

Why this shape give the hull these properties I don't know...

 

more planing

I think we are into a semantic problem. It has been a classic argument about when the planing state commences. Gerr has some comments on the subject. If we try to define the term, it might (or might not) bethe point at which dynamic lift exceeds bouyant lift. At that point the usual description is semi planing. If we try to define true planing, whatever that is, we have a dilemma.

It is my unqualified belief that the ability to plane is a function of weight versus support surface area. You might relate that to unit pressure. Have you ever seen kids at the beach skidding along on a round disc ? The disc is usually a piece of plywood about 30 to 36 inches in diameter. The drill is to place the disc in a few inches of water, then run a short distance, hop on the disc and away it goes. The velocity of the the disc plus passenger is quite small, but the weight/surface ratio is better than we usually do with dinghys.

A very unsophisticated boat is the Windmill. It is a hard chine cracker box of a boat that is just under 16 feet LOA. Its bottom has a small amount of vee deadrise on about 45 inches of chine width. It will plane nicely if crew weight is not too large. It's polite about the process too. I would not call it humpless but it goes from one mode to the other without fuss and it does so frequently.
The point is that this particular boat is certainly not in the same league as a 49er but it does the job with equal aplomb, though not as speedy. Put two 200 pound crew in the boat and it will not plane in less than gale force wind. That, among other observations, suggests that weight/area is the determinant. But we all knew that didn't we ? One could argue that the sailboard sailors plane with very little support surface. True, but they have some aero lift to play with and the effective weight is often less than static weight.

The shape of the run is a major factor in the propensity of a boat to plane at all. Long straight runs are the goal. Gerr describes the angle of the quarter beam buttock as a deciding factor. He wants the angle to be less than 4 degrees. Me too. We also want to get the transom somewhat above the water surface. If we are to meet that need then the boat has to be long or very light. When we design for planing we almost always sacrifice light air performance. We have a long wide boat with too much wetted surface. Woe is me ! Why can't I have it all ?

 

Mill

Interesting comments about the Windmill. I raced one for about 4 years in the 60's and again in it's ripped off "US 1" form in the late 70's. Now that you mention it transition to planing was humpless in that boat or at least very smooth and very early. And yet it's light air performance against other boats was terrific. I always enjoyed beating Flying Dutchmen the most.
Back then I dreamed of turboing the boat with twin trapezes and a lot more SA but never got
around to it. Great little boat...

 

"A well set-up boat with a good helmsman should never notice a "hump" between displacement and planing modes. The boat should almost be "flown" through the transition. It takes practice, but is possible.

Tim B."

I'm not so sure that all good boats (or craft) have un-noticeable "humps" when sailed properly. For a start, the ability to notice many things on a boat depends on the skill of the sailor - a top Olympic-level sailor on a strict one design can notice 4-5mm of outhaul ease whereas coaching club-level sailors proves that most don't notice 10 times that amount.

Secondly, some boats have a big "hump" even when sailed well, whereas others don't. An Australian-rules Cherub has a massive "hump" and a British Cherub has a noticeable "hump" because both are short and have a fairly high DLR but are very effective when planing. The national champ notices that the Cherub has a big hump and I think he's a "good helmsman". In one class I sail, two guys (both national champs a couple of times over) disagree about the size of the "hump", because they came into that boat from different classes. Same with windsurfers, to use an example - a world-class slalom board sailor will feel much more of a "hump" than a national-level longboard sailor. The modern Formula and RSX boards have much more of a "hump" than the original Windsurfers, but they are much faster in many winds.

So the feel of a "hump" seems to rely on many factors and sailor skill is not really one of them. Hull design certainly is; a long light (in DLR terms) boat like a Canoe or FD has a hard-to-discern hump but a very advanced shorter boat (Cherub, short windsurfer) has a very large hump (according to national champs who are "good helmsmen").

It seems there's also "feedback" in that some rigs and hull designs are just average (or even slow) below a certain boatspeed and then very fast above that speed...this is not related to the hump per se but to the lift/drag characteristics of the rig and other factors. Such an effect is seen in foiler Moths, which are amazingly fast when foiling and very slow when not foiling. The difference is partly in the lift/drag characteristics of the rigs (which work well at the higher apparent windspeed of foiling speeds but not at low apparents), not just the hull and foil designs. Same thing happens in windsurfers....modern windsurfer rigs (from which foiler Moth sails are partly derived) are qwonderful at low lift/low drag high speed sailing but rotten at low speeds when high lift and high drag are needed.

Paul Bieker says that the "hump" in related to dimensions like DLR, maybe we should listen to him until we can beat his designs an d win worlds. A skiff designer here said that his old boats had huge humps....that guy has won a worlds or two. Surely he is a "good helmsman" yet he feels a big hump. Therefore it's not just a matter of sailing skill.

I'm not sure if the Windmill is "unsophisticated"; it seems fairly sophisticated in that it is an example of the realisation that a lightweight (well by US standards) narrow low-drag hull is more important than the apparent "sophistication" of tinkering with round bilges.

 

I agree with CT on this, it isn't possible to get a smooth transition to planing on all boats, but obviously good sailing skills will help. I believe that the faster a boat is theoretically capable of displacement sailing (for a given length), the smoother the transition to planing will be (even though the boat may well be planing before it reaches it maximum displacement speed).