P35. Prop walk puzzle

[gs11gk]

Hi All.

It’s been a while since I’ve posted here but hoping to get some insight from the community. I purchased a 1977 Pearson 35 last year, spent much of that year doing updates, and now I’m spending more time on it. The boat handles well in forward but she doesn’t like reverse. I’m well read on prop walk but it’s not translating to practice and I don’t understand why. Here are my observations.

The boat appears to have a right hand prop. When in reverse, the wash is clearly on the starboard side. I can successfully do a back and fill turn to starboard (clockwise), and do it every time I put the boat in the slip. The usual maneuver is to pass the slip, leaving it to port, veer slightly to starboard, back and fill 270 degrees and then come into the slip. I have a private boat dock behind my slip, which complicates things a bit.

I was out with a friend today and we were practicing the idea of backing in a straight line, approaching a buoy in open water. We tried for over an hour and never really got it. Short bursts of reverse had minimal success but the bow tended to go to port… which I would correct with a burst of forward with the helm to starboard. The bow moving to port seems backwards to me. So question 1, why was that happening? Wind was minimal.


We also had the idea of just holding the helm straight and using reverse. The thinking was that once we had speed, we would be able to steer the boat in reverse. What ended up happening was the boat going backward, quickly, in a counterclockwise circle…. and it didn’t seem to matter where we put the helm. I would think that the prop walk would turn us the other direction.


One thing we did not try was to go into neutral and see if we could steer… coming out of the fast counter clockwise circles.


Everything I have read on prop walk says you will only be able to back and fill in one direction, based on your prop. I tried a back and fill to port (opposite of my usual docking maneuver. No problem. So I can back and fill both ways, but I can’t go straight back… and my boat likes to do counterclockwise circles, despite the propwash on the starboard side.

I’m wondering if the P35 hull design is contributing to it, specifically the long overhanging stern. It seems like at high reverse speeds the effect is on the front of the boat.

Any insight would be appreciated. I fear the day I ever need to back in a straight line. Is this something that can be done with this boat?

Thanks!

David

[GILow]

With that cut away forefoot and keel hung rudder you are condemned to a subset of yachties who only get to watch others reversing their boats.

It’s a fundamentally a-stable shape in reverse and any number of random factors will induce a turn to one side or the other that will simply increase in rotational velocity whatever you do.

Think of it as trying to fire an arrow backwards.

I feel your pain though. I really thought I had our docking manoeuvre perfected after twelve years of owning this boat (with the same general underwater shape as yours.). Then last week we approached the pen, I initiated the propwalk turn that should have rotated us 90 degrees and into the pen. Instead we did a 180 so tight we had to hang on for dear life. Ended up motoring straight back out into the fairway and starting again from scratch.

[gs11gk]

Seems I need to accept that backing up will be a crapshoot and master my back and fill turns. Need more maneuvering practice to confirm if I’m better turning clockwise or counterclockwise.

I’m still curious about others ability to back and fill in either direction. I was reviewing a gps plot from yesterday and it looks like my back and fill pivot is tighter going counterclockwise, which is contrary to what should happen with a right hand prop. Need to repeat that experiment.

[Iron E]

Originally Posted by GILow

you are condemned to a subset of yachties who only get to watch others reversing their boats.

Such a great comment.

[Dennisb2]

I had a similar design boat with an attached rudder and centerboard. The attached rudder does not provide sufficient leverage in reverse to easily steer going backwards. It a basic design issue. It was solved by using a spade rudder or a skeg mounted rudder mounted a distance behind the keel.
I used a feathering 4 blade Variprop that would fit in the aperture with reverse pitch flattened which helped. Also I would get the boat moving in reverse then go to neutral which allowed some steering capability. And bumping reverse as needed. The prop would also feather which helped a little. Never a good solution but I lived with it for over 40 years.

[GILow]

Originally Posted by gs11gk

Seems I need to accept that backing up will be a crapshoot and master my back and fill turns. Need more maneuvering practice to confirm if I’m better turning clockwise or counterclockwise.

I’m still curious about others ability to back and fill in either direction. I was reviewing a gps plot from yesterday and it looks like my back and fill pivot is tighter going counterclockwise, which is contrary to what should happen with a right hand prop. Need to repeat that experiment.

I’ve been thinking about your experience some more, and I think I’ve observed the same thing.

I’m going for a bit of serious armchair naval architecture here, along with some wild arse guesses, but here is what I THINK may be happening…

Breaking it down a little….

1. Like you, our prop pulls to port in reverse and generally I can use that to my advantage to turn the bum to port and the bow to starboard. But if for ANY reason the boat has already got a bit of bum-to-starboard happening I can’t afford to let ANY aft speed build up or the prop walk can’t overcome it.

2. What I think is happening in that situation is that the flow along the keel face has already broken away and is turbulent and therefore offers no directional stability at all, instead some kind of chaotic water flow along the port side is breaking the flow away and the water flows around the front of the keel to “fill the void” and thus amplifies the turn.

3. Now, in the case where we induce a turn AGAINST the natural prop walk (in our case turning the bum to starboard with the rudder and some gentle reverse power) I think we create that breakaway turbulence more quickly and induce the tighter “chaotic” turn as a result. The wash from the prop in that case is contrary to the motion we introduced with the rudder and hence at a greater angle to the line of the keel and thus breaks away more readily.

Like I said, armchair level naval architect stuff, but I can really picture it when I consider that in reverse maybe only the front two or three feet of our keel are now “behind” the center of resistance and so only those two or three feet have a direction stabilising effect. Once the flow breaks away from the keel we lose the effect of those few feet and away we spin.

None of the above should be factored into your next boat design. Instead I recommend anchoring out whenever possible (no tight turns required in reverse) or cultivating a friendship with your marina manager so that you get first dibs on the most suitable marina berths. (We have a 150 foot runway available to get in and out of our berth. I’ve used at least 149 of those feet from time to time.)

[gs11gk]

If you just reverse with a neutral helm, in plenty of space, which way do you spin? My boat seems to spin counter clockwise. It doesn’t really spin but does circles.

My inner naval architect thinks that with short bursts of reverse, the prop wash is just acting on the starboard side of the hull, near the back, and the stern walks to port. However, the longer/stronger that burst of reverse, or if the reverse is constant, then maybe that cone of water expands from the prop and starts pushing the front of the hull to port as well. With the P35s modified keel, there isn’t much to resist that turning moment, creating the counterclockwise circle.

As I said, I need to practice these a bit more. I want to know if I really can reliably back and fill in either direction, maybe by giving harder/longer reverse thrusts when trying to pivot counterclockwise.


What confuses me I have read from multiple sources you can only back and fill in one direction. That’s not been my experience thus far.

[GILow]

Our boats are truly a mystery. Our Swanson seems totally random if we start of in reverse from stationary with a centered helm.

If it makes you feel any better, our new boat, a Kelly Peterson 44, seems unwilling to turn in ANY direction in reverse. I haven’t figured out how to reverse it into the pen yet.

[rslifkin]

Skeg and keel hung rudders often stall at a low angle in reverse, so steering can be a bit fussy and you won't be able to turn tightly. And you may not have enough rudder authority to overcome prop walk either. Start with the rudder centered and add some slowly when you're moving backwards and see if you get some steering. You'll likely find a point where too much rudder causes a rudder stall and loss of steering.

As far as back and fill, you can do it in either direction, but using it to spin the boat in place is always more effective in the direction where prop walk in reverse is helping to spin you in the desired direction (vs fighting you). If you're moving backwards and doing a back and fill just to adjust stern position and help you steer, it works fine in either direction.

Some boats also get pushed enough in the bow by a little bit of wind, etc. that the behavior isn't perfectly consistent in reverse unless there's truly no wind or current.

I generally find that accelerating slowly in reverse gives better results than the "gun it and go to neutral" method. Put it in reverse idle, wait for the boat to start moving. Then gradually add throttle. Too much throttle at low speed leads to a worse proportion of prop walk vs reverse thrust and makes it harder to get the boat to go where you want. Basically you don't want to get the prop spinning fast (lots of slip and possibly cavitation) and wait for the boat to catch up, you want to keep prop slip to a minimum while accelerating. Keep in mind that most props don't work as well in reverse as they do in forward.

[Smooth Move]

I would consider installing a bow thruster . Depending on luck with docking not a good solution . When in a narrow fairway I find helpful though a different hull design.

[Ryban]

If it makes you feel any better about the reverse performance of your boat, my Albin Vega also had a cutaway fore-foot, and the prop was mounted AFT of the rudder (the rudder hung off the keel fwd of the prop). That means I wasn't even able to back and fill efficiently as the boat needed to be moving to have any kind of steerage, since the prop wash didn't travel over the rudder.

I also noticed in reverse the propwalk wasn't very noticable, and the bow almost always fell off to whichver way the wind was blowing.

In our Westsail 32, with a barndoor aft of the rudder and no cutways in the keel, I notice the propwalk has a bit more utility, and more importantly we can back and fill.

[pcmm]

Have you tried hard bursts of reverse, then back to neutral in order to steer?

Idle in reverse can produce a lot of propwalk, I noted that you mentioned in your initial post that you didn't try to go into neutral. My boat is pretty bad in reverse. need hard bursts of reverse then neutral, and sometimes it gently steers in the direction I want

[rslifkin]

Originally Posted by pcmm

Have you tried hard bursts of reverse, then back to neutral in order to steer?

Idle in reverse can produce a lot of propwalk, I noted that you mentioned in your initial post that you didn't try to go into neutral. My boat is pretty bad in reverse. need hard bursts of reverse then neutral, and sometimes it gently steers in the direction I want

This works on some boats (particularly those that steer better once you take it out of gear). But depending on the prop, the hard burst of reverse can sometimes walk the stern so far sideways that you have no hope of bringing it back even once you're moving enough to have steerage.

Generally I prefer to get the boat moving gently, then once it's moving fast enough in reverse reduce to idle or neutral (depending on how far you need to back). So a similar concept, but with gentler acceleration. Basically it's a balance of not taking too long to get enough speed for steerage in reverse, but trying to find the balance of what acceleration rate generates the best balance of reverse acceleration vs propwalk.

[TrentePieds]

gs11gk:

Here is a clip of how what Americans call a "back and fill" is done:

https://www.youtube.com/watch?v=jBOJ...nnel=SveinHaDD

Vessels such as the one shown here had/have underwater profiles not unlike that of the P35 - without the centreboard, of course. In fishing harbours with restricted space and lots of vessels "in", this maneuver was absolutely essential, and boys were taught early how to do it.

I do one of these - sometimes two just for the sheer pleasure of it - every time I come back to my berth. It's all technique. It's all in how you hold your tongue. I call it "doing a pirouette" :-)

One of the subtleties often missed is that deflecting the rudder too far "stalls" it and kills the effect you wish it to have. What is "too far"? It varies from boat to boat, but in your boat try deflecting the rudder 10º when you are lying dead in the water with lots of clear water around you. Then lay the shifter to "Ahead" and when you are sure the tranny has really shifted solidly, come to high revs, like maximum revs, all in one swell foop. Count five "elephants" and bring the revs back to idle. Now take a deep breath and note two things: 1) how many degrees has your heading changed? And 2) how many feet (boat lengths) have you move ahead? Note those things! Remember those things!

If you pay attention, this establishes a datum that will permit you to build confidence in the maneuver. Every time you do it, the boat will react the same: So many degrees change of heading, so many feet motion ahead. Now those things will be "known quantities" rather than "random events".

The trick here is that every time the propeller turning in the aperture throws a great gob of accelerated water at the deflected rudder, the forces at work will be the same. Same deflection, same "weight" of water, same energy imparted.

It is a characteristic of the maneuver that the change in heading occurs BEFORE the boat accelerates! That only happens if you "goose" the engine. If you are tentative about it, the boat may begin to accelerate before it begins to slew, and that is obviously just the opposite of what you want.

Now go back and watch the video. Note that our Scowegian friend takes his time, and, in the nature of the particular ship, he really gets into shifting from ahead to astern and back again, as well as shifting the helm. But you see that the ship really doesn't move ahead or astern. That is due to the inertia of the heavy ship and despite the fact that she no doubt has a bejasus big reversing pitch prop.

Have a go at the exercise I set out above. Then we can come back and play with going astern.

All the best :-)!

TrentePieds

[rslifkin]

Originally Posted by TrentePieds

Now go back and watch the video. Note that our Scowegian friend takes his time, and, in the nature of the particular ship, he really gets into shifting from ahead to astern and back again, as well as shifting the helm. But you see that the ship really doesn't move ahead or astern. That is due to the inertia of the heavy ship and despite the fact that she no doubt has a bejasus big reversing pitch prop.

That goes to a common failure I see in back and fill maneuvers. Many people spend too much time playing with the rudder and wait too long to shift. Ideally it should be that as soon as the boat starts to move forward it's time to shift to reverse, as soon as the boat is stopped it's time to shift back to forward. And if the boat is never allowed to move backward during the maneuver, there's no reason to move the rudder.

For other maneuvering on sailboats with a small-ish prop, timing is critical for good turning when changing from reversing to moving forward. Typically when I'm just about ready to start going forward, I put the engine in forward idle, but still steering for reverse. Let the prop get spinning and start to move a little water in the forward direction as the boat slows down. After a second or so when you're ready to go forward, start moving the rudder and add a bit of throttle. Timed correctly you'll get very little time where the boat isn't steering in the desired direction. The timing is just more sensitive than it is for a powerboat, as a small sailboat prop isn't going to instantly reverse the water flow over the entire rudder when you put it in forward, so it takes some practice to coordinate the timing of shifting, throttle, and steering so you move the rudder quickly right at the point where the flow over it changes from reverse to forward.