Prop Shaft Measurements and Calculations

Boring the hole for the prop shaft is psychologically one of the most difficult tasks in building the boat. Getting it right is important and it is difficult to judge if you are doing it accurately until it is all done. So here are some details on how to do this based on my boat.

Before you start I strongly recommend buying and reading Inboard Motor Installation By Barry Witt and Ken Hankinson (available from Glen-L).

You will most likely want to have both scale drawings / sketches and a full size drawing. The full size drawing helps to visualize and double check the measurements and calculations.

You may also want to take a look at my quick Trigonometry Refresher

The dimensions are from my Riviera. The spacings are rather close. So if you can make them a bit bigger, go for it. In particular the spacing between the transmission flange and the stuffing box bearing in my boat is really too small. It is a major pain to change the stuffing box seals.

So you will need to modify these for your boat. In laying out the propshaft I had several limiting factors:

The clearance from rudder to prop is easy to forget. However you need to be able to get the prop on and off without having to pull the rudder out.

Unless you already have your prop shaft, I would recommend getting a 1" pipe to use in doing the mock-up. I did not order the shaft until everything was laid out (the Aquamet shafts are expensive) and I knew what the final length would be. If you are getting a double taper prop shaft (like an ARE brand), you need an EXACT length prior to ordering as both ends are tapered, not just the prop end as in a conventional shaft.

Starting at the stern of the boat, you need to lay out the components for the first pass. After everything is laid out, you will need to double check again "from the front " or in my case, it was from the middle bench seat back to see if everything will fit.

Rudder: For the rudder I used, it is just under 6" from the shaft to the rear of the blade. The rear of the rudder hangs slightly past the transom as seen in the photo. You do not want this to go out too far as then there is the possibility of cavitation due to air getting sucked down from the surface, and loss of steering control at speed. The rudder stuffing box is mounted just forward of the stern framework.

Prop: Allow 1" from the strut to the forward edge of the prop. My 13" prop has a 2.75" thick hub. Add the allowance for removing the prop. Don't forget the end of the prop that will have the nut as well which will add at least another 1" . So the minimum distance is 1+3+3+1.75 = 8.75" between the strut and the rudder as measured along the prop shaft. As you can see in the photo ,mine is about 9.5" from strut to front of the rudder. In my original sketch, I had this as 4" as I did not yet have my prop purchased.

Strut: I used the 16 degree strut recommended by Glen-L for the Riviera. This has an odd shape which made measuring a bit of a challenge .

Strut to center of hull entry point: This needs to be calculated (below). Be sure to allow for the offset of the base vs. the shaft openings.

Distance through the hull: You need to know how thick your hull and keel beam are where the prop shaft hole will be going through. Hopefully you measured the thickness of the keel beam after shaping it prior to planking.

Stuffing box: With the stuffing box I used, the distance from the center of the shaft, where it would go into the hull to the end of the seal was approximately 10.5".

Clearance form stuffing box to flange: This should be no less than 1" and 2 or 3 would be nice if you have room. Remember the longer this is, the higher the engine will need to be mounted. The Riviera deck is very low, for fitting in a V-8 so I made this 1" (it really should have been 2).

Prop shaft flange: How thick is your flange? Mine was approximately 4"

At this point you have reached the flange of the transmission. Now you need to know how high the transmission flange is. This will vary significantly depending on the engine selected, transmission angle (mine is a 7 degree down angle transmission), oil pan height and contour, oil drain fitting (if present) and the mounting angle of the engine and transmission assembly. The top of the flange is 5" above the keel beam, the center is 3" above the keel beam.

How thick is your keel beam and hull? If I recall correctly the keel beam was 1.5" thick (I lost the measurement along the way) and the hull thickness is 3/4" at that point. This makes for a total of 2.25" thick for mine.



How much length (stem to stern direction) is used in going through the hull? Assuming a 16 degree angle and thickness of 2.25". Use the tangent calculation. The rise is 2.25", the run is the length = 2.25 / tan(16) = 7.84" (using calculator enter as 2.25 / 16 tan=).

How far from the hull side of the strut to the center of the entry point through the hull? The center of the strut opening is 5.5" high (rise). Run = 5.5 / tan(16) = 19.18". However the base of the strut is offset, so this needs to be taken into account in your measurements

Check of the angle calculation

Length of prop shaft from center of exit point to end 35.5" (hypotenuse of the triangle). Center of end of shaft to hull 9.25".

Angle = inv sin(9.25/35.5)= 15.1 degrees. This means that the strut will have to be bedded at a very slight angle of less than 1 degree which is easy with thickened epoxy.

Now to improve your accuracy, you may want to go through and update the calculations that used an angle of 16 degrees with 15.1 degrees.

The calculated strut measurement now changes from 19.8 to 20.4".

Length stem to stern of hole through hull now changes from 7.84" to 8.34"



Here is the sketch I used with the verified measurments. This sat in my briefcase for years, as I was gong to do these pages when I got some time travelling and redraw the sketch as well. So here is the original sketch