Aqua Dynamics

[OGITD]

There have been many discussions and criticisms regarding a sailing vessels ability to point to windward…… but I have not seen anything pertaining to the aqua dynamics of hull design.

But having a look round any hard during the winter lay-up you will notice many designs where flow rate, pressure, and control of have obviously been part of their design factors….. and some which obviously haven’t.

I am talking about bilge keel designs where the keels have been splayed and toed out to improve their sailing efficiency, plus some have the benefit (possibly from the aircraft industry) of a wing shape (rounded thick forward edge, thin trailing edge, flat on one side and curved on the other). And others showing no real design at all, with just what appears to be large slabs running straight fore & aft…. which surely cannot be anywhere near as efficient when pointing or beating into the wind.

So in the 21st is there anyway of establishing the best aqua dynamic design, and the figures which apply to the formula proving the efficiency when under sail? Or have these been lost with the design notes and original blueprints / plans of most vessels?

And what (if any) is the applied formula?

 

[penfold]

You have traditional tank testing, and added to that these days you also have whizzy computational fluid dynamics modelling software

 

[sarabande]

I've never been convinced of the reasons behind putting an asymmetric shape into a bilge keel.

OK, you refer to 'lift' profiles - which is fine on one tack, and will generate a bit of lift into the windward direction and reduce leeway, but while that is happening, the other bilge keel is generating lift 'downwind'.

Fifty year ago we had racing boats with trim tabs on the aft edge of the fin keels - and they worked, but were taken out of design by measurement penalties.

The Scheldt (sp?) keel has a slot half way along to improve flow over the after half, and endplates were much researched in the 12s.

The main efforts in hull design are surely to be found in reduction of wave making -there's no point in creating a giant v shaped wake - and in surface coatings to reduce friction/drag by imitating e.g sharks' skin, where a very small layer of water is carried along next to the hull, enabling laminar flow to 'grow' within millimetres.

Short of developing some form of super-cavitation as in the Sqval torpedoes (with which you might be familiar ) to overcome it, drag is the main burden of the designer

 

[Signed Out]

As I see it Sarabande, the lift effect of the windward bilge keel is lessened (in the splayed designs, as opposed to the "square" ones as described by the OP) by any heel, and would have (relatively) more downforce. Better in these cases (as I see it, and have mentioned in other threads) is the lifting asym bilge plate, as seen on the Hunter/Red Fox 200 amongst others.

And I believe it is dolphin skin, not shark. Shark being very course, and IIRC there was research into a form of dolphin skin type micro grooving (?) in the 80s, for the Americas Cup. Not a hydrodynamic boffin, but looking at it in layman's logic, I can't see it being a great deal of use, when it is simply more displacement of water, and the bulk of the hull's mass (I mean it is a bulk, not the larger proportion) is still being displaced, which is where energy will be lost. But then I have doubts (until proved otherwise; a link to a good explanation, or a simple one would do...) about the idea of a lightly abraded surface "to reduce surface tension" helping either. I guess as you say, laminar flow is increased, but...

And surely trim tabs, such as those you refer to, have been around far more recently than 50 years ago. Indeed are not canards essentially the same principle?

As ever, my tag line stands.

 

[snowleopard]

I've never been convinced of the reasons behind putting an asymmetric shape into a bilge keel.

OK, you refer to 'lift' profiles - which is fine on one tack, and will generate a bit of lift into the windward direction and reduce leeway, but while that is happening, the other bilge keel is generating lift 'downwind'.

With splayed keels when going to windward the lee keel is approximately vertical and with an asymmetric shape it is producing lift to windward. At the same time the weather keel is nearer the horizontal so lift from it is tending to reduce heeling. Sounds like the best of all worlds to me.

 

[CAPTAIN FANTASTIC]

. At the same time the weather keel is nearer the horizontal so lift from it is tending to reduce heeling. Sounds like the best of all worlds to me.

Also, the weight of the horizontal keel helps to balance the boat.

 

[snooks]

With splayed keels when going to windward the lee keel is approximately vertical and with an asymmetric shape it is producing lift to windward. At the same time the weather keel is nearer the horizontal so lift from it is tending to reduce heeling. Sounds like the best of all worlds to me.

Yup that was the way a boat designer explained it to me

You do also get twice the drag, unless they are skinny thin keels like the RM

 

[PeteCooper]

The Scheldt (sp?) keel has a slot half way along to improve flow over the after half, and endplates were much researched in the 12s.

I think that you are getting your keels confused. If you mean Scheel Keel then that has no slot but is a variation on a shallow keel with a bulb.
The Collins Keel(designed by Warwick Collins) is the one with the slot in the middle.

 

[snooks]

The Collins Keel(designed by Warwick Collins) is the one with the slot in the middle.

Isn't this also know as a tandem keel used on Etaps?

 

[Bosun Higgs]

It's hydrodynamics not aqua, and its a branch of cluid dynamics. Look up Bernouilli and follow the trail from there. There is a lot of science available and a lot of it is put into practice on the expensive racers.

I guess the problem with old bilge keel designs is a different one. Yes the science was available but did the designers have the skills themselves or were they simply boat builders with manual skills. Was it economically feasible to build in good design or are you better off making a glitzy inside to sell to SWMBO than a fine shaped keel to sell to HWMBO. To put it another way, we all know that lead is a better keel material than cast iron but how many boats are sold with lead keels for which the punter has to pay more?

 

[sarabande]

I remain unconvinced about the 'righting' effect of assymetric bilge keels.

Assuming that they are both immersed at a large angle of heel, one will be providing an uprighting moment, while..... the other one will be providing a heeling moment. You cannot have fixed bilge keels which choose to to optimise the heeling angle of their own accord (and yes, I know in a well heeled bilge keeler, one plate will be almost horizontal and providing a good righting moment, but it's the hydrodynamics we are discussing. )

Movable or lifting keels are different animals, and work. The primary advantage of having a symmetrically shaped bilge keel is to reduce drag by establish linear flow for as long as poss.

 

[sighmoon]

Also, the weight of the horizontal keel helps to balance the boat.

No more than the weight of the leeward keel helps the boat to fall over. The centre of gravity is midway between the keels, whichever way they face.

Some bilge keels are 'toed in' a few degrees, so that as the boat heels and the bow goes down, and the stern goes up, the keel starts to face the right way.

The main efforts in hull design are surely to be found in reduction of wave making -there's no point in creating a giant v shaped wake.

I wonder why there's no sailing equivalent of a bustle bow? Sure, it would have to be dynamic, but worthwhile, I'd have thought.

 

[Signed Out]

Snooks, I don't see how the drag will be twice, with bilge keel typically being far shorter than a fin. Only the fatness and incidence with the hull could possibly cause this increase, but not sure I see that as so.

As for windward bilge keels being horizontal- do you normally sail at such high angles of heel, or are they splayed far more on some models I am not aware of? I suspect closer to 45-55° at normal angles. The downward force could possibly be considered rotational, thus aiding righting moment.

Warwick Collins keel was known as the Warwick Collins Tandem Wing keel back when it came out. Typically referred to in full mouthmashing detail. Seen on many boats back in the 80s-90s; well, in the media anyway- Laser 28, Gallant 29, Freedom 21, and so many more. And Tomorrows World.

Anyone remember the wierd design idea for AC boats in the 80s that had long skinny "bilge keels" that looked like antennae sticking out at crazy angles? The future apparently.

And BH, surely you mean fluid... hehehe.

 

[sarabande]

I wonder why there's no sailing equivalent of a bustle bow? Sure, it would have to be dynamic, but worthwhile, I'd have thought.

there is. I have seen a pic of a Colvic Watson with a bulbous bow. I'll try and find it.

 

[Signed Out]

There is of course-

http://www.boatdesign.net/

Quite nice to reference this stuff rather deeper.

 

[OGITD]

Crude paper model

With splayed keels when going to windward the lee keel is approximately vertical and with an asymmetric shape it is producing lift to windward. At the same time the weather keel is nearer the horizontal so lift from it is tending to reduce heeling. Sounds like the best of all worlds to me.

I just had to make a crude paper model to allow me to work this one out....

Plus I’ve just stumbled upon this document which has some interesting information including asymmetric keels, trim tabs, air lubrication and a whole lot more…. so I’m now off for some anti headache tablets & a cup of tea!

 

[Signed Out]

By the way, I've not heard the expression "bustle" bows before, but I like it. As for sailing ones, the only bulbous type bow I've seen, as I recall (and sailed on) was a Lock Crowther catamaran. A very fast but wet boat.

Some examples of similar on the following-

http://www.offshorecats.com/lockcrowther.html

 

[snowleopard]

the only bulbous type bow I've seen, as I recall (and sailed on) was a Lock Crowther catamaran.

You tend to see stuff like that around DIY boatyards - mostly people who have heard that bulbous bows make ships go better so think by putting one on their pride and joy it will go faster/smoother/cheaper without really knowing anything about the science or what shape they should be. I have seen them on a Wharram cat.

 

[Signed Out]

Possibly so, Snowleopard. But I think Mr Crowther was rather more than a home bodger. Surely as a Cat sailor/racer of some experience you know of his pedigree. But not being so familiar with the big cat world myself, maybe his reputation isn't all that, and/or his theories have been surpassed as with so many scientists (not quite Mr Einstein recently).

Of interest to this thread perhaps is an interesting design on the other forum I just linked to-

http://www.boatdesign.net/forums/sailboats/quant-28-foil-assist-keelboat-dss-38421.html

Another way of using side foils as a righting mechanism.

 

[windfan]

Bulbous Bows

While building my Crowther Spindrift 45 the late Lock Crowther visited me and suggested I added the new shape to my build. The reason to incorporate them was reduced pitching which Lock said his latest design cat was achieving. I checked out with my son who had access to Marine tank testing results and the consensus was no real improvement except in a very limited range of wave size. The problem that was evident was in a fast Cat overtaking seas the bow dived in and was slow to lift, not a safe scenario. Prout Cats did offer add on bulbous bow kits but proved to be just a passing Fad.