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Old 06-07-2018, 18:40   #31
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Re: Boat length vs chop size and period

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I think most of you are missing exactly what the OP was asking.
He was asking about relationship of pounding to boat LENGTH.

As I said before, the critical relationship of Boat Length to Wave Period Length is around 1 to 1 for the worst pounding

Speed, or wave height, or hull configuration, or longitudinal weight displacement (which affects longitudinal centre of gravity) (tipping point) ALL affect the severity of the pounding, but they are secondary.

It is getting near that critical wave period relationship, which initiates the worst results for all the same secondary conditions.
Yes, there are lots of factors besides length and yes, boat length relative to wave length is possibly the most important factor. However, it is the boat length relative to the ENCOUNTERED wave length that is important. If the boat is not moving, encountered wave length is the same as the actual wave length. But if you are going into the waves you are effectively shortening their perceived wave length. (It is more intuitive to look at it from a wave encounter period viewpoint, but that requires another layer of explanation.) The point I am trying to make is that the posts observing that changes in boat speed and direction affect pitching and slamming are correct, because those actions change the encountered wave length. The earlier posting of a rule of thumb about boat length to wave length lying between (from memory) 0.95 to 1.1 is roughly valid, but only for encountered wave length.
In summary, this is quite a complicated naval architecture subject which has no short simple answers. That is partly why all boats are different and we naval architects are consequently still in business!!
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Old 06-07-2018, 20:47   #32
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Re: Boat length vs chop size and period

Yes, another key factor is inertia, which unfortunately they are unable to scale in Tank testing ship models although they have developed some pretty amazing computer simulations
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Old 06-07-2018, 23:18   #33
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Re: Boat length vs chop size and period

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Yes, another key factor is inertia, which unfortunately they are unable to scale in Tank testing ship models although they have developed some pretty amazing computer simulations
Actually you can scale pitch inertia in the tank (I have done it), though it is quite tricky.
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Old 06-07-2018, 23:53   #34
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Re: Boat length vs chop size and period

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Actually you can scale pitch inertia in the tank (I have done it), though it is quite tricky.
My experience with model testing inertia, was related to measuring the free surface effect from a flooded "float in" drydock recoverty system for a large tender via the stern .

After all other hull resistance tests were done, they flooded the drydock and applied the scaled waves.

Most interesting as it ended up sinking the model [emoji4]
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Old 07-07-2018, 05:21   #35
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Re: Boat length vs chop size and period

I fished the Texas Gulf coast for years in center console go-fast boats. The predominant conditions were ESE winds (on shore) 10-15 kts which made 2'-4' or 4'-6' waves with approx 6 sec. period. In a 30' center console you find out quickly that a wave period of less than 6 seconds is brutal to anyone in a hurry to get to the fishing grounds. But a 34'-36' CC bridges the waves nicely. Any boat is going to pound under the right conditions, a few of which are in your control.
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Old 11-07-2018, 00:54   #36
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Re: Boat length vs chop size and period

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Longitudinal centre of Gravity (also called longitudinal centre of floatation) is often forgotten by pleasure boat skippers when loading their boat.

Critical when you are a cargo ship but for us, it is something we should understand

Longitudinal & Transverse Stability
Actually, the longitudinal centre of flotation is the centre of the waterplane area, which is something different than the LCG and LCB, longitudinal centre of buoyancy.
LCG must equal LCB by definition as the vessel simply trims until they are equal.

So, if loaded unevenly, the vessel ends up trimmed by the bow or stern. Correctly designed displacement hulls have their LCB at 53-54% of LWL, measured from the bow. A practical consequence of this is that, as there is commonly more available hull volume aft than forward and yet both must be loaded evenly to maintain correct trim, the front of the boat ends up fuller in terms of storage.
The hydrodynamic penalty for having the LCB in the wrong place increases quite quickly.
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Old 11-07-2018, 01:11   #37
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Re: Boat length vs chop size and period

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Correct luckyknot, I was lazy in my explanation[emoji20]
Deja vu [emoji4]
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