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Catalina/Capri 25/250 Sailor's Forums

Catalina 25 Specific Forum

What is Hull Speed?

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Cate
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199 Posts

Initially Posted - 05/29/2007 : 08:15:04

Now that our knot meter is working again we are having fun using it to trim the sails and see how fast we can go. My question is, how fast can we go? Or does it depend on the boat?
Average?

1983 C-25, Standard rig, swing keel

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dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 05/29/2007 : 08:37:07

How fast can you go? My personal best is 8.4kts, but on a moderately windy day I'm usually in the 6-6.5 range. On days with winds a little higher, on occasion I can get to 7kts or so.

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Sloop Smitten
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Response Posted - 05/29/2007 : 10:10:55

The following is the formula used to calculate The Theoretical Maximum Hull Speed:
Maximum Hull Speed = 1.34 * sqrt(LWL)
LWL: Boat Length at Water Line (C25 = 22.17')

The theoretical maximum hull speed of the boat is: 6.31 knots.

You can go beyond that with an assist: mechanical, current, wave, gravity, etc.

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Dave Bristle
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Djibouti
10005 Posts

Response Posted - 05/29/2007 : 11:04:20

Theoretical hull speed (THS) is the speed at which the boat must be able to climb up on its own bow wave in order to go any faster. To do that generally requires more power than a typical cruising sailboat has except under ideal conditions. Planing sailboats (starting with Sunfish and going through the 60' sleds that race around the world) can easily exceed their THS. The C-25 is not a planing hull, but it can climb over its bow wave on a broad reach in a good breeze, or "surfing" on the front side of a large wave. The evidence is the spray coming off the bottom near the bow--part of what must happen as you exceed THS.

Under sail, exceeding THS on a C-25 is great fun--under power, it's more difficult and a huge waste of fuel and noise. So, on a nice breezy day (15 knots should do), get on a course of about 120 degrees off the wind, move your traveler to leeward, ease the sheets just enough to get the telltales flowing aft on the sails, and let her rip!

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John Russell
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Response Posted - 05/29/2007 : 11:40:18

Dave,
You wanna go sailing with me? You know, once a week for maybe the next twenty years or so? I enjoy reading your common sense posts peppered with sound sailing knowledge and a dash of humor. Wish I lived in CT.

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Steve Siefken
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Response Posted - 05/29/2007 : 14:26:59

Cate,

All the above is true, however, please remember that your knot meter will tell you how fast the water is moving by the boat, not necessarily tell you how "fast" you are going. A GPS will give you speed over ground. If you are sailing where there is a current (i.e., river) or tide change (i.e., salt water), you might be fooled by your knot meter.

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At Ease
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Response Posted - 05/29/2007 : 15:06:29

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Steve Siefken
 Cate,

All the above is true, however, please remember that your knot meter will tell you how fast the water is moving by the boat, not necessarily tell you how "fast" you are going. A GPS will give you speed over ground. If you are sailing where there is a current (i.e., river) or tide change (i.e., salt water), you might be fooled by your knot meter.
<hr height="1" noshade id="quote"></blockquote id="quote">

Agree.

Also, you can be fooled by the GPS for the same reasons

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Dave Bristle
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Djibouti
10005 Posts

Response Posted - 05/29/2007 : 15:09:51

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Steve Siefken
 If you are sailing where there is a current (i.e., river) or tide change (i.e., salt water), you might be fooled by your knot meter.<hr height="1" noshade id="quote"></blockquote id="quote">
True, but not regarding THS... That's a thru-the-water speed barrier. When trying to determine how you're doing relative to THS with a current present, your GPS will mislead you and your knotmeter will tell the story. The same can be said for most "tuning" and experimenting for maximum speed.

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dlucier
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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 05/29/2007 : 15:21:05

[url="http://en.wikipedia.org/wiki/Hull_speed"]Hull Speed[/url]

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Dave Bristle
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Djibouti
10005 Posts

Response Posted - 05/29/2007 : 15:45:55

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by John Russell
 Dave,
...I enjoy reading your common sense posts peppered with sound sailing knowledge and a dash of humor. Wish I lived in CT.
<hr height="1" noshade id="quote"></blockquote id="quote">
The CT shore is hard to beat, especially on the eastern end with its ocean breezes, protected waters, and nearby destinations! As for my knowledge, I've been known to say more than I know.

You should've seen the argument over water ballast 6-7 years ago--me versus some Stanford math professor (?) who said he was backed up by his brother, a Purdue physics professor... (I think everyone else ran for cover!) I don't know about his "backup", but he stuck tenaciously to his story, and he was wrong.

Edited by - Dave Bristle on 05/29/2007 15:46:59

Johnny Whaler
1st Mate

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30 Posts

Response Posted - 05/29/2007 : 20:59:46

I had my 1980 C25 at 8+ knots the day I bought it. My buddy has been sailing for 30+ years and he went with me to sail it home. I nevr sailed before and I was never so scared going less than 10MPH in 30 knot winds!

The GPS and the knot meter were both over 8 Knots. The rail was in the water and I just held on! If my old keel bolts didn't break that day they nevr will!

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DaveR
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2015 Posts

Response Posted - 05/30/2007 : 23:19:00

Good story Johnny sounds like way fun and a little worry! I'm seeing it! Hull speed on this boat is somewhere close to 6.5 knots as far as I know but your GPS will give you the true "mass over land" reading, which is the only answer that's relevant.

Edited by - DaveR on 05/30/2007 23:21:13

Jmurfy
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Response Posted - 06/03/2007 : 00:29:01

I hit 7 knots as verified by my GPS last week!

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Derek Crawford
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Response Posted - 06/03/2007 : 14:03:45

I had 12.4 knots reading on my knotmeter a while back - which, with its deliberate 30% over-read converts to 8.68k. We were just on a 110% jib and full main, beam reaching.

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Dave Bristle
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Djibouti
10005 Posts

Response Posted - 06/04/2007 : 12:26:20

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Derek Crawford
 ...8.68k. We were just on a 110% jib and full main, beam reaching.

<hr height="1" noshade id="quote"></blockquote id="quote">
...with a nice little Kevlar jib and a 6-month-old main, right?

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Derek Crawford
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Response Posted - 06/05/2007 : 08:58:26

Strangely enough Dave, it was an old worn out 110% and a 6 year old main...this was before I got serious about racing

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Dave Bristle
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Djibouti
10005 Posts

Response Posted - 06/05/2007 : 10:40:17

I guess on a beam or broad reach, it doesn't matter... Full sails can be an advantage. I got Passage on plane (all things being relative) several times with 6-8 year old brand-x sails, but didn't get a speed measurement. Now all I do is say, "Hang on--here we go."

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At Ease
Admiral

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Response Posted - 06/08/2007 : 14:43:19

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Jmurfy
 I hit 7 knots as verified by my GPS last week!
<hr height="1" noshade id="quote"></blockquote id="quote">

The GPS speed has nothing to do with hull speed...GPS is ground speed only, hull speed is thru water speed. If you had a 5 knot tide/current tail sea, your thru water speed was 2 knots.

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stampeder
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Response Posted - 06/08/2007 : 17:29:51

The other day while out picking up tree branches from a wind storm...I told my next door neighbour that I was going 100 miles per hour just as that storm began to build and while trying to get back to my mooring ball.
It did however, feel like 100 MPH.

So Cate, to answer your question, hull speed is around 6.7 knots unless you are relating a harrowing yet exciting event that only you could have sailed through, in which case, hull speed is 100 MPH.

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