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UNCIVILIZED · 05-12-2014, 23:14

Quote:

Originally Posted by magentawave

I have never sailed on an Iriquois but used to see one sail and they look nice and I know they are built like brick-sh#t houses. The capsize thing must have been an issue because you can still see a few out there with the float at the top of the mast. Maybe they've had a bad wrap, you know? So it would be interesting to read each account of why they capsized.

Does anyone know what the ultimate stability of a narrow beam Iriquois with its relatively small sail area would be verses a super wide cat like a Shuttleworth with its massively huge sail area? I mean, wide beam is good but at some point there has to be a tradeoff when the boat has so much sail area. I'm sure some computer nerd could run the numbers and come up with something.

It's pretty simple math really. Odds are there's an explanation on Shuttleworth's site, which is a site that any would be multihull owner should read in DEPTH, thrice.
But you just compare the stability numbers of the cat or tri vs the heeling moment of it's sailplan.

The stability (aks Righting Moment) is the vessel's weight x (1/2 beam - 1/2 a hull's width).

And the simplified version of the force of the sailplan trying to flip her is:
Sail Area x Height of Center of Effort x Wind Pressure on the Sails.

The latter being: Sail Area (in sqft) x Wind Speed Squared (in knots) x .00432 = Pounds per sqft exerted by the wind.

There are other factors which play into it of course, but assuming that they're about the same, on the two vessels in question, that bit of math should do it. But if you're truly curious about multi's, go & read Shuttleworth's articles, it's interesting & educational stuff.

Also, keep in mind, for the most part he, & thus his boats, come from/are designed for performance. They're not slugs, like 90% of the multi's out there.
I mean most of the production cats nowadays weigh at least time & a half of mono's of the same length, despite the mono's hauling around several tons of lead. Let alone the windage they have, UGH!

UNCIVILIZED · 05-12-2014, 23:27

Sorry, didn't quite catch this one in time, as it went up when I was typing my other post.

Quote:

Originally Posted by ZULU40

I seem to remember these as centerboard boats, and that this contributed to its capsize history which it certainly has.

Another McAlphine Downie boat was the 32ft Comanche, which I have a bit of a soft spot for, but somewhat rare.

On a catamaran, if you have the leeward board down (be it a daggerboard, or centerboard), & you're suddenly overpowered, the board prevents the vessel from sliding sideways to bleed off the extra wind force, & thus over you go.
- Assuming that is, that some idiot cleated the sheets down in gusty conditions, while flying too much sail.

If, under the same circumstances, you only have the weather centerboard down, then when the same gust hits, the weather hull & especially, board, lift clear of the water & the boat is able to slip sideways. Thus, in a fair number of incidences, avoiding a capsize.

Though the narrow beam of those old multi's surely didn't help things regardless.

ZULU40 · 05-12-2014, 23:28

Quote:

Originally Posted by UNCIVILIZED

It's pretty simple math really. Odds are there's an explanation on Shuttleworth's site, which is a site that any would be multihull owner should read in DEPTH, thrice.
But you just compare the stability numbers of the cat or tri vs the heeling moment of it's sailplan.

The stability (aks Righting Moment) is the vessel's weight x (1/2 beam - 1/2 a hull's width).

And the simplified version of the force of the sailplan trying to flip her is:
Sail Area x Height of Center of Effort x Wind Pressure on the Sails.

The latter being: Sail Area (in sqft) x Wind Speed Squared (in knots) x .00432 = Pounds per sqft exerted by the wind.

There are other factors which play into it of course, but assuming that they're about the same, on the two vessels in question, that bit of math should do it. But if you're truly curious about multi's, go & read Shuttleworth's articles, it's interesting & educational stuff.

Also, keep in mind, for the most part he, & thus his boats, come from/are designed for performance. They're not slugs, like 90% of the multi's out there.
I mean most of the production cats nowadays weigh at least time & a half of mono's of the same length, despite the mono's hauling around several tons of lead. Let alone the windage they have, UGH!

which is all true of course
but there is a historical record too, and thats something to keep ones eye on. It makes up for the quirky things like variable geometry (keels etc) that need multiple mathematical accounts and a greater understanding of the mechanics at play.

There is one thing going for older plastic boats too. Back then the basis of a quantifiable measure of the strength of composites and plastics didnt exist in the way it does now. As a result most things seen through the looking glass of toady were overbuilt where they needed to be. Where nowdays it would be a calculated strength X a safety factor, and that be that.

magentawave · 06-12-2014, 10:47

Nice boat and CONGRATULATIONS!

Quote:

Originally Posted by Catamoron

I just started browsing this thread tonight, and then I had a thought: I wonder if my boat is actually listed on this thread as a "Cheap Multis and Project"?

And what do you know - there it was talked about for the first time by Sand Crab on entry #636. The date is even right, because I saw the listing the same day, and within about two days we had an offer in on it and ended up buying it as-is, where-is.

If you want to go back and see everything that was done to this boat over the past year getting it ready for the water, check out my blog at Catchin' Rays I even have a cost break-down on one of my entries.

And we just splashed her 2 weeks ago and am writing this entry from the saloon. Do I think this crappy old boat was worth it? Absolutely! We've taken this old and forgotten boat and transformed her into a beauty.

Here is the link to the entire video documentation that includes 5 refit episodes and a couple other how-to videos and also includes my most recent splash video.

https://www.youtube.com/user/kevtamus

magentawave · 06-12-2014, 10:51

Huh?! Why would boards contribute to capsize?

Quote:

Originally Posted by ZULU40

I seem to remember these as centerboard boats, and that this contributed to its capsize history which it certainly has.

Another McAlphine Downie boat was the 32ft Comanche, which I have a bit of a soft spot for, but somewhat rare.

ZULU40 · 06-12-2014, 10:57

Quote:

Originally Posted by magentawave

Huh?! Why would boards contribute to capsize?

a lowered board can trip the boat over
see
http://www.cruisersforum.com/forums/...ml#post1693903

clockwork orange · 06-12-2014, 10:58

Quote:

Originally Posted by magentawave

Huh?! Why would boards contribute to capsize?

Read post 1202.

magentawave · 06-12-2014, 11:19

Yes of course centerboards and daggerboards UP allow the boat to side slip but I'd still like to hear back from ZULU40 as to why he thinks boards can contribute to capsize.

EDIT: Sorry but we keep overlapping here so I didn't see ZULU40's latest reply!

Quote:

Originally Posted by UNCIVILIZED

Sorry, didn't quite catch this one in time, as it went up when I was typing my other post.

On a catamaran, if you have the leeward board down (be it a daggerboard, or centerboard), & you're suddenly overpowered, the board prevents the vessel from sliding sideways to bleed off the extra wind force, & thus over you go.
- Assuming that is, that some idiot cleated the sheets down in gusty conditions, while flying too much sail.

If, under the same circumstances, you only have the weather centerboard down, then when the same gust hits, the weather hull & especially, board, lift clear of the water & the boat is able to slip sideways. Thus, in a fair number of incidences, avoiding a capsize.

Though the narrow beam of those old multi's surely didn't help things regardless.

magentawave · 06-12-2014, 11:28

Sure a lowered board could trip the boat but having the ability to lower and raise a board can make the difference which is something you can't do with fixed keels. A cat without keels or boards will be a dog when it comes to pointing so what am I missing here?

Quote:

Originally Posted by ZULU40

a lowered board can trip the boat over
see
http://www.cruisersforum.com/forums/...ml#post1693903

ZULU40 · 06-12-2014, 12:01

Quote:

Originally Posted by magentawave

Sure a lowered board could trip the boat but having the ability to lower and raise a board can make the difference which is something you can't do with fixed keels. A cat without keels or boards will be a dog when it comes to pointing so what am I missing here?

Im not certain, Im not sure anyone is or could be ...

as we are all aware, variable geometry keels bring with them benefits that full keels are without. But I think perhaps the fully down higher aspect ratio boards may give it so much less lateral slip than that with longitudinal keels that they experience 'or are prone to' tripping. Higher aspect ratios bring with them cleaner flow, and they travel deeper in the water. Their grip would be so much greater per surface area as to overcome forces to drift laterally.

Take a scenario with confused seas, a frightened inexperienced crew, what if the keels are correctly configured but the boat suddenly gets caught aback. The instance described in our friends post becomes immediately apparent and great peril potentially ensues.

Sure the beam is unfashionably narrow, and the rig quite high, but the record suggests that something is amiss with these boats that has been catching people out.

ZULU40 · 06-12-2014, 12:07

Quote:

Originally Posted by magentawave

EDIT: Sorry but we keep overlapping here so I didn't see ZULU40's latest reply!

I think because Im a new poster, Im on some sort of time delay in case I have some dastardly mission to do God knows what

Therapy · 06-12-2014, 15:36

Quote:

Originally Posted by ZULU40

I think because Im a new poster, Im on some sort of time delay in case I have some dastardly mission to do God knows what

Nah. It's your location. Everything has to be inverted and or reversed so it comes up right on your screen. Sometimes it takes a bit.

**s/v Beth** · 06-12-2014, 15:53

We can't be reading stuff upside down here in North America ya know....
Besides, who can trust a person where the toilet swirls in the wrong direction.