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With propellers there is to consider diameter and pitch but the term "RAKE" is somewhat of an unknown to me. Any discussion available on rake when associated with propellers? What is it, other than something to push around the yard and get a sore back? How is performance effected from different degrees of rake? How is it measured?
it has ben my experience that a high rake lifts the nose of your boat and low rake lifts the transomQuote:
Originally Posted by david bryan
Agreed, but why?
Jeff
mine is not to reason why. But is to due or dieQuote:
Originally Posted by Fastjeff57
As it was explained to me years ago when the prop is at its low point the trailing edge want's to be vertical and forcing the bow up. On my old inboard hydro we tried a prop with about 2deg rake, barely got on plane and when it did just plowed. best prop was around 18deg rake planed quick and flew the nose so I had to ride it like a outboard. A low rake prop already being close to vert. is just trying to climb out of the water.
In summary it is related to the shape of the thrust cone of the water leaving the blades. More rake has a smaller concentrated thrust cone that can be directed for bow lift (think narrow ice cream cone shape). Less rake wider thrust cone (think paper paint strainer funnel) thus less concentrated thrust vector with greater spread force up on the rear bottom. Assume for simplicity that the prop shaft angle relative to the bottom remains the same for high and low rake.Quote:
Originally Posted by Fastjeff57
Details here
http://continuouswave.com/whaler/reference/prop3.html
More on rake
http://www.rundquist.com/how_rakes.htm
Excellent explanations! Thanks.
Jeff
So what I'm reading is that high rake lifts the bow. If trim is used on the motor to trim up or down is rake really something to consider when selecting a prop? Would a cleaver prop be classed as low rake and is why they are used on tunnel type boats and the chopper prop is for the vee bottoms? Maybe I'm still in left field on this.
Ill be glad to help on this. Props are my thing.Quote:
Originally Posted by A Mosny
Now, about high rake...It is basically the leaning of the blades from the hub rearward toward the aft at the blade tips. The reason for this is to accommodate a bigger grab of water while attaining the need of a smaller diameter for high rpm performance (racing). If you see a high rake prop, you will notice that for its respective diameter, it has a longer leading edge on the blades with a smaller trailing edge to grab a lot and stream it down, and having a smaller diameter can wind up. A low rake of the same diameter would have less leading edge slightly longer or equal to the length of the trailing edge in some cases, of which is for recreational (general purpose use). That is all it really is. A cleaver prop is simply referring to its shape as having a straight trailing edge. A round ear has rounded edges of different varieties. The term "chopper prop" is simply a nickname given to props used for racing that are partially or half way out of water thus chopping the water to reduce unnecessary friction for free horsepower at high speed (not for a regular boat though). A chopper can be round or cleaver. The choice of a rounded or cleaver is just a matter of personal preference. One isn't necessarily superior over the other. Both kinds are used on High performance and/or racing V-bottom, flat bottom, or hydros of different types. I Hope this helps.
One thing about cleavers is that they are usually high rake. The straight edge helps to accommodate this configuration. However, rounded props are also often made as high rake, as a racing application, simply having a rounded tip instead of a sharp pointed blade tip like a cleaver though they may be a bit more limited in how much rake they can have by design. Trim has nothing much to do with it. If you need it for one, youll need it for the other either way.Quote:
Originally Posted by champ20B
I'm not any kind of expert on this but I might can ease some confusion ( or make it worse, no promises ).
As far as low rake lifting the stern and high rake lifting the bow, that is only partly correct. A cleaver will lift the tail end or the stern of the boat due to design. Now, a chopper or a high rake prop doesn't actually lift the bow. It does the opposite of lifting, which is actually pulling the stern into the water. The prop actually digs the hull into the water. As speed increases though, the stern tries to dig into the water. The more you speed up the water resists the stern from digging in the water and it picks the boat out of the water. Since is is still trying to dig the stern down, the bow gets "pulled up". It makes sense to me, so I hope that isn't too confusing or worded bad.
As far as trimming the motor, if you think of it as a force, you will get the most efficiency pushing something if the force is parallel to the direction. If you trim up, or down beyond parallel of your traveling direction you are wasting some of your efficiency and will run slower. A parallel trimmed motor (with a slight adjustment up/down sometimes) will be fastest and most efficient. That's why they make props with different rake.
I hope that helps some.
next question then from props 101
why does the steering pull one way trimmed in and the other trimmed out?
Holy cow! Can't wait to hear this one "explained"!
Jeff
The prop thrust force (vector for the math folks) is generally not in the same direction as the centerline of the prop shaft. The direction of that force vector is in 3 D space and its direction is dependent on forces generated on the prop of: water thrust, lift and drag that are related to prop depth, prop shaft angle (trim), prop blade design (pitch, rake, shape, diameter), lower unit shape and other factors. When you trim the prop shaft you change the direction of the prop thrust force and a net change in the other forces results in the boat to veer off course. Trimmed in (depending of prop rotation) the net thrust force is say left of the prop shaft centerline (and will pull right) and trimmed out the net thrust force is the opposite direction (to pull left). The difference can be small or large depending on many variables (some noted above). Ideally the net thrust force needs to be close in direction to the prop shaft centerline, not easy to do. For a more complete detailed description of the forces I can provide the interested with some tech articles that I used back in the days when working at a marine research facility from the "Naval Engineers Journal" and "The Society of Naval Architects and Marine Engineers" on the work they did on surface piercing semi submersible props.Quote:
Originally Posted by Powerabout
sure but whats changing?Quote:
Originally Posted by zul8tr
( easy to explain with a prop in your hand)
Lets say you are running a right hand rotation prop (clockwise). Typically due to the water column, the blades when at the 6:00 will push water harder since the water pressure is greater on the blades. The blades more towards the 12:00 position experiences less water column pressure. Since the blades on the bottom seem to "push more water" than the top, they are the ones doing more work and makes the prop want to operate like a paddle wheel and walk to the right. This makes the motor pull to the right. To counter that most motors have a trim tab anode below the cav. plate that acts like a little rudder to balance out the pulling force. Some lower units like Mercury torquemaster gearcases even have some angle to the skeg to help with this. As you trim the motor up or down, you change how the trim tab works in the water, and depending on the depth of the prop, it pushes water differently because of the water column pressure.
Powerbout, the idea of a wheeling propeller sounds a bit doubtful to me though I have heard this before. People who run props partially out of water don't often complain of this and if any wheeling occurs, it would from that if anything. Often times the torque of an engine under full power causes this, particularly if your steering is a bit light duty for the motor your running. Now, if it does it either way trimmed in or out, you might want to check the mounting of your outboard. If it is a bit loose or moves (pivoting) when you turn or change the kick out, the problems you have could occur and it doesn't take much at all. Under this circumstance, the force of water could be working against the misaligned skeg making it want to pull right or left.Quote:
Originally Posted by Powerabout
well yes and no, the paddle wheel effect will always try to make a rh turn right and vice versa.
Ask a tunnel boat racer, thats why F1 have power steering.
We still need to find out how a RH/LH prop can pull right and left when changing the trim and will do it whether fully submerged or surfacing
( trimmed in means under not just level)
I know about trim....but if everything such as how well the motor is presently fastened to the boat, so it isn't tilting side to side or anything like that, and the general performance is on the level......It may just be something that you have to live with as long as you can control it regardless and its not too much. Sometimes a boat just has a kwerk that cant really be worked out unless you upgrade something else. Driving F1 boats are much about power Tilt and Trim operation throughout a turn, entering and exiting just right. They need power steering to reduce distraction of these operations, I'm sure. In fact, power steering might be a good idea for your boat.Quote:
Originally Posted by Powerabout
Thinkng about it though, I never really got the relation of these boats to F1 race cars, if that is what they named these boats for. Hydros and tunnel boats remind me more of sprint cars than anything. Wave runners are like motorcycles. River racers are like Baja or buggy racing.......The only boat I ever saw that really reminded me of a true Lemans Gran-Prix car was the kneel-down open cockpit runabouts. In fact, the Top'o'Lake Michigan marathon is very much like the old Gran-Prix race in Europe like in the old movies, except it is boats (classic roll-up runabouts). Oh well.
may be weight distribution has something to do with it,more weight on the left and it will pull left more weight on the right and it will pull right????
on some of my old tinny's I used to run this would happen especially when trimmed in.
no its got to do with relative pitch of each blade in relation to the water flow...almost given the answer away