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Post by casablanca on Apr 8, 2009 15:41:35 GMT
My centre board is very loose in it's slot, is there a way of sorting this out? also when it is fully down it apparently points forward, is this normal? also I need a new tiller extention, any suggestions for a sensible length?
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Post by BMBA Admin on Apr 8, 2009 22:31:28 GMT
Pointing forward I'd guess is not normal. Mine doesn't quite extend down to the fully vertical. Others have had theirs modified or designed to do just that. I've not heard of anybody wanting to go beyond that however?
Wobbles up and down or side to side? How's the board fitted? Is it a case of putting in some washers? I used to use CD's to such an effect on my old Mirror's rudder. Wonder if the same idea might help you with your centreboard?
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Post by casablanca on Apr 9, 2009 7:30:05 GMT
Centre board wobbles side to side, I also have to use a rope and cleat to hold it down or it float back into case, really weird. It's fixed with a bolt through the case, this is normal as far as I know. I think we need to drop it out and have a good look at what's going on with it!
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Post by Pie-eyed on Apr 9, 2009 7:50:04 GMT
Sounds like the first thing you need to do is adjust the rubber friction brake to stop the board floating up. The side to side wobble is trickier because if you pad the board out enough to stop the wobble you wont be able to get it in or out of the slot as they tend to have a lip top and bottom of the casing. Worth looking at the bolt hole to see if it has worn?
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Post by colin on Apr 9, 2009 9:48:43 GMT
Mast Rake and Centreboard Problems
The classic method of measuring mast rake is to put the boat onto the water (preferably on a calm day). Hang a weight onto the main halyard and measure the offset between the halyard and the rear of the mast at the gooseneck. The start figure is 150mm (6 inches). It is worth placing a spirit level on top of the centreboard case and see if it is level. If not pack the low end and make a note of it. This will allow you to make adjustments on dry land. Once you have sorted the mast rake out you can then determine the centreboard position.
There are too many variables in design and quality of boards on Moths to give an exact answer. The correct position is that when sailing to windward with the boat flat (not heeled) there should be a small degree of weather helm i.e., the boat should slowly luff up to windward when you let go of the tiller. If the board is way too far forward (down) you will get very aggressive weather helm. Too far back and the boat will steer off.
Other variables are stretchiness of the sail, amount of Cunningham tension, boat heel and the wind strength………………..
The board should not slop about in the case. I find that a good Mylar slot gasket provides more that enough friction to hold the board down. If not the board must be producing an awful amount of drag.
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Post by bertfatal on Apr 13, 2009 12:43:09 GMT
I am planning to replace the centreboard slot gasket in my moth, are there any rules and regs I need to be aware of before I glue on the new Hawk Marine polyester and dacron type strip?
Richard.
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Post by scruff871 on Apr 14, 2009 8:41:43 GMT
Richard No Rules and Regs only to make sure it sticks on well !!!! Not aware of the New Hawk product but from recent experience last season with "Wild Bill" and "New Boy,s" gasket its all in the Glue....make absolutely certain that the Glue is fresh and nice and runny.If its gloopy ditch it !...the glue will not work. Make sure the key is good and the surfaces are clean then you should be OK. Take special care preparing the front of the slit in the gasket.Drill a hole or burn a hole at the beginning of the slit and ensure you get a nice seal around that nice new Higson C,Board.
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Post by moth868 on Nov 12, 2009 18:36:12 GMT
What is the optimum size for a centerboard? I ask this because following a remark by a fellow Skinner owner, that his boat went much better when he replaced the original dog leg board with a conventional one, I decided to make a conventional one. Having obtained a pattern for a conventional board, the first thing I have noticed is how much bigger it is - immersed area of my dog leg is 1600cm sq. the area for conventional board is 1900cm sq. (board at 45deg). Approx 19% increase, is the dog leg really this more efficient. The books, and Colin in the latest Chrysalist, recommend rudder area to be approx 30% of centerboard area. My rudder is approx 1200 cm sq. - 64% of conventional and 75% of dog leg so should I be making a new rudder as well? With it's current foils the boat will turn within it's own length about the centerboard which is nice.
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Post by casablanca on Nov 12, 2009 18:48:18 GMT
I also have a Skinner moth with a dog-leg centre board, I've not had it long but what I notice is that I slip side-way before I get much forward motion when I tack, I'm not sure if this is the centre board or not. I know Jeremy Higson has replaced his dog leg board & has improved results. I also know he made the new centre board himself.
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Post by Meat Pie ... on Nov 13, 2009 9:10:47 GMT
Lots of advice or thoughts on C/Board profiles,shapes,positioning,angles and thicknesses exist both on here and in past articles in Chrysalis. Suffice to say that narrow foils can work very well but need to generate lift[hence forward motion/drive]before they start working efficiently.So the slippage that has been referred to is perhaps to be expected.As for an optimum shape or size for a C/Board do not believe there is one.To my knowldege all the current quicker boats have different shapes,profiles and their respective Helms use differnt rig set-ups and as a result different angles when the C/Board is set.A few persist with dog-leg versions to some effect but the basic Goffo C/Board[slightly modified by most including myself]seems to give good all round performance.This shape was used by 846,871 and 817 at the 2009 Nationals and all have good upwind speed and height. Have no idea of the surface area but what concerns me a little is that you have checked your surface area at only 45 degrees.Suggest that before you commission new foils you do some tests with the board at greater angles and guage/evaluate the results. Whether to use or opt for a dog-leg C/Board is down to personal choice and experimentation. Alignment of sail center of effort and that of the main foil/hull is the target and consideration in all displacement boat design.The Moth is a relatively slow boat and perhaps more area than you currently have,together with a steeper angle upwind may minimise the slippage. As for the Rudder area thats a whole different subject. Again current quicker Moths seem to have narrow reasonably deep Rudders perhaps only 150-200 mm wide and 25mm thick and perhaps 600-700 mm deep measured from the lower edge of the transom.These tend to be parallel with a generous radius on the bottom leading edge.Kicking the trend is the Jezza Higson "Propeller" blade which is eliptical,narrower and deeper. Foil profiles,leading and trailing edge forms is reasonably well known technology but surface areas is not so well known or discussed. My new boat will have thinner foils than the standard 25mm for sure,and will also be trialled with a narrower[narrower than the Goffo shape]C/Board for more Open water conditions but a wider chord board may be used in more restricted waters.
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Post by graham7 on Dec 15, 2009 17:24:46 GMT
The Higson" Propeller board" is an exact replica of Phil Morrisons Spitfire wing rudder from his highly succesful Burton winning Whisper National12,which I own ,so it has an excellent pedigree and worth looking at.
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Post by Fatboysfly on Jan 25, 2012 9:59:47 GMT
Firstly, thank you Ian Edwards for announcing my BMBA membership looking forward to meeting you all on the circuit. Back to 713, in between coating up I was planning to go to work on the foils, my current c'board is of the wide and short type ( like me short and fat ) was planning to strip and lay up fibre glass matting as there is enough width to the case, extend to max to fit case and reduce width. Not sure how far to go with reducing width ? should I leave bolt hole in place and reduce front leading edge so I have a small ( 100mm ) dog leg. Is there a sketch of photo with average dimensions on as a guide ? Rudder blade is going to be made from one of my old mahogany merlin blanks, probably 600mm deep below transom and between 175 - 200 wide parallel. Help required from the BM oracles ! Thanks JH
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Post by colin on Jan 25, 2012 12:59:51 GMT
I would like to say there is some consensus with regard to foil dimensions but that is not the case. However high aspect ratio (long and slim) are generally more ‘efficient’ that short and wide. You should also consider the following.
Assuming you will be using one of the NACA sections, say 0009, by definition the maximum thickness will be 9% of the cord. So assuming that the board is full width (25mm) as it exits out of the case the cord will be a little over 270mm.
One of my boards is a NACA0009 and is 270 at the root but tapers down to 120 at the tip so to maintain the section the maximum thickness at the tip is about 11mm.
The leading edge of most foil sections will be a radius blending into a parabola and you will have to make a call on that radius. In general it will be something like 1 to 1.5% of the cord. So for example if you go for 1.5% (probably a safe option) the tapered board above would have a radius of 4mm at the root and 1.8mm at the tip.
Producing a foil that accurately is a major problem. Parallel foils are far easier to make due to the constant section. As the leading edge radius is constant, you can make up a scraper by drilling the appropriate size hole in a piece of steel. You will need to file away most of the circumference so that it does not undercut the parabola.
Incidentally I have written a piece for the next Chrysalis on foil cavitation and what can happen if you get in wrong.
Colin
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Post by Fatboysfly on Jan 25, 2012 17:38:46 GMT
Thanks for the info Colin. I have a 25mm slot so I will work as close as i can to that. I think I will work from the leading edge and forget about the small dogleg, my board is only 20mm thick so plenty of room for glass and epoxy after reshaping. This will be a learning curve for me, i like to have ago myself, then you tend to appreciate what goes in to a nice milanes etc foil. Look forward to reading your Chrysalis article. Thanks again. Jeff H
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Post by colin on Dec 11, 2013 14:50:21 GMT
Whilst eagerly awaiting Grahams build a boat thread I have been making yet another centreboard. This is following a similar quarter ellipse profile as the rudder blade. The core is Western Red Cedar sheathed with single layer of 320g biaxial glass, which I am trying for the first time. This should be as strong (?) as two layers of 200g 2/2 twill as there is no crimp. Handling the cloth requires a bit more care than twill as biaxial is only lightly stitched together. I used the wet method with the normal vacuum bagging layup of glass; peel ply, perforated release film and breather cloth. I do not normally work on the kitchen table but the garage is far too cold. One handy tip from the West System website: To calculate the amount of resin to mix for sheathing, weigh the cut cloth and mix the same weight of resin, plus 15% for wastage - I used EL2 Laminating Resin from Easy Composites which is nice and runny, so wets out well. I have also gotten into the habit of post curing where possible. Colin [/img] Attachments:
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