PDA

View Full Version : Center of Gravity



harleypauls
05-04-2004, 07:49 AM
Does anyone know how to find the center of gravity on a boat?
1600#
1200#
1650#
28' warlock
Any help would be appreciated. Thank you, Paul

harleypauls
05-04-2004, 07:50 AM
Sorry. 1600 port rear
1200 bow
1650 starboard rear

Infomaniac
05-04-2004, 09:27 AM
CG weight x arm = moments.
add up all the weights add up all the moments
Moments /weight =cg.
need the arm. actual distances in inches where weights were measured from a datum (reference point) That can be anywhere on the boat as long as all of the weight points are measured from there. Try the bow eye as the datum.
For example: Bow eye to bow weight point is 10" and bow eye to aft weigh point is 336"
1200 x 10 = 12000
1600 x 336 = 537600
1650 x 336 = 554400
------- -------------
4450 ------ 1104000
110400/4450 = 248" aft of the datum = CG
Clear as mud?

Infomaniac
05-04-2004, 09:40 AM
You can add or remove any component or fuel and see what it does to the CG also as long as you measure everything from the same datum point.
Oh yea - any weight measured fwd of the datum must be expressed as a negative number.

Sleek-Jet
05-04-2004, 09:44 AM
Oh great... a mechanic that can figure weight and balance...
Info, you are truly one in a million. :D

Infomaniac
05-04-2004, 09:47 AM
Originally posted by Sleek-Jet
Oh great... a mechanic that can figure weight and balance...
Info, you are truly one in a million. :D
SHHHHH Sleek-Jet
I would rather dazzle them with brilliance than admit that this is basic airplane mechanic stuff.
Did I say that out loud?
:) :)
Oh yea and don't think I did not catch the pilot - mechanic slam. :rolleyes:

Infomaniac
05-04-2004, 09:56 AM
Did I mention leveling the boat or the plum-bob and chalk lines for accurate measurements?
:D :D

Dr. Eagle
05-04-2004, 09:58 AM
Originally posted by Infomaniac
CG weight x arm = moments.
add up all the weights add up all the moments
Moments /weight =cg.
need the arm. actual distances in inches where weights were measured from a datum (reference point) That can be anywhere on the boat as long as all of the weight points are measured from there. Try the bow eye as the datum.
For example: Bow eye to bow weight point is 10" and bow eye to aft weigh point is 336"
1200 x 10 = 12000
1600 x 336 = 537600
1650 x 336 = 554400
------- -------------
4450 ------ 1104000
110400/4450 = 248" aft of the datum = CG
Clear as mud?
Dude, you cheated!
You must be looking at an aircraft weight and balance sheet.....LOL.....;)

MRS FLYIN VEE
05-04-2004, 10:01 AM
the hubby usually makes me stand in the middle of the boat.. does that help.. J/K :D :D

Infomaniac
05-04-2004, 10:06 AM
Came straight out of my brain Dr.
I taught weight and balance among other aviaton subjects at Spartan School of Areonautics and Tulsa Technology Center for about 7 years.
Must have stuck I guess :)

welk2party
05-04-2004, 10:11 AM
The only math I can do is adding and subtracting the number of cocktails required for the job.

Dr. Eagle
05-04-2004, 10:20 AM
Originally posted by welk2party
The only math I can do is adding and subtracting the number of cocktails required for the job.
that job being what it takes to arrive at a nice buzz of say .012?

LaveyJet
05-04-2004, 10:27 AM
Originally posted by Infomaniac
CG weight x arm = moments.
add up all the weights add up all the moments
Moments /weight =cg.
need the arm. actual distances in inches where weights were measured from a datum (reference point) That can be anywhere on the boat as long as all of the weight points are measured from there. Try the bow eye as the datum.
For example: Bow eye to bow weight point is 10" and bow eye to aft weigh point is 336"
1200 x 10 = 12000
1600 x 336 = 537600
1650 x 336 = 554400
------- -------------
4450 ------ 1104000
110400/4450 = 248" aft of the datum = CG
Clear as mud?
Agreed, the CG will be on that axis.
But not knowing how it applies in boating, is the 3 dimensional answer of interest?
That is, does one need to know the CG in 3 dimensions?

harleypauls
05-04-2004, 10:56 AM
The CG is important to racers. We need the CG to calc where to add or remove weight. Paul

Essex502
05-04-2004, 11:03 AM
To calculate the CG accurately you will need to know the location of the masses relative to a known location, i.e. the bow eye as mentioned above, in three space. Model the boat using a CAD software package and run the mass props on the CAD model and you're all set. :D

Infomaniac
05-04-2004, 11:16 AM
You can do the longitudinal CG as mentioned above.
Do the lateral CG the same way but 90 degrees to the longitudinal.
No problem.

Essex502
05-04-2004, 11:20 AM
It's the third dimension that will be difficult with the method proposed.

fourspeednup
05-04-2004, 11:27 AM
Originally posted by Dr. Eagle
that job being what it takes to arrive at a nice buzz of say .012?
That'd be like 1/2 a beer, now .12 on the other hand....that'd be in the neighborhood of 4-5
(since this is a technical thread and all):D

LaveyJet
05-04-2004, 12:00 PM
Originally posted by Essex502
It's the third dimension that will be difficult with the method proposed.
Just turn the boat on it side, ;)
I'm wondering if there's someway of using the waterline.
Have to think some more

Dr. Eagle
05-04-2004, 12:04 PM
Originally posted by fourspeednup
That'd be like 1/2 a beer, now .12 on the other hand....that'd be in the neighborhood of 4-5
(since this is a technical thread and all):D
I knew I got that little dot thingee in the wrong spot.......:rolleyes:

fourspeednup
05-04-2004, 12:06 PM
Originally posted by Dr. Eagle
I knew I got that little dot thingee in the wrong spot.......:rolleyes: You wouldn't have let that go, so I didn't want to dissappoint by not replying:D

Dr. Eagle
05-04-2004, 12:07 PM
Originally posted by fourspeednup
You wouldn't have let that go, so I didn't want to dissappoint by not replying:D
Touche! (and you're right).......... LOL.........;)

Infomaniac
05-04-2004, 12:26 PM
Originally posted by Essex502
It's the third dimension that will be difficult with the method proposed.
I really don't see a need for that.
How much Yaw does any boat get?
Front to back side to side should be sufficient.
Plus looking at his boat. He might need this info to hoist it in and out of the water.

rrrr
05-04-2004, 12:34 PM
Good explanation, Dr. Info. Much clearer than I've heard a CFI or two mumble it out.
Of course, me and math are bitter enemies anyway. A ninth grade Algebra II book viciously attacked me and left me for dead in 1971. :D :D

harleypauls
05-04-2004, 12:38 PM
Lifting is usually not a prob. Races almost always have a crane on site so boats can get in and out quickly. Basic reason is to clear the slate so to speak. No weight on boat. Find CG and run it. Add weight to front, changing CG, and run it. Do the same to the back. Play with different weights and different CG. See where this particular boat runs fastest, Win more races!! That's what we want it for. Paul

Infomaniac
05-04-2004, 12:51 PM
Originally posted by harleypauls
Lifting is usually not a prob. Races almost always have a crane on site so boats can get in and out quickly. Basic reason is to clear the slate so to speak. No weight on boat. Find CG and run it. Add weight to front, changing CG, and run it. Do the same to the back. Play with different weights and different CG. See where this particular boat runs fastest, Win more races!! That's what we want it for. Paul
Once you get this baseline CG. You do not have to weigh the boat again.
Calculate from that point on.

harleypauls
05-04-2004, 12:54 PM
Thanks for the help, I hope to see some of you out at the offshore races. I will post the when's and the where's when they are finalized. Paul:) :)

Dave C
05-04-2004, 01:09 PM
No thats so he can get a good look at the ( . ) ( . ).....:D
Originally posted by MRS FLYIN VEE
the hubby usually makes me stand in the middle of the boat.. does that help.. J/K :D :D

mickeyfinn
05-04-2004, 01:17 PM
Do it the simple way. Put a small pole in a level flat concrete surface, get two or ten guys to help lift the boat and move it around on the pole until it balances. Now you know where the center of gravity is....;)

harleypauls
05-04-2004, 01:21 PM
Originally posted by mickeyfinn
Do it the simple way. Put a small pole in a level flat concrete surface, get two or ten guys to help lift the boat and move it around on the pole until it balances. Now you know where the center of gravity is....;)
I can't find any flat level concrete in so. Cal. J/K:D :D :D