Cat hull design

[WildHare]

I'm an amateur boat builder and am planning on building a small coastal cruising cat. I purchased plans and began construction on an 18 ft cat, but then recently came across plans (of similar size)that appear to have much more comfortable accomadations and a much greater weight capacity. I am considering scrapping the current project and beginning again.The thing is, the hulls on this new boat are asymmetical. These are in fact what aids to the roomieness of the plans--hulls that are large enough to be slept in comfortably, more weight capacity, and a longer distance between the keels that lends to a longer boat that can still be legally trailered.

I know that this style hull appears to be "yesterday's technology" but I have not been able to find much information on the pros and cons of symmetrical hulls vs. asymmetrical hulls. I would greatly appreciate it if someone could explain some of pros and cons of both style hulls or direct me to some helpful info. Thanks!

[grob]

The point of an asymmetric design is that it is a hull shape that can go upwind without needing a keel or daggerboard, this is good for a boat that wants to operate easily off a beach, but a boat without daggerboards is not as efficient upwind as a boat with daggerboards. It then depends how much of a performance loss you are willing to tolerate.

Your specific question was about asymmetric hulls versus symmetric hulls. And here the performance distintion is probably not so great, there are other factors which are probably more important.

There is no doubt that some hull shapes are faster than others, It has often been stated that assymetric hulls are slower than symmetric ones and this is certainly true in some circumstances. But you must be careful that you are comparing apples with apples. You often get this statement when people are comparing an assymetric hobie 16 with more modern designs. However the things that make a H16 slow compared to modern cat designs is not necassarily the asymmetric hull shape, it is the lack of dagger board and heavy weight. The H16 is one of the fastest cats on a broad reach.

To be honest on a cruising boat out and out speed probably won't be the deciding factor and one of the main reason that asymmetric hulls are "yesterday's technology" is that you cannot get both hulls from the same mold so they are expensive.

Gareth
www.fourhulls.com

[Toby P]

Asymmetric hulls only work if the windward hull is flying (or at least, is more out of the water than the leeward one). If the boat is dead flat, the asymmetry of the 2 hulls cancels each other out leaving no benefit. The idea is that the curved hull acts as a foil to produces lift to windward, thereby negating the need for centreboards. However, the efficiency of asymmetric hulls is very low due to their low asect ratio. Centreboards are MUCH better for sailing to windard.
The Hobie 16 hulls are quite narrow and therefore have a reasonable amount of draught. I suspect they work by acting as skegs more than lifting foils, in which case the Dart/Topcat style hull would be simpler.
As you are planning on cruising two thoughts come to mind. One, you are unlikely to be flying a hull, therefore there will certainly be no advantage to asymmetric hulls. Secondly, you may not need to do much windward sailing, therefore centreboards may be unnecessary (but you would still need skegs).
BTW, accomodation on an 18' cat? Must be very tubby.

[Toby P]

The above notes only refer to the sailing ability of the boat. If the asymmetric hulls allow for more accomodation, this may be an over riding factor for you. it depends on your priorities. The extra width between keels will also increase stability marginally, all other things being equal (which they probably aren't e.g. displacement, sail area etc).

[JonathanCole]

Originally Posted by Toby P

The above notes only refer to the sailing ability of the boat. If the asymmetric hulls allow for more accomodation, this may be an over riding factor for you. it depends on your priorities. The extra width between keels will also increase stability marginally, all other things being equal (which they probably aren't e.g. displacement, sail area etc).

If the line of the inside of the hulls is straighter, than the outside, then it stands to reason (I think) that a larger bow wave is formed on the outside and a smaller wave on the inside of the hulls. This may help to prevent wave to hull interference between the bow wave and the opposite hull. Of course this interference factor would only be a factor with both hulls in the water.

[grob]

Originally Posted by Toby P

Asymmetric hulls only work if the windward hull is flying (or at least, is more out of the water than the leeward one). If the boat is dead flat, the asymmetry of the 2 hulls cancels each other out leaving no benefit.

With the greatest respect Toby, this isn't true and easily proved, hire a H16 on a light wind day, put your weight on the upwind hull to sail the boat flat, even if the boat is leaning slightly to windward you can still sail upwind. its not as good but it proves the point.

Originally Posted by Toby P

However, the efficiency of asymmetric hulls is very low due to their low asect ratio. Centreboards are MUCH better for sailing to windard.

But this is also true of symmetric hulls without centreboards also, that was my point.

There are many features that generate lift, different hull shapes do it in different ways. Traditional designs use centreboards, skegs or asymmetric hulls. But there are cats around that have none of these and still go well up wind. The G cat is a symmetric hulled boat that does not have any of these features nor does my own four hulled boat. see www.fourhulls.com. There is no doubt that the centreboard or daggerboard is the most efficient because of the high aspect ratio, but don't dismiss asymmetric out of hand. The best you can do is find someone who has the boats you are considering building and try them out.

Gareth
H16 owner

[grob]

Originally Posted by JonathanCole

If the line of the inside of the hulls is straighter, than the outside, then it stands to reason (I think) that a larger bow wave is formed on the outside and a smaller wave on the inside of the hulls. This may help to prevent wave to hull interference between the bow wave and the opposite hull. Of course this interference factor would only be a factor with both hulls in the water.

Good point I wish Michlet could handle assymetric hulls so this could be looked at.

Gareth

[WildHare]

Thanks for your replies, but I should clarify the shape of the hulls. They are actually flatter on the outside and curved on the inside, the opposite of the asymmetical hulls I've seen. This is what concerns me. If a boat like this had ever been built, I would try it out. Is there a program where I can attempt to "test" these hulls?

[fhrussell]

WildHare,
This IS a sailing cat, correct? If so, then yes, the flatter side is the outer side and the curved side will be inside.... like a Hobie 16. This is the configuration for sailing cats. It is the opposite for powerboats if they are assymetric.
Take a look at CSK cats and the early cats on Waikiki by Woody Brown; they were all assymetric and probably the earliest examples of modern asymetric hull design. Also look at K-designes for more info:

http://www.ikarus342000.com/Catpage.htm

Here's another thread you should read:

http://boatdesign.net/forums/showthread.php?t=14269

[rxcomposite]

Originally Posted by WildHare

They are actually flatter on the outside and curved on the inside,

Jon Cole "thinks" but he knows he is correct. A flatter hull on the outside would create little disturbance on the shore. This might be the rationale for this. The inner wave would cancel each other at certain speed and certain hull spacing. Leo Lazauskas has a good paper on this.

Is there a program where I can attempt to "test" these hulls?

Try Michlet. I am not sure if it could "test" assymetrical hull but it should be close.