Hull Speed

Why do some displacement vessels sail well past their hull speed. The following yacht as an example has a waterline of 38' a draft 6' 6" and a displacement of 45,000 lb's dragging a turning Prop of 28"x17". Sailing over 200 miles per day on ocean passages once or twice a season.

 

Were they sailing with a strong ocean current like the Gulf Stream?

With a 38' LWL the hull speed would be roughly 9.5 MPH, so 12.1 kt, basically 14 MPH, taking advantage of such a current might be doable.

 

Will also happen with a heck of tail wind too.

 

These sailing runs are mostly mid ocean no or little current and have occurred over the past 25 years. The yacht easily sails up to 10 and 12 knots - with the wind forward of the beam, all the way aft, including wing and wing She requires only 25+kn's to obtain a 200 plus Nm day. Do some wooden boats break the rules ?

 

Is this a translation problem?

200nm requires 25Kts?

24hours x 25 = 600NM

24hours x 8.3Kts = 200NM

8.3 is under the hull speed shown above of 12Kts.

Hull speed is dependent upon length and displacement, not material construction.

 

Requires 25+ knots of wind over a 24 hour period to sail 200 plus NM in one day. This not always being easy as there is no weather information on board. Hull speed is the square root x 1.34=8.2 knots .You have to keep hitting 10+ knots under sail to average the 8.5 knots to cover 200 Nm day. This you only learn by doing the miles at sea.

 

Because 'Hull speed' is a misleading term. People imagine that it’s a boundary but it’s just a sensible design point for powering. At this speed the driving energy requirement is increasing exponentially for a linear speed increase and it's just an economic and logistic tradeoff (that limit doesn’t worry many naval vessels).

Most cruising sailboats will operate around 1 rather than 1.34 factor for most of a passage but when the wind gets up the speed of a sailboat isn’t limited by any set design point the way a power boat is, but by her hullform and power to carry sail.

Given favorable course, sea conditions and the stability to carry sail area the displacement boat with a suitable hullform can exceed the rule of thumb ‘hull speed’ generally by anything from 15 to 25%, when the wind blows enough. The drag is rising rapidly but the driving force also increases non linearly as a function of wind speed squared. Speed is just a balance of drag and driving force.

For the craft pictured these speeds will be on a reach with the wind above a fresh breeze.

 

Years ago the AYRS came up with a simple method of figuring "hull" speed for a variety of boats.

The old formula was created by looking at fat boats of the time , and doesn't work well for skinny fast boats.

Speed = L/3b X SQRT (L)

When run for some theoretical hulls it may be a bit fast , but seems to work.

 

"...Sailing over 200 miles per day..."
I just don't believe it!

 

Hi Phil, is that the Windora yacht which is currently in the South Africa area?

 

Absolutely correct; there is no limit to hull speed except power. See my old post on the subject.

http://www.boatdesign.net/forums/motorsailers/expensive-hull-speed-11342.html#post97099

 

I see that hull speed formulas are more a guideline for vessels under power than sail. Windora is currently in Saldanha just north of Cape Town. Sailing from South Georgia to Cape Town we only managed to do two 190 Nm days noon to noon. When sailing in strong winds and large seas 4-6 metres the yacht looses to much speed in the bottom of the troughs .She has only managed to exceed 200 Nm per day, nine times from midday to midday. The naval architect Athol Burns designed over 700 vessels and like Herreshoff and other prolific designers one or two proved to be exceptional.

 

Then I guess that these length and beam data are not correct: http://www.marinetraffic.com/en/ais/details/ships/shipid:699307/mmsi:512004381/imo:0/vessel:WINDORA ?
If they are, the speed potential of a 15m (49') hull is higher than that of a 11.5m (38') hull. That can be part of the explanation for excellent speeds you have recorded.

 

Windora - LOA including bowsprit 15m X LOD 13.1m x Waterline 11.59m x Beam 4.0m x Draft 2.0m x Displacement 21,000Kg's

 

are we talking about noon-to-noon runs, over ground, veryfied by GPS???
if not GPS: also interesting the adjacent runs (day before & after...)
how much current? (3kn would add 72nm...)