Catamaran Evolution

Rick,

Thanks for the PDF for comparison. I have a question. Why do you think the Green Lantern requires less HP than the OPT34 up until about 6.8 Kn's? Does the program publish numbers for the designs? I would like to compare the percentages of increase for the Green Lantern and the OPT34. I see a pattern where the Green Lantern always requires more HP past 6.8 knots than the OPT34, but a little bit beyond that point, the graphs seem parallel although as already stated, the OPT34 requires less HP.

To answer the above...yes, the Green Lantern has more displacement because as I began to evolve the design from the other two, I decided I would keep the draft consistent at 1.39' regardless of where the CG and CB were located. I have no doubt in my mind it will come in lighter but my difficulty right now is trying to get the design to the next stage which is to the point where it can be sailed bare minimum hulls with the CG and CB where it should be. Once that is done it's only a matter of tweaking. For mass purposes offshore she cannot be too light, but she will never exceed the current draft. She is at 4959#'s right now.

The Green Lantern is a sort of compromise between the CR33 and the SR34 so it appears that it will have less performance than the SR34...but that is now. I want to measure it when it is completed and at displacement. Rest assured that I am on a mission to trim her up good and I am working towards minimum displacement once all is said and done. The first cut took off over 100 square feet from each hull alone.

Thanks again,

J

 

Two things:

Bit maps are very inefficient image files. They are very large for what gets displayed. Accordingly they are not shown as an allowable format for upload to Boatdesign. You should convert the file to a jpg image for posting. I have attached a view of the SR34.

Below 6.8kts the curves have some humps and hollows. This is most likely due to wave interaction between the hulls. There are points at lower speeds where Opt34 requires more power than the other hulls. I would consider 6.8kts ghosting for a 34ft cat. If you want good ghosting performance then a 34ft mono will be far better.

Rick W.

 

Rick,

I tried it and it didn't work. Can you explain how you saved it to jpg from FreeShip?

That view has changed a lot in the Green Lantern! Wow.

No mono-hulls. I'll tell you though...I wasn't on board nor do I have unequivocal evidence, but if my eyes didn't deceive me, I saw a 35' cat doing about 6 Kn. Winds were light...but I would have been motoring.

J

 

You have to save the image in bmp format from FreeShip and then open the bmp file in an image handler (my computer has Microsoft Picture Manager loaded as part of operating program) then Export or SaveAs to a jpg format.

If you can move at 6kts then you are at least covering ground. With my shorter mono hulls I would start motor if under 4-5kts over the ground. The moderate displacement monohull I had was very good at ghosting. It was the only time I could outperform similar sized cats. These are times when I appreciated having a crew member who smoked - I would encourage him to light up. It is amazing what you can do with crew weight to set light sails if you can detect airflow. Once the apparent wind gets going, other boats wallowing about think you are motoring.

Rick W.

 

Hello Richard...

Here is the latest on the Green Lantern. Not done yet, but working hard.
Sorry it took so long.

J

 

Here is the next update for those that have been following it. It is taking a little longer than expected because I made changes to quite a few components, so I figured I throw it out here and see if any comments are forthcoming yet. It isn't done.

Right now she is displacing 2.822 tons placing her at a draft of 1.302 feet with a LWL of 32.690 feet and a LWL BM of 2.449 feet. It has not been easy keeping the weight down while keeping that CG near the CB. It looks like final adjustments may come from stretching the waterline again and maybe shifting the fresh water tank forward a bit. Back to the board.

Rick...thanks for the info. I thought there was a way to create the JPG in FreeShip.
Question. Is there any way to download the bulkhead station x, y, z coordinates that are in text into the Fresship Model without having to input each and every single one? Thanks

J

 

JCD. It's a good looking boat. I'm just wondering if your bridge deck clearance is too low. The support beam (or whatever you call it) in those images looks very low. Or is it just the angle I'm looking at?

 

Hello Richard...

Thanks.

It may be the angle that might be throwing you off. At the lowest point...which is the aft beam, the clearance is 2.45 feet above the current draft of 1.39 feet. For slamming purposes on the bridgedeck sole, the lowest point is 2.56 feet above the same draft of 1.39 feet. I believe there is plenty of room right now and if her final displacement is lighter there will be even more clearance.

But it doesn't look like she will be lighter right now.

I'm still working on the stations for the bulkheads since I have to input every single x,y,z, point. I'll have something to upload soon.

J

 

Am I doing it wrong?

Oh boy...

Was working on the Green Lantern and I found what I believe may be huge errors on my part. I can only hope that I can be given clarity and guidance to fix it.

I was looking at the hydrostatics and saw that the sail had double the area than it was designed to be. I then began to look at everything else and figured out some things that threw me for a spin.

The hull was mirrored. That meant that strake 1-8 were actual areas and their displacements did not have to be doubled.

All other items that were drawn post mirror command, had to have their displacements doubled.

Any other items that had displacements "set" opposed to having them based on area also had to be doubled.

Finally, any single items such as mast, boom, center beam etc, does not have their displacements doubled.

Any of this make sense? Anyhoooo...I did a spreadsheet with areas and displacements and was shocked to find that the hydrostatics weighed in at (1.546 x 2) = 3.092 tons or 6,926 pounds. The displacement that was calculated with all of the above taken into account was 5,169 pounds.

Can someone shed some light on this and give me some clarity? I don't believe its the program and it could have been me creating components post mirror but, if the spreadsheet is correct, then what kind of false data can I expect from the program due to false data designed and can I fix it by entering different thicknesses until the accurate weights are displayed??

Any advise from anyone with more experience in Freeship would be much appreciated. The spreadsheet is attached as well as the hydrostatics.

Thanks

J

 

If you have the symmetric box ticked in the layered controls then everything will be doubled. The displacement figures I provided before were correct for the given hulls. If you recall I doubled everything in the first analysis I did this because I had not unticked the symmetric box in the first instance.

Also you can set stations and just produce the offset text files. These can be imported into Excel. Typically if you are making hard work of something in FreeShip you are doing it the wrong way.

Rick W.

 

JCD, I understand that bridgedeck clearance is a matter of personal preference. It's up to you to decide at what point the boat starts to slam. But going by the stuff I've read (and I'm no boat designer....just a hobby to me) I think your clearance is a little low for a boat that length. I know a lot of modern cats have very low clearance...but that's often to fit more room in the cabins and is just a sneaky way to sell the boat.
Ofcourse , if you're happy with your clearance then fair enough....but for a 32 foot boat I personally would prefer to try to get as close as possible to 2 feet from waterline to bridgedeck sole. Having said that, I have never sailed a 32 ft cat and have never experienced wave slamming myself.

In the following weekends I would like to get familiar with freeship and start to visualise my own design properly. I look forward to showing it to you and Rick and anyone else who is interested. I won't be showing the top secret bits though. lol
Unlike your design, cabin space is a low priority on my boat, but high weight capacity and passenger comfort on deck is high priority for me.

Regards
- Richard

 

By the way... I think the aft beam is a critical point because thats where the waves can be worst. The height of the rest of the bridgdeck may become irrelevant??

 

Oh geezzz...

No doubt about making work harder for myself than FreeShip.

With this symmetric box, if the box is ticked, is the area for the component displayed already doubled in the hydro or do I then still need to double it? For example, the transom steps are 65.32 sq. ft., symmetric is ticked. Is that the actual sq. ft'age displayed for both sides or do I need to double that to get the actual area and weight? Would it be better to just remove all symmetrics and then just double everything at the end?

I tried the stations offsets from the text file. I put 1 on the first line, eof on the last, imported and everything came in but for some reason all my other layers did a Houdini. Then I figured save the design as a part, do the text file first and then import the part, but, Houdini lives on. I'm sure I'm doing something wrong but I couldn't figure it out soooo, input time.

Thanks AGAIN Rick. I appreciate it.

J

 

Richard,

Are we discussing the same thing? I think I'm missing something. The design is 34 feet. The LWL is 33 feet. The bridgedeck clearance at its lowest point is 2.45 feet or 29.4 inches. This point is at 81% aft of the waterline entry. That translates to 7.5% of the LWL or 1.58 inches of height for every foot of LWL BM, (18.5 feet). I don't expect that slamming will be a problem with this design.

Most designs today that try to pack a duplex condominium into a studio are about 1" of height for every foot of LWL BM. That is very low and with all the other protrusions beneath the deck such as nacelles, step flares etc...you are going to be beaten senseless before making port.

If this design is to have any kind of a chance of being a capable cruiser, I have to have the cabin space because on deck there is very little for protection from the elements. The dodger will shield most from just about everything, but it is strictly sitting headroom and good headroom in the cabin to take the crook out of your back is a must. One could always make sure that excellent foul weather gear is on board for different weather and do without the dodger.

Lots of sitting room in the cockpit and if you look closely...a 4' x 6.5' convertible for sleeping under the stars when weather permits...oh yes, there is all that trampoline area too.

I have a top secret idea incorporated that I am going to discuss very soon! I'm researching to see if anyone has come up with it before I offer it to the community.:idea:

J

 

If you have symmetric ticked it is impossible to see if it is drawn twice. If you untick it then you can see what is left. If everthing is still there then you do not need to have symmetric ticked. Once you get used to this it is easy to work with. I usually keep it ticked and draw one half if it is symmetric.

I find doing cats is very tricky particularly if you want to get the hydrostatic information. There is a bit of a trick in the way you separate the hulls. If you put in two complete hulls then you have to untick symmetric and you cannot get hydrostatic information. You should have a feel of the hull surface area. It should be reasonably obvious if something is out by a factor of 2. As originally presented you had hulls repeated.

I often set the basic hull as a part so I can do analysis on it and keep importing it into new layers so I can break it up into different parts. If you did this you could do sections from this. Only problem is that if you make changes in the new hull they are not reflected in the original part. But it is easy enough to regenerate the hull part.

I am busy this week so do not have much time to spend on sorting out offset issues. There are others I am sure who could give you pointers. You might need to make a specific topic on FreeShip question to spark their interest.

Rick