Cataraft drop floor depth question please - Mountain Buzz
 



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Old 01-30-2013   #1
 
Yukon, Alabama
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Cataraft drop floor depth question please

New to this forum. I'm having a lot of trouble trying to decide how far to drop my cataraft floor. I will be using 28" diameter tubes and will be heavily loaded for a very long trip down the Yukon River. I will be using a 15 - 20 hp 4 stroke. I am planning on having some form of very thin splash guard material attached to portions of the underside of the frame. I've calculated that my draft will be between 8 1/2 to 9 inches deep. In trying to determine how far to lower my drop floor, for me the question boils down to how much distance do I want to maintain between the "bottom" of the floor and the "water" when the boat is fully loaded. If you folks were going to doing a very long trip down the main Yukon River in a cataraft occasionally using a 20 hp motor, how much clearance would you want between the "bottom" of the floor and the water surface? Many thanks.

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Old 01-30-2013   #2
 
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Sandy, Utah
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What kind of frame will you be using? The amount of drop in your rails will help determine your floor depth to a certain extent. Also, will this be high water, or low and potentially rocky? That will also factor in to floor height. For me personally, I want a floor that I can stand up on easily to scout and the move around the boat a little while loading & rowing. I don't want my knees in my nose so to speak. I've never measured, but I'd guess I have at least 6 inches of clearance? Here's a picture that shows the clearance. My Cat is 14 X 24, and I carry a medium gear load. My guess would be to maintain 4-6 inches of clearance.
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Old 01-30-2013   #3
 
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portland, Oregon
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4.62" is the optimal clearance

You are running a motor so i'm assuming their are few hazards in the water. Otherwise your question should be "How many spare props should i bring for my trip down the Yukon"

Make sure your draft estimate is accurate and you can balance your load. Are you hunting?
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Old 01-30-2013   #4
 
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Hi,

I run a large motorized (18 hp four stroke) Jack's El Tigre Grande with 30" tubes, an 11" drop on the frame, and a solid floor. In addition to GC trips, I've also spent a fair amount of time on the Mississsippi River with this setup.

A lot depends on the meaning of the words "heavily loaded." When I look at pictures of my rig with what I consider a fairly heavy load, it easily has 6" of clearance under the front of the frame. Your situation will differ, starting with the fact you have smaller tubes.

You should consider that when under motor, you will be kicking up two wakes that converge between the tubes. At some speeds, it will interfere with a smooth flow of water to the lower unit and prop. To account in part for this, I added a "jackass" adjustable transom to give me some flexibility in how deep I could run the prop. But you also could build in some margin here by adjusting the floor height.

This wake business also is dependent on speed. If you are just putting along adding a few miles per hour to the current, then there won't be much of a wake. But if you're thinking of making speed (good luck with that in any rig of this kind) then the wake convergence has to be considered.

On my frame, I actually can vary the drop on the floor, but actually never have done so. It's essentially the same as you'd find on a standard NRS cat frame. One sure thing -- a test run with a simulated trial load would help a lot.

Hope this helps.

Rich Phillips
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Old 01-30-2013   #5
 
East of the Pine beatle, Colorado
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28" tubes means a 14" drop would still give you 14" of potential draft before the floor was awash. 14" is a pretty decent drop- and your seat will be elevated as well. I'd do some mock up with a cooler, some lumber- whatever. Lay out how far the seat will be above the floor and start tweaking from there. Pat attention to other cats you see and gauge how much below 1/2 way the drops lay. I'd say you're going to end up around 16" or so- but make sure it's comfortable before you pull the trigger. If your drops are modular, rather than welded, start a little long and cut 'em back?
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Old 01-30-2013   #6
 
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portland, Oregon
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Quote:
Originally Posted by Osseous View Post
28" tubes means a 14" drop would still give you 14" of potential draft before the floor was awash. 14" is a pretty decent drop- and your seat will be elevated as well. I'd do some mock up with a cooler, some lumber- whatever. Lay out how far the seat will be above the floor and start tweaking from there. Pat attention to other cats you see and gauge how much below 1/2 way the drops lay. I'd say you're going to end up around 16" or so- but make sure it's comfortable before you pull the trigger. If your drops are modular, rather than welded, start a little long and cut 'em back?
The nrs drop is about 11" --- i run this with 26.5 leopard tubes. It gives me about the right clearance. But i run pretty shallow rivers a lot. That being said osseous numbers are pretty close
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Old 01-30-2013   #7
 
Yukon, Alabama
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Thanks everyone for the responses.

richp - sounds like we have a very similar set-up. Thank you for the information. PM sent kind sir.
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Old 01-31-2013   #8
 
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here's a shot of my 18' aire cat, nrs frame however 120" side rails. the boat was loaded to the gills being the only raft for a 25 day Grand trip last December. All the food and gear for three onboard. my bottom rail was an inch or two above the water. as long as it wasn't plowing water in the flat water sections I was happy. not too many rocks near the surface to worry about. most of the worrying is reserved for the rapids.
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Old 02-01-2013   #9
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This is all good advice.

One more thing I consider when designing a cat frame that has not been mentioned:
Most Cat tubes have the inside D-ring centered on the tubes. If they are 2” D-rings, then when they are in the up position to attach to the frame the drop will be 2” shorter. I like at least ½” of space between D-ring and frame to help get this tight. If that D-ring is not up and tight it will allow the tube to loosen and rotate until it is tight on the inside but loose on the outside and will have your tubes on an angle. With your 28” tubes, 2” D-rings and ½” space this would make your drop 14” – 2”- ½” = 11 ½”. That’s probably why NRS standard drops are 11”.

Also not enough space between frame and water with a heavy load is HELL. Too much space is no problem. You will raise the center of gravity with a higher load but that is not a significant problem. Raising the seat is the solution to knees in the face. But that can require higher oar towers.

The last comment is about the double wake. This can create havoc with your floor clearance and motor depth. Alaska Raft and Kayak Rentals drag a piece of poly under their frames in front of the motor to flatten this wake out and solve this problem. They are very helpful and will give you more details about this wake flattener.

Good luck, happy boating, and I wish I was coming with you.
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