Last weekend I sailed with my Nacra F18 (2005) together with other two F18 boats (one Tiger and a Hawk). The wind was maybe 15 kts (or more), upwind the two other crew were on trapeze, with one hull flying. In the same wind, and same tack, I was not able to fly the hull, even we were just sitting on the hull. We had considerable less power, consequently less speed…
If I sheeted in the main correctly, it was bending the top of the mast, even I had no power on the Cunningham. The top of the main became flat. With less main tension the top of the main remained open.

My setups was:

crew weight 160kg == 355 lbs
spreader rake 35mm == 1.4inch
diamond tension 39
mast rotator a little bit in front of the dagger board case
Jib sheeted hard,
jib slot: 1-2 inch between the end of the spreader arm and the jib

Any idea what was wrong with my setups?

Ps. Sorry for my poor English.

Hello Sanyi

I am an A Cat sailer, but I think our rigs have a few things in common. If you are looking for power the sail should not be flat on top, that should only happen when the cunningham is pulled hard and the main sheet is tight. I sounds as if you could have too much prepend in the mast which is causing the sail to be too flat. I would start with straightning the mast a little. Maybe the maufacturer of the main sail can tell you how much luff curve the sail has and how much prebend is recommended for it. 15 knots and both hulls still wet is no fun. Here is an article by Andrew Landenberger that might help you too http://www.landenberger-sailing.com/spip.php?article22
Regards

Daniel

Sanyi

Sounds to me like you have too much prebend in the mast as stated above.

One measurement you do not give is the prebend.

There is usually a class or builder defined method. But it is usually something like:

Take a line of cotton (or whipping twine) and pull it tight between the base of the mast at the gooseneck or black band if you have one and tape it down. Do the same at the top of the mast by the black band or another "constant" place. Now go to the mast by the spreaders and measure the bend between the back of the mast and the line using a tape. In 15kts, you should both the trapping and starting to de-power with the downhaul.

Thanks for the fast replies !

I'm still have not measured the prebend, but I will, and give it. (Today I'm far from my boat)

Any case, I think the prebend depend on two factor, the spreader rake and the diamond tension. (Is it true ?)

The spreader rake recommendation by Nacra is 35-60mm, mine is 35mm, really the minimum.

The diamond tension recommendation is 34-40, mine is 39. It is maybe too much... (But I was afraid broken my mast)

If you use a spring loaded gauge, it is possible that your unit have a different stiffness than the one used for initial measuring. Measuring with a string is much more reliable.

I'm using Loos Tension Gauge PRO PT1 , I think.

You should try to compare the readings from your unit with other units. I.e. measure the same diamond wires and compare the readings. These gauges dont always give the same measurements between models or even within the same batch.. If you can, set up the boats next to each other and compare the shape of the sails, then start fiddling with diamond tension and try to replicate the shape.
Measuring with a tape measure and a string is very reliable and cheap <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />

Our masts pre-bend is set up between the tang fittings, and the luff curve is cut into the mainsail in the same way. The part over the upper tangs is not influenced by pre-bend, but by mainsheet and downhaul. If you lack shape in the upper area, it might be that your mainsails luff curve dont match your mast. We have just gone trough the same process with our Tornado mainsail.
When you compared speed/power with your friends, how was your twist and how hard did you sheet your main compared to them? Perhaps they just have better sails for the conditions or did something else to power up their boats? Have you looked critically at your mast rotation and how that changes the sailshape?

AFTER you have looked at your prebend, consider that you battens may be to stiff in the luff area. Also, your sail may have been made with the wrong luff curve.

I have that same Loos guage - given your crew weight, I think your diamonds are a bit tight for the conditions you describe. First, measure your pre-bend as other have recommended. Then loosen your diamonds to about 35 on your guage and remeasure.

Every mast and sail combo is a little different - on my brand of F18, I run about 34-35 on the diamonds in light to moderate breeze and increase to around 39 only when the wind is over 20-knots.

Let us know how your changes work out.

The F18 in question does not have the wingmast so the tensions should probably be a little less than the Cap. I run about 400lbs of tension (using the same guage...sorry I don't know the number...My thought process for that is in lbs) and go up from there when we need to start depowering stuff and am experimenting with less on mine (I have a 2004 Nacra F18 with a mast that is probably a little softer than yours). If I remember correctly, my spreader rake is at 1.5".

I used to run a lot more tension because I was warned that the mast might invert...but I'm running with less now and finding better speed in the light stuff.

You describe so different power situations (double trap vs. two sitting on the hulls) that I can't imagine a few pounds more or less in the diamond could make such a huge difference. I would guess that either there is something REALLY wrong (mast not stiff enough so it bends back way too much or not enough main sheet tension) or you just didn't get into the same groove as the other boat. For checking trim, you should foot enough to get both out on the trapeze with the windward hull flying and then try to head up slowly, maintaing speed and the windward hull dry. Now by changing trim, try to improve the upwind angle you sail with respect to the other boat, maintaing more or less the same speed. It is here where the diamonds, cunnigham and all that come into play, imho.

Where you using soft or hard top battens?
And where all battens tensioned properly?

Also, if you are "pinching" (pointing too high) and the others are not, you will not fly a hull while they do. Do you know their crew weights? They might have been sailing with lighter crew.

Was your traveler centered? Was your mast rotated enough? For more power, rotate the mast a bit more, (or less if it is way out already) see what happens. Cunningham, on just enough to remove wrinkles, no more until you get over powered. Where was your out haul, out tight or loose enough to give you some power? Also, if the top of the sail is flat with little pull on it, try pushing the top batten in tighter, to force some curve into the sail.

I Asked a friend to measure my mast prebend, and it is only 2cm (less than one inch).

My mast is the original Nacra F18 mast 2005 (not wingmast), and the main is the original Performance sail (2006), topsquare 90cm (35 inch) approx.

A month ago I changed the two original top battens to softer; the original was 4kg (8.8lbs) the softer is 2.9kg (6.4lbs). The battens are in the pocket in order, not over tensioned, but stiff.

The other 2 teams sailed on the same tack, I was not pointing higher.

The other two teams are lighter then us, let say, 140kg (310 lbs), but just approximately

160 kg x 2,6 m = 416 << 140kg x 3,6m = 504

It is more than 20%

I have not seen too much F18 mast but mine looks really soft.

Traveller centered.

Mast rotator: in front of the dagger board a little bit. Later I tried with less rotator, because in this case the bending at the top of the mast became less (because the other axis of the mast).

Outhaul was tight, It was maybe a mistake.

Consider comparing your mainsail, especially the luff curve with the other boats.

Less rotation will help. Too much rotation and the mast bends back, flattening the sail.

Almost 15 knots and not flying a hull? He had to have been pinching too much. (or in a different wind) Most any catamaran talked about on this forum will fly a hull in 15 knots if the traveller is centered and the main sheet is tight..........

Keep it simple before going to all that technical stuff. Improper diamond wire tension / spreader rake usually affects performance in a different way. Like if the prebend is say too loose the boat won't drive forward when hit by a gust.......it will just fly a hull and go really slow.



Tom

Maybe 15 knots, I'm not sure, as it was not able to measure it on the water.

The main point is that my boats seems to be underpowered compared to the other two, and I wantted to check my setups, and find out the reason(s). Of course, one of the reasons (or just The Reason ) can be my boat handling.

Any case, Thanks for the answers to All!

Sheet out a little and head off the wind a little to gain speed. Then slowly sheet back in. Continue to do that going upwind to match your opponents speed and point.

All of your settings look about right.

Your weight is a little more and that does affect performance.

Sanyi,

The reason I said that you were probably pinching is because I used to do the exact same thing. I was bound and determined to sail the same line as the lead boats and would end up flat and slow while they were flying a hull and fast. If your setup is in the ballpark, then just forget about it and what the boats are doing in front of you and sail your boat in the wind you have and you will probably end up on the same line as the leaders. But, don't expect to stay with them if you do not have the experience they do. There are lots of little tweeks going on constantly (with the tiller, sheets, rotator, travellers, outhaul, and downhaul)with some of those lead guys. Also, they all do not necessarily sail the same way.................so copying one doesn't guarantee that it is the best way. Do not fear developing your own style. Also, sometime you have to learn before you can learn. I found that you have to learn to sail with the faster guys in steps..............so don't beat yourself up if you are not among the leaders as fast as you think you should be. Some of these guys have lots of tiller time.

Tom