How To Measure PDRacer Hulls
Because of the unique shape of our boat and class rules, we can use a very simple method to measure the rocker shape of our hulls. It is sort of an unusual way, but after you "get it", you will see it is a very simple method that can be performed on a hull very quickly. It will take you more time to read this page, than it will to measure a duck hull.
Basically we make a "string line" which is parallel to the line drawn on the plywood that we used when creating the rocker shape of the hull. Then we put the 6" part of the measuring tape on the string, and measure where the bottom of the hull is. Your hull's dimensions should then match the Defined Hull Shape drawing, or be within the tolerance range.
Tools You Will Need
Here are the tools I used to measure my hull.
I was working alone, and the whole process only took a couple of minutes, including taking photos for this essay.
String
Masking Tape
Rock
Measuring Tape
Paper
ShortyPen (or other writing instrument)
Step 1 - Affix "String Line"
The "end points" are the corners where the bottom meets up with transoms. Affix a string between the bow and stern end points. Then attach a couple of pieces of tape along the string to help hold it in place.
If you are working with others, volunteers can simply hold the string in place or use a measuring tape to create the line.
NOTE: It is very important to place the string line in the correct location. Most of us round the edges of everything, making it difficult to find the exact end points. You need to be careful to to find the proper location to place the string line. If you put the line in the wrong place, the entire rocker will look like it is out of tolerance. Or if you place the line diagonally, then half of the rocker will look out of tolerance.
When in doubt, adjust the placement of the line and try again. When placed properly, the entire rocker will be in tolerance, in relation to one straight string line.
Step 2 - Distance Between End Points
When measuring the distance, it helps to use a rock to hold the end of the tape onto the hull.
Step 3 - Calculate Your Horizontal Shift
We are going to use the stern end point to measure across to the station marks, but first we must figure how far in we have to shift this endpoint, because the hull grows in length when you add the transoms.
The horizontal shift is (length - 90" / 2)
Example:
Shorty used the defined hull shape as a cut plan.
The freshly cut side panel has a distance of 90" between the end points
Then Shorty added a 1/4" thick plywood bow transom, and a 1/4" thick plywood stern transom
The distance between end points is now 90-1/2"
So the length is 90-1/2"
Minus 90" equals 1/2"
divided by 2 equals 1/4"
So Shorty's horizontal shift is 1/4".
This means that Station 1 is 12-1/4" in from the end point
Station 2 is 24-1/4"
Station 3 is 36-1/4"
etc..
Measurement Format "Inches, Eighths"
To make the table of hull measurements easier to read, we use a classic style which indicates the measurements in eights.
You can see this measurement technique in action, turn to page 22 of Phil Bolger's "Boats With An Open Mind".
So our measurements are expressed "inches, eighths" like this:
| 90-1/2" | = 90,4 |
| 2 - 5/8" | = 2,5 |
| 1/4" | = 0,2 |
The drawing to the right shows station 3, and what the measurements would be above and blow the defined rocker shape. Note that the negative numbers are more than 6" from the string line.
Step 4 - Take measurements at each Station
Notice how the defined hull shape drawing measures from the edge of the plywood up.
To mimic this, place the 6" mark of your measuring device on the string, and read where the bottom meets the side.
Most of us round the edges, so figure where the corner would have been before rounding.
If the hull is perfect, it will match the dimensions marked on the defined hull shape.
Round measurements to the nearest 1/8"
Step 5 - Measure the bow angle
This measurement is to check the bow angle. From the bow end point, we are going to measure "up and over" to the face of the bow transom, which can also be said as "rise and run". If you "rise" up from the bow end point by 4", and then "run" over to the edge of the transom, that will tell us the bow angle. We all are using the same rise "up" dimension of 4", so the run "over" dimension is the one we are interested in to see if the transom is at the correct angle.
First measure "up"
From the bow corner, place the 6" mark on the string.
Go up on the hull, and make a little mark where the tape reads 10".
It would be better to locate this with a right angle tool, use one if you want, but a measure tape with your best guess of 90 degrees is accurate enough for our purposes.
Now we measure the "over" dimension
This is the "over" dimension is what determines if the bow is in tolerance.
Measure from that small mark forward to the bow transom edge.
Round to the nearest 1/8".
4 Volunteer Measuring Method
If you have a crew of 4 volunteers (such as at an event), the measuring process is very quick to perform and can be done in less than a minute. Instead of a string, the volunteers use a measuring tape for the line, and a second measuring tape for the distance from the line to the bottom of the rocker.
volunteer jobs:
1 - hold one end of the "line" measuring tape
2 - hold the other end of the "line" measuring tape
3 - read the distance from the line to the rocker
4 - record the measurements and confirm they are within tolerance
Measure With Hull Side Template
You can use a hull side panel template to make measuring very quick. The problem with the template is they don't tell you if the shape is withing the tolerance range, but they WILL tell you if the hull shape is right on. So, something you can do is slap a template on the side of a hull and if it matches, then presto you know the rocker is fine. If it doesn't match, you can pull out 2 measure tapes measure to confirm it is within the tolerance range.
Hull Measuring Comb
In an effort to make a tool for us to use which will make measuring the hulls quicker and easier, here is the "hull measuring comb". It is a simple jig that can be made which mimics all the steps of measuring a hull with the string method. The difference being the string is replaced by a straight bar and each station mark has the edge of another board or bar sticking down (forming the teeth of the comb) so those edges are used like measuring tapes.
Please note the importance of having the main bar be straight. Most boards have a slight warp to them and can easily deform due to humidity, temperature or storage conditions, so it would probably be best to use some kind of aluminum or steel bar as the straight edge.
As of early 2012, the measuring comb is currently being evaluated and tested. Hopefully no problems are found and we can move on to trying it at the next championship to further evaluate if it is a viable tool.
