Prop Shaft Rebuild CD 36

[John Nuttall]

Propwalk is a result of the shaft angle.

.... more angle, more walk ? ....

J.

[Skylark]

Yes. On a Cape Dory 36 the shaft angle is 15 degrees.

[John Stone]

Propwalk is a result of the shaft angle.

.... more angle, more walk ? ....

J.

If you are talking about the downward angle of the shaft...then I don’t think so.

1. The prop on a CD 36, so far as I know, which is suspect since I don’t have a prop shaft, is 0° laterally, i.e. the shaft is oriented and aligned fore and aft with the boats centerline. It is angled down vertically a little but I think that has little to no effect on prop walk.

2. Prop walk is primarily caused by water coming off a rotating propeller being pushed against the hull. I have read that the bottom of the blades are more effective than the top blah blah blah but I don’t know by how much since we are talking about inches. Maybe it’s the effect of the hull. No question though its effect tends to be greater in reverse because the boat steers better going forward. And don’t forget, there is prop walk effect going forward too.

The only way shaft angle can influence prop walk, that I know of, is if you off set the shaft laterally as in offset from the centerline...like an Albin Vega. Also the Centurion 32, Bloodshot, I was just on last week has its prop shaft off set to port maybe 10° to counter prop walk.

3. I suspect a bigger prop with great pitch? will move more water and thus have a more pronounced prop walk. Perhaps there are other propeller design criteria that can influence it. I’m not a propeller engineer or that well versed in hydrodynamics. But, shaft angle up or down would not make a lot of difference I don’t think.

[Skylark]

Yes the downward angle on a CD 36 is close to 15 degrees. The limit for a Perkins 4.108. Some sailboats angle to the side a few degrees so the shaft can be removed without having to remove the rudder. Prop walk is actually something that it useful/desirable when docking. At speeds above 2 knots steerage controls and is much stronger force than propwalk or any torque induced side force.

[tvangilder]

On our CD 36 (#53- 1981) we have what appears to be a bronze tube that goes through the sternpost.


-Thad

[tvangilder]

Our 36 has the bronze tube around the shaft with the packing nut threaded onto it. we have a 1" shaft. I was thinking of replacing the shaft while we do a repower and have everything apart.

Does anyone know if there is room in there for a 1 1/8" shaft or 1 1/4" if you replaced the cutlass bearing?

[tvangilder]

There is definitely not room for a shaft larger than 1" on the 36.

[wikakaru]

If you are talking about the downward angle of the shaft...then I don’t think so.

Do a little thought experiment of the "edge case": a prop angled down at 90 degrees from horizontal (that is, the shaft is vertical). The prop would serve only to spin the boat and provide zero forward thrust. Now slowly angle the prop up from that position and imagine how the rotational force on the boat decreases and the forward thrust increases.

There's a nice little article here: https://tradewindssailing.com/wordpress/?p=365

Smooth sailing,

--Jim

[John Stone]

If you are talking about the downward angle of the shaft...then I don’t think so.

Do a little thought experiment of the "edge case": a prop angled down at 90 degrees from horizontal (that is, the shaft is vertical). The prop would serve only to spin the boat and provide zero forward thrust. Now slowly angle the prop up from that position and imagine how the rotational force on the boat decreases and the forward thrust increases.

There's a nice little article here: https://tradewindssailing.com/wordpress/?p=365

Smooth sailing,

--Jim

Right. Like a helicopter without a tail rotor. Only that does not explain prop walk in a boat with zero down angle. There are others factors at play. Some props seems to have more walk than others. I think the underwater profile of the boat has more to do with it than a a few degrees difference in shaft angle. But that’s just my opinion.

I definitely think prop walk is useful, especially on a full keel boat. It will be interesting to see how my boat is different with the offset design than it was as a stock 36. I have four degrees down angle vice nearly 15° and the prop is now behind the rudder. I don’t expect to have much control in reverse. It will be interesting....