Vertical Center of Gravity...how low is it?

[er9]

I know the math can get very complicated and not such an easy answer but generally speaking...is it possible to guess the approximate area where the center of gravity is on a monohull? for example would one estimate its just above the top of the keel, floorboard height, waterline height?

reason i ask is that adding gear , fixtures, tanks etc above this point affects stability so ideally we want to add heavier loads as low as possible. would adding 500lbs directly above this point drastically affect stability or marginally?

im considering moving my fuel tank a foot higher in the hull (40 gall tank) but dont want to make a drastic change to offshore stability. trying to guestimate the limits of what i should and should not do.

any insight would be appreciated.

[john61ct]

Would vary with each boat design, may be spelled out in the drawings.

Some initial terminology here:
http://www.tedbrewer.com/yachtdesign.html

[Alan Mighty]

Although generalisations are dangerous, any boat with a mast allowing a high aspect sailplan has a surprisingly high VCG. That's precisely why big money goes into carbon fibre masts and other ways of managing the VCG.

One question that could be asked is: what problem are you seeking to solve by raising your fuel tank.

John61ct is on the money: your designer/boat builder should provide you with a Stability Information Book that tells you the location of VCG in lightship/arrival mode and in loaded/departure mode. With the software available to boatbuilders, designers, and naval architects these days, generating that info is simple. A few boat designs also come with Stowage Rules or Stowage Guidelines that complement that SIB info.

Note that diesel fuel has a specific gravity of 0.86 tonnes/m3. So raising a diesel fuel tank does not have as serious consequences as raising a water tank of the same volume or an engine. Nevertheless, it is not something I could suggest you do without meditating on the consequences.

You should also be aware of whether your fuel tank is baffled or not. A SIB for your design should tell you about the free surface effect from your tanks at 50% slack and whether the free surface effect is significant for your boat (in general, unless your tankage is more than one thousandth of your vessel displacement, it won't be).

From the Stability Information Books of cruising yachts I've read (you should be able to find a few on the Internet using appropriate search terms such as "SIB"), VCG is usually above the waterline.

On my s/v Led Myne, the vertical shift in VCG from arrival mode to departure mode is only a few centimetres (which likely equates to "an inch or so" in your brain). And the Stowage Guidelines for the design include things like stowing anything that would fail a sink test a low as possible.

[john61ct]

In looking for a small blue water cruiser, I came across

Twister 28, with sa/d = 12, 10,000 lb dispacement, most of that lead ballast in the full keel, total 5' draft.

I bet its VCG is below the waterline
8-)

While a light racer or coastal trailer sailor would have a completely different weight distribution.

[Jim Cate]

Another way to look at it: do you worry about adding 3 crew persons in the cockpit? About the same mass, considerably higher up...

Jim

[Alan Mighty]

Originally Posted by Jim Cate

Another way to look at it: do you worry about adding 3 crew persons in the cockpit? About the same mass, considerably higher up...

With one or two differences: the meat is mobile.

Meat can be rail meat or go below to a bunk. And Rail meat performs the opposite of tankage: tankage always sloshes to leeward but rail meat can be canted (or prodded) to windward.

The question remains: what problem is going to be solved by lifting the tankage 30 cm?

[er9]

Originally Posted by Alan Mighty

Although generalisations are dangerous, any boat with a mast allowing a high aspect sailplan has a surprisingly high VCG. That's precisely why big money goes into carbon fibre masts and other ways of managing the VCG.

One question that could be asked is: what problem are you seeking to solve by raising your fuel tank.

John61ct is on the money: your designer/boat builder should provide you with a Stability Information Book that tells you the location of VCG in lightship/arrival mode and in loaded/departure mode. With the software available to boatbuilders, designers, and naval architects these days, generating that info is simple. A few boat designs also come with Stowage Rules or Stowage Guidelines that complement that SIB info.

Note that diesel fuel has a specific gravity of 0.86 tonnes/m3. So raising a diesel fuel tank does not have as serious consequences as raising a water tank of the same volume or an engine. Nevertheless, it is not something I could suggest you do without meditating on the consequences.

You should also be aware of whether your fuel tank is baffled or not. A SIB for your design should tell you about the free surface effect from your tanks at 50% slack and whether the free surface effect is significant for your boat (in general, unless your tankage is more than one thousandth of your vessel displacement, it won't be).

From the Stability Information Books of cruising yachts I've read (you should be able to find a few on the Internet using appropriate search terms such as "SIB"), VCG is usually above the waterline.

On my s/v Led Myne, the vertical shift in VCG from arrival mode to departure mode is only a few centimetres (which likely equates to "an inch or so" in your brain). And the Stowage Guidelines for the design include things like stowing anything that would fail a sink test a low as possible.

Unfortunately the builder went out of business in the 80's but i do know who the naval architect is for my boat...it might be worth trying to contact him i guess to see if drawings are available or if he may have an opinion. other than sailboatdata.com i cant find much more specific info on my particular boat yet.

I'd like to replace and move the tank (which is currently under the starboard quarterberth) to create access to my engine and behind it which has none at the moment. i have a lot of work to do behind the engine and currently can not get to that area without pulling the engine. beyond that, access to the shaft seal, batteries, scupper seacocks, water heater and heat exchanger are almost impossible at the moment. god forbid my shaft seal or scupper hose starts to leak as i would have a very, very tough time stopping it.

there is a nice big space foreward of my steering quadrant behind the engine a couple feet if i move my water heater. i could easily fit a custom made tank in the space but as mentioned would be raising the tank maybe 8" or so above water line. Pearson does this on their 365 which is almost an identical boat in almost every way to mine 'EXCEPT' the Pearson has 1700lbs more weight in its keel than my Mariner although my keel is a foot deeper.

I do have a second option that is i could divide the fuel tank into two smaller 20 gal tanks and put them under the starboard and port settee's which currently house two 52 gal water tanks under each. This is safer option as the tanks would be under the waterline but very expensive to do as i would have to have four custom built tanks instead of one.

ultimately though safety and stability are most important but if its no issue moving the tank to the stern would give me more below water line storage option mid ships...tough call....need more info...

[er9]

Originally Posted by Jim Cate

Another way to look at it: do you worry about adding 3 crew persons in the cockpit? About the same mass, considerably higher up...

Jim

good point...absolutely not. im probably way over analyzing this but thats just because of my inexperience but i dont want to make a costly mistake either...

[Dave22q]

Easy rule is do not raise weights without strong reasons to do so. I cannot envision a reason why raising a 300 lb fuel tank a foot is desirable. It is a small factor but why do it?

[danhunt]

Are you raising the tank to install something beneath or just to have more space? What are the demensions? In essence unless the tank is only as deep as you are raising it you would be subtracting 12 inches from the bottom and adding 12 to the top.

[TrentePieds]

There were two Mariner 36s, I see, one by Peter Canning and one by Billy Garden. They had/have entirely different hulls, and the Canning boat is a 7 ton boat, the Garden one a 10-tonner. Both have ballast at about 1/3, though the Canning job is a fin-keeler, while the Garden job is a full-keeler.

I really don't think it matters which you have. As Jim sez, you are proposing to shift weight equivalent to asking three winch-gorillas or four wizened geezers to get on their pins up off the settees and join you in the cockpit. Why worry about it?

The Canning version is likely to be harder in the bilge than the Garden version and therefore to be stiffer initially, but more "problematic" at high angles of heel than will be the Garden job. IMO you are unlikely to even notice any effect by the weight shift on your righting moment. In case it still keeps you awake at night, here is a luvverly practical test that will give you your answer: Get four innocents, lash them to the cabin top, go out in 30 knots of wind, strap 'er down hard on a beam reach and give 'em the thrill of their lives. :-)

That would replicate quite satisfactorily the shift in the CG of moving your tank.

Have fun :-)!

TP

[Cheechako]

It doesn't sound like a major thing. As Jim Cate mentions, loading is pretty variable, and often boats get rearranged. You might ask yourself a few questions though:
-How tender is the boat now?
-Does it "squat" much in the stern? you are moving the tank more aft right?
-I like the two smaller tank option. It will likely be easy to find 20 gallon tanks premade of plastic. Another advantage is you draw from one tank, and replace that. Draw from from another tank next time. Contamination in one doesn't mean you are stuck either.
- Does your boat sit even in the water now? If so it may be designed to do so with the weight of the fuel on one side.
I have trouble visualizing your problem though. How does the tank under a quarter berth obstruct engine access? Are you considering removing the 1/4 berth entirely? Otherwise, the berth construction obstructs anyway doesnt it?

Another possible option. I did this on my 30 footer before going offshore. I installed a big watertight aluminum Freeman Hatch in the cockpit floor. This allowed me to actually crawl into the area behind the engine for service etc. Although these are expensive, probably no more $ than all the work you indicate.
The type I used had screw dogs on the perimeter for occasional access. No hinges, the lid just removed. Not sure they still have those, this is similar but with a big handle opening type. Mine sat about 1/2" above the cockpit surface, so I put a wood grate in the cockpit to make it flat everywhere.

[captmikem]

I think what you are interested in is Metacentric height, (GM), which is the distance between the center of gravity of the ship and it's metacenter. This gives you the stability of the ship. We calculate it on ships each time we load or discharge cargo. Sailboats normally have a high GM which causes them to snap around when rolling without sail up which is not a pleasant motion. Cruise ships ballast for a low GM which gives the a slower roll period making it a more comfortable ride.

It is basically the stability of the ship; High GM, very stable, low GM, less stable.

It is pretty easy to calculate based on the roll period of the vessel, or using a simple incline test at the dock. (Race boats do this test often)

Here is a link to a PDF for calculating GM on small fish boats that you can use to get a relatively accurate idea of the stability of your boat. If your boat is small (say under 60 feet or so) you can get her rolling at the dock just by stepping aboard and ashore a number of times, then accurately measure the roll period.

https://ir.library.oregonstate.edu/downloads/0r967460r

Or, you can do a search on line for incline testing which will show you how to do that with a bucket of water out on the boom.

M

[Olddave]

Originally Posted by er9

I know the math can get very complicated and not such an easy answer but generally speaking...is it possible to guess the approximate area where the center of gravity is on a monohull? for example would one estimate its just above the top of the keel, floorboard height, waterline height?

reason i ask is that adding gear , fixtures, tanks etc above this point affects stability so ideally we want to add heavier loads as low as possible. would adding 500lbs directly above this point drastically affect stability or marginally?

im considering moving my fuel tank a foot higher in the hull (40 gall tank) but dont want to make a drastic change to offshore stability. trying to guestimate the limits of what i should and should not do.

any insight would be appreciated.

The rule of thumb to remember is “the centre of gravity will move towards any weight added or moved”
Therefore, raising and moving the tank aft is bound to have an impact on the CG.
It is a fair amount of weight you are talking about and the three crew member scenario remains after you have moved the tank. Ie. you raise the CG and then add theee solid crew members which raises it some more.
I think it’s a valid concern, and certainly worth considering
Dave

[cyan]

This is a great learning thread, as Metacentric Height explains so much of what I always suspected. I just didn't know what it was called.
Thanks for all the good information!