Please send your comments to doug@zingads.com so that I can add them to this page.
Contents:
T-rudders, Zach Berkowitz (27 March 2000)
RE: T-rudders, Doug Culnane (03 April 2000)
Yacht Design, Ian Ward (07 March 1999)
Moth Design Comments, Ian Ward (07 March 1999)
moth design etc, Andrew Paterson (11 February 1999)
International Moth theoretical design, Emin Gun Sirer (15 February 1999)
RE: International Moth theoretical design, Doug Culnane (16 February 1999)
RE: International Moth theoretical design, Emin Gun Sirer (19 February 1999)
Centreboard stability when moving, Charles Crosby ( 15 February 1999)
Axeman experience from Australia, Daen Dorazio ( 13 September 1998 )
Composite Volume Fraction, Doug Culnane (13 September 1998 )
From: zachb [mailto:zachb@email.msn.com]
Sent: 27 March 2000 07:21
To: doug@zingads.com
Subject: T-rudders
Doug,
I just read your article on moth design and while it was very interesting, it was a bit to technical for my understanding.
I am a I 14 sailor from San Francisco getting ready for the worlds in Beer this June. I have finished in the top five of in the last three world championships. As you would figure in a development class like the moth, we are continually looking for something extra. Today we tested a T-rudder for the first time. We pushed it hard on the bay and results were interesting.
First and foremost, the rudder never stalled and the boat seemed to pitch less than normal. I am curious to hear the effects of your T-rudders and how you set them up(angle of attack, size, foil shape...).
Look forward to hearing from you and possibly meeting you in Beer this june.
If your are not the correct person to talk with about rudders, can you lead me to the right person.
Thanx
Zach Berkowitz
From: Doug Culnane [mailto:doug@zingads.com]
Sent: 03 April 2000 20:12
To: zachb
Subject: RE: T-ruddersDear Zach,
What are the factors to consider when using a T-foil rudder on a 14?
I would like to know more about this and I would be better informed if I was not so lazy and had tried it out. I was going to experiment with an old rudder on a 14 but never got round to it. I am therefore extremely interesting in your practical conclusions. However here are my theoretical thoughts for what they are worth.
T-foil rudders on Moths are fantastic. They make the boat much more stable fore and aft allowing it to be driven harder and therefore you get around the course quicker. They do however generate drag. They are not as efficient as a well designed flat rudder but the have the additional benefit of making the rudder effectively longer by reducing tip losses.
Very few boats are as unstable fore and aft as a Moth but boats like a 14 would benefit form more waterline length (effective of physical). This would make them less twitchy and enable them to be driven harder. However a 14 planes and a Moth does not. Therefore a badly set up foil may interfere with the planing attitude of the 14 and slow it down. It may also enable you to move your weight aft so that the bow is up and the foil would generate vertical lift. This would maybe reduce overall drag, because you have a foil assisted planning craft.
I would make the angle of attack so that it is neutral upwind. (How you measure that I have no idea.) This would mean you have minimal drag during important upwind legs.
One lesson from Moths is that the T-foil does not need to be very big at all. Since force is related to speed squared the force that you can get out of a small foil at the speed a 14 will go will be large. So I would make the T-foil no more than 25cm long to start with.
Another benefit of T-foils is that pitching in dampened. This is nice upwind and probably makes the rig more efficient as it is not waved around so much. However a 14 may benefit from going over the waves rather than through them. We are back to the hull shape difference.
In conclusion I would say that T-foils are very good on modern Moths and would be very lively to work on a high performance catamaran because they have a similar hull shape. They may work on a planning sail boat but it is difficult to say with out trying it. Like all good sailing development I think it will be up to people to make it work and make up their own theories why. The bloke that wins the P. o. W. will probably be believed, whether he is right or wrong.
I hope the above was of interest and may explain a few things that you are experiencing. If you have sailed with the rudder against other 14’s then you probably have a better idea of whether it works or not, so please do let me know. Well done for giving it a go that is what development classes are all about, but do not expect to get it right first time.
All the best,
Doug
In addition to Moths and Foilers etc covered in my other Email, I have an interest in Yacht design, hull balance and performance comparisons. On searching the Net, I find little useful information and seem to be drawing blanks, perhaps you can comment and give some direction as to who would have relevant information.
1) Hull Balance
Removing rig effects, a forward moving yacht hull heeled to leeward, will generally steer to windward ie: have weather helm. In some cases this helm is so great that steerage is lost and the boat broaches. This problem is highly prevalent and is why today we have balanced rudders, manned mainsheet travellers, moveable crew balast, flexi rigs etc. in an effort to keep the hull upright.
A heeled hull at say 30 degrees can typically produce a 150% increase in resistance, and even more if the rudder is hard across the boat at the same time. A large proportion of this total heeled resistance can be due to hull imbalance. Given this, an improvement in balance could yield a 20-30% increase in performance when heeled! A rather lucrative target when compared with the sub 1% improvements obtainable by expensive keel modifications.!
I would like to know what information is available about measuring and elimenating hull imbalance. I found nothing in Marchaj and most others. In Skene’s original version of his Elements of Yacht design, the Metacentric shelf principle of Admiral Turner is described. Unfortunately this does not work for short keel designs. Any ideas, articles or contacts would be of great interest.
2) Directional Stability
Many yachts today are highly directionally unstable. When moving ahead and upright, you cannot let go of the helm, it needs constant attention. If you let go, the boat quickly turns in circles of ever decreasing radius. What controls this? How to fix it? and why don’t they?
3) Performance Comparison
If you design a faster Moth, the result is clear. You win races. In the world of Yachts it is quite different. The current philosophy is to handicap out any design improvements to make all boats "equal" (eg: IMS). This means that real advances are not encouraged, and the art of making any boat appear slower than it really is is rewarded!.
How then can we, in a simple way set a performance yardstick for comparison of various designs and determine if "real" design advances are being made.
I have a proposal.
Yacht performance is principally defined by Displacement, Waterline Length and Sail Area. By using Taylors series of resistance curves, for any such combination, it is possible to calculate the potential speed of of a boat in a given wind strength. In this way, we can provide a simple yardstick. By comparing actual with predicted performance, we can rank boats as "good" designs or otherwise.
In line with open development philosophy, I believe it is up to the designer to decide appropriate hull shape, proportions, surface area keel size and shape, rig configuration etc.
The other major factor is stability. Today we use all types of moveable ballast, usually human. The comparison above does not account for this and applies to conventional yachts where the moveable crew weight does not exceed 5% of the boats displacement. Movable ballast will need to be treated separately, for which I have some proposals.
I have actually tried to set up such a comparison by curve fitting Taylors data and applying it over 150 actual yachts. In this way, I found I can predict IMS ratings within +/- 2%....just by knowing their Displacement, Length and Sail area!
What I would like to do, is repeat this excersize more accurately and on more up to date resistance data. The eventual aim being to produce a simple nomogram, so that anyone can easily compare their boats performance, whether rated or otherwise. It would also highlight "Good" designs ie: "Real" improvements in performance, not just those based on beating a rating rule. I suspect this could make a great thesis topic, and should really be the measure used for International open design yachting competitions!
Looking forward to your comments and any contacts you know of who could help me with the latest design techniques.
From: Ian Ward [ian.ward@sandvik.com]
Sent: 07 March 1999 10:35
Subject: Moth Design Comments
As always, real breakthroughs generally stem from a combination of dreams and hands on experience, ....not analysis alone. Although you have not pointed us to any specific improvements, you have however alerted us to some good analysis tools which could be used to evaluate new design proposals. For years I have had such dreams of tilt rigs, narrow hulls and hydrofoils. Most of these I have tried with varying success, always building on the bank of experience. I currently have the narrowest (BWL = 250mm) and lightest Moth hull around (10Kg), and it is the only one to successfully leave the water entirely on hydrofoils. It also now has a deck sweeping foot on the mainsail!.
Interestingly, none of the "complicated" tilt or raking rigs have worked effectively in practice, and it is the simplest boats which win regattas! There is a message in this for us somewhere! In spite of this I have now decided to go quite radical and have many ideas to significantly reduce the rigged weight, and improve stability and performance and most importantly reduce drag. To do this I have designed a simple foiler Moth which is now under construction. I believe this will be my contribution to a new breakthrough in the class.??!!...one day! I will certainly be using your analysis tools in the initial design. If the principle works, I would very much appreciate your assistance to further refine the design. .........more of that later. Here are some comments on your published work:
2.1 Transverse Stability
I campaigned my current experimental hull, called Oopsa Cat in the 1993 Japan worlds. I won the last race by 3 minutes from Emmett Lazich, so it is by no means a slow boat!.
It has a waterline beam of just 250mm on a U shaped hull. My rough calculations are based on the stability being proportional to the third power of the water plane beam. Using the waterline beam as a guide, an Axeman is some 2.7 times more stable than my boat!. In practice however, I have found no significant stability problems upwind in above about 4kts of wind. So I agree with your analysis when there is something to lean against. You have not analysed the situation for running square downwind, and there I can inform you that 250mm is far too little. In fact it is virtually impossible to sail square. I have to reach downwind, which is not slower, unless it is below 5 kts of wind...then I am in deep shxx! Based on this experience I can confirm that the minor stability loss in the WSF design is insignificant.
2.4 Transverse Stability
I am most interested in the effect of windward heel. While I agree that stability is gained, also forward thrust is lost, and so a balance must be maintained, usually at around 10 degrees to windward I would estimate. It is also important to promote to sailors to sail upright as often as possible as this is the most efficient situation. It is only when truly overpowered that heeling to windward helps..and then it is a huge advantage.
Your conclusion about lilting the hull and rig in opposite directions is interesting and more like the detailed info we need to assess an opportunity..even if in this case it will be difficult mechanically to take advantage of.
On the old Scow Moth designs of the 70’s, it was found that heeling the rig to leeward 20-30 degrees in light winds was very beneficial. We always thought it was just reduced surface area of the hull, but in fact the stability did seem to increase, especially as the centreline of the hull moved some 400mm to leeward. I would be interested in your calculation and conclusions regarding the scow design. I can provide details if required.
3.0 Longitudinal stability
This is also rather interesting. What I have found is that "fat" sterns cause nosediving. In fact the only way fat sterns work on most Skiff classes today is that the spinnaker provides sufficient lift to stop diving, otherwise they are dogs to sail!!. The addition of flares on Moths was an attempt to stop diving, but in fact accentuated the problem as you point out. I removed flares altogether 10 years ago and have since worked on getting the displacement distribution correct. While the Oopsa Cat hull is finer than any other, it still does not nosedive, because of the narrow, sinkable stern. In fact the whole hull submerges a little in a gust, accelerates and lifts again with no hint of diving. (No T foil on rudder required!).
I believe therefore it is not the absolute value of longitudinal stability which is important. For this reason your WSF design is actually better than the Axeman, and therefore does not need the T foil to prevent nosediving, quite the opposite, it is needed to stop the stern sinking and therefore maintaining a level planing attitude of the hull. It also stops pitching upwind..a great bonus!
Another aspect is that you can use less foil depth when a T foil is used and we have noted no noticeable increase in resistance due to the T foil in light conditions. What is the absolute value of the drag produced by the foil, and how does this compare with Total drag? Perhaps there is no need for a socond foil for light winds, especialy as pitching is also reduced. This is an important area not touched on in the analysis.
Hull Design
I would like to see your theory as to why your WSF design should have less resistance than the Axeman. It appears this is true. Perhaps we could take this further still if we knew why?!
4.3 Rig
The latest Moths are faster than sailboards in up to 10-12 knots of wind, and are pretty quick in 15-20 kts, however there is no doubt that a board will easily outperform a Moth on a reach in strong breezes. This is the performance gap I would like to bridge. Using hydrofoils reduces the hull drag and that is my current development. I am also now interested to reduce rig drag, therefore I would like more detail about the reverse circulation theory for fast sailbord rigs.
On the topic of comparing sailboards to Moths... it makes no sense to me that a Moth cannot beat a board, when it has the same weight, and more power! There is a fundamenatal design fault here...but what is it?? This is something all Moth designers should be working on.
4.4 Rigs
I hope you will be pleasantly surprised to find that Pocket luff rigs were developed on Moths in 1963, well before sailboards were invented! Not only that, they also had so-called "walking stick" masts, which give an eliptical sail plan. Sailboard have only just started to do this in the past 12-18 months! I used a simple pocket luff rig in the Christchurch worlds, which seems to have revitalised current interest. It was Emmett Lazich who further developed the rig and got the most out of it recently.
I would also like to take issue with the idea that standard rigs are highly inefficient due to the "separation bubble" formed behind the mast. I think this concept arises from comparing sails to aircraft wings. While it is true that both deflect wind, there the similarity stops. A sail, upwind is ususally maintained at the point of luffing, a completely different angle of incidence to a wing indeed!. This always maintains flow on the lee side of the sail and hence separation is not an issue.
I will grant you however, that the smoother and usually stiffer and flatter sail produced by a pocket luff generally results in a slight performance improvement in stronger winds. I estimate the improvement at about 40 seconds per hour of sailing, based on performances in the last Sydney World championships. While worthwile, I would hardly say it is a "great" improvement.
Mast Rake
Your calculations show 5 degrees of mast rake is best. You then go on to say raked rigs are less efficient, but more stable. I take it that a flat upright rig is still more efficient than a raked full rig. It seems you are drawing conclusions to support current practice, rather than support the theory. Which is wrong?
Foils and Mast
You have made some calculations and expressed some preferences but have not provided clear conclusions as to what we should aim for. It would be great to know if there is a benefit in a thinner heavier walled mast, or long thin, eliptical tipped fins etc. How about some firm recommended dimensions!
From: Andrew Paterson [bloodaxe.boats@telinco.co.uk]
Sent: 11 February 1999 20:47
Subject: moth design etc
I'm afraid my designing is more empirical.... latest is Axeman 7 which
I've just started.. simple slab sides. details on my site http://members.aol.com/~bloodaxeiw
Some comments :
My very simple rule of thumb for centreboard position : about halfway along boom foot length. ( for big head moth type sails, well raked )
Sleeve luff sails ..... avoids separation bubble on lee side.... no it doesn't because of the round shape of the mast. ( it needs a parabolic section )
Sail twist means that in unstable/gusty conditions, the sail works better . In theory the correct twist ( i e low ) is faster, but to get the max lift, the sail needs to be at the correct trim angle. More twist evens out the very high and low forces, and so makes the boat easier to sail, and faster.
When Emmett designed the aussie axeman, he may have moved the mast and board position from the earlier aussie designs, but these positions/mast rake ( and the rest of the hull concept ) were 'borrowed' from my original 1988 Axeman design.
T-foil rudders are essential for wavy sailing, so the hull's resistance to pitch down is less important. I’m sure the downforce from the T-foil must be much bigger than 10 kg..... it has more effect than moving 70 kg of body weight aft . Without t-foil, you need to move fore and aft a lot, and hang out the back, and still pitchpole. With t-foil, you can sit in the middle , and the t-foil looks after trim, and not pitchpole ( hanging out at the back helps in extreme cases! ) I think Jo Richards has worked out that on formula 40 Full Pelt with T-foils, loading is > 1000kg!
I’ve more confidence in empirical results than some of the theories, but the theories do give insight into the principles etc. eg Marchaj did wind tunnel tests on C-class cat and X boat rigs, and concluded that the X boat rig was more efficient! A lot of his tests also used solid plates to simulate old fashioned bendy Finn rigs and stretchy sailcloth. But as you can see, I’ve read and digested his books.
The more up to date Bethwaite book is more empirical ( but somewhat "clever me! " ), and technically rather unsatisfying.... lots of tests described, but few results or conclusions.
From: Emin Gun Sirer [egs@cs.washington.edu]
Sent: 15 February 1999 21:06
Subject: Re: International Moth theoretical design that may be of some interest?
Nice work, thanks for making it available on the Internet. One nitpick though: I don't think that equating the nose diving moment with the shroud tension (S') in the mast design section can be justified. The longitudinal stability section examined the boat when it is going into a pitchpole, and found that the boat+sailor weights created a righting moment of 1405 N. The boat is not in a state of equilibrium just prior to going into a pitchpole - one would not expect it to sail with the bow down 20 cm and the sailor aft (this may be where we differ - does the moth actually sail in this position in a stable state, or does the bow pop up soon after the boat yaws like this as a result of waves ?). Whenever this situation occurs, the boat will lift the bow, move CB back and go into a balanced, stable position. Since the boat is not in a state of equilibrium, one would not expect the shroud tension at the forestay to equalize the turning moments due to weight. Overall, there should be a net righting moment on the boat, and balancing righting moments assumes that the boat will just dive and go under when the bow is down 20 cm. This may explain why the working loads on the mast turn out to be too conservative.
From: Doug Culnane [doug@zingads.com]
Sent: 16 February 1999 18:59
To: Emin Gun Sirer
Subject: RE: International Moth theoretical design that may be of some interest?
It was common practice, before the advent of t-foil rudders, to sail hiking out over the rudder off the back wing bar and watching the nose getting very wet on a run. The forestay would be slack and you would wait for the inevitable trip up over the nose. If you were lucky you fell out the back as it happened, if you were unlucky you ended up getting flung through the air. I once came to the surface after such an event and grabbed my mast to they and give me some sense of orientation. I was then surprised to find I was half way up the capsized rig at the hounds.
The loads during such events are very hard to quantify and the stories get more and more dramatic as the beer in the bar flows. (Especially mine.) What I attempted to do is try and model a very complicated situation and the only way to do that is to simplify it. This model may be too simple to have any merit but it is there for what it is worth. I do however agree that the situation is not a simple static one it is very dynamic. I would therefore not take my work too seriously. I welcome any suggestions of better modelling techniques.
From: Emin Gun Sirer [egs@cs.washington.edn]
Sent: 19 February 1999 00:15
Subject: RE: International Moth theoretical design that may be of some interest?Hi Doug,
I agree - calculating the realistic lods on the rigging is pretty difficult. When I took a boat design course about seven years ago, we calculated the total sideways force at the CE of the sail (looking at the boat from the back, boat going close-hauled, upwind). Then it was a matter of using some tables for aluminum tubing of a given thickness to find the required compressive load on the mast that would keep the mast from bending. I recall making some assumptions on how the total sideways force would be distributed along the mast. Then we just worked backwards from the compressive load to find the tensions in the shrouds, taking the spreaders and diamond wires into account. Of course, some kind of fluid dynamics/discrete analysis software should be able to calculate the forces due to the sail much more accurately, but for some reason such programs are hard to find in the public domain.
From: Charles Crosby [ccrosby@mweb.co.za]
Sent: 15 February 1999 20:56
Subject: Centreboard stability when moving
A reason for more stability when moving, as mentioned by Daen Dorazio. It's not really stability, mostly just damping. As the boat rolls (i.e.. it has an angular velocity around the longitudinal axis) the centreboard (and actually the rig as well, especially to windward) develops an additional angle of attack which varies linearly along it's length, being greatest at the bottom (think in terms of radius X angular velocity). This creates an additional side force, which causes a rolling moment AGAINST the angular velocity. Hence a damping moment. It is exactly the same as roll damping on an aircraft, which limits the maximum roll rate.
From: Daen Dorazio [daen@tartarus.uwa.edu.au]
Sent: 08 February 1999 10:56
Subject: Axeman experience from Australia
I assume that the hull in question is the Lazich/Dovell boat that Emmett won all his titles in. I had one of these boats for a couple of years. The section on "sail balance" assumes about 2 degrees of mast rake. From experience, I can say that this isn't quite right. 12 to 15 degrees would be more accurate. When Emmett designed this boat he moved the mast step forward and the centreboard case backwards from the position in the older designs. The idea was that the boat could be sailed with a lot of aft rake to stop nosedives, but still have a roughly neutral helm balance. Most Australian boats are set up to sail upwind with significant windward heel, so sailed flat tend to have a lot of weather helm. Even in light air, correct helm requires more rake than any other class would ever use. Despite this, without a winged rudder the Lazich boats are still a handful downwind.
The other thing is that as the windstrength increases, the Aussie sailors use less and less centreboard. In over 20 knots of breeze, there is only about 6 inches of board in the water when sailing upwind. In addition, fore and aft trim changes as windstrength increases. As a result the centre of lateral resistance in these boats changes quite significantly. Interestingly, I don't remember ever seeing an Australian boat with a raked centreboard. I have a feeling that most of the boat's lateral resistance comes from the hull, not the centreboard, in strong winds.
Another interesting idea is that the stability of a Moth is significantly higher when the boat is moving. This is (according to Emmett) because thecentreboard provides considerable stability when it is moving through the water at high speed. I don't know how to model this, but I do know it is a very real effect, because if you pull up the centreboard on a run in strong winds the boat is very hard to control. It's a bit like riding a bike. Try sitting on one that's stationary. The same machine is a lot morestable when on the move.
From: Doug Culnane [doug@zingads.com]
Sent: 13 September 1998 17:06
Subject: Composite Volume Fractions
Since gaining more experience and knowledge I realise that the Volume fractions used in the composite for the mast design are not very realistic. A reasonable mast lay-up could hope for a fibre to resin volume fraction of 40% and the figure of 63% can only be achieved by the use of pre-preg and pressurised ovens.