"Freeship" hydrostatic stats analysis of three ships

[jedclampit]

hydrostatic stats analysis of three ships

I’m working on a design and I need an expert opinion of the Freeship output from my three models.

Can anyone interrupt the following data into layman’s terms to tell me what here works? What needs to be tweaked in the second case study?

Test number one is a proven design from an architect the second is my design and the third is a blank test case generated by Freeship. I was told by several professionals that case number two will not work...they have not seen the numbers just the concept.

Thanks for your help!

CASE STUDY One Two Three units

Design length 40.00 35.00 35.00 [ft]
Length over all 37.92 34.22 34.22 [ft]
Design beam 14.33 10.33 10.33 [ft]
Beam over all 14.24 10.33 10.58 [ft]
Design draft 4.00 3.50 3.50 [ft]
Midship location 20.00 18.13 17.11 [ft]
Water density 64.0 64.0 64.0 [lbs/ft3]

Volume properties

Displaced volume 525.29 272.41 380.15 [ft3]
Displacement 15.01 7.78 10.86 [tons]
Displacement 33,613 17,432 24,324 (pounds)
Total length of submerged body
31.79 29.97 27.34 [ft]
Total beam of submerged body
13.62 9.61 9.18 [ft]
Block coefficient 0.303 0.270 0.433
Prismatic coefficien 0.658 0.625 0.537
Vert. prismatic coefficient
0.389 0.368 0.614
Wetted surface area
414.76 318.55 251.29 [ft2]
Longitudinal center of buoyancy
19.74 15.31 16.54 [ft]
Longitudinal center of buoyancy
(0.83) (9.43) (2.06) [%]
Vertical center of buoyancy
3.01 2.57 2.19 [ft]

Midship properties

Midship section area 25.13 14.55 25.89 [ft2]
Midship coefficient 0.461 0.432 0.806
Waterplane properties
Length on waterline 31.79 29.97 27.34 ft]
Beam on waterline 13.62 9.61 9.18 [ft]
Waterplane area 337.74 211.67 176.95 [ft2]
Waterplane coefficient
0.78 0.73 0.71
Waterplane center of floatation
19.15 14.65 15.87 [ft]
Entrance angle 71.88 73.32 30.41 [degr.]
Transverse moment of inertia
4,001 1,203 881 [ft4]
Longitudinal moment of inertia
20,784 10,601 7,211 [ft4]

Initial stability

Transverse metacentric height
10.62 6.99 4.50 [ft]
Longitudinal metacentric height
42.57 41.49 21.16 [ft]

Lateral plane

Lateral area
97.60 93.66 64.55 [ft2]
Longitudinal center of effort
19.66 14.83 17.60 [ft]
Vertical center of effort
2.33 1.83 2.11 [ft]
Hull Data

Hull Surface Area
778.64 629.56 889.63 [ft2]
COG X 19.16 14.76 16.70 [ft]
COG G 4.10 3.74 6.25 [ft]

Please let me know if case number two is way out or does this have a chance?

[Gilbert]

Hi,
Did anyone say why it wouldn't work?
I've never thought of evaluating a boat design or comparing them in the format that you have presented.
The only item that seems really strange to me on a casual look is the longitudinal center of bouyancy (%). If that term means what I think it does, then all the numbers for that item appear wrong to me.
I have always liked to judge a design by looking at the lines. So my post has probably not been much help to you. I've also thought the very best way to judge a design is to use the boat for about ten years or so.

[Pelle]

Its impossible to tell without knowing more. First what type of boat are you designing?

[MikeJohns]

It is a lot of detective work for any of us to try and picture the hull from these numbers alone. They should accompany a lines drawing to be of any significance.
You also need to say what the intended use is,how she is to be powered and what construction material you intend. Then we may be able to give some critical analysis.

[jedclampit]

Thanks All...

The file Test case one is an original Bruce Robers 40' Spray Sailboat that was designed in the seventies...a proven design.

Test case two is a modified version of the same hull just tweaked a bit by simply removing a 3' section from the centerline and rebuilding her keel...

She will be a Gaff Cutter sail rig with about 18000 lbs full displacement.

Thanks for the input and I hope these files, although crude, will help.

JED.

[PAR]

Spray relies on a lot of form stability (typical of most shallow draft vessels) to remain upright and provide acceptable stability figures (which it doesn't excel at in the first place) You've removed most of her form stability, in short, she'll be an extremely tender craft to say the least, unlikely to withstand any reasonable press in her sails.

[Pelle]

What PAR said.... You have made the vessel very narrow for a boat of this kind and it is unlikely that there is enough displacement to support the amount of ballast you need to make it stay upright.

[jedclampit]

I am sorry, but your answers are too generic and opinionated. I’m trying to learn what hydrostatic properties in the two models makes you believe that test case one is stable and test case two is unstable, if at all.

I’m an engineer and I tend to understand numbers over opinions. If you could enlighten me as to what numbers in the modeling results would make you believe that test case one is stable and test case two is not. If you did not know that one is a modified version of the other would you have had the same conclusions? Did you even really look at the numbers? I understand that the models are not perfect and far from a real test case, but come on… The second boat is 8 feet shorter than the first.

Someone once said that boat design is an art and a feeling more than just numerical analyses, but what is so wrong with trying to understand the numbers?

If you want to add your two cents, please explain what differences in the results of the two different models would make you feel or believe this?

I am trying to learn how to design the best case.

Thanks

[Pelle]

The main number to look at is the transverse metacentric height. The initial stability of a ship is a function of the metacentric height and the vertical centre of gravity. I would suggest studying a basic textbook on ship stability. "Teach yorself Naval Architecture" by Brian Baxter is good if you can find it. Trying to explain the desing process on a forum is a bit too much but bsically the hull must provide enough stability to carry the press of sail. What enough is is a matter of judgement and in some cases regulation. In your case you have reduced the metacentric radius by almost 40% without providing a viable mean to compensate by lowering the CoG so I think that your proposed boat will be too tender.

There are several good text books on yachtdesingn available too. You can find a lot if tips if you search these forums.

[yago]

Originally Posted by jedclampit

...but what is so wrong with trying to understand the numbers?

Nothing... ;-)

the problem is that if you want to design a boat, you would need to understand these numbers to begin with, at least the most important ones, and you would then not need to ask here.

There is no right or wrong, they simply desribe and analyse a boat in such a way that you can say if this boat is suited for the purpose intended.

In Practice, you would actually start by defining the characteristics of the boat you want first, express this in numbers (and there is lot of empiric data out there to judge them against) and then design a hull to fit them. For this you obviously need to learn what they mean - there is not much sense in designing a boat, or in significantly changing an existing design to your taste, then have a program pop out some numbers and ask people if they are "right".

Freeship is a neat program that's getting better all the time - but it's just a tool to help you design a hull. If you do not learn how to calculate these numbers yourself with pen and paper, what they mean, what other similar boats would show and so on, then freeship or any other hull design program will not help you either.

[yago]

just remembered: there is a very short intro at http://www.tedbrewer.com/yachtdesign.html

After that there's lot's of good books.

[jedclampit]

Thanks for the input guys…

I am looking for good books on the subject. I’m not really trying to design a project from scratch but to utilize an existing beautiful hand built hull without keeping its original design. I’m not married to the keel in fact I don’t much like it. I have this hull, which I would not have built myself, that I’m not crazy about, but I want to use the canoe because it was made by a friend of the family. And because it’s a solid 1-1/2”x1-1/2” mahogany strip plank hand built hull and she is very beautifully done. I don’t like the original design being at 14 feet wide and a full displacement hull. I am simply trying to discover the problems so that I can find the best solution to making a useable boat without keeping its full beam.

I’d like to build this into a sailboat but I’m not married to a shoal draft or a full keel. I could even go strictly power as in a trawler. In fact if I have to plant the thing in the ground and make children’s play gym out of it so be it.

I’m just trying to understand why some people are so against cutting her middle out. I only see potential improvements like less drag etc. If you look at the three models the TMC in case two is larger than in case three which is a generic hull designed by Freeship using the same characteristics as in case two. So that in fact would mean that my design is better than the computer generated model. The mid-ship cross section looks to me to be a problem. I like the narrow beam and I’m going to try to find the best design with that in mind. I’ve seen many 33 to 36 footers that have comparable beams.

I’m going to cut her in half and haul her across the country anyways or the owner will cut her up into a million pieces and have her hauled to the local landfill. With that said…I’ll look for assistance in books or elsewhere. I’m not going to accept noway as an alternative because as an engineer I pride myself in using my creative talents to solve problems by first identifying and understanding any potential obstacles. And this is just that…a small stumbling block that needs to cut down to size.

Thanks again for your input.

The US NAVY also has a great website for the training of their seamen architect want-ta-bees... Let me know if you want that link...

Jed.

[jedclampit]

http://web.nps.navy.mil/~me/tsse/Nav...le2/basics.htm

[yago]

Originally Posted by jedclampit

...to utilize an existing beautiful hand built hull without keeping its original design.

So it's a real existing boat you are going to cut? Meaning you will add a serious structural problem to the already mentioned design issues? ;-)

I have seen boats cut across the beam to lengthen them (usually also not a very good idea) but cutting a piece lengthwise out of the center beats it all. I am afraid you won't find a lot of support for that here, but you should keep us up to date ;-)

Books:
Easy intro: Understanding Boat Design by Ted Brewer

A good read to get the first ideas: The Nature of Boats: Insights and Esoterica for the Nautically Obsessed by Dave Gerr

Principles of Yacht Design by Lars Larsson and Rolf Eliasson
Elements of Yacht Design by Norman L. Skene


and before you do any cutting:
The Elements of Boat Strength by Dave Gerr

[Gilbert]

Now that I understand a little more about what your goal is, let me suggest that you use the scale feature and the "save as" feature of freeship to arrive at a 35' version of your 40' spray. Then, being sure to have the waterline in the proper scale reference as the original, do the hydrostatics calculations for the 35' version of the Spray and compare them to the 35' number two model and then you will know how the numbers compare to the design of the Spray. I am guessing the Spray will look really good in the comparison.

Despite what others have said about the Spray it is an excellent boat. Bruce Roberts may be the most famous boat designer in the world. Why? Because he has based an incredible number of his designs on the Spray AND because he has pioneered selling detailed plans with good instructions for home builders through good advertising. If the Spray was not a good boat, Bruce Roberts would be the most vilified boat guy in the boat world instead of the most famous.

The Spray makes a great powerboat. Bruce Roberts has a whole line of powerboat designs that are based on the Spray. And they are good boats. I've seen one of these boats and been aboard and it was a very good boat.
Perhaps the guys that bad mouth the Spray are just jealous that they haven't designed a boat that is as good or as famous.

If I were you I would do everything I could to avoid cutting the boat in two. My reasoning is that cutting it in two is proceeding backwards, not forwards. And everything you undo has to be redone at who knows how much trouble and expense and lost time. I know the logistics problem is real, so you have to do the best you can.