Posts: 3479
Location: Elk River, Minnesota | Hiya,
I'll do my best to answer this to the best of my knowledge.
Starting with your first main question, There are two basic things that will help to increase bow lift: Rake angle and cupping toward the tip of the prop. A good example of this would be to look at a ballistic prop which has a modified tip (like it has been cut off) then cupped. I think the reason why this happens is the tip cupping is forcing water inward to the prop fin enough to allow your trim to be more effective.
If memory serves me correctly, Rake angle helps to lift the entire boat both bow and stern since it encompasses the entire blade fin. Stern lift usually is helped with cupping on the trailing edge of the blade more toward the hub. When the prop is rotating and the fin is traveling downward as it compares to the boat hull, it will essentially push the stern up since the cupping adds a little resistance to the water flowing over it.
In terms of hole shot, you are correct...smaller vent holes and/or more fins/larger fins will help to curtail excess venting. With more fins, you gain surface area (more contact with water) to push the boat forward. The same holds true for a larger diameter prop...more blade surface, more thrust potential. The trade off is loss of top end on either larger diameter or more blades since the extra surface area also means more resistance in the water as well. That is why you see so many pontoon boats with 4 blade props, and in some cases big cruisers with inboard/outboard setups that have upwards of 5 blades to essentially a double-prop design with what looks like two props stacked one after the other.
With regards to venting the prop, they all do the same thing. Most over-hub designs are used on high speed applications where hole shot is still premium. When those are set up well, the boat jumps on plane and is off like a rocket. But...you would NOT want something like this with big boats that will be in any sort of wind where having to slow down is needed for safety reasons. The most cost-effective choice is a hub vented prop that has different sized plugs you can put in to set up the prop to the way you would like it. Pulling skiers, larger holes for that quick hole shot to get them up. Fishing walleyes in chop, smaller holes to be able to run the boat at midrange speeds without losing bite. It is definitely a fine line.
I hope this helps some, and my memory is at least somewhat on the mark about what is going on down there. Lots of physics taking place....
Steve |
