I'd like to understand the basic theory behind mast shape and settings - in particular, the effect of spreaders. For instance, if I have 1.5" of pre-bend, does it matter whether the spreaders are in their forward or backward position? As far as I can see, I'd need more diamond tension to achieve a given amount of pre-bend if the spreaders are in the forwards position rather than back, but does that affect sailing performance?

Let the debate ensue!

from what i gather if the spreaders are not swept back than you would not get any prebend at all... ie on my Supercat 19 i have spreaders that are inline with the mast and only help the mast from bending sideways when your mast is rotated

Sail makers cut a constant luff curve. In order to get your sail to flow well at top, middle, bottom, the mast must generally follow the luff curve of the sail. Lets say there is a amount of luff curve your sail maker has placed in your sail (X). In order to match X in your mast you can play with your spreaders and/or your diamond tension. You can place your spreaders forward and put huge amount of diamond tension and get to that magic number, X. The result of this would be a mast that would be very stiff down low and bend a lot up higher ( NOT a constant curve). Now lets put a lot of sweep in the spreaders and less diamond tension to get to X. The result will be a mast that bends more down low than up top (NOT a constant curve). If you play with both in relation with other you can come up with a curve that is more constant (even) and therefore better matches your sail. This will allow you to get your sail to flow at top, middle, bottom. That is a good thing.

Ken

I think what he is asking is if a given amount of pre-bend can be obtained with 1.5 inches of spreader rake and 500lbs of diamond wire tension and the same amount of rake can be obtained with 1 inch of spreader rake and 750lbs of diamond wire tension (for example!!!), is one setting preferable over the other?

The amount of spreader rake will affect some of the mast bend difference between fully loaded and lightly loaded. As the rig is loaded through wind pressure and righting moment (trapeze), the diamond wire(s) are in additional tension as the mast tries to bend away in the middle.

The amount of rake in the spreaders affects how much additional forward bending pressure is applied to the mast as it loads sideways from these dynamic loads. More spreader rake means that the load on the diamonds will translate more into a forward bending moment on the mast as the load increases and you will experience a greater difference in the flexing of the mast from 8 knots and sitting on the trampoline to full trapezing in 20. Less spreader rake translates into less forward bending moment on the mast yielding a smaller range in the flexing of the mast in the same situation.

Perhaps it makes sense for the dynamic mast bending characterics to be very similar between two teams of different weights. The lighter team would want more spreader rake because they dynamically load the diamonds a little less than the heavier team. To see the same dynamics, the heavier team would want less rake because their diamond wires are actually going to carry a little more load.

Lighter teams will probably want to see the mast flex through a wider range as the wind increases. Heavier teams typically want to see it flex less to accommodate the extra weight.

The preferable setting is the one that best matches the shape of the sail.

Thanks for the feedback. I can see the case for matching the shape of the sail's luff - this must be the 'best' position, but that would imply everyone would set their mast up in the same way. I have had advice from the designer / manufacturer to go to the "fat boy" setting, with minimum spreader rake, and maximum mast pre-bend. I am trying to understand the theoretical advantage this offers over the middle setting for each. I gather that with the spreaders raked less (putting them further forward), I will get a stiffer mast, at least in the lower section, and this will generate more power. And if I want even more power, I can ease off the diamond tension and get less mast pre-bend.

Just a small thing. Luff curve is not constant, it is supposed to change according to how the mast bends. Above the hounds it bends very differently compared to around the spreaders.

I am no expert, and could be VERY wrong,... but i thought pre-bend was mostly a result in spreader rake.. not diamond wire tension .

Everyone, please correct me if i am wrong

i believe its a combination of both... if you have alot of spreader rake you can adjust the mast prebend by loosening/tighting the diamond wires.. so the looser the wires the less the prebend...

Pre-bend is a function of spreader rake and diamond tension. Less spreader rake and tension and the mast will be slower to react to gusts. More rake and tension and the mast will depower faster when you pull on downhaul or even when you go out on trapeeze. As always it is a compromise, but if you are racing it is critical to get a luff curve matching the bending characteristica of your mast. You can adjust it a bit with pre-bend, but getting the sweet spot where the mast do the work for you and depower in gusts is harder then.

I always have to remind myself that more power comes at the cost of more drag, it will not always be faster to have more power!

So to add to the confusion...when setting spreader rake, we typically measure by placing a straight edge (ie batten) across the two diamond cables at the spreader tips, then measure the distance from the straight edge to the mast track. When we're at the rake distance we want, then we crank on diamond tension (using a Loos guage). Typically, if we re-measure the rake setting as above, it does not change after diamond tensioning. So, to check pre-bend, you're suppose to stretch a line from mast top to mast foot and measure from the line to the mast track at the spreaders. This should change with differing tensions (the ends of the mast are flexing in the fore/aft direction with less/more tension, respectively).

I have never done the pre-bend measurement...since it can't be measured easily without tipping the boat on it's side and I'm too lazy :-)

Ahh, so how do you know that you have a good setup <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />

Well, I will repeat my question from the F16 forum about sail stalling here:
So far, I understood that I can adjust the twist of the main with pre-bend. But what is the best light wind twist? Should my main stall at the top of the mast/sail, in the middle or just above the boom? I am used that pin head sails always stall at the top first, however the square top sail which I have now stalls above the boom first. Do I still have to much twist?

Cheers,

Klaus

Set up technique is here:
http://www.hurricane59.com/rigging_sailing.html

Cheshirecatman

Klaus, you are the aeorodynaimist, you should be telling us <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />

In my opinion the sail should stall over the whole at the same time when you have perfect trim. If your uni-rig stalls over the boom first, I would say you should try less twist. But, that would depend on conditions. In chop you probably want more twist. That is my opinion..

Hi Rolf,


Well, a theory without practical back up might be wrong, hence I ask.


That's what I think as well. But I see that you use subjunctive. Did you ever achieve this 'perfect trim' (I assume you use tell tales at the leech)?
Actually I have already a quite relaxed dimaond wire tension, maybe the leech is cut to long <img src="http://www.catsailor.com/forums/images/graemlins/confused.gif" alt="" />...

Apart from confusing sail trim, my wife and I have had great time on the boat already. My wife did helm under spi for the first time and really pushed it to the limit and now she is really thrilled (she dived the bow up to beam, nothing happend, but a big shower for us).
You will be surprised, how sensitive these boats react on the smallest changes. I hope you get your boat on the water soon, so you can share this.

Cheers,

Klaus

Klaus,

Dependent on what you call light winds.

Stalling of the sail in light winds can also because by having too much draft in the sail; as in contrast to having a too large angle of attack. I think this could be the case here as the bottom part of the sail is widest and therefor the flow needs to have relatively alot of "internal energy" to overcome the "long" negative pressure gradient at the back of the sail.

In really light winds you'll want a flat sail with a considerable amount of twist. The flatness assures the sail won't stall due to negative pressure gradients that are too large and the (relatively) large twist on the leech compensates for the windshear.

However at some point in windspeed the wind itself transitions abruptly from laminair flow (with a significant windshear gradient) to a turbulant flow (with much less windshear) and then you'll need to bring the leech back in. When the wind increases more then the increase the draft; as the airflow is becoming more energized. You are now in medium wind conditions typically 6- 12 knots.

And only in high winds due to increase twist and decrease draft again.

In simple terms ; very light wind sailing has an almost identical sail profile as high wind sailing where the sail trim in the medium conditions is with a relatively full draft and tight leech.

Flat and rough water are influence factors as well. In flat water you can typically sail with a leech that is a little bit more tight and with a relatively unform draft profile. In rought water you sail with more twist and a draft that is located more in the lower part of the sail.

You use mainsheet, downhaul, diamond wire tension, spreader rake simultaniously to adjust the mainsail to these different conditions. Sadly it is not like one control only affects one adjustment. For example when you sheet the mainsheet tighter then you reduce twist and reduce draft in the top of the sail. Only be also using the downhaul at the same time can you increase twist again while flattening out the top even more. So if you only want to reduce draft in the top of the sail but keep the same twist profile then you have to adjust both settings simultaniously. This is sadly one of the more easy ones. Setting the draft profile properly also requires resetting the spreader rake and or the diamond wire tension together with the mainsheet, downhaul AND mast rotation.

As an aerodynamists you have more then enough learning and experimenting to do on your F16 !

But that is all part of the fun right.

Wouter

Hi Wouter,



That's one of the reasons way I changed from Dart 18.

Thanks for the details, so far they are clear for me. We have noticeable wind shear only a couple of days per year, hence 'my light wind', is what you call medium wind. My concern is, that (maybe) I have used already all trim to reduce twist and still the lower part stalls first, even with maximum outhaul (e. g. not to much draft). Or is it maybe not possible to close the top of a square head sail? So what is with your sail with maximum closed leech, stalls at the top or bottom?
Next week-end I will try to relax the dimaond wires even more (spreader rake is not adjustable on my boat <img src="http://www.catsailor.com/forums/images/graemlins/frown.gif" alt="" /> ).

BTW: Are you a soccer fan? Excellent play of the orange team, maybe we will see another great match between Geramy and the Netherlands <img src="http://www.catsailor.com/forums/images/graemlins/blush.gif" alt="" />

Cheers,

Klaus

Ok, to be more accurate: On a reach or downwind in light winds (not yet heeling up), I would like to stall all over the sail at once, so I maximise lift. Upwind I would like to stall in the middle first. That is because I would like to have a good lift distribution over the mast height just before the stall to maximise drag/lift. That's my theory, not necessarily the truth.

Cheers,

Klaus