Trimaran early stage design

[julien]

Hello everyone!

I am a young naval architect with no much experience and I am currently working on a Trimaran design.
This trimaran will have a length overall between 70 and 80 m. the cruise speed will be 18 knots and the max speed 25 knots
I will try to find “the best” compromise between the resistance, seakeeping and space onboard. To help me reach this target I have developed a fully parameterized hull and plan to use optimization techniques rather than the conventional design spiral approach.

As I know that some people on this forum have the experience that I don’t have, I was thinking that you could maybe help me with my starting point (baseline design).
I send you the lines plan:
If there is anything that shocks you, a parameter that you think is clearly not optimum or simply if you don’t like it, please let me know!
Your comments will be very useful.


The parameters are:
LOA=73 m
BOA=20m
Displacement=1000t
Distance between CLs =8.1m
Displacement of outriggers= 3.5% of total displacement


Thank you for your help!

[Fanie]

I will try to find “the best” compromise between the resistance, seakeeping and space onboard.

On that size boat that is not a challenge, you can almost get a town on it.

Try doing it on a 9m trimaran, foldable and trailable. Don't copy other designers ideas.

[Tcubed]

Could you please draw in the LWL and explain what its purpose is and what propulsion it will use.

[julien]

This is a case study for training purpose.
I should indeed have told you that this is a megayacht
the propulsion will be diesel electric with traditional shaft- propeller configuration.

I ask you to comment on this first hull form in order to make sure that I am on the right track.
I am not and will not try to copy other designers Ideas.


Thank you again for your help!

[bill broome]

1. it's a motorboat for mega-rich posers. go away.

2. it's big enough to be a catamaran, with improved stability over tris.

3. maybe you should get way out there and go for a monohull with active management of floats/hydrofoils like a water-skier, to maintain stability.

[Chris Ostlind]

Originally Posted by Fanie

... Don't copy other designers ideas.

With all due respect, Fanie, all designers copy the work of each other. Hopefully, in the process of lifting the original inspiration, the next guy puts his own inscrutable touches on the matter and sends the genre spinning off in another, different direction.

Some call it emulation, some inspiration, some just flat-out fess-up and say they "borrowed" the idea. Still, it has everything to do with copying and in so doing, it goes in one end of the creative loop of the designer and comes jetting out the other as a new take on things... sorta.

The creative manipulation of known ideas; You see it everywhere you go. With boats, some of the things that the boating community think are inspired works of the highest order are pretty much well-known features in an entirely different discipline.

[Rick Willoughby]

Julien
I refer to this concept as a faux-tri. This is intended to convey the idea that it is a trimaran below the waterline and monohull above. There is another thread on the idea. Chris O provided the linked image of a military version:
http://www.boatdesign.net/forums/att...5050813627.jpg

As far as you design goes:
1. I would go for the amas to be straight on the waterline rather than curved.

2. I would aim to have the main hull inherently stable and set the amas right on the water line. You might only have 3.5% weight on each but they will contribute a lot of drag. You can play with their location a bit based on how the triangular wave develops from the bow. You can actually set them to ride the bow wave so they do not add to drag.

3. You do not nominate the intended speed. I expect that such a long narrow vessel will operate predominantly in displacement mode, in which case the main hull will likely be most efficient with a canoe stern. If you download Michlet/Godzilla it can provide the optimised hull shape for the target speed.

4. If you want efficient propulsion you need large diameter prop/s and you need to think about where they are located at this stage.

I believe the basic concept is very sound for a fast efficient cruiser of moderate displacement.

Rick W

[Ad Hoc]

Julien
"...I ask you to comment on this first hull form in order to make sure that I am on the right track..."
There is no 'ideal' hull form. But there is an ideal hull form for the purpose that it is intended! The only things to consider is the length displacment ratio, the distance of separate and location longitudinally of the outriggers/amas/side hulls. The waterlines aft look like they need flattening out (transversely), a bit too pointy.
Being deisel elec...this will be heavy!

Big Q is, why choose a Trimaran hull form?

[Fanie]

With all due respect, Fanie, all designers copy the work of each other

True Chris, to some extent. If you just copy something then you can just as well just find a suitable boat and use that as a project as is.

The idea of designing is to find new and innovative methods of using space etc. On my own boat I'm going to use somone else's idea to get something done, but I changed it so it will suit my app more and there is a bit of space saving as well.

Anyway, this project is for a megayacht, it's out of my league. I'll leave it to you big guys

[Tcubed]

<<2. I would aim to have the main hull inherently stable and set the amas right on the water line. You might only have 3.5% weight on each but they will contribute a lot of drag. You can play with their location a bit based on how the triangular wave develops from the bow. You can actually set them to ride the bow wave so they do not add to drag.>> Quote from R willoughby

This is bang on except the amount of inherent stability in the main hull can be reduced to almost nil with the amas just grazing the water. But if the main hull's stability goes to negative then it will make the boat want to lean on one or other ama, creating strange rolling motion which is not a popular feature amongst megayacht clients.

<<2. it's big enough to be a catamaran, with improved stability over tris.>>Quote from B broome

A cat would have worse wetted surface and it is a fallacy to think huge righting moments are somehow desirable on powerboats, on the contrary, large righting moments create uncomfortable motion.

Myself, i would carefully scrutinize the forefoot area, to see what advantages you hope to gain from that shape. As is, especially with the amas raised for efficiency, it will be very difficult to keep on a straight course.

Also you stated speeds that mean it will mostly be going at or a little above hull speed, which is the worst speed regime you could be aiming for. For those speed/length ratios i would strongly suggest making it considerably lighter so the entire boat is much less affected by wave making resistance. It will work out such that despite having to reduce engine size to keep within the new weight constraints, the efficiency will be so improved as to make this lesser engine larger than required for those speeds. Win-win. Generally speaking, a length/Wbeam ratio of ten to one or greater will smooth out the typical transition speeds resistance wall to quite acceptable levels.

Remember, the mega yachts of the future will no longer be interested in spending 50 000 on fuel for a little trip around the islands........We hope!