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[Will_Lee]

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.  

I'm guessing that you are suggesting the optimum weight has gone up from pre 2005 rules.

I do not follow this connection.

[Banshee Ambulance]

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

Back on topic. Does anyone have any ideas about this? I can think of several reasons for not doing it but there are a couple of reasons for doing ot as well. I would be interested to hear other peoples views.

[Neil C.]

[quote/] Back on topic. Does anyone have any ideas about this? I can think of several reasons for not doing it but there are a couple of reasons for doing ot as well. I would be interested to hear other peoples views.
[/quote]

First of all, I'd like to make it clear that what I know about foiling could be fitted on the back of a postage stamp with room to spare. Also, my engineering / hydrodynamics knowledge is only a little above Ladybird book level.

However, using my completely unfounded logic, I reckon it might go a little bit like this:

1. Upward lift from a centreboard T-foil is a function of forward speed (actually squared in some way I think).
2. The foil would need to lift something in the order of 200kg (boat and crew combined).
3. Say for example the critical speed at which the foil produced 200kg of lift was 20knts. At 15 knots it might produce 150kg of lift, but this isn't enough to lift the boat out of the water at all, not even a little bit. All it would do is produce drag.
4. At exactly 20knts the boat would lift neatly out of the water and start travelling faster.
5. At 21knts it would get completely airborne and the t-foil would break the surface.

However, I had a thought a while ago about whether an inverted christmas-tree arrangement might work. The idea would be that you would have say 4 horizontal elements on the daggerboard, each about 30cm below the one above. The one closest to the hull would have the widest span, getting to a progressively narrower span as you go down. As the boat accelerates and starts to lift out, you would get progressively less lift as more elements break the surface. As long as the bottom element was small enough to not break out in anything less than 30 knts, you might  be OK.

N.B. in reality I have no desire to go full foiling in a Cherub, and this thing would never work due to too much drag, inability to raise the daggerboard etc.etc. But just an idea I thought I'd share with you.

[Banshee Ambulance]

[quote/] Back on topic. Does anyone have any ideas about this? I can think of several reasons for not doing it but there are a couple of reasons for doing ot as well. I would be interested to hear other peoples views.

3. Say for example the critical speed at which the foil produced 200kg of lift was 20knts. At 15 knots it might produce 150kg of lift, but this isn't enough to lift the boat out of the water at all, not even a little bit. All it would do is produce drag.
4. At exactly 20knts the boat would lift neatly out of the water and start travelling faster.
5. At 21knts it would get completely airborne and the t-foil would break the surface.


[/quote]
You would however, reduce your displacement by 150kg. Which is the aim. My question is if the functions of speed and drag make this lift worthwhile at lower speeds.

[Will_Lee]

The foil you describe would be tiny and have the control problem of angle of attack changing every time you hit a wave.  The central issue is the foil being in front of the centre of pitch: it is a bit like putting the flights on the front of an arrow. Even if it is possible to go at reasonable speed without a trip to Davy Jones, it will be Dragsville Tennessee.

I have been told the lead mine americas cup boats did this, but they are not big pitchers and their speed does not vary v much.

[phil_kirk]

I wish it was that warm here.

[simon_jones]

It is.......... Sometimes.

[Phil Alderson]

To see if a semi foiling cherub was worth while I think that you would need to do a Lift/drag curve for the centreboard and rudder foil (with the centreboard smaller than the rudder to give lift control)

then for the range of speeds you were interested in plot the drag curve for the hull at the displacements you would get including the lift from the foils across the speed range. Add the drag from the foils at each speed to the hull drag and compare with the bare hull drag figures and possibly one with just a rudder foil.


Of course this would not tell you about any dynamic problems like Will has suggested.

[phil_kirk]

from memory the generic resistance speed curve for a hydrofoil craft has a hump of high resistance before take off occurs.  weather this is higher or lower than the non foiling or rudder t-foiling modes is as Kevin suggests of benifit in only specific conditions. 

What I am saying is that the resistance dosen't drop away until you are fully foiling so the benifits of semi foiling are small. 

Lets concentrate on building good boats and sailing them.  Summer is coming. Honest!

[Ben Howett]

Interesting read as always Kevin.
Could you tell us a little about the data/assumptions your modified program is based on? I wrote a VPP for dinghies last year and would be interested in hearing the aproach you took given the pitfalls I encountered.

[Banshee Ambulance]

Some interesting ideas from some of our Moth cousins:

http://www.moth-sailing.org/download/CSYSPaperFeb09.pdf

Certainly worth a read. I would be interested to hear Kevins thoughts.

[phil_kirk]

Interesting link. 

i noticed that the helmesman equates for nearly a half of the airodynamic drag.  Should we all be wearing lycra when sailing? 

I was surprised by the quote (It’s hard to justify working hard on a foil, that may well not last through your next sail). 
So the message is don't fair your foils because they aregoing to break. They are not going to win many races with that attitude.


I also liked the quote;  "It is possible for a motivated amateur to build a boat on
par with the commercial offerings (or nearly so)". 

if that is not encouragement for home building I don't know what is. Go on you motivated amatures out there

[Banshee Ambulance]

It is important to note that the drag from the rig etc. was missing in that experiment but the findings were still very interesting. Another reason for trapping as close together as possible, as well as the more obvious inertial reason. Also, this almost makes the rash vest over everything look sensible apart from the more obivous "try not to get caught up in anything".

[phil_kirk]

It is surprising that we aren't all wearing those tight lycra outfits that most of the winter olympics athletes have.

They don't leave much for the imagination though.

[simon_jones]

what instead of the tight rubber outfits most sailors wear?

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