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Displacement to length calculations. Waterline length?

roblpm

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I was nearly going bonkers...... 😂

New lock down. Comparing boats. Using sailboat data filters.

However it appears wl length is used in the displ/length calculation which makes it impossible to compare old and new boats.

The newer boats have a waterline length nearly the same as LOA.

Now I can understand when heeled an old boat's wl length will increase. However when I am looking at the displacement to see how much the boat is going to bounce up and down I don't understand this approach.

Example:
Nicholson 39 (1975) LOA 11.89m Displacement 8165kg Disp/len 353
Elan Impression 394 (2012) LOA 11.9m Displacement 8000kg Disp/len 222

Seems completely bonkers to me or am I just wrong? When I am bobbing about in the middle of the ocean are these two boats gonna Bob up and down completely differently? (BTW I'm not too interested in the keel, rudder debates. Well actually I am but maybe we can ignore them on this thread! 😂)
 

dunedin

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To a degree, yes just ignore over simplistic metrics. Particularly when comparing very different boat types.
Some can be useful when comparing more similar boats - like sail area, to see whether a boat is under canvassed.
 

roblpm

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And just to add,
To a degree, yes just ignore over simplistic metrics. Particularly when comparing very different boat types.
Some can be useful when comparing more similar boats - like sail area, to see whether a boat is under canvassed.
Well this particular metric seems nuts!! Will carry on without it! Just look at actual displacements for similar sized boats!
 

roblpm

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Weight/length is always going to depend on units. I think weight/length cubed is more useful. This has units of density which can be compared to the density of sea water to give a dimensionless ratio.
Displacement–length ratio - Wikipedia
Sure but which length to use! The traditional formula uses LWL. New boats have LOA nearly equal to LWL. Old boats don't. So the results are not comparable.
 

Chae_73

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My guess would be that the Nic and the Elan would indeed bob up and down completely differently, one reason being that the Elan has a much higher volume hull compared to its displacement.

I'm sure there are lots of other reasons as well.
 

roblpm

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My guess would be that the Nic and the Elan would indeed bob up and down completely differently, one reason being that the Elan has a much higher volume hull compared to its displacement.

I'm sure there are lots of other reasons as well.
Yes interesting. So this metric is really more about volume than weight. Maybe that is what displacement actually is! I really should have paid some attention at school! So I am sure someone will be along to explain it in a minute.....
 

roblpm

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I seem to have ocd on this now.

I can see why waterline length matters now as this is supposedly an indication of the volume under water. But this ratio is still silly as it doesn't take account of the beam. So a skinny yacht would yield the same result as a beamy one which can't be right.

With modern computers you would have thought there would be a way to calculate the bobability ratio more accurately. I may trademark this idea!
 

TernVI

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A 70s Nic will bob like a 70s Nic.
A 21st Century Elan will bob like one of those.

You can't reduce that to one number for any useful purpose.

The most useful number in used yacht purchase is cost per ft of lwl. Because very few people take the mick out of you for having a bigger boat.
Apart from chandlers of course.....
 

roblpm

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A 70s Nic will bob like a 70s Nic.
A 21st Century Elan will bob like one of those.

You can't reduce that to one number for any useful purpose.

The most useful number in used yacht purchase is cost per ft of lwl. Because very few people take the mick out of you for having a bigger boat.
Apart from chandlers of course.....
Ok quite right. I am enjoying a bit of reading about Dellenbaugh angles etc though! 😂
 

James_Calvert

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Not sure exactly what you mean by bobability...

This would be my expectation, not having sailed either of them.

The two boats mentioned will behave similarly in calm water, except the Nic may heel more.

On choppy water the Nic may carry its way slightly better having slightly higher displacement. But there'll be a difference in comfort as the seas increase. With its shorter waterline and finer ends the Nic will tend towards hobby horsing, but the motion will be quite soft with little slamming. The Elan will be much stiffer, with sharper motions, and increased risk of occasional slamming.

Reducing speed, or altering course to take waves at a different angle, will mitigate both undesirable characteristics.

The Elan will probably reach its destination quicker. The crew in the Nic won't be concerned to the same degree about the length of their passage.

Both boats will bob about much less than similar but smaller boats. And the difference between each of them is probably much less significant than the difference from smaller, or larger, boats.

I cede the floor now to those with some actual relevant experience...
 

roblpm

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Not sure exactly what you mean by bobability...

This would be my expectation, not having sailed either of them.

The two boats mentioned will behave similarly in calm water, except the Nic may heel more.

On choppy water the Nic may carry its way slightly better having slightly higher displacement. But there'll be a difference in comfort as the seas increase. With its shorter waterline and finer ends the Nic will tend towards hobby horsing, but the motion will be quite soft with little slamming. The Elan will be much stiffer, with sharper motions, and increased risk of occasional slamming.

Reducing speed, or altering course to take waves at a different angle, will mitigate both undesirable characteristics.

The Elan will probably reach its destination quicker. The crew in the Nic won't be concerned to the same degree about the length of their passage.

Both boats will bob about much less than similar but smaller boats. And the difference between each of them is probably much less significant than the difference from smaller, or larger, boats.

I cede the floor now to those with some actual relevant experience...
Bobability is my new measure of comfort at sea. Soon to be patented. Obviously there is Ted Brewers comfort ratio too:

34.11 for the Nich
26.26 for the Elan
 

Laminar Flow

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I seem to have ocd on this now.

I can see why waterline length matters now as this is supposedly an indication of the volume under water. But this ratio is still silly as it doesn't take account of the beam. So a skinny yacht would yield the same result as a beamy one which can't be right.

With modern computers you would have thought there would be a way to calculate the bobability ratio more accurately. I may trademark this idea!
Well, the bobability ratio is called heave or waterline plane (area) loading and signifies how rapidly a boat reacts or accelerates to wave action. Boats with higher waterline plane loading tend to be more comfortable in a seaway.

Waterline at rest is not the same as effective waterline and consequently also affects displacement length ratio.

Effective waterline also influences speed/length ratio

Sail area / displacement ratios have also changed over time. In the era of the IOR boats had small mains and very large overlapping head sails of 150% or more. Sail area for determining SA/Displ ratio was the max sail a boat could carry to windward.
Now that we have roller reefing headsails and large mains headsails with an overlap of more than 130% are rare as they do not reef effectively.
SA is now calculated as fore triangle, i.e. the area between mast and forestay, and mainsail. This can make older designs appear underrigged by comparison.
 

TLouth7

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LWL is the correct measurement to use here, because it is the underwater profile of the yacht that affects its hydrostatics. The entire point of the long waterlines of modern yachts is to reduce Disp/Length so as to increase the speed potential of the boat!

If you want to get a really good measure of the pitching action of a boat you would need to know the moment of inertia of the waterplane (sorry for all the jargon) but I doubt you will ever see that as a filter on a website.
 

dunedin

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LWL is the correct measurement to use here, because it is the underwater profile of the yacht that affects its hydrostatics. The entire point of the long waterlines of modern yachts is to reduce Disp/Length so as to increase the speed potential of the boat!
The point of the increased waterline length is certainly partly to increase speed, but more to do with the wave drag factor I suspect, rather than any concern about the displacement/length ratio which is a bit meaningless. Clearly it also increases space massively (I reckon the OP’s examples, the Elan will have twice the interior space of the Nich).

As old working boats and early yachts tended to have straight bows, it might be more accurate to say that the long overhangs / short LWL fashion came in due to racing rules which measured LWL rather than LOA, so were a ratings dodge.
 

TLouth7

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The point of the increased waterline length is certainly partly to increase speed, but more to do with the wave drag factor I suspect, rather than any concern about the displacement/length ratio which is a bit meaningless.
I think we are approaching the same thing from opposite directions! Increasing waterline length reduces wavemaking drag (roughly), and so improves the speed potential of the vessel. This is what is being expressed by the displacement/length ratio.

If you have a vessel with long overhangs, and you chop off those overhangs above the waterline then you would expect the performance in flat water (which is what hydrostatics are all about) to stay the same. The LWL has stayed constant, but LOA has changed drastically. So disp/length based on LWL is clearly more useful.
 

RJJ

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If you want to get a really good measure of the pitching action of a boat you would need to know the moment of inertia of the waterplane (sorry for all the jargon) but I doubt you will ever see that as a filter on a website.
Wouldn't the prismatic coefficient be a good approximation? Higher prismatic equates to a flatter bilge, less rocker and broader/flatter stern quarters with more buoyancy at the ends, which is where the slamming comes from. Lower prismatic equates to pointy ends and a deeper bilge, like a Contessa, where it's intuitive that the slender and overhanging bow (and stern) can be sliced into the water without doing a belly-flop.
 

Laminar Flow

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Wouldn't the prismatic coefficient be a good approximation? Higher prismatic equates to a flatter bilge, less rocker and broader/flatter stern quarters with more buoyancy at the ends, which is where the slamming comes from. Lower prismatic equates to pointy ends and a deeper bilge, like a Contessa, where it's intuitive that the slender and overhanging bow (and stern) can be sliced into the water without doing a belly-flop.
Looking at any one single ratio in isolation is not helpful and prismatic coefficient on its own does not necessarily indicate the qualities you describe above. The Cp describes the relationship of the distribution of volume of displacement to that of the midship section.
A quick example: a normal displacement sailing yacht optimized for sailing to windward in medium winds would have a Cp of between 0.54 and 0.56.
A Cp of 0.49 would be very low and over 0.6 would be high.
My tub has a Cp of 0.61 and a Displ/length ratio of 360. Assuredly, there is no slamming or belly flopping involved even in rough going.

It has been found, that each Cp has, in terms of form resistance, an optimum relative speed range. Higher Cp benefit a higher relative speed, at a lower relative speed a low Cp has less resistance.
To produce a well-rounded performance under sail, designers choose a middling value.
Motorsailers, supposedly able to choose their optimum speed whether under sail or power, tend to have a higher Cp; it also indicates a vessel better suited for reaching in strong winds.
A high Cp entails less of a resistance penalty when going slower than it's optimum speed than trying to go faster with a low Cp.
 
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