# Finding trim and mean wetted length-beam ratio using Savisky's method

Discussion in 'Boat Design' started by Brynjaminjones, Mar 18, 2014.

1. Joined: Mar 2014
Posts: 10
Likes: 0, Points: 0, Legacy Rep: 10
Location: Cornwall/Plymouth, England

### BrynjaminjonesJunior Member

Hi All,
I'm new to the forum, so please be gentle!
I've tried searching, but couldn't find the answer to my question.

I'm trying to find the power requirement for a planing hull using Savitsky's method, but I can't for the life of me figure out how to calculate the trim and mean wetted length-beam ratio (lambda).

All of the equations I can find for these values seem to include both of them, so without one the other cannot be solved!

I have a feeling that I'm being stupid - could someone please point me in the right direction on how to find these!?

Cheers,
Bryn

2. Joined: Dec 2003
Posts: 334
Likes: 50, Points: 38, Legacy Rep: 460
Location: Filia pulchra Lubecæ

### Olavnaval architect

Bryn,

the Savitsky resistance estimation method calls for an iterative approach, i.e. you guess an angle of incidence (=trim angle), calculate all the equations and get a trim moment, which will most likely be ≠ 0. A reasonable starting value is about 4 degrees.

After the first iteration you change the assumed trim angle, do it all again and the result will be (hopefully) a residuary moment closer to 0. Repeat until this moment is 0 or very close to 0. Then you've found the running trim, resistance, wetted keel length etc. for that speed and load condition.

It's best to do this with a small computer program to save the large amount of donkey work associated with all the calculations over and over again. There's a very good excel spreadsheet available on this forum written by a gentleman called Dingo.

Hope this helps.

3. Joined: Mar 2014
Posts: 10
Likes: 0, Points: 0, Legacy Rep: 10
Location: Cornwall/Plymouth, England

### BrynjaminjonesJunior Member

I had wondered if that was what I had to do - at least now I know where to start!

I'm already using Excel, as this is for university and our spreadsheet is going to be marked. Unfortunately that does mean that I can't use Dingo's spreadsheet!

Thanks again,
Bryn

4. Joined: Mar 2014
Posts: 10
Likes: 0, Points: 0, Legacy Rep: 10
Location: Cornwall/Plymouth, England

### BrynjaminjonesJunior Member

Okay, my apologies for asking another (probably daft) question, but what equation/combination of equations am I supposed to use iteratively with my estimated trim angles?

All of the equations I can find with relation to the trim moment include the drag force, but I can't find a way to calculate that without lambda!

5. Joined: Dec 2003
Posts: 334
Likes: 50, Points: 38, Legacy Rep: 460
Location: Filia pulchra Lubecæ

### Olavnaval architect

λ also has to be found iteratively. Have a look at the two formulae for the lift coefficient cl0. You have to adjust λ until you find a cl0 that matches cl0 from cl0 = (m*g) / (0.5*ρ*v²*b²).

Do you have Savitsky's original paper from 1964? Very well worth a read! Find a copy attached to this posting.

#### Attached Files:

• ###### Hydrodynamic Design of Planing Hulls - Savitsky 1964.pdf
File size:
6.6 MB
Views:
3,131
1 person likes this.
6. Joined: Mar 2014
Posts: 10
Likes: 0, Points: 0, Legacy Rep: 10
Location: Cornwall/Plymouth, England

### BrynjaminjonesJunior Member

Thanks, I really appreciate the help. One thing though - you say that's the formula for cl0, but in the paper it says that formula applies for cl0 and cl beta.

I've gotten myself a bit confused now!

7. Joined: Jan 2005
Posts: 2,656
Likes: 535, Points: 113, Legacy Rep: 1110
Location: Philippines

### rxcompositeSenior Member

There is a program called Arneson for Arneson surface drive. It should be here in the forum or SNAME or somewhere else that can be downloaded.

Although it is designed for surface drive, it is also Savitsky based. You just have to guess which drive model you have to use to find the correct drag. It is very simple to use and allows simultaneous evaluation/comparison of two parameters. Once you get the hang of it, you should be in the ballpark.

There are two Savitsky method of analysis (excel based) available here in this forum as mentioned by Olav. More inputs required and you need to have a firm understanding of the terms. More inputs, more accuracy. It is iterative and gives you the correct trim for the speed desired. Great program.

Note that in the two programs, power is given in EHP. You have to use a powering program to find the SHP or the BHP.

8. Joined: Mar 2014
Posts: 10
Likes: 0, Points: 0, Legacy Rep: 10
Location: Cornwall/Plymouth, England

### BrynjaminjonesJunior Member

Thanks for that, you guys are very helpful!

I've managed to get my spreadsheet working now, and have used it to calculate my tau and lambda values. I've compared them with the results from this calculator http://illustrations.marin.ntnu.no/hydrodynamics/resistance/planing/index.html
and they're pretty identical.

Where I'm struggling now is with calculating Df, and therefore the power requirement.

I've used Df=(ρ*(Vm^2)*λ*(B^2)*CF_total)/(2*COS(β)), and calculated D using (Δ*TAN(τ)+(Df/COS(τ))), but my resulting power values start in the right area at 30 knots, but get much higher than those on the linked website quite quickly as I approach 40 knots.

9. Joined: Dec 2003
Posts: 334
Likes: 50, Points: 38, Legacy Rep: 460
Location: Filia pulchra Lubecæ

### Olavnaval architect

Bryn,

just a guess: There are basically two Savitsky methods around; the so-called "short form", where it is assumed that all forces act through the centre of gravity of the vessel, and the "long form", which gives a more accurate idea on where the centres of effort of the acting forces actually are.

The software by Arneson Marine (mentioned by rxcomposite) uses the simplified "short form"; maybe this is also the case with the website you brought up, which may be the cause for the different results.

10. Joined: Jan 2005
Posts: 2,656
Likes: 535, Points: 113, Legacy Rep: 1110
Location: Philippines

### rxcompositeSenior Member

Olav,

I have doubt it is a short form since it is an arneson, a variable thrust line. That means the thrust line will vary depending on what the program says % propeller immersion. So maybe it is not "short form".

Not too sure however since the code is hidden and I think it is a DOS based program.

11. Joined: Dec 2003
Posts: 334
Likes: 50, Points: 38, Legacy Rep: 460
Location: Filia pulchra Lubecæ

### Olavnaval architect

rxcomposite,

somewhere in the depth of my harddisk I have a copy of said software, as well as the corresponding manual. I'm quite positive that the manual states that it's the short form.

I'll have a look tonight, but I'm sure it is the simplified method. ;-)

And yes, it's a DOS software, written in 1988 or 1989 by Paul Kamen (I think). It's a bit of a hassle to get it running with a 64 bit OS, but still a nice program.

12. Joined: Jan 2005
Posts: 2,656
Likes: 535, Points: 113, Legacy Rep: 1110
Location: Philippines

### rxcompositeSenior Member

Olav. It is a very old but useful program. The first time I got it was on a floppy disc and we were using IBM 486 computer then. I still have it on my hard disc and it works on Windows environment except that it is not in full view, just half screen.

Will take a look at my manual. I know I have a hard copy somewhere.

Got it. It is a modified "short form" version 1.1

13. Joined: Dec 2003
Posts: 334
Likes: 50, Points: 38, Legacy Rep: 460
Location: Filia pulchra Lubecæ

### Olavnaval architect

Yes, definitively! That's why I still have a copy on my computer.

Yeah, I just found it as well. You can download the software including the manual here.

14. Joined: Jan 2005
Posts: 2,656
Likes: 535, Points: 113, Legacy Rep: 1110
Location: Philippines

### rxcompositeSenior Member

Bryn- What does "Df" mean?

15. Joined: Mar 2014
Posts: 10
Likes: 0, Points: 0, Legacy Rep: 10
Location: Cornwall/Plymouth, England

### BrynjaminjonesJunior Member

Cheers for the help - no matter what I do I can't change anything! Using various different forms of equations I can change the magnitude of power, but the graph never changes shape!

Df should be D subscript f, and is the frictional drag-force component along the bottom surface (according to Savitsky's paper).

In response to the possibility of the online calculator using the simplified method, I have a feeling that that is not the case as it asks for values for a, f and epsilon.

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.