Which Way Does Your CD Want to Turn in Forward?

[John Stone]

This question is for CD boat owners with an inboard engine and a right handed propeller with the shaft on the centerline.

As I consider installing an engine in the Far Reach there is an issue that suddenly I am unsure about. So, I am asking for input. The question is this: When you are under power only and moving forward in a straight line at normal cruising speed and you take your hand off the tiller which way does your boat want to turn? I want to say to port but I have not had an inboard in the Far Reach for 18 years so I don’t remember....

I do not intend to open the aperture up. And this question is not about prop-walk in reverse. This is about what happens with the boat moving forward at normal cruising speed. For the installation I am considering, the propeller will exit slightly behind and above the rudder. To see a picture of this plan see the thread under projects "Installing an Engine in the Far Reach."

To install the engine and shaft the way I want I need to pass the shaft on the starboard or port side of the rudder post. I will need to angle the engine about 5-8 degrees to accomplish that. I know that if I pass the propeller shaft on the port side of the rudder it will want to push the stern to starboard and the bow to port. Conversely, if I pass the shaft on the starboard side of the rudder post with the engine angled slightly to port the stern will want to go to port and the bow to starboard.

If I do this right, ideally, the slightly offset propeller shaft will counteract the torque of the propeller in forward. But, which way does the boat naturally want to turn going forward with a right hand prop?

I appreciate your input.

Thanks.
John

[Joe CD MS 300]

John,

From Wikipedia (for my own sake and clarity)

“A propeller is called right-handed if it rotates clockwise in forward gear (when viewed from the stern). A right-handed propeller in forward gear will tend to push the stern of the boat to starboard (thereby pushing the bow to port and turning the boat counter-clockwise) unless the rotation is corrected for. In reverse gear, the turning effect will be much stronger and with opposite direction (pushing the aft to port). A left-handed propeller acts analogically to the right-handed but with all rotation directions reversed.”

Per above, a right handed prop will tend to push your bow to port in forward gear. To offset this to some unknown degree, the prop should exit the hull on the starboard side of the rudder post. I guess how much the angle should be is the question. My gut thought is that it should be as small as angle as possible since prop walk decreases as speed increases.

An unintended consequence of you plan could be how it handles in reverse if that is a concern. I would think The relative strength of the prop walk vs side ways thrust might be opposite in reverse vs forward.

[Steve Laume]

I have never really noticed, Raven wanting to stray in either direction while under power and going forward. I do know she wants to go to port when I go into reverse.

One of the things I admire about the CD boats is that the prop is in an aperture and the rudder is attached to the keel. This is a very good design and I don't think it can be approved upon. You could hang a prop and rudder in many ways and have them preform their primary function. Fin keel and spade rudder boats have all sorts of advantages along with some down falls. It is when you start to introduce things like, kelp, rock weed, logs and lobster pots that a well protected prop comes into it's own. Having the prop in line with the rudder also has the advantage of being able to give a forward blast of water over the rudder surface while maneuvering to kick the stern around. There are also some times when you might be counting on your motor when the seas are high and the boat is heeled. A deep set prop will stay in the water. A high offset prop may well be sucking air at times. I know this is not a common situation but it does happen. Some anchorages and harbors offer no room to dowse sails once inside and are very exposed to ocean swells where you might be motoring into strong wind and seas while trying to get your sails under control. I would not want to add a cavitating prop into the mix of whatever else was going on.

Some areas are pretty forgiving and you could easily sail without an engine. Other areas present all sorts of hazards and you just wouldn't be able to, safely, venture into them without an engine. This is especially true when single handing. With only one person on board you lose a few options, Steve.

[Tod Mills]

I find it helps to realize that the flow near the tip of the prop is inhibited by the presence of the hull and is therefore fighting hull rather than being free to thrust against the open water. Kind of like paddling a canoe sideways against the hull (not effective) rather than in open water (more effective).

The harder question is, to my mind, to which side of the center of directional stability will the prop axis pass (if not parallel to the boat centerline)?

An experiment you might perform, if you haven't already experimented in this regard, is measuring the angle your existing outboard makes with the centerline when the helm is neutral (boat tracks straight). Couple that (pun intended) with the outboard's location along the length of the boat and distance from the centerline, and you should get a decent idea where your center of directional stability is located.

As food for thought, here is a 27' Albin Vega. The prop shaft passes to the port of the rudder stock:

[Paul D.]

Femme goes to Starboard in forward. When reversing, all bets are off, but most times to port. I sort of like the mystery.

Also, you may wish to take a look at the Tartan 34C bulletin boards, the old one. That model was an S&S design and had a Farymann diesel under the settee with an offset prop.

[Tod Mills]

Femme goes to Starboard in forward. When reversing, all bets are off, but most times to port. I sort of like the mystery.

Also, you may wish to take a look at the Tartan 34C bulletin boards, the old one. That model was an S&S design and had a Farymann diesel under the settee with an offset prop.

On the 34C, isn't the prop centered, but angled about 3-4 degrees to port?

That makes sense...normally a prop will tend to push the bow to port in fwd (ccw turning)*. By angling the shaft to port, the line of thrust creates a small clockwise couple to counter the ccw couple created by the lateral differences due to hull interference.

*a prop in an aperture can be an exception, as the lateral thrust is affected by the aperture at the bottom in addition to the top. Additionally, a prop in an aperture is sometimes further away from the hull.

[John Stone]

Femme goes to Starboard in forward. When reversing, all bets are off, but most times to port. I sort of like the mystery.

Also, you may wish to take a look at the Tartan 34C bulletin boards, the old one. That model was an S&S design and had a Farymann diesel under the settee with an offset prop.

Paul, I’m familiar with the Albin Vega. That’s a variable pitch prop in the photo. I have seen an AV with the shaft same as above but with a fixed RH prop. It’s one of the reasons I got confused. The guy said it was tough to handle. Maybe that’s why—shaft angled to port should have LH prop. So the boat may have been repowered with a RH turning engine when it should have been left hand turning—which appear to be rare these days.

[Tod Mills]

Here's a 34C. The prop is centered, but angled to port, creating a c.w. couple (tending to turn the boat to stbd) to counteract the RH prop, which tends to turn the boat to port.

This is as it should be.

[John Stone]

Thanks Tod. That’s interesting. I would say it’s angled to starboard though (forward to aft). But as you said, it makes sense. Having learned more about offset propeller shafts than I ever wanted to know, it seems sometimes this was done simply to allow removal of the shaft without contacting the rudder or skeg.

I don’t think I’d want my engine below the cabin sole. That’s an interesting looking two blade propeller.

[Neil Gordon]

In less than ideal (for a scientific experiment) conditions, LIQUIDITY will tend to go beam to the wind/waves. I'm not sure that a few degrees of offset, at cruising speed, would make a noticeable difference. (I'm not sure that I notice a difference when towing a dinghy, which I never tow from the center line.)

There's an engineering answer to this. Even if anecdotal evidence gives you a directional answer, I'm not sure you'll get the offset angle right without expert calculations.

[John Stone]

Even if anecdotal evidence gives you a directional answer, I'm not sure you'll get the offset angle right without expert calculations.

Neil, concur. It’s just a consideration. But may not be practical. Bigger more important issues to address. However, I notice things like that...especially with a tiller. Like driving a care that always wants to pull to the right because the front end is out of alignment.

[rorik]

Mathilda goes slightly to port.

[Paul D.]

OMG Look at that T34c engine access! You have to admire those old S&S designs. I used to charter a T34c. I think sail like a dream. One finger on the wheel beating into 30 knots. You've got to admire that.

[fmueller]

John

My boat walks to port in forward and reverse. Obvious in reverse, subtle but always present in forward (at the end of the tiller; 1-2 fingers to port correction needed). If I release the tiller motoring forward the boat will turn to port at about 1-2 degrees per second.

I can understand not wanting to put your proposed prop into a rudder aperture but you will be giving up the ability to do a very low speed "k" turn to starboard under power because your prop wash will be behind the rudder and in fact you might not have the pretty amazingly good forward low speed steerage that aperture mounted rigs have ...

On the other hand you might find that you actually have some extra steering control in reverse because there may be some prop wash passing over the top of your rudder in reverse. That would be a good thing. You may find you really have to hang on to the tiller when you go into reverse.

Fred

[John Stone]

That’s good information. I need to determine how much to offset the shaft log.

The aperture makes the boat more maneuverable under power but less under sail. There is no going back to the aperture at this point. See my most recent post on installing the diesel.

“Alea iacta est.”