michlet/rocker

[ediestel]

I want to change the rocker of a monohull; I assume that you have to set the f3 restriction, which relates to the horizontal view of the ship's forward.

I want to set a forward rocker of 2.5 inches; do I have to set:
2.5, 2.5, 2.5 for f3 ?

can anybody help me out with an example ?
I posted this under 'design', I guess it is better asked here.

[Maxim Projet]

Originally Posted by ediestel

I want to change the rocker of a monohull; I assume that you have to set the f3 restriction, which relates to the horizontal view of the ship's forward.

I want to set a forward rocker of 2.5 inches; do I have to set:
2.5, 2.5, 2.5 for f3 ?

can anybody help me out with an example ?
I posted this under 'design', I guess it is better asked here.

The Michlet parameters for shape functions are not dimensions. They just change the shape of a part of the hull, like the waterplane or the section shapes or the rocker.

[ediestel]

Thank you.

What does the allowed range relate to ? Is it the radius of an arc, is it a fraction ?

Example: What does it mean if you put f3 at 0.75 ?

[terhohalme]

Yet an other too lazy to read the manual...

[ediestel]

terhohalme

The time it takes for you to post, you can also use to share your expertise.


Do you have to export the offset to a design program and measure the rocker in inches/metric there or can
you approximate the rocker ( in/cm ) while still in Michlet ?

[ediestel]

Series 1

This hull series takes three shape parameters. All must lie between 0.0 and 1.0, inclusive. The series is such that if a parameter has value 0.0, then the shape is rectangular, 0.5 corresponds to an elliptical shape, and 1.0 creates a parabolic shape. Intermediate values will produce shapes intermediate between these familiar shapes.


Series 7

f0: Forebody waterplane shape (min. 0.0, max. 3.0)
f1: Forebody cross-section shape (min. 0.0, max. 3.0)
f2: Forebody sideview shape (min. 0.0, max. 3.0)

The first three parameters (f0, f1 and f2) are the same as for Series 1, but are relevant to the forebody only;

The maximum value of the first five parameters is 3.0; cusped shapes are possible with this series.



The forebody sideview shape at f2 = 0 is a straight line, at f2 = 3 spheroid. So far so good.



Is there a way to relate this to a forebody rocker expressed in inches, taking into acccount the length of the forebody, the draft and f2 ?

[Guest625101138]

Eckhart
You are looking at reasonably minor change in rocker. For such a minor change, I would allow the hull parameters to have their full range and optimise for a certain speed.

Once Godzilla has the hull shape I import the offsets produced into Delftship and then modify slightly to get closer to the desired result. I then retest the shape in Michlet to see what my modification costs in terms of performance. I have once or twice get a lower drag result than Godzilla produced but normally the drag is higher. This sounds tedious but if you have set up the right folders it only takes a few minutes.

I do force a hard chine in Godzilla if I want a very simple hull for plate development but I have only forced rocker once. You could do it by trial and error just by setting the maximum limit and ensuring the initial setting was within the range.

You do need to know what power level you aim to operate at. Even with Godzilla you will still need to iterate as you want the optimised speed to coincide with the target power level. Hence it is important to know what power level you are going to operate at.

Rick W.

[ediestel]

Thank you, Rick

I have read this advice from you here on the forum before and have been trying to go back and forth between Godzilla/Michlet Freeship as you discribe.

I quite often get an error stating that certain coordinates ' 0.018, ..., are not submerged' and FreeShip does not calculate the hydrostatics under those circumstances.
The manual does not comment on this. What can I do to avoid that ?

[Guest625101138]

I will play with parameters for a design and send you the input file. You can then run Godzilla and watch a hull evolve.

Rick W.

[ediestel]

Thank you very much.

I suppose it is the formula in the following article that can be used:

http://en.wikipedia.org/wiki/Air_resistance


related reading :

http://www.keelhauler.org/khcc/seakayak.htm

Eckhart

[Guest625101138]

I have attached the No-Compromise in.mlt file for a hull to do 3.5m/s with displacement of 110kg.

You can use this as a starter file to see what Godzilla optimises for the requirements. If you do not know how to start Godzilla then I will provide more detail.

The result is the underwater shape for a no compromise hull and performance prediction. You then have to decide if this will do the intended job.

My experience with OC1s suggest that you need a little stability in the main hull otherwise you have to load too much onto the outrigger or you have a boat that is too twitchy to handle. Loading weight onto the outrigger kills performance. That is why I use balance outriggers on my pedal boats. The aim is to not load the outriggers. They are insurance primarily.

So the next step is to adjust the minimum GMT so you get stability. If the GMT is higher than 0.3m you would not require the outrigger if you sit low. This is probably more stability than needed and will cost in performance. I have already looked at GMT of 0.2m and it was wider than most OCs I have seen. It costs a lot in performance. I think GMT of 0.1 might give a nice result.

As for rocker I have mixed view. I do not have enough experience in large waves with slender hulls to know what works best. I think most seakeeping qualities can be built into what is above the waterline. You will see the optimum hull has reasonable amount of rocker.

I have done a design for an ocean pedal boat and I have increased the optimised speed above the design power level to take advantage of mostly following sea. So there are circumstances where you might benefit from a longer hull. If you get Godzilla going you will see that the NC hull is just under 8m long. This aligns well with a heavy weight rowing scull. Are you able to transport an 8m boat!

Rick W.

[ediestel]

Rick,

thank you for the file and suggestions.

Indeed, we are trying to paddle the OC like a surfski, rigging the ama as light as possible. The use of balance outriggers would be within the rules for OC 1.

If length improves performance, than it should be tried; unless increase of weight and wetted surface area negate the benefits.

The idea for open ocean racing is of course to take advantage of the waves and the speeds while surfing. Important is further that the boat has the ability to 'connect' the bumps, that is, maintains speed as much as possible without loss of energy due to pitching or rolling.


In my experience, rolling and trim changes are very expensive, so I'd like to avoid them.



I will run your file in Godzilla and see what happens.

[Guest625101138]

Eckhart
Let me know if it does not get the expected result. When you start Godzilla you should see the hull changing shape. The NC optimum is almost 8m and 250mm beam (say 10"). Michlet works in metric so it is easier to do all the files in metric.

If you are allowed twin outriggers it may be worth a try. The only disadvantage I can see is slightly more difficult to right if you do roll it.

The NC hull is very easy to make light and strong because it is so narrow and almost cylindrical so is inherently strong.

Rick W.

[ediestel]

The current design - 7.90 m, beam 0.25, draft 0.11 - under water body requires about 12 % more power at speeds up to 5 m/s.
About 22 N per KAPER.

These speeds are reached while surfing though, so the paddler does not have to generate the power continuously.

This is compared to designs that I tried earlier with waterline length closer to the currently available OC 1 of about 6.00 - 6.50 m, max body width minimum 14",
beam at waterline is mostly unknown, but I don't see 2 inches of flare on each side on my boat, at the most 1, likely a little less. That would put the beam at waterline for
a OC 1 Hurricane at 12 ".

Rocker is likely more in other designs. So I would be inclined to increase the rocker a bit. This decision will be the most difficult one as nobody seems to have specific knowledge for this scenario about this.

How responsive would a 7.90 m boat be to rudder action ?

I have no experience with seakeeping features above the waterline:

I think volume in the bow in a predominantly vertical distribution is one thing.
The Epic V 10 has a lot of volume in the foreship, too much it seems as they built the Epic L with a flat foreship. Pearling can mostly be avoided once you know how to paddle a certain boat.

Flare may be needed in the midsection where the paddler is sitting ? My fear would be that it increases pitching ?

The stern should be a canoe stern, but not be too narrow, I guess. I actually would like the wave to pick up the boat early and easily.


I'd like to upload the linesplan for anybody interested, but I am not sure about how to do it.

[ediestel]

Some specs for those that are following this thread:

Hull:
from outside to inside:
carbon - thin core - carbon - thin core - carbon or glass. Duratec primer. By preference urethane paint.

Weight:
should be as light as possible, but rigidity and strength have priority.
target < 30 lbs., better 25 lbs.

Seat:
molded into the deck, height 5 - 7 " above heel.

Sit-on-top

Footwell:
Single footwell with a 'chambered' Venturi drain. The Venturi drain is at the bottom of a separate compartment; the compartment is open at the top. Overall this prevents water to flood the footwell retrogradely through the drain.

Steering:
T-bar

Rudder:
usually about 30" from the stern, minimum about 10" with 4 " chord, mostly eliptical.

Freeboard:
I would love to have the freeboard as low as possible in the seat area. Higher in the aft. I prefer a 'wet' design as long as it does not impact performance too much.

Race duration ranges between 60 min and 5 hours; the seat and footwell have to comfortable. Stability must also be good enought to alloow relaxed paddling over that time in following seas that can be quite irrregular.

While this boat is supposed to be fast, it is supposed to be fast specifically in the following sea.