Performance of a fully foiled Cherub

[kevin_ellway]

Ross asked why 'a cherub was not really a good platform for a foiler'

I have just adapted a program (courtesy of Alan Smith (Aus)) for predicting what a fully foiled Cherub would do with a 140kg crew.

1) It would take between 7-8 kts of wind to foil on a reach, but you won't really get any useful vmg upwind until it's around 10kts.
2) In 12 kts of wind, it will do just under 12kts upwind, but at a low angle (about 55 deg). It might be around 5% (maybe 7) quicker than a non foiled cherub in vmg terms. But this will only be realized if the wind is steady and you can foil tack!
3) In 15kts of wind the boat will be overpowered upwind, you will still only do just under 12kts speed at 54 TWA. Not much quicker than a non foiled Cherub. To sail in anything more, you'd need to rig smaller sails.
4) Downwind in 12kts, the boat with a kite would get just under 17kts if you twin flat wired and had a very flat kite (AWA would be around 45 deg!) . But as the wind increases to 15kts, the speed will drop to around 14kts as you would have to sail really deep.  At this point, the boat would probably be slower than a non foiled Cherub. This is mainly because the current kites are too big / full.

So why isn't a Cherub a good foiler platform?
a) The hull is too short. You need 7kts + of boat speed to lift out, what's needed is a long slender hull. To scale a Moth to a Cherub sailing weight would require the boat to be at about 14ft long.
b) The rig is too short - a high AR rig is required because foiler sail at small AWA.
c) The hull is too big - in a non foiler, the hull drag dominates the overall drag except at very low speeds. For a foiler Moth, for example, the foil drag, the sail drag, and the windage on the hull and helm are all of about the same magnitude. It is really important to have a low windage hull. This means shallow and narrow.

The low weight of a Cherub means it could foil, but to make it work even half way decently:
a) A new kite would be required
b) 2 sets of sails ( a big and small rig) would be required

These costs are on top of the cost of the foils. For this kind of expense, it would have been better to have designed a 2 person foiler from scratch.
The foils, incidentally, would require major engineering as the front foil span would need to be around 1.4m  - you won't get it in your car and it would need a lot of carbon to make it structurally strong enough.

Without the multiple rig and screecher kite option, the boat would go best in about 12kts of wind. In anything more than 15kts it would be pretty nasty. In any event, it would be a lot slower than a current cherub in less than about 10kts of breeze.

Given the cost, the fine nature of the existing boat, and our sailing conditions, I don't see this an an attractive option.

Hope this helps

[Banshee Ambulance]

A very informative analysis. It is nice to see some scientific thought going into the issue rather than just speculation. What are your views on the R-Class' direction of full foiling? And do you think that some sort of reduced displacement foiling might work on a Cherub? eg: a small centerboard foil. 

[john_hamilton]

beat me to the question rob

[kevin_ellway]

A very informative analysis. It is nice to see some scientific thought going into the issue rather than just speculation. What are your views on the R-Class' direction of full foiling? And do you think that some sort of reduced displacement foiling might work on a Cherub? eg: a small centerboard foil. 

Thanks - I have not carried out a really detailed analysis, but it is sufficient to give the general characteristics.

Re the R class:
Firstly, Rs are sailed in the windy bit of NZ. This is why the restriction on sail area is 13m^2. I have some wind stats from Paul Roe - from memory, typically 10kts by noon, soon increasing 15kts then 20kts as the afternoon goes on. So they have pretty nice conditions.
Secondly, the Rs are a very small class with 2 fleets located quite close to one another. It's quite easy for them to decide which way they want to go. It's not like here where some people sail on lakes, others the sea and in fact Chew, for example, has banned foilers.
Thirdly, the R is 3.9m and very light. It has no rig height restriction. So it is slightly better suited than the Cherub. That said, the hulls are still way to big.
I think, for the R fleets, if that suits them, then great.

On a slightly different tack, it is important to use 15 - 20 even 30 degrees of windward heel when overpowered. This is so that the lifting foils resist the side force from the rig. In addition, the weight of the boat is now to windward of the centre of lift. This produces a significant increase in righting moment.
Now consider trapezing. If you heel about 20 deg to windward, your feet will be weightless. Any more and your feet would leave the side of the boat. The Rs have no beam restriction. It may well be that they ditch trapezes (or at least the helms) and just make the boats wider.

Re Cherubs. At anything over 7kts of boat speed, the more load you can get on a T foil, whether it's on the centreboard or rudder, the quicker you'll go. There is a possible advantage of having the foil on the CB. You could get more load onto it, but you'd need a wand and flap to stop the boat taking off.
My personal view is that it would be wise to amend the rules to limit the foil area, and to state that the foil may only be carried behind a vertical extension of the transom. In fact, given this constraint, if the wheelie bar lengths were restricted, there is probably no need to limit the area.

The reason for this is that the centreboard can then be removed when afloat. Yes I know you could put it in first, but if the boat inadvertently tacks or gybes you end up in a right mess.

Anyway, using a T foil rudder of about 0.1m^2 area and AR of 10, will give you about a 20% reduction in drag at 10kts. That's massive. It's like removing 40kg of weight out of the boat, or increasing the rack beam to about 2.8m. The new crop of boats are pretty quick once powered upwind and are not slow offwind with respect to anything.

If we want to make some more speed improvements over time, I suggest increasing the beam to 2m (not more than 2.25m), and getting rid of the kite area rules. Just go for a simple perimeter rule of about 19m. The kites are currently a bit small in my view..

Anyway, I hope it's food for thought.

Cheers

Kevin

[Banshee Ambulance]

With a racked boat the maximum beam is easier to change so there is potential there for width extension. Though if anyone would want to is another matter. The modern boats seem to be a good package. The only thing I would change is the pole rules. I think as a high performance class fixed poles would set us apart from the crowd and generally simplify everything. Or at least remove the 4.3m from transom bit. That just makes converting older boats more hassle. But I am sure these rules were included for a reason.

As far as the center foil goes, what stops you designing your foil to lift the boat and crew fully at something silly say 35knots? That way some of the weight is taken on the foil but you will never lift off and dont need a wand. Not as efficient as the wand system but far simpler

[phil_kirk]

Interesting stuff.

Beam is a very powerfull performance parameter as Kevin suggests. I would have a practical problem with a beam increase though. I would not be able to store the boat on it side in the garage or when on the trolley i would not be able to get between the boat and all the other stuff I have.   The merlins have shown that 7 feet wide is practical.  Anything more than 2.25m would require adjustable racks for towing.

But were a wider beam to be considered without any change to the rig would this lead to competitive crew weights dropping to the point that Sarah and I would be the optimum.  Would this change the type of people that sail cherubs? and possibly lead it to have to compete more closely with buoyant youth classes and perhaps the class would become incompatible with the sailors more likely suited to sail it.

[JimC]

The merlins have shown that 7 feet wide is practical.  Anything more than 2.25m would require adjustable racks for towing.

I don't know that I'd go as far as that. Merlins are not home built and tend to be sailed by folk with more financial resources than Cherub sailors. This has a big impact on how much space you have - single or double garage for instance.  Also a merlin is not full beam for much of the hull, whereas our boats are.

The Plus Plus was, as I remember, 6 foot 6 across the hull flares and 9 feet across the wings, which folded in for transport and storage.  It was just about possible to work with one wing out in a normal single garage, but a right pain, two wings out was impossible. I only ever worked with a wing in the down position when actively working on it and had to shuffle the whole plot across the garage to work on the other side.




Really I would say that 6'6 (or 2metres if you insist) is about the outside that's practical for folks like us to homebuild, and 1.8 is a damn sight better on the practicality front - if you want to be able to walk each side of the boat while working on it for instance.

[daryl_wilkinson]

But were a wider beam to be considered without any change to the rig would this lead to competitive crew weights dropping to the point that Sarah and I would be the optimum.  Would this change the type of people that sail cherubs? and possibly lead it to have to compete more closely with buoyant youth classes and perhaps the class would become incompatible with the sailors more likely suited to sail it.

I'm guessing that you are suggesting the optimum weight has gone up from pre 2005 rules. And suggesting that a slightly older and wealthier ( and slightly heavier ) sailor is the ideal candidate for Cherub ownership.

Interesting indeed. More interesting in someways is the fact that the key instigators of the change to twin wires didn't sail Cherubs much at the time and didn't do so afterward either ( Kevin, Simon, Bob etc... ) preferring to move on to other things having complained that they were underpowered. I supported the idea strongly in Committee due to the dwindleing membership / turnouts we were suffering due to the recent introduction of the 29er. Which offered an alternative lightweights skiff. The move offered us a clear space in the market. I also only agreed with the original increase to 14.5sqm and not the half cocked 15.5sqm calculation that we ended up with delivered to us by a strange technical officer set up at the time. Feeling that 14.5sqm maintained the position of the boat that was for lightweights. And that the older boats wouldn't end up with awkward and badly balance rigs in the interim period. Meaning a more sailable boat with less handling problems.

You may ask why I'm bringing this up now? Well there are somethings in a class that a class has to stand by or you might as well chuck out the rules completely or join with another class. One of those is the design weight of the crew and the basic principals of the boat, that in this case John Spencer designed. The name 'Cherub' hardly depicts what we currently sail. Daemon ( one of mine that was highjacked BTW ) better discribes it. And as we are discussing foiling again and beam it seems the right time.

It seems to me that those original design principals are being eroded. I'd love to see the beam go up to favour the lightweights. But at the same time this opens the door to making the boat more extreme ( see my other posts ) which ultimately is not a good thing. As well as proving an opportunity to increase sail area further in the future, with rule changes driven by the heavier crews. Which is an even worse idea.

I ( like Jim came to the conclusion at the time ( 2004/2005 ) now think we might as well chuck in the the towel and join with the R-Class.

Feel free to 'flame' the crap out of me.

[JimC]

Bob

Not Bob C if my memory serves me right... He was involved in earlier experiments with two strings but not with the 2004 era.

[Phil Alderson]

If you are planning on comparing the sail areas and performance of the R-Class and the UK-Cherub it might be worth examining  these pages

Wellington Wind

Weymouth wind

UK Wind map at 25m elevation in m/s

Just watch out for all the different units,

[andy_peters]

Given the beating Pete gave the class over the past two Nationals might everyone not be better served by leaning how to sail the current crop of E5/6 and Banshees to their full performance?  IMHO only Pete and the Lee's to date have got close to realising the potential of these boats.  If you haven't been lucky enough to sail one in a breeze believe me you wouldn't be thinking 'what this boat needs right now is some foils'.   

A wider beam and bigger kite?  Perhaps a bigger kite but I'm used to an RS200 so the current one seems plenty big.   As to width - we only just get the E5 into a space at QMSC/our garage so there are some practical considerations. 

For my twopenneth I would say the current crop of boats are mighty fine, I'm loving sailing the E5.  We should build some more to the current rule and get out and race them together more.

[ross_burkin]

Has anyone done a crew weight/wind speed results comparrison?

[JimC]

Has anyone done a crew weight/wind speed results comparison?

Last time I had a go, some years ago, as with most of these things, there was far too little (as in order of magnitude too little) data to be able to extract statistically significant variations in boat speed (which are small) from variations in crew ability (which are ginormous). But hey, Mr Lee is far better qualified and capable than I am to do a study on these lines!

[kevin_ellway]

For my twopenneth I would say the current crop of boats are mighty fine, I'm loving sailing the E5.  We should build some more to the current rule and get out and race them together more.

I'm totally with you on that one, Andy, and not just because you're sailing one of my designs!

With regard to the beam thing, the point I was attempting to make, was that if people want to go faster (aside of course from being better sailors!) adding beam is less drastic and costly than adding foils and would not adversely affect light wind performance. Pete had also mentioned that Ronin gets overpowered in over say 16-17kts with less than a 146kg crew weight. But Ronin's rig and centreboard are all about light/marginal wind power.

Also on the issue of beam, the SK4 has a beam of 2.8m and 16m^2 of sail. The wide beam makes it impossible to roll the boat over on the beach unless you remove one of the racks (18 style). Tacking takes commitment - it's a long way from one side to the other, and the extra leverage makes it easy to stall once on the new tack.  In contrast, the Cherub is small, light neat and manageable. If you consider the leverage to length ratio you get something like:

Cherub 51%
49er 50%
800  47ish %
18ft skiff 56%
SK4 60%

So the Cherub is actually quite favourable in those terms. Adding still further leverage will make the boats harder to sail, especially the bear off in a breeze.

Finally, yes if you're light you'll get overpowered. So you need a flatter/softer (perhaps  smaller) rig than the big guys. This is good because such a rig won't develop the power that a full/stiff rig will in the light stuff. So the 1.8m beam allows the class to be sailed reasonably equitably by a fair range of crew weights.

[Will_Lee]

The message is you're much better off doing an experiment to assess causality if you can than using observational data. This is quite within our reach using a cross-over design and GPSs.

The dataset Ross proposes:

Too few data points is a problem. More boats would be good, but more races is the real requirement. Also, how are the conditions recorded? (No - you may not look at the results to guess the conditions!)  It's a surprisingly complex problem - the killer is individual-level clustering (ie some people are better than others).