UK-Cherub Forum
Cherub Chat => Tech Chat => Topic started by: Banshee Ambulance on January 25, 2010, 12:08:35 PM
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The time has come for me to order some foam. What grade do I buy and who do I buy it from? I am Southampton based. Any advice appreciated.
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impag in dursley, gloucestershire are the main supplier of structural/core foams. i'm sure they have a website.
Arex or divinicell 80kg/m^3 is commonly used for shells and bulkheads. Green in colour. Bulkheads and hull pannels are built from 8mm thick foam where as hull sides can be reduced to 6mm. This option really depends on what you have allowed for when making your plug. 8mm 2.4m x 1.2m sheet costs in the region of £40.
1 sheet does the spine and bulkheads of an elway 5. you should need 1 &1/2 sheets for the deck but should have some off cuts. Probably 3 more sheets to do the hull shell.
final no. of sheets depends on the dimensions of your hull. You will be fine if you buy 6 but if you're really carefull and you have a skinny hull you may do it with 5.
order thicker foam of the same or a higher density for the foils. A sheet of 20mm thick will do your rudder blade, t foil blade and centreboard. if you bond a blank of 8mm thick to a 20mm thick sheet.
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And to finish my last sentence: to get a centreboard blank thicker than 20mm you can bond an 8mm thick peice to a 20mm thick peice. I know that foils have been successfully made from 80 kg/m^3 foam however you can go to higher densities if you want it to be a little more rugged.
a 20mm thick sheet of 80kg/m^3 will cost about £100.
Since I made the cross beams and C/b case pads out of high density foam £180 per sheet (6' by 2') i bought 2 and made all the foils as well.
If i did it again I would make beams from carbon tubes with no foam core and make the foils from a medium density possibly 120kg/m^3.
You can collect. or pay postage. I would pick up fo you but wouldn't easily get more than 1 of 8' x 4' sheet in the focus.
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Airex comes in a confusing array of letters and numbers. What sort do I need?
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rob could i order a sheet with you if there is discount for bulk? high density for foils,etc...
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I think it is the Airex C70 foam that you want. appears right from the data sheet on IMPAG's website.
Please confirm when you enquire and ask what they supply to Ardvark or bloodaxe. They should be using the same stuff as you need.
I previously found a product sheet which showed the different sheet sizes and densities. might be worth googling Airex C70 and working out how much you need.
john - check the sheet sizes and that you can get all the foils out of 1 sheet. Don't forget that the thickness is proportional to the cord and the naca section. Our board was tapered from a cord of 0.3m at the top so needed to be 30mm thick. I had to cut out two blanks glue them together before routing all the contors. This produced a lot of wastage. It may be more efficient to get 1 sheet of one thickness and another of a second thickness.
My Point is work it so you have enough and save the hasel of a second order and postage. I'm not sure if there is a discount until you buy a whole box. that's a lot of foam! but you would save on postage.
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John, My first set of foils will be done in Cedar I think. Its easier to work than foam and the core will be more structual so I can use less of the black stuff.
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Paul Croote may be able to help you with the cedar if you talk very nicely to him.
Cedar is close in density to the expensive high density foam so a good alternative.
We re-sheathed Slippery's cedar foils with 1 strip of uni 2/3rds the way down the centre of each side and 1 layer of weave over the whole lot. I would suggest adding an epoxy trailing edge to the cedar and fairng this before putting the weave on because the grain in the wood makes it very easy for the trailing edge to get damaged. Because foam dosn't have a grain it's better than wood in this respect.
I agree that it's a pleaseure to work with wood. Cedar has a nice smell to it.
Wood sniffers of the world unite!
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I would also suggest leaving a bit of thickness to your trailing edge.
The tinner trailing edge increases the bending arm and causing cracks that would not have been possible with a thicker edge (i.e. it breaks at a thickness that would have been ok if the edge had ended nearer it, not tapered to nothing).
Have learnt this the hard way and am still picking parts of what was left of "knife" trailing edge out of my hands - not to mention the mess it made of the inside leg of my dry suit.
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I would suggest shaping your core right down to a feather edge (that way the two layups touch each other fairly), but shaping the final foil it so that there is thickness of the edge you go sailing with to avoid damage (see above).
A bit less than a mm seems to be the norm.
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whats the best way to shape a foam core....router???
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I normally just mark the thickest point of the section 1/3 back from the LE and use a belt sander to remove material judging by eye and template if it is the right shape.
Some people use a router to cut grooves to the appropriate depth, and then sand the rest down to the grooves.
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Phil you are a god when it comes to foils if you only need to do that.
For the lesser mortals:
you can draw the foil in cad or crunch the same numbers in a spreadsheet.
First you need a profile.
then at equal intervals down the foil mark lines. measure the cord of each line and scale your chosen foil section to the cord. this will give you a 3d shape which you can cut with planes at each thickness. 1mm intervals work well. you may need some 1/2 mm intervals near the center cord line.
Do we need a sticky weekend on this?
Remember to convert each of these thicknesses into depths from the surface of the foam.
I did it in a spreadsheet and drew the contors onto draughting film. I then (or rather sarah then) transfered the lines to the foam by dotting along the lines through the film with a fine point pen. join the lines up. route each contor working from the outside to the inside.
Then the easy bit is sand the foam down until the grooves just dissapear. Use a long board for the flatter bits to ensure fairness. wood or high density foam requires more sanding but is less delicate.
A paralell section is much quicker to do because all the contors are straight lines.
Some of our photos will show the practical steps.
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Do we need a sticky weekend on this?
i think we do, well me anyway, i just dont quite understand how to get the section diagrams,etc. people give me the NACA sections but i have no clue where to find them
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Phil you are a god when it comes to foils if you only need to do that.
The final shape is more approximation than perfection, besides the router I have has over 1.5mm of end float so is useless for doing any sort of accurate work
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i think we do, well me anyway, i just dont quite understand how to get the section diagrams,etc. people give me the NACA sections but i have no clue where to find them
http://www.pdas.com/profiles.html (http://www.pdas.com/profiles.html)
http://en.wikipedia.org/wiki/NACA_airfoil
Google is your friend you basically you either make a spreadsheet, scale the points and plot onto square paper or;
put into a cad programme, draw to scale and print out.
If you are routing contours you need to make an allowance for the thickness of the laminate, and you still need to make a template for the final shape to check things after the laminate.
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On one of the previous threads there are some Rhino files. Ifyou can't find them send me a PM with an emial address adn I should be able to dig out some examples....
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After some great advice from Phill K, I have settled on one of Martin Hemples (model planes) sections for t - foil lifting part. Low drag and operates at Similar Reynolds numbers to model planes so makes sense (apparently - I havent done the maths). It operates at a low angle of attack so less of a worry of stalling. Then a NACA for the center board and a chunky NACA for the steering bit of the rudder. Interestingly no one has mentioned aspect ratio. I assume you have the same drag/stall angle compromise there as well?
John, I wouldnt get too held up on the fluid mechanics. I studied it at degree level for two years and still dont have a clue, its not easy to get your head around. Im just copying what the clever people do on the issue of foils. But if you have a head for physics then good luck, you can explain it to me!
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rob, fluid ddynamics is what i want to do at uni so i thought id get started soonish :)
on a more confused note, martin hemples seems to be blocked at school on the internet so ill do some more in the weekend. phil k and kevin ellways advice seem to match up quite well so ill go with a NACA 10% rudder, NACA 03010 centreboard and the Martin Hemples 50 plan with a bit of asymetry for the t bit of the foil.
right... aspect ratio, what is goodd for an E5 as i imagine it spends more time at higher speed than cheese so the board could be different in aspect ratio for less drag with a slight comprimise on lower speed stalling???
sorry for asking so many questions... first lot of carbon ordering soon!!!!
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If you are in Southampton, talk to Tim at Marineware for foam. Corecell A500 for general stuff and they have a HD version too but can't remember the code...
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this bit of software may help...
http://www.dreesecode.com/index.html
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Sounds like you have a grasp of the design side for foils.
Aspect ratio is a consideration but if you talk to Kevin Elway he will explain that the downwash angle also affects the efficiency.
The rudder is posibly the greatest compromise of design because it has to work at all speeds and you want the most stearage at the lowest speed i.e. manoeuvring for the start. A T- foil gives you the benifit of an end plate effect which increases the effective aspect ratio. The cords of current t foils is about 120-150mm so the rudder blade will need to be a similar cord or greater.
I oppted for a rudder blade cord of 230mm to aid the low speed stearage issue but coupled this with a 9% low drag section to offset some of the drag of the larger surface area.
Slippery's rudder is 18cm cord at the top and tapers down to 15cm at the bottom but has no sculling effect and does stall out on poorly balanced bear aways. The optimum maybe somewhere in between the two.
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That is very interesting, thankyou. What is downwash angle?
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I can't remember the exact explanation of downwash angle but i think it is associated with the flow round the tip of the foil and the drag associated with it is controlled by the planform of the foil. The dagger boards/centreboards on Antidote and E-numbers and I think the other Elway 5's are shaped to minimise down wash. As I suggested before Kevin may be able to explain the theory.
Alternatively ask an aironautical engineer and keep asking why?
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is it simlar to tip vortexes on the sails, i.e. why the elliptical shape is so effective as it doesnt allow vortxes to take hold and so lose lift?
edit: like kevin designed will's centreboard as elliptical to allow aspect ratio without a too low CLR for less heeling
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John
Aero/hydro
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There are 2 forms of drag; a) profile b) induced. They are practically independent of each other.
The foil shape you use affects the profile drag. Profile drag is caused by viscous effects. You need to choose the shape based on the lift coeff and the Reynold's no. Typically CL~0.3 for a centreboard and Re 400k. You can use 2D drag predictions to look at this. eg JavaFoil
If you use a NACA00 section for the rudder, sharpen the leading edge where it goes through the water surface - it will ventilate otherwise. There are lower drag foils (as already advised) which may suit you better.
The induced drag is a 3D phenomena and is the drag that is produced directly a function of lift.
CL=Ao x AR/(AR+2) x aoa where AR=e*span^2/area, Ao is the 2D lift slope of the foil (0.11 CL/deg for most foils) and aoa is the angle of attack and e is the Oswald efficiency factor
The induced drag is given by Cdi=CL^2/(e x pi x AR) .
So you can see that the lift for a given aoa increases as AR increases and as e increases. Similarly the induced drag decreases as a function of AR and e.
So what effects Oswald efficiency?
e is a function of the end terminations of the foils and also what is called the downwash distribution.
When you stick a foil in a fluid and give is an angle of attack of angle A, the flow is angle is increased ahead of the foil by an angle ai and decreased behind the foil by an angle 2*ai. Ai is called the induced angle of attack or upwash and 2*ai is the downwash angle. Upwash from a mainsail makes the wind freer for the the jib. Downwash from the jib makes the wind head for the main. This is why the jib is sheeted at say 10 deg and the main at 0 deg. It is also why when you get a boat ahead and to leeward of you, you fall down. Your boat frees the wind for him, and his boat makes the wind head for you.
Anyway, ai is given by the local section lift CLs/2*pi - A.
A German scientist called Munk worked out that the induced drag was at a minimum when the downwash angle along the foil was uniform. So if CLs is constant along the span, so is ai, and thus so is the downwash angle.
For an untwisted symmetrical foil like a centreboard or rudder, it turns out that to get a uniform ai and the CLs, the planform needs to be an ellipse. If it is anything other than this, eg a triangle, the lift will not be constant along the span - in fact it would theoretically go to infinity at the pointy tip. In consequence ai is not constant and so the induced drag won't be at a minimim.
So, for a completely elliptical foil (like a spitfire wing) e = 1.
Now, if you use 1/2 an ellipse and put it under a boat, the hull forms an end plate and e=2.
But this is not the whole story...
The centre of resistance of a 1/3 ellipse is at a depth of about 42% of the span. But for a triangular tapered foil it is at 33% of the span. For the same AR, the elliptical board will give less induced drag, but more heeling moment. When you're fully powered, what you want is a board shape that gives the best ratio of induced drag to heeling moment. In essense, you can have a longer (and thus higher AR foil) if it is triangular. So there must be some optimum.
Another scientist called Jones was interested in this for gliders - higher AR, less drag, flatter glide slope. But the bending loads at the centre span get get really big. So he wanted to get the minimum drag for a given bending load - just as we want min drag for a given heeling moment. He found that this was achieved the the downwash distribution is tapered linearly. For a centreboard, this is a board shape which is much more tapered than an ellipse.
To cut a long story, you can increase the span by about 15% compared with an elliptical foil and keep the centre of resistance in the same place. If the board shape is such that the downwash tapers linearly to zero at the tip, the induced drag is also some 15% lower than the ellipse. This is what was designed for Will.
There are some detractors. On a centreboard, we can't twist the tip. This means that section lifts at the tip is higher than the root - we have a 'tip staller'. This adds some drag, but should mean the board stalls progressively along its length rather than all at once. You'll have to consult Will to see it it works!
Hope this helps. Try looking at www.tspeer.com and www.onemetre.net and www.desktopaero.com
Kevin
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Wow Kevin you really do know your stuff. I haven't done a proper analysis of the equations yet as I have an interview tomorrow, but I will try and have a proper look when I get back.
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That is very interesting, thankyou. What is downwash angle?
Wow! And there was me thinking it was something to do with Maverick getting too close to Iceman and heading out to sea in a flat spin. :-[
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Neil, you're mistaken my friend, that was backwash from Icemans thrusters!
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We will be doing things a bit differently on the t-foil front. We will use the lift from the t-foil to lift part of the blade out of the water, through the top of the stock once moving, and when we stop the blade will slot back down with a big clunk thanks to lots of bungee. This should give us a large rudder when we need it and a small rudder when we dont.
Trailer is coming on nicely, photos to follow.
I thought for a first boat I would actually like to buy a shell, as it is several less things to go wrong so I have got my hands on Hardly Sluggish. Shell being delivered over easter hopefully. I know the Slug is an old design but it should hopefully still be quicker than an E6 in the top wind ranges in flat water. Not that hull shape matters a great deal really.
At last things have go off the ground.
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Picked up some more bits today! More to fiddle with when the time comes. Loads of uni work at the moment so the build is still on hold.
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Update: Finances have put the build on hold a little longer than expected but the postie brought me some presents today. They are ex I14 so may be a bit big but it is easier to make them smaller rather than bigger! Will Aardvark supply a bare t-foil blade to add to this rudder? I have been promised workshop space close to my new job so I stand a chance of turning this boat into reality in the near future!
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good stuff
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John, Do you still want some tube? I can post it if it helps and you pay the postage?
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i do still wnat some, where are u based, i am going to get my boat (exams finally over) next weekish???
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3 sheets of foam on order. Should be here by monday!
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At long last things are getting going!
Edit: Wrong picture.
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I did not know that laying cloth at 45 degs was so hard! Here is a picture of the port topside if anyone is interested.
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I was going to post:
If you are not vaccing I would recommend using gravity to hold things in place!"
(i.e. spin the shell and light the lamp, not the rat)
[/b]
Then I noted that the floor was vertical in your shot ::) Looks good.
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I did not know that laying cloth at 45 degs was so hard! Here is a picture of the port topside if anyone is interested.
Big sheets are tricky, however for going around sharp corners and edges 45's are much eisier
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Took the peel ply off the topsides today and can confirm that they are amazingly stiff. However, due to the lack of vac bag facilities they will need to be faired more than I would have thought is usual.
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Topsides don't need to be very fair. Only aesthetically fair. The bottom panel is the important one!
Good effort on the progress. I might be joining you and john for you boat building goodness in a couple of weeks.
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Got the bow on and the snout tube test fitted. The actual bow will be a low density foam fairing added to the flat bow you see here at the moment. This is because in my experience this area takes a beating especially from the trailer so the theory is it would be nice to have a non structural area that can just be filled etc as required. I think the bow will be like Loco Perro but I have not studied it in great detail so I can not be sure. Perhaps Simon or someone else with some knowledge of the boat can help me out.
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A faired in snout gives a little more buoyancy when you stick the bow in as apossed to a dolphin stricker which doesn't.
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Although this is true, I think that when you have got to the point that you are relying on snout buoyancy to save you, you've probably gone swimming anyway!
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The snout on Loco although fairly narrow low down becomes wide quite quickly due to the hull shape. This has the effect of slowing the boat down quickly in waves. If anything this was more of a cause for our many capsizes than a lack of bouyancy in having a dolphin striker to support the bow tube. On the other hand it also made for some very funny moments when the hull stopped and the rig and crew carried on at 14 knots.
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A built in snout allows the aft pole support to be further forward. This in turn makes it easier to retract the pole past the mast for a given mast position and pole taper.
Consider adding a carbon stay to the tube snout shown in the photo to provide vertical support to the jib tack.
The snout fairing that you are suggesting will take a bashing from the trolley etc so I would cover the front of it with 2 layers of carbon. very little additional weight and you wont have to keep fairing it after every outing.
Once you've finished building a boat you will want to sail it and not keep re-building it.
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any news rob :)
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No news, sorry. Boat is on hold due to other things at the moment. Need to get a car and other imortant things first!
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The snout on Loco although fairly narrow low down becomes wide quite quickly due to the hull shape. This has the effect of slowing the boat down quickly in waves. If anything this was more of a cause for our many capsizes than a lack of bouyancy in having a dolphin striker to support the bow tube. On the other hand it also made for some very funny moments when the hull stopped and the rig and crew carried on at 14 knots.
Yep.
Hull shape flare is a bit different to a built in snout in terms of increasing pitching, and as Phil says there are other benefits to a snout.
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I have gone for the Loco style snout for a number of reasons but only because my build is a bit strange. Firstly and fore mostly, I had to cut the shell down to remove the flair and it made locating the foam and re building the topsides easier. Second, I am sort of continuing with Ians plan to comply with 12ft skiff rules. If I was going to build straight for a mould I would have stuck with the built in snout.
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I know it may seem that I have lost interest in taking this project to the start line. Fear not, new job, home and lack of transport has put the beast on hold. I now have a driving licence and have been promised workshop space so things are looking up. Thunderbids might be 'are go' in the near future!
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Had a look at the boat over the weekend when I was back at my parents It seems there is the odd bubble between the foam and the lamiate, presumably due to not vac bagging. What do I do to get around this? I was thinking of cutting out the area with a scalpel and re laminating the affected area? Is that wise?
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I can't think of any other options. Be sure to align the fibres and give it an inch overlap.
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Cutting around bubbles and re-laminating is a good idea.
If you have not done so already get a 50p peice and tap all over the hull to map any sections that have not stuck properly, you should be able to hear the difference. Use a marker pen and some masking tape to mark out all the patches.
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Have you found a workshop/garage/front room for the build?