Hull Shapes and Behaviour in Heavy Seas

[Lost Horizons]

Sorry, don’t have pearls of wisdom to offer, perhaps only more questions. Cat A means the STIX is above 32. One opinion I remember seeing is that the STIX number should be normalized to the boat length, which if true or close to being true means that larger boats that barely made Cat A are in fact unstable. The other point I don’t quite get is, Cat A means average wave highs of up to 23 feet with maximum heigth twice that value. There was a research that a boat would definitely capsize when hit broadside with a wave with the height half the lenth of the boat. Which means that Cat A certified boats under 46 feet will capsize as soon as the waves are on the beam. This even does not take a hull shape into account.
Anyway, more questions.

[Paul L]

Originally Posted by NevisDog

AVS and STIX combined make up Cat A - Americans don't seem to want to recognise Cat A much so I guess they'd be even less enthused over STIX. Feel free to offer pearls of wisdom though - just don't expect much enthusiasm.

I guess it could because they are "Americans". Could also be that the traditional boats being touted as far superior in this thread often don't have available either STIX or AVS values easily accessible.
See any in this IRC rating list? Know where a better list is?
https://www.ircrating.org/images/sto...web_latest.pdf

[Steve Bean]

This is not a troll or attack, ad hominem or otherwise. It is a technical point. Several pages back someone mentioned "hogging" as the situation when the two ends of a boat are on two adjacent waves. Actually this is properly called
the "sagging moment." The "hogging moment" is when the midship part of the boat is supported by a wave, and the bow and stern unsupported. A vessel that has suffered distortion from this is referred to as "hogged." The extreme case of sagging or hogging is called "breaking," probably resulting in "sinking."

[NevisDog]

Lots of questions - glad to see folks finally paying attention to stability data - it is the most critical aspect of any boat, bar none, when the chips are down, but is often barely recognised when we look at the galley layout, cabin upholstery, and that wide expanse of deck.

LH, you have more info than I have currently; remember that a well-constructed boat is not 'lost' just because she is hit broadside and rolls - with a good AVS she will pop right up; with a marginal AVS you'd better be able to hold your breath in that inverted cockpit for a very, very long time. Larger boats get away with very poor AVS IMO, which is why designers tell us to go to a higher standard. The great thing is that the standard is not a pass/fail - the data must be published (for any boat sold in Europe) so we can compare and decide how low we individually are prepared to go.

The RYA had a fairly extensive list of AVS/STIX a year or two back when I was comparing yachts on the market. A guy in Europe also offers to calculate the data for any boat we have a particular interest in but I no longer have his contact. What I did is look at similar designs to the one I'm interested in and make an educated guess. You can get a feel for different boat types with practice. Be suspicious though if a builder doesn't want to share his data - there are plenty other builders on the market.

[Metal Boat]

Hi all



I am still unsure how to load pictures



just put some pics of a robert perry design into my personal gallery




can I get some comments on the seaworthiness of this hull shape please


cheers

[NevisDog]

Someone here will recognise her but Bob Perry is one of the finest yacht designers ever - I just checked on sailboatdata.com and of course he designed the Valiant 47 and the Tayana 47, two of the best loved cruisers ever (except for that comment earlier about a Tayana having iron encapsulated ballast, not a great idea if correct).

Your boat looks like a totally conventional, conservative cruiser of the type I'd love to own - no excessive beam, long cruising fin and skeg - perfect.

[Metal Boat]

Thanks Nevis Dog



finally found domething practical with pedigree and just a short hop from home




going to get down there quick smart

[JPA Cate]

Metal Boat,

See if you can't get the relevant pictures to this thread, please? Do be aware, there will be differing opinions, from those of us who've sailed fin keeled boats for 30 + years over those who have enjoyed full keeled boats.

Ann

[Metal Boat]

I only have a photobucket account and forum will not let me use it? I dont know how to insert a pic into the thread

[mako]

Originally Posted by NevisDog

- glad to see folks finally paying attention to stability data - it is the most critical aspect of any boat, bar none, when the chips are down, but is often barely recognised when we look at the galley layout, cabin upholstery, and that wide expanse of deck.

Brother, I agree with your statement, but in reality I think that AVS and other stability data is of little importance to the majority of all boat owners. Most boats sit at a dock or are used locally and minimally.

I read earlier in this thread someone mentioned the suggested minimum of 120 degrees for a sailboat. That sounds fantastic, but being a powerboater, the AVS is generally accepted to be about 70 degrees for a passage-maker.

There was a builder who I was very serious about signing with, but his very wide beam fishing trawler designs had an AVS of 50 degrees. That always made me nervous. Once again, in reality you could spend an entire lifetime and never be in a survival situation, but still that marginal level of stability wasn't making me comfortable, especially with intentions for open ocean work.

[Paul L]

Originally Posted by Metal Boat

Hi all



I am still unsure how to load pictures



just put some pics of a robert perry design into my personal gallery




can I get some comments on the seaworthiness of this hull shape please


cheers

Most of Perry's boats of that era are good sailing boats. The Taiwanese metal work is troublesome, so you need to see whats been replaced. Chain plates and tanks are the big ones.
While Perry made his name in the Pacific North West and is still active there, he was born in Australia.
He has, or at least used to have, a service where for a few hundred dollars you could consult with him via email during your search and he would give lots of good and bad on any of his designs.
His book is an interesting read
https://www.amazon.com/Yacht-Design-...s=Robert+perry

[boatman61]

Originally Posted by Metal Boat

I only have a photobucket account and forum will not let me use it? I dont know how to insert a pic into the thread

If you can save/download it to your phone/comp as a file you can upload it that way.

[Metal Boat]

I have the files on my machine and I can upload them to my gallery but cant seem to upload them into the thread. I like this boat it has a good hull shape

[Metal Boat]

[Metal Boat]