Which prop

[CARL]

Hi to all, again after reading to much (i think) about cp,feathering props i am hopping for some advice as to which make has pleased or disspleased .I am hopping to help if not cure my yachts tendecy to turn which ever way it chooses in astern regardless of angel of rudder.The extra sail speed is appealing ,though i would still like to motor at the same , the boat an off shore 47 ,85 hp perkins with a large 3 blade ,any comments from personal experiance much appreciated . regards Carl.

[Weyalan]

It seems to be the general wisdom that 3-blade feathering props are the best compromise between performance under motor & performance under sail. With the newer deisgns of feathering prop, you can set the pitch of the blades in reverse separately from the pitch in forward, which seems to result in better reverse performance and slightly less "prop walk".

[Alan Wheeler]

Hi Carl. OK lets look at this in a little more detail. Question, So you have a large fixed three blade prop on there now??
A boat will not easily steer in a direction when going astern. There must be sufficient water speed over the rudder before the rudder has an affect. When going forward, the prop wash is supplying most of that water volume and speed to allow the boat to turn. When going astern, it is only the boat speed flowing over the rudder that is affecting the turning ability. Add to that the fact that the rudder is not shaped to go that way correctly and add again the fact that some rudders are of a stilleto design with little surface area, thus you have difficulting making the boat turn in reverse. No Prop will solve that problem for you. The only thing a prop changes is the ability of stopping forward motion and turning that into reverse motion.
Once the boat has reached such a speed that reverse steering control has been gained, great care is required. There is tremendouse force being applied incorrectly to your rudder. A large area rudder can damage steering gear if you are not careful. So the best practice is place the rudder in the direction you want to go when in forward. Use the reverse to go in a straight line and use the forward gear to then turn your bow. allow boat to get into the line you want and apply reverse. Place into forward and allow the bow to swing and imediatly place in reverse again. It takes a little practice, but once you master it, you can swing a boat on a dime so to speak.
It is one of those manouverse you go out somewhere away from other boaters and practice.

[CARL]

Wheels, thanks for your sugestion, i understand exactly what you are saying ,and beleive me have tried .Iused to be a skipper on a vivier crab boat in English channel,and could relatively easily manover these boats in confined spaces, however this yacht does seem to have a mind of its own in astern. I have checked for rudder play ,have tried increasing speed astern, but the way it swings is so quickely and there is no correcting . In the offshore encyclopedia Steve Dashew does talk about this problem with 3 bladed props on some boats and says you have to be very skillfull, which it seems im not or extremely lucky, which also seems to be avoiding me,He then goes on to say ,that by using a feathering prop a lot of the problems with going astern are aleviated .I was just currious how other people got on after changing props and which make they had used,regards Carl.

[KaptainKen]

Carl;
I have a 3 blade Max-Prop which replaced my OEM 2 blade fixed prop. I did this 'cause I planned to motor 950 miles down the ICW in the fall and sail offshore north in the early summer (when the weather is more stable and predictable). I over pitched mine slightly at 17" x 18". (the old fixed one was 17" x 17").

Works like a champ! I've used it four round trips up and down the ICW. I putter along at about 5 knots at 1800 RPM and burn about 8/10ths of a gallon an hour in my Perkins 4-108 powered Endeavour 37. It has great reverse thrust too. In reverse the blades "flop" over to present the correct foil to the direction we want to go. Yes ... it still has prop walk in reverse, but the increased thrust in reverse gets us moving backwards, faster, sooner, which then provides water flow over the rudder. As soon as the boat is moving (in reverse) I reduce RPM's and steer. As the boat looses sternway, I increase RPM's and repeat as often as required to back out of a slip or dock or whatever.

It feathers like it's supposed to if, after getting the sails up and driving the boat, I put the transmission in reverse after shutting down the engine.

Get a 7mm "Zerk" fitting and one of those old fashioned pump type grease guns. Fill it with "Lubriplate" and use it to pump out the old grease (and water that has seeped in) every time you haul out.

There was a interesting discussion on this subject a couple days ago on "SailboatOwners.com Message board". Follow this link: http://www.cs-bb.com/forums/CSBB/index.cgi?read=60668

[Alan Wheeler]

OK cool, good to know the experiance I am dealing with. So a quick question, I am assuming from your original description that the stern is going uncontrollably in either direction, correct?? As you would know, prop walk will be one direction only, so I am a little perplexed by that. It may help further to know the hull, is this full keel and a rudder hung directly of the Keel, or is it a seperate keel and rudder with rudder hanging back some distance from prop?? or????
I am not going to pretend I am an expert on the variuose hull handling quirks, so experianced ones, please pipe in.

[KaptainKen]

Alan;

From a dead stop stern of my boat goes to the left when I begin reversing. Interestingly enough, the bow swings right. I usually have to ask a crewperson to fend off with a boat hook from the bow.

Endevours have a 3/4 length shallow draft keel, with a cutaway forefoot. This is probably what causes the bow to swing right. The rudder is not attached to the trailing edge of the keel. There is a plan drawing at http://www.endeavourowners.com/boats/e37/e37sloop.html

I suppose you can use this sketch to compare with your hull / keel plan.

KaptainKen

[Alan Wheeler]

Right, thanks Ken. OK this looks like classic prop walk. Sadly, there is little you can do about it, bar just learning the boats behavour and living with it.
It's a little complicated and I will try and keep this simple. Prop walk actually has nothing to do with the number of blades on the prop. the correct term for this problem is called P-effect or Aysmmetric thrust being the more correct term. It is to do with a combination of three aspects. The Downward angle of the drive shaft and the Diameter/pitch of the prop. Changing angle is not usually possible, but changing pitch and diameter is of course. This is often why a new prop changes the way a boat "walks". But if the daimeter and pitch are correct for you boat/motor, then changing the prop may not benifit you at all. In fact, it is just as possible that it could make the "walk" worse. I would be sad to hear you spending a great deal of mony on a new prop only to have no change or an even worse outcome.
If you want to solve other issues with a feathering prop, such as better pitch and load on the motor and maybe better sailing speed, then by all means, it's worth a shot and hopfully the walk problem may improve. But I am worried about trying to solve a walk problem only.

[Dunkers]

On my Westerly 33 the stern kicks to port when the engine is put astern. So when reversing, say out of a marina berth and wanting the stern to to to starboard I put the engine into astern with plenty of revs and, as soon as the boat begins to move backwards put the engine into neutral. Then I can steer astern without too much difficulty. In a fairly confined space I may have to do a three point turn manoeuvre but it does work.

My previous boat was an Elizabethan 29 with a long keel and she was dreadful when going astern and you never knew which way she would turn - making for some very embarrassing moments with lots of revs and exhaust smoke with the accompanying dash about the deck fending off! She would always swing her bows down wind in weather cock fashion and refuse to do much else. Chosing the right berth was always difficult and even if the perfect one was found to effect an orderly exit Sod's Law would prevail and the wind would shift. Whenever possible I would walk her aft and turn the bows by hand and get going as quickly as I could before the wind took charge of the bow.

[Alan Wheeler]

Yep that is the best way to manoeuvre. Plenty of reverse RPM and then let it coast. Many are afraid of gunning the engine, but it is the best principle. It gets the max amount possible of prop wash over the rudder blade, which is reducing the "walking' effect and then you stop the prop turning which is removing the "walking" effect and allowing the rudder to aid direction. The forward gear can help with bow placement and checking speed, but ultimatly, wind on the bow is difficult to controll and you just have to use fenders and someone up on the bow if you think you are going to swipe another boat of jetty finger.

Going back to my other statment of what causes Prop walk, there is a lot of mis-information out there on what causes it and some of that info is coming from soem rather well repected names in the industry. But it is one of those subjects that is much like "how a wing/sail works" that has long scince been filled with wrong information. Not that those respected people are stupid or anything (poor choice of word) but that it is what we all believed and has been parrated as to being the way for years. Now we have the "Pluto" knowledge and like "how a wing works" the concept of prop walk needs to be updated.

[CARL]

Hi Wheels, for me all responses are interesting also to see alot of people struggle also, The keel on the OS 47 is a fin and best way to describe ,is shapped like a shark fin, the rudder is skeg hung though not verticaly but raked back ,quite sharply, the prop comes straight out of yhe back of the fin, and distance between prop and rudder must be at least 6ft .

[Alan Wheeler]

That is probably the worst Carl, as the prop wash is not easily flowing over the rudder even in forw3ard, let alone in reverse. Jeff H would be best to comment on this (you reading Jeff?? jump in)