Thornbury files

[roland_trim]

Sunday:
The team EJ camped in order to be ready for the 9am start. Serious airbed malfunction at 3-4am meant canceling the alarm and the inevitable sleep in till an hour before the start. We binned racing for breakfast and had a cup of tea watching the start. After the boats headed out of sight the wind follwed them, spent an enjoyable couple of hours in the sun re-commissioning the rig and trying different sails on the mast. We appear to be nearest the Hyde...

Project garage is progressing. Spent the afternoon tidying up the site, we now have a stop to work whilst our neighbors work out how their property actually stands up (Gabion retaining wall). Although none of the new walls is higher than 3ft, the garage currently contains a BMW, a double stacker, Badgers and there is still room for a few hundred bricks in piles.

[phil_kirk]

The garage is huge and will make a brilliant sticky weekend venue. We hope the retaining wall issue doesn't take too long to resolve and R &H can feel sure that next door won't suddenly come to stay in their garage.

[TSC]

Thanks Phil for not going into too much detail about my efforts on Atum.

So the story goes that during a conversation a month or so ago Paul (Croote) asked if I'd ever sailed a Cherub. I said no and that at 84kg was unlikely to ever be competitive, so he said after the nationals he'd let me have a go. I was on business in Paris last week, when out of the blue, I get a txt message asking if I'd like to helm Atum, so I said yes.
Turning up at TSC yesterday morning with an 8kt forecast it wasnt looking great, but then there had been more wind than forecast on Saturday and it was picking up from nothing when we arrived.

Anyway we got going or rather drifting off of the line and it became clear that Atum is not a light wind boat! Up the first leg we held Born Slippy behind us and stayed with the 29er but the cross tide leg killed us. At times we were sailing head on to other boats on opposite tacks such was the variance in wind direction. We lost out about 1/2 mile to Phil & Sarah, but on the next downwind leg reeled them in, holding them all the way to the bottom mark where we managed to get a couple of boat lengths in front. The long upwind leg back towards Olbury power station caused us a few problems. Paul Croote getting stuck between the jib and the mast causing the first swim of the day! Eventually we got our act together and in one 20second puff managed to get twin-wiring and upto 11.7kts before sailing into a hole and binning it to windward. Boy do Cherubs stop quick and tip to windward quickly! A couple of tacks on windshifts later, and we had recovered to catch the 29er and were gaining on Phil and Sarah. The final downwind leg to the club saw us gaining quickly on Phil and Sarah until another hole and another windward flop dropped us in. Luckily we recovered from that and scraped past both the 29er and Born Slippy to just beat them over the water.

So what did I think?

Atum is a lovely well sorted boat and is clearly quick once there is enough wind to twin wire, but if your sitting in, it doesnt like it.
The lightness of the rig means that sailing into holes can cuase problems in that the boat comes over on you very quickly, and the hull shape and rig-layout on Atum produces enormous amounts of lee-helm as the boat comes over on you, making it even harder to get back in the boat. I'm assured by Paul that these problems go away when sailed in decent breeze, so I will be back for another go.
Sailing upwind at the back of the wing felt odd with the t-foil wound on and perhaps because the mast is short, the forward pull on the wire is strong. On my 700 I've got much longer distances between the front and back of the wings but the fwd pull is nothing like it. The only way to keep leverage on is to engage the toe loops which is an alien thing for me. I only use them on the 700 when maxed out downwind in big breeze.

Before sailing the boat, I thought it would be twitchy and unstable, but its not. It actually ok, certainly no worse than the 29er. Its got quirks like the 700 but then that may be particular to the Slug shape in terms of chine steering but once up and running I reckon you could get it into a groove comfortably.

So I'll definately have a go again, although Paul is selling Atum now, (great boat for anyone interested in getting a well sorted boat without all the hassle of building your own).

Paul
RS700 844

[paul_croote]

The light flooky conditions at Thornbury on Sunday was most Cherub unfriendly I have sailed in this year!  of course it would be the day that Paul had a helm of Atum "D'oh!". He did a good job despite my ham-fisted crewing. Hopefully his next Cherub experience will be in better conditions.

Thanks Phil for not going into too much detail about my efforts on Atum.

So the story goes that during a conversation a month or so ago Paul (Croote) asked if I'd ever sailed a Cherub. I said no and that at 84kg was unlikely to ever be competitive, so he said after the nationals he'd let me have a go. I was on business in Paris last week, when out of the blue, I get a txt message asking if I'd like to helm Atum, so I said yes.
Turning up at TSC yesterday morning with an 8kt forecast it wasnt looking great, but then there had been more wind than forecast on Saturday and it was picking up from nothing when we arrived.

Anyway we got going or rather drifting off of the line and it became clear that Atum is not a light wind boat! Up the first leg we held Born Slippy behind us and stayed with the 29er but the cross tide leg killed us. At times we were sailing head on to other boats on opposite tacks such was the variance in wind direction. We lost out about 1/2 mile to Phil & Sarah, but on the next downwind leg reeled them in, holding them all the way to the bottom mark where we managed to get a couple of boat lengths in front. The long upwind leg back towards Olbury power station caused us a few problems. Paul Croote getting stuck between the jib and the mast causing the first swim of the day! Eventually we got our act together and in one 20second puff managed to get twin-wiring and upto 11.7kts before sailing into a hole and binning it to windward. Boy do Cherubs stop quick and tip to windward quickly! A couple of tacks on windshifts later, and we had recovered to catch the 29er and were gaining on Phil and Sarah. The final downwind leg to the club saw us gaining quickly on Phil and Sarah until another hole and another windward flop dropped us in. Luckily we recovered from that and scraped past both the 29er and Born Slippy to just beat them over the water.

So what did I think?

Atum is a lovely well sorted boat and is clearly quick once there is enough wind to twin wire, but if your sitting in, it doesnt like it.
The lightness of the rig means that sailing into holes can cuase problems in that the boat comes over on you very quickly, and the hull shape and rig-layout on Atum produces enormous amounts of lee-helm as the boat comes over on you, making it even harder to get back in the boat. I'm assured by Paul that these problems go away when sailed in decent breeze, so I will be back for another go.
Sailing upwind at the back of the wing felt odd with the t-foil wound on and perhaps because the mast is short, the forward pull on the wire is strong. On my 700 I've got much longer distances between the front and back of the wings but the fwd pull is nothing like it. The only way to keep leverage on is to engage the toe loops which is an alien thing for me. I only use them on the 700 when maxed out downwind in big breeze.

Before sailing the boat, I thought it would be twitchy and unstable, but its not. It actually ok, certainly no worse than the 29er. Its got quirks like the 700 but then that may be particular to the Slug shape in terms of chine steering but once up and running I reckon you could get it into a groove comfortably.

So I'll definately have a go again, although Paul is selling Atum now, (great boat for anyone interested in getting a well sorted boat without all the hassle of building your own).

Paul
RS700 844

[roland_trim]

Armed with Badger's main, team EJ headed down to Thornbury this evening.  Hmmmmm. Blowing a steady 20-30mph and gusting the odd old boot and poorly secured dog. I am assured there were flying sheep, but to see them you'd need equipment to see across the channel.

We rigged Born Slippy, but come launch time we watched the RS700s depart the shore and took the sensible decision to pack EJ for her move to her winter club instead. Strange as 12 months ago we would have been 1st on the water in winds like that...

[Will_Lee]

Hi Paul,

Boy do Cherubs stop quick and tip to windward quickly!

The lightness of the rig means that sailing into holes can cuase problems

Before sailing the boat, I thought it would be twitchy and unstable, but its not. It actually ok, certainly no worse than the 29er.

So what did I think?

Two opposite things at once? 

Glad you had a good time. The lightness of the rig is because lightness is a good property in a rig. The forward pull is because the trim angle can be less when maxing it downhill. Before T foils loops were only rarely used, now needed all the time. Also if you lengthen your trap loads it can make being right back much easier. Also the ride is much more violent on a Cherub (It is noticeable how much tamer the SK4 is, and that is only a foot longer - and loads faster). The other thing is there is no crew on a RS700 to interfere with where you want to put your front foot!

As to the twitchiness, it is all relative: The rig is  (much) less than half the weight of similar sized SMOD rigs. There are huge differences in roll inertia. Its more like sailing a board, though it looks like a boat.

[phil_kirk]

All Paul's comments are relative to previous experience of which he has lots in Skiff type boats. The differences betwen a cherub and a 700 are therefore different to what someone else might experience coming from another class.

I think Sarah found that the helm on a cherub is much more sensitive than the byte probably due mainly to the greater boat speed making the rudder more effective.  The range of boat speed is also higher than she is used to as a helm.

The feel of any class of boat can be changed by setting up the rig differently or using different sails.

I have noticed at low speed (drift mode) that the T foil provides a lot of roll damping requiring you to move to correct a roll before you think you need to. Consequentially you can move too much and set up an osscilation. At speed this effect appears to reduce or perhaps the planing forces on the hull masks the damping provided by the t foil.

[TSC]

Your right Phil, if I'd sailed the boat having come from Enterprises or Lasers my comment would certainly have been different. For me the speed certainly isnt an issue and neither really is the sensitivity of the rudder. In fact in hindsight I go as far as saying the rudder was suprisingly un-responsive, but then I'm used to the 700 and the 600 where the blade is enormous and twitchy as hell at speed. What you will see with the T-foil is that any roll should be damped out when you are moving at any speed, as roll will increase the apparent AoA on the correct side of the T-foil therefore increasing lift and providing a restoring force. For the brief period where we were really going the boat felt rock steady to me, with a little less feel in the rudder than I'm used to.

Having sold my 49er to he who shall not be named, and then caught up with him at the following Dinghy Show when he proclaimed that he'd sold the 49er because it was too stable, I assumed the Cherub would be super-twitchy, but I dont think it is. As I said the only issue is trapezing in the loops upwind feels odd and could be more comfortable with the trapeze attachments moved further up the mast. That and a bit of time in the boat!

[Will_Lee]

What you will see with the T-foil is that any roll should be damped out when you are moving at any speed, as roll will increase the apparent AoA on the correct side of the T-foil therefore increasing lift and providing a restoring force.

Leaving aside whether it is actually damping or risking 'terminal oscillation', can you explain the above please? I have expended some energy trying to work it out and I can't....

...thanks!

[dave_ching]

yeh I got the boat is more stable with t than with out. But that is like saying a plain is more stable with wings.
I got lost at the explanation.

[TSC]

Explanation time:
If the boat experiences a sudden roll to leeward in a gust, the windward half of the t-foil will effectively be lifted up in the flow, and the leeward side will drop in the flow. The flow vector on the windward side will then have a vertical component to it thus reducing the apparent angle of attack that the foil has to the oncoming flow, and therefore reducing lift on this side of the foil. On the leeward side the foil is pushed down and therefore the apparent flow vector is coming from below and ahead of the foil rather than straight on. This therefore increases the apparent angle of attack and thus increases the lift on this half. The combination of these forces provides a rolling moment which opposes the roll induced by the gust. There will be other effects due to the junction between the horizontal and vertical foils but in principle you will see some form of roll damping. It is however feasible that if you have full T-foil wound on and experience a violent roll then the leeward side of the T-foil may stall, which will reduce the damping effect and will probably pitch the nose down harder and give you a load of lee-helm.

[Stuberry]

On a slug hull, the shape of chine line and the chine angle to the topside, mean that when the boat heals the bow is lifted. As the bow lifts this increases the AoA.

Maybe too simple?

[Phil Alderson]

Explanation time:
If the boat experiences a sudden roll to leeward in a gust, the windward half of the t-foil will effectively be lifted up in the flow, and the leeward side will drop in the flow. The flow vector on the windward side will then have a vertical component to it thus reducing the apparent angle of attack that the foil has to the oncoming flow, and therefore reducing lift on this side of the foil. On the leeward side the foil is pushed down and therefore the apparent flow vector is coming from below and ahead of the foil rather than straight on. This therefore increases the apparent angle of attack and thus increases the lift on this half. The combination of these forces provides a rolling moment which opposes the roll induced by the gust. There will be other effects due to the junction between the horizontal and vertical foils but in principle you will see some form of roll damping. It is however feasible that if you have full T-foil wound on and experience a violent roll then the leeward side of the T-foil may stall, which will reduce the damping effect and will probably pitch the nose down harder and give you a load of lee-helm.

That sounds about right, I am not sure if the effect would be greater with the T at the bottom rather than the middle. I noticed more of a difference in roll damping with the Patterson 7 and t in the middle compared with Pocket Rocket with the t at the bottom.
It also shows why there are so many issues with T's breaking off, with the twisting of the t joint.

When I was sailing with a damaged T joint at the nationals, I could hear some cracks if the boat rolled a lot, also when I did any sharp steering, as there was a difference in speed between the ends of the T

[phil_kirk]

Our first outing at thornbury this year.  Following three days at Weston we were glad to see similar conditions at thornbury in which to further practice Light air cherub sailing.

The race was one of Thornbury's long distance series the Bernie Scoop.  This involved starting at TSC and sailing to wollaston Pill upriver and on the other bank of the severn.  Once all boats had crossed a temporary line at Wollaston we re-started and raced back to TSC.

In a northerly it was a beat out and a run back although to make things more interesting we started the outbound leg with a running start to a mark before heading upwind.

The first leg involved lots of gybing to avoid the tide and all the other boats.  We rounded the turning mark behind the YW Dayboat.  Then it was single string and then twin string sailing across the river.  With t foil on and the flat water we were really shifting and started closing on the higher pointing and better starting 700's.  It became gusty and shifty on the other bank and in the strongest tide quite bumpy.  At times Sarah was on the fore deck and others we were twinning.  We reeled in the 700's finishing in front of the 3rd one sailing by Tim U.

While waiting for the other boats to finish we landed on the mud banks of woolaston pill.  Some made a better job of this than others. most people and most boats got quite muddy.   

As the 5 minute gun for the re-start sounded most of us were still on the mud.  Amazingly we launched quite quickly (although not with any finnesse), Sarah having to jump for the boat before being left behind as the tip of the rudder detached it'self from the mud.  We had a late start but a brilliant Kite run all the way back.  After 19 minutes and 3 gybes we crossed the line as first boat to finish.  We later learnt that on corrected time for both legs we had won.  Something to do with not capsizing and few corners me thinks.  A great race if a little muddy.

[Will_Lee]

Nice one!