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Thread: Prop talk

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    Question Prop talk

    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?

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    David Bryan

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    Quote Originally Posted by david bryan View Post
    it has ben my experience that a high rake lifts the nose of your boat and low rake lifts the transom
    David Bryan

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    Agreed, but why?

    Jeff

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    Quote Originally Posted by Fastjeff57 View Post
    Agreed, but why?

    Jeff
    mine is not to reason why. But is to due or die
    David Bryan

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    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.

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    Quote Originally Posted by Fastjeff57 View Post
    Agreed, but why?

    Jeff
    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.

    Details here

    http://continuouswave.com/whaler/reference/prop3.html

    More on rake

    http://www.rundquist.com/how_rakes.htm
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    Excellent explanations! Thanks.

    Jeff

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    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.

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    Default about props

    Quote Originally Posted by A Mosny View Post
    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.

    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.

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