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Old 11-08-2020, 06:16   #1
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How Displacement Works, How much is needed?

As I do not have the experience that many of you do, I would be appreciative to get some feedback on how displacement affects sailing, especially for Bluewater crossings or sea states that are uncomfortable and unavoidable...

I see that light displacement hulls sail around the world regularly, so how much value is there in displacement and for what reasons?

Can you simply load up a light displacement hull as you would being a liveaboard situation, virtually increasing the displacement to go from light to moderate displacement ratio, all things balanced of course?

I’m not talking anything multihull, just mono. Assume sailing seasons to avoid all major cyclones but reality of early running away if needed.

Every vessel is different, attached are the specs of the one that caught my eye. Dufour Atoll 6. Obviously a charter design, but could be outfitted with proper gear.

Appreciate your insights... I’m an open book to any thoughts you share. Thank you!
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Old 11-08-2020, 07:29   #2
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Re: How Displacement Works, How much is needed?

As a general rule, the lighter a boat is to start with, the more negative the effect of adding weight will be.
The reason is assuming the weight added is the same (cruising stuff) there will be a greater increase in weight from a percent of change in a light boat.

Depending on how the boat is equipped to start with, and the level of comfort you expect and can afford, you may be adding more weight than you think.
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Old 12-08-2020, 17:51   #3
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Re: How Displacement Works, How much is needed?

In your attachments; the "calculations" section is a better place to begin your research that raw displacement... the 122 number there for disp/length used to be considered a fairly light boat, but nowdays not so much... and as a64 note, just loading up a light boat doesn't turning it into a heavy dispplacement boat -- it then becomes a light displacement boat that is overloaded -- the load (people, stores, spares, whatnot...) needs to be matched to the type of boat it is...
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Old 12-08-2020, 18:45   #4
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Re: How Displacement Works, How much is needed?

this is a over-simplification but it will do until you know more :

generally a light displacement boat will be faster and respond more quickly. it will also be more 'bouncy' ie be pushed around by waves etc. up to a point, all these things will reduce as displacement increases

most racing boats are 'light displacement' and most cruising boats are 'moderate' to 'heavy' displacement

as regards loading a boat, all seacraft have a designed waterline. this is where the designer intends the boat to float and it's where the boat will perform best.

most (not all) light weight craft will quickly submerge the designed waterline when you start to load them up, as the extra weight is a greater percentage of the displacement. conversely a heavier displacement craft will usually be able to accept greater added weight - however this is ultimately more a factor of hull shape than displacement

catamarans are a good example. our 7.5mt catamara will not carry the same extra weight that a 7.5mt mono will.

any fairly recent design - esp one where the designer is known and accessible - will have whats called a TPI (tonnes per inch) factor. this is the immersion figure and will tell you how far added weight will sink the vessel

good luck !

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Old 12-08-2020, 19:04   #5
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Re: How Displacement Works, How much is needed?

What can be a little eye-opening is how sensitive the performance ratios disp/LWL and SA/disp are to displacement. Add 5-10% to the displacement and see how much the ratios change from the design values. Then consider that published displacements are usually pretty optimistic to start with.
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Old 12-08-2020, 20:30   #6
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Re: How Displacement Works, How much is needed?

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Originally Posted by sandy stone View Post
What can be a little eye-opening is how sensitive the performance ratios disp/LWL and SA/disp are to displacement. Add 5-10% to the displacement and see how much the ratios change from the design values. Then consider that published displacements are usually pretty optimistic to start with.
Published displacements are generally for the lightship condition, engine and maybe sails but no water, fuel, batteries, dinghy&motor, provisions, personal gear, crew (crew weight can be significant significant).
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Old 12-08-2020, 21:44   #7
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Re: How Displacement Works, How much is needed?

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Published displacements are generally for the lightship condition, engine and maybe sails but no water, fuel, batteries, dinghy&motor, provisions, personal gear, crew (crew weight can be significant significant).
interesting, i always assumed the weight listed was for the bare fiberglass hull and deck only. dont know why i thought that.

would listed displacement include all the cabinetry and interior fittings plus exterior fittings like rigging, winches, etc?

my boat is listed at 16,000 lbs and weighed 19,000 on the travel lift scale. i estimate i had about 1500lbs of water, fuel and batteries. i suppose an extra 1500lbs would add up quickly from miscellaneous stuff.
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Old 13-08-2020, 18:40   #8
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Re: How Displacement Works, How much is needed?

Almost the weight of tools and spares
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Old 13-08-2020, 18:58   #9
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Re: How Displacement Works, How much is needed?

Your 16000lb boat never weighed that, except in dreams...the 100 gallons of water weighs 800lbs. 40 gallons of fuel weighs 280.....

Any batteries more than one, pumps, strainers, anchor & rode, swim ladder, Bimini, dodger, roller furler, etc. etc. Every bit is extra, and your Hull absorbs water over time as well. Bottom paint - how many layers of copper are on the hull. Dock lines, fenders. Battery charger. On and on


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Old 13-08-2020, 19:51   #10
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Re: How Displacement Works, How much is needed?

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Originally Posted by mlydon View Post
Your 16000lb boat never weighed that, except in dreams...the 100 gallons of water weighs 800lbs. 40 gallons of fuel weighs 280.....

Any batteries more than one, pumps, strainers, anchor & rode, swim ladder, Bimini, dodger, roller furler, etc. etc. Every bit is extra, and your Hull absorbs water over time as well. Bottom paint - how many layers of copper are on the hull. Dock lines, fenders. Battery charger. On and on


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OK. not sure im following you on why reputable boat makers would give a false weight of manufacture but im sure it doesnt really matter that much one way or the other. i cant imagine how 1,000 lbs or so give or take really matters that much on a moderate or heavy displacement boat.
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