Why can't most catamarans get over the hump ?

You can't make that supposition unless you know the displacement and waterline length of each hull.

 

We have been through this same debate several times: for example:
http://www.boatdesign.net/forums/hydrodynamics-aerodynamics/canoe-sprints-44227.html

That sounds right for the S64 results. There is more doubt about the
experiments at higher Froude numbers. Add error bars of +/- 5% and
they are all essentially within the same fuzzy cloud.

There are many other experiment results and references I have compiled
in the reports at the bottom of the page at:
http://www.cyberiad.net/flotilla/flotilla_mono.htm

Each had their own "sweet" spot, but all very consistent.
View attachment 91224

There are doubts about these plots too. In that paper, there is no
description of how the experiments were conducted, or the lengths
of the models, or even the scale of the drag on the y-axis. Again,
if we add +/- 5% to the experiments, then there is very little to
separate the different shapes, except the "crude hull".

Trying to pick up small drag differences between slender hulls in
experiments is very difficult. Scragg and Nelson tried that in their
work on rowing eights and concluded that the results were too muddied
by experimental error to be useful. The trends were much easier to
discern using thin-ship theory and the ITTC line for the skin-friction.

It should also be remembered that most of these experiments have been
with bare hulls in calm water. Add some appendages, roughness, and other
"realities" and I doubt that anyone could confidently predict that one
slender shape is clearly superior to another.

Those who use Froude scaling should also remember that it is a hypothesis,
not a hard physical law. There are discrepancies when scaling up to full-size
because of many omitted "realities".

 

Ad Hoc says if the length/displacement ratio is the same, those two hulls will have the same resistance, no mention of what speed range though. You tell me !

 

I used the assumption they were constant between hulls.
I should have said so explicitly, apologies.

I haven't kept up with who's saying what, but that sounds wrong to me. The increased beam will increase wavemaking resistance.

 

I'd have thought so, but no mention of speed makes it all a bit academic.

 

Indeed.

Concur

Exactly...it is all about trends, not absolutes.

 

To J. Perry:

In post no 93 you show the Michlet calc result in a diagram, with blue line for the deep hull and red for the wide one. Unless my eyes have gone colorblind, the red line is below the blue, meaning that the wider hull has the lower overall drag, which is according to my expectations with the hull shapes shown.

But in the text you state that the deep hull is the one with the lowest drag; am I misinterpreting something there?

As to the issue of having the same transverse immersed transom area in both cases: The transom drag is not a simple question of "the size of the hole"; the aspect ratio of the hole is very important as well. The enthalpy in each streamline at the transom has the form g*H, where g is earth acceleration and H is the local depth. Its dimension is (m^2/s^2), ie a velocity squared. Now, if you integrate the enthalpy available over the transom width, you will find that the enthalpy difference between the two transoms is substantial. Thus you cannot manipulate shape just by scaling up vertical dimensions and scaling down the horizontals; it has direct implications for the combination of the various loss components.

 

If you were to plot resistance against beam for hulls where speed, LWL, displacement and WSA remained constant you'd see clear trend of increasing resistance as beam increases.

 

You are right, I have made a silly error, thank you for pointing it out. I have checked my files and I find that the graph I initially drew using Excel together with the output file from Michelet is the black and white one attached below. When I made my first post to this thread I was not sure that the markers on the curves would be visible to readers using small screen devices, so I altered the Excel file to remove the markers and add colors to the curves, I also removed the skin friction line since I thought that might just add confusion. Unfortunately, in changing the Excel file I inadvertently switched the legends for the curves!

I hope the graph as now attached is correct, but do say if you see anything else wrong.

Re your points about transom immersion, I understand what you are saying. The complications resulting from introducing an immersed transom are the reason I used canoe stern hull shapes for this initial comparison, I was trying to keep things simple, at least to start with.

 

And the reason why? Its the wave making drag... look at the component drag results from michlet. Increasing beam increases wave drag all else remaining constant.

So when the OP ask`s "why cant most catamarans get over the hump" as vague a question as it is, the answer is most likely "many catamarans have fat hulls"...and most definitely in combination with "also have poor length to displacement ratio"... Unfortunate product of designers and operators trying to squeeze too much into a restricted length catamaran...

The next question begs to ask - what is the separation between hulls? Fat hulls, close together = big wave negative interference drag whilst at hump speed also...

 

For many, it's probably even more rudimentary than that, as Eric notes in an article he wrote for Proboat Magazine, which he posted here.

 

fat hulls close together describes most of the cats that are sailing dogs

 

That's not very surprising.
See Fig. 14a on page 30 of the report AdHoc mentioned and I linked to:
http://eprints.soton.ac.uk/46442/1/071.pdf

The experimental resistance results for the NPL 3b catamaran with the closest
hull spacing (s/L=0.2) could not be completed for Fn > 0.6 because of problems
with swamping. Note that the resistance at Fn=0.5 is quite a bit higher than
the line fitted to the experimental data.

 

Yes, in the main.
As AdHoc has said, it's about "trends", and these can be seen easily by varying
the beam to draft ratio while keeping L, D, L/D^(1/3) and WSA constant.

 

My thoughts exactly!