A few Michlet/Godzilla questions

Ay up. Figured it wasn't worth starting another thread.

Current pondering is about sinkage and trim in Michlet. How to estimate?

I suppose one could start with the wave-heights-at-a-given-speed plots from Michlet, then try to figure out how the immersed volume is affected by the wave heights along the hull, then try to get some software to calculate displacement and LCB for a wavey DWL, then try to figure out what the actual sinkage and trim would be for said speed, then use that for another run through Michlet.

I can see how that would work, but it sounds horrible. Anyone got any better ideas?

 

That wouldn't work because Michlet only produces waves aft of the stern.

I am currently writing the manuals for Flotilla version 6.0 which will
give reasonable estimates of sinkage and trim for thin vessels but I am
in no great hurry to release it.

I guess you could look at similar vessels to get a rough estimate of how
they behave. I attached a number of pdfs in a zipped quite a while ago
which contains many different hulls and comparisons of my predictions
with measured resistance and sinkage and trim. Most of the papers
also contain tables detailing the principal parameters of the hulls. See:
http://www.boatdesign.net/forums/at...66646414-michlyt-demo-flotilla_1st_drafts.zip

I emphasise that these are first drafts, so they will contain some errors, but
the experimental results should be useful.

 

Thanks. Took a look at those, and although there's quite a bit of information in them they are all hull series which aren't very close to the shapes used in recreational rowing boats. So, I have a feeling that trying to extrapolate from that information could well lead to GIGO for me.

Looks like I'd better wait for that manual to be released, which is ok because at the moment I'm actually building something anyway. I asked mainly out of general technical interest, and in case I wanted to try running more accurate predictions sometime down the track.

 

You are quite right: using experimental data for the transom stern hulls will
be of no use to you.
I have a large collection of rowing shell data, but they are Olympic class not
recreational. I have been working with an Australian shell manufacturer to
estimate the sinkage and trim of hulls taking into account the varying
location of the mass of the rowers on sliding seats, but this is probably a
step too far for your purposes.

 

Yup, that probably is taking it too far, at least for fixed seat boats. Some of the Olympic classes would be fairly close in general terms. IIRC things like coxed pairs (if they are still used) have a fairly high DLR by racing shell standards.

 

The boats used by elite classes are probably a bit narrow for amateurs who
need a little more transverse stability.
Some of the shells used by school kids are a bit wider and more stable. It's
tedious to do, but I found it worthwhile to measure up some of their boats to
add to my database a few years ago.

 

Yeah but my guess (which is obviously infallible, like all guesses) is that sinkage and trim wouldn't be much affected by variations in B/T, and would be mainly dependent on DLR and prismatic for hulls like shells and recreational rowboats.

 

Sorry, I forgot you were after squat trends.

For what size hull do you want sinkage and trim? I have a lot of results for
rowing shells that might help.

 

Well for 5.5 to 6.5 metre DWL, ideally, and about 100kg or so displacement, at somewhere around 6 knots give or take. That's not really in the range of any Olympic classes, but I suppose singles and coxed pairs would be the best to start with for getting an idea of trends.

It's been yonks since I looked at displacement/length ratios for Olympic classes. In old terms (Imperial units) displacement/length for fast recreational is somewhere around 15-ish and speed/length around 1.4-ish or so, depending on amount of Weetbix consumed.

 

The attached pdf shows some trends of LDR and L/B for Olympic class shells.
If you give me an idea of the beam and draft of your shell I can run Flotilla
to estimate the sinkage and trim. Squat is quite small for Froude numbers less
than about 0.35.

 

Sure. Straight from out.mlt..............

HULLS
Quantity , Hull 1
D (m^3) , 0.112000
LPP (m) , 5.693000
T (m) , 0.067000
s (m) , 0.000000
w (m) , 0.000000
B (m) , 0.681754
S (m^2) , 2.566042
Awp (m^2) , 2.311136
LCB (m) , -0.152410
LCF (m) , -0.108301
VCB (m) , 0.038182
KG (m) , 0.067000
BML (m) , 29.029214
BMT (m) , 0.519337
GML (m) , 29.000396
GMT (m) , 0.490519
At (m^2) , 0.000002
Bt (m) , 0.008021
Tt (m) , 0.000419
Cb , 0.430700
Cm , 0.735576
Cp , 0.585527
Cwp , 0.595466
L/B , 8.350521
L/T , 84.970149
B/T , 10.175431

 

I'm really making a welter of this!
If you give me the in.mlt file I can use the offsets directly in Flotilla.
(Or if you don't want others to see it, pm it to me).

 

Mitchlet help

Hi, my name is Albert, and I´m trying to design a catamaran... it´s drawn and working in freship, but I have been in troubles to work with michlet.
When i change the name file (in.mlt) and try to run the program, there is a out file and advice me that: ERROR: Bad Nx, in sometimes Bad trim speed as well. Somebody knows what it´s refer and how i can fix it.

Thanks a lot