design draft vs navigation draft

**B.Smitty** · 10 May 07,, 01:08

Can anyone explain to me the difference (if any) between the design draft of a vessel and its navigation draft? Are they the same?

Is the first the physical distance from the waterline to the bottom of the vessel, and the second some minimum water depth required to operate the vessel?

thanks in advance

**~~SnowLeopard~~** · 10 May 07,, 08:43

I'd have to go back to my nautical books before I could answer that directly BUT ..........

One of the Cunard liners, I think it was the QE2, once went aground. What happened? They didn't calculate for squat. Squat? What's squat? Squat is a condition where a large ship at high speed pushes the water under the hull out and away. This results in the ship having less water to travel in than normal and the amount of water is, of course, key.

Based on that, I would say, yes, design draft and navigation draft could be different ....... but I should check the question against the books first.
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("You know a wise man once said, I think it was Attila the Hun, "It is not enough that I succeed, everyone else must fail."--Ross Webster, (wtte), "Superman III")

**kato** · 11 May 07,, 00:23

iirc isn't the navigation draft the draft within specific waters? i.e. also adjusted for the local temperature/density and salt content of the water.

**RustyBattleship** · 11 May 07,, 00:53

Simply speaking, the design draft of a ship is what the designers consider will be the median hull draft between high and low loads.

Navigational draft is calculated from what the ship's HULL draft is and adding anything sticking out below it such as a SONAR dome, unretracted manuevering propellers (such as on FFG-7 ships), a Pitometer sword that refuses to be cranked back up, etc.