Survey: For Sailors Who Have Converted from 2-Blade to 3-Blade Prop: PropWalk

[rardiH36]

Here goes:

For those that have actually converted boats from 2 blades to 3 blades (or know someone else that has converted) has prop walk in reverse been?:

a) very improved?
b) moderately improved?
c) somewhat improved, but not really worth the cost of the conversion?
d) couldn't detect a difference?

No need to respond if your boat always has had a 3-blade prop. But if you for some reason switched from 3 to 2 blades, was the prop walk worse after?

I acknowledge that I've asked this prop-walk in reverse question before, but it has been within already started threads on other but related topics (such how can to get more speed from an existing engine). Responses, although useful, have been from too few for me to feel sure that converting from two to three blades will be worth the effort/expense/improvement. So I thought to get a larger response pool with this posting’s title.

Reason for the question is that my boat's 2-blade prop walk is severe. Looking over the side when engaged in reverse and when the boat isn't moving yet, the prop wash is thrusting out sideways at even more than 45 degrees away from fore-aft axis. Great for back-fill pivot turns, but I can't reverse out of my slip without employing a difficult-to-execute slip spring/stern line knot, or an impossible to coordinate throw-off of the spring/stern lines, or first pushing the boat out to get a little backward movement so that the rudder has some influence, then stepping on board. (The push-out method works, but the boat is 13,800 lbs and its a high step up to get on the moving boat and then a scramble to the helm position). Other boats in my marina simply engage reverse and back out. They might drift a bit towards the side of the slip opposite the prop walk, but the majority of initial motion is backwards, after which they get steerage with the rudder. My slip is 3.4-4.0 feet wider than my beam, but engage reverse and my boat's aft port side kicks up against the left side of the slip (opposite the prop walk thrust) before (enough) any backward movement is established to get steerage. This whether throttle in reverse is slow, fast, or in intermittent bursts. I sail solo, and most often, my passengers aren’t nimble enough to help guide the boat out and then step on the moving boat safely. So for long-term safety and for convenience, I need a solution.

Thanks.

 

[Stu Jackson]

9 out of 1`0 times most people who "complain" about

prop walk do NOT use enough reverse throttle to get the boat moving and the rudder to engage - the rudder only works with water moving past it. I guess you may already know this, but it's sure high on my list of priorities when people ask about prop walk.

Our PO removed the 2 blade and installed a three, which I've always used, so can't answer your question directly. he advised he moved to a 3 because the 2 gave him too much prop walk, as you've suggested. As I recall, that's the whole reason to move to a 3, as discussed here many times.

Be interesting to see the responses to this one.

 

[RichH]

Tip clearance ....

Prop walk is due to many things; the probable chief factor is the distance of the tip of the prop to the hull and the shape of the blade at the tip.

Get the prop tip too close to the hull and you get excessive prop walk. Put on a prop with a more 'triangular' tip than a board 'flat' tip and you get less prop walk. Martec folding props are good at reducing prop walk in reverse ... plus dont 'drag' through the water when youre sailing. Tip clearance is just as important when sizing a prop as is pitch.

A better solution is to learn to live with prop walk and enjoy the benefits. Some tricks include: applying shorts 'bursts' of HIGH rpm with the rudder at a 45 degree angle 'away' from the direction of prop walk .... and then letting the boat 'coast' backwards. Most folks seem to avoid HIGH rpm in short bursts to steer a boat with a prop ... as it appears that they try to backup like they are driving a bus instead of a boat. Practice, practice backing up out in a channel so you learn how far the rudder has to be held over to get the boat to go straight in reverse, etc. Once mastered, prop walk is actually an advantage ... in docking, in doing a "u-turn" in a tight space, etc.

Go to a Watermans 'docking' contest (for single screw boats) and actually SEE the advantages of prop walk and how to use it effectively.

 

[rardiH36]

Stu ---

Stu:

Thanks. But yes, I have tried all reverse variations including initially with agressive throttle. Both sustained for a bit or short burst. Result; Unfortunately just causes the boat to kick up against the opposite slip side faster. Then it does go out but with the hull scraping along the side. The bumpers tend to ride up onto the dock and don't protect the hull much. Don't like doing that too often.

When I first got my (newly acquired) boat to my slip last October and discovered that getting out neatly was a problem, I took a 2-hour private maneuvering lesson from a well regarded sailing instructor (and long-term large boat owner) at the local sailing academy. He wasn't successful either at backing out without usually making some contact with the dock. We together employed several times slipping a stern line around a dock cleat to keep the boat centered until backward motion was established. Yeah, it worked, but even for him, coordinating each time the slip-off release single-handed was not predictable. So we adopted the push-out method as the best compromise.

Aside from the safety and convenience concerns, you can appreciate that I am quite skittish about taking my boat to unfamilar marina's and moorings. Under sail, the Cherubini 36 seems just great and a few friends with long term big boat ownership experience have commented that its very well-mannered nice to sail boat. Very pleased with the purchase, except for the back-up performance.

 

[sailaway78]

Prop Walk

Since I have never owned a boat with a two blade prop, I can't really comment on the difference. What I can comment on is technique. I have seen lots of people with the exact same problem as you have. The very best cure for this is to back the boat into the slip. If you back into the slip, you have more room in the fairway to gain steerage way before you run into something. Almost any boat, two blade or three, has the same initial tendency to go to the left, or right, before you build up steerage way up. Backing out of a slip with no place to gain steerage way is tough. Pulling out if you back in is easy. I have some friends with a H36, and he runs past his slip thirty yards or so before reversing. Gains some momentum and into the slip nice as can be.

 

[Ed Schenck]

Definitely better with 3-blade.

My experience with the H37C now includes four props. The original two-blade was so bad that I generally walked the boat back as far as practical and jumped over the lifelines and ran back to engage reverse. Followed that with a Flex-O-Fold 2-blade which was an improvement, the goose it and then neutral trick worked to some extent. Then the new engine with 3-blade Flex-O-Fold and the prop walk is almost unnoticeable. Then the 3-blade Michigan fixed prop after loosing the Flex-O-Fold in the Erie Canal. Back to some noticeable prop walk.

Having written all of that I would not change props for that reason except that yours seems excessive. It really could be a diameter problem, the tips are too close to the hull. That can also be caused by a shaft that is too short. The closer the prop to the strut the less distance between the tips and the hull. At any rate a Cherubini deserves a 3-blade Flex-O-Fold.

 

[rardiH36]

To RichH

Dear RichH:

Thanks for your well-worded and experienced reply.

Your comments do encourage me to practice again backing up in open water, and the next time I can get my still spry brother over to try again a few times to exit my slip under power -- with him on the dock to fend off the boat as needed.

In my response to Stu Jackson, above, I forgot to mention that putting the wheel to starboard to counter the effect of the prop walk was a feature included in my maneuvering lesson. But with the prop wash pushing sideways at more than 45 degrees off the starboard side and not enough reverse motion yet, the rudder couldn't compensate enough to keep us from first twisting the aft side up against the left side of my slip and the starboard bow up against the right side of the slip.

Situation remains that I think my boat is at the extreme bottom tier of "badly backing" boats. Improved skills are a must, but better handling performance of the boat is also necessary.

When returning into my slip, I keep enough speed to maintain steerage until my bow is in. Then I power-up in reverse and turn my wheel to the left to swing the aft starboard to counteract the reverse prop wash thrusting out sideways from my starboard side (pushing the aft to port). Nonetheless my port aft side is usually hard up against the left hand side of my slip by the time forward momentum is stopped. I'm lucky our marina has individual slips and that I don't have another boat on my port side.

Again thanks.

 

[Chris Burti]

Prop walk has less to do with the number of blades

...than with the pitch of the blades. All things being equal, a steeper pitch produces more speed and a flatter pitch produces more power. The more pitch to the blades, the more a prop operates like a paddle-wheel until the boat gets moving and as a result there is more initial lateral movement resulting from prop walk.

Generally, if one sizes the prop on a diesel powered displacement hull for maximum speed at maximum RPM on a given boat, the pitch will be greater on a two-bladed prop than on a three.

In theory at least, a properly pitched two bladed prop will exhibit more initial prop walk than a properly pitched three bladed prop.

In my experience, in practice, people "try on" props for size, so any practical experiences reported must be tempered by the fact that they may not have experimented with optimum pitched props or have run them for enough time to give you any meaningful data.

Expensive feathering props are often recommend where prop-walk is a problem because they are variable pitch and the pitch in reverse is designed to mitigate the effect.

Try this trick in the interim. Using as short dock line as will work, cleat it to your stern cleat on the off side (I'm guessing starboard) and pass it around the stern piling on that side to your winch. Only use one wrap on the winch. Snug it up enough to hold you off of the dock and then reverse. Once you start moving in reverse let it play out just enough to let you move while keeping you off the dock. As soon as you are moving and the rudder is keeping you off the dock, release the line and then gather it in from the cleat before going into forward. This requires far less coordination that rubbing your tummy and patting your head and about the same as chewing gum and walking. practice it a few times and it will stand you in good stead in strange harbors with wind and current issues as well.

 

[Joe]

Just a thought, since you're having so much trouble

get trolling motor!! ....drop it over the side and let it pull the boat out of the slip. You can turn the motor to compensate for prop walk. If you have the bucks, get a foot operated model.

 

[rardiH36]

Thanks and For Ed at the Bottom of this:

You are all writing great and helpful replies, so I can't not acknowledge your assistance. Thanks. (Sorry about the double negative, but it reads ok in this instance.)

Just I hope I don't cause not offense that I'm responding with a "yes ...but".

I'm heading to my boat this afternoon.* I will rev up the engine in reverse and see if I can take a photo to post that will capture the angle of reverse prop-walk wash.

* For Ed, I seem to recall you mentioned within one of your posts that stanchion repair is on your to-do list. So you might have interest that my current project is removing the stanchions to repair and stop the severe corrosion to the aluminum toe rail due to 28 years of stainless steel vs. aluminum corrosion. Four down, Four to go. Underneath one of the stanchions, after scraping out all of the built-up aluminum oxide corrosion residue, I found that only 25% of the aluminum still remains. This was the stanchion furthest aft where maybe more salt water is sprayed. And the deck is more level at this spot, so maybe the salt can't drain away. The toe rail under the other stanchions has been less severely 25%-50% eroded away.

 

[Ross]

Rardi, just one more note on this subject. My boat started out with a 13x7 two blade

prop on an Atomic 4. I repowered to a diesel with a 3:1 reduction gear. The prop simply wasn't enough so I bought a used 13x15 three blade. That was too muchbut I lived with it for a couple of years. My boat backed to the starboard with both of these props. This spring I had the 3 blade repitched and recomditioned. Now it is 13x13 and all the blades are the same. Prop walk is substantially reduced and for the first time I can back out of the pumpout station in one try. The distance there is about 100 feet and with a hard acceleration in reverse and the helm hard over I can throttle back to idle and have enough boat speed for the boat to respond to the rudder.

 

[Steve Dion]

Good prop guy in your area.

Rardi:

You have one of the best prop guys in the country located in your back yard.

Contact Kilian Props in Alameda and talk to them.

Kilian Propeller
2900 Main St.
Alameda, CA 94501
Phone: 510-614-0596

My understand is that lack of tip clearance is major culprit (as Rich H mentioned). Most of the time when they replace a two bladed prop with a 3 blade unit, they also decrease the diameter. Consequently the new prop has more tip clearance and less prop walk.

 

[Les]

Max prop results

rardi, et.al.,

I've owned a number of sailboats and have taught sailing on an equal number of boats with inboards. It is interesting that some boats have more prop walk then others.

My last boat was a Hunter 380 and it started with a two blade fixed prop and it had a lot of prop walk. For one reason or another I elected to put on a three blade Max prop and eliminated not only prop walk but any difficulty going in reverse. We also picked up some speed at the same RPMs and in my opinion we also seemed to drive through chop better. Going in reverse is the same as going forward except your rudder needs much more wheel control since it is now in front of the prop.

I was so impressed with that prop that we are now buying a new Hunter and I ordered a new 3 blade Max prop on it. I haven't got the boat yet, so can't tell you how it works but I suspect it will be like my old one.

Les

 

[RAD]

Interesting thread

Cause when I bought my 32 ODay in 2003 the boat was tough backing up with the prop walk to starboard and so I ordered a 3 blade Cambell sailor's prop and was quite impressed with its performance going foward and reverse untill last season after I replaced the broken tach and noticed the rpm's would not get over 1500 but the boat would go like a bat outta hell,hmm prop must be over pitched so I dig out the original 2 blade prop and have the boat hauled and now its happy @ 2500 rpm and I learned to back it up using lots of throttle and the rudder slightly over.
One of these days I'll have that three blade prop repitched

 

[rardiH36]

Photo 1 of My Prop Wash in Reverse.

Forum comments about narrow prop hull clearance and propensity for prop walk in reverse are probably on the mark. At the pre purchase survey, the surveyor did note that the clearance between the prop and the hull was much much less than typical. He commented that somewhere along the line a larger than manufacturer specified prop must have been fitted. Later I checked and the prop size is per Hunter's spec for the model. So the narrow clearance is in the design.

The attached photo is taken from the dock, transmission in reverse, about 2000 rpm showing on the tac.

My boat's centerline is parallel with the dock. As reference how far aft this picture was taken, the rudder in the water can be seen and the stern line is cleated only about 1 foot from the stern.

The bubbling prop wash is flowing right at the camera. More like 70-80 degrees off the fore-aft axis line. The prop probably still is throwing lots of water forward, but its the sideways thrust that's causing my difficulty.

See also picture 2 on my next post.

 

[rardiH36]

Photo 2 of My Prop Wash in Reverse

Center of the photo is area of maximum side thrust from the propeller in reverse. Note how far aft the area of side thrust extends. Direction of the water flow is almost straight towards the dock.

(P.s. -- Sorry that the photo captured my sorry looking shoe. Retired a few months ago active tennis duty and put out to pasture as the preferred foot wear for boat maintenance days.)

 

[Steve Dion]

Simple explaination but not a complete answer.

Here is another explaination. It is a simple explaination, but does not take all of the hydrodynamics into the problem.

The normal clearance of the blade tips to the hull is supposed to be at least 15% of the diameter of the prop.

This somewhat explains why many sailors "claim" that the three bladed prop has reduced their prop walk. Many times the prop diamater is reduced by 1" when going from a 2 blade to a 3 blade.

 

[Calif. Ted]

I've docked same model boats with

both props and my experience was more walk with a 3 blade. A two blade you can back to starboard (a little) at an idle, three blade the walk was stronger than rudder affect.

 

[MoonSailer]

Is your Prop Clean????

A few barnacles on a prop can destroy it's efficency but seems to have little effect on prop walk.

 

[rardiH36]

From Rardi to MoonSailor ... Yep the Prop is Clean ..

Strong reverse prop walk was from the day the boat went in water after my purchase. Not a crustacion (sp?) on it.

And its been cleaned a few times since launch.

Today I stopped to watch a boat in my marina back out. The reverse prop wash did exit the boat on the starboard side, but near the beam and the wash direction was streaming only at a slight angle from the centerline axis. My boat, the wash exits at the stearn and the current is almost straight out to the right .

regards,
Rardi