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 Boat Design Forums How low will she go.

#1
06-22-2006, 11:00 AM
 CMARSHALL Junior Member Join Date: Feb 2005 Rep: 10 Posts: 18 Location: tEXAS
How low will she go.

Hey boat guys, answer this for me. I am building a 40'Lx12'Wx4'D scow with outboard(s) that will be used to transport equipment up and down a local river. The boat will weight in right at 5000 lbs. with another 5000 lbs in cargo. At 10000 lbs. total weight how low will the boat sit in the water and how much engine am I going to need to push this thing 10-15mph.

Thanks

CM
#2
06-22-2006, 11:19 AM
 SeaSpark - Join Date: Mar 2006 Rep: 96 Posts: 593 Location: Holland
Scow plans

Hoi,

If you post a plan of the scow you are building i am sure someone will be willing to calculate the displacement for a given draft for you. Without a plan it is impossible to make a somewhat accurate estimation.
#3
06-22-2006, 11:41 AM
 CMARSHALL Junior Member Join Date: Feb 2005 Rep: 10 Posts: 18 Location: tEXAS
Basically the boat is a true rectangular box. The sides, bottom and transom are flat with the bow having a flat 50 degree angle. At the water line the dimensions are actually 34'Lx12'Wx4'D.

CM
#4
06-22-2006, 12:13 PM
 lewisboats Obsessed Member Join Date: Oct 2002 Rep: 1517 Posts: 2,000 Location: Iowa
You will draw 6" at 10,000 lbs, and to get it to 10 mph you will need lots of hp (350+?). You are describing a planning hull form which will chew up a bunch of hp with drag. Angle the back of the boat and reduce your hp by 80% or more. You will still need a hundred or so hp with a big and slow (read efficient) propeller. A Diesel engine with a reduction gear should work well and be fuel efficient. The shape below will draw about 8" but will use much less power and fuel than your shape. Barges and outboards do not bode well for fuel consumption. If you are determined tho...use the hull shape below and mount ob brackets.

Steve

PS: About the best you could expect is 9 mph or so, given the waterline length. This is max speed of a displacement hull at that DWL.
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#5
06-22-2006, 12:17 PM
 SeaSpark - Join Date: Mar 2006 Rep: 96 Posts: 593 Location: Holland
metric system

Fortunately i live in europe where we use the metric system, in this system 1 cubic meter displaces 1 metric tonne of fresh water, a great help for naval engineers.

So,

1 feet = 0.304 meter

34'Lx12'Wx4'D=

10.3x3.6x1.2= 44.4 cubic meter of displacement
i will substract 4.4 cubic meters for the angled bow, this is a bit to much but will compensate for beer.

So you boat will displace 40 cubic meters = 40 metric tonnes when 4' deep in the water.

one metric tonne = 2,204 pounds

so 40x2,204= your craft can carry 88160 pounds plus some beer.

By the way 4' draft for a flat barge 34' long is a bit much. If you make it longer and less deep you will need a less powerful outboard.

(edit)
Oops cross post with Lewis.
#6
06-22-2006, 03:23 PM
 CMARSHALL Junior Member Join Date: Feb 2005 Rep: 10 Posts: 18 Location: tEXAS
How much angle for the bow and stern would be optimum for power to speed tranfer.

CM
#7
06-23-2006, 10:55 AM
 CMARSHALL Junior Member Join Date: Feb 2005 Rep: 10 Posts: 18 Location: tEXAS
If I shift plans to pontoons at 40'Lx 4'Wx4'D each and still assuming 10000 lbs total weight. Would there be any benfits in regard to HP. I really need 10-15 mph.

Thanks
CM
#8
06-23-2006, 11:36 AM
 lewisboats Obsessed Member Join Date: Oct 2002 Rep: 1517 Posts: 2,000 Location: Iowa
If you need at least 10 mph, you will need a waterline length of about 60 ft for a barge shape. You might get away with 50 ft with pontoons. Otherwise you will need a significant amount of HP and fuel to get to that speed.

Steve