Prop talk

[A Mosny]

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?

[david bryan]

Attachment 57163

[david bryan]

Attachment 57163

it has ben my experience that a high rake lifts the nose of your boat and low rake lifts the transom

[Fastjeff57]

Agreed, but why?

Jeff

[david bryan]

Agreed, but why?

Jeff

mine is not to reason why. But is to due or die

[Stuart]

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.

[zul8tr]

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

[Fastjeff57]

Excellent explanations! Thanks.

Jeff