Is it possible to quantify the effect of extra fuel carried strapped to the rail?

I have never liked the practice but many medium size cruising boats carry extra fuel in jerry cans strapped to the rail amidships.

Is it possible to quantify in general terms how this extra weight might effect the vessels stability?

Say five cans of fuel each side of boat on a 35' sail of moderate displacement?
It is not so much weight~ 180 lbs per side..

I like to mutter under my breadth when I see a boat outfitted this way and would prefer to see an extra tank installed below decks.
Am I off base, being too much the curmudgeon or do I have a dock to stand on?

 

You can use standard stability calculations. Add the weight of the fuel at whatever location it's at.

 

bn.......

Yep, it is possible to quantify the effect.......as it happens I was just messing with the Swain 36 sheets so I added 360 pounds on the deck aft of the shrouds. Displacement was at half-load, 19000 pounds, the addition brought her to 19,360, draft increased from 5.06' to 5.08' (about 1/4").......

This addition raised vertical center of gravity by 1", thus the max righting arm (RA) was reduced by about 5%, and angle of vanishing stability (AVS) reduced from 131 to about 128 degrees......

By itself not a huge difference, but there are always others as well.....and they add up quickly.......

 

Many of the small yachts I meet on the ocean are poorly operated . All those fuel cans on deck... a seagoing gypsy ship. Messy practice when a tank heats up in the sun and starts drooling slippery diesel fuel on deck or youre working to windward with the tanks dragging thru the water on the lee side.

Well conceived Small cruising sailboats should have adequate internal tanks for 400 or 500 miles of motoring range . Add sails and its easy to greatly extend that range motorsailing.



Bladder tanks for fuel are expensive, messy and obnoxious to put away after the trip.

If you must use a deck tank make it permanent. A small French cruising yacht just passed thru. It had two outboard motor poly tanks permanently installed aft in an open style transom cockpit. The tanks rested in a lipped SS tray and had A nice teak seat built on top of them . When needed the vessels diesel tanks were gravity fed by plugging an outboard style fuel hose with primer bulb into this deck tank to start the syphon and feed the internal tank. When not needed they doubled as gas tanks for the tender. Clean and seaman like.

 

How do you know the deck tanks are full on these boats you have seen? The idea is to use that fuel and put it into the tank ASAP first.

If your in difficult circumstance you cut them loose.

 

Yes, the tanks have all sorts of disadvantages on a practical level, but most likely, heavy seas will carry them away (along with the stanchions) before they can too much of a stability problem in a gale.

But as a rule of thumb, the same weight on the rail has many times more influence on stability of a yacht of average form, in normal sailing trim (0 - 45 degrees), than the same weight added to the keel. That's why stacking the crew along the rail (or sails and kit to windward below) always pays when racing regular boats. It's also why on boats like the JOD35WBs, even limited water ballast is worth the hassle.

Now if the weight is on the leeward rail, its negative effect will be less marked as the lever arm between it and the centre of buoyancy is less when heeled.

At angles of stability where the rail is immersed, then its influence is much harder to predict. If the cans are only on one side as is common, you certainly can't do it by "adding 360 pounds" to the table of weights because the fuel cans are outside the modelled 'envelope' of the hull. You also have to decide whether they are rigidly fixed and likely to stay there when inverted. If you think they are (however unlikely that is), then their volume also has to modelled, and as diesel in plastic cans are neutrally or slightly positively buoyant, then this will result in an improvement in AVS when capsized towards the side with the cans, or a slight reduction in AVS if capsized away from them as they will be still be airborne when the boat reaches it's AVS.