6 knot hull speed in a 6 knot current..?

Discussion in 'Boat Design' started by Caldera Boats, Mar 3, 2008.

  1. Caldera Boats
    Joined: Nov 2005
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    Caldera Boats Beer4Ballast......

    6 knot hull speed in a 6 knot current..?

    Will the boat just stop moving forward??

    My hull speed is 6.4 knots
    I tried going upstream where the current was over 7 knots and the boat continued to do 2 knots over land? Why?

    Can someone explain this?

    Thanks for your input.
  2. Guest625101138

    Guest625101138 Previous Member

    Hull speed has nothing to do with ho fast a boat will go it is simply a function of the waterline length.

    If you mean your boat speed was 6.4kts and opposing current was doing 7kts then there is something not adding up if you were making 2kts.
  3. jehardiman
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    jehardiman Senior Member

    First of all, there is no such thing as "hull speed" as I, and many others, have pointed out in several threads. There are "humps" and "hollows" in most displacement vessels powering curves and eventually the installed prime mover cannot push the vessel over one of the "humps" (usually when strating to climb out of the third "hollow" to get over fourth "hump" for most hulls; something which generally occurs at S/L ~1.34-1.45, Fn ~ .35-.40). What happens is that the power required to clear the fourth hump is 2-5 times the power to get into the third hollow. As the difference in speed between the bottom of the third hollow and the top of the fourth hump is 2-3 knots for most boat sized hulls, that generally becomes a "break point" for fitting an engine of a given size/cost. So in actuallity, the power required to move at a given speed eventually reaches the point that no more power can be delivered to the water and the vessel no longer continues to gain speed. You can make a brick go very fast if you strap a huge gas turbine to it!

    So, if the actual maximum speed of your vessel was 6 knots and you were attempting to breast a 6 knot current dead on the nose, then speed over ground would be zero.

    That said, it is unlikely that the maximum speed is exactly the "calculated" hull speed. An ever trickier point is what was the available power. Let us suppose that it was exact, that with my engine at WOT, I was just breasting the current, speed over ground zero. Then I rasied some sails (or held out my arms or had a tall cabin) because the wind was behind me. What would happen? Well, because I was going slower than the wind, I would get some thrust (i.e. power) from the sails, and because I increased the inflow velocity to the propeller, I would lose some thrust (power) from the engine, so as long as the composite thrust (power) generated by the sails and engine is greater than the drag (i.e asorbed power) I would accelerate and move upcurrent. Additionally, it is unlikely that the current was also dead on the nose or that the water you were in was actually going "over 7 knots" (especally if you were hugging the banks or in shallow water).

    For about US$100,000.00 I could specifiy, buy, and install the equipment that could accuracy determine the instant current and wind velocity and direction relative to the hull and, if we could get bottom lock (normally a given unless the water is over 100 ftms), true speed over bottom better than GPS (subject to an accuracy limit of ~ 2mm/sec). For another US$100,000.00 I could accuracy determine the drag and power/thrust of both the hull and the prime mover. With these two studies done, we could attempt to replicate your voyage and we could get a NRT output that could show predicted and actual overground performance

    As it is right now, bask in the glory of your stupendious seamanship as you spread the tale far and wide.

    (Fairy Tales start "Once upon a time....", sea stories start 'This is Gods honest truth...." ;) )

    (Hull speed comment crossposted with Rick)
    Last edited: Mar 3, 2008
  4. Caldera Boats
    Joined: Nov 2005
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    Caldera Boats Beer4Ballast......

    I can explain this a little better:

    All readings are taken with Garmin 60CSX GPS (knots over land)

    On still water (no current), I can push my boat to a max of 6.4 knots with the little 4hp motor

    On the river, If I turn off the motor and drift, my GPS says 6.8 - 7.1 knots .

    If I motor upstream with full power, I can keep between 1.8 - 2.0 knots.

    There must me other factors involved because this does not make sense..
  5. Guest625101138

    Guest625101138 Previous Member

    How do you know the still water reading is "still water"?

    Do a test with the boat going with the current and against the current? See if theis makes more sense.

    Is there a significant change in depth between still water and against the current reading?

    Was there any wind present in any of the tests you carried out?

    Rick W.
  6. TollyWally
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    TollyWally Senior Member

    Actually sea stories start with "There I was..."
  7. erik818
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    erik818 Senior Member

    I presume that the river with a 6 knot current wasn't very deep and that the bottom wasn't perfectly smooth. A possible explanation could then be that eddies in the water disturbed the waves generated by the boat so the wave pattern resulting in "hull speed" didn't occur at the same speed as for undisturbed deep water. Waves look different on a river with flowing water than on a lake.

  8. ghostwriter
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    ghostwriter Junior Member

    somebody correct me if im wrong but hull speed is always the same regardless of what the current is. That being said in still water 4 hp makes boat go 6. In a 6 knot current you simply need a lot more horsepower to get up to the 6 knot speed.

    In a current it simply takes more POWER to reach HULL SPEED. These are separate issues.
  9. charmc
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    charmc Senior Member

    Measuring current and streamflow accurately is more difficult than it seems at first. USGS has a guide for such measurements; the basic whitepaper describes 9 policies to be followed for accurate measurements, one of which calls for averaging results from a minimum of 4 different measuring points. If the difference between maximum and minimum readings is more than 5%, then 8 must be averaged. Another policy requires differentially correctable GPS units certified accurate to sub-meter tolerances. The paper references 5 other publications, and cites 4 courses to be taken to become qualified to make the measurements. Yikes!

    Caldera Boats, the "other factors" you mentioned are probably the inherent inaccuracies common to simple measurements of complex systems. I suspect you're not opposing a constant force equivalent to 7 knots when you make 2 knots over ground. Water flow in most rivers and streams is a lot more complex than we boat operators think.
  10. lazeyjack

    lazeyjack Guest

    jerdi an charmie, i do enjoy your posts
    Now the other day i was rowing my 12 foot dory down river, with the tide,
    As I came to the breakwaters the tide and current were so great I turned upstream and then had to row up against tide and current
    Easing over to one bank I pulled like mad and just stemmed her
    Question is, at my est the combined was 6 -7 knots, so how strong am I?:)) or how much bhp am I generating? wl 11, boat about 60 lbs, me 200
  11. charmc
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    charmc Senior Member

  12. timgoz
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    timgoz Senior Member


    As usual, you are correct. As a former whitewater river guide, I agree as to the complex nature of river currents. We had to make the most use of eddies and such as possible, many times when human lives were at stake.

    1 person likes this.
  13. jehardiman
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    jehardiman Senior Member

    First of all, a quick look at the GPS that was used shows that it has a SPEED function, but not a VMG function. So there is some suspect data there.

    As Rick suggests, a set of upstream-downstream runs will actually give you much more and better data. Try to find a straight section of river, set two waypoints that have a straight course line between them, drive the straight line coure between them as you time the down and up current runs (even better to do each direction 3 times). Work out the average of all the speeds up and down and this is your true boatspeed. True river speed is then the average of downstream speeds minus the true boatspeed.
    Last edited: Mar 6, 2008

  14. SamSam
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    SamSam Senior Member


    I believe hull speed is in relation to the water. A 6 knot speed over the bottom against a 6 knot current is equal to 12 knots in relation to the water but only 6 in relation to the land or bottom.
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