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Easy way to find the C.G on boat

Old 08-30-2012, 10:17 AM
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Default Easy way to find the C.G on boat

While I was support on a race team, we could pick up the boat using two hoists, a scale and get the weights.Then calculate the C.G.
Can it be estimated using the water line ? any other backyard mechanic method or formula ?
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Old 08-30-2012, 11:37 AM
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Sure. It's easy. Of course, you'll need a commercial drive-on scale, or some other way to measure your weights.

(1) Weigh your trailer empty. Record the weight readings at the axles, and then at the tongue. With this you can get the CG of the trailer by itself.

I'm just making up numbers here, but let's say you get 1800# at the axles and 200# at the tongue. Therefore, your trailer weighs 2000#.

The CG is going to be 10% (200 [light end] / 2000 [total]) of the distance from the heavy side (the axles).

Or conversely, 90% (1800 [heavy end] / 2000 [total]) of the distance from the light end.

So, let's say it's 20' from the axles to the tongue. Your CG is 2' from the axles, or 18' from the tongue.

Either way you do it, it will work out the same.

(2) Repeat the weighing with the boat on it this time. Again, record the weight at the axles and at the tongue.

Do the same math with the new numbers to arrive at the CG position of the trailer-boat combo.

Let's say it was 7,800# at the axles and 2,200# at the tongue. The total is 10,000# (2,000 Trailer, 8,000 Boat).

The CG from the axles of the combo would be 2,200 / 10,000 X 20 = 4.4'

From knowing those two positions you can precisely calculate where the boat's CG is.

(3) We're going to do a summation of moments at the axle position, I'm calling that the zero position.
(A moment is also known as a torque expressed as Distance X Force[Weight], i.e. FT-LB, IN-LB, etc.)

So, the moment generated about the axles by the boat-trailer combo is 10,000# X 4.4' = 44,000 FT-LB

We know that the moment generated by the trailer alone is 2,000# X 2' = 4,000 FT-LB

So, from that we can compute that the necessary moment that the boat needs to generate is 44,000 - 4,000 = 40,000 Ft-LB.

Thus: 40,000 FT-LB = 8,000# X the distance from the boat's CG to the axles.

40,000 / 8,000 = 5 FT.

So, in this example the distance from the axles to the boat's CG is 5 feet.

Last edited by CigDaze; 08-30-2012 at 11:41 AM. Reason: sPeeling.
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Old 08-30-2012, 11:58 AM
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That’s a pretty cool method! When we were setting up one of our boats in the past we just put a heavy duty jack under the keel with a 4x4 piece of lumber and just kept moving from front to back a few inches at a time until the boat tried to lift evenly off the trailer, just to be sure we did it from the opposite side back to front and made sure that we had the same result. Not sure how accurate that was but we never had a problem towing the boat again.
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Old 08-30-2012, 12:11 PM
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If you have the equipment to do so, that's a perfectly valid method. The center of gravity, is simply that, the center - equal mass in front of and behind that 'balance' point. That could get dangerous or awkward, though. I'm trying to picture how that would work without tipping the boat.
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Old 09-03-2012, 08:27 AM
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What Mercury used to do at Lake X was just lift the boat about 1 foot out of the water with a single strap. Just raise and lower the boat and move the strap/hoist until balanced. Any marina with a travel type lift can do this, it shouldn't take but 15-30 min. Make sure that the crew and only the race gear is in the boat and the fuel is at (avg load?).
The next question is what do you do when you find the CG?
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Old 09-03-2012, 04:23 PM
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Hi Ben, Long time since Orlando PRI....Hope all is good !!

Yesterday, we took the Motion to Texoma. We had changed the fuel circuit and it worked perfect. (electric aeromotives set low for back up/prime and white spring CV billet as primary). Hilborn return to tank regulator plumbing was added to a Hilborn flapper valve. The flapper valve isolates the electric/mechanical pumps and smooths frequency waves.

That all sounds as a Thread High Jacking, Sorry guys...Just saw Ben's Handle.

Ben, the sling strap solution has always concerned me. Others suggest using a large padded pipe and rolling the hull fore and aft on that fixture until finding a balance point. Both solutions are similar, but concern is the pressure point on the hull.

Your question of what to do with the CG findings is valid. However, subject to the boat being a step bottom, the manufacturer may offer recommendations for locating the CG relative to the last step. Or, if not a step bottom, make recommendation measuring from the transom. Regardless, the boat should be set up to fly flat or slightly stern heavy, to avoid stuffing.

CigDaze, IMHO, your instructions are correct. One additional point, consider the trailer design. The trailer should be jacked up and a sturdy stand placed at the trunnion. The axle springs and tongue height may otherwise create an erroneous number. An example of proving that statement is the small amount of weight required to change the tongue weight when setting on a stand Vs: setting on the tires.

Armed with physics, bath room scales and a beam, you can find CG of the Boat/Trailer combination. The trailer is first placed on trunnion stands, the beam is then placed cross-ways under the receiver hitch ball. ( example would be a 6' beam setting on a stand 12" to one side of the ball and the scale set at 60" from the other side of the ball). (The trailer and beam should both be level to a bubble). The tongue is then adjusted until all of the the tongue weight is setting in the beam. (a small diameter pipe placed between the beam and ball will improve accuracy.) The scale reads 200 pounds...The lever arm is 12/60 =20 %, 200/.20=1,000 pounds. (you can play with the arm lengths as desired)

Then, have an assistant walk fore and aft on the deck. Watch the scale as it goes from a positive to negative number. That change from positive to negative will be CG.

Once you weigh the total combination on a platform scale, the same procedure can be added to more math to find Boat CG. However, the simplest is what CigDaze suggests. The above can be value added for fine tuning.
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Old 09-06-2012, 11:02 AM
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Originally Posted by CigDaze
If you have the equipment to do so, that's a perfectly valid method. The center of gravity, is simply that, the center - equal mass in front of and behind that 'balance' point. That could get dangerous or awkward, though. I'm trying to picture how that would work without tipping the boat.
This was all done with the safety of the trailer under it. When I said lift it I wasn’t doing anything more than just lifting it until it left the bunks no more than an inch or so and the part about tipping wasn’t that bad either because it acted like a three point lift until you were close to lifting from the CG but you were able to tell before it ever had the chance for a problem.
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Old 09-22-2012, 09:40 AM
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Wow, time got away from me with other projects, like my Grand Sport bike carburation. This thread all started because unlike any of the other boats I have owned, I have a boat that that does not sit Bow high. It traps water and the bilge pump empties what it can. If I have the boat on the lift, then I can pull the transom plug and drain 6 gallons out ! I went back to my race days to figure out how this boat hull and weight distribution was designed. I also researched CG and there are several different variations. Depends what you are looking for. I was looking for the simple fore/aft . Thanks to all for good reasoning on this subject.
Now back to my original thought.
If a boat is sitting in the water and a "water line" is scribed on the hull, can't the CG be estimated by measuring the length of the wetted line,(horizontal, call it "A") the depth at the transom, (verticle line "B") and form a triangle by connecting the two diagonally? ( "C") Then divide the formed triangle into equal AREAS. Displacement would be synonymous to area by slideing the horizontal line to find equal area, not length.
Another way might be: To have a 200 lb. person walk for and aft while afloat with someone in the water to chalk the change in the water line. Adaption from a previous reply

Last edited by stillcrazy26; 09-22-2012 at 09:47 AM. Reason: Try to clarify last sentence
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Old 09-22-2012, 07:22 PM
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The Bow Low makes one wonder if the Bow is saturated. Though, I realize that is not your question. Your last remark causes curiosity to ask if anyone has tried an inflatable cylinder. The cylinder would be placed crossways, under the hull and inflated. The result may be similar to BenPerfected Lake X/Mercury strap solution.

Subject to the cylinder capacity, at some point the cylinder should become the fulcrum. Then, it should further verifiable/testable by the 200 lb. person walking the deck.

Variable force lines may otherwise cause difficulty for finding a specific CG location, using only your solution.
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Old 09-24-2012, 07:33 PM
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Basically what we are trying to accomplish by all the suggestions is find the "Balance point", or Pivot Point mechanically. That's why when the boat is afloat, it IS at balance in the water. The heavier end sits deeper aft because of the engines, drives etc. That is counterbalanced by everything ahead of that. The water accounts for any difference in hull configugation by displacement.
If it sat level like a barge, then BP or PP would be 1/2 the length.
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