Swimming platform

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Jamestj
Posts: 63
Joined: 25 Aug 2004, 17:50
Location: Building one at the moment

Swimming platform

Post by Jamestj » 07 Sep 2006, 12:54

Most IOMs have a forward rake resembling the swimming platform on a full sized yacht. This means that the backstay does not connect to the extreme rear of the hull and the effective hull length is reduced. Since long boats tend to sail faster than short ones, what is the advantage of a forward sloping transom?

Andy Stevenson
GBR NCA Officer
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Post by Andy Stevenson » 07 Sep 2006, 13:28

Following your theory to extremes, does a boat with no backstay have infinite length or no length at all? Sorry, couldn’t resist that!

As I understand it, it’s water line length that determines ultimate hull speed, that’s not dependant on the attachment point of the backstay.

A sloped transom will give, among other things, less weight. Most IOMs these days seem not to have much of a transom at all, the deck aft of the mast being as dished as possible.

Cheers
Andy Stevenson
"A little pain never hurt anyone!" Sam, aged 11

Jamestj
Posts: 63
Joined: 25 Aug 2004, 17:50
Location: Building one at the moment

Post by Jamestj » 07 Sep 2006, 17:00

Full sized boats with swimming platforms have a backstay which divides so that people can get to the back and so that the wires don't get in the way. The backstay connects either side the back off the hull. I understand that a bifurcated backstay is not permitted under IOM rules for some reason.

Would a flat rather than concave rear deck save more weight?

Would a rear turtle deck not cause less air turbulence and encourage air to lift into the mainsail?

A rear turtle deck would allow water to run off rather than be carried along in the boat.

A backstay attached to the extreme rear of the hull would reduce the need for such a large mast crane to clear the mainsail leach.

A vertical transom would not result in reducing the effective hull length when the boat was healed so the hull speed woud be greater.

The currentv IOMs look very attractive and my suggestions would perhaps make the shape of the transom rather boxy. Are current IOMs designed to look good because the psychology of racing a pretty boat overcomes the advantage of a faster but somewhat ugly boat?

cfwahl
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Post by cfwahl » 08 Sep 2006, 04:53

Most designs I sail with (Ericca, Cockatoo, Disco, Bagheera, various others) actually have a fairly plumb transom viewed from the side, though the concave afterdeck("skiff style") is the norm. The Disco has a thwart across the upper corners of the transom, to which the backstay is attached, raising it nearly to the top of the gunwales. I don't think that the rules would prohibit any fitting on the hull that would raise the anchorage of the backstay, but the structural loads are something to consider.

Some Discos (and others?) have also been seen with thin plastic covers between the gunwales behind the mast, to provide a "wind/water tonneau" of sorts, while still dumping the water from around the low-mounted mast/vang assembly. That's the real reason behind the skiff configuration: so that the booms don't end up 75 mm above the gunwales, which would reduce the wind speed at which one has to change from A to B rig, because of heeling. I suppose if you sail in perennially light winds, mounting the rig higher might be an advantage, but the angle of the backstay would also be steeper.

I know that some of Graham Bantock's hulls (Italiko, Topiko) have a forward-sloping transom, but I'd say that's the exception rather than the rule in IOM hulls. Windage might be a justification (freeboard is not always free), but I really don't think that this is a major performance issue -- might as well do what you think looks fast.
Charles Wahl

Jamestj
Posts: 63
Joined: 25 Aug 2004, 17:50
Location: Building one at the moment

Post by Jamestj » 08 Sep 2006, 09:57

Many thanks.

I don't mind making a boxy hull if it has a theoretical advantage. I can understand that manufacturers would be keen to make the hull look realistic since it attracts buyers. For a one-off this won't be a problem.

Unfortunately we don't live near the coast and inland lakes can be rather sheltered. Raising the boom a few centimetres might be an advantage.

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