Skinny Class 40

The low rider scow Moths died out because the skiffs were a better all round proposition.I think what needs to be considered in the context of this discussion is the amount of wetted surface as an Open 40 heels as the chine digs in and the windward section of the bottom lifts clear of the water.It has to help and in addition,the point around which the boat heels moves to leeward,which automatically increases the righting moment.Perhaps it might be easier to visualise as a catamaran having the bridge deck at the keel line.The Merlin Rocket analogy describes the situation quite nicely as the weight of the hull,ballast and crew are all to windward of the waterline.

 

My sailmaker friend has a Moth like that, its almost all stern, and i had never seen so much string to pull on a small boat. Much fun, apparently, but not my idea of sailing if one has to wear a wetsuit..

 

I think in the last 40 years I have sailed without a wetsuit on about six occasions.It beats getting cold and I prefer it to a drysuit for flexibility.I don't like the idea of three or four layers beneath traditional oilskins.Which leads to another advantage of a wider hull;when it heels,the occupants are lifted a bit higher and should remain dryer.I know a man who used to have a Tumlaren and he described it as about as close to a sailing submarine as you can get.

 

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Thats funny, as the same friend also had a Tumlaren. Sailmakers seem to gravitate to boats that perform well given the appropriate rags.

I much prefer a full on hard cockpit dodger/canopy if not similar to the latest IMOCAs. Getting cold and wet was no fun back in the day, thought i expect modern oilies have come a long way, i prefer to stay dry, stay warm, stay alert.

 

The performance of the DSS-assisted version is really amazing (at least my VPP says so). Without DSS it is not bad, but not as great.

I once found an article about the drag increase due to the heel:
The influence of heel on the bare hull resistance of a sailing yacht | TU Delft Repositories https://repository.tudelft.nl/islandora/object/uuid%3Ad68e748f-6c80-480a-8e21-2bd116332009
It seems that triangular hulls with a flat bottom and vertical sides are relatively robust to the heel (for the price of bigger wetted surface). On contrary, drag of wide hulls with a sharp bow and narrow waterline grows considerable with heel and leeway.

The first approximation is to calculate drag of both halves independently. Then, a 4.5m wide hull of class 40 boat, when heeled by 30deg, can even double its wave-making drag. For such hulls, the flap on the keel should be very beneficial.
The problem is, the rules allow only two movable appendages, so the flap requires a use of a single rudder. With a wide hull, twin rudders are more appropriate - but I guess a single rudder with a sufficiently deep blade could be sufficient.
For such a hull, you need as big as 20deg flap angle to maintain the symmetrical flow around the hull. This is a big value, yet reasonable.

For a skinny hull, say, 3m wide, you don't need twin rudders, the flap needs only 10deg angle, but the benefit is also smaller - several per cent of wave drag.

For scows, the negative effect of heel is much smaller. Actually, you can imagine a hull without such an effect.
BTW, I didn't check a skinny scow hull - may be interesting.
What I played with, are two variants of a "barrel cone" and a "barrel scow". All of them maintain the geometry of the submerged part when heeled up to 30deg. Obviously, for the hulls with sharp bows the submerged part rotates (in "wrong" direction) when heeled. In my scow hull, the submerged part rotates in the opposite direction by a few deg. Look at them (plus a hard-chined, old vande Stadt-like one):

No idea: is slamming better or worse than diving? Modern monohulls are slamming, while multihulls are wave-piercing...

Technically, it is possible. However, the class rules restrict the maximum (not only minimum) righting moment at 90deg heel. On the other hand, the maximum moment is quite big: 323kg x 19m mast height.

regards

krzys

 

It’s never quite straightforward is it? Great post. The smaller spiral does reduce things, but the flap, for example has less %. I’ll have to dig up some dinking around I did on my Vacanti, but IIRR, wave drag for wider hulls was astonishing compared to skinny hulls: friction drag was commensurate with wetted surface.

As far as construction strategies (heavy low, light higher up) this might be interesting, for a slender ish boat

Expedited Delivery: The Longtail 30 - Professional BoatBuilder Magazine https://www.proboat.com/2021/07/longtail-30/

 

Oooh, yeah that Longtail is right up my slipway. Why carbon spars have to come from New Zealand is a surprise though.

When you refer to "flap", are you refering to a trim tab on the trailing edge of the keel?

 

Yep. I’ve been staying at aero on multi element wing stuff, so I forget marine nomenclature from time to time.

 

Yeah no worries. The design i took across the Atlantic had an option for one of those on the build plans when it was orginally designed for the Junior Offshore Group in the mid 60s, but the builders had not done so on the one i bought, and i had only seen three others, also without the flap/tab.

 

A Scows are skinny!

 

Historical pointy bow scows were skinny 2:

Pt 1.14:”A radical departure”: the scow https://sailcraftblog.wordpress.com/2016/10/08/scows-21-sept/

 

That Longtail 30 has reminded me of the Yachting World Diamond, also 30ft.


Diamond Keelboat - Wikipedia

 

Cheap to build for a new single hander with sporty pretensions.

 

Are you speaking of this :
www.balta.fr/Classe 40.html

PS : in 2023 class 40 french rules, daggerboards and lifting devices are not allowed.
In french :
"Nota : les dérives sont interdites. Les plans porteurs, c’est-à-dire tout appendice visant à générer une force verticale, sont interdits."

 

Looks expensive........ I heard RM went out of buisness (plywood yachts) while having a full order book!