Displacement

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Cotillion

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I'm looking at a Dolphin 31. The Brokers spec tells me the displacement is 4 tons and the ballast ratio is 44%.

What does this tell me about the boat, i.e heavy, medium?
Will she be quick, or seakindly in the rough stuff?

kim
 
I don't know that boat but it sounds like a fairly heavy displacement boat. The high ballast ratio indicates the boat is relatively narrow; beamier modern boats rely to greater extent on beam to provide stiffness rather than ballast.
You can calculate your own ratio if you know the water line length:
The displacement to length ratio tells you how ‘heavy’ a boat is.
The formula is: D/L ratio = Displacement in long tons/ (0.01 x waterline length) ³

An extremely heavy displacement boat would have a D/L ratio of over 400, a moderately heavy displacement boat would be 300 to 400, a moderate displacement boat 200 to 300 and a light displacement boat would be under 200.

There is an article on this subject on my website.
 
4 tonnes would probably put you on the light side of medium. My own 32' foot boat is 5 tonnes and that is medium. A boat like a Nic 32 which is perhaps the architypal heavy displacement boat is more than 6 tonnes.

A ballast ratio of 44% is pretty good by modern standards and would tend to suggest that it is fairly stiff with good characteristics to windward, but perhaps also suggests a comparatively narrow beam (10' or so maybe).

That would make her more sea-kindly than many modern mass-produced boats but probably not as fast in light airs.

However that is very much guesswork based on the figures you gave so it could all be completely wrong
 
It has a displacement/length ratio of 248. This is a moderate displacement. Beam/length ratio is 3 so, again, quite moderate. 44% ballast ratio is high, so I'd say this boat would stand up to a lot of canvas but could have a quick motion.
 
Two separate reference books ('Daytons' and the 'Good Yacht Guide') give the wedgy-decked cruiser-racery Dolphin 31 at 3808 lbs displacement, which seems too low. Probably one copied the other's mistake. Yachting Monthly A to Z quotes 8892 lbs, with a drawing that does not look that much like the boat itself.

Could there have been more than one boat called a Dolpin 31 - there are similar cases such as two entirely unrelated Contessa 25 designs?
 
There is a centre consol power boat called a Dolphin 31.
I found the displacement of the wedgy sail boat quoted at 4.6 tons on a WLL of a bit over 26 feet. 3808 lbs on that length would make it a lightweight flyer!
 
From http://cruisingresources.com/

The best measure of a boat's relative size: displacement compared to waterline length. An analytical ratio, not a dimension. Rough classifications:
Ultra light racer: 60-100
Very light racer: 100-150
Light cruiser/racer: 150-200
Light cruiser: 200-250
Average cruiser: 250-300
Hefty: cruiser 300-350
Very hefty cruiser: 350+
 
Yes, I'd agree with those categories but perhaps the assumption that cruising boats are heavy and racing boats are light is a bit simplistic.

Here's my classification:
An extremely heavy displacement boat would have a D/L ratio of over 400, a moderately heavy displacement boat would be 300 to 400, a moderate displacement boat 200 to 300 and a light displacement boat would be under 200.
 
Assuming I did my sums right I came up with a figure of 264, which puts it on the cusp of moderate to heavy. Exactly what I was looking for.

For those who are interested the beam is 10' 3"

3808/2240/(.01x26.5) cubed

Thanks all for the info

kim
 
The displacement isn't 3808 lbs. That would make the displacement 1.7 tons which would be impossibly low. This would give you a D/L ratio of 34, which is off the scale.
I reckon the displacement is around 4 tons (I found a reference to 4.6 tons in the archives) and on this basis the D/L ratio is 215 or, on the higher displacement, 247.
 
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