waterjet powered Catamaran for rec use

[videorov]

Some need to design a PowerCat that uses waterjet motors like is used
in the new Yamaha 24ft ski boats.
You design guys need to think of some ideas. I think there is a large
market for such a thing. Then the cat could get in shallow water and not
have to worry about catching a crab trap line etc. No thru hull drive to
corrode away and prop shafts to get bent.

[Village_Idiot]

Check out the Flatscat.

No need for a jet drive and the issues associated with it.

[Stumble]

Or just a standard outboard cat. A 20' Twin Vee draws something like 7 inches, and my larger 26' only draws 10. I can't imagine that a jet drive would improve on this at all.

[Mr Efficiency]

Originally Posted by Stumble

Or just a standard outboard cat. A 20' Twin Vee draws something like 7 inches, and my larger 26' only draws 10. I can't imagine that a jet drive would improve on this at all.

I'd like to see that. How does a boat operate in water half as deep as the propellor diameter ?

[keysdisease]

http://www.youtube.com/watch?v=3Mqpvn9WEhE

Shallow running flats boats and tunnel boats are common in Florida through the Gulf States to Texas. Some seem to run on wet grass.

Steve

Originally Posted by Mr Efficiency

I'd like to see that. How does a boat operate in water half as deep as the propellor diameter ?

[Mr Efficiency]

Originally Posted by keysdisease

http://www.youtube.com/watch?v=3Mqpvn9WEhE

Shallow running flats boats and tunnel boats are common in Florida through the Gulf States to Texas. Some seem to run on wet grass.

Steve

Airboats though mainly, aren't they ? A catamaran hull will typically draw more water than mono by simple reason of the smaller water plane area, although tucking the drive up inside the tunnel saves draught. The trailable cats I had would draw at least 30" with the motors down, more than a deep vee.

[Mr Efficiency]

Another snag with jet drives in a cat is the fitment of engines into narrowish sponsons, in twin installations. Very cramped.

[keysdisease]

There are indeed plenty of airboats in use in the Gulf States, but they typically run in the Everglades of Florida or the marshes and delta's of the Gulf States.

A flats boat is a monohull and the real skinny water version a tunnel hull, they are used in the shallow fisheries of the Gulf States. The video I linked to is a monohull outboard powered boat running in ankle deep water.

Steve

Originally Posted by Mr Efficiency

Airboats though mainly, aren't they ? A catamaran hull will typically draw more water than mono by simple reason of the smaller water plane area, although tucking the drive up inside the tunnel saves draught. The trailable cats I had would draw at least 30" with the motors down, more than a deep vee.

[Mr Efficiency]

Originally Posted by keysdisease

There are indeed plenty of airboats in use in the Gulf States, but they typically run in the Everglades of Florida or the marshes and delta's of the Gulf States.

A flats boat is a monohull and the real skinny water version a tunnel hull, they are used in the shallow fisheries of the Gulf States. The video I linked to is a monohull outboard powered boat running in ankle deep water.

Steve

I see what you mean, the prop was carving into the mud a bit, I assume their is a step in the hull allowing high mounting of the engine.

[keysdisease]

And the engine is on a jack plate allowing the operator to raise it vertically while underway. I have watched tunnel hulls from behind and their props seem to bite into the combined wakes from the inboard side of the tunnels which actually allows the props to run in water above the waters static surface.

Steve

Originally Posted by Mr Efficiency

I see what you mean, the prop was carving into the mud a bit, I assume their is a step in the hull allowing high mounting of the engine.

[Stumble]

Efficiency,

I think the power cats we are discussing are quite different. I bought mine specifically because it could fish the shallow bays and inlets of the Louisiana marshes, and at the same time fish the offshore oil rigs out to 40 miles or so.

Certainly some cats draw a lot more, but mine is pretty shallow, and while it can't get into the truly skinny water of a flat boat, 10" of water is good enough that I can get away with only owning one boat instead of two.

I mentioned this in a different thread to you, but seriously go take a look at Twin-Vee's website. They have quickly displaced a lot of fishing guide boats here in Louisiana simply because they work well in so many different environments.

[tom kane]

Waterjet rescue boat in New Zealand. Jet propulsion is not necessarily shallow draught propulsion, they need draught to operate well.

[DennisRB]

Originally Posted by tom kane

Waterjet rescue boat in New Zealand. Jet propulsion is not necessarily shallow draught propulsion, they need draught to operate well.

Just make sure the tide is up when you need to get rescued

[Mr Efficiency]

Originally Posted by DennisRB

Just make sure the tide is up when you need to get rescued

Never underestimate the ingenuity of those kiwis.

[Village_Idiot]

Originally Posted by Mr Efficiency

Airboats though mainly, aren't they ? A catamaran hull will typically draw more water than mono by simple reason of the smaller water plane area, although tucking the drive up inside the tunnel saves draught. The trailable cats I had would draw at least 30" with the motors down, more than a deep vee.

True, a catamaran will always draft more than a similar-sized monohull, all else being equal, when both boats are at rest. The cats that are designed for shallow-water operation are designed to channel the water up into the tunnel between the hulls so that the prop can run above the ambient water surface elevation. So, when on plane, they can run just as shallow or shallower than a similarly-equipped (pocket tunnel) monohull. The trick is to not shut down in a really shallow area because you may not be able to get up on plane again. The (pocket tunnel) monohull has a slight advantage, then, depending on the tunnel designs of both craft. The bar of measure for a good flats boat is to be able to jump up on plane in water less than a foot deep.

There are a lot of different designs out there for pocket tunnels, but it seems that there are only a few really good ones. Some of the best seem to have a wide-shallow "pre-tunnel" similar to an inverted-V (aka Hickman sled) to gather the water, and then channel it up into a smaller pocket tunnel to allow the prop to run even higher. The designer can also add a vent to the leading edge of the tunnel to break vacuum and regain most of the speed lost by adding a tunnel to the design in the first place.

Water coming through the tunnel will always be disturbed, aerated water. A prop with plenty of cupping and blade area is paramount for getting a good "grip" on the water and maintaining good performance, with a downside of losing reverse thrust with the cupped blades. The good news is that you can use a good stainless steel prop with little worry, since you typically won't hit anything in the water that you can't see.