Power efficiency

Hello friends
I have modelled a landing craft hull in maxsurf and i did resistance analysis in maxsurf resistance by considering 60% efficiency to engine selection.
Is it a good way to estimate engine power?

 

For an outboard, 60% seems about right to me. A waterjet could be somewhat more efficient and a traditional geared axle line somewhat less.

 

It depends what type of landing craft you have designed.
Can you show the lines/GA?

Also the efficiency of 60% you cite, is that for the whole vessel, i.e the PC, or just the 'estimate' at the prop?

If your LC is a traditional heavy/slow speed, then you'll only get around 40% PC if you are lucky...but if it is a more 'higher speed' type, which has more of a Vee hull, it will depends whether you have inboard or outboards and whether props or jets, and hence the water flow into the propulsion unit.

Thus, more data is needed to really provide a better answer.

 

In Maxsurf - what method you used for resistance calc? Start from here...
Maxsurf, in my opinion, is a toy for dummies. Easiness of calcs makes them think they are getting correct numbers. But this is not always true.

 

I used KR BARGE method.
Please introduce another method that you believe is better.

 

Size of landing craft
80x16x4.5m
I used KR BARGE method .
Engine power=RxV/total Eff.
I mean above total efficiency.
Considering 60% for total efficiency is ok?

 

It depends of course on your hull shapes and size but a normal LC can be closer to a planning boat, or to a normal hull, than a barge (see waterplane shape). Given the size of your boat I would use Holtrop.
As far as I know, Maxsurf does not have its own procedure for calculating power-speed. Therefore, the results obtained with this software are as good or bad as those obtained with any other software since the methods used by all of them are always the same (Leaving CFD methods aside, of course).

. You should use 45% at most.

 

The design speed and loading are critical factors for determining both the transmission efficiency and the possible propeller efficiency. In addition, the hull shape has a major impact on the propeller (or jet) inflow.

 

L 80
B 16
H 4.5
Draft 3.2m
Speed 10kn

 

I agree with 45% .

 

That is a low Fn.
However, the key element is the flow into the prop....if you don't have this, the efficiency will be very low.....as also noted by Baeckmo

 

I have looked at our data from previous LC's of this size, and we got PCs around 0.30

 

You mean 30% efficiency?

 

The PC... yes

 

What is your idea about using kort nozzle in this landing craft?