drag calculations for large catamaran

[fredscat]

The vessel under construction is a 97ft fiberglass SAILING catamaran. I need help in deciding on the HP needed for the "EMERGENCY" engine. I would like to design for a 6 knot tide and a head wind of 15 knots. I am capable to calculate the wind resistance but not the hydrodynamic drag of the vessel. I plan on having two props. My maximum draft will be near 22" rudders and center boards partially retracted.
Fred
P.s.: details are on my website: CODEVCO.WS

[gonzo]

Without trying to be too negative. You are designing a 97ft vessel and can't calculate the drag? It looks like a NA is needed urgently.

[fredscat]

what I need is the formulas for the skin drag, suggestions on how to calculate "wave drag" on non displacement non planning hull forms. I am going to ignore the interference drag between the two hulls. I am able to calculate rudder and center board drags based on Hydrofoil date.

[gonzo]

That usually takes three years of college. I think you need to hire someone to do the calculations.

[fredscat]

Hey, I have more than that!! I am just looking for a reference manual. I had a real good hard back book - lost in Hurricane IKE. And the name of the book is lost someplace in my feeble mind

[War Whoop]

Clue start with the surface area.

[fredscat]

Thanks I have all that. I need a reference manual listing viscosity numbers drag numbers for various surfaces etc

[Tim B]

Right... I'm with Gonzo on this one.

Yacht design is not actually that complicated, but it is vital that you understand the interactions present. This shouldn't be too onerous for a good aeronautical engineer. However, yachts and aircraft have a few significant differences. The most important of these is that a vessel operates in TWO fluids. That gives rise to wave drag, and on a multi-hull, interference. It also produces environmental waves (distinct from the wave pattern produced by the hull(s)), which the vessel has to handle in an operationally safe manner.

So, without going into too much theory (as there are many papers on these areas), you won't find one simple quick-fix solution. You seriously need to consider the resistance and sideforce prediction (otherwise how do you size the rig?) and also the stability and vessel motions (dynamics) which is of particular importance on a multi-hull to avoid excessive bridgedeck slamming.

I would seriously advise the you contact a Naval Architect and discuss the project with them. Proceeding on this size of project on what appears to be limited knowledge of the subject area may end up putting you out of business, or worse, causing someone's death.

Yes, we all mess about in boats, but only a few actually know how dangerous the sea can be. It is a fool who does not respect that and act accordingly, whatever part you play.

Tim B.

[fredscat]

I do thank you for your advise. This is a large but "Quite simple" design and both hulls have been built and the design is "locked in" I am a structural engineer with a reasonable knowledge of aero- dynamics. What I need are reference manual(s) for the various formulas and numbers. I am going to take chance on my proper interpretation of the data.

[PAR]

Is this the same "engineer" (my assumption) that on another thread, wants to use house hold wiring for the 12 VDC system(s) - this 97' cat? If your engineering "expertise" level is this modest and your ability to research a particular topic this meager, then you should have grave concerns over this 97' cat being able to safely take friends and family father off shore then they can swim back to.

It would be easier to just tell us what your are trying to do Fred. Are you attempting to design a 97' cat (how may million have you budgeted?) or do you have a 97' cat in need of some engineering expertise, that you currently lack?

[Ad Hoc]

I'm struck by this:

Originally Posted by fredscat

I had a real good hard back book - lost in Hurricane IKE. And the name of the book is lost someplace in my feeble mind

Would not be better to focus on buying another copy of this book? If you felt this book has been your saviour in the past, you should ask questions about this book so you may buy it again.

Having said that, it is clear you're out of your depth. A professional (no matter what type/discipline) knows when they should stop and seek advice, an amateur continues on blindingly believing their own ability. As PAR says, just ask the basic question, tell us what it is you're trying to do.

[Tim B]

So you've built it, but have no idea how it will perform. Brilliant! Have you consulted a Naval architect at any stage? How did you do the stress calculations for torsion in the cabin? What about hull bending, rig loading, rudder stock strength, wave impact pressure?

As a structural engineer you should know that accurate load prediction is the first step in a good structural design.

If I were you I would be VERY worried at this point (and I mean halting the project pending a design review). If you continue with this attitude then you'll either have a yacht with no performance, or someone will get hurt.

However, if you wish to continue, a quick google for "Catamaran Resistance" produces a few relevant documents.

Tim B.

[MikeJohns]

Originally Posted by fredscat

The vessel under construction is a 97ft fiberglass SAILING catamaran. I need help in deciding on the HP needed for the "EMERGENCY" engine. I would like to design for a 6 knot tide and a head wind of 15 knots........................

Presuming this is going to sea? But I'd like to see your full design brief.

The usual mistake is to forget the oceans wave generated resistance and concentrate on wind and smooth water self generated wave resistance which is almost useless for real life in a seaway.

When you really need that emergency thrust is in the teeth of a gale and worst case in relatively shallow water wave spectra.

Little factors so much more important than designing a slippery smooth water hullform like how high is your bridge deck? how 'clean' are the internal hull topsides? (Have you eaten into this space for access stairs?)

Short of actually testing a model the best advice is that for real emergency use into the weather you want the largest slowest turning prop you can match to your engine.

As a very rough rule of thumb iif you find the smooth water resistance ( self wave and friction) at your desired speed ..6 knots then double that resistance it for an attempt at approximating an RAO powering off a lee shore then add the wind resistance you'll get a glimmer of something practical.

[Alik]

I've published a diagram for power/speed estimates of catamarans here: center of flotation calculation and implications?

But of course, this is just preliminary estimate, one needs to perform detailed calculations to get resistance and propeller characteristics.

[mydauphin]

Build it and it will float... perhaps.
Powered it and it will move, how well who knows.
Anchor it in a storm, and pray... Pray very hard, very hard indeed.

A 92' foot cat is major engineer feat. It is way beyond a 50' monohull. The stresses involved are astronomical. In a storm it will thrown about as easily as 30 footer, but because of its large size the stresses will be much larger. Don't think that because it is large it is immune to the weather. I have been on 800 foot ships that have been thrown around like toys, literally thrown sideways 100 feet, and this wasn't even in the open ocean. It happen just north of the Virgin Islands

I am not trying to put water on your fire, just slow down a bit, get a naval engineer to review everything and see if it makes sense. It is the best money you can spend. I am not a NA, but I have built plenty of boats without them. But nothing on the scale that you are doing. It is kind of like a one of us building a 747... it ain't going to happen. It won't work. But you could fix problems may be early on.

Ps. Naval Architects aren't that expensive. Pay their fare, feed them, take them to the stripper place and many will come for a few dollars. They all live boring lives anyway...