Jet Inlet Design

Hello, I am in the preliminary design stage for a small power hydrofoil catamaran, something a bit bigger than Chris Maas' hydrofoil. I want more range than is possible with electric, and the vessel size is probably too small for an inboard with running gear. I could go the same route of customizing the midsection of an outboard, but I wanted to investigate using a jet pump instead. It seems that high power to weight ratio PWC engine and pump packages are readily available.

My question is: Is it feasible to integrate the inlet of a jet pump with the vertical strut of a hydrofoil?

What should the geometry look like?

1. horizontal inlet, above the top surface of the horizontal hydrofoil
2. horizontal inlet, below the bottom surface of the horizontal hydrofoil
3. vertical inlet, integrated with the bottom surface of the hydrofoil

In design 1, the pump would be accelerating the flow across the upper surface, giving more lift. However, by being closer to the surface, it would be more susceptible to ventilation. By putting it below the bottom surface, it would reduce lift, be less likely to ventilate, and also marginally increase draft. However, options 1 and 2 would require either a sharp bend right behind the mouth of the inlet, or a long duct with the foil being substantially forward of the pump. This would complicate CoG issues, and there may be substantial friction losses from the length of the intake duct.

Would 4 feet or so of suction lift be a problem for a typical PWC pump? Would the ram effect of a horizontal intake at speed provide any positive effect?

I've attached a rough render of option 1. Thank you for your input.

 

I believe that the inlet is covered in Heorner's Fluid Dynamic Drag.

 

I found a copy of the text online. Thank you for the reference.

 

One of the issues with the intake, is that if you catch air, it will loose its prime. For it to be primed the impeller needs to be below the waterline, which means the will slam down and slow down to almost a standstill. Also, there will be a significant loss with a 4 foot head. The impellers/pumps are not designed for high negative pressure (suction) but for high positive pressure to accelerate the water. They operate very differently from a well jet pump.

 

Wouldn't he lose all lift instantly, ramming water in as it dropped the 4', repriming immediately, all in the blink of an eye, BANG!

I would think a small scoop-ram might make up for the 4' head losses but comes with a drag price-tag.

EDIT: Flip the scoop (in your rendering) under the foil, no?

 

For this size vessel (20-25ft LOA) I had imagined a single engine in the pod to be powerful enough. It would obviously be above the static waterline if placed in the pod, but I really hadn't thought about the priming. I think the PWC engines could still work for something large enough to have an engine in each hull (30 to 35ft) but I think it would be overkill for what I am looking at. I will pursue the outboard idea instead, unless anyone has some tricks in mind. Thank you all.

 

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No, once the pump is dry, it needs to be primed. I doubt that water will be rammed 4 feet upwards vertically and then have enough volume to fill the impeller housing.

 

This would be right up @baeckmo alley but he hasn't been around since February it appears.
He's likely commented on something similar before.
@ziper1221 try the search engine, top right corner.