Michlet

Rick

OK also I do the change in scale. I scale the boat 1/10 and naturally I have the same results as yours.

What you do not do, is export the new data from Freeship to Michlet.

Do this.

And what you see there?

Cp = 0.62129

Is not my hull!

Without talk in wetted area that in previous, not scaled hull, by Michlet is 0.155 m2 and in this new boat scaled 1/10 Michlet gives 14,3692, more correct, but for me should be 15.5 m2 for consistency.

In change the scale by software, in Freeship, we can have a distortion in precision.

If you see the original archive hydrostatics you see that before 0,004 I have ten informations that give me 0.003 for the displacement each 10 of that wl are spaced 1 - one - mm.

You think that Freeship have a precision of 0.00356/0.004 = 0.89 or a dimness of 11% in such level ?

I do not know.

What I know is that my boat do not have a Cp = 0.62

I do manual calculations to inclined waterlines and the Cp is compatible with a Cp 0.58 for not inclined wl. I do not calculate without inclination.

Michlet do something that I do not know. Approach some known surface to the points or something similar, that changes the Cp. I do not have confidence in something that say me that my Cp is 0.62.

And this is for a boat that have 10 meters length. What is the problem now?

 

Rick

The new beam for Michlet now in the L = 10 m is 1,505m ????????????????

 

Fred
Yes, you can see how poor the Freeship data is if you do the hydrostatics at 1mm interval. It has zero displacement up to draft of 17mm. The displacement jumps from 3 to 4kg when going from 53mm to 54mm. So the error could be as high as 0.5kg in 3kg - an error up to 17%.

The wetted area given in FreeShip at 10X is 14.216sq.m. The value that Michlet produces from the export data is 14.35sq.m. So within 1%.

In FreeShip the prismatic coefficient is determined from the midship section based on the length you have set in the Projects settings. In your case midship is at 0.5m.

Michlet does not see the overall length. It is only concerned with the underwater part only. So the midship location will be different to what has been set in FreeShip.

If your hull had an overhanging bow such that you set the overall length in the FreeShip file to 1.2m the Cp changes to 0.628 rather than the 0.582 you have with the length set at 1m. So FreeShip is producing a result that has nothing to do with the underwater shape. The Cp is a meaningless number in this context. It could be whatever you want without changing the hull shape; just by adjusting the project length - it can be completely unrelated to the underwater portion of the hull.


Rick W

 

I do not rely much on hydrostatic data from Freeship or Delftship.

I use Godzilla to generate the hull shape within the constraints I set and then import the offsets into Delfthip for rendering above the waterline. I often also do some fairing in Delftship for ease of development but I always check the end result in Michlet to confirm I have not added drag.

I have learnt to rely on the Michlet data. Typically it is very reliable for displacement regime and Godzilla can literally condense years of trial and error to a few minutes.

I have not not tried to generate a hull that is intended to perform at angles of heel like you get with a keel yacht so cannot comment on how well it does here.

Rick W

 

Rick

No, you do not look well.

See there:

H = 0.014 --volume = Cp*Am*L => Vol =0.574*0.001*0.511=0.0003 m3
H = 0.54-- volume = 0.5807*0.006*0.932 = 0.0032
H = 0.55 -- volume = 0.5820*0,007*0.936 = 0.0038

Why Volume for H = 0.55 is not 0.004, because Am have only 3 digits, but for the software the others digits are there.

If Am = 0,0073 Volume = 0.004 m3

`` Yes, you can see how poor the Freeship data is if you do the hydrostatics at 1mm interval. It has zero displacement up to draft of 17mm. The displacement jumps from 3 to 4kg when going from 53mm to 54mm. So the error could be as high as 0.5kg in 3kg - an error up to 17%. ``

The displacements that you do not saw is that not show - logic - 0,0033 - 0.0035 - 0.0038. Why?

Because only 3 digits are displayed.

Apparently the change is 0.003 to 0.004, but is not. Your thinking is not correct.

The digits are not displayed but for the software they are there. The software is made for boats more than one meter and I think that the origin for not display more than 3 digits is for this, for these displacements 4 digits is not necessary, but they are there.

``In FreeShip the prismatic coefficient is determined from the midship section based on the length you have set in the Projects settings. In your case midship is at 0.5m. ``

Remember, submerged midship is not in 0.5 m, who are in 0.5 m is a maximum submerged area and this is correct. The maximum transverse area submerged gives the Cp not the midship area submerged. In my case he is maximum on 0.5 m - amidship only for total length, not submerged length.

Final detail = If you go to Menu - Project - Hydrostatics you see there

`` Coefficients based on = Actual dimensions of submerged body``

I am the oyster.

Ok Rick thanks very much for your replies. I appreciate yours replies.

I will calculate in some days the real volume by hand. When I have time and I will give you the results.

Best regards

 

Rick

Up to now I do not use Godzilla because my system is Linux. Michlet go well with Wine, but I have problems with Godzilla.
What I learned about Godzilla is that we have some families of hulls and with the input that are given it search for a hull with less resistance in the family.
Is correct this?

 

No Godzilla does NOT rely on a family of hulls. The most common version has 7 hull functions that are continuously adjusted to arrive at the lowest drag hull.

To confirm the inaccuracy in the Cp value produced by Freeship I have attached the hydrostatic data for your hull but I have changed the Project length setting in one of them to 1.2m instead of 1m. You can see how this has affected the value of Cp. If you want the Cp to 0.7 you can do it simply by specifying the length. It has little to do with the hull shape.



Rick

 

The information that is exported to Michlet is rounded to three decimal places - the same FreeShip displays in its hydrostatic data. Hence Michlet has been sent a volume of 0.004Cu.m. If you open the export file you can see this value. The actual value is 0.00356 but Michlet is not given this more accurate value. It has 0.004Cu.m to work with and I presume Leo regards the displacement inviolate so the beam is adjusted.

Rick

 

Fred
I have gone through the process of using Godzilla to determine the hull shape for various constraints similar to what I do for my low power hulls.

The attached drag curve shows drag for four hulls all having a displacement of 3.5kg. I have included the hull you sent me, which I have termed original. The lowest drag is for a completely unconstrained hull for minimum drag at 1.3m/s. It ended up at 1.7m long.

The green curve is for the minimum drag hull at 1.3m/s and constrained to 1m long.

The purple curve is for minimum drag hull for 1.3m/s, which is 1m long and has the same initial stability as the "original" hull.

The data on the chart is only applicable to zero angle of heel.

For interest sake I have attached a linesplan of the hull with both length and stability constraints. This hull would require about 50% less force to push it at 1.3m/s than what your original hull requires.

The version of Michlet I have does not allow analysis of a heeled hull so I cannot do comparisons for other than zero heel.

I chose a design speed of 1.3m/s because I knew this would be getting into heavy wave drag for a 1m hull. You could use other values to see how the shape changes in response.


Rick W

 

Rick

Thank you very much for yours replies. Leo is a nice guy, but you is a very very nice guy. Is difficult to obtain a person disposal and with time to exchange ideas. I am very thankful with your disposition. And I hope that we obtain a conclusion about Michlet. I will calculate by hand the characteristics of my hull and I will pass for you.
I am surprising with the results of Godzilla hull. When I do a design for one meter hull I have in mind principally Cp, wetted area, and bow and stern forms.
For bow I need the stations open above water line to reduce the tendency to nose the bow down, a problem that we have in non helled and helled condition in one meter class. By same motive the stern need have right stations above water line and can not be wide.
These are by now my rules to design a one meter.
I am surprising, because with single rules like these my hull has the minimum drag curve in the velocities that we operate.
Our maximum velocity remain in 1.2 m/s = 2.33 knots ==> v/L^1/2 = 1,34, but the boat do not have always this velocity. Normally the velocity is below this. Our motor is the wind and we depend from his force.
Looking for the graphic I am very surprising in see that my hull up to 1.07 m/s =~2.1 knot, we have the lowest drag with exception in the range of 0.8 to 0.9 m/s.

What think about this? I do not know.

Well, I need a time. I am going to calculate by hand the non heeled parameters from my design to conclude something. With the time that I have I think that Monday I have it.

Fred

 

Fred
Setting the constraints for Michlet/Godzilla is the key part of the design. You need a good understanding of the operating conditions.

Also Michlet does nothing with the hull above the waterline so you need to design this to suit the forces that the hull will resist in wind and waves.

I have attached the linesplan for a hull with same length and stability constraints as before but it has a minimum drag speed of 0.8m/s. You can see how the shape has changed. You can set multiple speed objectives if you desire.

I have updated the drag curve and expanded the scale more in your area of interest.

Rick W

 

Fred:
Rick has done a splendid job of answering your questions, however I should remind you of a couple of points...

1. Michlet has not been formally validated for hulls with L/B as low as 4.5.
Some hulls are reasonably well approximated, but I can show you many where Michlet is not appropriate. Also, people do not show results where the approximation is poor, so the "validation" sample is very biased.

2. I am not confident of Michlet's ability for very small model hulls. The wave resistance will be Ok, but the skin-friction is almost a complete guess.

The ITTC line (and most others) are not reliable below about 5X10^6. If the draft is very small, the meniscus of the water could also add a significant amount to the wetted surface area. Should one add that to the wetted area? Or should one treat it specially in some other way?

Good luck with your project!
Leo.

 

Leo
One thing that has come up in this discussion is that Michlet holds volume as set but will change the offsets to produce the set volume within the set length.

Did you have a reason for doing this?

Rick

 

Leo

Thanks for your replay.

The information change with Rick was very profitable for me. Was an incitement to look better Michlet, for me.

The file exported from Delftship and Freeship is not correct when the transom is above DWL, at least in my drawing. The table of offsetts last row is not do by zeros, and Michlet assume that the boat has an immersed transom.

In Michlet we note that Cp, B, wetted area, do not agree with originals.

When I see the result in Michlet that show, different B, Cp, wetted area, we think: is not my hull. How we explain this?

L/B in my case is 6.3.

Fred

 

Rick

Let's go wait for Leo to continue our conversation, meanwhile I do the calculations.

Regards