Asymmetric hulls

[Trevlyns]

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?

[Raggi_Thor]

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

[Trevlyns]

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

[yipster]

In 1986 we integrated what we call the "Chamfer panel" into the design work for a 10.6m (35') LOA open bridge deck cat which we were designing at the time and which was to become the Alfresco 1060. The chamfer is quite simply a 45û bevel between the bridge deck and the inboard hull side and our original intention was to create a more uniformly stressed structure by minimising the loadings on the hull to bridge join but it had a surprisingly beneficial effect on the motion of the boat in a seaway and significantly improved access into the hulls when applied to bridge deck catamarans. At the time there were two of the Alfresco 1060's building side by side and one of the owners elected to employ the straight bevel (chamfer) as designed while the other rounded it out, effectively creating a large diameter radius between the hull and the bridge. When the two boats were sailing side by side it was possible to detect a distinct difference in the motion of the two boats in short lumpy conditions, with the boat which employed the chamfer as designed demonstrating a more even motion as it encountered wave action. The chamfer panel effectively has a dampening effect on the motion of the boat and this is especially noticeable in quartering and beam seas. When the rising wave encounters the chamfer, the wave acts to lift the boat slightly and if the wave is large or steep enough to strike the underwing it will eventually do so with less force and create a minimum of vertical acceleration to the boat. A steep beam sea is potentially the most uncomfortable sailing environment for a catamaran, with wave forces at 90û to the hull side often creating a sharp jerky motion. The inboard hull side causes most of the problem here as waves are effectively trapped in the corner created by the hull side and the bridge. In this situation the horizontal accelerations are dampened in a similar fashion to the vertical accelerations with the chamfer panel lifting the hull and easing the wave under the leeward hull. The structural form created by the use of the chamfer panel was ideally suited to the integration of composite technology to produce an evenly stressed structure which was also simple and economical to build. Unidirectional glass fibres are laid across the bride deck and splay out down the inboard hull side, thereby removing the highly stressed corner join which is otherwise formed, and effectively distributing the loads from the main bulkhead into the adjoining structure. In 1989 we were commissioned to design the Azure 37 production catamaran (also known as the G37) and in this case the chamfer was a major attribute to the design quite apart from it's structural and sea keeping advantages. The chamfer panel allows the steps into the hull to be moved closer to the hull centreline, thereby making access from the hull into the bridge a reality without having to stoop and without having the upper coach house extend too far across the boat thereby stealing valuable deck space and limiting access to the foredeck.

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

[cleblanc]

Originally Posted by Trevlyns

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?

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/

[Trevlyns]

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

[fhrussell]

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.

[fhrussell]

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.

[Trevlyns]

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.

[Jimbo1490]

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

[Trevlyns]

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.

[fhrussell]

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?

[ron17571]

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.

[Jimbo1490]

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

[ron17571]

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.