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 Hull Speed (and beyond?)		  New Topic  Topic Locked
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Ben
Master Marine Consultant

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USA
1234 Posts
Initially Posted - 05/13/2007 :  11:37:45  Show Profile
Hi folks...

This season so far in Central Ohio has seen some very favorable wind conditions for the Catalina 25. In one of these 15 mph wind days, we were out for a pleasure cruise. We had the full main up and the 130 jib.

Last year we hit hull speed (6.2 knots, right?) once, and we were happy to do that. On this pleasure cruise last weekend, we easily hit 6.2 knots, regularly hit 6.3, 6.4, and even 6.5 knots, and we maintained 6.5 knots for many seconds.

Am I missing something? Where I sail is a small inland reservoir, so there should be no current, and obviously no tidal effect. It seems to me that we shouldn't have been able to go above 6.2 knots. Maybe I'm not understanding something.

The speed reading was taken with a handheld GPS, and it was/is calibrated for displaying speed in knots, rather than mph.

Thanks all!
Ben
Beneteau 361
Viking Kitty
Columbus, Ohio
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Derek Crawford
Master Marine Consultant

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USA
3324 Posts
Response Posted - 05/13/2007 :  11:43:00  Show Profile
Ben - what point of sail were you on? Theoretical hull speed is only the maximum when going to weather. A beam reach can be much faster - I've hit 10k on occasion!
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Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts
Response Posted - 05/13/2007 :  12:21:07  Show Profile
As I've been led to understand it, theoretical hull speed is the speed at which the angle of your bow wave makes its wavelength, relative to your hull, extend from bow to stern, so amidship you're sitting in the center of the trough. In theory, the only way to go faster is to climb over the bow wave, becoming at least "semi-displacement" (if not outright planing). This normally takes more power than a sailboat has unless (1) you're surfing down the front of a larger wave, or (2) you have enough drive, usually on a beam reach, to get your bow up. It definitely can be done.

I also think the formula is a bit arbitrary, and doesn't consider all variations in hull form. Rowing shells seem to be able to exceed the theoretical limit with ease (without planing). Barges don't seem to be able to come close to it. However, I know well that kayaks are limited in speed by their length, although I can beat the theoretical limit for a few seconds on mine by really leaning into it and getting the bow up.

I once encountered a tug (no tow) that was pushing its bow wave as hard as it could, creating two vertical walls--he was right at hull-speed and trying to beat it. As we passed him about a quarter of a mile away, Passage went airborne off the first wave and then dove into the second, with green water over the cabintop and into the cockpit. Fortunately, before we got to the wake, I had a friend dog down the forward hatch, or we would've taken a BIG load of water into the cabin! (We were motoring in no wind with the hatch open.)
Edited by - Dave Bristle on 05/13/2007 12:30:20
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Ben
Master Marine Consultant

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USA
1234 Posts
Response Posted - 05/13/2007 :  12:21:48  Show Profile
Hi Derek...that may be the answer, we were mostly on a beam reach, going a little bit to windward.
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ct95949
Captain

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Aruba
300 Posts
Response Posted - 05/13/2007 :  14:39:58  Show Profile
<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by Derek Crawford</i>
<br />Ben - what point of sail were you on? Theoretical hull speed is only the maximum when going to weather. A beam reach can be much faster - I've hit 10k on occasion!
<hr height="1" noshade id="quote"></font id="quote"></blockquote id="quote">
I thought I read that hull speed was about the maximum a particular displacement hull could be moved through the water, without surfing or planing, regardless of point of sail. This is the first time I've ever heard of a C-25 doing over 8 knots. My sail trimming must be way off since I've never done more than low sevens .
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Champipple
Master Marine Consultant

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USA
6855 Posts
Response Posted - 05/13/2007 :  15:14:02  Show Profile  Visit Champipple's Homepage
Last weekend during the transport to the new owners location the Speed over ground on the GPS hit 14.2kts. This was on the back side of an 8 foot wave in gusts to about 28. Harvey actually had the boat surfing one wave for a good 20 seconds. It was fun all the way up until the point when the rudder split in two but that is another story for another day.

Upwind with decent windspeeds and flat seas we probably could get the boat to around 6.5 or 6.8kts. Our downwind speeds with full symetrical Kite without planing where probably in the high 8's.
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Andy_334
Navigator

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USA
206 Posts
Response Posted - 05/13/2007 :  20:29:30  Show Profile
Duane: you can't leave us in suspense with a tease like that..........do tell....
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ClamBeach
Master Marine Consultant

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3072 Posts
Response Posted - 05/13/2007 :  21:41:16  Show Profile
"...variations in hull form... even 6.5 knots, and we maintained 6.5 knots for many seconds..."

There is some 'black art' involved in determining true hull speed. When heeled, the waterline length changes which effectively makes the boat hull present a bit longer footprint in the water. Hence a slightly higher hull speed will be possible. Your 6.5 kts sounds quite reasonable if the boat was heeled a bit.

"Rowing shells seem to be able to exceed the theoretical limit"

Extremely narrow hulls don't follow conventional hull speed formulas. Hence the greater boatspeed available in catamarans, trimarans and racing shells.
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At Ease
Admiral

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672 Posts
Response Posted - 05/15/2007 :  19:02:05  Show Profile
The key word in theoretical hull speed is 'theoretical.' I routinely exceed what it is 'supposed' to be...which is 1.34 X the square root of the water line length. Also, it is a speed through the water...GPS is ground speed which has nothing to do with hull speed (unless the water is totally still, no tide, no current, no wind, no wakes, etc).
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Notice: The advice given on this site is based upon individual or quoted experience, yours may differ.
The Officers, Staff and members of this site only provide information based upon the concept that anyone utilizing this information does so at their own risk and holds harmless all contributors to this site.