Hull Speed

Well, according to this chart (which is not strictly pertinent to your hull, but can be used for a first guess):

your boat requires approxim. 3.0-3.5 t of sail thrust at 11.5 kt.
It doesn't take into account the full keel which you have down there, and which will add something to this number.

If your GPS tells you that you have occasionally attained that speed, it simply means that you have had the required drive force in those occasions.

 

I believe "Hull Speed" is far more a sailboat concern than a power boat issue.

With limitless free wind power (sometimes) the ability in a blow to go "faster" is always great fun.And dock strutting stories.

For marine motorists speed = fuel burn , so most will understand "hull speed" and go a K slower at 1/2 the fuel burn.

 

One armed fishermen, when dock strutting, cannot catch as big a fish (it's a visual joke).

 

Over the last 25 years of sailing the log speed over water and the GPS speed over ground show a daily runs within 5 Nm with some very rear exceptions.
If you can find 1kn of current in the mid southern Atlantic ocean good luck to you
Starting 9 of March 2016 daily runs at 47S 20W - 126 - 190 - 158 - 190 - 171
The passage was 2890Nm in 20.5 days with being hove to the first 5 nights because of ice and Sailing only no motoring.

 

This is one of the exceptions French Guyana to Tobago
From Windora's log 1988 Sailing in the Gulf Steam 620 Nm in 3 days
These don't count as 200 Nm daily Runs

 

Slavi
You can't apply invalid data sets and get anything sensible.

Although you might

 

Of course. But I wanted to express a point in a visual way.
The point is - any given speed can be attained if the required drive force is available.

 

Slavi

Here's a similar type as a rhino model. Can you or anyone else run a delft resistance prediction easily?

It would be close enough and should be interesting.

 

More Data Windora's Log 2001
Wind just forward of the beam
Galapagos to Easter Is
2000Nm in 12 Days

 

IMO, Delftship would give as uncertain results as the Series-62 drag curves, because it doesn't account for the wine-glass sections of full-keel hulls, which have important effects on all hydrodynamic parameters of the hull.

I did something different instead. I had an old simplified Freeship model of a cutter, so I have grossly changed its length, draught, form and displacement to match the shape of Windora. Grossly indeed, because I don't have all that time to dedicate to this issue. Then I have run it through Michlet. The Freeship model is atached below too.

This is the model:


And here are Michlet results for this hull, overlayed to the Series 62 graphs seen before:


At 11.5 kts, Series 64 gives an Rt/W=0.16 , Michlet gives 0.18 . And the qualitative shape of the two resistance curves are also pretty close. Michlet curve is shifted slightly upwards, and that can possibly be the drag effect of the long keel with a big wetted area.

Now, this similarity of the two curves is interesting.
IMO, yet again it gives force to what Ad Hoc keeps saying over and over again - the single most important parameter for the resistance of a well-shaped conventional hull is the slenderness ratio L/Disp^0.333 .

So, at 11.5 kts:

Rt/W=0.16 gives 0.16*21 t = 3.4 t of required thrust force;
Rt/W=0.18 gives 0.18*21 t = 3.8 t of required thrust force.
​

- that's even beyond what I intend when I say "ballpark".

Cheers

 

I didn’t mean to make you work so hard Slavi :

The Delft 1 or 2 resistance prediction I suggested ( not Delftship) would be a good illustration of the urban myth of a hullspeed limit. Particulalry around the speed required for 200 mile days. The Delft series would be good enough for that.

It would be more interesting to show a resistance or powering curve up to say 9 knots.

To get that 11.5 Knots would certainly be hard on the sails !

 

I would agree, but look at those screenshots in the OP.

For sure, this is the first time I hear that a sailboat with that hullform, that type of keel and that L/D ratio can attain such high semi-displacement speeds. I admit to having hard times accepting the claimed data as facts. But on the other hand, I want to believe that Phil C. is telling the story correctly.

So I have limited my comments to simply estimating the drag and assuming that the required power was there, without trying to venture into deeper explanations. Because I have no other explanations.

 

"... I admit to having hard times accepting the claimed data as facts..." - second that!

 

<<shrug>> I don't doubt that someone could snap a shot of a GPS at that speed. I personally have seen a Santana 39 consistently post speeds in the upper teens....while surfing down swell off San Francisco. Hell, while surfing in the Gate, my 26 footer would post 10-12 knts. 200 NM day? That's only slightly over 8 knots, not that hard in the southern ocean, especially if you're going east.

 

Yes JH, a snapshot of the log while surfing down a swell could be the most probable explanation. Otherwise I just cannot see that boat sailing so fast in a relatively flat sea without a consistent external power input (talking about approxim. 280 HP required).
At this point, I would like to hear Phil C. about conditions in which those snapshots were made.