Multi mast quiz of the week :)

[Guest]

This came up in conversation the other day.

 

I am with this understanding

 

http://www.a-cat.org/id38.htm

http://www.tspeer.com/Wingmasts/teardropPaper.htm

[Megwyn 2]

Surely the ideal is a mast than can be rotated in increments, so you can increase power as required?

[Guest]

Less or more than what, I understand you have to keep a flowing curve upwind (this is hard with a old sail)

Or down wind let the rotation off and up wind pull it on

[Guest]

I find in stronger winds, that less rotation produces a smoother power curve.

I think it is less drag, and this allows more usable power.

[Guest]

Less or more than what,

 

that is a good point,

 

but lets say upwind, taking mast rotation from 30 degrees to zero. (zero being on centre line).

[MrWolf 0]

Too easy.

 

Max rotation is maximum power provided the mast isn't rotated too far and the sail stalls.

 

Minimum rotation is generally the mast axis parallel to the boom, which may mean its on the centreline but actual

under rotation can also cause the sail to stall, and that is least power.

 

rotation on my rig gives an overall sail/mast depth range of 6-15%.

 

But here's a more challenging question.

 

1.Why on the TWU will the rake of the diamonds stop them preventing the mast bending sideways ?

 

Clue : It has to do with basic physics.

 

2. If you put two sets of raked diamonds on TWU will that stop the mast bending sideways (assuming the same rake as the first attempt) ?

 

3. If the original configuration of the raked diamonds (single set) had a forward facing dork would that have worked ?

 

4. Would you go to NZ Rigging for a multi rig ?

[Guest]

 

1.Why on the TWU will the rake of the diamonds stop them preventing the mast bending sideways ? ..

 

With the diamonds angled back when you tighten them they just prebend and mast. Then when you go sailing and sheet on the mast comes back further and guess what....the diamonds go slack again as the spreaders are angled back.

 

2. If you put two sets of raked diamonds on TWU will that stop the mast bending sideways (assuming the same rake as the first attempt) ?..

 

Two sets of diamonds should reduce initial prebend..... will it help....will tell you in a week.

 

3. If the original configuration of the raked diamonds (single set) had a forward facing dork would that have worked ?

?..

 

.....will never know....but might be able to tell if 2 dorks on 2 sets of spreaders will work ....

 

4. Would you go to NZ Rigging for a multi rig ?

 

NZ rigging are the local agent for Allyacht spars www.allyachtspars.com.au

and have some good extrustions available to them for multis at competive prices compared with carbon rigs.

They also have a great bunch of guys working for them that are friendly co-operative

 

but we all know that aluminium has no place in the marine environment

[samin 0]

im thinking of the same issues with my rig for the new death trap. Not sure the carbon section is grunty enough to withstand fore and aft bending without a front dork, and also wondering about twin or single spreader with a 11.3m section to deal with. (masthead rig)

Doesnt Crowded House, and Sundreamer have twin spreaders with twin front dorks? both rigs still standing.

[Guest]

both exodus and crowded house use twin diamonds and front dorks on what are esentially eleptical tubes.

Exodus tube is around 180mm x 120mm

 

attitude and epsom salts though, identical boats, but one has swept spreaders and dork, while the other has no dork.

[ScottiE 174]

after watching my mast sway all over the place with Bill's main - I think I might be a bit of a dork for not having a dork!

[Megwyn 2]

after watching my mast sway all over the place with Bill's main - I think I might be a bit of a dork for not having a dork!

 

well, I can't be a dork. but wondering what one is? Not a terminology I have heard before. Can someone enlighten this newby?

[ScottiE 174]

forward pointing strut at same location as spreader with stay to mast base and forestay termination - at least in the context of a mast!

[Megwyn 2]

forward pointing strut at same location as spreader with stay to mast base and forestay termination - at least in the context of a mast!

LOL

 

so it has a stay to the mast base, and the top/3/4 way up of the mast where the forestay terminates? So how would two work then? Is it supposed to give forward tension where the spreaders are - as oppossed to only at the forestay termination?

 

Thanks for answering my dumb questions.

[MrWolf 0]

Answers:

1.

 

Because if the diamond stay takes any load it forces the mast to bend fore and aft, thus causing the diamond stay to go loose, thus allowing the mast to bend sideways.

 

The only way that set up could work would be if the mast were so stiff fore and aft that it could withstand the extra fore and aft bending force being applied by the poorly engineered diamond.

 

The idea of the sweep is to prevent the mast inverting when reefed, but even this is flawed as the diamonds will not stay tight enough for them to work.

 

Diamonds are supposed to stop the mast bending sideways (in a single plane), and if they are basically straight across the length of the stay doesn't alter if the mast bends fore and aft. They can have a little sweep to prevent inversion.

 

2.

 

Two sets does not change the same basic physics as above.

The stays are still trying to bend the mast fore and aft, the more load on the stay the harder it tries to bend the mast,

highly likely result is the mast will be better behaved but ultimately the Engineering is still flawed.

 

3.

 

The Forward facing dork would solve the initial problem, see explanation above.

So would the set of diammonds having less sweep, providing the mast is stiff enough fore and aft.

If the forward facing dork beats up on your jib you can look at those little wheel thingy's

 

4.

NZ Rigging is a great company I'm sure, and you are right they have a great bunch of guys working for them.

But they have a poor track record on Multi rigs, and their first effort on TWU is very poorly engineered.

 

Hopefully the next version will be a lot better.

[Megwyn 2]

Thanks Mr Wolf - wonderful comprehensive answer - I will have to read it a few times lol.

[johnMi 2]

Well Mr Wolf

I'm about to find out how good NZ Rigging really is....

[robdavies 0]

This is all extremely helpful. What is the general concensus on which rig setup is currently working the best? There has been some mention of works in progress so what is the set up to copy? I know there are a lot of variables, materials, weight etc. but which rigs seem to work the best currently on an 8.5?

[samin 0]

Hi Rob, when you say which rig works the best what do you mean? which is the fastest 8.5? racetrack will clarify that pretty quickly, however the top ranked boat would most likely be even faster with a carbon rotating wing section as opposed to a fixed carbon round tube they have.

 

Lucifers the newest 8.5 and has a carbon rotating wing section about 250mm by 100mm with single spreader, no front dork. It is a very stiff section, and most people would probably agree if money is no object this is the way to go. His forestay is 1m from the top of the mast.

 

Another feature is the hound should be very high to get max genoa area, like 15/16 or something.

My new 8.5 Tri will not be fractional rigged, the forstay will go to the top of the mast for max genoa area.

[MrWolf 0]

Another feature is the hound should be very high to get max genoa area, like 15/16 or something.

My new 8.5 Tri will not be fractional rigged, the forstay will go to the top of the mast for max genoa area.

 

Well I've thought about this and I don't think the mast head rig is a very good idea at all.

I reckon the 7/8 option on Lucifer is about right.

 

With the big square tops we use there is quite a load on the top of the mast, if you can get the tip of your mast to fall off a little sideways then when a gust strikes, the leach opens, the tip falls off, you accelerate nicely and off you go.

Below the hounds, little changes.

 

same gust on a mast head rig !

Mast must bend so bends below the hounds, but what opens now ?

 

Leach opens much lower (not what we want at all).

 

Second big problem is that on a 7/8 rig the square top is working in basically undisturbed air, unlike on a mast head rig, where it will be backwinded to billy oh.

 

Like SB alluded to, what other modern boats have mast head rigs ?

 

I even think it could be a huge hands up when furling the Code zero, which i guess is mast head, but how do you run a fractional screecher or gennaker with a mast head rig ?

 

You are right that the trend is to shorter topmasts and stiffer as well, that is to do with the square tops and the high code zero loads, but I think the mast head rig idea is a step in the wrong direction.

JMHO of course !

[Guest]

Ok - how about a biplane rig a la Shionning Radical Bay. Two full sized mains would be more sail than a main and jib???????