Can I Sail/motor upriver in 5 to 7 knot current if hull speed is less?

[scott hall]

Have a Hunter 23 with a 9.9HP mercury. I believe theoretical hull speed is about 6.5 knots. Live in Nashville. Marina located by both a lock and a Dam. Desire to take by sailboat down river to downtown Nashville about 25 miles. Return trip against current is 5 to 7 knots depending on flow of water released thru dam. Can I do this? Not sure of science of hull speed verses current.
Thanks so much Scott

 

[Kings Gambit]

Have a Hunter 23 with a 9.9HP mercury. I believe theoretical hull speed is about 6.5 knots. Live in Nashville. Marina located by both a lock and a Dam. Desire to take by sailboat down river to downtown Nashville about 25 miles. Return trip against current is 5 to 7 knots depending on flow of water released thru dam. Can I do this? Not sure of science of hull speed verses current.
Thanks so much Scott

Marginal, maybe. But look at 0.5 kt (mph) speed over the ground for 25 miles = 50 hr return trip if you don't end up further down river.

 

[Gunni]

Take an Uber.

 

[smokey73]

The first question to answer is can your Hunter 23 motor at hull speed in still water? If not the max speed you can consistently get throught the water is the max speed in still water. If the current is 7 knots and you can only do 6 kts with your 9.9 it is a losing battle. Even if you can get the full hull speed of 6.5 kts and the current is 5 kts against you then you will only be doing 1.5 kts against the current. While you may catch an eddy out of the main stream now an then that can help a little you are still talking about a very long haul to do 25 nm at 1.5 kts or less. And you'll have the outboard at full throttle all the time. Better take a lot of fuel and have an anchor ready for immediate deployment if things go south with the motor

Maybe you can trailer the boat back? Will probably take less fuel and certainly a lot less time.

 

[pateco]

My boat has a hull speed of ≈ 6.5 knots. when we are returning to port with an outgoing tide of ≈ 4 knots through our inlet, we make about 2 knots over ground. If your current is higher than your boat speed, you will go in reverse.

 

[rgranger]

...simple vector math....

Add the vectors up head to tail and see what the net looks like by drawing the hypotenuse. In the case of two vectors in direct opposition, it is simply the remainder of the larger vector. Also a good analysis for cross currents.

 

[Meriachee]

Let's see, the water flow is 7kts because you have to plan for the worst possible scenario, and at wide open throttle, the boat might do 6.5, downhill, with a tail wind, and we're talking about 25 miles?
Might be a good idea to see about getting a slip around Nashville.

 

[David in Sandusky]

You hull speed is 5.9 knots. As others have said, in the best case, you will need over a day to return upstream. If the boat has to come back from the marina, you need a trailer.

 

[shemandr]

Even going with that much current could be a hair raiser. There could be eddies, whirlpools, standing waves and clumsy steering. Going by objects can be tricky if a venturi effect comes into play and the boat may be sucked closer to an object than you have planned. I avoid going through the Shinnicock Canal when the locks are open for these very reasons.

 

[Johann]

Where did you find the 5-7 knot figure? The present mean velocity of the Cumberland at Nashville is 2.9 fps (1.7 knots). The highest flow in the past year was about 5 fps (3.0 knots).

https://waterdata.usgs.gov/nwis/uv?...od=&begin_date=2017-12-27&end_date=2018-01-03

 

[scott hall]

Thanks: will review where I got figures but thought it was government site. May have misinterpreted readings: If you are correct I feel much much better. Thanks Scott

 

[Capt jgw]

Short answer is no you can't. If the opposing current is faster than your boat is capable of in still water, you will be going backwards. Upstream of a dam that could be really bad. Conversely, going with the current you'll be doing around 12 knots over ground.

 

[scott hall]

Just curious. If hull speed max is say 6.5 knots and going down river in 7 knot current than science used to determine max hull speed is only applied in still water otherwise if use motor to achieve 6.5 knots in still water and than add current of 7 knots than will I be going 13.5 knots downriver which is faster than designed hull speed/

 

[jssailem]

Scott. There is a difference in Hull Speed. The speed that the boat will travel through the water. And ground speed. The speed at which you and the boat are traveling over the ground.

If you can move the boat at Hull Speed (6.5 knots) then you would be moving through still water at 6.5 knots and over the ground at 6.5 knots.
If the water is also moving over the ground then you add the water speed if you are moving with the water flow or subtract the water speed if you are moving against the water flow to get your speed over the ground.

 

[Benny17441]

Hey sail her down and trailer her back home. The science is pretty straight forward if you can motor over the water at 6.5 knots and the current is flowing against you at 6.5 knots then you are standing still above the ground. Any progress under 1.5 knots is just a tedious slog not worth entertaining. Get a friend in a powerboat to tow you up river or trailer.

 

[DayDreamer41]

Theoretical hull speed - 1.34 * SQRT(LWL) or in your case 1.34 8 SQRT(19.58) = 5.9 knots even against a 3 knot current you will not be making much headway and will take you quite a while to return to your home port, would be a nice weekend zoom down the river for dinner and then listen to your engine screaming for about 8 yours on the way back.

 

[Johann]

Theoretical hull speed - 1.34 * SQRT(LWL) or in your case 1.34 8 SQRT(19.58) = 5.9 knots even against a 3 knot current you will not be making much headway and will take you quite a while to return to your home port, would be a nice weekend zoom down the river for dinner and then listen to your engine screaming for about 8 yours on the way back.

Unless your trip is between late April and mid May, the typical current is less than 2 fps, or about 1 knot. June looks like a great month for the trip with a mean current less than 1 knot.

https://nwis.waterdata.usgs.gov/usa...iod=&begin_date=2017-2-27&end_date=2018-01-03

 

[scott hall]

Thanks: Will check with weather as it is a hydro dam and will open more with high elec usage and/or excess rain if I am correct. Must have misinterpreted chart for FPSecond for Knots per hr. I usually take my motor boat down river and dock at downtown landing and walk around city and local music joints. Correct about a 8 hr trip back. If have problem at least I have tow insurance. Thanks again for advice

 

[Joe]

Just curious. If hull speed max is say 6.5 knots and going down river in 7 knot current than science used to determine max hull speed is only applied in still water otherwise if use motor to achieve 6.5 knots in still water and than add current of 7 knots than will I be going 13.5 knots downriver which is faster than designed hull speed/

You're confusing SOG (speed over ground) which is what your GPS measures... and STW (speed through water) which is what your in the water (usually a through hull paddle wheel device) speed transducer will measure. If you're sitting idle, where hull speed is zero, in a 5 knot current your boat's gps will show the boat moving over ground at something close to speed of current.

I will mention that hull speed is "theoretical" .... don't fall in love with it... most modern sailboats can regularly exceed theoretical hull speed when sailing. HOWEVER... in your case we're talking about motoring... and the hull speed theory will be much more applicable in this case.

Finally, a 7 knot current is exceedingly fast.... no small sailboat should be recreationally sailing in a current like that...it's very dangerous because you're essentially out of control. Check your data... do some research.

 

[Capt jgw]

Think of the current as a treadmill. You're running as fast as you can with the treadmill set at the same speed. You actually are standing still. Turn the mill up one more notch and you fall off the back because you can't keep up. The boat will do the same thing with an opposing current.