Prop wash advise

[nashwen]

Hi
I?m attempting to build a prop wash for a 75hp Marina outboard I have on a RIB. The area I want to work is between 2 - 4 meters deep with fairly compacted bottom sand, I hope to only need to excavate up to about 3ft.
Would a 75hp be powerful enough or am I wasting my time with this size engine? Any advise or pictures of fitting would be helpful?
The Prop is about 15" diameter, what diameter cover would work best?
Thanks

 

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[CaribbeanDiver]

Personally, I would use a good 24 volt scooter like an Aqua Sub..... 3 feet down is no problem, but how large of an area is the question.



Good luck!

 

[nashwen]

We have several good hits with a magnetometer most of which seem to be buried below the sand, generally there is poor vis in the area, so we hope to clearholes around the objects quickly before diving on the occasions when there is good vis. Not sure what area can be effectively cleared with a prop wash, larger the better I suppose, any advise info would be most welcome. I hope 3ft or so would be no problem but the sand is compacted in this area.

 

[gord]

What you want is 1"-1-1/2" clearance around the prop, so 2"-3" on top of the 15" diameter puts it @ 17"-18".
Gord

 

[Stroover]

Not to change the flow of your post, Nash, but I wonder if it's possible to make a prop-wash work in reverse as to suck sand up and blowing the wash away from the site, as opposed to blow water at the target? Or would that potentially mess-up the prop? It sure would help to prevent murking up the water. But what do I know?

 

[fldiver]

Stroover you could do it by reversing a propeller and making a "impeller", instead of pushing you are pulling. I would think It wouldn't be worth the hastle when you could use a pump unit for the same purpose. I've seen it done with these and screen systems to catch broken pottrey etc. If it was done one would definately need a sealed stainless steel screw because it would sandblast bushings down fast lol. just my two cents worth. Take care, fldiver

 

[nashwen]

Thanks for the info guys. I think I'll continue with the prop wash idea at the moment, I have not come across any examples of them being used here in the UK. i think our sand is more compacted here compared with what you may be used to in the US Florida area. It would be good to hear if someone has some first hand experience with these sort of conditions and if a 75hp outboard would do the job.
Any further info much appreciated.

Nashwen

 

[Sandman]

The prop wash from a 75 Hp. motor will do it easy in soft sand or silt, but it would take longer in compacted bottoms. I have used the "mail box" with larger engines and they blow a hole and create a crater like on the moon with smaller objects blown away or destroyed. The water from compacted areas stays cloudy for days with out a lot of tide or current to move it. After a short time the motor will over heat if your using an outboard from the silt.

 

[J@J salvors]

The Prop Wash or Mail Box Does work with a 75 horse engine at that depth just make sure your boat is secured with at least 4 Four anchors to keep the boat from shifting around the top of the hole you are working and use at least three feet of pipe downward from the elbow joint on the Mail box.If you slightly close the open downward side of the pipe like a funnel you can force a large amount of water through a smaller diameter pipe just be sure that the propeller can handle the water flow.

Also if the water at the top is clear it will force that same clear water to the bottom where the divers are
Make sure all of your gear is securly teathered to you The blast of water will take the tools right out of your hand
Good Hunting