hull speed power calc

hi guys,

If i use an overall efficiency of 65% in hull speed while culculating power. Doesnt it mean that it is the rewuired load to deliver the necessary power to the propeller and will this curve be used for engine selection.

also is 65% efficiency too high for a 30m yatch with twin engines with a speed of 14 knots.

Thanks

 

I'm not sure what your "65% efficiency" is implying, but a 30 meter yacht, traveling at 14 knots is carrying a speed length ratio of about 1.5 which is just above it's theoretical hull speed limitations.

 

I assume you are referring to the propeller efficiency %age used in a number of the performance prediction methods. Most however are for planing hulls, and as PAR quite rightly points out, your vessel is operating only just above theoretical hull speed.
The value you use for efficiency is also greatly dependent on the type of drive system. For a vessel that size, and operating at those speeds, one would assume you are talikng about conventional shafts / props. In which case an efficiency of closer to 50 -55% is more likely. But, as I said, you wouldn't normally use methods like Savitsky for this speed range....

 

Thanks for your replies

I didnt use Savitsky, though I used the the preplaning methoth. And then i used holtrop, comptom, slender body, and for a comparison again lahtiharju and they all gave failry the same results. The propulsion will be with two small engines with convetonal shafts and propeller.

In analysis there is a value to enter overall efficieny. I took the overol efficiency as 65%

 

OPC = EHPbh/DHP

EHPbh = Rbh*Vk*0.51445/75
DHP = BHP * ETAM

OPC for such vessel can be estimated 0.575

 

thanks. Ranchi from the chart it seems eta 0 is .75

say
eta(h) is .97
eta(r) is 1
eta(o) from graph is .75
eta(s) is say .96

the overall efficiency comes to something like 69 %

 

ETAH = 0.92/0.97 = 0.948
ETAR = 0.970
ETA0 = 0.660

ETAD = 0.607

The figures in the graph are max. values... an ETA0 = 0.660 seams to me more realistic.


The propulsive coefficients are derived from tests in model basin (propulsion and resistance with and without appendages).

 

ok Ranchi, Thanks again

 

2 diagrams, one about OPC for submerged propellers, OPC = 0.575 is a good starting figure.

The second is based on sea trials results, KR coefficient vs speed.

where KR = (BHP)^0.551/(displ)^0.476
BHP metric hp
Displ. metric tons

 

Ranchi you are beign very helpful. where is the data from. Since this is an acedemic project I should mention where these source data is actually from. Thanks again a lot.

 

Whats a KR coefficient ? ???

 

OPC from
SSPA "HIGH LIGHTS" #2 1989 (Swedish Model Basin)

KR (Ranchi Koeffizient) is derived from an empirical Clement formula.
I've collected the KR derived from sea trial results and so doing it is possible to check in a few seconds an approximated speed for various propulsion systems. (I've the same for waterjet or surface propellers)

If you need some other inputs........

 

wow cool .. can you check my other post in Deisel Engines. I need a propeller to get a suitable propeller and rudder

 

In order to save time inform me about your questions in this thread...

 

Hi guys. I did a power analysis in HullSpeed for a 30 m cruise at 14 knots. and the power required seems to be like 600 KW (about 1000hp) considering an overall propulsive efficiency of 50 %

and i am choosing 2 of these engines

http://www.volvo.com/volvopenta/sing...9-500/spec.htm

Now I checked one of the propcalc data sheets from the forum. And it says i will require 2400 Hp to achive an speed of 14 knots. Isnt this way too much power.

Can someone pls help me with this and also guide me to estimate a suitable propeller. If some have pdfs or anything that could be of help pls send me

Thanks to all your help