center of gravity, center of buoyancy

[jcaesar625]

Oh, the old Simpson's Multiplier.  Kind of like Avagadro's number...the R in PV=nRT...

 Bob, neglected to mention...Barely made a "C" in high school calculus. Avoided it like the plague in college.

The other subjects that involved talking/bullshitting....good there

Sure he even made a C? R is a gas constant (8.314 joules per Kelvin), where Avogrado's  (6.022x10^23) which is the number of atoms per unit mole. And Avogrados number is also why October 23rd is known as Mole day in some geek circles. But what do I know, I'm just a dumb Civil E that didn't do well in Chem.

[gran398]

Yep I made a C...and as stated, barely.

Not smart with calculus like you guys.

[Capt. Bob]

Oh, the old Simpson's Multiplier.  Kind of like Avagadro's number...the R in PV=nRT...

 Bob, neglected to mention...Barely made a "C" in high school calculus. Avoided it like the plague in college.

The other subjects that involved talking/bullsh!tting....good there

Sure he even made a C? R is a gas constant (8.314 joules per Kelvin), where Avogrado's  (6.022x10^23) which is the number of atoms per unit mole. And Avogrados number is also why October 23rd is known as Mole day in some geek circles. But what do I know, I'm just a dumb Civil E that didn't do well in Chem.

Thank you.

[T Race]

Why is is it never called animal wifery?

It IS at U.C. San Francisco ....

[orb]

I did not make it to calculus:) That is why I am asking you guys. I thought Seabob would have some good info, possibly even the info from the original design.
I am putting a larger fuel tank under the deck and adding a paralift. These additions will alter the location of the weight the boat is carrying.  I think proper placement of this weight is key to the boat operating as it was designed. The LCG should be located so the hull will sit at an even trim at  rest, and will run at a few( Eric Sorenson, in his book, Sorenson's guide to powerboats, says 4 degrees) degrees -bow high at cruising speed.

I think I can figure it out, by playing with it, finding the LCG by determining where the LCG was with the original tank and engine placement, but it never hurts to ask for more info.

[flounderpounder225]

All right fellas, I figured it out... CG and CB are both directly relative to the precise placement of the beer cooler... had to go there...

[seabob4]

orb,
No offense intended in my response regarding calculus, physics, chem, statics, dynamics, all that stupid chit.  Yes, as an engineering major, (University of Illinois, University of Central Florida), they were required courses, so thus I took them...do they help me today?  In a certain few areas, a bit.  But not really.  When I switched to accounting, I was able to take more interesting classes that counted towards my degree, a lot of psychology classes...now THEY help me more today than anything!!!

This is how we used to determine longitudal center of gravity.  Crude, and requires the ability to lift the boat off the ground at both ends, but it works.  Pick up the boat, place a long 2 by abeam at the point where you think COG should be.  Set boat down on 2 by.  If the bow drops to the ground, pick boat back up, move 2 by forward, set bat back down.  Visa versa if the stern drops (motor must be on boat).  You'll obviously need to support the sides to keep here from rolling.  When she more or less stays balanced fore to aft, that is your CG, centered on the keel, as this is longitudal.

Center of buoyancy, well, that's a horse of a different color.  Buoyancy exists in a totally dynamic, totally unpredictable environment.  Hence, simple equations, IMO, just don't work.  Example, I have a boat that exhibits this behavior in a trough between 2 foot seas at 3 second intervals, but what behavior will the boat exhibit on a crest in 4 foot seas with a 5 second interval?

This might help, might not...but it's the best I can do...

[seabob4]

All right fellas, I figured it out... CG and CB are both directly relative to the precise placement of the beer cooler... had to go there...

Think I'd listen to Marc!!    

[orb]

No offense taken.
Thanks Seabob, that is what I was thinking. But trying to work it a little in reverse. Find what should be the LCG mark that on the hull, then arrange the weight accordingly, so I will end up with the LCG at the intended location.

I have everything out the boat now, just down to the bare hull, so doing the balancing act will not work to find the original CG now.
The LCG should be somewhere in the area of 40% of the planing surface --measured from the stern. SO that is probably under the console or near there.I was planning on moving the fuel tank a little forward from original location to compensate for the additional 12 or so inches the outboard will be hanging off the transom. I just don't want the boat to run stern heavy. I have seen to many boats, originally designed for transom mounted outboards, with brackets and the stern dragging low in the water.

[gran398]

orb,

Club member and professional builder Group W Bench is completing a closed transom 22-2 with Bob bracket at present. He moved the tank forward, not sure how much.

Shoot him a PM he'll be happy to help you.

Best :flower:

[seabob4]

Typically, and I emphasize typically, the CG will fall just aft of the console, between the console and the seating position.  But keep in mind, as fuel decreases, CG changes.  That is a given, and can be accounted for by shifting weight to compensate fuel decrease.  What you CAN'T compensate for is permanently mounted structures/hardware/whatever.  

Look at a lot of guide boats these days.  Moving the console WAY aft to accomodate passengers forward.  Somehow, I don't think all these guys are going through all the calculations to ensure their boats are going to be properly rigged per CG and CB efficiencies...

Just sayin...

[orb]

Yes, I agree. I think that aft seating is more comfortable than riding on the bow:) , but when you add all the weight aft, it does affect the cg, which effects the trim.

I read the book I referred to earlier, Eric Sorenson's guide to power boats. He gives some excellent ideas in that book. I think he recommends putting the fuel tank centered over the LCG, that way, as the fuel level goes down, the LCG does not change. Ideally I think this would be great, but I don't think you can always mount it there, due to space requirements, and such.

[dburr]

Orb, what type of scales do you have access to? Load cells, 1000 lb hanging scale etc. or just a bathroom scale..

How that question is answered will either make the CG quest easy or hard.. You get the hull jacked up at three known points and leveled. Prior to this you have measured the hull and know exactly where the jacks are from the Forward Perpendicular (furthest point forward of the hull and it is assigned the number 0). You have the forward jack on centerline (CL) at a known station, the aft two a measured distance from CL and from FP.

If you are lucky enough to know an aircraft mechanic that has jacks and load cells, you can weigh it quicker then me typing this..  

If not then solving the jack problem gets interesting.

Once you find the empty and dry CG then as you build you can add the weights and the location of the weight you add and you will get a good idea of where you end up CG wise.  You can get a rough idea of the theoretical CB by some fancy math with the area of the water planes and shape of the underwater sections that I do not remember off the top of my head, been 25 years since I have done any stability and trim calcs.  If you want the info let me know and I can see if I can dig it up.

[Capt Matt]

Balancing these boats is the key to getting them to ride right. But a few inches either way on the fuel tank and console will not make a world of difference. Now if you planned on doing 70mph it might be more important. You should be able to get enough rough measurements from other rebuilds on here that you will be good to go. Looking back at my rebuild I did a lot of over thinking but thats just how I am.
Good luck
Capt Matt

[GoneFission]

OK folks, busy this weekend - but here goes:

Center of gravity is the is the weighted average location of all the mass in a body or group of bodies.  It is the mass center of the body.  In a perfect sphere, the center of gravity is in the exact center of the body.  Put weight (mass) in the bottom and the center of gravity moves down.  

The center of buoyancy is the center of the gravity of the volume of water which a hull displaces.  It is usually below the center of gravity and also changes based on list and lift.  A vessel with a center of buoyancy above the center of gravity is unstable and prone to capsize.  That's why transport vessels in rough seas add ballast in the bottom of the hull - it moves both the center of gravity and (more importantly) the center of buoyancy lower in the vessel to prevent capsizing.  It may sit lower in the water, but with waterproof topsides, the vessel will not sink and is less likely to capsize.  

Sailboats have a bigger issue under sail, as the center of buoyancy changes with the heel of the boat.  As the boat heels over due to wind from the side, the center of gravity does not change, but the center of buoyancy moves up and to the side.  If the center of buoyancy goes too high beyond the center of gravity, the sailboat is likely to capsize.    

Hope this helps!!