Larger lenght to beam ratio vs weight

So, lets assume that we have two single engine round bilged semi displacement boats both around 11m (~36ft) long, and have a bottom loading of ~ 250kg/m2 or 51lbs/sqft. (calculated with my very rough formula: ((lenght x beam) - 10%) : weight
But the other one is narrower, and lighter, because we wanted to keep the same bottom loading.

Boat 1:
Lenght at waterline: 11m / 36ft
Beam at waterline: 3.6m / 11ft 9.7 in
Lenght to beam ratio: 3.05
Weight: 8900kg / 19 621lbs
Bottom loading: ~250kg/m2 / 51lbs/sqft

Boat 2:
Lenght at waterline: 11m / 36ft
Beam at waterline: 3.1m / 10ft 2in
Lenght to beam ratio: 3.55
Weight: 7700kg / 16 975lbs
Bottom loading: ~250kg/m2 / 51lbs/sqft

So, the question is, which one of these two would be more seaworthy/comfortable in rough weather?

 

I don't think your question can be answered with the data you give, since everything will depend, to a great extent, on the shapes of the boats. but while someone risks an answer, could you tell me what you call "bottom loading" and what causes it?

 

https://www.google.com/url?sa=t&sou...gQFnoECAwQBQ&usg=AOvVaw3NhePk7YZ-IBLVaboz926u

I think this is a pretty good definition of bottom loading.

 

ToMy, TANSL is correct when he says that there is not enough information to tell. Seaworthiness is dependent on too many other factors. Additionally, because the volumetric Froude numbers are too different between the hulls; bottom loading for planing comparisons is rather moot. Realistically, from Savitsky's work, Series 62, and Series 64 hull forms, I'd say a 11m x 3.1m 8900kg hull form would have better seakeeping at speed rather than some arbitrary pixie dust bottom loading comparison. I would suggest you get a copy of High Speed Small Craft by Peter du Cane (revised 1974 edition) for a discussion of what makes a seaworthy planing vessel.

 

I didn't know that ratio. Frankly, I always thought that estimating a hull's ability to plane was a lot more complicated. Everyday you learn something new.
Thanks a lot.

 

For what purpose?
And by "bottom loading" - you are really saying - the same hydrostatic pressure on the bottom??

But this:

Suggest so...which indicates a change in draft....so i assume you simply 'played' about to get the same hydrostatic pressure per unit area?
But the hydrostatic Pressure = rho.g.h.
h = draft.
So if the boat no.2 has a different draft.. how can it have the same "bottom loading" as you call it?

In addition to the comments above.
Seakeeping is terrible complex and involves many variables. If it could have been dumbed down into a single one liner - then it would have been done so decades ago. But it hasn't!

Boat no.1 has an LD ratio of 5.31 and boat no.2 has an LD ratio of 5.57....peanuts between them in reality.
Why is this important?
Because the higher the LD ratio the lower the vertical accelerations. But the difference between the two hulls is extremely minor.



So, bottom line is.. as noted above.. insufficient data and you're over generalising a very complex subject.

 

looks similar to wing loading for planes !

 

Obviously, he speaks about planing craftys, so dynamic pressure load, not hydrostatic !

 

Sorry, crafts ,

 

It may be similar in some aspect but, in reality, they are totally different effects, those that occur in a body moving totally submerged in a fluid (in the case of the wing of an airplane) and that of a partially submerged, static, object (in the case of the hull of aboat). The article is a demonstration of how words can be put together to give a pseudoscientific explanation, as if some new idea were being contributed, when in reality they are only talking about hydrostatic pressure (I think it doesn't even take hydrodynamics into account).

 

Edit: Xpost.
Airplanes are nothing similar to high speed planing vessels.... S**t, caught by a troll. Suffice to say, totally different, especially in a seaway. Wait, TANSL said it much better.

 

Agree with both of you, in addition, bottom pressure loading on planing crafts depends on velocity !

 

Not if you look at the link he provided as 'his' definition of such:

"...Definition
Bottom loading is the displacement mass divided by the area of the planing surface..."

So that is a no to hydrostatic pressure and a no to dynamic pressure.

 

He must define also the area of the planing surface : wetted bottom area at speed or at rest ?
the first depends on the velocity , the second don't make sense as planing area !

 

Looking at the website ToMy quotes...

This is similar to many other quaint and curious design ratios of yore....i.e. it works well with empirically derived coefficients dealing with a single hull shape of given geometry but is totally useless given modern methods. Might as well use Circle C or Admiralty Coefficient....just as useful.