Hull shape design advice for 11.5m, 8kts, Fn=0.4

[ram68ocean]

This is a rather skinny vessel, with a L/B of approx 4.4 - the 'average' type of power boat usually has an L/B ratio around 3.

Thanks for pointing this out. Still the beam could be wider and the length shorter to bring down the L/B ratio. The idea behind making it slender is to reduce fuel consumption.

So does this mean that the difference between the lightship and fully loaded conditions is approx 8 tonnes?
If so, then that is a heck of a lot of difference!

I know. I was having issues with the trim at both conditions. Perhaps a water ballast tank(s) is needed.

Re the 'offshore heavy seas', bear in mind that there is a relationship between vessel beam and wave height re waves capsizing a boat - I think it is something like a capsize scenario could happen if the wave height is equal to the beam, and the vessel is caught beam on to the seas.
Which would suggest that your autonomous vessel could capsize in 11' high waves out in the ocean.

Interesting point. I wasn't aware of this direct relationship between wave height and beam.

Yes a catamaran was considered but disregarded due to higher roll period.

 

[Mr Efficiency]

Obviously the round bilged version will have less wetted area per unit of volume, so that more or less mandates itself. If any differences in roll behaviour don't matter, the chined version would be a little more roll damped by the shape.

 

[DCockey]

Re the 'offshore heavy seas', bear in mind that there is a relationship between vessel beam and wave height re waves capsizing a boat - I think it is something like a capsize scenario could happen if the wave height is equal to the beam, and the vessel is caught beam on to the seas.

Any source for this?

 

[DCockey]

How do you set the DWL ? At what loading condition, 50% fuel ?

DWL is up to the designer, and depends on how the boat will be used. It can be useful to evaluate the vessel at several waterlines, ranging from light with minimal fuel, stores and crew to maximum displacement.

 

[bajansailor]

Any source for this?

No source, that is why I said 'it is something like' - I just remember reading articles where they mention this.
I realise of course that it is not as simple as height versus beam, as some waves are steep, and others have long wave length.
And of course it depends on the stability characteristics of the vessel concerned - if the boat has positive stability through 180 degrees then it should not be a concern. And maybe the autonomous vessel will have a watertight buoyant superstructure to give it self righting capability if a wave does knock it over.