Blower barge?

[ropesfish]

Since this forum seems to be in the doldrums here is a thought I've had while landlocked...
Has anyone tried a pontoon boat/barge with a midships located diesel engine running into a blower setup? Big old pontoons in need of deck repair can often be had for a very reasonable price. A Cummins truck engine ($750 at the U-Pull salvage yard), a right angle drive, a prop, some well thought out bracing and downward ducting...
Reasons in favor:
The 90 degree turn in stern mounted mailboxes comes with a significant cost in efficiency.
Much easier to maintain a deck mounted engine than one mounted below-decks.
Less wear and tear on the propulsion units on your boat.
If the blower motor quits, you can still get home with the towboat/tender.
Fuel costs. Fuel costs. Fuel costs.

Been done? Won't work? My brain is suffering from too much time out of seawater?

[stevemc]

Its been done. Back in the 60s the Real 8 company made one like that. I am sure there is a picture around somewhere. The main problem was - air would get sucked down and cause severe cavitation. I dont think the motor was very large. Maybe you could have a pivot at the engine, and have a fairly long propshaft going down, and using a jackshaft to extend back and lift the prop end up while it is turned off and traveling, but on site it is lowered down and level. You could use an outdrive from an inboard/outboard that has a 90 degree downturn, and where the foot attaches have a prop shaft going straight down, instead of back. Unless you could find a 90 degree turn V drive, or something like that.

[Au_Dreamers]

Check this thread, there's some of the same info there.

http://forum.treasurenet.com/index.p...tml#msg2691996

It's something I've wanted to build for years since hearing about the one my old Captain Harold Holden built. I relate more in the listed thread.

A baffle will help minimize the vortex.

Steve my designs play with the idea you described.

I've spent hours searching the Internet and keep getting new/improved ideas. Technology in many industries makes it easier, more practical to build.

Yeah I agree Ropesfish with the cost of a big twin blower boat, fuel and wear and tear an independent blower system seems ideal.

I'm hopefully going to get it built for next season or the season after. If I ever get this move back to Ft. Pierce underway....

[Au_Dreamers]

Ya know it's actually ironic because I've spent about the last 3 hours or so doing more research and modifying.

Here's something that came up. What would you think would be a minimum distance you would want to be away from the bottom? If you're too close wouldn't you basically just drill a hole about the diameter of the "prop" maybe slightly larger?

[stevemc]

Yes, if it was too close it wouldnt blow away a big hole. I guess even if it was 3' away, and you ran it slow, it would eventually make a huge crater. The force required is a touchy subject. I remember early on when the ashcan as it was first called, now a prop wash blower, was invented, people were using them full blast and blowing stuff up in the berm and down in cracks, and actually loosing stuff. So the theory was to run the props just so much that you just blew the sand away, not anything larger or heavier than a coin. Kind of like using a fire hose or hand fanning the sand away. The problem is it takes a long time if you have deep water or if the sand is real deep. A diesel driven prop even at idle has a lot of water moving, just leave it at the dock in gear and watch how much water is moving past. So you really have to watch it if you rev up the motors as you could loose all the stuff in the hole, so you check it every so often to see how it is going. I feel that 10-20' feet away is good with big diesel driven props. Any farther and of course you have to blow harder and longer if the sand is deep. An elephant trunk will help with deep blowing. With the single downward prop barge, the prop should be housed in a tube with 2" clearance or so, and the top of the tube not so close to the surface that it would cause suction to pull air in. Those props really have a lot of pull, so it must be pretty deep. Maybe have a large flat surface so it cant suck air. And remember too, that when a big prop is turning, it will lift the boat up in the air a lot, 6-8" or more is normal.

[Au_Dreamers]

Stevemc,
Hmm I also have been thinking that 3ft might be the nearest you would want to be. The first Spanish treasure coin I ever found was a 1/2 reale and it was out of the hole up on the brim. It was my first season and with a completely new boat and blowers so there was trial and error.

RPM's are one thing people tend to overdue. You can get it in your mind that you want to dig dig dig and find treasure but blowing too hard is counter productive.

When new, you have to blow, dive, check the hole and adjust and it changes when you move to a different spot.
As you said, water depth and sand depth are variable factors so until you know your boat and the site(s) very well you're on a learning curve. I think most people don't realize just how much of that consumes their first salvage season. That's why people who take over an operation are lucky if they have someone from the previous crew and/or if a new operation they can get someone on that knows the site(s) to help accelerate that time frame.

Technically since it's all about uncovering bottom area, the key is how much bottom area exposed in what time period.
Of course other factors are involved like start-up/construction costs, fuel/energy costs, amount of crew needed but on initial planning I try to keep the former in mind as a primary attribute.

[ropesfish]

Thanks Steve and Au for the replies. The other thread was very helpful and I'm going to go back through a few pages and see what else I've missed.
I've been doing some calculations...(good thing I saved my fluid mechanics book)....and I need an idea of how many prop rpms would be normal during blower operations and what boat speed would correspond to that rpm. I'll need the prop size as well, if you wouldn't mind.
I want to find how many cubic feet of water move through the mailboxes.
I'll post what I find.
Thanks again.

[stevemc]

Working with Seahunter this summer, when he is in deep sand, he blows at 2000 RPM maybe 2400 sometimes and his boat is very similar to mine with similar power, he has Cat 3208 with 250 HP and trans are 1.5 to 1, props are 24" diam with 24" pitch. That is a lot of RPM, I think when he gets close to rock bottom, he slows it down, or he should anyway. I am sure at 2000 RPM his boat will be going about 15 miles an hour. My boat I normally cruise for best economy and still going over hull speed is about 1700 RPM at around 11 kts or about 12.5 miles an hour. Any more and the bow starts coming up and you can hear the engines starting to work, and its a bigger wake. My boat is a lot bigger and heavier than Seahunter, and mine are Cat 3208 210 HP non turbo, and his are turboed. Trans and props are the same I am sure, except his are 3 blade maybe 60-65% and mine are 4 blade, about 85-90 percent props.

[Salvor6]

A few years ago I built a vertical blower and attached it to a Hobie Cat. It had anh 18hp vertical shaft motor turning a 6" diameter prop. The tube is adjustable. It worked pretty good in shallow water but it was a hassle towingt the cat around.