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Loading of small boats for long trips

andlauer

New member
Joined
15 Mar 2007
Messages
310
Location
Paris France
Bonjour
I would rise the problem of the weight and balance of the boats.

The wave effect are dynamics and forces the oscillation of the boat.

The inertia of the boat is very sensible to weight and even more to the position of the weight. (M*L*L where M is the weight of the object and L is the distance of the object centre of gravity to the boat centre of gravity).

The inertia of the boat create a pendulum effect with its own period. (remember a mooring at Alderney).

While crossing on a small boat the temptation could be to think loading in a car spirit in terms of volume. I think it would be more wise to think loading in an air-plane spirit in terms of weight and balance.

The maximal "useful" weight of a JC boat is in the magnitude of 500kg. Probably less for some of us. The temptation to move the stuff at stern or bow to keep living area is great.
The balance should favour the most heavy stuff (man, anchor, water food...) next to the centre of gravity.

If the boat is overcharged or un-properly balanced the inertia of the boat will be great. In some conditions (different periods of the pendulum and the waves) the pendulum effect of the boat may be in opposition to the wave forced oscillation. The movement for the hull in the wave is then opposing the boat inertia. It will create, on a very short time, huge accelerations and efforts, specially in the mast (L is great). the efforts have to be supported by the rigging and the structure.
If you re-enforce the rigging the weak point is moved to the structure (chain-plate, keel or hull)

With a limited centred weight the boat is more free to follow the waves and the efforts on the structures are then limited.
Eric /forums/images/graemlins/wink.gif
 
Joined
20 Jul 2001
Messages
205
Location
Southampton, UK
Hi Eric

Thanks for that! Food for thought. Makes me wonder if adding ballast to the very bottom of the keel is such a good idea now. I'm keeping the boat very much in cruising trim for the aftermath of the Jester Azores.

Once (God willing) I make the Azores, I'm planning a leisurely return via the Med, so the boat is a compromise in terms of living space. Also, she has a lot of opening ports and vents for the hot climate. All potential weaknesses, of course.

In terms of oscillations, I have tried to bring all the heavy stuff as near the centre as possible. The batteries are by the mast base (rather than under the cockpit) , and the anchor chain has also been bought back from the bow. In the Lazarette, the fuel and water tanks are up against the inner bulkhead. I have a raised goal-post arrangement and a large hatch, and am compensating for this top-hamper with a winged bulb on the keel, packing around 45lbs of lead each side. There is a risk this could induce deeper oscillations in a seaway, but I'm hoping the increased stiffness will allow me to carry more sail and create less leeway.

When my partner and I took the Corri across the France a few years ago, the boat didn't sail too well. We then took on two trolley loads of wine(!) in Cherbourg and put it all up forward. The huge weight of all those bottles bought the nose down and compensated for the two of us in the cockpit. Just trimming her bows this way (accidentally) added another knot to our return journey. I now take trim very seriously.

Thanks again for the insight.
 

2nd_apprentice

Well-known member
Joined
18 Mar 2007
Messages
2,480
Location
Berlin
Thanks for your thoughts Eric!

While talking about weight distribution, loading capability and trim secrets, what about all the fresh water? Nothing against that occasional glass of Beaujolais of course! A lot has been written about splitting the water tanks in order to break up the free surface area and keep the center of gravity higher thus reducing rolling motion. Some even going to the extrem of exclusively using jerry cans. Sure, nowadays one could just go for a water maker substantially saving weight but that seems like opening pandora's box to me. I'd really like to hear about what solutions to this obvious problem participants of the last JC had. Especially those in the very small boats!

Patric
 

JREdginton

New member
Joined
15 May 2006
Messages
155
Baffles in any tank to reduce sloshing are a good idea, water being no different to any other fluid but... Water in split containers is a must for a long passage. If anything happens to your store (contanination, leakage) you have a fall back position, eggs in one basket and all that.

John
 

andlauer

New member
Joined
15 Mar 2007
Messages
310
Location
Paris France
Bonjour
On Sterenn (30 ft), I don't have any water tank. I only use 1,5 l plastic bottles. They are easy to store under the banks... and when they are empty I just add them to the air ballast.

In case of crash, they are not dangerous weapons.

An other advantage is that the use of bottles help you in the management of hydratation (a major risk in single handed). You have to open two and preferably three bottle in a, of course 24 hours, day.
Eric /forums/images/graemlins/smirk.gif
 

MIKE_MCKIE

New member
Joined
5 Sep 2002
Messages
515
Location
Me Hants, Boat Gosport
Whilst putting the additional weight in the middle does tend to lessen the inertial impact, it also detracts from course keeping ability. If the boat can rotate easily about it's axis, it will require more input to steer in a straight (ish) line. The additional movement of the rudder required will reduce speed and may well overcome the theoretical increase in hull speed due to the additional waterline length occasioned by the increased weight. As with all these things, the loading should be a compromise to give a relatively easy ride, while retaining at least a semblance of space below, and good course keeping ability.
 

bumblefish

Active member
Joined
22 Dec 2002
Messages
1,572
Location
Brighton
I have two large (too large!) flexible tanks under the saloon berths. These supply the wash basin and sink. I was planning on emptying these and filling the void with 1.5l bottles. I should be able to fit more than enough in the spaces. Does that sound a reasonable exercise?
 

andlauer

New member
Joined
15 Mar 2007
Messages
310
Location
Paris France
Bonjour
It seems sensible. It avoids any water polution risk and it helps in water management (consomption and deshydratation).
You may also use the flexible tanks as flotablility reserves by filling them with air.
Eric /forums/images/graemlins/smile.gif
 
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