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Shroud attachment position?


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I am building a new mast for my MX14 IOM and the old one had become damaged. I am using a PG tube from potter solutions.

I have managed to gain a nice pre-bend and am marking out the positions for fittings.

There appears to be two schools of thought about the best position for the shroud attachment. Brad Gibson's rig build document recommends a single forward attachment point, where Sailsetc rig guide suggests the sides of the mast. Others rig builders also have differing views.

So before I set drill to mast what are the benefits or negative points of each? On my full size yacht the shrouds were always on the side of the mast.

Thanks Michael

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2 hours ago, Michael Thomas said:

I am building a new mast for my MX14 IOM and the old one had become damaged. I am using a PG tube from potter solutions.

I have managed to gain a nice pre-bend and am marking out the positions for fittings.

There appears to be two schools of thought about the best position for the shroud attachment. Brad Gibson's rig build document recommends a single forward attachment point, where Sailsetc rig guide suggests the sides of the mast. Others rig builders also have differing views.

So before I set drill to mast what are the benefits or negative points of each? On my full size yacht the shrouds were always on the side of the mast.

Thanks Michael

Michael, there is only one school of thought for a PG mast and that is a single hole in the front.

The hole in the front still offers the same support athwartships as a hole in the each side however it does limit excessive fore and aft bend.

The other thing to bear in mind that the weakest link in any alloy tube or cause of failure down the line will be due to holes in the mast tube and corrosion.

The long and short being the less holes you drill in the tube the less likely there will be a failure.

A single hole for the shrouds is the current status quo for the majority of the IOM class, BG's mast layout is the adopted norm now for most top skippers.

http://www.bgsailsanddesign.com/toptips.html

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I have managed to gain a nice pre-bend

Years ago (when I sailed high performance dinghies) there was a politically incorrect joke about someone froma certain part of Great Britain sending his mast back to Proctor's to have some pre-bend put in it.  The joke being that pre-bend wasn't about putting a static bend in the mast but using the ring tension and standing rig set-uo to bend it according to the wind strength - under no load the mast is still straight. This is not the first time I've heard of IOMs using masts that have a static bend.  I really don't see the point in this.  If you need to bend the mast that much to get a good sail shape then I would think that the sail has been made with too much luff round.

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1 hour ago, John949 said:

Years ago (when I sailed high performance dinghies) there was a politically incorrect joke about someone froma certain part of Great Britain sending his mast back to Proctor's to have some pre-bend put in it.  The joke being that pre-bend wasn't about putting a static bend in the mast but using the ring tension and standing rig set-uo to bend it according to the wind strength - under no load the mast is still straight. This is not the first time I've heard of IOMs using masts that have a static bend.  I really don't see the point in this.  If you need to bend the mast that much to get a good sail shape then I would think that the sail has been made with too much luff round.

I'm with you... put pre-bend in with the rig set up!!!

Larry

IOM sailor

North Essex

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13 hours ago, Darin Ballington said:

Pre bend in an IOM mast is to counteract the limited strength of the material allowed in the class rules.

By putting in pre bend you can achieve a reasonable amount of rig tension and match the luff curve of the sail.

I'd be interested to see the metallurgical analysis of the effects of rolling in a bend rather than relying on the elastic properties of the alloy

 

Larry

IOM sailor

North Essex

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Both Darin and Gavin are correct in the need for pre bend. What might not be clear is that the talk of pre bend in an IOM rig or similar is not what would normally thought of in a full size rig. Simply, light wall IOM masts on their own are not stiff enough to give enough forestay tension without a significant amount of backstay resulting in a large bow forward in the mast centre. To match a sail luff curve to this would give you reasonable upwind performance but a terrible downwind performance with an incredibly tight leech and a huge pile of luff bunched against the mast.

A straight, or near to straight setup fore and aft under tension gives best performance both upwind and down. To get that, forward pre bend is added to the tube section prior to rigging. Depending on tube used and mainsail luff cut, amounts from suppliers will differ.

I have found that with any style of tube used that the hole in the front offers the ability to add, through shroud tension, better fore and aft support to the top of the mast. When placed at the correct height in tandem with the spreaders, this allows the bare minimum of pre bend to be added, giving better adjustability through a rig range.

i.e. If the wind goes light you can ease tension in the rig for a softer jib luff setup and feel without mainsail distortion, or alternatively firm up the rig for the upper range for a firm forestay and flattened sails. This is something that excessive, or zero pre bend can not achieve, easy to setup and just works.

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Hi Guzzilazz, there is no need for the complexity of metallurgical analysis here; it is simply a question of mechanics. The pre bend requires  backstay to just to bring it straightish and this is the starting point. The forestay tension to get to this point alone is the bonus that pre bend gives and the reason it is so important. More pre bend = more forestay tension for a given luff shape plotted down to less tension for less.

We are stuck with a very flexible alloy tube and it is very important to match the pre bend to the sailmakers luff profile and this is not easy. Get it in the wrong place and you will have problems. That is a good reason to ask the sailmaker, or get them to do it, what the mast bend position and amount should be.

Like so many settings, there are no empiric values and settings provided by experienced sailors will not necessarily suit all; but are invaluable as starting points. Personally I prefer side shrouds , but have yet to win the World Championship of course.

Experimentation is great fun and the best way to understand the value of any adjustments.

 

Richard

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I'm never quite sure about matching mast to sail. Brad's post above advises a straight or near staight mast but in his tuning notes he says bend the mast to suit the mainsail luff with a gentle curve. I allways thought that the luff curve was there to push some camber into the sail along with broad seams. If this is the case why take out the luff curve by bending the mast or is the luff curve there to accomodate the predicted mast bend similar to putting a concave curve in the jib to allow for forestay sag?

I guess the obvious answer is to just tweek the rig until the sail looks nice with no folds or creases.

Roger Threadingham 

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Apologies for any confusion. You pretty much answer your question when you mention bending the mast to suit the luff cut in the sail. Now if a luff has say 8-9mm of static luff curve cut into it on a flat table, then it will reduce when set in its flying shape by a few millimetres, dependent on how much seam shaping and where. That sail matched to a mast would result in a pretty straight mast setup by my reasoning, with the slight mast bend to suit.

You can induce small amounts of depth through mast bend but your base setting will be matching the luff cut and letting the seam shape do the work, with small tweaks either side of this. As a reference my backstay on an IOM A rig will not have more than 2-3mm adjustment across its sailing range. Getting an IOM rig to work well on any design is not difficult. Your sailmaker is the best person to talk to on getting the best rig setup to suit their cut of sail. Notes as to why I set mine the way I do and reasoning can be found on a link Gavin posted above. Other manufacturers may prefer a different setup.

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Having started this thread I have now finished building the rig and fitted it to the boat today.

I used Frank Russel's pre-bend suggestion of 12 mm for the A rig and Brad's mast building guide measurements.

However when I rigged the boat and set everything up I noticed that looking down the rig from the top, the pre-bend has not straightened out or formed a nice curve to match the luff of the main sail in the top section. I have a put on a lot of backstay tension but can't get the mast to form a nice even curve.

Now I bought two masts at the outset and although I used the recommended pre-bend I'm using a PG mast which I understand is considerably stiffer than most available. Although the sails appear to set nicely I'm tempted to build a new mast without pre-bend just to see how it sets.

Should I do this or just wait and see how the boat sails?

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Hi Michael,

My first question would be how far down the mast does your pre bend go? Then, is this a gentle even curve of pre bend without any hard points. It is very important that pre bend be a smooth transition from the bottom section to then gently curve forward. For an A rig our standard has been at approx 12-14mm of pre bend with it starting 600mm down from the top of the mast.

Now if the above is somewhere similar to what you have, then I would say you are over bending through the lower section with backstay applied as the bend point is transferring down to the non pre bent section that will have less resistance to bending.

You need to bring the bottom section back in column with both your mast ram and side stay tension and spreaders. Looking down from the top of the mast, start winding the mast ram on so that it pushes the lower mast backwards. If your side stays are loose, firm these up a little so that when the boat is on its side the underneath one still has some tension.

Your spreaders should start set square through the mast and if all is correct, end up with the slightest sweep aft by a degree or two as a last adjustment.

Essentially you need to straighten the lower mast through your controls that will then force the top to start bending. Getting all of the controls working together, then matching the sail is the key, and pretty much the difference between a nice rig and a fast one.

One final experiment. Take a drinking straw and apply gentle pressure on it by standing it vertically and pushing down with a finger from the top. The straw will eventually bow out of column and needs ‘shrouds’ to support it. Where it bows and buckles is determined by how and where the shrouds and supports are fitted and how they are tensioned to support your fingers increased push on it...

By all means send me a picture down your mast if unsure but I’m certain your rig is within a careful trim. An unbent mast will not be  up to the task.

Cheers

Brad

Edited by Brad Gibson
Correction
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Thanks Brad.

I'll have another go tomorrow, mast pre-bend starts at about 600 mm from the top, spreaders are square and in tension so everything is about as you suggest apart from the ram which I set up after setting the downwind sheet positions and vang to give an even twist to the leech of the main in the close hauled setting. I'll reset this tomorrow with more ram and send a photo if I still can't get it to work. Thanks for your advice. I'm sure many will read it with interest.

Michael

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Hi Michael, just to add, if running a standard body gooseneck you will need to add 2 x 5mm wide deck patch strips across the bottom of the gooseneck body between it and the mast to offset the axis point on an A rig set to our spec. Standard axis is fine for B and C. This will give you the correct leech set from run to beat with the mast bend sorted.

Cheers

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I can confirm that what Brad recommends does work. I used his settings for standing rigging and bent the mast accordingly. Then a little trial and error with the ram and shrouds produced the desired result. Boat "Alternative" Brad Gibson, Sails BG.

Can only reiterate that pre bend is essential and will not necessarily be the same values for different sailmakers sails and rig set up.

Richard

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Thanks Brad & Richard.

I'm actually using one of Dave Potters goosenecks which have an adjustable axis. I'll adjust it a little to change the axis. Busy at the moment but will try again later today or tomorrow. Thanks again.

 

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  • 2 weeks later...

Hi, one of the hardest things to appreciate when 1st sailing Radio boats is the point that Brad touched on, which is the lightness of adjustments- no need for an arm full of extra tension, or massive changes in rake. Once you have found a sweet spot, mark the positions and come back to these during an event. The strength of the IOM class is in the skill of rig setting and it takes time and practice to get this right on a regular basis. This is another reason for looking at how you build the rigs and rigging, in particular pay attention to the direction of pull on bowsies, you want to be able to adjust these by 2-3mm easily whilst holding the boat or dealing with noisy flapping sails not having to have two people hold the boat whilst you adjust in 10mm chunks!

Finally amongst his many skills if you look at Brad’s boats on the water it doesn’t matter if he has changed rigs 3 times in 3 races his settings are always good and repeatable, although I usually only see them from behind 😉

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