IOM Do It Yourself DESIGN - Page 226 - RC Groups
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Sep 12, 2017, 08:03 PM
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just ordered materials to start new build in the next few weeks. Excited!
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Sep 12, 2017, 09:41 PM
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Paul.

No the mast is not constrained by any blocks at the deck level. But since my last post I have devised a turnbuckle arrangement behind the mast which pulls it aft. This has been fairly good as it takes most of the adverse bowing of the mast out of it at least 2/3rds of the way up.

There is a U shaped slider track on the forward deck with blocks in it, but that is short enough that it is not in the way of the mast pocket.

I would have to assume from your response about your own Nimbus that it sails fairly well and is balanced more or less. I am thinking now that I may have to move the fin forwards in relation to the mast so that the rudder does not get overpowered in a blow. Even though my boat is more or less balanced with a slight weather helm, it still kicks to weather if the wind is gusty and you are on a beam reach with the A rig,

So one last question for you Paul. How far forward of the leading edge of your fin is the rear of the mast? (assuming that you could draw and imaginary line vertically down from the rear edge of the mast?
Sep 12, 2017, 09:44 PM
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Paul.

No the mast is not constrained by any blocks at the deck level. But since my last post I have devised a turnbuckle arrangement behind the mast which pulls it aft. This has been fairly good as it takes most of the adverse bowing of the mast out of it at least 2/3rds of the way up.

There is a U shaped slider track on the forward deck with blocks in it, but that is short enough that it is not in the way of the mast pocket.

I would have to assume from your response about your own Nimbus that it sails fairly well and is balanced more or less. I am thinking now that I may have to move the fin forwards in relation to the mast so that the rudder does not get overpowered in a blow. Even though my boat is more or less balanced with a slight weather helm, it still kicks to weather if the wind is gusty and you are on a beam reach with the A rig,

So one last question for you Paul. How far forward of the leading edge of your fin is the rear of the mast? (assuming that you could draw and imaginary line vertically down from the rear edge of the mast?
Sep 13, 2017, 10:49 AM
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Crunchy Frog's Avatar
Most boats seem to have the aft edge of the mast in line with the leading edge of the keel, or very close.

If your boat is balanced in moderate wind, but not in stronger winds, I'd suggest that the keel probably does not need to be moved. You just need to adjust the tuning as the winds increase to reduce the weather helm. More sail twist, open up the main sail sheet 1/2 inch, decrease forestay length to add more mast rake (more commonly required on older hull shapes), flatten both sails with outhauls, etc.
Last edited by Crunchy Frog; Sep 13, 2017 at 04:18 PM. Reason: got mixed up about forestay length increase/decrease
Sep 13, 2017, 11:55 AM
Lucas
Isn't it normal that the helm of a boat changes with heel angle since the center of effort of the sails moves to leeward as heel increases. When well heeled, weather helm develops, when upright, lee helm is present. The reason that the center of effort is forward of the center of lateral resistance is to account for the turning moment that develops with heel.
Sep 13, 2017, 11:42 PM
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Quote:
Originally Posted by Tillerman6
Paul.

I would have to assume from your response about your own Nimbus that it sails fairly well and is balanced more or less. I am thinking now that I may have to move the fin forwards in relation to the mast so that the rudder does not get overpowered in a blow. Even though my boat is more or less balanced with a slight weather helm, it still kicks to weather if the wind is gusty and you are on a beam reach with the A rig,
The changing shape of the hull, as it heels, can also have a marked effect on weather helm. Way back in the day IOM designers were greatly concerned with what was termed "snap weather helm" when a boat on the wind at a certain angle of keel would round up no matter what the skipper did. This was all down to hull shape.

Also, it is normal for any boat of any size to develop weather helm on a reach. The cure is to ease the main sheet, preferably before the end of the main boom hits the water due to the angle of heel, causing the more or less inevitable broach. In other words, balance the helm against sheet pressure. Moving the keel will just bugger the boat on the wind - a much more important leg than reaching, which is pretty much a thing of the past with the predominance of windward/leeward courses anyway.
Last edited by Emintaka; Sep 13, 2017 at 11:48 PM.
Sep 14, 2017, 12:22 PM
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Emintaka

I'm not sure that very many IOM designers were ever at all "greatly concerned" with so-called "snap weather helm". A few designs, and even individual boats, evidently suffered from this, but there were more factors involved than just the hull shape.
Sep 14, 2017, 03:52 PM
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Quote:
Originally Posted by RW1966
Emintaka

I'm not sure that very many IOM designers were ever at all "greatly concerned" with so-called "snap weather helm". A few designs, and even individual boats, evidently suffered from this, but there were more factors involved than just the hull shape.
Well, put it this way: at one time there was a lot of chat about it. And hull shape was the determinant as all the other factors have remained constant up to and including today.
Sep 14, 2017, 04:01 PM
"consumerwil"
denisoni1's Avatar
... maybe because I'm such a newbie but the more I think about IOM, the more I think the displacement hull is the constant, and the combination of sail, keel fin, bulb shape, and rudder are the main determinants. Not picking a fight or disrespecting the comment, but my 1991 Tinto did really well against more modern IOM's just this year... and if I had better fins and sails (and winch), who knows how much better it would have been... there has to be some credit given to the fins as it reacts with the motion of the hull in the water. And chords in sails surely is not constant?

Quote:
Originally Posted by Emintaka
Well, put it this way: at one time there was a lot of chat about it. And hull shape was the determinant as all the other factors have remained constant up to and including today.
Sep 14, 2017, 05:10 PM
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Quote:
Originally Posted by denisoni1
... maybe because I'm such a newbie but the more I think about IOM, the more I think the displacement hull is the constant, and the combination of sail, keel fin, bulb shape, and rudder are the main determinants. Not picking a fight or disrespecting the comment, but my 1991 Tinto did really well against more modern IOM's just this year... and if I had better fins and sails (and winch), who knows how much better it would have been... there has to be some credit given to the fins as it reacts with the motion of the hull in the water. And chords in sails surely is not constant?
I misspoke. Forgive me. I didn't mean that the many other factors involved in IOMs are constants. I meant that they continue to exist in all their glorious variability, and that therefore when talking about snap weather helm it is the hull which is at issue. I do hope I have now made myself clear .
Sep 14, 2017, 05:14 PM
"consumerwil"
denisoni1's Avatar
Thanks for clarifying Emintaka

Quote:
Originally Posted by Emintaka
I misspoke. Forgive me. I didn't mean that the many other factors involved in IOMs are constants. I meant that they continue to exist in all their glorious variability, and that therefore when talking about snap weather helm it is the hull which is at issue. I do hope I have now made myself clear .
Sep 14, 2017, 05:19 PM
Registered User
An extensive (and ongoing) discussion of balance in full-size sailboats can be found on the Boat Design forum:

https://www.boatdesign.net/threads/a...l-boats.59106/

Cheers,

Earl
Sep 14, 2017, 05:33 PM
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Crunchy Frog's Avatar
Graham Bantock says - it's the foils in the water and the foils in the air. The hull shape is negligible.

That said, it's been my experience that modern hull shapes generate less weather helm as they heel. I'm convinced this is due to the roll center being close to, or at, the centerline of the boat.
Sep 14, 2017, 05:45 PM
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Quote:
Originally Posted by Crunchy Frog
Graham Bantock says - it's the foils in the water and the foils in the air. The hull shape is negligible.

That said, it's been my experience that modern hull shapes generate less weather helm as they heel. I'm convinced this is due to the roll center being close to, or at, the centerline of the boat.
So why does Graham keep updating his designs, with considerable success?

I work on the basis of 40 per cent tune, 40 per cent the skipper, and 20 per cent the hull. Meaning that if you can get a 1 per cent improvement in the hull (which is about what the Britpop achieved over existing designs of the time) you're really talking about four-fifths of five-eighths of you know what. Nevertheless, we continue to persist in searching for the elusive break-through as under the IOM rule, the hull is the only thing we can materially change, and that not so much. Anyway, what else is a silly old bugger, who can't sail toy boats any more, let alone real boats, to do with his time?
Sep 15, 2017, 12:03 AM
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Any suggestions? Please take a look!


If anyone has an idea about why this boat wants to spin up into the wind in a puff, please let me know what you think.

The problem is worse if the wind is stronger or if I have the boat on a beam reach . There is also some tendency to nose dive down wind.

The helm is balanced as much as possible, but the helm varies drastically depending on whether you are going up wind or down wind.

The rudder is overpowered in a puff and especially if the boat is on a reach. When this happens and the only way to get control back is to slacken the sheets.

This is a Graham Bantock Nimbus design and the sails are BlackMajic. I am sure it is not a design issue, but I have tried moving the base of the mast all over the place and this position that you see in the picture is about as close to neutral as I can get it.

If the wind is light then everything is OK, but anything over 5 knots or so and the boat wants to spin ip into the wind.


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