My Cherub Design

[JP233]

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 I have also seen the foiling 49er swap the hull to a cherub and it would be better! Bethwaite Foiling 49er

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

[roland_trim]

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.

[JP233]

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!

Thanks
Jamie

[Phil Alderson]

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.

[JP233]

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)

[PaulClements]

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.

[phil_kirk]

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.

[Clive Everest]

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

[Phil Alderson]

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.

[ade white]

sounds really interesting - and confusing. the confusing bit to me is, why would you bother with wave piercing bow on a foiling craft?

[JP233]

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

[Phil Alderson]

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.

[JP233]

http://www.boatdesign.net/forums/attachments/sailboats/30661d1239230770-new-high-performance-monofoilers-foiling-49er-cover-sa.jpg

[roland_trim]

If I read it correctly that foil configuration is legal in the Cherub rule set.
As is the "tri-foiler" arrangement

[phil_kirk]

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.