Motorised Catamaran

[Fanie]

Hi guys,

I want to ask something for a change please.

You know the wide sailing cats, usually sailers. How bad would they be if you fit a couple of nice outboards on each hull. Why not.

I know they are usually displacement only, but surely you can get them to plane same as the skiboats does. I was wondering if there would be any handling problems caused by the cat's width.

Power boats usually have no rocker while the sailing cats do. How much less power will the hull shape couse.

Ok I basically know the answers already but still want confirmation.

Thanks for taking the time.

[hoytedow]

It is a good question.
I would not even attempt to try to plane a hull or hull pair designed for displacement.
I would choose instead an economical engine that can safely maintain necessary headway in various meteorological situations.
As always, I post these remarks as an interested amateur and await confirmation by someone more knowledgable.

[marshmat]

Fanie,

I can't tell from the post whether you're thinking of something like an 18' Hobie, or a bigger full-bridgedeck cruising cat, or any of a hundred other possibilities.

Speaking very generally, sailing cats can easily be powered to the speeds at which they cruise under sail. Also speaking very generally, sailing cats won't plane and won't take kindly to being overpowered- it's hard to put an exact figure on it, but more than about 10 hp/ton would likely be excessive.

[Fanie]

Hi Matt, if you say won't take kindly to being overpowered, what does it mean ?

Lets say I have a 10m cat and two 60's or even two 80's on the butts. The hulls are flat bottomed to walk on, not very wide though, max 600mm.

Surely when you exceeds the 'hump' in the hull speed you should begin to get lift and they would tend towards planing ? Or do you think the motor prop's would begin to run on the water at a certain speed if the hulls lift ?

[marshmat]

Even in flat-bottom / hard chine sailing cats, the hulls tend to narrow as you approach the transom and the bottom slopes upward. This shape won't lift very well- you'll get lots of bow-up trim, but not enough lift in the stern for the boat to actually climb.
To plane, you'd want the full waterline beam carried all the way aft, and no rocker aft of about station 6. Such a shape, of course, won't sail worth a damn.

[Doug Lord]

Hey, Fanie take a look at this little power cat-two Tornado hulls:

http://www.biekerboats.com/Bieker_Boats/Skeeter.html

I can't post a picture for some reason...
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[ancient kayaker]

Originally Posted by Fanie

Hi guys,

I want to ask something for a change please.

You know the wide sailing cats, usually sailers. How bad would they be if you fit a couple of nice outboards on each hull ...

I don't see why it wouldn't work if the cat hulls were symmetrical. After all that would be just a pair of canoe bodies and canoes move along pretty smartly under power, although they don't plane safely. However, with hulls asymmetrical hulls I can see a potential for resonant heeling leading to a potential catastrophic result.

[garydierking]

Tell us more about "resonant heeling" :-)

[ancient kayaker]

Heeling results in righting moment which is proportional to heeling angle, at least initially. This is a good definition of a pendulum and has a natural frequency which can be resonated. I am sure you already know that. If the hulls are asymmetrical, when the boat is heeled there is a sideways force that, conditions permitting, can feed back into that resonance. Lots of boats tend to do this when sailing downwind, nothing new at all.

[garydierking]

In the 40 years that I've been intimately involved in multihulls, both sail and power, I've never heard of or experienced anything like that.
An asymmetric cat sailing downwind rolls less than the average aircraft carrier.

[catsketcher]

That reminds me of the story of the beggar who asks a bridge crew for some money and when they tell him off he gets his violin out. He plays the resonant frequency of the bridge until it falls down - a la Tacoma narrows - you tube it.

Most bridges don't fall down as they have lots of internal damping. I have never seen the effect of resonance in a boat, unless you are talking about my cups on the table at certain engine revs.

cheers

Phil

[Fanie]

I think you're right Matt, it will remain a displacement hull then because of the rounded hull shape aft below the waterline.

Ancient, the heeling you refer to, do you mean the rocking ? effect..., I can imagine if the hulls rock in a certain size swelll the props could come out of the water half the time. The hulls are fatter towards the stern to prevent too much rocking.


What I would like is for the cat to have a bit of speed and power when you run the motors. I also want to be able to use the sails on long voyages where the outboards will be too expensive on fuel.

Can these two options be mated somehow ?

[Tantalus]

Hey Fanie,
Check out the Prowler 10.4

[Fanie]

Hi Tantalus, that is what I want yes, but with the option to sail it. In the gallery you'll note the hull has no rocker, which makes it a power hull only. She's not going to want to tack well with sails...

[ancient kayaker]

Sailing monohulls are subject to death roll when sailing downwind, an uncontrollable side-to-side rocking that builds up until it ends in a broach. It's not inevitable even in monohulls but it does happen if the boat is fast enough and has enough power, and it can be minimized by raising the daggerboard.

Does it happened to multihulls with enough power? Of course it does. The racing powerboat crashes I have seen (on TV) seem to fall into a number of distinct categories. Ignoring whatever triggers the event these crash types seem to be common:

1. the boat and flips backwards
2. the boat becomes airborn and digs in when it hits the water
3. the boat starts to yaw uncontrollably leading to a sideways skid or rollover
4. the boat starts to rock violently with a zigzag course which increases uncontrollably until disaster occurs

These events happen routinely despite the fact that the boats are designed expressly to avoid them, whether the boats are cats, monos, three-point hydroplanes or whatever. Of course the boats are being driven on the edge in order to win. It seems to me that outcome 4 particularly affects multihull designs but I don't have statistics so I could be wrong.

Of course sailing multihulls don't do this. Either my theory is hogwash, or it doesn't happen because nobody is dumb enough to park a couple of big outboards on the ass-end of a sailing cat and crank them up.

Gary: it's a sailing mono thing that does not affect multihulls because of their greater lateral stability and the limited power of sails.
Fanie: it's just a cautionary note.
Catskeller: it might be surprising what you can do with a 200 HP violin ...