Thread: Gwen 12 and hull design evolution.

Strike one for the old symmetric!

Originally Posted by Boatmik

But gosh it looks cold and windy.MIK

Bit of a cold spell just now



Brian

Aww, that's pretty

Richard

Absolutely Bruce! Almost all the Asymmetric boats find that if the waters are restricted in any way and they have to lots of gybes downwind that they are not even in the ball park against smaller, or rather conventionally sized symmetrical kites.

The bigger and more complex the boat, the more this seems to be so. The thing I do like about asymmetrical kites is the simplicity of the gear, particularly if the bow extends far enough forward of the jib tack to reduce the necessity for a long pole/prodder.

MIK

Hi all,

Mik, after mythbusting jib slots, you've started on planing!

My imperfect understanding of planing is that the boat stops climbing 'uphill' over the bow wave, climbs on top of the forward moving wave crest, and reduces its surface area.
But you're saying that that lifting effect is a continuum (?) - it starts to happen to all boats as soon as they start to move, just because some water is inevitably displaced downwards, pushing the boat up... So 'planing' versus 'non planing' is just a matter of degree...?
The speed of that wave crest under the boat is limited by the laws of physics - 1.32xsquare root of wavelength in feet - and the boat on top of that wave crest can't go any faster than the wave crest... So the trick of planing (or fast multihull sailing) is to make the wavelength of the bow-wave independant of the length of the boat - which some boats do better than others - by creating that smooth hollow behind the transom, where the second wave peak may be some distance behind the boat, allowing a higher wave-train speed, and therefore a higher boat speed...?

Bethwaite's book seems to claim that the difference between planing dinghys (like the GIS or a NS14) and non-planing multihulls is the amount by which surface area decreases as you break that hull-length/wavelength link. Skin friction increases with the square of speed, but in a planing boat (according to Bethwaite) the reduction of area is directly proportional with the increase in speed (which means that surface drag then has a linear relationship with speed)- whereas in a non-planing boat there's significantly less reduction in surface area and consequent reduction in skin drag.

Multis don't plane in Bethwaite's definition, because their surface area (and skin friction) doesn't significantly reduce as the speed increases - they are able to 'sail up' the small bow waves that their slender form produces in what Bethwaite calls 'forced mode', without significantly reducing their displacement... Even though they do break that hull-length / wavelength link by forming that smooth hollow behind the transom, their speed is limited by surface drag increasing by the square (or close to it, given the continuum of lift you describe).

So in a way, breaking hull speed (by smoothly separating the stern wave) and planing (reducing surface drag by significant lifting) are actually two separate phenomena...?

My brain hurts.... again. Keep them coming, Mik - this is so much fun!

Jack

Originally Posted by Coucal

Hi all,

Mik, after mythbusting jib slots, you've started on planing!

My imperfect understanding of planing is that the boat stops climbing 'uphill' over the bow wave, climbs on top of the forward moving wave crest, and reduces its surface area.

Not quite. The bow wave always is near the bow. It has to be because that's where the boat first disturbs the water. There are ways of cancelling it out to some extent for particular speeds using bulb bows.

As the boat goes faster and planing lift starts reducing the displacement required the wavemaking will reduce and bow and stern wave will reduce. Also they become less visible - at 10 knots the stern wave is about 50ft behind the bow wave (mental arithmetic) and on light displacement boats it can be very hard to see such a small amplitude on a long wavelength. Where is the stern wave with a hydroplane going at 150 mph? is that about 14400ft behind?

But you're saying that that lifting effect is a continuum (?) - it starts to happen to all boats as soon as they start to move, just because some water is inevitably displaced downwards, pushing the boat up... So 'planing' versus 'non planing' is just a matter of degree...?

I am really saying that "planing" as a mode doesn't exist as the same effects are present to some extent (sometimes positive and upwards sometimes negative and downwards) as soon as the boat is moving.

The speed of that wave crest under the boat is limited by the laws of physics - 1.32xsquare root of wavelength in feet - and the boat on top of that wave crest can't go any faster than the wave crest... So the trick of planing (or fast multihull sailing) is to make the wavelength of the bow-wave independant of the length of the boat - which some boats do better than others - by creating that smooth hollow behind the transom, where the second wave peak may be some distance behind the boat, allowing a higher wave-train speed, and therefore a higher boat speed...?

Not independent. The bow wave is about where the boat contacts the water and the stern wave is always the right distance behind that for the equation (Froude equation).

The boat can reduce the wavemaking drag - which is the stronger drag effect by far at higher speeds - by either being very slender (multis) or by reducing displacement by replacing it with planing forces OR A COMBINATION. Wavemaking is related to displacement.

So that means as the boat goes faster the stern waves moves back. At some point the trough is around the back of the boat which is where the weight and volume is concentrated with most motor boats so they squat badly. A sailing dinghy doesn't squat so much because the volume in the stern is not so great, the weight is more central (assuming the crew are not dumb enough to move further back - because they think it promotes planing) and the propelling force is well up in the air holding the nose down.

Bethwaite's book seems to claim that the difference between planing dinghys (like the GIS or a NS14) and non-planing multihulls is the amount by which surface area decreases as you break that hull-length/wavelength link. Skin friction increases with the square of speed, but in a planing boat (according to Bethwaite) the reduction of area is directly proportional with the increase in speed (which means that surface drag then has a linear relationship with speed)- whereas in a non-planing boat there's significantly less reduction in surface area and consequent reduction in skin drag.

I think he puts forward the two normal reasons. That the boat has to overcome both wavemaking and friction resistance. Friction resistance is the easier of the two to deal with as it is progressive but the wavemaking is very low at low speeds but becomes very significant at hullspeed. The two conspire to create a drag "hump" at hullspeed which is hard or impossible for some boats to get over. Don't think for a moment that drag reduces after the hump. It is always going up with higher speed but there is an accumulated bump where the drag goes up quicker than anywhere else when the increases in wetted surface and wavemaking drag coincide.

Wavemaking can be reduced I think by less rocker but I am not sure that this is beyond anecdotal - like an older NS14 was a lot harder to get through the "transitional" mode at a bit faster than hullspeed than the later low rocker models.

A part of this ... just thinking through - is that lifting force comes from the speed of the boat, the area of the surface and its angle to the direction of travel (angle of attack). The old boats with the wide planing area aft had to go really bow up to start getting lift from the back end of the boat - particularly if they had more rocker. The Goat and other modern designs have more surface area or flat surface or footprint forward than the older veed boats and that is always at the right angle to start generating lift. Then the lower rocker means that with slight bow up trims more of the bottom is at the right angle to develop lift compared with a boat that has more rocker.

The old style of boat is very draggy through the transitional zone because of its high angle of attack (dragging the transom, blunt hull entry made worse by any waves (sea state)

Multis don't plane in Bethwaite's definition, because their surface area (and skin friction) doesn't significantly reduce as the speed increases - they are able to 'sail up' the small bow waves that their slender form produces in what Bethwaite calls 'forced mode', without significantly reducing their displacement... Even though they do break that hull-length / wavelength link by forming that smooth hollow behind the transom, their speed is limited by surface drag increasing by the square (or close to it, given the continuum of lift you describe).

It is headgame. He has made the mistake of making a claim without making any sort of measurement. I haven't either, but from observation of the volume of the depression in the water behind their transoms I can see some multis are creating some lift. I would agree it is not really significant for most multihulls - though the hottest A class cats had very flat hull bottoms a few seasons ago - going to a box shape almost - so maybe they are not discounting lift either - I doubt they have measured anything either but just assume some sort of effect and will say it is true if the boats win on the racecourse.

So in a way, breaking hull speed (by smoothly separating the stern wave) and planing (reducing surface drag by significant lifting) are actually two separate phenomena...?

Exactly! The stern wave will always be the right distance behind the boat but doesn't explain any lift or even really explain any drag - except for when the stern is undermined and the boat squats (in the case of particular hullforms)

My brain hurts.... again. Keep them coming, Mik - this is so much fun!

Jack

Me too, but it is interesting.

So here's another question...
How would a PDR hull go backwards (compared with forewards)?
And why?
AJ

The 125 class also looks interesting. Aimed at low cost racing, what do you guys reckon?

Promo video from 1985 here

[ame="http://www.youtube.com/watch?v=0kxm7m3IrdU"]YouTube- Broadcast Yourself.[/ame]

make sure you watch till 4 minutes - super quick S=T illustrated.

Brian

Originally Posted by b.o.a.t.

So here's another question...
How would a PDR hull go backwards (compared with forewards)?
And why?
AJ

Work it out for yourself using the lift criteria and knowing the curvature of the hull is greater at the (normal) back compared to the front.

MIK

Originally Posted by keyhavenpotter

The 125 class also looks interesting. Aimed at low cost racing, what do you guys reckon?

Promo video from 1985 here

make sure you watch till 4 minutes - super quick S=T illustrated.

Brian

The 125 is a pretty nice little boat in many ways. It has suffered from gaining all the high tech stuff they would use on a fireball or 470 though. I think the Proctor adjustable spreaders they use are around the $3-400 mark - about 350 from the Australian distributor. Just for the spreaders.

The class is slowly disappearing.

MIK

Originally Posted by Boatmik

Work it out for yourself using the lift criteria and knowing the curvature of the hull is greater at the (normal) back compared to the front.

MIK

Slower in lighter conditions but with potential to plane flatter & at slightly lower wind speeds?
AJ

I think that is a reasonable guess. maybe slower upwind in rough water.

It seems a German company is importing NS14 hulls from Aus and putting an expensive rig on them. Called the Flight 4.3 here is a video of them

http://www.youtube.com/watch?v=SQ1Zp...ayer_embedded#

and the web site for info

Flight 4.3 - Landenberger OneDesign

Very extreme hull and costly rig - ughhhhh

Brian

Yes ... that is a current NS14 including rig. Interesting that they have gone One Design - I think it is a mistake.

The Tasar was state of the art - an NS14 with a bit of extra sail area a long time ago but now the NS14 with 18sq ft less sail will burn it off in most conditions.

They did introduce Mylar sails for the Tasar a few years ago, but more from a sail cost perspective (I would guess) but allows some talking up of the performance advantages.

So the Flight will be similarly frozen. If you get a chance to sail one, do. They are a lot less stable than the NS14s that I used to race in the 70s and early 80s but faster too.

MIK