Catamaran hull difference sail / power

[Fanie]

Hi, thanks for taking the time out to look here.

Fistly I'm not interested in a ski boat catamaran. The other members in our group also suggested that they feel the same way about this.

The boat type that did get our attention is something like the sailing catamaran. Maximum size would be 8m x 5m, foldable to be trailable.

Questions -

Are there fundamental differences between a sailing catamaran's hull's and an outboard or inboard's hull design ?

If there are, can one compromise these to be suitable for both conditions ?

[Jarrod]

Dear Fanie,

Yes there are fundamental differences between sailing and power catamaran hull forms. Firstly sailing cats require rocker in the hulls for tacking etc, where this is not required as such for power catamarans, it will hinder the performance/efficiency of the power catamarans.

Note: Boats are all about compromises.

What operational speed would you require for the power catamaran?

[Fanie]

Hello Jarrod,

Sorry for the slow response - meeting. Yuk.

Well, of course it would be nice to have something that can do 134.6035445km/hr, but that's unrealistic of course.

Ideally if it's possible to achieve a decent planing speed would be nice. Since you mentioned the bowed or rounded hulls it's actually quite logical, but also not. The power boat would like a streighter (and more flat) rear hull in contrast, I imagine for easier planing since a planing boat is kept on the water only on the rear aft part of the hull. The streight hull would cause a yacht to tack more difficult and may cause more drag.

If this is correct, what would be the middle way or a way around this ? Assume one does go for the power boat rear hull, but uses oversize rudders for tacking when under sail ?

On the other hand if the sailing hull's curved rear is maintained, you may get that the outboards begin to run on top of the water when the hulls pushes up on the water to begin planing ? while if adjusted to the correct hight for planing at slow speed would tend to drown the motors.

In the one case the boat gets dragged and in the other it gets pushed. It's like an aeroplane prop and a wind generator's prop.

How about a small planing hull that tilts down with the outboard that would push up against the hull of the yacht to prevent water from rising up. This could force water out at the motor's cavitation plate level ?

Any ideas ?

[tuks]

Im no yacht designer, but I thought that cat hulls are narrow enough that hull speed does not apply. Essentially they can go much faster than a wide hull and dont experience the "hull speed hump" and therefore dont plane.

I think there will be a big difference in the beam between motor and sailing catamarans. You will need widley spaced hulls for stability to counteract sail forces, and this is probably not necessary for a motor cat.

Infact Disregard all that and take advice from somebody who knows what they are talking about.

[Fanie]

Hi Tuks.

As I have it you may find a yacht hull may tend to skid more sideways when turning and may not turn as sharply as a ski boat will as the sides will byte in the water while the motors force the turn for the ski boat.

[Fanie]

Here... seems it's not really that new either.

http://www.2hulls.com/used_power_catamarans_2006/Katmando.html

http://findarticles.com/p/articles/mi_m0EIN/is_1999_Jan_29/ai_53669145
The Moorings 37 Power Catamaran allows guests to cover more cruising ground in less time and is appealing to both power boaters and sailors, said Hans Birkholz, vice president of global sales and marketing for The Moorings. The power cats offer stability, long range, a shallow draft and ease of handling. They provide our guests with another excellent choice when selecting a yacht for charter

A-ha !

In Durban -
http://www.maximyachts.com/maxim400powergallery.htm

[Fanie]

Ok if we can step off the issue of how wide a catemaran should be, can anyone suggest a hull type or form that may be more or less suitable for sailing as well as with an outboard ?

What would be the golden middle way if possible ?

[Basjan]

Hi Fanie

Stay with your idea.

An alternative will be a trimaran.
It will give you the benefit of "breaking it up" for trailering and allow you the benefit of having an outboard motor in a central position with the steering and other controls.
You'll just have to find out what size motor would be sufficient.
A Trimaran will give you more deck space as well, so you might even want to add a fighting chair for those big marlins.

[Fanie]

add a fighting chair for those big marlins
I like the way you think !

[Trevlyns]

Ok Manne! I’ve got it!

Let’s all chip in together and get something like this… :D :D :D

[Fanie]

Nah ! Not enough deck space :D

[Basjan]

Hi Fanie

Try this site, the designs look outdated, but gives you an idea of where you want to go.

http://www.hartley-boats.com/

[Fanie]

Hi Basjan,

I have an idea of what I want. The tri has some advantages, like you need to fit only one motor and it can be a big one for speed, best is you mount it in the centre of the vesel which you cannot do with a cat unless maybe you make something special for that. Two motors for sea going req could become somewhat crowded especially if you want to add a rudder system with it

The other problem may be space in the tri, the cat does offer two hull's and a cabin with more space and should be more comfy.

To get back to the catamaran hull shape. If one flattens the rear part of the hull, how will it behave as to the rounded shape ?

[tuks]

Hey Fanie

Have a look at some of this stuff
http://powermultihulls.com/magazine/articles/why-multihull.htm
http://www.solarnavigator.net/hull_speed.htm
http://www.multihullsmag.com/magazine/articles/cat%20hulls/performance_of_cat%20hulls.htm
http://www.sailcopress.com/elusive_cruising_catamaran_perfo.htm

Is there really no boat out there that satisfies your requirements?

[terhohalme]

Still one:

http://www.glacierbaycats.com/

[Fanie]

Thanks all, Tuks you have outdone yourself with these links ! Looking at the article on the hull shapes, why doesn't sailing catamarans also adapt the Bobcat hull with HySuCat foils ?

[tuks]

You mentioned foils, Douglord will be here soon.:D I think for sophisticated and highly developed craft foils might be an option, but im a firm believer in the KISS principle. Less stuff to maintain and replace when it breaks.

I hope those links will help. I find this stuff very interesting and have learnt a little bit of the basics of why hulls are shaped the way they are.

I found it interesting is to look at drawings of existing boats to see the difference between hullforms of powercats and sailingcats. The rockerline is very different. There are a bunch of drawings on the the Schionning designs website. www.schionningdesigns.com.au

[Fanie]

After some reading and more reading and then some I've come to the conclusion that it would be better to keep the sailing hull's. While you may not be able to plain the cat at speed with outboards, the water piercing shapes of the sailing hull's still can make good time.

The next question would be what size outboards should one use. Yeah yeah I've seen the new 300'ds, but hardly peactical in my case :D

It seems most of the cats have 9 or 10 hp outboards, but that probably is just for maneuvering, still, it would be nice to know what speeds a couple of those can achieve on a cat.

Smaller motors tend to be more fuel efficient and easier handled if you don't have power trim and tilt and I don't think one needs to overpower the idea. I was thinking in the range of two 28 to 40hp motors. Don't worry about the steering for now, I'm sure there's an easy way to do it.

Could any one dare an opinion as to the speed one can expect ? 8m x 5m sailing cat hull

[Basjan]

Hi Fanie

First get the design you like and then worry about motors.

The hull design would be the factor that influence your decision iro the amount of power required to move the boat.

In general most local manufacturers will advise that minimum power for a 21' powercat is 2 x 50hp.
I'm no expert, but it stands to reason that a sailing vessel will require less power to get moving than a power boat and you would therefor be able to down scale on the motors.

PS - If you realy want your boat moving, bring her down here. We have wind, fish, sunshine, wind, wind, wind and more wind.

[Fanie]

Hi Basjan, I'm attempting to bring a couple of things and ideas together to paint the picture. Be patient, there's a lot of unknowns that still needs coloring in. I have in principle already decided what I want, but not carved in stone.

Personally I hate the wind. When lure fishing it is the only single element that can bugger your day up. Now I know it's different at the coast. If there's no wind there's no fish. If the wind blows the fish bites, so it's different. Best sea fishing ever was in rainy, wind blowing, really miserable weather. Never been so wet in my life ;) Amazingly how much water some clothing can hold. Best way to lose weight quickly is to get undressed.

Bottom line is if you keep the wind down there where it counts I'm happy with it.

I still have the question - What speed can one expect with wat size motors on an 8m x 5m sailing cat hull. Surely someone must have an idea of at least their own rig ?

[Basjan]

Hi Fanie

Were trying to get rid of the wind here, can i send you a couple of truck loads to get you started?

[Fanie]

Getting rid of the wind is really easy !

Just put up anything that will work off the wind, like a wind generator - the wind won't blow for at least three days.

[Basjan]

Their planning to build a windfarm here to generate electricity so maybee we will have more nice fishing days.

Have a gander at Super Cat's new "Sliver 29". It's a displacement cat but resembles a sailing cat. Minimum is 2 x 25hp, the test model was fitted with 2 x 40hp and reached 22 knots.

http://www.supercat.co.za

[Fanie]

Thanks Basjan, nice link and I must say nice boats too. I'm not even going to ask what such a boat retail for. My Yamaha 115's CDI packed up and I'm really PO about it. After working the rubber potting away I can see about 6 solder joints that was never soldered. The spares used will probably come to R100, yet a replacement CDI is around 5K ! Pathetic and rediculous.