# Estimating Power

Discussion in 'Boat Design' started by jmercer, Oct 21, 2009.

1. Joined: Oct 2009
Posts: 6
Likes: 1, Points: 0, Legacy Rep: 26

### jmercerJunior Member

I am currently trying to estimate power for a project vessel. It is a 65 meter displacement motor yacht. So far I have estimate Admirality Coefficients from SOTA data and browsed through Harvald diagrams, but am still looking for an alternate method of estimating power(shaft). Anyone have suggestions?

2. Joined: May 2004
Posts: 5,372
Likes: 246, Points: 73, Legacy Rep: 3380
Location: Italy (Garda Lake) and Croatia (Istria)

### daiquiriEngineering and Design

Can you post a drawing of the hull lines, together with some more data - main dimensions, estimated displacement, propulsion type (for SHP), etc.?

3. Joined: Oct 2009
Posts: 6
Likes: 1, Points: 0, Legacy Rep: 26

### jmercerJunior Member

the whole design is still very preliminary. what i can get you is:
LWL = 57m
BWL = 11.2m
draught = 3.2m
estimated dis. = 1100 tonnes
Cb = 0.48
Twin Screw

4. Joined: Aug 2002
Posts: 14,150
Likes: 540, Points: 123, Legacy Rep: 2031
Location: Milwaukee, WI

### gonzoSenior Member

What is the speed target? Is that loaded displacemnt?

5. ### Guest625101138Previous Member

The question is akin to how long is a piece of string. Also your parameters are inconsistent. The block coefficient for saltwater operation is 0.52 for the dimensions given.

If you consider a 1100T displacement vessel with WL of 57m then attached curve will give you an idea of the power to push the hull at various speeds. Depending on how well it is propped, appendages and windage the shaft power could be around double that shown for any required speed.

Take this as indicative. If you have an actual hull shape then the analysis should be done on that not some hypothetical hull.

Rick W

#### Attached Files:

• ###### 1100T_Vessel.pdf
File size:
27.7 KB
Views:
208
6. Joined: Oct 2008
Posts: 6,390
Likes: 509, Points: 113, Legacy Rep: 2488
Location: Japan

jmercer

Your target speed is what is required.
Since your length displacement ratio, is not overly excessive, roughly 5.5, hence the hull is not that "draggy".

However are you trying to estimate the EHP naked, or the total power requirements, taking into account losses (mechanical and hydrodynamic) etc to arrive at a total installed power required to go XX speed?

7. Joined: Feb 2002
Posts: 511
Likes: 27, Points: 28, Legacy Rep: 394
Location: Denmark

### sorenfdkYacht Designer

Why don't You ask the designer - he should know! And if he doesn't, then I wouldn't trust him to design a ship like this!

8. Joined: Oct 2009
Posts: 6
Likes: 1, Points: 0, Legacy Rep: 26

### jmercerJunior Member

hmm, I think I may have been too brief with my request. I'm not looking for numbers. This hull is hypothetical. I'm looking methods of finding a preliminary power estimate, not the actual estimates.

9. Joined: Oct 2008
Posts: 6,390
Likes: 509, Points: 113, Legacy Rep: 2488
Location: Japan

In that case, apart from the methods you have already tried, read the excellent and now classic paper "Some ship design methods" by Watson and Gilfillan, published in RINA 1976. Has all you'll need.

10. ### Guest625101138Previous Member

Assuming you intend to operate it as a displacement vessel then you need to decide on a design speed. From that point you have all the data necessary to use GODZILLA to produce the lowest drag hull shape for the envelope you have settled on.

There are quite a few references to GODZILLA on the forum.

Rick W

11. Joined: Oct 2008
Posts: 6,390
Likes: 509, Points: 113, Legacy Rep: 2488
Location: Japan

You're missing the point again Rick, as always.

He is not looking for numbers, which you like. He is looking for ..methods of finding a preliminary power estimate, not the actual estimates...
This means words and equations and theory and understanding where the theory comes from and why. Numbers are simply an output and irrelevant.

Plugging "whatever" into a computer program does not answer his Q. If it were that simple, he would just select 4 or 5 software programs that perform resistance calculations! That isn't rocket science.....anyone can do that.

It would do you good too, to put down your endless computer programs and try and do the same as jmercer.

12. Joined: Oct 2009
Posts: 6
Likes: 1, Points: 0, Legacy Rep: 26

### jmercerJunior Member

Thank you for all the input. I ended up reading back through a resistance and propulsion book from Harvald and came up with a few methods. Im still trying to get my hands on that RINA paper, the library at my institution is looking into it.

Thanks again

1 person likes this.
13. Joined: Oct 2008
Posts: 6,390
Likes: 509, Points: 113, Legacy Rep: 2488
Location: Japan

jmercer
...if you can't get the RINA paper just PM with your email. I have it electronically, but it is some 12MB, so cant email via this website.

14. ### dskiraPrevious Member

You are absolutly correct about everything as to bigger in the US, the D/L here will be 165
Cheers
Daniel

15. Joined: Oct 2009
Posts: 920
Likes: 46, Points: 0, Legacy Rep: 732
Location: NW Washington State USA

### Easy RiderSenior Member

For a yacht I have always used power/displacement. Compare a boat/hull with numerous known vessels (more the better) and soon you will observe what other people think is correct power loading. Consider the differences in hulls, speeds and applications. Some yachts operate at 2hp per ton .. what I consider a minimum for boats that aren't extreme in some way. I think 3.5 to 4.5 hp per ton is very close to perfect but you will find many (perhaps even a majority) that are well above that. Frequently manufactures over power their boats for marketing reasons. It's hard to tell people that are accustomed to 200hp in an automobile that 70hp is enough to power their 16 ton boat. When comparing avoid commercial boats that do not have a fixed load and sailboats.

Easy Rider

Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.