[Public-List] hull speed

[George Dinwiddie]

Mike,

Not so much heeling when running wing and wing, but, yes, the hull line 
does lengthen on our boats when heeled.  The square root of 30 is a bit 
under 5.5, which, multiplied by 1.34 gives a hull speed of 7.3 knots.  I 
know you've been on an A30 going faster than that.

Even with a displacement hull such as ours, it WILL exceed hull speed if 
given enough power.  The power curves look something similar to 
http://www.todddunnmicroyachts.com/tortuga/LE_power_curve.JPG  The power 
required goes up tremendously as the boat moves through the hull-speed 
region.  It's the knee of the curve, where the shape changes 
dramatically.  Because hull speed is not an easily identifiable point on 
that curve, different people use slightly different coefficients to 
calculate it.

  - George

On 11/15/11 7:22 AM, Mike Lehman wrote:
> Scary times huh George? But consider this...when over-powered by sails ,
> you are healing over 20, 30, 40 perhaps more degrees. This increases
> your LWL and reduces your wetted surface. In fact, when the boat is
> laying on her side she starts to take on the properties of a planning
> hull, which is why she is able to achieve the speeds under sail. I
> wouldn't recommend reaching with a spinnaker in 35 knots, it's too....
>
> -----Original Message----- From: George Dinwiddie
> Sent: Monday, November 14, 2011 8:46 PM
> To: Alberg 30 Public List -- open to all
> Subject: Re: [Public-List] hull speed
>
> Jeffrey,
>
> On 11/14/11 6:45 PM, Jeffrey wrote:
>> Gordon,
>>
>> With respect, I don't think it is possible to power an A30 hull to 7-8
>> knots. Hull Speed is somewhere around 6.2 knots. My 20hp can power the
>> boat
>> to just over 6 knots at max rpm, but the noise and fuel consumption is
>> wicked. I've hit 7 knots and more for very brief periods of time under
>> sail
>> when hit with a gust, but never under power. These speeds clocked with
>> GPS.
>
> Oh, you can go faster than hull speed. It's just that the power
> required to do so goes up quite fast as the boat starts climbing it's
> own bow wave.
>
> I've been on an A30 going in excess of 10 knots (sometimes pegging 12 by
> the knotmeter), but they get pretty squirrelly with that much of the
> boat out of the water. This was night-racing on Kittwake in 40+ knots
> going wing-and-wing with a reefed main and the #2 genoa poled out on the
> spinnaker pole. It tore the spinnaker pole track off the front of the
> mast and the tiller head twisted off the rudderpost twice.
>
> And ask Mike Lehman about the 10 knot spinnaker deathroll.
>
> - George
>

-- 
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   When I remember bygone days                         George Dinwiddie
   I think how evening follows morn;            gdinwiddie at alberg30.org
   So many I loved were not yet dead,           http://www.Alberg30.org
   So many I love were not yet born.                          also see:
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