Boat cross section determination

Instead of piecemeal selecting the design considerations you don't understand, you'd be best advised to contact an NA or designer, qualified to draw up your boat. You just and simply don't have the understanding necessary and learning what you need to know, one question at a time, will require decades of forum posts. Buy "Performance by Design" by Blount and work from there.

 

Knowledge

Since I lack the knowledge, I should go out and but a 100$ book? Yes, granted that I would learn a lot but have a little mercy Sir!
Should I think about a hybrid design like a cross between a cigarette style and a RIB? That way I can keep the rigid structure within 2m but have ample room outside for the inflatable units.

 

Um, compared to the cost of a 40 foot cigarette boat, $100 is nothing. Depending on where you are and who you're asking, "harsh ocean conditions" could be anywhere from 5 to 25 foot waves! Or higher. I suspect even 5 foot waves will seem pretty rough at 25 knots in a 12 meter long boat. Unless it's some kind of hydrofoil.

If you really want to ride around in a 12 meter boat you designed, at 25 knots, in rough conditions, you will need to develop some expertise. It won't happen overnight. 25 knots is probably just fast enough to die if your boat breaks the wrong way. I don't say this as a naval architect, which I'm not, but I've used some motorboats in the past, and I have some mechanical engineering background. Design and build some other boats first. Carefully study successful boats that are intended for similar tasks. Learn how to actually engineer such a craft, calculations and all.

At some point you will have to start strengthening your knees and your back, as you're likely to experience quite a bit of pounding. The kind you don't want to be sitting down for. (Unless your seat has shock absorbers or something.)

If this is just a design exercise for educational purposes, that's much better! Do you have a good library near you? Also, used copies of David Gerr's books seem to be relatively inexpensive on Amazon. Perhaps those other books are too.

 

P.S. You may find this article of interest:
http://www.proboat.com/2015/12/patrol-boat-in-a-box/
Note that this boat is considerably slower and is intended for "riverine and inshore" use, not rough ocean conditions.

 

rnlock : Thank you! The article is very cool. I stated rough or harsh conditions because we see the Arabian sea and the Indian ocean behave strangely pretty often. The waters in Goa are fairly calm but during the 'Monsoon' season, things change dramatically.

 

Agreed, if you aren't willing to learn what you need to learn, in regard to design and engineering principles, then I don't know what to tell you, other than good luck. My personal library includes many dozens of books and yes, some were costly, but a necessity, given the requirement. A $100 book is a pretty small price, given the total cost of your proposed project, don't you think?

 

Why mistake my inability to make a quick decision on a book for an unwillingness to learn.
I guess i'll just have to go out and get it.

 

No one is forcing you to do this, though you did ask.

The education you need, for engineering a structure such as this is substantial and you'll need more than a single book.

 

PAR : Take it easy will you. Yes, I will require an education for this task and a single book or a library full will not substitute for it.
I am trying to plan a way forward. As much DIY is possible by me, I will go ahead and do it. Spoke to a few colleagues at my office and they will do the whole CFD and stability using Hypermesh in their free time.

 

sudeb.roy : if you need it, I can do all the naval architecture calculations you want. It is not just to make calculations but know how to interpret results and whether the boat meets the criteria of security and stability to be met. And if it does not, know what to do to get it (or if the ship can not comply in any way) .And that, my friend, is the most complicated in this whole thing.
If you give me a GA and some other indication, I can start work immediately. This, however, does not mean that I assume design responsibility.

 

I downloaded a few GAs and am adapting them to dimensions I want. I understand that this process of learning will be pretty complex and long but I'm game. I surely won't bother everyone here with all the basics henceforth but will ask a few questions here and there.
Did my answers for a SOR make sense or should I be adding more details or elaborating a bit more?

 

While that chart shows general trends, I think it contains many assumptions. For instance, an unshrouded air propeller of sufficient size should be just as efficient as a water propeller, just completely impractical in most cases. It might even be more efficient if the shaft of the water propeller isn't parallel to the direction of travel. So I imagine a diameter constraint was used for the air prop. Furthermore, the speeds given are arbitrary unless we know something about the vessel assumed in the calculations, or at least the power and rpm supplied to the prop. For low levels of power and large diameter propellers, high efficiency can be reached at much lower speeds.
For instance, this boat isn't exactly "practical", but it met it's design goal with an air prop, at 18 knots:
http://lancet.mit.edu/decavitator/
I seem to recall that the air prop was adopted because the "lower unit" for a water drive would have had too much drag. It may not be coincidental that the guys who did it were aeronautical engineers. I had a chance to see Decavitator in action. It was awesome.

Another factor, for water props, would be how deep in the water they are. Submarines at depth have much less trouble with cavitation than ships do, because the extra water pressure keeps the bubbles from forming.

You are correct that "awesome" and "gun it" are "Western" terms, common in the USA at least. Both are colloquial. Awesome used to be appropriate for a volcano or a tsunami or maybe just the Matterhorn viewed close up, i.e. tremendously impressive and maybe scary. These days it just means that you really like it. Gun it usually means to open the throttle quickly, and is a bit older than the new meaning of awesome.

 

sudeb.roy:

Someone else may know better than I do, but I seem to recall a book named Principles of Naval Architecture that was a general introduction, available from SNAME. Maybe it's just as expensive as that other book by now. Also, as I recall, it was kind of a survey book that covered a lot of different topics.

You might want to check out MIT's Open Courseware. For instance:
http://ocw.mit.edu/courses/mechanical-engineering/2-019-design-of-ocean-systems-spring-2011/

Also, if your local library isn't up to it, maybe the Navy has a library that does? Or maybe your father knows some people?

 

Yes the Navy has more than a few books about it and the Naval Architecture institute is located near by as well. I will try accessing material from there.
Thank you for your inputs.