Sailing Dinghy Design

[Tim B]

I would suggest, that the size of the 'hump' is also subject to the boat's trim at the time. For instance, I have sailed the LARK in marginal conditions a fair bit over recent months, and sometimes the humpis obvious, and sometimes it isn't it tends to be smaller when the boat is totally trimmed and flat. It is then planing on the aft section of the hull.

In response to your comment about PYs, Chris, remember, that it is only a general handicap. Different boats are betterat different speeds. Run a VPP for say a beam reach on various boats and you'll see what I mean.

Cheers,

Tim B.

 

[Phil S]

At leastI got the debate back to design issues, sorry Tim.

If someone wants stressed ply moth designs see www.moth.asn.au

If planing is faster than slicing why do tornados go faster than 18s.

It is easy to argue a moth has a speed limit which is probably more like 18 or 20 kts now we have GPS, but it is only 11ft long. Why not compare a moth with other boats of the same length and then consider how far advanced it is. How fast does a Flying 11 or heron plane?

 

[Another guest]

Tim, while I understand that the PY is an average, I was using it to underline the vast difference in speed between the Lark (basically the 1953-ish Mike Jackson "March Hare Nat 12 with spinnaker and very heavy centreboard).

I was referring to your comment "the fact remains that in
In full planing conditions, though, I know for a fact that the Lark is faster than the Laser (we've tried it), and by virtue of reduce drag, I would expect it to beat the Moth."

So I was merely trying to point out that the Moth can beat the world's best Flying Dutchmen and Contenders around the track in full planing conditions, and there is NO WAY a Lark will do that. Therefore the Moth is much, much faster in full planing conditions.

This is not a computer VPP, but tbe best VPP; the world's best boats of different classes, sailing against each other.

PS; I agree, Andy, that Phil S is someone to listen to. Interesting points about planing; but is it the difference between planing and not planing so sharp? Is it a straight either/or between support from dynamic lift and support from buoyancy? The only vessel I know with no buoyancy lift at all is the George Greenough "Spoon" bodyboard, about 1/8" of solid GRP. Even a waterski floats.

And while the Moth may (or may not) have the peak speeds of a 14 or skiff, it is only 11' with one sail. Former Moth world champ and top 14 er Peter Moor says the Cherub, very much a planing machine, has a higher peak speed than the 14, yet there's no doubt the 14 is quicker around thje course.

Mind you, I'm betting a Lechner style planing Moth would be very quick, but I just meant to posit that the planing hull for top-end performance is very different from the "semi-planing" hull for ultimate race performance.


Chris

 

[Another Guest]

Andy;

Sorry about the loose expression on these posts, the browser doesn't agree with this page and it's really hard to write on it. So my spelling and writting is stuffed.

I think we may be agreeing, largely, about humpless boats. However, it's not just the Bethwaites who are achieving it as far as I can find out. Other OZ designers tend to have a lower wetted surface and lower wavemaking resistance shape with centreline flat planing panel, so they reduce the wavemaking and skin resistance; the "displacement hull" route.

The Bethwaite's flat vee (rather than ellipical section) hulls would have more wetted surface than an ellipticxal section, per volume. The vee panels of their bottoms have less dynamic lift than those with a flat centreline panel, per area. BUT they make up for this by designing long boats; for example, the 29er is similar in most dimensions to a Cherub BUT it's 2' longer. This has a considera ble effect on the DLR and on the hull speed, and this makes them, as Frank puts it, plane before they reach the hull speed hump. (Mind you, sinche the Oz Cherub rules prevent "displacmeent" hulls, the Cherub has a HUGE hump).

Or that's as I understand from talking to many top designers. The following are from my notes;

Julian O'Mahoney - "A 12 doesn't seem to have any hump at all in its performance. I suspect it must, in theory, but perhaps the amount of power has an impact." - designer/sailor of champion 12 foot skiffs and Cherubs

“A 12 is so light and narrow you’re not going to notice anything (of the hump). “There’s less distinction between planing and not planing – we’ll be going bloody fast even before we break out into a planing situation”.” - Jim Walsh -designer of champion 12 foot skiffs.

"The concept of the hull changes drastically with displacement/length ratio" notes Paul Bieker. "The lower the displacement to length ratio, the less noticeable the resistance hump. The Bethwaite boats all tend to be long for their weight, which goes a long ways towards acheiving their "humpless" hulls. Boats with higher displacement to length ratios have to deal with the "hump" - it's unavoidable due to the laws of hydrodynamics. I think that's what makes the shorter boats more of a design challenge."

Michael Nash (designer of champion NS 14, 16s, 12s - VERY influentual) believes that the displacement hull largely solves the "hump" problem; "As far as I know, research by the Navy into high-speed shapes showed that the better a shape was as a displacement hull, the easier the transition from displacement to planing".

The low-resistance displacement-style hull doesn't have a significant hump, says Stu Friezer (Naval architect and NS 14 designer)........."

I'm interested in the comment "the way the drag is managed throughout the speed range is very smooth as the lifting forces move from buoyant to dynamic lift. This is manifested in different ways - in the 49ers case its a lack of a speed bump."

Have you got any info on what/how this is done? My understanding is that Frank doesn't actually know how the "humpless" effect works, although I haven't chatted to him about it for a while.

My brother, an ex 16 foot skiff and 49er sailor, has commented that the 16 has a SMALLER hump than the 49er, because the 16 follows the other (elliptical/U section "displacement" route. That's interesting when the 16 carries an extra crewman.

He also preferred the 49er feel, I might add, he much prefers them generally ovedr the 16. Then again many 14 foot sailors prefer 14s to 18s.

 

[Andy]

Phil,

In terms of comparing planing versus moth style designs of a similar size, Chris is correct in identifying the higher top end speed of the Cherub, a reasonable comparison in power to weight and so on. Tornados may indeed be faster than 18s in straight line speed in most conditions, but evidence of +30 knot bursts by 18s in survival conditions (+35 knots if you believe Frank Bethwaites book) still suggests that the top end ultimate speed of the planing 18 is higher. I dont think anyone could pretend to have gotten a tornado anywhere near 35 knots.

Chris hits the nail on the head when he says "I just meant to posit that the planing hull for top-end performance is very different from the "semi-planing" hull for ultimate race performance". Speed around the course in a variety of conditions remains the goal.

There are different levels of planing efficiency, and the observations Tim made on planing angle are correct. Chris's body board may support his weight fully with dynamic lift at slow speeds but the lower the speed the higher the angle to create the lift and the greater the resultant drag in comparison with the lift. Tim's comment that "sometimes the hump is obvious, and sometimes it isn't... it tends to be smaller when the boat is totally trimmed and flat. It is then planing on the aft section of the hull" says a lot for practical observation of planing, and the flatter (but not too flat) after sections of the lark may be ideal for a humpless transition. You could find the optimum trim angle for a hull and determine the increase in drag for a range of non optimum angles by adjusting the trim during tow testing.

 

[Guest]

Just saw the next post from Another Guest - who are you?!!! A lot of very interesting material here, from some very influential people!!

A basic explanation of how the hump is 'managed' would be to say that as the overall resistance increases with speed, efficiency is maintained regarding all the componenents of resistance. This seems to happen predictably and is fairly well understood at displacement speeds, and similarly at full planing speeds. The difficulties arise during the transition period when the boat is operating between the modes, with the drag characteristics of both modes varying constantly in proportion and magnitude with one another. CFD could yield some more concrete results here, but it seems that designers (unless someone is keeping a big secret) are designing these boats using empirical data more than understanding exact numbers when choosing hull shapes.

A letter in Seahorse some time ago from a member of the Wolfson Unit claimed that what Frank Bethwaite had said was not an exact science was, in fact, fully predictable, if one understood fully the drag components in the manner mentioned above. A later article by Bethwaite about the 29er suggested that he had developed a system for designing hulls with these characteristics using mathematical techniques, so perhaps people are closing down the science of it all.

Personally, though, I like the idea that a bit of theory, some empirical data and a touch of inspiration is what is driving these developments. This 'messing about in boats' is certainly more fun than staring at numbers on a computer screen!

 

[Andy]

oops that last post was me...

 

[Tim B]

Ok, I've got to try to understand the problem, before I can even start to solve it. So I'm going to run a 'typical' (ie. 14' Lark (ish) fine bow flat stern) hull through a drag/power prediction at a few trims and heels, I'll let you know the results.

 

[Another Guest]

The quotes came from notes for a book on design I'm working on. I've written 40 or 50,000 words so far, plan to self publish in a few months. I have no claim to be a designer (the cloest I;ve come is building sailboards and re-rigging/re ruddering my ancient scruffy 1/2 tonner) but someone had to cover this interesting area.

I also think that asking a whole lot of experts a whole lot of questions gives at least as valid a result, as one expert giving his opinions and knowledge.

I agree, empirical ideas in yacht design are very valid. What worries me about using VPPs is the proven problems in something as expensive as the IMS VPP, and the fact that we are often dealing with dynamic systems an d turbulence flow and who the hell can model that? Some formulas are interesting, though; I hadn't realised how heavy skiffs are, in DLR terms, until I ran the numbers. It then revealed (I think) a link between low DLR and consistent (ie all weather/all angle rather than "dash") performance but I still have to check this.

When Thorpey was designing the Hungry Tiger he spent a lot of time on the computers at the Murray Burns and Dovell office - with Andy Dovell cautioning him that their America's Cup CFD and VPP programmes were not valid in this area, and he shouold use his gut reaction as the main guide. Similarly, when I asked Andy a while ago about doing a boat design course, he just said "don't worry abouot it, just keep on doing what you're doing" ie looking at boats and asking annoying questions.

Phil S did some 12 footers that were ahead of their time, by twisting ply models into good looking shapes. Andy Patterson "cuts and shuts" intuitively and comes out with very quick boats. The empirical approach is very valid. Worlds-winning A Class cats, 18s, top 12s, they are all people using empirical design. So are succesful IMS / offshore boats like the J/24 and J/35.

I'm not a designer, engineer or a scientist, but I look at the empriical evidence of the racecourse as being like epidemiology. The epidemiologists knew scientifically and definitely that smoking killed people, long before the biologists or chemists could work out why it was happening.

Mind you, I'd still like to find out about the Wolfson Unit info - who wrote the letter?

Also about flat transoms and humpless hulls; not sure of this relationship, the Bethwaite Tasar has a fairly flat transom and one of the few faults of this excellent boat (I'm biased, I admit) is its very large hump. Then again, that may be exxagerated by the rig; with no spinnaker it has less "power" (to use the term loosely) but once the apparent wind is high (when you are a gust and planing) the low drag of the wing-mast rig comes into play.

The same thing occurs when you switch from an old-style windsurfer rig (like a Mistral's, with depth and apsect rations similar to a skiff-type boat) to a very flat, twisted modern slalom/Formula type sail. It feels like the board develops a huge "hump" with the modern rig, but the board hasn't changed. It's just that the new rig is very efficient once apparent is high, but cruddy at low speeds. It's hard to get planing, but once you can get onto the plane, you're gone - because the drag of the rig is so low.

Or that's my impression, may be totally wrong.

About tow testing; I have been told that the idea of using a spring to measure doesn't work, because it moves around too much. Is there anything else that would do the job that can be easily obtained?


Chris

 

[Tim B]

Drag

I have not yet got anywhere near the VPP software, though there is some free software on the net. Anyway, starting by looking at resistance plots, I have used Michlet 8, and come up with the following drag curves. The dimensions used were roughly those of a 2-man 14 foot dinghy, and generic as it may be it proves a point. The 'hump' is quite clearly evident on all the curves. The change in total hull resistance with speed, is really what we are interested in. Rig drag is possibly an issue, but if we assume a standard rig, then the problem returns simply to the hull design, whilst, quite rightly chris, it is unsteady, can be solved by a mixture of empirical and momentum formulae. Anyway, here's the picture:

Cheers,

Tim B.

 

[Phil S]

Tim,
Can you reproduce those curves for different weight boats of the same size. It might back up or not the idea someone had that the hump was displ/length dependant.

From my observations it may not be and I will offer three empirical case studies.

Chris said the Tasar (70s Bethwaite one design Dribbly design NS14) has a distinct hump, but a Flight 15 NS14 which I once owned had a very subtle hump and it was only a few years more modern than the Driblbly/ Tasar design. I suspect that Michael Nash was the first with a Humpless design, probably about early 80s Aero 6 or 7 NS14s. This continues in all modern Nash or Feizier NSs. All these boats weigh about the same and have the same power, so on this basis it is only hull shape changes which have reduced the hump.

Similarly the 1970s 12ft skiffs I built had a huge hump. Basically they did not have enough volume to displace cleanly so they dragged heaps of water. But the big rig meant you got up and planing early, a lot like todays sailboards. Michael Coxon and Iain Murray followed with big volume boats which didn't sink so baddly and planed early, but then Nash and others created the current deep style of 12s with narrow waterlines and which dispace well but transition easilly to the plane. They also can be healed to plane on the topsides when pressed. While the weight of 12 rigs may have come down the present min hull weight is similar to my old ply boats because mine were about half the freeboard. So the change again is not due to Disp/L ratio but shape.

Ironically now I find my moth to have the easiest motion and speed transition of all with no transition or hump. It is almost identical to the feel of a 12ft catamaran I built in 1965, and which must have weighed at leat 3 times the weight of my moth.

Some one with a good computer and some fancy software might try to disprove all this but you can not argue with performance on the water. I suspect that the types of boats we are talking about are off the scale of all the sotfware about which was designed for big boats with lots of lead.

That is what makes small boat design so interesting. We are all amateurs because there is not enough money for good performance design in small boats to pay for any real science or research.

Its a shame that all the commercial small boat research dollars goe into mass produced, mass appeal, heavy boring tubs and the necessary associated marketing.

Phil S

 

[b14maniac]

NS14s... dynamically humpless?

Phil,

While I wouldn't go as far as saying that Michael Nash designed the first dynamically humpless hulls (and neither did Julian Bethwaite... he even goes as far as to say that maybe the 1960s-70s 16 foot skiffs may have been the first...), I'd say that he designed the first dynamically smooth hulls (still show a distinct turn on the speed/displacement graphs, but it's of an incredibly large radius that it may as well not be there). I owned a Nash Aero 8 until about 6 months ago before transferring to a B14. These two boats give a great comparison of dynamically "smooth" (basically humpless) and a well reknowned skiff/"humped" boat. The Aero 8 was essentially designed for the purposes to making an Aero 7 with a far improved volume distribution, making the boat both quicker and far more user-friendly (try sailing an Aero 7 in no breeze... get forward, lack of volume... sink the bow, loss of efficiency, slow. Sail it normally, too much wetted area, slow.)

The B14 and Tasar feature a very similar wide stern (for extra planing area) and Veed forward sections. This sacrifices wetted area of the incredible accelleration and top speeds that these boats are capable of. In my opinion (and I am no expert, just an informed amateur in this game); the big issue with both these boats in my opinion is that the traditional Bethwaite hulls lack the ability to use the fine veed bow sections in any sort of a breeze due to the way that most (pre-9er) Bethwaite designs have a poor weight distribution (all down the back) which counters the volume in the bow sections. This contrasts wildly with the modern approach to fast development boats, such as your beloved wooden skiff moths (or Andrew's carbon creation!), or the NS14s. The NS's contrast by means of following the moths in many ways: the performance of an NS and a Moth in 0-10 knots of breeze is very similar... the NS14 can "plane" (dynamically displace?) to windward in less than 5 knots and off the wind (jib set wing and wing), the NS rarely planes, but more slices through the water like a knife does butter.

The volume in the modern narrow moth is very uniform in all the boats before the Axeman 5 (UK) or Hungry Tiger in AUS. Then the pintail became fashionable, at the expense of making it difficult in the light stuff. It is the uniformity of the volume distribution as well as the conical bow sections of most modern Int. Moths that causes the moths not to plane. Instead, they follow a pattern of dynamic displacement, induced by the sheer narrowness of the boats. Maybe the next thing for moths is to introduce aeronautical wing structures so that a moth hull can "fly" and induce a planing-like condition (in the right conditions) without needing these crazy hydrofoils that have appeared recently (in the last 5 years).

As I said before: I am no expert, but only an informed amateur, with future ambitions to be a naval architect, so I am in no position yet to go quoting mathematical results or graphs proving my points. However, as boating in general is a science, I'm meerly putting forth my observations on the subject.

 

[Pieetry]

i just spent half a day reading this whole thread and id just like to say wow and good job to all who posted

 

[Tim B]

So it could be a question of volume distribution... I'll have a look at that. It'll take me a while, though, since I have to mend a rudder I split yesterday. I'll post a picture later, since the next question will be foil design.

Many Thanks Pieetry, It's getting to be a bit of a mammoth job to read, but I think there's some good stuff.

Cheers,

Tim B.

 

[Tim B]

Well, this came off a LARK, I'm currently repairing it, but it gives some idea of the impact speed. The split is clearly visible down the length of the blade.

Cheers,

Tim B.