CG Location Calculation
 

Is there a service or does someone have a formula for finding CG ?

Procedure includes weight of loaded trailer trunnion (axle centerline on single axle, equalizer on tandem and centerline of center axle on triple axle) and weight of the tongue jack. The jack weight and trunnion weight are separate individual weights and found one at a time. The two weights are then added together to find total weight of loaded trailer.

Measurements are then taken, trailer ball coupler to trunnion, boat bow tip to trunnion, overall boat length, trailer overall length.

Empty trailer is then weighed using same procedure used when trailer was loaded (trunnion and jack separately).

Some believe the CG should be located relative to the aft step on a stepped hull.

What would be a fair price for such a service ?

why not just CG it?

Hey guys,
I don't know anyone who does this work nor do I have any experience on which to base the price.

John

Sean, What boat is that ?

We tried doing that, I guess the floor wasn't level. The boat rolled through the wall, down the street, into traffic and didn't see it for a week. When it finally came rolling back it was hung over and looked like it had even been in a brawl !! Some lady HUH ? :boat:

John, your welcome to send us the numbers, gratis.

38 DW

Yes, flat concrete is a requirement for putting it on a roller. :drink:

now i dont care who you are,THATS FUNNY CHIT.:lolhit:

Want a science project ? Try doing the overall C.G. which includes vertical and horizontal axis plus centerline offset.
We did this once with a 62 ton Army tank !
Loadcells, overhead crane and rotating pivot plus many H.P.calculators ( pre-laptop/P.C.)
ed

Thanks!

I'm completely lost. I thought CG was Coast Gaurd. I was trying to come up with an alibi ( i'm sure i spelled alibi wrong ).

Has anyone bought a custom trailer and had the manufacture ask for the boat CG to find axle location ?

Sean, was the picture taken with the boat rigged or un-rigged ?

rigged, weight in the seats, race ready.

Cat,
If you plan to relocate the positions of the axles, you still have to be aware of the potential impact on the tongue weight. Is your goal equal weight on each axle with maybe 8-10 % tongue weight?
What if you just position the boat on the trailer at the targeted tongue weight. Now jack up the trailer and loosen and re-position the axles using scales. When you are done with this, you can post the easy way for the rest of us :drink:
Ben

Ben, I'm curious to know if there is concern for loaded trailer tongue weight and also, boat CG.

We know the trailer pulls better and the boat runs better if weight is properly located.

Considerations for finding trailer CG (loaded or unloaded ) start with a level trailer. Level can be found with a carpenters level or laser. Then, using a single bathroom scale, three sections of round stock (fulcrums) and lever arm (angle stock for rigidity) 4ft.+/- long,we start. Place the lever arm on one fulcrum located 8-10" from another fulcrum centered under the ball coupler, then the long end of the lever arm on the last fulcrum, centered on the bathroom scale.

Using the trailer jack, raise the tongue to fit all components under the coupler ball. (Blocking may be required on each end of the lever arm). The lever arm assembly should be relatively level from end to end. Note the scale weight (lever arm and fulcrum weight) , measure distance from coupler ball centerline to each end of lever arm. Divide the short end into long end of lever arm to find ratio.

Lower trailer jack until it is off the ground. Read scale weight and subtract assembly weight. Divide net weight by lever arm ratio===Trailer ball weight.

Notes: Respect wt. stated on receiver hitch, hitch insert, ball and trailer coupler. A "sharpy" may look at those items in the event of an accident.

Notes: Personally, 500 to 700# is a good coupler ball wt. Subject to, proper components including 2 5/16" trailer coupler and other components rated for that load.

This has been one leg of finding boat and/or trailer CG.

Ben, did we ever tell you about the boat going out on the town for a week ? :drink:

Is the boat over 21 years old :drink: I believe our tongue weight is in the 900-1000 lbs range....we never thought about repositioning the axles but it doesn't surprise me that you have! Are you still planning on a trip to FL?

thats allot of weight in that one small spot.do ya do it with fuel and driver in boat?

When I took my rub rail off for painting it has a black magic marker mark 'arrow',its close to what Chip Fendt told me it was,which was the back of the drivers bolster.

You can scoot a sling back and forth until the CG is revealed.

No the boat isn't quite legal, someone else probably bought :drink:

The CG thing has evolved from a sequence of events ...it seems to be a topic with limitations. However, from time to time a person might wonder if that extra weight is in the stringers, transom or somewhere else. Or, perhaps it's a new rigging, problem or just fine tuning the set up. Were there to be a need we might consider setting up shop.

My wife and I are going to Key West for the races. I may well be in FL before that ! Want to go the races ?

West Texas is found to be nothing special.

No on the races, but I am going to the PRI Show for the day on 12/10

Hang the boat in a travel lift with just one sling. Lift it off the trailer and move the sling until it lifts level. That is the boat CG. Start a couple of feet forward of the engine bay.

That's a good method. What do you consider the "sweet spot", centerline, ahead or behind the rear step ? Then, as drive box's may be added, aluminum cylinder heads replace iron, interior is changed and etc., do you use math to stay at the "sweet spot" or the sling ? Further, what is the simplest method to rig a bare hull. Some start with drive line length, but that seems crude. After everything is cut to length and bolted down it's a bitter pill to move things. A freight scale to weigh each component and math should save a lot of time and other resources. The finished project can be verified with the sling.

If you know the bare hull cg and where you want the target cg to be, then simple math and known weights will let you determine driveline length (along with proper fuel tank, batteries, trim pumps, oil tank, etc placement).

Sean, in your opinion, why do two or even more hulls from the same mold with the same components run and handle different from each other ? Is it anything obvious ? And, is the CG or prop shaft depth more critical ?

The human factor... I would say no two custom boats are identical (lay up/rigging/etc)

The cg and prop height are both critical, it would be hard to determine correct propshaft height without a boat that runs balanced. Also, remember there is a dynamic CG as the boat runs and the tunnel creates lift.