UK-Cherub Forum
Cherub Chat => Tech Chat => Topic started by: JP233 on April 14, 2013, 09:35:20 PM
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Hello, I'm a design student; my Final Major Project is in full swing.
I need some questions answering, and some opinions on what I am doing.
I decided to design a hydro foiling cherub (can you tell I’m a mothie?) I have seen pictures of Aquamarina with a T foil on the dagger board http://www.yachtsandyachting.com/news/21935 (http://www.yachtsandyachting.com/news/21935) I have also seen the foiling 49er swap the hull to a cherub and it would be better! Bethwaite Foiling 49er (http://www.youtube.com/watch?v=bPPUwvUwEN8#)
At the Chew valley open meeting I went round with my camera taking pictures of hull shapes, and features that I liked. The hull I was going to design was not meant to actually 'work' as I don’t have access to those computer programs, so if the average sailor looked at it, it would be a cherub to them. That was the plan until Clive Everest kindly sent me the CAD files for his new cherub.
I have slightly modified Clive’s hull, the main thing is the reverse bow, for those that don’t know, a reverse bow allows the hull to cut though a wave rather than going over it and hobby horsing on the other side, I also tweaked the curve on the front of the wings that I based on my ‘experience’ in the front of a 49er.
The hydrofoils would be the same style as those seen on the foiling 49er, or the latest Americas cup AC72’s as with my first plan, these wont actually ‘work’ as I am not a hydrodynamisist (?)
The model I plan to make will be 1:6th scale (2ft / 61mm) it will be 3D printed as we have just acquired one at College
Progress/Picture/Questions to follow
Thanks
Jamie
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Bow thing is fashion, not sure it is applicable to such a short boat - Cherub rules have a 200mm vertical bow thing going on...
GEEK BABBLE
If your 3D printer is a MakerBot Replicator 2 then I'd suggest:
- Use ReplicatorG (from replicat.org) to generate your G-code, works better than the MakerBot programme and appears to gives a better logic to the print (especially for circles).
- Use the "print bed" option for anything that is even near big (any dimension more than 1/2 bed size)
- Replace the feed straw with one about 3 inches longer.
- Look to buy the feed kit ($10) to replace the plunger.
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Bow thing is fashion, not sure it is applicable to such a short boat - Cherub rules have a 200mm vertical bow thing going on...
Why is that? Too many rules in this class! :P
Thanks
Jamie
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The rules allow a snout, which is needed to fit the sail area in. As the overall length is allowed to be more than 3.7m (including the snout.) there needed to be a way of keeping the hulls to the 3.7m length so a simple way is a requirement for there to be a 200mm vertical section of the bow.
It could be an interesting structural challenge to use a reverse bow, and snout,
The foils in AquaMarina were just slotted in as a joke/trial, the boat never sailed with them.
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Thanks for that Phil, i had given a little thought to the snout problem, i thought about adding a snout like the 18ft skiff have, the non solid triangle, but it does ruin the nice curves on the rest of the boat, to regain the sail area lost a square top jib could be added as seen on the lumix 18 this year, but then this might infringe the mast thought a hole rule? i may have to step back from the cherub rules to allow for more 'design freedom' (whatever that is)
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With respect i believe the perfect foiling Cherub hull form already exists. Shiny Beast is a moth at heart. It is the grean boat on the Chew report photos. You cannot get narrower lighter or simpler. Yes you guessed it I want to fit foils for fun. Perhaps we should meet up for a chat. Funds may suddenly have become available for this rash project.
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Aft the Phonicians fitted ram bows to their sailing war galleys the reverse bow or wave peircing bow next appeared on a sailing craft on the team Philips catamaran and has since been added to cats to reduce speed loss in chop. It works well with slim hulls. While the modern cherubs have a fine bow entry angle they are not slim enough to remain in a displacment mode beyond about 7 knots. The Moth has about 3 times the wtaerline length to beam ratio of a Cherub and therfore can achieve higher hull speeds due to the reduced wave making drag.
As BS suggests wave peicing bows on planing boats are a fashion and apart from perhaps improved aerodynamics add little to the performance of the boat.
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Hi Jamie,
I think that you will find that the Cherub rules offer a lot of design freedom.
Far more than anyone is currently using.
If you design and build within the rules you will have a boat that has some value.
You will have a class to race against.
You will have a bench mark for performance evaluation,
and you will get a lot more credit and publicity succeed or fail.
I have built boats that are not part of any class. They have all ended up at the tip.
Whilst they are fun to do and teach you a lot, if you can, stay within a class.
I would not put a reverse bow on a Cherub. However I would never discourage experimentation and development. It is very possible that it may have gains that I cannot see.
Clive
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Open triangle snouts have been done before, and can be made to work, however if you have the snout and bowsprit sitting above your reverse bow how will they affect the wave drag?
Another thing to consider is space to put the kite. Having the sort of clipper bow that most of the modern boats have gives you somewhere to put the spiniker chute. With a much narrower bow, the chute mouth moves further back, which can give handling problems.
It can be easy to be carried away with ideas, however to learn what works it is often worth changing one thing at a time.
So if you have a radical hull, and radical rig, and radical foils. How do you know which one is making you go fast, and if one is making you go slow?
Designing within a ruleset is gives you a benchmark, but it also gives a challenge, given the same amount of material as goes into a Cherub I could eisly make a boat that was faster around the course, but designing a faster Cherub is more difficult.
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sounds really interesting - and confusing. the confusing bit to me is, why would you bother with wave piercing bow on a foiling craft?
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the reason of a wave piercing bow on the mach 2 was that it reduces windage, i have no idea if it would work on a cherub, it is a trend, but i think it looks good
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Do you know what configuration they are using for the foils on the 49er? It looks like a bow foil, with wands, and a large main foil.
I could see corners being a bit tricky, unless they are steering the bow foil.
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http://www.boatdesign.net/forums/attachments/sailboats/30661d1239230770-new-high-performance-monofoilers-foiling-49er-cover-sa.jpg (http://www.boatdesign.net/forums/attachments/sailboats/30661d1239230770-new-high-performance-monofoilers-foiling-49er-cover-sa.jpg)
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If I read it correctly that foil configuration is legal in the Cherub rule set.
As is the "tri-foiler" arrangement
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It makes more sense on a Moth where the speeds are higher and areo dynamic drag will be more important.
It would be interesting to take some GPS records of a Moth and cherub racing in similar conditions and plot the distribution of speed. This kind of research would provide evidence for the final design approach. I suspect the most common speed of a Cherub is notably slower than the simialr most common speed of a moth.
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What amazes me is that nobody has tried it on a Cherub before now. Foiling Moths have been around for years but there has never been a fully foiling Cherub. Whether or not you think it would be a Good Thing is another question of course.
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The rule "NO FULLY FOILING BOATS" at chew has restricted our efforts in that direction :-)
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The rule "NO FULLY FOILING BOATS" at chew has restricted our efforts in that direction :-)
Come to Draycote then ;)
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Thanks for all of you input so far.
Phil A, 'Another thing to consider is space to put the kite.' i have looked at the ovi aura skiff from the womens olympic skiff, my thinking is that a 3patch kite would fit in the boat, IF ANYONE KNOWS THE LENGTH OF A KITE SOCK, PLEASE CAN THEY SEND IT TO ME
'it is often worth changing one thing at a time. So if you have a radical hull, and radical rig, and radical foils' i did try and do a 'safe' design, but got told it needs to be much more unique, i agree with your point, it is my usual mantra, but for this project it’s not allowed.
Phil K, come to Draycote with a GPS and we can do some testing.
Neil C, there is the R Class skiff, (basically a 12fter with moth foils) https://www.youtube.com/user/RClassDotOrg?feature=watch (https://www.youtube.com/user/RClassDotOrg?feature=watch)
IMO, the R class is too big of a boat to foil, the hull with wings is 29kgs (what's a Cherub bare hull?) but it is too tall, and deep, could just be me though.
Thanks
Jamie
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Jamie, Will we be looking forward to having more of this conversation at Weston?
Would be a good subject whilst the wetsuits are drying in the sun and the BBQ is warming up.
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The r-class custom made their own foils. The first ones were over 1.5 metres wide! much bigger than Moth foils. There are now about 6 of them foiling and because of this, I understand the cost of joining the class and being competitive has gone up by one pair of foils and a control system. (not an inconsiderable amount as you know from moths)
According to the rules, the Cherub minimum hull weight is 50kg (with all fixed fittings). Although as the r-class guys show, you can make a 12ft hull a lot lighter than this but in order to race as a cherub, you will need to have enough lead to bring this up to the correct weight. Your rig will weigh between 15 and 20kg bringing an all up sailing weight of about 70kg.
So 70kg + 2 people ~ 200kg if you are kind about the people. Typical upwind speeds are about 9-10kts when fully powered up. Generating this much lift at these speeds is pretty tricky. You will probably need to make your own custom foils too as there isn't going to be anything off the shelf. These will either be very expensive (if someone else does it for you) or maybe you have the kit to make your own, accurately.
Does the 49er have a wand system? Whenever they seem to be going well, the front foil looks like it ventilates and the nose instantly drops.
Cherub kite socks go fairly well down the boat, mine is about 3m i think give or take for a 3 patch kite. The feedback from the rclass guys is that they use a smaller kite when they are foiling as the apparent wind will have more of an effect and you can therefore get away with a shorter pole too.
As a Mothy you probably enjoy the smooth quietness of foiling but a lot of us like the wet and wild contact with the water you get from cherub sailing. They may not be the fastest round a course but they are sure as hell one of the most fun.
I look forward to see how you get on. I hope you manage it as I have thought about it myself a fair bit but not had the time to put any serious work into it.
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BS, yeah possible, although it will be near the end of the unit, it depends on if i can get a boat/crew/money/time out travel. we will have to see.
Ben, Thanks for the info, these some good bits i can put in the 'research development' so i can score some more points
im not sure i cant find a lot of reliable info on the foiling 49er
Thanks for the sock length, much apreciated
'As a Mothy you probably enjoy the smooth quietness of foiling but a lot of us like the wet and wild contact with the water you get from cherub sailing. They may not be the fastest round a course but they are sure as hell one of the most fun'
you havent seen me sail have you...
Thanks
Jamie
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Should have put these up before... Ooops
The first pic is basically A+E. This is the hull Clive kindly sent me.
The second picture is my version of her.
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Here is my version
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Bow thing is fashion, not sure it is applicable to such a short boat - Cherub rules have a 200mm vertical bow thing going on...
GEEK BABBLE
If your 3D printer is a MakerBot Replicator 2 then I'd suggest:
- Use ReplicatorG (from replicat.org) to generate your G-code, works better than the MakerBot programme and appears to gives a better logic to the print (especially for circles).
- Use the "print bed" option for anything that is even near big (any dimension more than 1/2 bed size)
- Replace the feed straw with one about 3 inches longer.
- Look to buy the feed kit ($10) to replace the plunger.
Are you playing with 3D printers? I am going to a lecture by the chap behind rep-rap tomorrow. I will let you know if I find out anything interesting.
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E-Number's sock is 2.6m from the bow and 2.3m from the shute and we can get a 2 patch kite into it if the head and tack stick out by 0.3m. Our kite used to be three patch but was harder to hoist.
Your wave peircing bow concept works if you don't want full sail area but you can still race in 05 rules if you have less than max sail area. Many boats do.
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You can always get the area back up by making the boom longer or by going back to a non self tacking, overlapping jib.
We seriously considered this so that the jib tack could be put on top of the kite hoop making hoists and drops much easier.
Is there any chance of this boat being built or is it just a design exercise?
Clive
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Normal jib at the weekend was a dream. Am seriously considering it for the one in the shed....
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BA, yeah, i went to a talk about them on tuesday, i think rep-rap was there, but we left before his lecture.
PK, thank you. True, i hadnt thought about 05 rules, i will have to see. i quite like the square top jib idea
Coxon says the theory for the square-head jib is the same as for the square-top main: It twists out at the head in a gust to de-power instead of relying on mast bend. “In the past you had to ease jib sheet to depower the jib. With the square-top the crew should not have to move the jib sheet as much.â€
taken from http://www.mysailing.com.au/news/square-top-jibs-and-other-sail-innovations (http://www.mysailing.com.au/news/square-top-jibs-and-other-sail-innovations)
This way you can have a big jib and big main, but you would need a prodder, as seen on moths to push the fore-stay forward, would this fit in the class rules?
Clive, its just a design exercise unfortunately, maybe some day when im older it could be built, but the model will be pretty big, just under a meter, with foils, hopefully a full rig!
BS, its tempting, but then gybes become a pain, and light wind tacking, would an inverted jib track work so you can keep the self tacker?
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but then gybes become a pain
Actually we find the opposite with the gybes.
Next time you are anywhere near BS look at the jib. If you put the clew on the forestay the sheet is very tight between the cleats with the knot to the mainsheet midships. This means you can sheet in on either tack by pulling on the same bit of main.
For the gybe you simply uncleat on the way in and re-cleat when you are on the other side. Uncleated the short jib sheets mean it sets from one side to the other very nicely.
All that said - I am buying a self taker for the next boat...
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true, could be i have just got used to the self tacker.
What are the thoughts on an inverted jib track?
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Ah, I didn't get that you were just doing a model of the boat. Is the model supposed to sail? or is it purely a design exercise?
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just an exercise, as much i would love to build it, i have no budget :(
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W-tracks have been tried to get the clew of the jib further back but if the W shape is too extreme you end up with the car not getting to the end of the track as the sail fills. A straight track with curved ends will give you nearly the same benifit.
Some of the jibs have a bit of roach to add area meaning that they do twist like a square top.
Bethwait identified that for a given sail area the ratio of main to jib should be something like 73/27 which gives you a much smaller jib than the current 05 rules rigs. This was practical research done on 18's in the early 90's and is described in his second book. Perhaps the Square top allows the jib area to be increased without changing the parameters that gave them the performance they measured.
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Finaly printed my hull, or at least on section of it, it took 30 hours to print.
All it needs is a rub down, sticking to the other sections then a bit of paint.
Pictures to follow
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so here are the pictures of the rear third of my final model
the first is while it was being printed
the second is pre rub down
and the third is post rub down
All i have to do now is print the other two sections, rub them down, glue it all together, make some foils and stick them on. i have 3 weeks to do this, if by a miracle i have some spare time, i will make some sails.
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These bits emerged from the bed of our printer this weekend. Replica of crash test dummy parts. The elbow will follow later in the week.
Like being back at school I have the new work printer in the sitting room (except I was never trusted with the pets following an incident involving my little sister freezing the school Siberian Gerbils). Bought it home so I can free some space on the queue during the week to print off some more rack end fittings when I have finished drawing them.
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Unfortunately I have will have to miss this weekends cherub sailing.
Reason being that I have printed my bow, I have a fair bit of sanding to go as well as a couple of layers of filler primer.
The bow section took about 25 hours.
Next questions are, what colour should the finished model be? single tone or two tone?
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Not suprisingly I'd say paint it red, white internals.
If you paint it white it will hide more of the defects though.
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How dare you suggest there are defects in my work!
i agree the red does look good!
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I agree that white does hide more :)
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I have gone for 'Ford Electric Orange'
I didn't have enough time to make the rudder a t-foil, and i still have a jib track to make.
If you a have any questions/thoughts/comments please fire away!
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Can someone tell me what the diameter of a cherub mast is at the bottom, and the kite pole.
Also the length and chord of the rudder
a ball park figure will do!!
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Hi Jamie, I have just noticed that your scale 'cherub' seems to be a solid mass, no wonder it is taking so long to print! If you are ever in the position that you are having to pay for your materials you will be a lot better off that you put in some sort of internal structure to support the shell (a lot like a real boat) as it will be a lot cheaper and faster to print.
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Hi Tim,
There are two skins in my model to bring up the wall thinkness.
The ends are colosed off, but the sections are hollow.
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I stuck her together today, she comes to a huge 616mm, all i need to do now is fair in the joins and paint, i have a week to do this, so its not impossible, but it will take a lot of effort!
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Was supposed to be my week off it being half term, but no, dues to someone putting a hole in the transom i have had to repair that, which too an age to get right. as i type this the final layer of white primer is drying so that on Monday i can apply some colour to her.
i am making some sails, (well, trying to) but if anyone has got some vague geometry for a main and jib, i would be very grateful if you could send them to me.
Also spreader height, a rough estimate would be lovely.
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Finally got the colour coat on. Its looking pretty sexy if I do say so myself. Tomorrow I will paint the deck and make some sails for her and we should be almost finished!
I don't know how you lot make the real things!
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Really, really beautiful...
...when will full-size 3D printing in carbon fibre become feasible?
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...hang on, I think I've got my Star Trek replicator going...here goes...
....my friend who is a GP had a 'Dr Beverley Crusher' hand held scanner which he used to use on kids in his surgery...it really worked!...well the lights flashed anyway...
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you need a CNC machined plug, and one of these VIPER® Fiber Placement System.wmv (http://www.youtube.com/watch?v=zP-sukcKJ9A#ws)
job done!
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i need more/some money for that! :P
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That is a lovely looking model. Well done Jamie. Don't let Clive see it or he will have Alex test sailing it before the weeks out!
nice in orange as well.
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Jamie, we make the full size ones in bigger pieces and there is more sanding to do.
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Well its all done and dusted (literally!)
This is her all set up in the exhibition space.
The main has too much return in the head, this could be because there are no controls on so there is a really floppy leach, might be able to fix it by canting the rig over, but for non sailors, it does the job.
Thank you everyone for your help, especially Clive Everest for letting me use his hull, and Roland for some 3D printing/lots of answers to stupid questions
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Well done Jamie,
Hopefully you will be inspired to build a real one soon.
Clive