Asymmetric hulls

Looking at the P95 plan view at http://www.ikarus342000.com/P95page.htm, - which seems to have quite radically asymmetric hulls - I got to thinking what hydrodynamic effect (if any) would be present because of the Bernoulli Principle. Any thoughts?

 

It could add some lift to minimize leeway, but I think it also adds more resistance than lift generated from keels or centerboards.

 

Thanks for the quick reply! Looking at the larger picture between the hulls, you also have the "pinched tube" effect where pressure would be greater at the bows, but suction greater at the stern. Would that have a performace effect do you think?
Regards
Trevor

 

could also be relevant ( from http://www.graingerdesigns.com.au/considerations.php )

 

I do not think that the Bernoulli effect will come to play as long as there is sufficient water dept under the hulls. The asymmetrical hull shape is designed to provide some lift with a small angle of attack

Also, don't forget that the hulls shape is not as dramatic below the waterline than on the top of the hull. I purchased the plans and I have the line drawings and I even reproduced the drawings with Rhino3D.

I think that the asymmetrical hull shape is an interesting and simple way of preventing leeway. I did sail on Hobie16 and Prindle16 catamaran and I can tell you that there are very fast and competitive in speeds with catamaran* equipped with daggerboard on every point of sail except an upwind beat. I have also sailed other type of beach catamaran with daggerboard like the Nacra 5.2 and with an integrated fin like the Nacra 5.7.
*(I mean with catamaran of the same generation with a similar sailplan)

On catamaran, hulls are usually equipped with LAR (Low aspect ratio) keels and I do not think that a LAR keel would be much more efficient than an asymmetrical hull to prevent leeway.

The asymmetrical hull really simplifies the hull construction and decreases the weight of the hull. (no LAR keel, no centreboard trunk, no centerboard) and you have to consider that this is essentially a quick building boat designed for amateur boatbuilders.

You can come and visit the following Yahoo group
http://groups.yahoo.com/group/k-designs/

 

Thanks to all for your valuable input. I enjoyed your website, Yipster and have picked up on a few more ideas from the articles. Gotta rush back to the drawingboard and make some changes!
Cheers guys

 

99% of the CSK cats were asymetric. The very first ones did not even have daggerboards. Most of the big beachcats you see in vintage Wiakiki photos were designed and built by Woody Brown and Rudy Choy and they all had asymetric hulls, were very fasy, and had shallow draft.

There is no Bernoulli effect taking place. The effect is more about the high pressure on the outboard side of the leeward hull creating lateral resistance. The windward hull is slightly lifted, while the leeward hull is slightly depressed, therefore the lift effect is stronger toward windward. Many designers and critics claim the hulls negate each other when sailing flat, but a heel of only 2 degrees changes that. Besides, a cat sails flat only on those offwind tacks where you would lift the boards anyway, reducing wetted surface.

Another plus of the asymmetric hull is its resistance to broaching in following seas. You should pick up Chris white's book and the new book by Gregor Tarjan.

 

another thought on asymmetric hulls. Talking with Roy Seaman, (son of Warren Seaman (CSK), designer of several Nacra Cats, multiple winner of the Worrell 1000, batten maker, and helmsman on Aikane X5 when they broke the TransPac record) he is not a big fan of asymmetric hulls. His comments were that up to a certain speed they make sense. Aikane X5 was capable of speeds above 30 knots. Roy claims that at about 25 knots the boat would 'shudder'. The Randy Smyth designed rig was very powerful and wanted to push the boat faster than 30 knots, but the hulls, being asymmetric, wouldn't go through the water as efficiently as the rig needed. It was a planing issue...he said the hulls wanted to 'break out', but couldn't because they didn't plane at all, like a D-section hull would ala Tornado, Nacra, etc.

Roy did agree that asymmetric hulls are great for cruising, not having to worry about skegs, boards, etc and having a very shallow draft. Although, you can not load up an asymmetric hull as much as a D-section hull. I, for one, am very interested in Bernd Kohler's 'anti-vortex panels'. What a great way to add lateral resistance without the draft!

One other plus..... Due to the design of an asymmetric hull, a boat realizes more perceived beam in a given length. The centerline of each hull is moved outboard slightly giving a wider centerline to centerline beam than a symmetric hull with the same overall beam.

It's all a trade off. I love the asymmetric design, aesthetically and in practice. In large cruising boats, the interior has a unique hull layout that makes for a very ergonomic hallway in the outboard side; and with the head and all shelving, bunks, etc, occupying the area in the extreme curve of the interior side.

 

Hi and thanks for a different point of view. It's always good to see all aspects. The boat I'm designing is a 26 foot conservatively rigged "coast hopper" so it'll definitely be more of a cruiser than a speed machine. Draft is an important consideration and the lack of appendages will make for hassle-free beaching when required.

 

Asymetric hulls have more wetted surface per unit of buoyancy. They are also much more sluggish in turns, making the handling of the helm much more tricky to avoid ruining a tack. I have found that in a rough quartering sea, I could not tack my Prindle 16 upwind. Repeated retries confirmed that the boat could not tack because it lost too much speed while turning. I was forced to head off and gybe. No such problem with my Prindle 19. It glides through tacks almost like a monohull.

Jimbo

 

Thanks for your point of view, Jimbo – as with all these responses they are greatly valued. Just putting the matter into perspective though, the yacht I am considering is a 26ft cruising cat which would be vastly different from a performance beach cat like the Prindle 16. My understanding is that the underwater profile and section shape – particularly in the forward sections of the hull - would affect the turning properties. The cruiser would have more rocker and flatter sections compared to the Prindle. Nevertheless, your points and practical observations are totally valid and appreciated.

 

It is true that asymmetric hulls are harder to tack than D-sectioned hulls. But, if you're tacking in a heavy sea, it helps to keep the bows down and backwind the headsail until the main fills,..then tack the headsail over. It takes a little getting used to, especially on how you round up. A 'hard over' maneuver usually stalls the boat, so you have to gradually round up, keep the speed, then hard over at the last moment,...and sometimes if the waves are really steep, even reverse rudders if you drift backwards...but that's an extreme.

One observation (and opinion) with beachcats (specifically H16, P16) ... It is more difficult to tack a beachcat because the helmsman must stay aft to swing the tiller extension behind the mainsheet blocks. If the crew (if there is one) isn't right up on the forward crossbeam, the bows are going to ride up high and the boat will never tack in a steep oncoming sea....any thoughts on this?

 

Funny my favorite cat has been the Prindle 16,i never had any problems sailing it any where or way i wanted,my only problem was hull volume(or my weight!)but this was on a lake,i really liked to wait for a front to move in and go out with white caps and hall butt.I guess if it was a problem on a larger boat a bow thruster would help to turn.i mean how often do you actually come about on the ocean.Oh yeah backwinding the jib is how you push the bow over on a prindle 16.

 

I really learned to sail on my Prindle 16 on Lake Murray, SC in the early 80's. Thought that boat could do anything until I took it into the ocean THAT'S when you figure out it has some faults! Great all around beach cat, though and miles ahead of the Hobie that it replaced.

Jimbo

 

Yes the prindle was much better than the bow burying cartwheeling hobie.Ive never sailed on the ocean,i think the bigger it is the better it would be would hold true for safety on the ocean.my neighbor years ago said he thought 600 foot was about as small as he would go on the open ocean(ex navy man)my parents with much sailing exp. rented a hobie 18 in hawaii and couldnt beleave how rough the water was.I like a lars type keel,i think daggerboards on anything else than a pure race boat are a pain in the butt.i think of going right up to the beach.i think of this stuff,mabe some day ill actually exp.it.