Prop Wash diverters

[bpedersen]

I am looking for a company to Manufacture 2 mailboxes for me. I have 46" props and these are the last things I need for my boat. Any help would be appreciated. Thanks.

 

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[SEAHUNTER]

THE GUY AT TREASURE COAST SCOOPS BUILT MINE AND HE HAS BUILT MORE THAN A COUPLE. HE IS GOOD. KNOWS WHAT HE IS DOING. I WILL TRY TO FIND HIS NUMBER AND GET IT TO YOU.
GOODLUCK

 

[bpedersen]

Thanks.

 

[Trez]

Ed Mathews...........AKA, the Scoopman
Treasure Coast Scoops.....
772-388-3507 is the shop number if I remember correctly.......
He builds em' good..........

Good Luck,

Trez

 

[FISHEYE]

im building my mailboxes out of carbon fiber and kevlar,im in the process of building the molds now.my mailboxes will be hydraulic operated so no pins underneath the boat to mess with.

 

[bpedersen]

So if my props are 46", what diameter should I be looking at?

 

[wreckdiver1715]

48"

 

[bobinsd]

A related question. Water flowing thru a pipe increases in speed inversely with the square of the diameter, yet every mailbox I've seen has a constant diameter. It seems that with much less fuel the higher exit velocity of a tapered tube would do the job faster, albeit expose a smaller area? Also, has anyone use the jet stream to "push" the silt/sand towards the bow while winching in on the "anchor/stabilizers" so that a nice straight line could be cut for a distance? The water would be clearer sooner so the the diver could start searching while the path was being plowed. What I have seen of the techniques (Mel Fisher's Macgruder) for instance, seem very innefficient.

 

[Cablava]

Hi

I have used blowers to dig trenches from the shallowest point we could get the boat into, as far out as the water depth would allow to be efficient. I was not treasure hunting but laying 1.2m pipes for desalination plant intakes averaging about 1km length, it works well but a lot depends upon the sand type.

The South African civil diving companies have been using them for many years for the same thing.

Mooring the boat correctly and having the ability to move the boat accurately on the anchor pattern is the key.
With GPS it should be easier but you will still need visual markers to keep a uniform line.

You need a second small boat to move the anchors for you so you are not repositioning the anchor pattern completely.

We tried cones to increase the speed of the water passing through the end of the blower once but did not get the results we wanted so went no further with it. Not conclusive as we did not have any time to spend on it. Time is lots of money when layng pipe.

 

[SEAHUNTER]

I HAD THE SAME IDEA ABOUT THE TAPERED TUBES, BUT WHEN I TALKED TO ED MATTHEWS OF TREASURE COAST SCOOPS ABOUT IT HE SAID HE HAD ALREAADY TRIED IT AND IT DID NOT WORK.
SEAHUNTER

 

[ScubaFinder]

OK, you guys are going to get me going here.... I work for Pratt & Whitney aerospace, we manufacture turbine engines which use almost the same basic principals you are discussing. The problem with the taper is that it builds considerable amount of back pressure in the tube while accelerating the water. Jet engines accomplish this by having a minute (<10 micron) gap between the blades of the impeller (propeller) and the shroud housing (mailbox).

This kind of alignment and clearance is not possible reliably in a swing down mail box, and would likely be more expensive to engineer and create than it would be worth. Theoretically you could get away with a slightly larger gap due to the fact that you are working with water as opposed to our air, but I'd have to ponder that one a while, because the backpressure would also be higher, possibly resulting in a lower allowable clearance.

However, if you built a simple 2 or 3 stage compressor into the mailbox that was powered by the water force of the propeller, you might get some worthwhile results for a smaller expenditure. I tend to think you could simply blow a hole in deeper water or blow it more quickly at a set depth with a higher velocity. With a simple step-down mailbox you'll just get less water through it at about the same velocity due to losses around the shroud.

Hydrodynamics and aerodynamics are very different, so I'm not completely sure what a stage compressor might do in water, but I do know why the funnel shape doesn't work on a mailbox. Ramblings from an aircraft engine guy...I'll shut up now

Jason

 

[bobinsd]

It would depend on the degree of taper, a function of the overall length, and the design of the end, which should straighten out gently to promote laminar flow. A straight cut-off of the taper could easily cause stalling/vortices at the circumference, causing dispersion rather than a directed flow. The pipe-laying guy probably knows the basic design specs.

 

[bobinsd]

Scubaseeker ...I was penning my post above while you were yours. The compressor idea would not work in water (incompressibility), but a secondary prop running at a higher pitch/velocity would help, but I can't imagine where it would get its power. Another solution might be to put stabilizer vanes radially inside the last 1/4-1/3 of the tube to help induce laminar flow AND reduce the stream rotation generated by the prop rotation, sort of like baffles in a LSWT. That would be an easy modification and ad strength to the mailbox, possibly allowing the use of lighter material. What are they made of anyway...steel, aluminum?

 

[ScubaFinder]

And here I was trying not to be too technical Bob

 

[bobinsd]

Fun, isn't it.

 

[ScubaFinder]

Most I've seen are made of Aluminum or Stainless Steel. I like the stator idea to straighten out the flow, you probably could get more depth and a cleaner hole that way. You would loose some velocity while straightening the flow out, but i think you'd make up for that in overall effectiveness.

Catia has some water modelling capabilities, maybe i'll get one of the engineers to run some simulations later this week and see what they come up with...your tax dollars at work

Jason

 

[SEAHUNTER]

MAKE SURE WHEN YOU BUILD THEM THE PROP IS HALF WAY IN AND HALF WAY OUT OF THE TUBE OR YOU WILL HAVE EITHER CAVITATION OR BACK PRESSURE PROBLEMS. ALSO DO NOT MAKE THEM EXACTLY 90 DEGRESS OR YOU WILL CREATE A SANDBAR ALL AROUND YOU. YOU WANT THEM TO POINT BACKWARDS SLIGHTLY, AS THIS HELPS FLUSH OUT THE HOLE YOU BLOW.
SEAHUNTER

 

[riobravo]

hi guys,
why wasn't technology as available for thing in the years ago.
its exciting to see the brainstorming for as much as mailbox's, when i have some more time i will talk aboutthe trials and errors or how we got to here and what didn't work in the road to get there, some stuff i think yall will will respect and maybe raise the bar on.

 

[ScubaFinder]

I'd love to hear about your trials Joseph.

I talked with one of engineers who specialized in fluid dynamics (originally for hydraulic controls on aircraft) and according to him, Bob hit the nail on the head. Water's incompressibility is the main reason the cone shape doesn't work so well also. His thought was that you could get deeper by adding stators to straighten out the flow creating less vortices just outside the mailbox. He also commented that you wouldn't want them blowing straight down, and then proceeded to talk way above my head in explaining why.

He was pretty keen on trying to find a way to lengthen the down tube a bit to keep the column more condensed further down. Something like for every 1 ft. of extension, you would get 20 feet of extra effectiveness by keeping the blast tighter. I only showed him the pic of SeaHunters mailboxes and explained that they were used to blow holes through 8-12 feet of sand at varying depths. He was of the impression that the only real way to increase anything was to increase the depth by extending to tube further downward. He likened it to the difference between a short barreled pistol as oppsosed to a rifle.

Not sure if that helped anything, but there's some advice from a seasoned fluid dynamics guy. Hope it helps someone some how.

Jason

 

[bpedersen]

Has anyone used a waterjet instead of a prop? It looks like you could could flush mount it to the exhaust and has have incredible flow towards the target. Can anybody think of a downside to this idea.