Calculations?

[bearwen]

I have a few questions that I am sure there are no simple rules of thumb to go by but please give it your best shots. First question when designing a boat (Power or sailing Catamaran) how do you figure the LCG? Second question what is the standard if any or how do you calculate the distance from the water to the wing bridge ( or bottom of the hull inside the tunnel not sure if I am using correct terminology)? Third question when you are calculating a length to beam ratio for a long and narrow round bilge discplacment hull for a cat (sail or power) do you calculate length by the width of each hull separate or the total width of both to the length? Fourth question what are the norm in length to beam ratio for the over all boat for power cats, sailing cats, fishing(center console type) and the high speed race type hulls? Last question what shape is the best for the bow for cats I notice that most of them are either straight from the water line to the Bow or taper back just slightly from the bow to the water line? Why do they not have a sharper angle like your V-hulls or high speed planing boats (both cat and V-bottom)?
I am looking at designing and building my own cat most likly a power cat. I am looking at the 30ish foot range. What I am looking to do is basiclly have a luxury/ high speed (race Type)/ fishing cat all in one boat. I now that I am going to have to give up some for a nother but I am looking to balance it all as best as possible.

Sorry so winded.

Thank you all.

Bearwen

[gonzo]

LCG is kind of tedious. You have to figure all the weights of everything: hull and deck materials, interior, machinery, tanks, supplies , etc. Then you calculate the center of mass for the added masses.

The bridge or deck height depends on the intended use and hull design. For example, a planing hull that produces a high bow wave needs more clearance than a displacement hull with little wave forming. A boat for protected water needs less clearance than on for offshore use.

Length to beam uses overall beam

They vary widely but between 8:1 to 10:1 are common

Bow shapes follow fashion.

I think you need to look at a design as a whole. There are no generic answers. The design spiral starts at any one specification and then goes on getting more complex.

[Robjl]

Try http://www.newavesys.com/spiral.htm
for a good overview on designing. I found this and other articles by the author useful.

[tspeer]

Originally Posted by bearwen

...First question when designing a boat (Power or sailing Catamaran) how do you figure the LCG?

For a sailing multihull, a key consideration is what happens to the LCB as the load shifts from hull to hull. So the LCB/LCF relationship is important.

Alexander Simonis decribes his evolution in thinking on this subject in "Sailing Yacht Design - Practice" (ISBN 0-582-36857-X). He places the LCB just ahead of the LCF and somewhat behind the CE. His approach may be applicable to a displacement power cat, too, since that's almost what a charter catamarn is.

John Shuttleworth talks about his criteria for arranging the LCF in http://www.john-shuttleworth.com/Articles/NESTalk.html. He doesn't talk specifically about LCG and LCF, but his design principles imply their relationship.

I've found it useful to think in terms of the multihull footprint - the location of the virtual c.g. as the rig is loaded. Here's an example:

The blue curves are the location of the center of buoyancy as a function of heel and pitch trim, and the black curves are the location of the virtual c.g. for different apparent wind speeds and angles.

Second question what is the standard if any or how do you calculate the distance from the water to the wing bridge ( or bottom of the hull inside the tunnel not sure if I am using correct terminology)?

I thought Charles Kanter discusses this in "Cruising on More Than One Hull" (ISBN 0-9618406-2-5), but I can't seem to find the actual criterion. Rick White talks about bridgedeck clearance in qualtitative terms in "The Cruising Multihull" (ISBN 0-07-069868-6). IIRC, a common rule of thumb is a minimum of 3 ft for a typical 40 ft cat. Avoiding a right-angle joint between bridgedeck and hull seems to be important, too - a sizeable chamfer or flare helps to deflect a wave riding up the side of the hull from slamming straight into the bridgedeck. As a benefit, this also reduces the stress at the hull/bridgedeck intersection and provides for nice space-preserving steps down into the hull from the bridgedeck.

You might be able to come up with a more rational criteria by considering the wave spectrum and the signficant wave height for wavelengths on the order of the beam of the boat. What you are trying to avoid is each hull sitting in a trough and the peak reaching the bridge deck.

Third question when you are calculating a length to beam ratio for a long and narrow round bilge discplacment hull for a cat (sail or power) do you calculate length by the width of each hull separate or the total width of both to the length? Fourth question what are the norm in length to beam ratio for the over all boat for power cats, sailing cats, fishing(center console type) and the high speed race type hulls?

Each hull separate when you're talking length:beam ratio.

But the beam between the hull centerlines is very important. Kanter goes into a lot of detail as to why excessive beam in this sense is not appropriate for a cruiser.

Last question what shape is the best for the bow for cats I notice that most of them are either straight from the water line to the Bow or taper back just slightly from the bow to the water line? Why do they not have a sharper angle like your V-hulls or high speed planing boats (both cat and V-bottom)? ...

Because it gives the longest waterline length for a given length overall. And waterline lengt is very important to reducing wave drag at displacement speeds. High speed planing boats operate in a regime where the wave drag is comparatively low and issues of wetted area, dynamic lift and stability, and slamming are much more important.