Best thru hull design to keep a gen set running at over 40kntsu Hulls

[ChrisN67]

I have had many opinions on this point. I have an intrepid 339with twin yanmar 6LY2 and surface drives. I am using transom scoops for engine raw water.

For the Air Con and raw water system I have a scoop-type 1" pickup facing forwards;

The book for the Paguro 4000 GENSET shows a scoop type pickup facing backwards...I would think this would create a vaccuum...

Does anyone have a way to install a pickup so that the genset inlet receives neither pressure or vacuum at any speed?




Thank you

[Landlubber]

Does anyone have a way to install a pickup so that the genset inlet receives neither pressure or vacuum at any speed?

Good question, the displacement boats generally have then facing backwards and planing types facing forward, but neutral.....interesting to see what someone comes up with here...

It can't face forward on a fast boat. On 25 knot boats they face aft with no problem but forty is another matter. I would simply put a sailboat thru-hull (flush). Let us know how you do - forty knots AND a running gen is new territory for me, at least!

[kenJ]

Perhaps use a pump that isn't effected by the high head pressure.

[Greybarn]

We use a flush seacock, with a Groco slotted strainer, facing forward, on the outside surface of the hull.

[Typhoon]

Why not use a forward facing scoop inlet, then just before the water pump, tee it off and run a return line overboard?
This way, you are assured of a full intake line and excess pressure is dumped overboard, the water pump will only take what it needs.

Regards, Andrew.

[jonr]

> the displacement boats generally have then facing backwards and planing types facing forward, but neutral.....

Sideways?

[ChrisN67]

If the scoop is facing backwards (for the genset) then would there not be a vacuum generated? A straight (not scoop) thru hull would also generate a vacuum?

[daiquiri]

Maybe you can try with a NACA duct (or scoop):

http://www.recumbents.com/wisil/nacaduct/naca-duct.htm

Don't know if it is correct to call it "the best", but it does meet your requirements (except for the simplicity of construction).
It is being extensively used in automobile and aerospace industry, but I've never seen it on a submerged part of a boat. That's probably because it is not a simple geometric shape - not just a circular hole in the hull, but rather a modification of an area around the inlet (plus a lack of information and the fact that not many applications have a need for it).

P.S. (edit):
A typo - highlighted word "never" was missing above.
Also, I forgot to say that these should be installed at the hull bottom... So now I'm not sure anymore they will meet the requirements - you decide.

Cheers!

[ChrisN67]

Thank you for your input, as a pilot I can tell you that NACA airfoil are great when angle of attack is constant (ask an F16 pilot what happens to a Pratt and Whitney under high G and angle of attack) The angle of the boat will change in the water, additionally to determine the exact geometry and orientation of the inlet would be difficult without model analysis.

On a side not, for my engine raw water; I am thinking about hardin marine dual rami pickups:

http://www.keitheickert.com/detail.aspx?ID=8909

But as with any transom pickup, you can develop 30+ psi in the raw water system. Disregarding issues that arise in the event of strainer or other plumbing failures; I am wondering if the additional water might constrict the exhaust system to a detriment.

[daiquiri]

Originally Posted by ChrisN67

as a pilot I can tell you that NACA airfoil are great when angle of attack is constant (ask an F16 pilot what happens to a Pratt and Whitney under high G and angle of attack). The angle of the boat will change in the water, additionally to determine the exact geometry and orientation of the inlet would be difficult without model analysis.

Good points. Thanks for the correction.

[singleprop]

here is a simple solution (if you have a steel boat): how about running a small open metal tube trough the keel with a T-piece in the middle - facing up (the thru-keel pipe must obviously have mesh screens on both sides of the keel). This T-piece would then be attached to your genset/equipment and would most likely not add neither pressure nor vacum to your system. If any, it would be minimal.

[Landlubber]

"would most likely not add neither pressure nor vacum to your system"


...please explain why, or do i miss the point entirely...

[singleprop]

It's a proven concept, I will try to explain the logic: Any aircraft that I know of have these static pressure sensors on the side of the body. They are placed perpendicular to the body and only sences the static pressure. They do this at any altitude (pressure) and at any normal speed range - say beween 0 kts and Mach 0.9)

I am thinking that the water intake acts in the same way, it's like a small seachest with only the static water pressure in it.

Since the "sea-chest/water intake" is perpendicular to the sea water stream (not exposed to 40 kts) then there would neither be created a vacum or a pressure to the intake water, just like the static air intake on the side of all aircrafts.

The T-piece adds another 90 degree bend that would further enhance this effect (I think), anyway the T-piece is only there to guide the water in the right direction (towards the genset).....

Does this make sence?

[Landlubber]

singleprop,

...so is what you are saying, the T piece removes somehow the actual pressure of the water(air), what about the flow past the opening...still lost, sorry mate.