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Posted

I have an old IOM, with a B rig which seems to have no spreaders on the mast/stays.

Is that unusual for IOM B rigs?

Would  performance be hugely affected if I sailed her just under stays?

Regards

David

Posted (edited)

Hi Dave,

As nobody has chipped in i’ll add my thoughts…

The short answer is yes, spreaders are most probably required to stop your mast from being to soft with sideways bend.
But thats a broad response without knowing:

- What boat you have i.e. narrow or skinny shroud connection point at deck?

- Mast step type i.e. does your mast step on the top of the deck, in a lowered cockpit or go down through a mast tube to the hull floor?

- Mast tube type and diameter i.e. is it a groovy or round 11mm diameter mast or 12.7mm diameter of either of those types. It may even be a more modern 11mm Round tube high tensile mast tube.

- Is there a mast ram at the deck level that will support the mast latterally, preventing any sideways movement?
 

As the class has evolved, a lot more effort has been put into rig setup to get the right amount of sideways bend to suit other aspects in total design. The answer is not as simple as it may have been before, if looking for best performance.

cheers

Brad

Edited by Brad Gibson
Posted (edited)

Thank you, Brad, for stepping in.

My IOM is a Cygnet 2,  very broad in the beam. Probably dates to c. 1990.
It’s my first ever IOM and I am only new to rc yachts.

The A rig has spreaders, the B rig doesn’t. The shrouds are at the widest extremities of the deck.

I just wondered if this was how it was designed.

The mast is round (no groove) goes through a tube to hull floor

No mast ram.

I accept that the boat will never be competitive, but I would like it to be the best it can be, without spending a shed load of money on it.

I know that the A rig fits that bill, but I guess I will just have to suck it and see with the spreaderless  rig.

Regards

David

 

Edited by David Norris
Posted

Hi Dave, 

Given the age and chance of heavier mast tube used and quite possibly thicker foils used. it may be just fine without spreaders. The mast bending sudeways in a gust could help it balance out. 
Id suggest you try it as is, then if you find the mainsail leech is flapping around a but on that rig in a bit of wind, then take a look at some spreaders.

cheers

Brad

Posted

Shrouds without spreaders doesn't make a lot of sense from a stiffness point of view.    Once you have shrouds, the extra weight / windage of the spreaders is small for a significant increase in stiffness.  Of course you don't need them in light airs, but if you find a crease running from the mainsail clew to about 1/3 up the mast then spreaders will help.  

Posted

Yes, I see the logic of that. It’s just strange that the mast should be pre-drilled for hooking the shrouds on, but that there are no holes for spreaders to push through as on the A rig mast. 
I guess there is always the possibility that this is not the original mast.

I plan to sail her this Friday (although there probably won’t be a strong enough wind to assess the job fully).

David

Posted (edited)

Don’t drill the mast, go for clip on spreaders, I have on my B rig.

also cut them to length for the rig, don’t fit A length ones onto a b rig,

how long should they be…. There is a % formula somewhere

Edited by Stephen B
Posted
4 hours ago, Stephen B said:

Don’t drill the mast, go for clip on spreaders, I have on my B rig.

also cut them to length for the rig, don’t fit a length ones onto a b rig,

how long should they be…. There is a % formula somewhere

Taken from BG set up detail

Spreader Length

If using our mast rigging guide your spreaders should be measured from tip to tip as a percentage of your shroud base on the deck. For narrower less powerful designs, flat deck older designs or those with thicker section foils creating a tendency of weather helm, a shorter percentage spreader length allowing the mast to bend and depower the sail by opening the mainsail leech earlier in a gust would benefit A Rig 75% (of shroud base) B Rig 70% (of shroud base)

Posted

That’s it…. I cut to this length on my Kantun B rig and same values for A rig calculation…

seems to work fine, boat quite fast,

 

maybe someone will verify

Posted

Great discussion! Sorry if this next point is a bit nerdy, but in selecting the right type of aluminium tubing for a mast, it’s worth bearing in mind that the tensile strength doesn’t have much bearing on the stiffness of the tube. Basically the tube stiffness depends mostly on two things:

1. The section dimensions (particularly the 2nd moment of area) and

2. The Young’s modulus of the material.

I understand that different grades of aluminium have widely varying strengths, but surprisingly very similar values of Young’s modulus (about 70 GPa, give or take).

In practice, this means two tubes of different alloys with identical sections (e.g. outside diameter and wall thickness) may have different strengths, but their stiffness will be pretty much the same.

By the same token, you might select a very high grade of aluminium tube, with a thin wall thickness to save weight, but a lower grade alloy with the same outside diameter and thicker wall (to give equivalent strength) will actually be stiffer, because the section will have a larger 2nd moment of area (but of course it would be a little heavier).

Again apologies if the above sounds a bit nerdy, but sometimes buying expensive materials doesn’t always have the outcome you’re hoping for! BG is absolutely right; if David’s B rig mast has a large section (diameter/wall thickness) and is well located in the boat, it may be adequately stiff without spreaders. It just needs to be tried out on the water.

 

 

  • Like 1
Posted

Wow, Martin!

From all I’ve read, it seems possible that my B rig was never designed to have spreaders. 
Given that, I plan to set the boat up with the mast in the upright position, with equal tension on both shrouds. …..and a bit of rake. 
As a point of interest, have IOMs, from the get-go, always had spreaders?

Posted

Some interesting points Martin.

I am certainly no metallurgist, nor am i an engineer, more a pupil of the school of experience, hard knocks and weeding out what can be relevant or put in the bin.

I understand the pricipals of pairing tubes of same wall thickness and diameter to give a near identical stiffness over a given length. Smarter minds say this is so. Over experience though through a past dinghy and skiff lifetime and on to transitions of mast types in IOMs, i believe all is not that simple in practice or actual use. Age hardened mast or heat hardened or tempered? Do they bahave the same on a Cherub with all else equal in dimensions? Not at all. 18 foot skiff masts in drawn state over the equal section spending a time being heat hardened. I can tell you one had a proponency to fall down, the other was spaghetti on identical geometry setups.

i have found the differences in IOM tubes just as noticably different. Some maybe be harder than others. Some much stiffer. Same measurable OD and ID. 
the largest noticable difference in the harder more commonly used 11mm tubes is their resistance to relaxation of pre bend. I do find them naturally stiffer for the same given geometry of rig setup, certainly over a longer period. Equally, some offered as harder tubes are quite dissapointing and overly flexible. Just my take as its the lightest masts being sought.

Not disagreeing with the science, but in the same way a hulls best VPP is in real practice, not figures, i think there is always an element of experienced trial and error to find what works best.

Cheers

  • Like 1
Posted
9 hours ago, David Norris said:

Wow, Martin!

From all I’ve read, it seems possible that my B rig was never designed to have spreaders. 
Given that, I plan to set the boat up with the mast in the upright position, with equal tension on both shrouds. …..and a bit of rake. 
As a point of interest, have IOMs, from the get-go, always had spreaders?

Hi David, I’m afraid I don’t have enough knowledge of IOM history to know whether they had spreaders from the get-go, but the class rules list them as optional items, rather than mandatory. The B rig mast is quite a bit shorter, so the bending of the mast would be less than for an A rig. I would think the location of the mast in the hull is quite important - especially restricting any free movement at deck level, as this can make a big difference in how much the mast will bend between its base and the shroud points.

Posted

Hi Brad, thanks for your reply. Yes, the behaviour of masts and rigs is a complex topic. My background in ship science and a lifetime in naval architecture does lead me more towards the science and engineering (and not always for the better!); and I totally agree there’s no substitute for hard-earned experience.

Aluminium alloy is a complicated material to use and the hardening process complicates it further. As far as I know hardening doesn’t change the elasticity (Youngs modulus), but it can significantly increase the strength. So a hardened section doesn’t become stiffer, though it does become stronger. Any aircraft engineers out there might be able to help on that one - they’ll know quite a bit more about aluminium than a naval architect!

There are several different manufacturing processes for tubes and mast sections, and production varies a lot. Manufacturing tolerances also lead to variations. The hardening process is also very variable and these together can lead to uncertain qualities of tube. In ship construction it’s standard for structural materials to be tested and certified against set standards such as classification society rules, so that the material properties are categorised and verified giving confidence in the materials used. Of course we don’t have that level of information for dinghy or model yacht components, so it’s a bit of a minefield.

I’ll stop now. Sorry if I rambled a bit off the original topic. There’s a lot more to add about cold work hardening and other thoughts, but that won’t help anyone’s boat go faster! Time on the water is a much better idea.

  • Like 1
Posted

There are several engineering principals that apply here. The “If it looks right, it probably is” theory is the main one. The S.I.A.S. (Suck it and see) technique refers to variations on a fundamental. The “Belt, braces and a piece of string” method of trouser support is always useful in the absence of supporting data. The “Law of diminishing returns” is the one that most applies in this case as the chance of improving your boats performance by adding something that was probably not there originally is negligible. 
Cheers

Trevor

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