# How do we compare two hulls?

Discussion in 'Hydrodynamics and Aerodynamics' started by tea, Apr 15, 2017.

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### teaNew Member

Say one hull is one hull is slender (∇/L^3)= 8, in nature and another one has slenderness ratio of 4.5. If I were to make them a full model of 40m and test for resistance and motions, This could be an unfair comparison. Say I may later one slender its length will decrease. SO whats are the parameters to be kept contents to make a fair comparison. Assuming The bulky hull's initial parameter are double the slender ones. ?

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### jehardimanSenior Member

Hullform is never selected in vacuo. Transport capacity, propulsion, accommodations, tankage are all in the trade space.

Also, for what it is worth, slenderness ratio is only relevant between like hullforms, not equal displacements. So depending on the two hullforms, slenderness ratio could just be a meaningless factoid with no relevance for comparison, like the admiralty coefficient between a planning boat, VLCC, and a torpedo.

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### oataruJunior Member

Note that Slenderness Ratio (SR=Lwl/Vol^0.333 ; SI units)

Once you fix the full model length as 40 m for both hull,
The more slender hull (SR = 8) would have displacement as 125 t in fresh water while the other hull would have 1,000 t.

Obviously, the smaller boat, which is the former one, would have lower resistance but larger motion at sea.
Anyway, you are comparing apple with durian.

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