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Aluminum boat design and selection

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Aluminum boat design and selection

Messagede Lei Lani » 17 Jan 2010, 12:05

A quite comprehensive article by Colin (wave-action.com) re. aluminum boat design and selection :

The full article can be read here :

A few excerpts :

The benefit of true shoal draft like this opens up many cruising areas, and is ideal for the exploration we have in mind. Not only that, in many wild places, the ability to anchor further in can make a huge difference to comfort and safety in bad weather

the vexed question of the supposedly limited stability of the OVNI was never an issue for us. I simply thought of Jimmy Cornell taking his OVNI 43 Aventura III to the Antarctic and Alaska, and then crewing aboard Igloo, an OVNI 39, to Spitsbergen. I also thought of many other well known high latitude boats, Pelagic, Seal, Parati (to name a few), all lifting keelers that have made equally illustrious voyages.

Nonetheless, I thought it would be interesting to check the numbers. The OVNI 435 has an EU category A (Ocean) rating, and that’s good enough for most people over here. Being no naval architect, the complexity of the formulae needed to come up with comprehensible figures made my head spin, so when I heard about the excellent Sailing USA web pages, for example, where simple data entry can be calculated to give a wide variety of figures that might be pertinent to ocean cruisers, it seemed a good place to start. So using the manufacturer's data from our owner's handbook, the website allowed some basic calculations to be effected.The OVNI 435 has a ballast ratio of 135%—about average for today’s cruising yachts, and has high form stability, as we found when we sailed her home recently. The stability value is 34.15, and the Angle of Vanishing Stability (AVS) is 126.57, within the boundaries deemed acceptable for ocean crossing. The capsize screening value is 1.84, anything less than 2 being considered good. So looking solely at the stability numbers there is little to be overly critical about.

Another factor we’ll be looking to exploit is the ability to lift the centreboard when going downwind, not just to gain extra speed (although that’s good, too) but to avoid “tripping” on the keel and broaching. Many experienced OVNI sailors report lifting the keel in bad weather, allowing the boat to slide when hit by bigger waves, rather like a multihull. Well, if and when we have to face that, we’ll find out how well it works!

But at the end of the day, these are all just figures, and as it’s not unknown for well respected ocean cruisers to get into serious trouble if caught in the wrong place at the wrong time, ...There’s nothing like empirical experience, as the sea soon deals with the complacent, and at the end of the day it’s down to the way the crew handle the boat in so many cases, and staying away from the most dangerous places and conditions if at all possible.
Bon vent à toutes et à tous !
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Re: Aluminum boat design and selection

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Re: Aluminum boat design and selection

Messagede cybelle » 17 Avr 2013, 21:54

and to reinforce the Morganscloud.com statements about (yawn) stability, electrolysis, lifting keels/rudders and that perceived demon, aluminium, in general....have a look at Jimmy Cornell's answers to the usual (predominantly negative) questions about aluminium centreboarders under 'OVNI Yachts FAQ' in Resources section of his site
OVNI 385 No.84 (2004) - Sud Bretagne
ex OVNI 30 (1989)
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