Propeller Issue

[Guest625101138]

Originally Posted by navalarchitect

....

Actually, the Owner wants 20knots speed. But, I'm only getting 19 knots for 810HP (2 engines 405 HP each) using 55% overall efficiency.

(Sorry, for my previous post for saying 11.5 knots, I only used 405 HP in my calcs, that's why).

If the props are absorbing 405HP at 20kts then you should be able to get substantially better than 55% efficiency with either 0.6 or 0.8m props. I get open water efficiency of 75% for the 0.8m prop.

I have not done the drag for 20kts as I would need better idea of the detail around the stern to get a good number.

My earlier power figure for 11.5kts would be a bit pessimistic because of the transom drag with the extra draft I needed to compensate for the lack of beam aft. But your numbers are within 2% of mine anyhow.

Getting back to your original question - It is a matter of keeping air out of the prop. I think Atkins designs are best known for their shallow draft and it seems this is what you are trying to achieve. Have a look around for Atkins tunnel hull or box keel. He produced designs with the prop tunnel above the waterline but was careful to seal off air. As soon as prop begins to push water it fills the air void. So it is possible to have the blades above the water level at rest in this design. Also note that bigger diameter props usually equate to higher efficiency so there is an advantage if the flow is to the prop is not too disturbed.

I could only find a small example here:
http://www.atkinboatplans.com/
Take a look at the Nanuk.

Atkins tunnel hulls are discussed on this forum. Your proposal is much bigger than I have seen elsewhere here. The Atkins hulls are very nice. You could do worse than developing along this concept.

I did the linked shape for a much smaller boat based on the Atkins design. It is a single prop but it gives you an idea how the transom overhang shrouds the prop.
http://boatdesign.net/forums/attachm...8&d=1199160374

Rick W.

[Jango]

Listing indicated 11.5 kts with 810 HP, I assumed Full Displacement Hull. Using "PropCalc" for a Full Displacement Hull, provides 9.9 Kts with 810 HP and 38 tons. I guess one shouldn't ass u me.

Jango