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ship's draft affecting speed


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Its interesting that the answer almost avoided the question, I suppose because draft has very little affect on speed and it is much more to do with design. The over enthusiasm to speed being determined by length is a little foggy though, especially as hogging of ships that are too long has a pretty drastic effect on speed as it distorts the hull lines and throws out dynamic efficiency.

Really draft has little to no effect at all and the important thing here is the design, material and construction strength, paired with how well a ship is loaded and where its centre of mass is. A shallow draft will likely force a small ballast and a lower mast and rigging, where as deeper allows for not only a higher masts and deeper ballast.

What it really comes down to is the general shape of the hull and the measurements a ship might have, which is somewhat more complicated than the OP asked for, but for example, generally a large 74, or an 80 Ship of the line sailed faster than a 64, or small 74 due to better comparative dimensions. Its also interesting that occasionally a razee was slower after losing their weight, leaving them higher in the water due to various changes in the dynamics of how they sit or sail.

Edited by Fluffy Fishy
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Easy one, this. High in the water is faster. With caveats.

It isn't necessarily true now as they can do clever things with hydrodynamics which I don't begin to understand, but in the age we're talking about I think they knew about hull speed (longer ships go faster) but the two biggest effects are:resistance and friction.

  • Resistance is basically having to push water out of the way of the ship. Hull shape outweighs everything here and a deeply-laden well-designed hull will easily outperform a lightly-laden poorly-designed hull, but all things being equal, deeper in the water means more water that needs to be pushed out of the way, without having any more sail force to do it with.
  • Friction is, having pushed the water to either side of the ship, it now has to flow past the hull. Weeds, barrnacles and such like have greatest effect here, but so too is the area of hull in contact. Deeper laden means more hull area in contact with water, so friction is increased.

Turning the rudder increases both effects.

Now for the caveat. Or rather two caveats. Hull shape trumps everything, and hulls were designed for a certain displacement. If you lighten the ship to less than this then you might upset the hull performance. The same thing happens if the weight distribution is wrong. All ships require weight to be evenly distributed side to side, but they also require the weight to be distributed correctly front to back as well, and the optimum distribution varies from ship to ship. A heavily-laden ship with the load well distributed may well outsail her sister, lightly-laden but porrly distributed. The second caveat is stability. If you remove all the ballast the ship might become unmanageable or fall over.

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2 minutes ago, Rickard said:

Draft = Length of the ship vertically, measured from waterline to highest point of the ship.

That's air draft where I come from. Draft is waterline to bottom of ship.

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2 hours ago, Rickard said:

Draft = Length of the ship vertically, measured from waterline to highest point of the ship.

Air Draft or Height generally refers to the total top to bottom measurement of the ship

Bilge tip Measures from the bottom of the ship to the upper most deck, its uncommon to see this being used.

Draft refers to the distance between the keel and the waterline

2 hours ago, Remus said:

Easy one, this. High in the water is faster. With caveats.

It isn't necessarily true now as they can do clever things with hydrodynamics which I don't begin to understand, but in the age we're talking about I think they knew about hull speed (longer ships go faster) but the two biggest effects are:resistance and friction.

  • Resistance is basically having to push water out of the way of the ship. Hull shape outweighs everything here and a deeply-laden well-designed hull will easily outperform a lightly-laden poorly-designed hull, but all things being equal, deeper in the water means more water that needs to be pushed out of the way, without having any more sail force to do it with.
  • Friction is, having pushed the water to either side of the ship, it now has to flow past the hull. Weeds, barrnacles and such like have greatest effect here, but so too is the area of hull in contact. Deeper laden means more hull area in contact with water, so friction is increased.

Longer ships don't go faster, ships that have a good ratio of length to width go faster, as long as they accept limitations to material strength and elasticity. This is why the Sepping's method and introduction of iron frames helped ships go quite a lot faster than their traditional framed vessels, especially when considering the reduction in hogging that slowed ships down quite drastically due to the amount of force being transferred into the the ships bending instead of going forwards.

Its also important to not forget the power associated with the ships and how well the force of the sail area can be made the most of with regards to roll, yaw and heel, something that is exaggerated in the use of square sails when compared to triangular sails, the square generally having a higher force pivoting nearer the top.

11 minutes ago, Lieste said:

Not really. At this period:

Draft = lines of the hullform, as drafted by the maritime architect.
Draught = depth of underwater hullform from the keel to the waterline, not including the false keel in most cases.

Draught is just an older word that predates setting words similar to "ye" and "the", it used to be that Draught was used in Britain and the Commonwealth and Draft comes from the USA but now Draft is now much more common the world over, although both are acceptable and mean the same thing.

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The less ship you have in the water, the less water is dragging and slowing your ship down. However the less water dragging across your side structure, the less stable, and more prone to tipping over your ship is. The more of your ship below the water, the more stable, however slower, due to water drag. 

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8 hours ago, Fluffy Fishy said:

Longer ships don't go faster, ships that have a good ratio of length to width go faster

not exaclty. And OneEyedSnake: not comprehensive. If it was so easy everyone could build ships that go fast

the lenght of the hull determines a theoretical top speed which you will never reach in RL. Youd need a lot of sails and perfectly balanced.. Smaller vessels carry more sails per hull lenght and thus they are able to get closer to that theoretical number.

dirty wikipedia

Width to lenght ratio is not very important. I had that discussion with Malachi. US ships were extremely long while very thin (essex, consti, others) - yet they generally speaken didnt sail faster than their foreign counterparts. But they were less stable.

 

In modern high speed sailing this issue is solved when the hull is beeing heaved out of the water and sailing on skits. Quite crazy but it works and sailboats reach up to 60kph

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Quote

the lenght of the hull determines a theoretical top speed which you will never reach in RL. Youd need a lot of sails and perfectly balanced.. Smaller vessels carry more sails per hull lenght and thus they are able to get closer to that theoretical number.

Hull speed isn't a top speed, just a number at which the amount of thrust needed to accelerate skyrockets.

Small fast vessels can exceed hull speed, as Pride of Baltimore II has done on multiple occasions. Having an enormous amount of sail area in relation to displacement is helps a lot.

Quote

Width to lenght ratio is not very important. I had that discussion with Malachi. US ships were extremely long while very thin (essex, consti, others) - yet they generally speaken didnt sail faster than their foreign counterparts. But they were less stable.

Who says they were less stable? Constitution was reputedly very stiff. If your length:beam ratio comes from great length, stability isn't compromised.

 

Something that was not understood during the Age of Sail is that at low speeds, the main influence on performance is the friction caused by water moving along the ship's sides and underwater hull. At higher speeds, most drag comes from the hull forcing water out of the way to create waves.

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