57 knots? 9 tonnes 2x820hp Seatek, chines&strakes dimension?

[AntonioCasas]

hi

I´m new in this forum, and I must say english is not my mother tongue, so my appologies for my spelling mistakes.

I´m involved in the design of a cabin cruise power boat. 15.5m LOA, 3.1 m chine beam, 21 degree deadrise angle at stern, about 9000 kg full load displacement, 2x820 hp Seatek engines , LCG full at 5.3 m from stern, estimated speed 56 to 57 knots at 2.4 to 2.3 trim angle, two stepped hull, and custom surface drives propulsion.
I do not have any Idea of how to dimension, shape and distribute Chines, and lifting strakes or spray rails. I read some where that chines+strikes should be around 20% of the beam wl. Also that chines width should be about 3 to 5% beam wl. Down angle for chines not higher than 5 degree, and down angle for strikes not higher than 15 degree. With the information I submited about the boat, can any one give me some more clues about dimensions, shape and disposal for chines and strakes?
Thank´s in advanced,

Antonio Casas

[Toplana]

Do you have some existing project for guidelines, maybe lines plane? How do you know height of the steps? I can tell you some details from Fabio Buzzi 42`one step hull. There are very little informations about chine width, spray rails position etc... I am in similar problem like you. I am working on 16m long 5m width and 15 tones displ. with 18 deg. deadrise. RRKaMeWa jets+MAN engines. I decide for jet propulsion because we make luxury boats. I also have problem with spray rails position and chine width. What are the engines and SD for you boat? Please check the LCG position, my opinion is that it is too forward, but i do not have to much experience with stepped hulls. Also have in mind L/B ratio which in your case is about 5. I supposed power to weight ratio is about 140hp/tones regarding top speed of 57 kn.

[AntonioCasas]

Hi Toplana,
The maximum beam is at the stern with 3.1 m, and smothly narrowes to the bow. You know, like a bullet shape. With this and the deadrise of 21 degree, you can figure the shape out.
The height of the steps I found it through a formula given in a book (the autor is Faltisen), that descrives the trajectory of a fluid when deataches a surface. The first step should be slightly forward of the CG, and the more steps you put, the shorter steps can be, and the cruise speed can be slower.
The engines will be 2x820 hp seatek which gives the best kg/hp ratio in the market for diesel. The SD is a inox, hydraulic vertical trimable transom made by my father, and I can tell you its much lighter, much stronger, much cheaper and much reliable than any other in the market, like Arneson. And its been proved for 15 years and thousands of miles.
The CG is at 3.68m from the "active stern", and 5.3m from the very end of the boat.
L/B ratio about 5 is good or bad?
the power to weight ratio in this case is 178hp/tonn , for this speed. Maybe I´´m conservative, in guessin the speed, but its a cavin cruiser, so aerodynamics are important aswell. I also believe that real speed in other boats in the market is about 15% less of what the boatbuilders claim, don´´t you think?
What makes you chose jets for your boat, why is it better for a luxury boat?

Salut,

Antonio

[Toplana]

First of all, jet drive has much more maneuverability at low speeds which is very important regarding our mahogany boats. With RRKaMeWa`s vector system you have fully electronic control with one single joystick. It is very easy for docking in marina. Also it is much more quiet and have less vibration. But we are custom made boat builders so if someone of our customers want SD he will get it, probably MTU+ France helices surface drive. Now i am clear with LCG because i always look from the active stern. You probably know but just to remind you to watch out on ventilation of the steps. I am not sure about L/B but for 57 knots you must watch at air resistance too as you said. Also I think you should maybe go to 23-24 deadrise (take a look Baja boats). When we talk about weight to power ratio have you take a look at Isotta Franchini engines?

[AntonioCasas]

Hi Toplana,

Looks reasonable the use of jets in this case, although they have losses between 20 to 30 % in thrust copared to a surface drive.
About ventilation inlet, its twice the height of step and 4 to 5 times in lenght the height of the step.
Regarding Issota vs. Seatek, the last one we believe is better... but you are right, IF is similar to Seatek in performance.
For the 23 to 24 deadrise, I want to risk with 21 to get some extra speed.

thank´s

Antonio Casas

[AntonioCasas]

By the way,

what do you have from Buzzi 42´?

[Nojjan]

It sounds like an optimistic speed estimation, however I don't know much about step hulls. How many % better than a normal hull can it get? Should not LCG be further back for such high speed?

Regards / N

[AntonioCasas]

Hi Nojjan,

to tell you the truth I don´t know the % improvement in speed I can get from a stepped hull, but I´m sure I´ll get some at this speeds... I hope!
The CG is 3.7 m forward from the "active stern" (I don´t know if this is the right way to define it).
For the speed estimation i used the Hydrospeed program from Hydromax. so far is the best I can do for calculation, which is based in Savitski formula for planning hull (no steps), and I guessed a 65% of performance for a supercavitating propeller + a frontal area of 9 m^2 and Cd of 0.9 for the aerodynamics resistance.

best regards

AC

[charmc]

Originally Posted by AntonioCasas

and Cd of 0.9 for the aerodynamics resistance.
best regards AC

Just a thought, but you should be able to reduce Cd. 0.9 sounds pretty high. A flat steel plate held upright has a Cd of 1.17, and a skyscraper building 1.2. Boxy autos are around 0.4. Reducing Cd should give you some speed improvement, although hull design is most critical, obviously. I agree that LCG should perhaps be more like 4 m from transom.

[AntonioCasas]

Hi Charmc,

yes, you are right, the Cd should be less than 0.9, but I allways like to be conservative, because I don´t like ugly surprisses.
About LCG, is not enough 3.7 m forward from active stern?
What about chine and strakes dimensions and distribution, any suggestion?
Thank´s

AC

[AntonioCasas]

I forgot to say:

what about propeller, do you think 65% in performance is a good guess?

[KCook]

Find a Fountain hull and study that ...

[Nojjan]

For a good surface prop system it is not unlikely to get 65% efficiency. A 3.7m LCG sounds alot better than 5.3m. What about chine beam, 3.1 m is really not that narrow. Compare with the super performance boats in the market, I think they are narrower. As far as the strakes I suggest testing. Fixing on a position for this type of boat will prove to be, if not wrong but at least less than optimal. Really high speed boats only become high speed because they are tested and modified extensively. A good starting point is to sneek a peek at other speed boats, two strakes evenly distributed transversely.

Good luck and have fun / N

[AntonioCasas]

I must admit that I allready "sneeked a peek" at Fountain boats, which in my opinion, are between the best, together with Michael Peters and Fabbio buzzi designs.
If you guys have some numbers for chines and strakes, or some "magic" rules that could guide me somehow... since I won´t be able to test.
3.1 m chine beam is the minimum I can afford becasue we want to alocate the fuel tank between the Seatek engines + its not a race boat, the design is very similar to the P56 Fast boat Patrol. you can find it at
http://www.victoryboats.com/?dom=2&r...lId=8&gapId=93

Anyhow, thank´s KCook and thank´s Nojjan

[charmc]

Originally Posted by AntonioCasas

Hi Charmc,

yes, you are right, the Cd should be less than 0.9, but I allways like to be conservative, because I don´t like ugly surprisses.
About LCG, is not enough 3.7 m forward from active stern?
What about chine and strakes dimensions and distribution, any suggestion?
Thank´s

AC

AC,

Earlier you said, "LCG full at 5.3 m from stern"; that's the LCG point to which I responded. 3.7 m from transom is even better. I'm with Nojjan and KCook on design of strakes and steps. Copy what's proven stable and fast if you're not budgeted for testing. Visits to the right marina, boat show or dealer showroom should yield good data. Cut weight wherever possible. If you use the forum search function on the toolbar above, you will find some good surface drive discussions, as well as links to mfrs' sites with good information on drive and prop data for high performance boats. That should give you a good basic education for your project.