new construction questions I just have to ask about fiberglass layers.

Im building a 40' power catamaran in a tri- level format and need to know the basic rule of thumb as to how many layers of glass is required using a balsa core design. Again I am looking for a basic rule of thumb not yet written in stone format. Also does the center of gravitity of a design exist true to definition? ie;( Does a 40' foot boat have its center of gravity exist @ 20')?
These may be simple questions But as a first timer, I must ask. We all no about the stupid question theory

Also Im looking for an engineer that may be able to help me with the conceptual design. I have most of my drawings complete. I am in the process
of transferring to computer where I will complete the total concept from technical drawing to layout and I guess what Im trying to say is that I need someone to make it float. any input, advise or opinions are greatly welcome.
with all you great people out there I think we can make this happen.
thanks for all your input

Donjames.

 

No such "Rule-of-Thumb" really, because skin thicknesses depend on location of internal structures and therefor on panel sizes.

Simple answer: NO. But, when the craft is completely symetrical in all three directions (a cube or ball) then, YES.
Center of gravity is also subject to the installed equipment onboard, therefor inevitably will it shift from its dead-center location .

THERE ARE ONLY STUPID ANSWERS (I think).

With to-the-point-questions on the subjects you're struggling with, there is always someone on this forum that will help you (I presume!!! Correct me when I'm wrong)

Good luck,


Peter

 

CG vs CB

The center of gravity is based on the mass of each individual part of the vessel. The mass of the vessel can be represented at a single point, the CG. To calculate the CG estimate the weight of each individual part and multiply the weight by the distance from a reference point (I use station 5) to get a moment. Sum all the moments and this gives you the CG in three directions longitudinal, transverse and vertical. If the transverse is not pretty close to 0 you will have a list if your hull is symmetrical.

The shape of the hull will determine the center of buoyancy, CB. This is a single point which counter acts the weight of the vessel. Although the CG is fixed except for variable loads such as fuel, water, gear and crew, the CB changes based on how the vessel sits in the water. In fact the vessel will always try to attain a condition of equilibrium where the CB is directly under the CG. This concept is simple to observe when comparing two similar boats – one with full fuel under the cockpit and one with no fuel. The full fuel condition squats at the stern. The fuel moved the CG aft and the boat responded by submerging more of the stern thus moving the CB aft. These days the way I calculate the CB is to input the hull shape into my hydrostatics program and let it solve for various loading conditions. Then make sure the boat is in equilibrium based on the weight study.

As far as help on your project I’m sure you will find it here. There are some members who are always hawking for work. Look in the services section of this site.

 

Donjames,

Just to come back to the laminate thickness. It has been written before in this forum that, although it is possible to calculate the required thickness mathematically. For small boats it's easier to copy from other (succesfull) designs and for larger vessels you can use the rules provided by the Classification Societies.
There are some books on the market that use these Class Rules, one of them is:
Dave Gerr's "The Elements of Boat Strength"'.

Good luck

 

thanks guys, you've been extremely helpful with your patients and skill to answer these ?'s. Each day I learn a little more and that helps to put everything into perspective and keeps things moving.