high speed boat

[amamini]

Dear friends,
For designing a high speed boat (Loa=12 m , B=3.5 m, D=15 m (main deck), V=25-30 Knot), I have a problem to choice a good lines drawings. In fact, with my model, our calculation (by Maxsurf software) present that, we need 900 hp for 30 Knot. With space limitation in my model, it is very difficult to install a good diesel engine.

Can you help me to solve this problem?

Thank you.

[Ad Hoc]

Make the boat bigger if you cannot fit the engine in!

[amamini]

Thank you so much. But according to owner requirement the boat's dimension was fixed and we can not very play by this constrain. I think a good way to change the model, but how? Is there suitable model?

Regards.

[Ad Hoc]

Well, welcome to the world of "design"...it is called compromise and creativity.

You need to do a weight estimate firsts, then you'll know if you can really do anything. Since the final weight will dictate what performance you are really able to achieve, regardless what the hull is actually like. Hull shape doesn't really make as much of a difference as people tend to make out. It is simply horsepower per tonne on such small boats...

[amamini]

Thank you so much Mr. Ad Hoc, good idea!
I will try to assess the "horsepower per tonne", may be we can change the weight!

Regards

[Ad Hoc]

For a boat of your size, just guessing here (not knowing a great deal about what your SOR is), you'll need around 80 horse power per tonne as a minimum. 100 hp/tonne would be better...but as i said before, you'll need to do a weight estimate first to establish IF, the boat floats and you can actually use engine with the given power and weight to achieve your gaol. If not..back to the drawing board...

[Grant Nelson]

I have little experience with launched designs, but a quick hull design with ORCA3D for your boat, with around a 11 waterline, at 5.5tons, and 50% propulsion efficiency says, using savitsky analysis, that you need 300hp for 25 knots and 375 for 30 knots. This is a 'V' hull with 12 degree deadrise and 1.5 degree rocker and a .15m chine which was not included in the calculations.

Thats less than half what MaxSurf gave you... I wonder why...

[Ad Hoc]

That's "why" you should never trust a bit of software...it has no frame of reference other than its own algorithms!

Originally Posted by amamini

Thank you so much. But according to owner requirement the boat's dimension was fixed and we can not very play by this constrain. I think a good way to change the model, but how? Is there suitable model?
Regards.

Which engine model did you play with? If any

[Grant Nelson]

Hi Ad Hoc, that was a pretty ad hoc reply.... its not the software manufacturers algorithm, its Savitsky method applied using the package, and while not perfect but its pretty good, so there must be more to the discrepancy.

I guess I was wondering what the design we are talking about looks like, and its displacement.

I think I was also wondering about the generality that a boat of this size would need 80 to 100 hp per ton...

I also applied Clement, Couch and an un-named set of forumulas and came out with between 31 and 34.75 knots for 400hp. So it looks to me as if the 900 it about double what is needed.

etc.

[Ad Hoc]

Grant
If it is using a Savitsky method, either it is using it correctly or it is not. If it is not then of course that has its own problems. If it is, then either the assumptions are beyond the parameters or the algorithms are coupled with the parameters are incorrect, and probably does not even prompt the user that such has occurred.

I never use software for my speed predictions.

The rough guide i mentioned above (as i also noted difficult to gauge with such limited data given) is based upon around 30 years of real hard data of vessel, not a program. I have a graph, given to me by my old boss (chief designer) nearly 20 years ago, who is now my business partner. It is a graph of various different hull forms of a simple hp/ton versus Taylor quotient.

After every sea trial, a new spot was added to the graph...after many decades of real actual boats, surprise surprise a smooth curve can be drawn through them. I have this for various types of hulls. Hard chine round bilge etc etc

Hence looking at my curves, and not knowing too much about the vessel, 80 hp/ton seems reasonable.

If you wish to place your trust in a computer program, then that is your prerogative. In naval architecture, i tend to trust hard reliable data.

[mydauphin]

This discussion is foolish, you never said what the boat is for. A race boat, sport cruiser, open fisherman, flats, river, bay, ocean. We know nothing about boat.

[Grant Nelson]

Ad Hoc, you are right - in the end its the hard data that counts.. of course since you have a nice curve, you could turn it into an algorithm, and put it into a program and that 'computer program' would generate the exact information you get from your hard data sheet of paper... but of course the sheet of paper will not crash, boots instantly, has no maintenance fee, etc.

btw, my results where for a 5.75 ton boat, and that times 80 = 460 hp, which is not too much more than I got

And indeed, we digress, as we have no idea what this hull looks like. It was fun to discuss however...

Cheeers

G.

[Ad Hoc]

Grant
Agreed, my paper never crashes, neither do all the tank test reports i have for many different hulls, just open up the report and hey presto, answer!

It is rather amazing the number of people who post on this site saying....i need help with my boat what is the solution, and provide no details, sigh!

[Guest62110524]

take look at ALIKS design gallery
You most important charecteristic, is the entry, it needs a very deep vee for high speed in choppy to moderate seas
The degree of deadrise aft can be as little as seven, in fact look at designs of DON SENIOR on the web, all his are very deep entry , shallow deadrise aft for ease of getting up on plane and stability down weather A boat should be able to go from wave top to wave top at high speed, landing softly, and thats what most do not achieve, because the entry is more spoon shaped
i tell you this from my experiences of building thsi type of boat The shape you want is also in my gallery, but those are smaller,