MartinRF
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Posts posted by MartinRF
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So I couldn't help myself and started modelling the board-hull interaction:
Very much work in progress and I can't guarantee the usefullness of the results but the idea is to load it up with some of those finite elephants and hopefully understand better where stresses go.
/Martin
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My intention now is to remove the sole roughly 3cm around the opening, grind out any fractured glass from the cracking and lay in many layers of triax and db. No core is compromised in this area as it is all glass. I will then replace the floor and let it make a horizontal column to further distribute the sideways load of the board.
Then I will also attack it in similar fashion from the outside, removing any fractured glass I find and rebuilding. Glass and more resin are on order!
Thanks all for your input. Martin - that is beautiful work on that trunk. I'm surprised you have no sole in that area (or was it added later?) I think the sole would be critical in distributing point loads from the dagger case....
No sole in my boat. None specified and my boat is small with some 1.3 m head room so adding a sole would not really add to comfort.
I see Tennant stuck to the same design we rejected back in 1985. There were a number of things we change in the structural engineering. The only things that have broken are things we did not change such as the cross beams.
Yes, the sole helps distribute the load but only from one side unless you wrap fibres around the dagger board case and spread them onto the sole.
Are you in a hurry? If not, why not spend some time on a bit of proper engineering before slapping on more material?
/Martin
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Four layers of glass? What kind of glass and why?
Here is what we did 30 years ago instead of following Tennant's instructions. This has worked but I (now) understand there are even better ways of reinforcing this area of the hulls.
Some years ago there was a very long thread on the renovation of a tri called Timberwolf. Unfortunately all photos are missing now.
Having longer boards with the upper support further up in the dagger board case also helps -- less crow-bar like. My dagger boards reach deck even when fully down.
/Martin
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Underengineered and underbuilt foils are far too common.
There is some information on this here.
I have enclosed a section that works real well. You can scale thickness a bit to fit your dagger board case. The first coordinate is cord and starts at the trailing edge (1) with zero at the leading edge.
Recommended for rudders too.
/Martin
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I am envious, I want to emigrate.
/Martin
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This four year old video demonstrates what impact on sail shape down-haul-induced mast bend can have.
/Martin (envious, I don't have this level of shape control)
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Both boards are fully down throughout this video. That was not perceived as a problem by the helm. When the biggest gusts hit he was not totally happy with rudder response (he is a former Swedish A-cat champion sailing Marström boats -- very skilled but a bit spoilt when it comes to build quality).
Afterwards I calculated bow-down and forward tilt and figured that during maximum acceleration even the lee rudder operated almost totally in the wake of the hull: the lowest point of the hull being almost as deep as the tip of the rudder.
Hence the plans for new rudder blades.
Note that this is sailing under full sails and spi. Doing these speed and more under storm jib and two reefs (on a lower rig) in much more wind many, many years ago there was no bow-down or control issue. The lower centre of effort and, I think, less fierce gusts relative to base wind strength meant the boat was just skidding along without nose-down tendencies. (I wish I had had a GPS to record that run but that was well before GPSs became available.)
/Martin
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Another factor I think is rudder blade twist. The leading edge start to 'gybe' through the water so it loads one way then the next. I have had this same thing on many smaller boats at high speeds but mostly SR26 and my GBE both capable of high speeds..
I broke 2 rudders on my GBE before building 2 new and very strong somes which helped a lot. the first set of rudders used to twist really bad and then eventually self destructed. I also found it while getting towed at about 17kts one day with no effective helm load.
Yes, bending and tosional stiffness are important, especially if the blades are raked forward: Bending will induce angle-of-attack increasing twist...
/Martin
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MartinRF, When you say balanced, are you talking about how far forward they are raked?
The rudders are supposed to be Naca0013 according to the drawings, but the leading edge does not match that profile. So I'm not really sure WHAT the builder built. I suspect he built a profile that SEEMED workable, but was easier to build than that 0013 with it's curved leading edge. At about the 30 second mark, you can see the profile of the rudder in this video:
Martin - I agree it is odd. I was getting a bit nervous, because I thought the boat was gonna spin out or crash jibe.
OK, here is what I think may be going on after having looked at the video above. I see you use the same 'geometry trick' I do for balancing transom hung dagger-board rudders. There is really only one mostly aestetic difference: In my case the rudder blade is vertical and the rudder axle is raked.
Watch the attached drawing: The left part shows the original design from Tennant. The right-hand side shows what I have had since 25 years or so. I have reduced the distance between the centre of effort and the rudder axle. Your geometry does the exact same thing.
The right-hand side also shows a potential problem with this design: If only the lower part of the rudder blade is active (disturbed flow around upper part, transom starting to lift out or whatever...) the centre of effort moves down and may eventually cross the rudder axle.
You can shape the rudder blade plan form to avoid this by raking the lower part of the leading edge backwards. New, deeper rudder blades designed like this are in their planning stages -- since several years
/Martin
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I was spending time at the other end of the speed spectrum today -- had problems reaching five knots. This was possibly the last sail of this season. The temperature had dropped to 3 C when I got back to my apartment.
Very nice, calm and quiet as I was all but alone on the water.
Tiller gets very light above 18 knots? That's odd I think. Do you know what wing section is used for the rudders? Are they balanced in any way?
/Martin
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> The failure mode seems to be in the details, such as around reef patches.
> This is where the original sail maker shows his worth, as the sail nears the
> end of its life.
Which is why I mourn the fact that the sailmaker of my last two main sails has closed shop. I had hoped to place an order with him for a new jib...
> Sure on dacron Tornado and PT mainsails we used lots and lots of luff tension
> to bend the mast, open the leach and move the draft forward. But I'm not
> convinced it it effective on bigger boats with radial sails.
It is an important tool in my toolbox. Inspired by members of this forum I beefed up the down-haul just a few years ago. Sailing singlehanded most of the time I am running out of stability at around 10 knots true when going to windward -- like last Saturday when the wind topped a mere 12 knots I had the main traveler 30 cm down from middle point, reduced mast rotation, downhaul and sheet fully on and jib sheeted flat. (GPS track plotted in enclosed file)
> Awaiting other opinions on this...
So do I.
/Martin
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When Kevlar sails were a new thing back in the 1980s I was told they held their shape until they disintegrated. I was not in the echonomical position to test this.
Now I have an almost ten year old jib built from a carbon/Kevlar laminate from Contender called Maxx. The material still looks very solid apart from the window which isn't very transparent.
Still, ogling the shape in lighter wind I see the draft is not as far forward as it used to be and the leech is hooking. If the wind is up and its sheeted and tuned flat it looks fairly good. (Disclaimer, when the wind is up I don't have time for close study of sail shape...)
So what's going on here?
Am I making this up? (I can pull the draft forward by increasing luff tension but my gut feeling is I need much more luff tension than when the sail was new -- but then, how good is my memory?)
Is the Mylar films aged in a way that shanges light wind shape but overruled when loads are up?
My roughly five year old carbon (Dimension Polyant laminate) main looks very good.
(Main and jib are both from Oz but from different sail makers.)
/Martin
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Check out Martin Schoon's tacking video - light airs but still achievable.
Yup the sheet setup is a pain with little kids.
My jib sheet is routed 'on top' from cockpit to cockpit via the jib clew: Forward from the winch to a cheek block on the main beam, then along the top of the beam to a block on the traveler, through a block attached to the clew and down to another block on the traveler etc.
The line I am giving a tug in that video is the barberhauler. That one is routed under the trampolines. I need to give it a tug as the traveler track of the jib is straight and the jib is only semi-self-tacking. Simplisticy over function?
/Martin
PS Nice photo paxfish.
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See, Paxfish, I am not the only one doing my own hardware.
/Martin (Looks like sail I could have a lot of fun with on my boat.)
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I think length = size is a mistake.
Sundreamer was, if memroy serves, designed as a very long fortyfooter: Width and sailplan as if she was 12 m long but with 18 m long hulls.
ORMA 60s were designed to a rule which limited length, width and rig height but not much else. They were designed for pro sailors and money was not an issue until it became such a big issue it killed the class. Designing them became a desperate pursuit of longitudinal stability, according to Nigel Irens.
All information I have indicate the MOD 70 trimarans are much better boats and in most cases actually faster despite being less extreeme.
So why are ORMA 60s faster than Sundreamer? Simply much more horsepower -- the wide stance makes it possible to carry a lot of sail. But they need pro sailors to reach their potential.
Look at these photos from 1992: http://hem.bredband.net/nschoon/galleries/Plymouth-92/index.html
I was out sailing with Richard and Liliand Woods and some friends on their Sagitta. Wind was 10--11 knots and very steady. The ORMA 60s were averaging 20 knots on a course that was almost close hauled! Arthaud, Poupon, Peyron, Bourgnon...
For the 8.5 class I think the horse power thing is boxed in so I would look at the latest in hull lines since length and weight are dictated. My ISAF insider tells me old Tornado aces claim the Nacra 17 is quicker than a Tornado... I am intreagued by the shape of AC45s, GC 32s and their ilk -- based on the lates in beach cat design I understand.
Foiling may be banned (I haven't looked) but if you are allowed to add some vertical controll a smother ride may be had and that means sails have better working conditions and will be more efficient.
Work with the areodynamics -- both drag and and lift.
In your own back yard: http://www.hpyd.org.nz/
/Martin
PS Point-to-point record runs are a bit special as they depend so much on the weather factor and simply having a bit of luck with that.
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No.
/Martin
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Been there done that twice. Once because of stupid underbuild and second time because I hit a rock. That time I missed a race but the boat was back in working order within a week.
By the looks of it I don't think you hit something. I think this is a case of too weak.
Remove damaged laminate (angle grinder and 40 grit 'paper' comes to mind). Rebuild and make it strong enough. You need a 'bandage' of fibres holding it together between the pin and the sleeve.
Idealy you should have fibres forming a figure '8' with the rudder sleeve in one loop and the bolt (pin) in the other loop + fibres wraped around a whole lot inone loop. The intent is to create something that can take shear and bending.
Well, I guess ideally you should do something a la Marström in carbon prepreg (50% of fibres parallell to rudder cord and 50% at +/- 45 degrees to cord) but since that is out of reach for most of us mere mortals...
Reinforce the one that did not break this time.
/Martin
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Their store locator does not feature Scandinavia or even Europe.
I have been told covered lorries are painted using 2-pot polyurethanes like the ones we are used to and you can get any colour you fancy. I will investigate this.
/Martin
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Now you can do the other side
Easier said than done since that yellow colour is not part of either International's or Hempel's offering any longer. I have some ideas on where and how to get it but some investigation effort is needed.
/Martin
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Lessons learned:
- Longboarding isn't quite as intimidating as I thought, but it is a good thing to be patient and not try to do it all in one go.
- From an Altex video I learned about applying tracer paint onto the undercoat before sanding. This really helped.
- Keeping the mixed and thinned (20--25% thinner) paint in a bottle and squirting paint into the roller tray as needed to wet the roller ensured consistent viscosity throughout the application.
- Don't count on weather being cooperative
/Martin
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Final result photo from about a boat length away.
/Martin
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I have had really good battens from day one but the first set of sails were undebuilt dacron sails (1986, remember?) and good battens can not save bad sails
We found another sailmaker, one who knew what he was doing, who re-cut the main and reinforced its leach and we ended up with a useful if not good sail.
/Martin
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Perfection was not the goal and I don't like adding weight to my boat so I added almost no filler -- less than 100 g for sure. I sanded until reaching high spots of the laminate and did not fill in lows. I then undid some of the longboarding as I removed the 60 grit paper scratches by machine sanding with finer paper: exposed filler sanded quicker than old paint. Still fairer than before.
/Martin
Sigh....
in MarineTalk
Posted
Using open source programs you mean?
I think that could be done using Freecad and maybe Z88 Aurora or Elmer or the tools collected in CEALinux. I will try that some day but for now I am lazy and use tools I am used to from work.
/Martin