rocker

for a planing catamaran hull is it better to have a flat bottom or should i include a lttle rocker, what is the effect on rocker for a planing hull is it just drag?
sean

 

Your term planing catamaran implies that it will be a power boat. Let the keel run straight, with no rocker, for planing power boats. Lightly rockered bottoms can be made to plane but at the cost of wasted power, a peculiar running attitude, and a tendency to porpoise.

If it is to be a sailing boat, use generous rocker, in which case it will not be likely to plane readily. Sailboat sections should be rounded or oval shaped and narrow. Power boat
sections should be wider and more nearly flat in the after sections.

 

ta for that
sean

 

I am interested to know whether in fact it is to be a power cat. I expect so. However I am fascinated by the concept of planing sailing cats and tris.
Rocker is necessary for turning ability particularly for narrow hulls. The position of the max. rocker may be very different depending on whether it is fully planing or semi-planing, and naturally also whether it is a high powered motor cat, or a sailing craft. More info please!

 

yes a powercat as i am nearly finished my 38ft sailing cat, i want something long(6m) and about 2m beam able to plane with 60 hp but not high speed, with assymetric hulls because of single motor installation, i want something that'll get me out to the reef in rough weather, its always rough here. i have drawn up some drawings of frames that i would use to build with as i have not been able to find plans anywhere else
sean

 

Sorry, don't know too much about power cats so better I do not make an opinion on this one. Regards---

 

Why do you want a cat for getting to, and hopefully from, the reef? You have said; " not high speed", "Always rough here". 2 meters is a bit narrow for a 6m length. A well designed cat or monohull of that length, unless carrying a very large load, will go like a bat out of hell with 60HP.

A cat for rough water will need quite a high center deck for rough weather use. A high center will subject you to excess windage, extra sheer height that equates to more weight and expense. Also there are structural implications with high decks connecting the two hulls. Why assymetric? The engine will not care and with assymetric hulls you'll get a boat that handles quirky in rough water.

A good mono will serve your purposes. It will cost less while being more practical, comfortable, handle better, and probably be safer as an all weather boat. Moreover, there a many current monohull designs that that are proven rough water boats. You'll be able to find those designs pretty easily. For example; a Calkins Bartender or something on that order. Bartenders have been running the wicked weather off the US Oregon coast for many years. There are many others with equal talent for rough going.

I have no quarrel with sailing cats if properly designed. They're a hoot to sail and often fast, nearly always more comfortable than a mono of 38 feet length. Cat sailors can rightfully brag about level or nearly level sailing. What's not to like about that. After all it is hard to eat your soup on a keel boat when it is heeled to 25 degrees. I presume that the reef runner is for fishing or diving and will not be required to have facilities to prepare soup.

 

with assymetric hull you get a boat that will lead into the corners like a mono and not have that awkward feeling of being thrown out. also with an assymetric hull their is less turbulence created in the tunnel meaning that the motor has better water to run in.

as for handling the rough water i have fished in a number of different deep vee glass and alloy boats and the catamrans i have been in beat them hands down for comfort in rough water, im not putting down monos of this length rather that i found the cats travelled faster and more comfortably.

i figured that 60 was on the small end of motors for a cat this size hence the narrow beam which allows me to have a narrower bridgedeck

also thanks for the really constructive reply

sean

i am curious about the quirky behaviour of assymetric cats in rough water however please elaborate as i am trying to learn as much as possible

 

Most of us are opinionated about our boats in some way or another. Maybe I am falling into that mode about power cats. To be sure, some of the fastest ocean racers in the world are cats. On the other hand , operators of those boats ought to wear G suits for they take a brutal pounding.

Take a look at the old Hickman sea sleds. They are almost like cats. They have a tunnel hull that transitions into a near flat afterplane. The sides were nearly parallel and the curvature was toward the center line. They had a reputation for speed and rough water ability that surpassed other boats of their day. Hickman is said to have built the worlds first aircraft carrier while using the sea sled design.

An assymetric plan form has flow that necessarily contains a velocity differential, thus a pressure differential. When the boat rolls there is more pressure on one surface than the other. That can cause the boat to respond to unbalanced pressure by autosteering one way or the other. Quirky. In smooth water and moderate speeds, there is little problem. Witness the three point Hydoroplane racers. The sponsons are most often assymetric with the flat side in the middle. When the water is rough those boats stay on the trailer or risk very unpleasant events.

Quirky is not reserved for cats. Certain monos will exhibit the same behavior and it is less than reassuring to be in one of that type. The problem is partly solved by slowing the boat to a manageable speed and/or getting the forefoot up.

Keep us posted about the reef cat.