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Episode 59 – Final Discussion of the Hammock Boat.

The Hammock Boat has been a very good exercise in learning many things, but this is the wrap-up post on it.  I don’t consider this really “done” until one is built, but I have done enough by the completion of this episode to decide for certain that I have no interest in building it.  However, there is enough information here that you may choose to continue on and actually build one.

This is one post where both the write-up and the audio will be useful.  I talk in a little more detail in the audio than I have in the actual blog and to attempt to write in that detail would bore both of us to tears.   However, the audio by itself would be useless without the pictures shown in the blog.

Where we have been:
Early on the first thing tackled we the problem of making it float.   We really had no idea of the weight of anything, but from the picture it is easy to see the original one used some kind of plastic pipe for the pontoons. Two posts were dedicated to calculating the buoyancy of the pipe. (Post 53A & Post 54A).  Because I knew we would have to use CAD several posts were dedicated to presenting LibreCAD.  Later, because I was concerned about the boat tipping over in a strong cross wind, we talked about Center of Gravity.  We are now ready to put everything together.

Picture 2 – Center of Gravity Table

The first concern was the weight of all the components including the operator do determine the final weight of the boat.   Where it was possible to determine actual weights that was done, but in most cases is is simply a guess based upon upon experience.  This is shown in the first three columns in the table on picture 2.

The next step was try to  determine the center of mass for each of these components.   Again estimates were necessary.   Since everything about this boat is based upon the hammock and the hammock frame it was also the basis for these estimates.  The reference height (zero) was the bottom of the frame.

Picture 3 – Side view sketch of the boat

Picture 3 was created by using a photograph of a hammock frame as well as the photograph of the existing boat. A advertisement for the frame stated that the frame could handle a 15 ft. hammock (180″) and another advertisement said the frame would be 188″ long.  The frame photograph was traced and then the tracing was scaled so the top was 188″.  Then the Center of Gravity (CoG) of the frame was estimated  on the drawing.  The hammock sag was estimated from the existing hammock boat picture.   The CoG of the operator was assumed the be about midway in the torso.  The height of the sunscreen was guessed at based on the picture of the boat. Dimensions of all these numbers was taken  from this picture and entered into the table.   The height of each partial CoG was multiplied by the weight of each component.  The sum of these calculations were then divided by the total weight to determine the Total CoG.   Because the frame has a low CoG and the weight of the PVC pipe is a significant weight the CoG of the total boat and operator was very low.   This is good news… the boat will probably be very stable.

Picture 3 – Front view sketch of the boat.

The next step was to create a front view sketch of the boat.  This shows an outline of the problem.  The wind will tend to cause the boat to rotate clock-wise around the down-wind pontoons.  This has a lever arm the length of the mast to the sunscreen.The weight of the boat will  push down from the CoG creating a counter-clock-wise rotation that would try to maintain the boat upright. This has a length of a lever arm equat to the dimension we labelled “D”.  I entered these values into a spreadsheet and solved for the wind force necessary to make the two rotations equal.  This is shown in Picture 4.

Picture 4 – wind force calculations.

It turns out that with our low COG and high weight it would take a lot of wind force to lift the boat at all the dimensions.   In other words, I have been “freaking out” over nothing. However, it is much better to put the effort in and understand than it is to assume and find that you have assumed wrong. I do want to note one thing.  In an earlier episode I talked about the COG of the operator would change if the boat lifted.   It looks as if the guy that designed the original boat  already fixed that problem.   If you look closely at the picture of his boat, you will see a rope going from the bottom of the frame to both ends of the hammock spreader bar.   This will prevent the hammock from rotating within the frame.

Picture 5 – Cost Calculations

The final estimates was the estimate of each of the components to come up with a total cost.   These costs were obtained by a quick web search.   The killer is the 8″ PVC pipe and the End Caps for that pipe.   If I was looking for an opportunity to improve this design, that would be the first place to look.

Now that we have done all these calculations and work what do we really have?  We have gained a lot of knowledge in a lot of things.  We know how modifying each part will effect the whole.  So although I don’t see me building this, I think the exercise has been a good and fruitful one.

Again I would encourage you to listen to the audio,  I explain things in a little more detail than I have done in this post.    Thank you for reading this.

Gary

Because my posting has been a little random lately, I encourage you to consider signing up for the e-mail subscription at the bottom of this screen.   That way you will become aware of each post as they are published.

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