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Old 16-11-2015, 11:21   #1
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Displacement and ballast

Hi,

Probably a very naive question: On the specs reported at sailboatdata(.com), does the displacement value includes the ballast value?

I have a Paceship P29, for which the displacement value is 6,500 lbs and the ballast value is 3,160 lbs. While it was in the TravelLift, the reported weight was 10,200 lbs. The boat was almost empty, being prepared for winter. Everything left on it couldn't add up much more than 500 lbs. (see below). So it seems that the overall reported weight is displacement + ballast + stuff (6,500 + 3,160 + 514 = 10,174 lbs)

For some reason, I always thought that the displacement value reported by the maker included the ballast value. Either I have been wrong all this time, or a have a whole family of refugees hiding somewhere in my boat.

item density lbs/gallon volume weight
water 8,35 20 167
gasoline 6,25 17 106,25
gas tank 10
sail 15
cushions 10
water heater (water) 8,35 6 50,1
water heater 10
running rigging 30
spares 50
electronics (VHF, radio) 10
anchor 1 25
anchor 2 30

Total 513,35
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Old 16-11-2015, 11:30   #2
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Re: Displacement and ballast

Displacement does include ballast. Often the designed displacement includes the ranks being half full as an average of the total actual displacement.


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Old 16-11-2015, 11:31   #3
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Re: Displacement and ballast

If it helps here it is in writing: displacement = net weight

P29 Specs
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Old 16-11-2015, 19:36   #4
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Re: Displacement and ballast

I have a similar boat, though not as beamy. I have 3120# in ballast. I have a feeling that the P29 specs are a little light (displacement includes ballast) or someone before you added ballast because they felt it was too tender or the travel lift doesn't have a very accurate scale, or all three perhaps?
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Old 17-11-2015, 08:23   #5
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Re: Displacement and ballast

The marina owner seems knowledgeable and assures me that the TravelLift reading is right. If this is the case, and that displacement usually includes ballast, then I'm baffled. A 3,000# mystery. Don C L: The Columbia 29 looks nice! Do you still have the A4?
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Old 17-11-2015, 09:15   #6
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Re: Displacement and ballast

Travelift measure is only very very very broad brush.

We lifted a 40 ton boat recently and a 60 ton lift operator came up to us to make sure the boat was 40 tons (???) The lift read 60, roughly 50% off the mark.

Then a 10 ton travelift readout may be anything between say 5 and 15 ton in real life. Or anything else too!

Scales used to weigh truckloads of grain can be used to exactly measure a small boat. Farmers tend to get very pissed off when they find any inaccuracies ...

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Old 17-11-2015, 10:20   #7
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Re: Displacement and ballast

Keep in mind that displacement and weight aren't the same thing too. As, for example a cubic foot of fiberglass (50% glass, 50% resin) weighs 96lbs on a scale on land. Yet in theory, when you put it into the ocean, & hooked it up to a scale, it'd only "weigh" 32lbs. As the cubic foot of water which it displaced weighs 64lbs.

It's the same reason why a PFD with 15lbs of bouyancy is plenty to keep most adults afloat under benign conditions, even though they weigh a lot more than that.
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Old 17-11-2015, 10:33   #8
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Re: Displacement and ballast

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Originally Posted by Bazz View Post
The marina owner seems knowledgeable and assures me that the TravelLift reading is right. If this is the case, and that displacement usually includes ballast, then I'm baffled. A 3,000# mystery. Don C L: The Columbia 29 looks nice! Do you still have the A4?
My boat has never known an inboard; the purist in me likes that. The pragmatist concedes the need for auxilliary power now and then. When my boat was built the inboard was an option. The C29 is a sweet little boat, thanks!
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Old 18-11-2015, 10:22   #9
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Re: Displacement and ballast

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Originally Posted by barnakiel View Post
Travelift measure is only very very very broad brush.

We lifted a 40 ton boat recently and a 60 ton lift operator came up to us to make sure the boat was 40 tons (???) The lift read 60, roughly 50% off the mark.

Then a 10 ton travelift readout may be anything between say 5 and 15 ton in real life. Or anything else too!

Scales used to weigh truckloads of grain can be used to exactly measure a small boat. Farmers tend to get very pissed off when they find any inaccuracies ...
A quick look about the net tells me that TravelLift are seldom inaccurate more than 10%, usually in the range of 2%. But, granted, this specific TravelLift could be faulty. Another possibility would be that my balsa cored deck is soaked, but I don't see any sign of delamination and, worst case scenario, I don't see more than a few hundred pounds of water residing there. This, plus the fact that I'm quite fast in light air, tells me that I'm not that heavy. At this point I'm suspecting the specs themselves. I posted something to that effect on the Paceship forum.

Wouldn't it be interesting to have a database, populated by owners, of actual weight of their boats? Since we all at some point do put the boat in a lift, it would be very easy to just take the weight reading and put it in the database. This could then be compared with "official" weight, or compared to readings of other vessels of the same make, or even to the same boat from one time to another. It would validate the published specs, put some pressure on the builder to be accurate, and even serve some diagnostic purpose if one is too far from the observed mean weight. Could be useful tool, don't you think?
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Old 18-11-2015, 11:47   #10
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Re: Displacement and ballast

A database could be good and a partial one is available from SailCalc and from sailboatdata.

What is the point of knowing the exact weight other than racing formulas?

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Old 18-11-2015, 12:04   #11
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Re: Displacement and ballast

Take a good look at the waterline. Is it where it used to be and whereabouts it was painted by the builder..

BR Teddy

ps. in your boat 3000lbs extra weight is about 4" effect on the waterline
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Old 18-11-2015, 12:05   #12
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Re: Displacement and ballast

Does the design displacement value of a new boat include mast, sails, winches, running rigging, Diesel engine, toilets, plumbing, tanks?

Or does it only mean the empty hull with nothing else?
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Old 18-11-2015, 12:31   #13
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Re: Displacement and ballast

Quote:
Originally Posted by UNCIVILIZED View Post
Keep in mind that displacement and weight aren't the same thing too. As, for example a cubic foot of fiberglass (50% glass, 50% resin) weighs 96lbs on a scale on land. Yet in theory, when you put it into the ocean, & hooked it up to a scale, it'd only "weigh" 32lbs. As the cubic foot of water which it displaced weighs 64lbs.

It's the same reason why a PFD with 15lbs of bouyancy is plenty to keep most adults afloat under benign conditions, even though they weigh a lot more than that.

This does not apply to things that float. A floating object will displace the same weight of water as the dry weight. If it displaced any less it would sink (as in the example of the fiberglass piece). Same goes for a person with a PFD. The PFD displaces additional water so the total water displaced matches the weight of the swimmer+PFD thus they don't sink. The PFD only has to displace enough water weight to overcome the swimmer's negative buoyancy not their entire weight.

Many (most?) small cruising boats weight at least a ton more than design displacement because of extra stuff such as sails, running rigging, ground tackle, fuel, water, etc. If a boat is only 3,000 lbs. more than dry design displacement that's not atypical.
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Old 18-11-2015, 12:36   #14
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Re: Displacement and ballast

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Originally Posted by Steady Hand View Post
Does the design displacement value of a new boat include mast, sails, winches, running rigging, Diesel engine, toilets, plumbing, tanks?

Or does it only mean the empty hull with nothing else?
Yes, displacement includes spars, winches, engine, etc.

Paul
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Old 18-11-2015, 12:41   #15
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Re: Displacement and ballast

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Originally Posted by Steady Hand View Post
Does the design displacement value of a new boat include mast, sails, winches, running rigging, Diesel engine, toilets, plumbing, tanks?

Or does it only mean the empty hull with nothing else?

It depends. The exact displacement conditions should be spelled out on the spec sheet. Some include full cruising gear with half full tanks and some are dry and leave out several "common" items.

Even half full tanks can end up a ton more when full.
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