Centre of Weight

[Poida]

I would like to know where the centre of weight is on my boat.

I was contemplating the see saw. It see saws because the weight is evenly distributed from the middle (fulcrum). A small amount of weight distribution makes it swivel.

If when my boat is in the water and I start pushing the bow up and down will the boat see saw at the point of the centre of the boat's weight?

Or does bouyancy also come into play here as well. I thought that if the boat was forced up and down it may overide bouyancy and rock in the centre of the weight distribution.

What do you guys reckon?

[PAR]

Any object, floating on water (not moving) is in equilibrium. The force of weight (exerted at the Center of Gravity) is battling with the hydrostatic force of buoyancy (exerted at the Center of Buoyancy). These two centers will align vertically and is the natural tendency of any floating object. A weight shift (movement of the CG) like a crew member walking forward on the deck, will cause the boat to adopt a new alignment of the CG and the CB. This may entail boat trim changes as the new equilibrium is settled on. In practicality, the two centers are in constant motion, readjusting with each passing wave, wind shift and even a crew member raising a bottle of beer to his lips.

On land the CG is fixed (unless you move something). A small boat could be lifted bodily, placed on a fulcrum and adjusted until it balanced, but I'm not sure how knowing this point is important to you.

What boat do you have (year, model etc.)? If it's production boat (manufactured) the center of mass will be known and likely available from the manufacture or designer. If it's a one off design, then the designer will have this information. If you designed the boat, then it should have been one of your calculations, during the design process.

[alpamis34]

The centre of gravity does not change by lifting up the bow,you have to displace the weight within the boat in order to achieve an LCG change or VCG.The boat will trim from the centre of flotation which is a point generally close to centre of buoyancy.But of course when you change the hull shape the LCB is going to replace itself.

[Eric Sponberg]

Alpamis is correct in that the boat trims about the center of flotation, which is the geographic center of area of the flotation waterplane. If you have a lines plan, and can plot on the lines plan where the flotation waterline actually is (by measuring the freeboards fore and aft and side to side) you can calculate where the center of buoyancy is, and the center of gravity will be directly above and in line with that. If you don't have a lines plan or other 3-D model of the hull, there is no easy way to determine where the boat's center of gravity is.

Eric

[jehardiman]

Originally Posted by Eric W. Sponberg

...snip... If you don't have a lines plan or other 3-D model of the hull, there is no easy way to determine where the boat's center of gravity is.

Eric

Alpamis and Eric have already pointed out that the boat will pitch about its center of flotation....and it will also heave and surge as it pitches unless the waterplane is bi-axialy symetric (i.e. a circle).

They did leave out one point though.

If you have the money, the capability, and the need; it is possible to determine the exact CG in all 3 axies without resorting to a lines plan drawing. To do this you need to suspend the vessel from two points on CL from a single pick point spreader bar and very good load cells in the working legs. This was always done as a check of the weight/buoyancy calculations for Deep Submergence Vehicles both in air and submerged. In a DSV, 5 lbs and 1 inch out of trim at the surface could kill the operators at 10k feet depth so the cost and hassle was worth it.

[Poida]

Thank you for your replies.

I didn't state the reason for wanting to know the centre of gravity as I thought it might confuse the issue. But I confused it more not telling you.

I am designing a new trailer and would like to know the centre of gravity for the boat to determine its position on the trailer.

Par you said the centre of gravity and the centre of flotation would align vertically (think that's what you said) the size of my brain is only a fraction of yours.

This would mean in relation to the length of the boat it would be in the the same position. If not would it be close enough to use the centre of floatation as the centre of gravity?

[PAR]

I suspect you and I have similarly sized brains, though I do have a substantial protuberance in my frontal lobe, but it was likely from banging my head into something hard.

I put boats on trailers several times a years. It's a guessing game to see how close I can estimate tongue weight, proper support and balance. The first thing I do is take a good look at the boat, with an eye towards where the weight will be if hung in slings, which is often the case when I place the boat on the trailer. Sometimes, I'll pick up a boat at a ramp so it's even more of a guess. Plastic boats can tolerate a poorly fitted trailer (for a short time) much better then a wooden hull. On a wooden hull I'll make necessary adjustments at the ramp, moving axle(s), rollers, bunks, supports, etc. to get it close enough to not spring planks or do damage until I can get it back to the shop.

On a plastic boat, I'll drag it up on just about anything, take it home and adjust it there, so long as nothing on the boat is getting badly distorted by the trailer.

Once in the yard, I'll lift the boat, which for most folks means a couple of slings from George's tractor or my chain fall. I usually just lift clear of the trailer, move things around for good support then drop her back down. Then check the tongue weight, which I like to be 60 to 70% of the hitch rating (Okay, it depends on the hitch). I like the weight to be about how much I weigh, because I can lift my own weight (on an average 18' plastic runabout), but larger boats will require heavier then my 150 pounds. If you're light, move the axle(s) back, heavy move it forward. Most axle(s) assemblies are mount on a track (usually a big piece of angle stock), which can slide forward or back on the trailer frame rails. The springs, their hangers, fenders, axle(s) and all usually move as a unit. Just make sure they're straight, by measuring from a fixed, centerline point at the front of the trailer to a common fixed point on the axle(s). Both sides should be the same distance from this forward point. If you do have to move the axle(s) assembly, then make a reference line on both sides at the slide thingie (marking on the frame), not the "U" bolts, so you can measure from these (assumes the trailer was properly aligned to start with). I use a come-a-long to drag the assembly forward or back on the trailer frame. It's a lot easier to move this assembly if the boat isn't on the trailer, but it can be done with it in place, if you use care.

To directly answer your question about CG and the trailer setup, it's in the axle(s) placement really. An axle to far back will make the tongue too heavy, but if it's well supported, she'll ride short distances, until you get it right. Conversely, if the axle is too far forward, the tongue weight will be light (not good) and it'll tow poorly, possibly dangerously if very far off. The boat will jerk up on the hitch after each bump and you could snatch the ball right off the hitch (seen it happen) or break the hitch. Trust me, at 50 MPH, you don't want to watch your boat pull into the passing lane without you. I've seen this happen more then once and those safety chains are great if you're going slow, but a good jerk at 50 MPH and those chains are history and you boat is off on a wander'bout, by itself.

Most outboard production boats will want the axle(s) nearly as far back as they'll go. An I/O setup may like it slightly forward of this, with inboards and sailboats wanting a more centrally located axle(s) arrangement. The exact location is frankly, guess work, though with experience you can eye ball it damn close.

In most fair sized cities, near the water, you'll find a boat trailer builder. They fit boats to trailers all day long and have the hoist(s) to do it easily and safely. They'll charge a small fee to fit your boat on a trailer properly. Considering the problems that can crop up during the process (like dropping the boat) you may be best advised to seek one of these outfits out.

[Poida]

Thanks Par

You've cited all the reasons that I was asking how to assertain the centre of gravity of a boat in the first place, for weight distribution.

I thought I posted a reply yesterday but I've just got a new computer and the reply seemed to have gone astray.

Telescoping drawbars, not happy with the strength when the drawbar is fully extended with the load distributed over a small area making the winch overwork to retract it. I also want to keep away from rectangular hollow section due to rust problems.

Extending the drawbar with a bolt in section. Good idea I think if you normally moor your boat and only need to take it out occassionally, but I don't want to have to do that everytime I launch the boat.

I've had quite a lot to do with trailers in the past but not boat trailers, and I'm in the engineering trade so manufacture of my design will not be a problem. Towing it might be

I might take my boat down to the ramps when it's quiet and pull my boat in and out and try different positions to get an idea of the centre of weight.

The trailer I have is definately not designed for this boat, is just about rusted out and the wheels are not in line with the drawbar. So I've got to build another one anyway so I may as well design something more suitable for my application.

Started drawing last night, which included long blank stares at the screen, but then the answers come to me when I'm in bed at night or in the toilet.

Catch up with you later

Poida

[PAR]

A removable tongue doesn't have to bolt in, it can just have a hardened pin or two to lock it down, similar to how a receiver hitch is secured. Pull the pins, swap the tongues and you're good to go, taking less then a few minutes.

I too dislike box or rectangular section trailer frames, prone to rusting out and holding moisture. Typical rectangular tube sizes for your boat may be 3" x 5" or 4" x 6", which is pretty stout stuff. I like the structural "C" section frames or "I" beam (typically aluminum) which will shed water and are easily cleaned.

Take a look at the construction details on some of the trailer builder's tandem and tripe axle rigs available. The parts are commercially available and the fabrication is minimized. No need to re-invent the wheel here, you're not alone with this particular set of issues. Axle placement will govern the balance of the yacht, regardless of it's particular CG location.

Another tip for difficult to launch boats is to set the boat on the trailer with it's bow down as much as 6 degrees (referenced to the LWL). This permits the boat to float free of the trailer all at once, in spite of the trailer being on a ramp (typically set between 6 and 8 degrees). Many boats, sitting relatively level on the trailer, will have it's stern floating well before the bow can get free of the trailer. With the boat, bow down she'll pop up when deep enough, without teeter tottering on the forward trailer rollers. It also makes loading the boat a lot easier, because the bow doesn't rise up (pulling away from the bow chock) as you begin the pull up the ramp, during recovery.

Some of my best work in done in the john . . .