Displacement Question

Maybe z is more important than y

50 feet is approx 15 meters.
Displacement could be 15 x 2 x 0.5 x 1/3 = 5 tons
or 15 x 4 x 1 x 1/3 = 20 tons

Look at some 50-footers you like.
Read some more books

 

High performance is dependant on engine or sail area relative to displacement. A fifty foot boat can be quite heavy, but have enormous engines or a huge sailplan. Light weight usually means better performance, but not always. On average, though, high performance yachts are lighter than others of equal length.
In light air, a sailboat goes faster for reasons it might be slower at higher wind speeds than average. At extremely low wind speeds, hull (skin) friction could be reduced to a minimun by making the hull more bowl shaped rather than needle shaped. Such a hull would have the least surface area to carry its underwater volume. Since a short, wide sailboat hull can carry a lot of displacement at low wind speeds and still perform well, it will often outperform a boat with a long narrow hull and lower displacement in areas where the wind doesn't blow very hard.
One could say the same for engine-driven boats. If you were interested in making 1/2 hp do the most in a 200 lb displacement boat, you would not necessarily make a 50 ft hull 12" wide. You would likely make a hull 19 ft long and 23' wide.
Reduce power to 1/4 hp, and the boat might be fastest at 15 ft long.
At a certain point, a very tiny amount of driving force would achieve the highest speed if the hull were shaped like a salad bowl.
So performance is all relative to available power. In areas with a lot of wind, a long and narrow (non-planing) fifty foot sailing yacht with a deep ballast bulb would be capable of high average speeds.
Planing sailing hulls are a different story. They are very light, enough to effectively climb over their own bow wave and jump on top of the water, reducing both immersed volume and skin friction. They could be considered, at fifty feet, to be super performance boats.
To make them light enough to do this, they are also super expensive to build.
So "performance" is not always attained in the same way, if performance implies the shortest time to cover the longest distance. And further, shortest elapsed time isn't necessarily what is meant by performance in the first place. A world record holding windsurfer would be amazingly slow in large waves for all kinds of reasons.

Alan

 

That's good. The boat I needed the displacement for was one I designed. It is a fifty foot high performance sailing cruiser. I did the calculations using Simpson's Rule and came up with 30,339.84 lbs, roughly. I also used the displacement to check displacement/length ratio, Comfort Ratio, Sail area displacement ratio, etc, and they all came out right. That was really good news considering I designed this boat for fun wiothout any traing as a high school senior. However, I am going to school for this at the Webb Institute on Long Island. Thanks again for yur help!

 

B, if you raised or lowered the hull (or the waterline if you like) a few inches that mean would some tons in change, you see what I mean? The question is if you can build the boat and fill it with gear and crew and everything and hit 13,5 tons.

 

I know exactly what you mean, and I did a little bit of research and found that 30,000 pounds seems to be in an aceptable range for high performance fifty foot boats, so my actual weight displacement can be a bit higher or lower and the boat will still perform roughly the same way I designed it to.

 

It's an interesting question

I usually think the other way.
1) Estimate a total weight based on type of boat and loading (crew, gear etc).
2) Select a Cp based on what speed I want to optimize for, 0.52 for light wind sailing, 0.6 for hard wind sailing for example.
3) Draw the displacement curve.
4) Draw some sections with corrrect area.
5) Modell the hull (on pc) or sketch a lines drawing with pencil.

 

Really good thread! It is a pleasure to learn about the design process this way, a Q and A format and many cumulative years of experience from all over the world distilling the process.
Edited properly, these threads would make one of the most comprehensive texts on (practical) current marine design available.
... Though I'm not sure if the forum agreement mentions copyright.


Alan

 

bfisher33

If you PM me your Email address, I forward a simple Excel spreadsheet I "composed" based on Simpson's multipliers before the advent of free and/or affordable boat design software during 1990 when I took up studying boat design. This helped me through troubled times.

All you need to add is each station's submerged area and the common interval, beam and max hull draft.

In return you get displacement, LCB, VCB, CP, CF, CWP, rate of immersion and moments to trim.

 

From the work that I did for the Westlawn program, which requires the by-hand method, versus repeating the analysis on the same boats expressed in Rhino3D and analyzed with RhinoMarine, I have to say that the CAD method is a lot more accurate. I have a hard time accepting less accurate ways without justification (cost of software for example) ...but the cost of software seems cheap after you do the analysis on a few boats ...and then again at varying degrees of heel to compute a righting-arm (GZ) curve. That said, I *do* think you should do it the hard way first... on ALL aspects of designing a boat, because it really internalizes what's going on. On other than unimportantly-small (flames to /dev/null) hulls, I don't think I'd invest much time or money into the boat if it weren't evaluated properly.

You might try taking a look at DelftShip ...I think they have a free version, or maybe it's just free for a trial period, that will do the hydrostatics for you. It's a lot cheaper than the Rhino+RhinoMarine combination (although that's what I'm sticking with ...having proved to myself its accuracy and abilities.)

By the way, I just paused for a look at Delftship (see http://www.delftship.net ...no affiliation whatsoever) and see that if you register that you can download their free version of the software. Looks like the free version is pretty capable and if you go with the pro version, the 3 components that I'd recommend only add up to about $350 USD. Not a bad deal if the s/w works OK and isn't too hard to use. DelftShip used to be FastShip BTW, so maybe someone can comment. I just downloaded a version to try for myself...

If you really want to do the job right, you'll want to thoroughly study how to determine not just the boat's displacement, but also the trim. Without the trim (both directions), you cannot accurately evaluate the submerged volume. Why not take the time to study those chapters in the Principles of Yacht Design (and other similar books) and try it out? It's all fun. After you've done a few boats, then you'll decide the software is worth the money and will buy your own! Dive in!

Brian

 

Thanks guys. I really like this method of learning new things as well. Also, like I said I am very new at this, and I don't have any training, so my metjods of doing things are basically wrong. And I slao can't afford software even if it is worth it. I am only 17, and a college bound high school senior. However, once I can feasably afford it, I will get it. I think that at the college I am going to, the Webb Institute, I think you either get for free or for not much a laptop to use while you're there with all those programs. But anyway, I am thinking about downloading the free version of Delftship to use until then. Once I get my lines drawing, sailplan, exterior and interior drawn on the program, I will have to upload them to sow you guys for advice.

 

Thanks. I have always loved drawing things by hand, and I didn't have access to a really good CAD program, but I did have a design book, some pencils, a ruleer and a calculator. It actually turned out pretty well. I am using the designs to make a 3D model out of fiberglass.

 

Also where can i freeship to download?

 

cool-thanks.

 

I tried downloading Fastship, but it doesn't seem to be working correctly. Do I need another type of software to use it? In other words, exactly how do you download it?

 

Nevermind.