Originally Posted by Bigmuz7
.... but I dont understand the importance of what it means ? and how it is relevant ?,
Ah well, it's not really slip at all or, at least, it shouldn't be, but it might be.... Your prop blades fly in the water just like a wing of a plane in air but instead of travelling in a straight line they are attached to the hub. For there to be useful lift*, the blade will need to be at an angle of attack to the water. For some reason it always appears easier to understand when considering the wing of a plane so I'll stick to that. Imagine a plane landing; to generate sufficient lift the flaps will be activated and the plane will be flying nose up to increase the angle of attack of the wing. This is kinda like your prop at slow speed. When a plane is flying fast, the angle of attack will be much less, about 3-4° would be good. So now imagine your prop; the blades are angled so when you start to move the angle (of attack) is steep because you're hardly moving though the water so you could describe this as high slip. When you are travelling fast, the water is approching the blades much more from the front so there is less slip. Therefore, the slip (angle of attack) varies with boat speed and load and it is plainly also going to differ prop to prop depending on the prop's blade section and to some extent on the blade profile.
* Some folk reckon there will be no lift without an angle of attack but I reckon there will because the blade (wing) section will generate lift by dint of it's shape. It might not be much lift but lift is lift.