Design:

The design of this device is very important.  It will dictate how the part reacts to the problem that is trying to be solved.  Therefore, the design of each and every part will first start with different calculations.  These calculations come from the different scenarios that happen during water skiing activities.

 

A very important design property with this device is that it will be able to handle different environments.  This device will likely be used in fresh and salt water.  Stainless steel can be chosen for its properties that will not allow it to corrode easily.  The device will likely only be put on when it is needed.  Once the device is not needed anymore, it will be taken off, cleaned, and put away properly.  Still it is very important to choose the right material.

 

It is important to keep the device as light as possible.  Extra amounts of weight on top of the tower can lead to a bigger amount of stress then it is designed to handle.  Therefore for the parts that are actually wrapping around the tower, the material will be aluminum.  A certain type of aluminum that also has non-corrosive properties was the ideal choice since it will still likely in contact with fresh and salt water.

 

The next part of the design was figuring out how to attach a rope to this device.  Regularly when towing someone behind a boat a boating ski eye is used and will also be used for this device.  This could be simply added to anything with the right size of diameter holes needed to slide the ski eye through.  The rope is hooked onto the ski eye with a very simple hook.

 

Since the flag needs to be raised and lowered with a simple change in tension, it was decided that a part would rotate about a pin.  While in the horizontal position, the flag would be rendered down which means someone is being towed behind the boat.  When the flag rotates up in the vertical position, this would mean there is a fallen swimmer in the water from the boat.  This part would need to also attach the boating ski eye as described before.  Therefore it would need to be wide enough to attach the two prongs on the back of the ski eye.  This makes the rope attachment part have to be at least 3.500” long. This ensures that it is wide enough for the ski eye to attach to it and also to withstand the forces that the bolts will put on the attachment.

 

This device needed some type of housing for the rope attachment to rotate about a pin.  It would also need to be hollow so that the rope attachment could fit inside and also rotate inside the housing.  It was finally decided to use stainless steel square tubing.  It was the easiest stock material that could be found.  A block of material would have been too hard to produce.  The square tubing makes it easier to. 

 

Since the housing would be the main part of the device, the other parts would have to be attached to it.  The bottom and top parts of the device that attach to the tower will be bolted to the housing and the rope attachment would be attached to the housing. This will be attached by a pin that would still allow the rope attachment to rotate.  As for the spring, it would be put through a hole on top of the housing with a cap, which would also be bolted to the housing. This would hold the spring in place. 

 

It was decided that a spring would be used to dictate the amount of forces that will allow a flag to be raised and lowered.  This spring will be inserted into the device’s housing where the back side of the rope attachment will swing up and make contact with the spring.  This spring will dictate the flags raising and lowering between the forces of 100 N to 2400 N.  Since the spring will experience such a high amount of force at some points when being used, the spring will have to be larger than the amount of space already provided in the housing.  These calculations can be seen in Appendix A on Green Sheets 9 through 12

 

An important factor with designing this device is that it can be attached and detached as the boat owner pleases.  Most boat owners like to keep their boats very clean and tidy.  When the boat is not being used to tow a skier, wakeboarder, etc., the device will be able to detach quickly so that the boat owner can put the device away for further use.  This will allow the device and the tower to not sustain any type of damage further damage if both can be cleaned and properly stored away.

 

One of the most important factors of this device will be that it does not do any damage to the boat tower.  Richard Jones is a stickler when it comes to his toys.  He likes to keep them neat and tidy.  This device needs to accommodate that.  An easy solution would be to install some type of protective filming or foam in between the device and the tower.  This material needs to also be corrosive resistant for the environment it will be used in.  It should be a material that will keep clean and can be cleaned easily if it ever happened to get dirty.

 

Calculated Parameters:

Below will be the calculated constraints for each part of the device.

1. Rope Tension Analysis: The most important part of this project was determining the type of tension a rope undergoes under regular circumstances when towing a skier behind a boat. This was found to be about 363 N. The person skiing or wakeboarding does not always stay just behind the boat however.  Most of the time the person being towed likes to sway back and forth in a circular motion behind the boat.  A centripetal force needed to be calculated to keep the person being towed behind the boat.  Once the person being towed starts to go back and forth, moments are created on the pin as well.  These moments will put different stresses on the pin.  Once this was found, the design of the device could be found.

2. Pin Analysis: An adequate pin was need to be found that would not fail under the force circumstances that it will encounter.  The full calculated analysis can be found in Appendix A for the pin analysis.  The first part of the analysis was to find the amount of force that would be put on the pin while disregarding the size of the pin.  From the previous rope tension analysis, the force that would be acting on the rope would be 360 N to 1000 N.  From this data, I could make a free body diagram of what the pin would undergo.  This again can be seen in Appendix A. The forces in the x direction and the forces in the y direction were found using simple sum of the forces equations.  A moment equation was not formulated because the pin will be rotated on by the rope attachment.  Once the forces were found for the pin, the stress was next.  The stress needed to be found to better understand the amount of loading this pin will undergo.  It is important that this pin does not fail so that the rope will not fall into the water and the skier can continue to ski.  A size of the pin was thought to be around 0.75 inches in diameter.  It was thought to be a reasonable size for a pin.  Once the pin stress was found, a type of material could be found next.  Most of the materials were found to properly exceed the amount of stress the pin in this device would encounter. The next step will be to find what kinds of materials are used best as a pin.

 

Device Assembly:

This device will assemble in seven different parts: the housing, the bottom attachment, the pin, the spring, the spring cap, the top attachment, and the rope attachment.  The device will first be assembled by attaching the housing to the top attachment.  This part can be set on the tower while the bottom attachment to the housing will be wrapped around the bottom part of the wakeboard tower and screwed into the top housing part.  The next part of the assembly will be inserting the pin through the rope attachment and the housing so that the rope attachment can rotate but will stay intact with the housing.  The spring can then be inserted into its housing spot. Once these steps have been done, the device will be completely assembled.

 

Tolerances:

There are some very important tolerances that will allow this device to function properly.  One big tolerance that needs to be accounted for is the pin.  This pin needs to be able to stay in the housing without moving or sliding out, but also allow the rope attachment to rotate.  This tolerance will be set at + 0.005 in.  Other parts of the device will not be as important for the size and the tolerance that will be set for the other parts will be + 0.010 in.