Boat length vs chop size and period

[soares29]

Depending on how long is the haul and the size and period of the swell you will pound - or not.
What is that relationship? For example, In the Mediterranean I noticed that sometimes our ( Charter ) 48 was engaging in the next wave and not pounding, other times would bang...
Any engineers input on this ?

[Sojourner]

In the mediterranean, everything will pound Short, steep little nasty waves, we call it the two-step

[atoll]

Originally Posted by soares29

Depending on how long is the haul and the size and period of the swell you will pound - or not.
What is that relationship? For example, In the Mediterranean I noticed that sometimes our ( Charter ) 48 was engaging in the next wave and not pounding, other times would bang...
Any engineers input on this ?

probably more to do with underwater profile of the hull and angle of attack to the wave.

[MartinR]

This is very complicated, many variables: wave length, waterline length, weight of the boat, course, speed, distribution of deplacement and buoyancy, shape of hull.

Falling off a bit from optimum often makes the ride much more enjoyable.

[Cat White House]

I agree with Martin R.

Too many variables. Also add-in salt vs. fresh water, salinity in general, water temperature, depth, waves vs. swell, tides, land mass, current, surface air pressure, =load balance + ++

Out in the open ocean, swell can travel 1,000+ miles emanating from a large storm (ie from the North Atlantic to the trades.

Angle of attack to the seas and speed you can usually control. Most modern hulls are well-designed.

[Dave_S]

This might help.

http://media.bom.gov.au/social/blog/870/ruling-the-waves-how-a-simple-wave-height-concept-can-help-you-judge-the-size-of-the-sea/

[thinwater]

Originally Posted by atoll

probably more to do with underwater profile of the hull and angle of attack to the wave.

This. Hull form is a major variable.

[Elie]

Mass distributions on the hull is a dominant factor. Long moments of inertia will render the boat slow to follow the sea movements. It might be considered a good damping factor. But it might slam when the waves are steep with a short period. Centering of mass in boat will make the boat more lively, it won't slam as much but it might be much more inconfortable.

[knockabout]

we only have one boat so whatever the chop is we adapt to suit / usually the bigger the boat the more sea it will handle except when the big boats break they get out into a small rubber boat designed to handle any chop

[Hasbeen]

I did a lot of sailing in a boat with a pretty full bow in Great Barrier Reef waters. With the strong tides generated there, the sea was very different & much steeper when the current was against the wind.


Fortunately the boat as fast, as I found it preferable to wait out a wind against tide period. Any advantage in speed over the ground with tide assistance was completely ruined by the pounding. Much better a softer passage against both tide & wind, even if it took longer.

[Pelagic]

On all vessels, the critical wave period (between crests) is 0.95 to 1.1 times the waterline length of the boat.

In tank testing ship models for dynamic forces the critical tests are at that wave period range and we have sensors placed in different parts of the model to measure G Forces at various speeds and wave heights.

Dramatic difference when the wave period stretches out to allow small vessel to recover.

[Kenomac]

Originally Posted by Sojourner

In the mediterranean, everything will pound Short, steep little nasty waves, we call it the two-step

Not all boats will pound, hull shape/attack angle can minimize pounding as shown.

https://m.youtube.com/watch?v=_f4Wf0qDdbs

[TeddyDiver]

There are only two variables, the longitudinal megacentric height of the vessel and the wave period. Both give a time which, if too close to each other cause pounding. If that happens there are multiple factors influencing to the amount of the amplitude.
The megacentric height is practically static but the wave period is not as it's dynamic according the speed of the wave train, the speed of the vessel and you can allways change your attitude to waves.
my 2c

Teddy

[Pelagic]

"Metacentric"

https://en.m.wikipedia.org/wiki/Metacentric_height

[MartinR]

Originally Posted by TeddyDiver

There are only two variables, the longitudinal megacentric height of the vessel and the wave period. Both give a time which, if too close to each other cause pounding. If that happens there are multiple factors influencing to the amount of the amplitude.
The megacentric height is practically static but the wave period is not as it's dynamic according the speed of the wave train, the speed of the vessel and you can allways change your attitude to waves.
my 2c

Teddy

These are very simplified the factors affecting the movement of the boat in waves. But the shape of the bow is defining if this results in pounding or not. Flat forefoot - more pounding, deep forefoot - less pounding. The Colin Archer I had many years ago never slammed, it was always soft. Modern boats are much more prone to it, but there are still big differences. And it is often avoidable just by changing the course and i.e. the apparent wave period.