# Michlet 9.32 Released

Discussion in 'Software' started by Leo Lazauskas, Apr 18, 2014.

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### Leo LazauskasSenior Member

You haven't specified the lateral separation for the 2nd hull.

Next time, just attach the in.mlt file.
There is no need to post all files.

Good luck!

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### malithJunior Member

Thank you very much!!!

It helped a lot

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### tspeerSenior Member

Only using source distributions results in a symmetrical hull for a monohull, but is that still true for a catamaran? The side force on each demihull would be zero, of course, but wouldn't the centerline of each demihull be much like a streamline of the flow around the other demihull? This would result in a small amount of camber in each hull.

Looking at the same issue from the opposite direction, if you had two symmetrical hulls in close proximity, there will be a suction force trying to pull the two hulls together. You can camber the hulls to neutralize the suction and get zero side force on the demihulls. But it would seem that vortices would be necessary to calculate the flow around the geometrically symmetrical hulls.

So do the source-only representations simulate symmetrical hulls, or do they really simulate side-force-neutral hulls?

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### Leo LazauskasSenior Member

Interesting and probing questions, Tom!

Yes, it would more like that, but it wouldn't necessarily be a simple
concave or convex camber.

Yes. For a proper linearised calculation a transverse velocity, phi_y,
is necessary to counter-balance the velocity induced by the other
demihull. This can be done by including dipoles or vortices, and it
should lead to more accurate wave resistance predictions. (Satisfying
the Kutta condition at the sterns is a nuisance in actual calculations.)

Of course, there is also induced drag to consider in a full treatment
of the problem.

The sideforce can act to repel the demihulls at some Fr and hull
separations, or to pull them together at other Fr and separation.

Couser et al say that "...the presence of one demihull modifies the
flow so that the other demihull is effectively operating at an
effective angle of incidence (or yaw) to the free-stream."
See:
Couser, P.RR, Wellicome, J.F. and Molland, A.F.,
"Experimental measurement of sideforce and induced drag on catamaran
demihulls",
International Shipbuilding Progress,
Vol. 45(443), 1998, pp. 225-235.

I think that is a slight over-simplification. The effect is likely to
be a yaw angle and a wavy form of camber, sometimes inwards, sometimes
outwards, along the hull. That waviness would also be Froude-number
dependent, and dependent on the hull separation. Think of the waves
created by one demihull impacting along the length of the other
demihull.

Their experimental results show that the induced drag can be ignored
for practical purposes as it comprises less than 0.3% of the monohull
drag.

They also showed that using an effective yaw angle, like one does
in simple induced drag calculations, gives reasonable approximations,
so their slight "over-simplification" is of little actual consequence.

I have never given it much thought, but I suspect the latter.
I now have the means to actually calculate the flow along each
centreplane and, in fact, along the actual hull surfaces. I'll
try that some time, but I am only spending about one day per week on
ship hydro. As a recent semi-retiree I can play more fluid dynamics
and artificial life games these days without feeling like I should
be earning money at the same time.

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### malithJunior Member

hi

i'm using michlet for a research project of designing a waterbike with planing catamaran hull.
i want to know the resistance calculation methods and theorys used in the michlet code.
because i have to validate my results in the final report.

Thank you

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If I understand you correctly, you're basically describing the Munk moment.

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### Leo LazauskasSenior Member

I haven't read much about the Munk moment, so I can't comment.
I suspect that I might need accelerations (to get forces) on the hulls rather than just the velocity components which are relatively easy to calculate.
If somebody sent me the actual equations for the Munk moment I might be able to program them up one day.

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A simplified summary is that when a simple shape such as a tube, like a torpedo, is in an ideal fluid and has a slight angle of yaw, the 'stream lines' of continuous pressure on the upper surface (away from the action of yaw) will have a neutral point no longer coincident with the centreline of the volumetric symmetrical shape, unlike where no yaw exists. The corresponding lower surface (facing the stream flow) has the same condition, identical just 180 degrees from each other. The problem is the this creates a couple, a moment.

That's it in a nut shell. Simplified maths attached.

#### Attached Files:

• ###### Munk moment.pdf
File size:
429.4 KB
Views:
268
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### JacobSimonNew Member

I am trying to get estimates for long planing catamaran hulls from Michlet as part of my senior design project. So long as I lift the boat and get the trim it to where we think it will opperate we seem to be getting results that match up with the parametric studies we did earlier. What I would like to know is what point will the hulls trim about in this software. If I know that I might be able to better adjust the draft at a given trim to show the correct amount of boat as being in the water at boat speeds where large trim occurs.

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### Leo LazauskasSenior Member

The hull trims and sinks around the LCB.

Although it's gratifying that you are getting reasonable results for these
vessels I think that they might be a bit of a fluke. Michlet does not model
planing in any way.

Be aware too that Michlet is not set up to handle large trim angles, say +/- 5
degrees at the very most.

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### TANSLSenior Member

But the reality is that the trim of the boat is produced by rotating around a transverse axis passing through the center of gravity of the water plane area. Is this correct?

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No.

A hull trims about its LCF. This is caused a couple between the LCB and LCG.

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### Leo LazauskasSenior Member

How a boat trims in reality has nothing to do with the question.
It is possible to specify trim and sinkage in several ways, e.g. around the
LCF, or the LCB, or midships, or, as is done in Flotilla, by specifying the change
in height of the bow and stern.

Dolfiman likes this.
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### TANSLSenior Member

That's what I meant. Or I misspoke or you have not understood me.

A couple occurs only between forces, not between distances. Perhaps, now, I’m who do not understand.

My question had nothing to do with theother question. I just missed your answer which, without explanation, did not seem right.

We agree but to avoid silly confusion, like mine, somewhere will have to explain it.

Thank you both for your kind explanations.

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### JacobSimonNew Member

I am aware that this is not the ideal tool to model the very high length to beam planing catamarans that we are considering for our project. Problem is that there is little out there to do estimates for this hull with. Are there considerations other then the correct rise out of the water and trim that need to be considered that Michlet does not account for in its computations. If that is all that is missing we can get estimates by mixing methods for finding the rise and trim of our boat and entering that information into Michlet.

Fortunately this is only expected to trim 4.5 degrees, but that is kind of dramatic looking when the boat has a max of 2.25 ft of draft when in displacement mode and a length of 60 ft.

Does Michlet recalculate the center of buoyancy for each angle of trim as the boat trims?

Thank you so much for getting back to me last time.

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