rigger 47 Posted August 5, 2010 Share Posted August 5, 2010 rich2 PostPosted: Thu Feb 12, 2009 7:48 am Post subject: folding Blade Prop We have probaly been over this before, but apart from kiwi props are there any others availble in the New Zealand market. May be a bit cheaper. 2 blade or 3 blade? ======================================================================== Murky PostPosted: Thu Feb 12, 2009 8:19 am Great topic! Bri-ski is one name that comes to mind although I have no first-hand experience either way. http://www.bri-skipropellers.co.nz/ ======================================================================== Apparition PostPosted: Thu Feb 12, 2009 8:40 am I have just been through this exercise and ended up with a Kiwi prop. It was the second cheapest option with the cheapest being a two blade folder without any teeth or mechanism to link the two blades which was basically the same as the one I was replacing. I'm very happy with the kiwi prop, having lots of thrust in reverse is great and no more problems with my prop not unfolding properly. I have used prop-speed on the kiwi prop and after 4 months its still clean as a whistle and working just like the day it went back in the water. ======================================================================== The big T PostPosted: Thu Feb 12, 2009 12:03 pm Have had a kiwiprop four nearly 5 years and love it (and my boat is at the top end size wise). Needs to be greased as per the manual, but apart from that is really good. Local back up is quick and not expensive. Apparition, I would suggest Propspeed is not warranted on a Kiwiprop - being a plastic/composite, it takes antifouling really well. ======================================================================== wheels PostPosted: Thu Feb 12, 2009 3:29 pm I also vote Kiwi prop. I wish they did one for my engine Hp. Which by the way, the largest they make will only handle 55Hp. They really are value for money. As stated above, the next cheaper is a really nasty version of a folding prop. To get anything better than a Kiwi prop, you are into some serious money. Auto prop would be one of the many choices to look at. When I looked at opne for my boat, I think the cost was somewhere around $7K. I decided to stick to the three blade'er I had. ======================================================================== idm28 PostPosted: Thu Feb 12, 2009 3:44 pm the 3 blade volvo penta is cheap and works well...really well ======================================================================== Knot Me PostPosted: Thu Feb 12, 2009 7:43 pm You guys leave your props in the water??? How quaint Wink I've heard many singing the praise of the kiwi prop. ======================================================================== Fish PostPosted: Thu Feb 12, 2009 10:26 pm We got a kiwi prop a year or two back, brilliant. 17in dia ona 37ft / 6T boat, 26hp. Use propspeed on it. Need to service them when you have the boat out but hay... bit of grease and I think we replaced a little o-ring thing last time. Fantastic bite in reverse, we are on pile moorings in the river, parking is a breeze now... ======================================================================== Fusion PostPosted: Fri Feb 13, 2009 8:45 am Kiwi Prop question. Can you create prop walk in reverse? My big ass 3 blade sailor dose 6kts at 600 prop RPM at 100 I can get Fusion to walk around on the spot, makes docking easy but only goes one way Very Happy ======================================================================== Apparition PostPosted: Fri Feb 13, 2009 8:53 am Yes, big time! More than a regular prop in my experience. ======================================================================== Jono PostPosted: Fri Feb 13, 2009 10:17 am I have a Volvo 3 blade on Revolution (1104). Very happy with it. 8kn flat out. 6.4kn cuising revs, 6kn into a 25kn SW no worries. They look silly but work well. I prefer 3 blade over 2 blade for playing launches in the rough stuff. ======================================================================== wheels PostPosted: Fri Feb 13, 2009 3:33 pm Quote: Kiwi Prop question. Can you create prop walk in reverse? Prop walk is not actually a factor caused by the prop itself. People often think Prop walk is to do with the pitch of the blade and it's resistance in the water, pulling the boat across. But that isn't the case at all. The blade exerts the same effort in the Horizontal in both directions, cancealling out any "prop walk" affect. Prop walk is actually to do with the angle of the prop shaft. The greater the angle, the more "walk". It is due to each blade taking a lightly different length of travel through the water between the top half of it's cycle and the bottom half of it's cycle. Because the prop blade is rotating at the same speed through the entire cycle, the result is actual water speed is greater over the blade at the bottom of it's cycle and slower at it's top cycle point. This results in a different amount of pressure on the blade at the lower cycle and thus a greater effort always in the direction of rotation. With a bit of practice, this 'walk" can be used to your benefit. But try to enter a berth with "walk" opposing you and it can get ugly real quick. Usually in proportion to how many are watching you berth of course. ======================================================================== rich2 PostPosted: Fri Feb 13, 2009 7:03 pm wheels wrote: Quote: Kiwi Prop question. Can you create prop walk in reverse? With a bit of practice, this 'walk" can be used to your benefit. But try to enter a berth with "walk" opposing you and it can get ugly real quick. Usually in proportion to how many are watching you berth of course. yup i have been here. and it was the owner on the finger DOH ======================================================================== Fish PostPosted: Fri Feb 13, 2009 7:50 pm Now I'm an engineer and I still don't get Wheel's explanation there... Always thought it was the blades at the bottom having a greater 'paddle' effect than the blades closer to the hull and the resistance from the hull (more hull, less 'paddle'. Not that I'm questioning Wheels, just can't handle angular momentum and circular velocities questions at beer o'clock on a friday.. But yes you can get prop walk with a kiwi prop. I say 'can' instead of 'do' because of the excellent bite in reverse, with the correct amount of (high) revs from the engine you can actually go backwards instead of sideways. With less stick from the engine you can generate more prop walk and less backwards and swing your arse around to your hearts content (hope yachtyacka doens't get overly excited by that...) Its not perfect but it is a lot better at backing up than a conventional three blade (lots of prop walk) and light years better than a folding two blade which thrashes about and normally doesn't actually go backward when you really need it to. Basically the blades flip around so you have the equivalent of a normal prop mounted backwards. ======================================================================== wheels PostPosted: Fri Feb 13, 2009 8:08 pm Quote: Now I'm an engineer and I still don't get Wheel's explanation there.. Yeah I don't blame you. It's a hard one to explain. I wish I could post a picture of it. I will have a dig around and see if I have such a picture. But anyway, if you picture in your mind the boat side (beam) on and the shaft/prop angled down. Lets take a blade starting the cycle at the 12oclock or upper most position. We will assume the prop has been turning and the boat is in a forward motion. Now as the boat moves forward, the blade as it travels downward is also moving forward through the water due not just because the entire boat thus the prop is moving forward, but because it's arc of travel means it is actually moving forward also. As the Blade now swings upward again, it is now moving slightly backward through the water. The boat still moving forward means the prop is also physically moving forward with it and the result is the arc through the water is actually slightly less than what is was on the downward leg. Hmmm, still not sure that explanation helps. Let me have a dig around for a picture. ======================================================================== wheels PostPosted: Fri Feb 13, 2009 8:44 pm The attachment is too big, so here is a link. http://www.castlemarine.co.uk/download/propwalk.pdf ======================================================================== Knot Me PostPosted: Sat Feb 14, 2009 7:07 am You get the same thing with aircraft props. When you 1st push the throttle in on takeoff you have to give many aircraft a hoof full of rudder to keep it going straight down the runway. ======================================================================== wheels PostPosted: Sat Feb 14, 2009 7:32 am That would be due more to engine/prop torque though KM??? You know, wanting to turn the plane over in the opposite direction to rotation? I know many found that out the hard way with Spitfires. Not used to the huge Hp and torque and the trainee Pilot could have the take off all come unglued if he wasn't aware and ready for it. Although I imagine you could experience "prop walk" when going through a Loop. The prop is in a state of continual arc and thus the plane would want to fly "sideways" as it pulls through the arc. Having never done that myself, I am asking this as a question. do you ever sense such a thing? or is it maybe something you don;t actually notice up there? ======================================================================== Knot Me PostPosted: Sat Feb 14, 2009 8:54 am Good point, as always, Mr Wheels. Still doing my Aerobatics rating so still at the stage of concentrating on not filling my undies and/or knot hitting the ground rather than the finer details like that. I'm at the 'it's fun but this upside down thing is still knot quite right' stage Shocked That's knot a smilie, that's what my eyes do look like when doing aeros at the moment. I'll ask one of the lads that already have the kevlar underwear Smile ======================================================================== Fish PostPosted: Sat Feb 14, 2009 11:54 am Thats a pretty good explanation Wheels, and I can understand the differences in relative speed of the blade tips going up on one side and down on the other when the boat is moving forward, and I think your correct that produces a difference in torque, but I don't think that explains the back of he boat buggering off sideways when manourvering at slow speeds. The way I've always looked at it, at dead slow speeds and trying to creep astern, stuff like that, there is no flow over the blades. Example, if the boat is stationary and you slip it into gear at low revs, the prop turns slowly but doens't have any forward (or backwards) motion through the water. So at low revs the boat always 'walks' more. When I was taught how not to crash the boat parking it, I was told to use aggressive throttle bursts to manuevor, so as to increase the flow of water over the blades and reduce prop walk while increasing forward (or backwards) motion. The logic being without water moving over the blades from forward motion, the blades act as a paddle until the water stars flowing over them (i.e. until the 'bite' and propel the boat). At the top of the prop the blades are closer to the hull so get less 'purchase' on the water (as the hull restricts the water flow a bit (via friction and all that exciting stuff) whereas at the bottom of the prop the blades are free to shift all available water, there is no friction / constraint from the hull so the prop exerts more force at teh bottom than at the top. thats why the boat will always move in the same direction as the blades at the top of the prop, if that makes sense. I think the aircraft prop thing is conservation of angular momentum. When the prop spines faster the plane will try and counteract the change in angular momentum by rotating the other way, hence the need for a bunch of flap when you put the hammer down. Must have been amusing watching those kids in the Spitfires put the hammer down and almost flip on the runway, guess the yahd to learn fast how not to sh*t themselves anyway back then... ======================================================================== wheels PostPosted: Sat Feb 14, 2009 4:03 pm Quote: but I don't think that explains the back of he boat buggering off sideways when manourvering at slow speeds. Arr good question. Here is another point many forget. The boat itself does not need to be moving. Remember the simple law of physics, every action has an equal and opposite reaction. That is what makes your boat move. The propeller moves water. The water moving one way is the thrust required to make the boat move the other way. More mass of water moving, more thrust and thus the faster your boat accelerates in the opposite direction. So before your boat can actually physically move, water has to move first. There is a slight delay in the movement of a Mass in a Fluid due to Inertia and viscosity. Thus the water moves first and shortly after, the Boat mass starts to move. So actually, the water is rushing past the prop well before the boat moves forward. And as the the blade speed through the water is actually very fast, Hull speed has very little affect on the prop. It is only in racing boats that water speed would start coming into play. But that is a different blade design completely. The distances of prop to hull have no affect at all on "prop walk". The issues there are compression of water between the blade and the hull. There must be a minimum distance between Hull and Blade tip so as not to produce a hammering affect. Remember water being a liquid can not be compressed. The plane prop situation is opposing Torque. It's just that the force can be counteracted by flaps. But a Chopper has to have the tail rotor to oppose the force. With out the tail rotor, the thing simply spins around and around. The affect of Torque can be seen in two ways. One is opposition to Acceleration of a Mass. It is seen in any revolving item when the revolutions are increased or decreased. Even the motor in your car wants to turn the car over if it could when you plant your foot on the go louder control. The engine mounts allow the motor to move a little, but the Mass of the car stops it from spinning the opposite direction. The other is opposition to the force required to make the what ever move. So for any propeller, whether it is air or water, the mass of the liquid or air it has to move is seen as an opposing force to it's ability to turn. ======================================================================== grant PostPosted: Mon Feb 16, 2009 6:27 am so a sail drive doesn't experience propwalk? anyone with one like to comment great topic BTW, just going to have to read it a couple of times... Smile ======================================================================== fng PostPosted: Mon Feb 16, 2009 7:46 am ours does ======================================================================== Apparition PostPosted: Mon Feb 16, 2009 8:02 am ... if that's the case why would I get MUCH more prop walk with a Kiwi prop than the old folding prop on the same shaft? ======================================================================== PaulR PostPosted: Mon Feb 16, 2009 8:41 am Kiwi props have a great BIG BITE when going astern. ======================================================================== Apparition PostPosted: Mon Feb 16, 2009 9:02 am http://www.kiwiprops.co.nz/prop_walk.html ======================================================================== Danaide PostPosted: Mon Feb 16, 2009 11:42 am As wheels alluded to with the Torque, you will usually get less walk with a sail drive as it is generally further from the rudder and the rudder is the key as it is all you have to counter the torque, but the trick that most don't get is you must keep your rudder centred until you are making way astern then with water passing over the rudder you will have steerage but until then turning the rudder reduces the resistance and increases the prop walk. ======================================================================== Ailys Comet PostPosted: Mon Feb 16, 2009 12:01 pm Sorry but I'm going to have to disagree with the explanations given for prop-walk. My far simpler understanding is that it simply arises due to increasing water pressure (and hence density) with depth. Hence the bottom of the propeller is passing through slightly greater pressure water than the top of the propellor and hence instead of the two cancelling out, the net effect is that the bottom of the propellor 'wins' by a small amount. Hence you get a slight walking effect - the direction being the same as if you imagine if the bottom of the propeller was actually touching the seabed. I have a saildrive and can confirm it exists for those too. So maybe shaft angle contributes to the problem but it cannot be the sole cause. ======================================================================== Fusion PostPosted: Mon Feb 16, 2009 12:21 pm I can generate Prop walk in forward and reverse. Although reverse is greater. ======================================================================== wheels PostPosted: Mon Feb 16, 2009 3:18 pm Quote: increasing water pressure (and hence density) with depth. 10m is 1Atmosphere or 14.75lbs/1"sq. The distance from tip up to tip down of the prop is not going to be enough to give much of a pressure increase. ======================================================================== island time PostPosted: Mon Feb 16, 2009 3:47 pm Interesting discussion re prop walk. Island Time had virtually none with a 2 blade folder - it is now noticeable with a kiwiprop. On a saildrive. AC - pressure of water changes with depth, but I don't think density does - isn't water incompressable? I'd always thought that it was caused by a percentage of the water (shifted by the propeller) being moved aside (tip vortexes etc) before the vessels intertia was overcome and the vessel started to move. Whatever the exact science that makes it happen, it does happen, and it is more with a kiwiprop than a folder! Also, my kiwiprop sometimes stalls the engine if engaged at idle in reverse - the instant full pitch in reverse is to much for the engine's available torque. Otherwise it's all good. Very Happy ======================================================================== Bogan PostPosted: Mon Feb 16, 2009 7:16 pm I'm not surprised that a two blade folder doesn't prop walk. You rarely use them at low speeds when you're not moving, because when in reverse they're: 1) near useless at low revs with the boat going forwards 2) marginally better at full revs with the boat going forwards 3) work OK at low revs once you're moving backwards When used in reverse the inertia of the blades balances against the force on the water flowing over them to set the blade unfolding angle. On one side: blade inertia (which is proportional to the square of the shaft speed) multiplied by cosine of the blade angle (increasing from zero as it swings out from the shaft). On the other: the force of water on the blade multiplied by the sine of the blade angle (increasing from zero as it swings out from the shaft). The water force on the blade includes a first component which is proportional to the shaft speed and a second component which is proportional to boat speed (forwards is positive). In other words: increasing engine speed tends to a higher angle (but never fully open) forward boat speed tends to a lower angle, backward boat speed tends to help with a higher angle. Question: If each blade hinges seperately they could follow an individual profile as they go around, which would probably have some effect I can;t fgure out. Does a two blade folder where the blades are geared together to hinge out together prop walk more than a prop where the blades are independent? ======================================================================== Ailys Comet PostPosted: Mon Feb 16, 2009 7:59 pm island time wrote: AC - pressure of water changes with depth, but I don't think density does - isn't water incompressable? Yes you are right, like nearly all liquids water is virtually incompressible. ======================================================================== rigger PostPosted: Mon Feb 16, 2009 8:47 pm Post subject: Reply with quote another description of propwalk or transverse thrust- Transverse thrust is the tendency for a forward or astern running propeller to move the stern to starboard or port.Transverse thrust is caused by interaction between the hull, propeller and rudder. The effect of transverse thrust is a slight tendency for the bow to swing to port on a ship with a right-handed propeller turning ahead. Transverse thrust is more pronounced when propellers are moving astern. When moving astern, transverse thrust is caused by water passing through the astern-moving propeller creating high pressure on the starboard quarter of the hull, which produces a force that pushes the ship’s stern to port. Rudder angle can influence the magnitude of this force. ======================================================================== rich2 PostPosted: Tue Feb 17, 2009 3:40 pm i think i get it now..... i know on one of the boats i sail we have a massive fixed 3 blade Prop. the wash on this really helps us put it into the finger. I find one of the hardest things to teach New Boaties is that the boat steers from the stern(back) i wonder the best way to explain Prop wash to a novice. I guess also how do you let them know how to use it or not. ======================================================================== PaulR PostPosted: Tue Feb 17, 2009 8:37 pm Another major effect of Kiwiprops is the fact their pitch changes. Fixed props and most two bladed folding props have a fixed shape (pitch etc) going forward or astern. The Kiwiprop changes its pitch when going astern and has a LOT OF POWER gained by having a coarser pitch. ======================================================================== Link to post Share on other sites
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