IOM Do It Yourself DESIGN - Page 222 - RC Groups
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Aug 10, 2017, 09:01 PM
Registered User
Crunchy Frog,

Yes, I don't know if you are a fellow IOM guy or not? but for light airs this is really nice. You can reverse the mod in 20 seconds by re-hooking the jib tack lines and fore stay to the original hole in the mast which is slightly below the upper limit mark. As far as loosing the ability to flatten the sail for heavier air you might have a good point there. It would take some 5 or 6 pounds of back stay tension to start the mast bending away from the main with the fore stay attached to the very top. And I don't really know if the residual camber built into the main is too much for anything over about 5MPH winds. That could easily be the case, but you could always hook up the jib in the normal way if that happens.

You can also add or remove camber from the luff by tension on the down haul and the out haul. plus maybe the cunningham to some slight degree, but it is much more normal feeling to me because the mast is not acting like a wet noodle any more.

I was sailing Tornado catamarans for a while and some other less exotic cats, but we never had a back stay. You had to tension the fore stay and that was it. The mast itself was also on a swivel mount. The side stays and the fore stay only went up the mast to the hounds which is about 3/4's the way up. The down haul has a 16:1 advantage and you can bend the mast from the bottom with the tension on the down haul. It works the same way, but probably more of a brute force approach than on the little IOM.

I am still trying to find that sweet spot where the neutral helm starts to show up. It's almost there, but very illusive. With the extra fore stay tension I have now and opening the slot up, it's wanting to fall off the wind again, so the mast wants to be moved farther aft again like where it was before I lengthened the mast box! Arrgh! Anyway, the mast sure is acting much stiffer without all that topping lift one sided bending force and the un equal mounting points of the fractional rig in play.
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Aug 10, 2017, 09:43 PM
Registered User
The problem is in the design I'm afraid. The mast is so small in relation to it's wall thickness and length, that it cannot stay in column (straight) with any sort of back stay tension on it when there is nothing to balance the out of column bending forces of the jack stay or topping lift (whatever you want to call it) except the tensile strength of the mast material itself. * with the possible exception of the mast ram or baby stays?
You need some back stay tension in relation to the wind speed to keep the jib shape under control. But bending the mast permanently to get extra stiffness has it's costs. You have to keep the sails on the rig and tension the rig or take it all apart to protect the sail shape. If you ever slack off the back stay with a bent mast it's going to wreck the main sail shape eventually.

I confess that I am a little on the skeptical side about the reason for the pre-bend and how it came into existence? Was it because the mast was too limber for the application? Or was it to sell more sails and masts? Or maybe some copyright infringement or patent law problem with a competing design? You never know about these things. And no one is going to tell you. So I guess the pre-bend stretches the A rig a little bit more as far as how much wind it can handle without going to the B rig, but I don't think I want to do it to my boat.

Especially when you are still at the mercy of the weather and you may already own a B rig and a C rig. So what are you really saving by not switching rigs?

What do most skippers use for a rule of thumb when they decide to switch rigs down to the B rig and the C rig? I don't think they can rely on the pre-bend to get much above 8 to 10 knots at the very most with the A rig?

And do they also per bend the B rig and the C rig.? If that is the case, then it sounds more like trying to save a couple grams of weight instead of changing the verbage of the class rules. And that eventually gets back to the 4KG minimum which is pretty hard to match and have any chance of building it yourself "on the kitchen table" I don't think so.


I didn't see anything in the rules about it being mandatory or illegal either, so I am at a loss to see why a "one design" boat should have to suffer this fate?

If you want to sail in 20 knots or more you have to go down a size or two anyway. There is no point in trying to de power an A rig that far. It will only tear it up.
Aug 15, 2017, 12:00 PM
"consumerwil"
denisoni1's Avatar

Long boat build


Just thought I'd post this as some techniques would be helpful for IOM Woodies... its a technically super cool project

https://www.bammrc.com/kobus-thai-longboat-build
Aug 15, 2017, 12:21 PM
Registered User
Tillerman, with all due respect, are you new to the class or a seasoned veteran? You are getting advice from people who know the history and understand the boat. The IOM class appears to me to have extremely high quality skippers a great many of whom are very successful big-boat sailors. They don't seem to think that the class is "suffering this fate" as you put it.

It is a fractional rig, but the mast ram (or check stays) have a huge effect on the mast bend. Without a ram, it is likely as others have pointed out that the 1/2" mast may not be sufficient, so the problem may indeed be with your boat. When you tighten the backstay, the whole mast bends evenly causing the boom to lift. The ram on most boats is right in line with the boom, which prevents the boom, vang triangle from rotating. Another way to think of it is the ram changes the mast butt from pinned connection to something more akin to a cantilever.

Btw, there is absolutely no point in suggesting rule changes to the class in this venue. Way too much inertia.
Last edited by lhurt; Aug 15, 2017 at 04:32 PM.
Aug 15, 2017, 12:42 PM
Registered User
hiljoball's Avatar
The mast ram or check stay provides a secondary but vital purpose, other than controlling bend in the lower half of the mast. Its other purpose is to provide a counter force to the pressure of the gooseneck.

When a gust hits the main, it tries to lift the boom and twist off the upper leach. The ram resists that push and allows the vang to hold the boom in a stable position.

Try this - rig your boat and then lift the main boom clew while watching the gooseneck area .

John
Aug 15, 2017, 10:24 PM
Registered User
Quote:
Originally Posted by lhurt
Tillerman, with all due respect, are you new to the class or a seasoned veteran? I am new to the class, but I have been sailing for a long time. Mostly in catatamarans. Never had a mast that would bend forward like this and never had a suggestion to bend it backwards to make the metal work harden to counter act the forward bending. This was my concern not to mention the stress on the sails if the mast is not rigged and the sails are still on it.



You are getting advice from people who know the history and understand the boat. The IOM class appears to me to have extremely high quality skippers a great many of whom are very successful big-boat sailors. They don't seem to think that the class is "suffering this fate" as you put it.

I think there is something wrong if you have to resort to work hardening the metal mast to get it to make the boat perform. So that is my suggestion.
I don't mean to offend anyone.

It is a fractional rig, but the mast ram (or check stays) have a huge effect on the mast bend.

So does rigging the tack of the jib to the tip of the mast!. That fore stay position and the un equal torque on the topping lift are where 99% of the mast bowing forwards is coming from as soon as you apply any rig tension with the back stay, fore stay or even the side stays, or even when the boom lifts. So everything is trying to bend the mast forward.


Without a ram, it is likely as others have pointed out that the 1/2" mast may not be sufficient, so the problem may indeed be with your boat.

When I took a close look at the mast the other day with no strain on it - it has taken on a permanent forward bend of about a 1/2" forwards already.

I never did the pre-bend operation on it yet.

So any and all down forces on the mast just add to this bending moment. I don't run much rig tension anyway, but even 1 pound of back stay tension and it starts to get out of control with the wrinkling of the main and the flattening of the main camber.

When you tighten the backstay, the whole mast bends evenly causing the boom to lift. The ram on most boats is right in line with the boom, which prevents the boom, vang triangle from rotating. Another way to think of it is the ram changes the mast butt from pinned connection to something more akin to a cantilever.

Btw, there is absolutely no point in suggesting rule changes to the class in this venue. Way too much inertia.
I'm not yet a class member, just trying to figure out this boat and trying to keep the sails from being damaged by the bendy mast. And I'm not trying to do anything but make some sense out of this rig.

If a mast ram will help without work hardening the mast itself, then I'm all for it!
Yesterday, 06:57 PM
Registered User
Quote:
Originally Posted by Tillerman6
I'm not yet a class member, just trying to figure out this boat and trying to keep the sails from being damaged by the bendy mast. And I'm not trying to do anything but make some sense out of this rig.

If a mast ram will help without work hardening the mast itself, then I'm all for it!

Way, way back in the day when bendy rigs were all bright and shiny and new, Mike Fletcher -- then head of the Elvstrom loft in Sydney and later for many years Australia's Olympic head coach -- took pity on an ignorant young pup working in the same building and putting together a Contender. He had a model rig to demonstrate to hapless clients how the bendy concept worked and explained it thusly: when the mast is bent forwards it pulls draft out of the sail, effectively flattening it and reducing power. However, at the same time, the position of the maximum chord depth (now reduced) moves aft, greatly increasing drag. To restore it to the correct position, you increase luff tension, usually with a Cunningham eye. There are other complicating factors such as leech tension and twist, but they don't change the basic mechanism i.e to reduce power, flatten the sail and then reduce drag by increasing luff tension.

On fractional rigged yachts from ocean racers to OD's, such as the Etchells, the Soling and the Star, to sport boats, backstay tension is one of the most important controls on the boat and it is usual for it to be constantly adjusted. If IOMs allowed more than two channels, my pick for a third would automatically be the backstay.

I might also add that I think the Contender class can claim to have invented check stays. We very quickly found that without them, vang tension would turn the deck-stepped mast into a financial disaster as in those days none of us had any money for replacements.

Any keel-stepped mast, be it on an IOM or Comanche, needs support at deck level, both fore and aft and laterally. On IOMs a ram is the most efficient way of achieving this as it will also allow adjustment. Work hardening is not and has never been an issue.
Last edited by Emintaka; Yesterday at 07:14 PM.
Today, 03:14 AM
Registered User

Forward mounted winch


Hi guys '
I'm in the middle of building an IOM that calls for forward mounted winch. I need to get the sheets through the mast bulkhead and down the outside of the hull deck. Do any of you fine gentlemen have a simple "cheap" way of turning the lines? Yeah I know simple and cheap don't go together.

Thanks in advance

Bas
Today, 05:41 AM
Steve Landeau IOM USA 12
ibldrc's Avatar
Quote:
Originally Posted by Gregg28
Thanks Frog. I get that. But he seemed to indicate that his problem was with the draft too far forward. "30% constant cord"??? That would indicate a little more bend was needed
Chord is the measurement between leech and luff (leading edge to trailing edge)
Draft is the measurement of the depth of the airfoil (chord thickness, vs chord width)
When the OP mentioned "constant chord", he was essentially saying that the luff has no curve cut into the lower 30% of the sail, so when that part of the mast bends due to backstay, he's losing shape uncontrollably in that area of the sail.