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Catalina/Capri 25/250 Sailor's Forums

Catalina 25 Specific Forum

Higher than hull speed - no, really!

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jandrus
1st Mate

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USA
79 Posts

Initially Posted - 03/18/2002 : 21:39:29

I know we've beaten this topic to death but I saw a link to site I hadn't seen before. http://potter-yachters.org/manyways/hullspeed/ The site lists a more complete way to calculate hull speed taking displacement into account that explains why many of us have exceeded the 6.3 knot figure that old 1.34*sq rt lwl. I did the calculation and for my fin keeled boat it gives a figure of 7.56 knots empty - about right for my experience.

Justin -

c25 #1942 s/v tako kichi

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Greg Jackson
1st Mate

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Response Posted - 03/19/2002 : 23:13:14

It's been at least a day since hull speed issues have been brought up, so I'll dive in without excuses.....

The title of the original post seems to suggest that hull speed is some magical number which cannot be exceeded, or that if it appears to be exceeded then somehow it was calculated wrong. This is not the case. If you imagine a plot of power versus speed, it would be a line with a positive slope. A little power yields a little speed, more power yields more speed. This plot is pretty linear up to a point, then it takes a steep turn up such that it takes a tremendous amount more power to achieve just a little more speed. Hull speed is simply the point where the curve takes a distinct turn up.

The Potter web page referenced is correct, but the equations do not apply to the hull form of the C25. The hull speed of the C25 is indeed around 6.4 or so. Once you get tuned into the boat it becomes quite apparent when the hull speed is being exceeded. The hull speed is fundimentally the natural wave speed for a wave which is about the length of the boat from crest to crest. At hull speed the boat hull fits into that wave very nicely. As you exceed it the bow starts to climb uphill, into a wave which cannot run faster than the boat, and the stern has to leave the water faster than the upswelling part of the wave can replace it. When this happens there is a distinctive sound to the water behind the transom. You can see an upswelling slightly behind the transom and you can hear a gurgling noises. The extra energy which it takes to drive the boat that fast is spent getting the water excited.

Getting the C25 to go 7+ is not at all uncommon. I have actually had mine going at 12.6 knots. I was surfing downwind in 25 knot winds for about 13 hours. By dawn there was more than 100 miles of fetch to the waves behind me and the waves had built to 8 feet with occasional higher. As the bigger waves would pass I would get surfing up into the high 7's and occasional low 8's. When the biggest waves would come by they would begin to break. A breaking wave is a transition where the water at the crest falls off and begins to travel at wave speed. Water normally just moves up and down for the most part, a breaking wave has forward movement of water. In the breaking waves the entire body of water I was sailing in would accelerate greatly. I was still moving at close to 8 knots in the water, but the water was doing around 5 or 6 knots on its own. It was like sailing in a current. It was pretty exciting to be in this sea of boiling foam and watch the GPS jump up above 10 knots while the knotmeter passed 8 knots. I had a series of incidents like that, with the best one putting the knotmeter to 8.5 and the GPS to 12.6. So much for hull speed limits.

G. Jackson
S.V Compass Rose

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Derek Crawford
Master Marine Consultant

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3323 Posts

Response Posted - 03/20/2002 : 00:10:23

Isn't the theoretical maximum hull speed always calculated going to weather?
I've often exceeded 7 knots on a beam reach or when running - but never when beating.
Derek on "This Side Up"

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Derek Crawford
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3323 Posts

Response Posted - 03/20/2002 : 00:13:14

Isn't the theoretical maximum hull speed always calculated going to weather? It also does not apply either when planing or surfing. I've often exceeded 7 knots on a beam reach or when running - but never when beating.
Derek on "This Side Up"

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Derek Crawford
Master Marine Consultant

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USA
3323 Posts

Response Posted - 03/20/2002 : 00:20:06

Sorry for the double post - but I can't find the garbage can! Anyone know who stole it? <img src=icon_smile_sad.gif border=0 align=middle>
Derek

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dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 03/20/2002 : 00:47:45

Greg

It is my understanding that the theoretical hull speed for a Cat 25 is around 6.4. Correct me if I'm wrong, but isn't that figure from a formula that is based on a displacement hull only. At speeds above 6.4, our displacement hull tries to become a planing hull(when surfing) and may exceed the displacement hull speed. But at this point the hull is no longer functioning as a displacement hull and therefore the formula for displacement hull speed no longer applies. So, yes, hull speeds greater than the theoretical speed are attainable, but this happens outside the realm of a displacement hull.

Imagine your hull speed as you surf done Niagara Falls. Now that's hull speed!

Don Lucier, Northstar
C25 SR/FK

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Gary B.
Admiral

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USA
969 Posts

Response Posted - 03/20/2002 : 05:40:10

I think you're absolutely right! Theoretical hull speed is figured on a displacement hull. When the boat planes, or surfs, the figure doesn't apply. My boat will actually motor at 7+ (in salt water, which provides more bouyancy, hence less wetted surface) with my Yamaha 9.9 high thrust wide open, but it's stupid to do it; I can do "hull speed" at 1/2 throttle or so; that extra half a knot takes a lot of power! I have seen 7.0 once while beating to the weather mark in a race, but I suspect my knotmeter. Actually, downwind with the chute, or reaching, I go over "hull speed" fairly often; to weather, 6.3 or so is a great goal.....

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Greg Jackson
1st Mate

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84 Posts

Response Posted - 03/20/2002 : 20:14:37

When a C25 gets to 7 knots or so it is certainly exceeding the hull speed, but it seems like quite a stretch to say it is on a plane at that point. Plowing is more like it. It is certainly attempting to plane, with the bow being pushed up and the transom being sucked down. Never the less, exceeding hull speed does not necessarily imply planning.

The clearest case of that is a submarine. A submarine will also have a hull speed and it can certainly exceed that speed, yet no one would accuse it of getting up on a plane.

An interesting related fact is that old wooden ships were in danger of sinking after a good run of speed. Exceeding hull speed puts pressure on the bow and suction on the stern. On older boats it could actually pull stern planks off the boat. This was not an issue at speed since the open joints were acting like the bailers on small dinghies. However, when the breeze faded and the boat slowed down the water would pour in the open joints and sink the boat.

G. Jackson
S.V. Compass Rose

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dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 03/20/2002 : 20:45:30

Greg,

In your post you state,

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote> When a C25 gets to 7 knots or so it is certainly exceeding the hull speed, but it seems like quite a stretch to say it is on a plane at that point.<hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

but, in my post I never said, "...it is on a plane at that point."

In fact, I said, <BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>At speeds above 6.4, our displacement hull tries to become a planing hull(when surfing) and may exceed the displacement hull speed. <hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

the key words here are, "TRIES to become a planing hull".

Where did you get the idea that I said it was "on a plane at that point"?

Don Lucier, Northstar
C25 SR/FK

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Greg Jackson
1st Mate

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84 Posts

Response Posted - 03/22/2002 : 09:57:31

Don,

Please don't take offense. We know that the C25 cannot really plane and I didn't mean to imply that you thought otherwise. Never the less, to say that any speed in excess of hull speed is outside of the realm of a displacement hull suggests that the C25 is no longer a displacement hull when it goes fast.

I think we all understand it is a matter of degrees. A pure displacement hull floats only because of the hydrostatic forces from the water it displaces. A planing hull is held up by hydrodynamic forces. Any boat which can float at rest is acting as a displacement hull when it's parked. Any boat moving has some fraction of both hydrostatic forces and hydrodynamic forces. When a boat gets above hull speed the fraction that is hydrodynamic starts to increase dramatically and the pure displacement based formulas do not fully explain what is going on.
Greg Jackson

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dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 03/22/2002 : 11:56:16

No offense taken, just spirited debate.

PS. Sometimes I get a little anxious the closer it gets to splashdown!

Don Lucier, Northstar
C25 SR/FK

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capfrank
1st Mate

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USA
31 Posts

Response Posted - 03/31/2002 : 11:22:51

About 9 years ago, with out the use of furling or auto pilot, one day I did 8.5 knots consistantly, with burts to 9.5 knots, on a beam reach, with 4 pieces of rail meat, for a total of 7 people on board. This was recorded on a Knot Stick, checking every 5 minutes for seaweed. I now have a knot log, auto pilot and furling. I can't remember exactly, but I believe I logged over 1,300 miles last year, in the short Maine sailing season. Speed log has been calibrated / checked by GPS many times, and 50-60% of the time, registers higher than the knot log and 40-50% of the time within one or two tenths of a knot. Last summer, I believe the highest speed was 7.3 knots. Just my 2 cents. Happy Easter, spring is almost here.

Fair winds and following seas, _/)
~~~~
Captain Frank P. Grande
Goddess of the Sea Cruises
Sailing Adventures in Maine
www.goddesscruise.com
capfrank@mint.net
207-877-7824

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