04-26-2018, 07:57 AM
To continue, the owner said it would "thump" progressively increasing with RPM, which he took as a bad shaft or alignment thing. Now picture a big, three bladed prop (yeah, he'll change down to two eventually), digging hard on some thick water (about 3' below the surface). The blades pick up a pile of water, likely in the neighborhood of a quart or more, per blade and starts to shove it rotationally. This is normal and expected on a boat of this size, but now picture trying to shove this quart of pressured water, through that 1.5" gap at the bottom of the blade aperture. Yeah, it's going to slam hard against the vertical portions of the skeg and rudder, 3 times per revolution.
I see this more often on engine retrofits, but the prop aperture is too small, not well shaped and the prop is too close to the rudder blade too. I can't carve much off the leading edge of the blade, but I can "relieve" quite a bit of material below the prop on the skeg and also taper the entry into the prop. This will eliminate (I hope) the thumping and possibly free up a few engine RPM too.
He has lots of other stuff to do, before we splash her next month for sea trials, but I'm betting I nailed on the thumping thing.
I see this more often on engine retrofits, but the prop aperture is too small, not well shaped and the prop is too close to the rudder blade too. I can't carve much off the leading edge of the blade, but I can "relieve" quite a bit of material below the prop on the skeg and also taper the entry into the prop. This will eliminate (I hope) the thumping and possibly free up a few engine RPM too.
He has lots of other stuff to do, before we splash her next month for sea trials, but I'm betting I nailed on the thumping thing.