hull speed power calc

[RANCHI OTTO]

The VP engine has 500 hp x 2 = 1000 hp

For me you need 2 x 600 hp at 80% of max. power i.e. 600/0.8=750 hp each.
To be installed 2 x 750 hp

Stabilizer ? better fixed bilge keel and a deeper central skeg to protect the propellers and to avoid lateral wind effect..

[nautical]

Ranchi you are killing me, I guess here is the guy. I was going to put only one of these at first . geuss I was naive,

Is there emprical or rule if thumb to put a Skeg. yeah stabilizer to prevent rolling

[RANCHI OTTO]

Fine!

Have a check to this for Dopt....but it would be better if you have the resistance, to use the Wageningen B Series...

The diagr. rpm at prop. vs Dopt(m) for various powers (hp)

[nautical]

kinda hard to read off the diagram. Wageningen b I will try that I guess. Thanks a lot

[RANCHI OTTO]

It is better?

[RANCHI OTTO]

Don't install active stabilizers, they will add resistance...put fixed bilge keel!

[nautical]

thats too bad. with such low Rpms the engine is giving no out put IT seems I have to Optimize for both Rpm and Diameter at the same time.

[nautical]

Do think it will ad a norticable about of resistance. I am thinking of 9 sqr feet fins at midship section.

[RANCHI OTTO]

Have a look on gearbox available reduction ratios and you will find for each propeller rpm the corresponding Dopt.

All appendages will increase the bare hull resistance...rudders, strut palms, brackets, shafts, fins....and take care with the high superstructure frontal area.

To evaluate appendages resistance:

Rapp (% of bare hull resist.) = 1/(0.005*Fvol^2+1.05)

To evaluate air resistance:

Raa (kg) = 0.5*0.125*(Vk*0.51445+2.36)^2*Aw*Caa
where Aw (frontal area m^2) and Caa =0.5

[nautical]

Thanks you are really helping me out. again i think i will need where the formuleas come from. I have got a some fomuleas too but dont have urs

Raa=Caa*0.5*ro*Aw*Vk^2
there are some complicated ones too

there is one for Skegs
Dsk=0.5Cf*Vm^2*Ask
Ask = area of skeg
Vm = average velocity at skeg
Cf = frictional coeficient depending on the lenght of skeg

[RANCHI OTTO]

in my formula for Raa....
ro=0.125
Vk is on meter/sec2 not knots
2.36 is the wind speed meter/sec^2 for Beaufort scale wind 2 and to be added to vessel's speed
Caa is an aerodynamic coeff. of penetration like for car....I've taken 0.5 (medium figure)

For skeg I usually add his wetted surface to the vessel wetted surface.

[Olav]

Ranchi Otto,

I regularly read your great postings with a lot of interest, thanks a lot for sharing your experience and graphs. Unfortunately we don't do those things at uni, so I have to educade myself by reading here at boatdesign.net.

But am I right that rho should be 1025kg/m³ (not 0.125) and Vk and wind velocity in m/s, not m/s²? Or do I get something completely wrong here?

[nautical]

Yea olav ro should be either 1.025 ton/m3 or 1025 kg/m3. And i think Rachi is talking about ^2 outside the brackets. But in his formulea Vk is in knots I guess because its multiplied by 0.51445 to make it m/s.

[RANCHI OTTO]

Danke schoen Olav,

I do of my best!