Where is the CofG of a yacht (roughly)!

seanfoster

seanfoster

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I'm going to be lifting my yacht onto a trailer for the first time tomorrow and need to get it sitting evenly as much as possible.

I'm assuming that the centre of gravity is at the mast, would I be correct (generally)?

The boat is a bulb keel Dufour 1800 and it's going on a twin axle trailer, I'm thinking that the line of CofG should be somewhere between the two axles.

Could anyone comment?

Any help gratefully received!
 
seanfoster said:
I'm going to be lifting my yacht onto a trailer for the first time tomorrow and need to get it sitting evenly as much as possible.

I'm assuming that the centre of gravity is at the mast, would I be correct (generally)?

The boat is a bulb keel Dufour 1800 and it's going on a twin axle trailer, I'm thinking that the line of CofG should be somewhere between the two axles.

Could anyone comment?

Any help gratefully received!
Click to expand...

On my boat the centre of gravity is certainly further aft than the mast. It is under the keel.

I think that if you set the keels centrally between your two axles you should be about correct.

The way to find it is to cut out a cardboard profile of a full side view of the boat including keel and rudder then using a pin on rhe cardboard profile find the pivot point.
You will find this is prety accurate.

Iain
 
I agree - usually somewhere just aft of the mast on a Bermudan sloop. Have you ever dried out against a wall? If so, assuming the boat didn't try to tip over backwards or forwards, it must be pretty close to the base of the keel. Can you get the crane driver to take the weight off the slings while you try pulling on the tow hitch of the trailer? You need to shuffle the boat backwards & forwards until the trailer hitc hweight is about right, I'd guess.
 
Strathglass said:
On my boat the centre of gravity is certainly further aft than the mast. It is under the keel.

I think that if you set the keels centrally between your two axles you should be about correct.

The way to find it is to cut out a cardboard profile of a full side view of the boat including keel and rudder then using a pin on rhe cardboard profile find the pivot point.
You will find this is prety accurate.

Iain
Click to expand...

Um, at the risk of being shot down in flames ... surely that will indicate the centre of lateral pressure (CLP) *not* the centre of gravity?

If I'm not mistaken, CofG is going to be much harder to predict and will depend hugely on the shape of the underwater volume displaced by the boat and therefore the centre of bouyancy which will vary a lot - a boat with a fine entry but broad flat run aft will necessarily have, in order the obtain the correct trim, a CofG well aft of amidships whereas a canoe stern boat with a fairly bluff entry will need a CofG miuch closer to amidships.

The critical factor on the trailer is going to be noseweight (i.e. the downward force applied on the hitch of the towing vehicle). Max nose weight varies from vehicle to vehicle and as a general rule of thumb you want something in the region of 90% of max nose weight for stable towing. First time any load goes on to a trailer, unless the CofG and mass are known precisely, there's usually some fiddling about moving the load backwards or forwards to get the correct balance. Good old fashioned suck it and see!

Bru
(Not much experience in moving boats but lots of practice shifting all sorts of kit by trailer and lorry ... and arguing, and winning, arguments with t'management about what a truck can carry and how it should be loaded!)
 
Strathglass, Thanks for that tip I'll try it.

Avocet, I'm intending on doing this (raising and lowering to get about right) just wanted a starting point to go from!

It's also important that the nose weight on the towing hitch is correct (from what I gather somewhere around 75kg for the weight of the load (boat 2000kg, trailer 500kg) if someone knows different please let me know!

Thanks for your replies.
 
brup - shooting you down in flames....

The OP is asking about weight distribution on a trailer, so it is just the weight (in air) that he's concerned with. He's not bothered (now, anyway) about how the boat reacts to wind and water, which is where the Centre of Lateral pressure, centre of effort, centre of buoyancy... would come in. Therefore it's the Centre of gravity, as he says.

I'd agree, by the way, somewhere just aft of the mast, certainly along the main keel, and just below the floor boards.
 
What I would do is lower the boat onto the trailer at your best guess, then before the crane takes the sropps off completley, check the nose weight of the trailer. If too high or (worse) too low, lift again and move the boat forward or backwards a bit.

This is an issue I have with my trailer at the moment, the boat sits too far back putting zero weight on the tow ball, so when the boat's in the water this summer and the trailer's empty, some modifications are needed (basically move the winch post further forward to allow the boat further onto the trailer)
 
Sensible advice from ProDave and Avocet. Position it correctly before releasing the slings.

With a crane the GOG will be directly below the top of the crane jib while it is in the slings and a decent crane operator used to lifting boats will be able to position it near enough spot on at the first attempt.

FWIW the SOG of my boat is a bit aft of the centre of the keels and well aft of the position of the mast.

I have to position my boat almost as far forward on the trailer as is possible in order to get it balanced with the right amount of nose weight.
 
Indeed - and the crane driver did just this when we first put our 30'er on a newly made trailer - it was a lot further forwards than we thought - the back of the fin keel ended up between the axels ...
 
Strathglass said:
The way to find it is to cut out a cardboard profile of a full side view of the boat including keel and rudder then using a pin on rhe cardboard profile find the pivot point.
Click to expand...

Wha?!?

Surely that will only work if all parts of the boat are of equal density? Given that parts of it are made of thin fibreglass and parts of it are made of lead or cast iron, that's a patently false assumption.

Wiggling the trailer as described sounds like the best way.

Pete
 
seanfoster said:
Strathglass, Thanks for that tip I'll try it.

Avocet, I'm intending on doing this (raising and lowering to get about right) just wanted a starting point to go from!

It's also important that the nose weight on the towing hitch is correct (from what I gather somewhere around 75kg for the weight of the load (boat 2000kg, trailer 500kg) if someone knows different please let me know!

Thanks for your replies.
Click to expand...

75kg sounds a bit light for such a big load, but you'll be limited by the capacity of the towing vehicle. If it was registered after about 1996, it ought (legally!) to be fitted with a type approved towbar which should have an approval plate on it. There's likely to be a "D" weight (or "S" weight - can't remember!) marked on it (again, a legal requirement) and that's the maximum permitted noseweight for any trailer that you're pulling. Failing that, it ought to be in your handbook somewhere. I'd be tempted to go for the maximum permitted!
 
Another important factor is the fact it is a twin axle trailor. I used one for a racing car and we found the tow hitch position was very important. In the end we set the trailor up level and then set the tow hitch to slightly raise the front and induce some nose weight.
Allan
 
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The centre of gravity is exactly in line with the centre of bouyancy. Which might soulnd blinding obvious but is the useful reference that you need.

Obviously if you move the centre of gravity by any amount the centre of bouyancy must move to bring the boat back into equilibrium and stop it rolling end over end.

To get the most accurate result you really need to add known weight at one end and remove the same weight at the other. But for the sake of generally establishing where it is on a 25 footer, it would be sufficient to just add two people to one end and measure the change in freeboard and then do the same to the other end.

Of course it does need to be pretty calm! But with the waterline length either measured or known, then the change in freeboard becomes the ratio along the waterline length at which the CofG lies.

Having said that the CofG will be around a point above the lower part of the fin - otherwise the boat will pitch backward or forward when drying out. Its vertical position on a boat like this may actually be quite high - around one third to half way up the hull depth.

Either way the critical point is to ensure the crane driver knows the boat cannot be simply lowered onto the trailer. It will need to be moved backward or forward to ensure the supports do just that and you already have plenty of advice on trailer nose weights.

Knowing where the CofG is can really help in positioning where the strops go - or if necessary changing their lengths to make the boat hang more of less horizontally - this will usually mean adjusting the chains supporting the strops but is well worth doing prior to the initial lift.

Adjusting the position of a boat which is not hanging horizontally on a trailer is really difficult, although from memory a Dufour 1800 should not be too bad as the stern is not that wide.
 
Allan said:
Another important factor is the fact it is a twin axle trailor. I used one for a racing car and we found the tow hitch position was very important. In the end we set the trailor up level and then set the tow hitch to slightly raise the front and induce some nose weight.
Allan
Click to expand...

Yes that's important also.

My boat trailer, even when empty has a high towball height, much higher than the standard towbar height on my car and much higher than any other trailer I tow.

My solution was to fit a drop plate "upside down" to give me a second towball about 8" higher than the first one.

People often ask why I have two towballs on my car.
 
Some good stuff on this thread.

Your Dufour may be a bit big but on my trail sailer I tend to put heavier things like the outboard and rudder infront of the CoG especially if I can lift the tow hitch a bit too easily. The nose weight doesn't have to be spot on if you have scope to move things, you can always move the winch post an inch or two for next time.

The extra weight in the nose stops the jockey wheel lifting on the slipway when the car is unhitched too, not a problem if you are lifting on.
 
AliM said:
brup - shooting you down in flames....

The OP is asking about weight distribution on a trailer, so it is just the weight (in air) that he's concerned with. He's not bothered (now, anyway) about how the boat reacts to wind and water, which is where the Centre of Lateral pressure, centre of effort, centre of buoyancy... would come in. Therefore it's the Centre of gravity, as he says.

I'd agree, by the way, somewhere just aft of the mast, certainly along the main keel, and just below the floor boards.
Click to expand...

Putting out the flames :) yes, I understand what the OP is trying to ascertain, it was the method suggested to determine the CofG (of balancing a profile drawing on a pin) that I was questioning - the method described would determine the CLP *not* the CofG

Runs and hides :)

Bru
 
brup said:
it was the method suggested to determine the CofG (of balancing a profile drawing on a pin) that I was questioning - the method described would determine the CLP *not* the CofG
Click to expand...

It would also determine the CoG, provided you had a boat of uniform density. Outside first-year Mechanics courses, that's unlikely to be the case, of course :)

Pete
 
The longitudinal COG (LCG) is not geometrically determined - but will be by definition at the same point as the LCB when floating.

It will be different with the rig up and the rig down. It will also vary with the fill of the tanks.

IMHO adjustments with the crane until you get a sensible nose weight will be the easiest way of positioning the boat. This is how I have seen the guys at one of the yards I work at put a motor boat onto a general trailer for transport.

The crane drivers at boat yards are generally experienced and will understand what to do. Have a chat with them as they will have done it many times before.
 
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