Discussion in 'Boat Design' started by Willallison, Nov 26, 2009.

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Will

"..The problem with a discussion such as this, is that you can see the noose before it begins to tighten round your neck.."

Whilst this may not appear pleasant, it ensures we are all on the same page, to begin the debate proper. (It is the only way to separate issues and/or myths for clarity).

So, lets look at the extension, which has just slight buoyancy.

What can this do to reduce motions.

The size of the extension is not large and the amount of buoyancy is also not large. Therefore, its add mass is also not large. As a percentage of the added mass compared to the whole hull...it is almost negligible. The chnage in vertical acceleration is proportional the Sq.rt of the added mass.

(Consider standing in the sea, hand out palm open, but fingers closed. If you have your palm and 'slap' it down onto the water and submerge it, then lift it immediately, then compare this to the same hand but now rotated 90 degrees, ie edge of the hand, which has greater resistance?...simplistic added mass explanation, since it is proportional to the exposed X-area on the waterline)

2) Water Plane Inertia
Again, the extension is not large, but does exhibit a waterplane. However the waterplane, as i the case of the buoyancy, is not significant at all. The inertia term in pitch is square root of Iy. So as unity that is only a 2.5% chnage.

So, in terms of affecting the motions, the added mass and the water plane inertia is not enough to significantly affect the motions. Since the percentage of increase would need to be in the order or 30, 40% etc, not 2.5%. This would require a large very large extension.

So, lets go back to the hull length.

Take a box, call it a superstructure, which carries, people, cars whatever. lets say it is 50m in length. and lets say the hull that supports this box, is also 50m in length. Along she goes with motions of XX.

Lets increase the length of the bow, but, since we can't obtain any real benefit unless the 'bow' is large, lets just increase the length with near zero buoyancy...ie have knife edges. So hull length + "extensions" like knives, literally like knives on edge...just added to the bow. Oddly enough like the picture attached....(I can find pix too ).

What do you think will happen?

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2. Joined: Oct 2008
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Ship motions are the filed of study called hydroelasticity. In defining the problem it uses very very complex equations, the ships mass/buoyancy is one of the major inputs when defining the oscillatory motions of a ship. The resorting force (spring constant use din the analysis) is proportional to the buoyancy, without such there is no resorting moment. No buoyancy = no motions!

Perhaps this is too analytical for the man from analytical only world?

Exactly, supposition, not fact. As usual, you're trying to subvert your supposition as fact.

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catsketcherSenior Member

into the lions den

I come to this from a different but maybe similar background. A trimaran float in a hard breeze certainly does pierce waves. In fact a well designed one must or it may cause the boat to be very greatly decelerated with accompanying bad things happening like pitchpoling.

What happens in hard reaching or windward work when the tri float is well immersed is that float still has reserve buoyancy if you can still see it. But when it hits a wave the reserve buoyancy cannot produce enough of a torque to rotate the boat much. This is a good thing as it means the boat does not pitch so much. A wave has a certain time to impart energy to the boat structure. If you make the hulls fuller so that the reserve buoyancy is greater you will pitch more and go slower.

Speed is a big part of the equation. All boats have pitch harmonics. Go to the back of a boat and give it a push and let it go. It will rock back and forth. If you get in certain waves that reflect this harmonic you will pitch a lot. But if the bow gets an impulse and the next impulse is applied before the boat can get into this pitch harmonic you stay much flatter.

I may have the terms slightly incorrect but my 7 years living on a 100% float Twiggy let me see this phenomenon often. Sailing fast was often much much more comfy than sailing slow - but usually much much wetter. We used to call it "magic carpet riding".

Speed and rotational inertia coupled with the hull buoyancy ability to produce torque are all related in this question.

cheers

Phil

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CDKretired engineer

This is clearly a copywriter at work. Words carefully chosen for maximum impact, characteristics exaggerated. I know, I made a living writing such texts for many years.

5. Guest625101138Previous Member

The speed also provides dynamic lift and this becomes more significant in the resultant forces acting on the hull as speed increases.

Rick W

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WillallisonSenior Member

Of course, but given that you are talking about a purely displacement hull, do you really think that the dynamic lift created is significant?

AH - I'm back ... and I've found the thread again.... now where were we....

Ok we've increased length, though not significantly and there has been minimal increase in waterplane or mass. So, in answer to your question, not much will happen.

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Welcome back

So, a small-ish extension will not provide much for pitch and heave accelerations, in terms of its added mass and water plane inertia. The only noticeable benefit is simply the length of the extension, in increasing the waterline length.

And as already noted and agreed, a hull with a Lwl of X metres where the hull length extneds to the pointy extending bits, or, is a "normal" hull also of X meters Lwl, will have the same motions, everything else being the same.

So, the picture of the catamaran above, with the hull, normal hulls but has an extension, knife like edges on the bow, so their only contribution can be by virtue of their length only, and no other redeeming feature.....yes?

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WillallisonSenior Member

...No

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So, what other contribution, other than the increase in waterline length, do you think is gained from those knife edge bows?

10. Guest625101138Previous Member

Who is talking about a purely displacement hull?

If dynamic forces were not involved the hulls would not trip up and pitchpole. The buoyancy would always prevail. That is not the case. The flatter and wider the deck the greater propensity to keep diving once the there is downward trim. The hulls design to wave pierce do not have flat decks.

Rick W

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WillallisonSenior Member

Sorry - gotta go - taking the kids to the flicks - back later

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WillallisonSenior Member

Why... they help the builder to sell the boat of course!

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ooooooooooooooo, that sounds like an agreement to the contribution, actual contribution in real physical known laws, of the extension known/called " wave piercing" bow...yes?

But is that what was intented, on the catamaran (pic i posted above)?

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mydauphinSenior Member

E=mc2....Simple but complex. That is why there are test tanks, computers and Trial and Error.

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bhnautikaSenior Member

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