Weight&balance records

Do any of you keep a weight&balance record for your boat.How many know where there static cg is? I'm thinking i'm going to start a weight&balance log,starting with the bare weight of the hull,and the bare center of gravity.Then weigh all parts that go into the boat,starting at the cg and asigning a station number every six inches for and aft of static cg.It would be similar to how we keep our aircraft within limits.Makes ajustments easier and to some degree safer,as well as keeping track of that ever increasing gross weight!!! Thoughts?

You need to get Jim Russell's Tunnel Boat Design Program, once you have all the weights and measurements you can make changes on the computer and see how it will effect performance.
As far as the ever increasing gross weight goes, go to the gym.lol

Dave:)

It would seem a "program"would be specific to a certain hull,in order to be accurate.As for the gym,no thanks my personal gross weight is down 147# from this time last year:eek:

Quote:

---

Originally Posted by outasite

It would seem a "program"would be specific to a certain hull,in order to be accurate.As for the gym,no thanks my personal gross weight is down 147# from this time last year:eek:

---

The program needs a ton of inputs and is very accurate. Ask Greg or Helmut, Jim is a Engineer and specializes in boat design. You need the program to really see how cool it is.

Dave

Quote:

---

Originally Posted by Dave Strong

The program needs a ton of inputs and is very accurate. Ask Greg or Helmut, Jim is a Engineer and specializes in boat design. You need the program to really see how cool it is.

Dave

---

Thanks for the info!

I have a full W&B on my Glastron, I use the transom as the datum. I gathered my initial data weighing the trailer with and without the boat with a set of digital race car scales. This method allows me to be in the boat so I could determine where to place the jack pump and battery as I was rebuilding her.
FWIW the boat with 5 gal fuel is 1622 pounds with the CG 51" forward of the transom.

When I first got the program to use to build up my Cougar had accurate info for my altitude and 1400' lower. Had all the info dialed in and got sick and broke have not got the Cougar back together. Input the info from my BAJA and it predicts speed within 1% if you add weight of a passenger or 2. ;)
It is for real.

Dave

Quote:

---

Originally Posted by CharlieN

I have a full W&B on my Glastron, I use the transom as the datum. I gathered my initial data weighing the trailer with and without the boat with a set of digital race car scales. This method allows me to be in the boat so I could determine where to place the jack pump and battery as I was rebuilding her.
FWIW the boat with 5 gal fuel is 1622 pounds with the CG 51" forward of the transom.

---

Static CG is one thing, tunnels really show how CG moves as speed, aero and hydrodynamic forces change as you go faster. 1deg in bow rise can make a big diff in dynamic CG.

Dave

Quote:

---

Originally Posted by Dave Strong

Static CG is one thing, tunnels really show how CG moves as speed, aero and hydrodynamic forces change as you go faster. 1deg in bow rise can make a big diff in dynamic CG.
Dave

---

This article on Dynamic Stability might be of interest: http://www.aeromarineresearch.com/dy...stability.html

Quote:

---

Originally Posted by Dave Strong

Static CG is one thing, tunnels really show how CG moves as speed, aero and hydrodynamic forces change as you go faster. 1deg in bow rise can make a big diff in dynamic CG.

Dave

---

I want to clarify something here, the CG does not move. The center of pressure and the center of lift are what develop and in cases, move. These are what act on the center of gravity.

how do you work out where the centre of lift is and is that with or without the prop effect?

I can calculate COL on an aircraft but have no clue on a boat.
The lift on a boat comprises the hydrodynamic lift, aerodynamic lift and the propeller forces. That is all beyond me mate.
Jimboat sure can do the hydrodynamic part, not sure how one can accurately get the rest though.

Quote:

---

Originally Posted by powerabout

how do you work out where the centre of lift is and is that with or without the prop effect?

---

Quote:

---

Originally Posted by CharlieN

I can calculate COL on an aircraft but have no clue on a boat. The lift on a boat comprises the hydrodynamic lift, aerodynamic lift and the propeller forces. That is all beyond me mate. Jimboat sure can do the hydrodynamic part, not sure how one can accurately get the rest though.

---

ALL the forces can be calculated and located, and their contribution to Dynamic CG accurately considered. Location and values of forces of hydrodynamic lift/drag of lifting surfaces change throughout velocity range; aerodynamic lift/drag values and locations (center of Lift) change with several factors that include aerofoil shape (type), thickness, aspect ratio and height (above water surface); center of force for lower unit & propeller is significant but location is generally stable. All these factors change (some very significantly) throughout the velocity range based on several design, setup and operating factors. All can be calculated accurately, and have a significant effect on dynamic stability of hull. Static CG is the only force acting that remains in the same location, represents the deadweight of hull only (this is why you can't dynamically balance your boat on the trailer).

Here's another article that discusses "How Weight and Weight Distribution is Important to Performance"

More detailed articles here: http://www.aeromarineresearch.com/pe..._articles.html

subbed