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Catalina/Capri 25/250 Sailor's Forums

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Hull speed

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coach
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USA
231 Posts

Initially Posted - 07/07/2005 : 15:43:38

Ok, quick question. I was told that hull speed on a cat 25 was 6 knots. People have argued that there is no way that a Cat 25 will go faster than the hull speed. At our club, this topic came up while at lunch and it was split 50/50. I called Catalina and the answer that I got was this.

The catalina 25 has been known to travel up to 10 knots. That is the fastest that one has been known to travel according to the people at Catalina. The boat on average will travel between 6 and 7 knots. But an experience sailor in the right winds can get it up to 8 or 9. 10 knots is very rare, but I was told is possible...just not recommended.

Hull speed is a fictional number when it comes to actual speed of the boat. Usually it is very close to what the average sailor can push the boat to.

If you put a 50 hp motor on a catalina, it will travel faster than 10 knots!!!! I was told that no matter what size motor you put on the boat, it will never go faster than 7 knots. Apparently this is not true.

Not that anyone would put a 50 hp on the back of their Cat.

"Yes I am a pirate, 200 years to late" J.Buffet

www.springfieldstallions.com

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ClamBeach
Master Marine Consultant

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Response Posted - 07/07/2005 : 16:31:21

>"...told that no matter what size motor you put on the boat, it will never go faster than 7 >knots"

With enough horsepower, you can waterski behind a bowling ball. If you can supply enough energy to get a body to plane on the water surface, the displacement hull speed limit no longer applies. Supplying enough power to get a C25 to climb out of it's bow wake might provide a frightening ride. (I think it would become quite unstable) ;>)

>"But an experienced sailor in the right winds can get it up to 8 or 9. 10 knots is very rare"

The only way I know to get a C25 significantly above hull speed (under sail) is to get it to surf down a swell... and that's temporary.

Edited by - on

dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 07/07/2005 : 16:44:25

Last week or so on a very wind day, I hit 7.1kts with no main and a hankerchief for a headsail. My top speed ever was 8.4kts on another particularly nasty day last year.

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Lightnup
Master Marine Consultant

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USA
1016 Posts

Response Posted - 07/07/2005 : 17:03:53

But the bowling ball has to have its trim tabs set just right.

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ilnadi
Captain

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452 Posts

Response Posted - 07/07/2005 : 17:17:37

I feel that this needs to go on the quotes page, if we only had one. Do you think a wing keel would act as a hydrofoil?
<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by ClamBeach
 With enough horsepower, you can waterski behind a bowling ball.
<hr height="1" noshade id="quote"></blockquote id="quote">

I thought hull speed was related to water resistance (both friction and hydrodynamic due to bow wave) on a displacement boat, not application of power. So the only way to exceed hull speed should be:
1. "fall" down a wave
2. heel and increase the waterline (see Bill Holcomb's [url="http://www.catalina25-250.org/tech/snkhelm.html"]weatherhelm discussion[/url])

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Steve Milby
Past Commodore

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USA
5932 Posts

Response Posted - 07/07/2005 : 17:34:59

In discussions on other forums some very knowledgeable people have said that the theoretical limitation on hull speed only applies when a boat is beating to windward. In my own experience, I've never exceeded hull speed on a boat beating to windward, but have done so many times on boats sailing downwind.

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TRogers
1st Mate

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USA
90 Posts

Response Posted - 07/07/2005 : 19:52:51

My understanding of hull speed is that it is not a "brick wall" limitation, but a matter of the amount of power necessary to achieve the next knot (or fraction there of) starts increasing dramatically as you approach/surpass hull speed.

Whereas 10hp might get you all the way to hull speed, and additional 10hp might be required to get you over the top for an extra knot. In other words given enough horses you can do just about anything, just a matter of whether its practical or not.

For the Trekkie types out there, that warp drive they use to go faster than the speed of light (well actually warp space around the ship) is actually supported by scientific theory, however the amount of power required to do it is totally unrealistic by any reasonable ability to generate it.

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ClamBeach
Master Marine Consultant

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3072 Posts

Response Posted - 07/07/2005 : 22:08:08

>"water resistance (both friction and hydrodynamic due to bow wave) on a displacement boat, not >application of power"

That's true up to the point where you apply enough thrust to get the boat to climb out of it's own wave and start to plane. Up to that point, application of more power causes the boat to throw a bigger and bigger wave and it digs itself a hole deeper and deeper into the water. (you apply lotsa power, with little increase in speed).

I can easily play with this in my powerboat... bring up the throttle slowly and she'll plow a trench all day... give it a little more gas and you hit a 'magic speed' and she climbs out... once it climbs onto plane, the speed increases radically without touching the throttle.

Latitude 38 had a photo a few issues back of a 20-something footer running downwind under spinnaker, wildly overpowered and digging a magnificent trench through SF Bay.

IMHO: Hull speed limits are a indeed bit 'fuzzy'... as speeds increases, you begin to get some hydrodynamic lift, which starts altering the equation that set the speed limit in the first place. Get enough lift and you plane. Unfortunately (fortunately?) with our hull design, we only see the first glimmerings of this phenoenma.

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ronrryan
Admiral

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USA
561 Posts

Response Posted - 07/07/2005 : 22:28:15

Here is a topic to daunt the most knowledgeable, really! Way beyond me, but here are two items to consider: (1) You can put all the power you want into a real tugboat, and it will never plane. (2) Years ago, a man named John Rowland designed a modified dory sailboat hull, I think I remember about 25 feet. He had a picture in his book "Wind and Salt Spray" showing Mouette under power at eight knots, leaving no wake. He said he could not explain why the vessel exceeded theoretical hull speed, he didnt expect it to, and then it did! He was an extremely reliable and serious fellow, and I believe him. So there seem to be more parameters to this topic than are easily explored, even by skilled designers. Fair winds, ron Orion srsk (awaiting Dennis, as we speak) in SW FL

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Douglas
Master Marine Consultant

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1595 Posts

Response Posted - 07/08/2005 : 00:34:08

It all depends on how you look at it. I had my C-25 up over 14 Kts when I owned her. BUT that was over the ground going through Tacoma narrows with an outgoing tide . LOL

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Turk
Admiral

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USA
736 Posts

Response Posted - 07/08/2005 : 07:08:40

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by coach
 
The catalina 25 has been known to travel up to 10 knots. That is the fastest that one has been known to travel according to the people at Catalina. The boat on average will travel between 6 and 7 knots. But an experience sailor in the right winds can get it up to 8 or 9. 10 knots is very rare, but I was told is possible...just not recommended.
<hr height="1" noshade id="quote"></blockquote id="quote">

Well, I've had my 250 up to 67 mph with no problem hooked to the back of my Ford 150 leaving Witchita for it's trip to it's new home. I encountered 10-15 headwinds, but nothing that seemed to slow the pace that much. Crosswinds outside of Chicago caused a little problem for the wing keeled 11 foot high package, but nothing a seasoned sailor couldn't overcome.

I can't figure how catalina got their numbers so wrong?

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dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 07/08/2005 : 08:13:43

While out last night is a nice stiff breeze, I hit 7.6kts on a beam reach with a quartering sea.

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ilnadi
Captain

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452 Posts

Response Posted - 07/08/2005 : 09:55:18

GEEK ALERT [the following post has a high geek quotient]
At the risk of taking this to another level, how many of these speeds are from a GPS?

An upper-limit case: GPS receiver 6ft above the CG of the boat swinging through a 60 degree arc in 1 second:

[6x(2pi)x(60/360)] -> 6.28ft/sec x 3600 sec/hr / 6076 ft/nm -> ~3.7kts
going downwind at 30deg, if you swing in the direction of travel this will add 3.7sin(30) -> 1.85kts to your speed

beware!

Edited by - ilnadi on 07/08/2005 09:57:01

Esteban
Navigator

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139 Posts

Response Posted - 07/08/2005 : 16:08:08

9.1 kts according to GPS baby!!

Top speed on Tally Ho!

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lcharlot
Master Marine Consultant

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Antigua and Barbuda
1301 Posts

Response Posted - 07/08/2005 : 17:05:11

My $0.02 worth:
Speed under sail: I have had both of my C-25's (#1205 '79SK/SR and #5857 '89WK/TR) up to about 6.5~6.6 knots a few times on Folsom Lake. This is speed indicated on GPS, on relatively flat, sheltered, non-tidal water, a clean bottom, new sails (full batten main and 135% genny), and standard cruising displacement (water/fuel tanks full, etc.). To acheive speeds above 6.0 knots, I have to trim the sails for maximum power, beam reaching in 15 knots of wind, and the boat is heeling between 35 and 40 degrees; not a comfortable ride and it would have badly frightened my wife had she been with me those times. Also, the few times I have pushed the boat to 6.3+, it never lasts more than 15 to 20 seconds (usually during a gust), before the rudder cavitates and the boat rounds up.
Speed while motoring: My Yamaha T8 outboard, with a High Thrust 11 x 9 prop, will push the boat to 6.1 knots at maximum throttle, but this is really wasteful, as a lot of the extra gas the engine is burning at full throttle is just being wasted in generating a deep wake trough and big bow waves. The motor will push the boat at 5 knots at 1/2 throttle, or 4 knots at 1/3 throttle; this is MUCH more fuel-efficient, and who needs to go faster than that? If I wanted speed, I wouldn't be mucking around on a 25' displacement-hull sailboat.
So, 7+ knots in a Catalina 25 might be possible in really heavy wind, or surfing down big ocean swells, but those conditions are not what the C-25 was designed for (or any sailboat with a pop-top for that matter).

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JimB517
Past Commodore

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USA
3285 Posts

Response Posted - 07/08/2005 : 20:44:35

the hull speed of 1.34 times the square root of the waterline in feet is an approximation based on some general factors about displacement hulls. Prismatic cofficent is the measure of how canoe-like the hull is. Canoe hulls will go a lot faster. The hull speed of a catamaran does not obey the above law because of this. Nor does a duck, whos hull speed should be about 1 knot. This is the speed when the wavelenght of the wave generated by the boat equals the length of the boat. A power boat on plane does not obey this law - with enough power a boat can climb out of this wave and surf in front of it. Large flat aft sections + a hundred horsepower or more allow a boat to do this. Trim is essential.

All that said, the formula applies to our boats and a value of 6.3 knots is generally assumed as a nominal top speed.

The catalina 25 acutally has a pretty flat aft section, and when conditions are right can easily exceed this speed through the water. (through the water, GPS speeds over ground are meaningless).

Conditions are right when there is plenty of wind power, the boat is on a beam reach or slightly aft, and large waves provide a powerful lift forward.

I've had Indiscipline up to speeds in the high 7s through the water for a short time.

When the boat is well heeled over it will "plane" on its side a little.

I really wish I had a Melges 24 or one of the boats that would really plane off the wind. I've seen Lasers pass me by like I was standing still downwind, with the sailors hiked out and pumping the main to get on plane with a following wave.

I've never really surfed a wave in Indiscipline, just gotten well lifted and pushed forward for several seconds.

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Dave Bristle
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Djibouti
10005 Posts

Response Posted - 07/09/2005 : 11:10:19

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by JimB517
 the hull speed of 1.34 times the square root of the waterline in feet is an approximation based on some general factors about displacement hulls. Prismatic cofficent is the measure of how canoe-like the hull is. Canoe hulls will go a lot faster. The hull speed of a catamaran does not obey the above law because of this. Nor does a duck, whos hull speed should be about 1 knot. This is the speed when the wavelenght of the wave generated by the boat equals the length of the boat. A power boat on plane does not obey this law - with enough power a boat can climb out of this wave and surf in front of it. Large flat aft sections + a hundred horsepower or more allow a boat to do this.
<hr height="1" noshade id="quote"></blockquote id="quote">
I was just about to say all of that till I got down to Jim's reply...

I've seen the "wave length equal to waterline length" explanation, and it has also to do with the angle of the bow wave, which becomes more acute as the boat gains speed, and therefore lengthens the "apparent wave length" relative to the hull. At some speed, the boat is sitting centered in the trough of the "apparent wave" and won't go faster until it has enough power to climb out. The power can come from wind or the Middle East, and can also come from gravity (on the front side of a swell). A knife-like catamaran hull just slices through its bow wave--given enough power. I've felt the hesitation on a Hobie 16 as it reaches hull speed and then suddenly accelerates as it breaks through its wave.

I recently went through the wake of a tug that was apparently trying very hard to break its hull speed--that was a scary moment--his wake was vertical and breaking (in deep water)... Passage went airborne (literally) and then submarined the second wave with the loudest "bang" I've ever heard in a boat. Fortunately, I had told my crew to dog down everything and close the companionway as we watched the curling wake approaching. So guys, watch that hull-speed--you might hurt somebody in a lesser boat!

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reuben
Navigator

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USA
202 Posts

Response Posted - 07/09/2005 : 16:07:43

<blockquote id="quote">quote:<hr height="1" noshade id="quote">GPS speeds over ground are meaningless<hr height="1" noshade id="quote"></blockquote id="quote">

Not for those of us on inland lakes. Sailing alongside other boats with paddlewheel-type speedometers, we're sometimes two knots apart while sailing rail to rail, but GPS readings were consistent.

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Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 07/09/2005 : 23:27:51

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by reuben
 <blockquote id="quote">quote:<hr height="1" noshade id="quote">GPS speeds over ground are meaningless<hr height="1" noshade id="quote"></blockquote id="quote">

Not for those of us on inland lakes. Sailing alongside other boats with paddlewheel-type speedometers, we're sometimes two knots apart while sailing rail to rail, but GPS readings were consistent.
<hr height="1" noshade id="quote"></blockquote id="quote">
Agreed... Paddle wheels are generally useful only in a relative sense--you can tell whether you're going faster or slower than before, but you can't be sure how fast you're really going--and that's if they work at all, which mine decide not to this year (until I go swimming one of these days). The best thing about GPS is it tells you where you're really going (in currents and such) rather than just what direction the bow is pointed. When you're in the vicinity of The Race, you want that!

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At Ease
Admiral

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672 Posts

Response Posted - 07/10/2005 : 14:20:41

It's true that GPS readings are consistent, but what they are telling you is ground speed, it has nothing to do with boat speed through the water. As for the initial question, I agree with Catalina's answer. I can routinely exceed the hull speed in my C320, without really trying. All this makes me agree with what I heard hull speed called when I heard it used for the first time..."theoretical hull speed."

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takokichi
Captain

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USA
321 Posts

Response Posted - 07/11/2005 : 11:08:23

For whatever its worth, yesterday we hit 7.2 through the water on a screaming reach in a race. We see 7 knots with a clean bottom and 18 knots somewhat regularly.

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