Association Forum - newbie question

Assn Members Area · Join

Home.

Association Forum

Ships Store

Cruising

Racing

Technical Tips

Photo Gallery - New

Boat Search - Updated

Manuals & Brochures

Swap Meet

Links

Officer Flag Line & Staff

Bylaws & Constitution

Members Area

Join the Association

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.

Association Forum

Username:	Password:
Save Password
Forgot your Password?

All Forums

Catalina/Capri 25/250 Sailor's Forums

General Sailing Forum

newbie question

New Topic

Topic Locked

Printer Friendly

Author

Topic

Page: of 2

Walt Oler
1st Mate

Member Avatar

USA
44 Posts

Initially Posted - 01/29/2009 : 19:28:47

As I understand it, there is no point in putting more than a 9-10 hp motor on a Catalina 25/250 because that is sufficient to reach the "hull speed" and it would take a lot more power to go any faster. Does the same apply to wind power? What is the necessary wind speed to reach the hull speed of a Catalina 25 and can the boat not go any faster in a higher wind?

Edited by - Walt Oler on 01/29/2009 19:29:04

pastmember
Master Marine Consultant

Members Avatar

2402 Posts

Response Posted - 01/29/2009 : 19:57:41

HUll speed in displacement mode is hull speed, more sail does not help. The only time that is not the case is when catching a wave and surfing a little, a C-25 will not plane so it is a momentary event. A big factor in getting to hull speed is the condition of your bottom, how clean and smooth is it? If you want to be a fast C-25 clean your bottom and foot off a little.

Edited by - on

delliottg
Former Mainsheet C250 Tech Editor

Members Avatar

USA
4479 Posts

Response Posted - 01/29/2009 : 20:07:35

Walt,
Hull speed on a displacement hull is calculated using the formula: velocity in knots=1.35 x #8730;waterline length. This works out to pretty close to 7 knots for our boats. How it gets to that speed, either by an outboard engine, or it's sails makes no difference, the boat can only go so fast, no matter what. There are caveats to this, you can go slightly faster by surfing down a wave or wake, and in an enormous blow, you can also pick up some speed, but it'll be short lived. I've seen my boat at around 7.5 knots on my GPS, but there's no way for me to be able to verify if that was boat speed, or combined current & boat speed.

When you're under sail, the boat can only absorb so much energy from the wind, the rest is spilled by heeling the boat, easing the sails, reducing sail by furling, reefing, or hanking on a smaller jib. That's why there's really no point in heeling beyond optimum angles to reduce wetted surface, you're simply not going to go any faster & it makes the crew antsy.

In order to go faster than hull speed, the power necessary to be applied to your hull increases exponentially. Interestingly, I've read that you can sink a boat by going past hull speed, the water is pushed out of the way faster than it can provide support to the hull, so the hull begins to sink. I've experienced something like this once on my old catamaran when I was out sailing in conditions I probably shouldn't have been. My port hull sank underwater for a short period of time, maybe ten seconds? Then the boat rounded up and the hull popped back up onto the surface. That was a fun/scary day.

Edited by - on

Deric
Captain

Members Avatar

USA
408 Posts

Response Posted - 01/29/2009 : 20:20:45

Walt,

I agree with Dave and Frank. I was able to get my boat to 7.2knots, but that included riding a wave - - lasted 2-3 seconds.

The boat is comfortable at 5.5 - 6 knots. The boat won't be a speed boat.
I have had my boat sail faster with a reefed main than with it unreefed. It depends on the amount of wind and the amount of heeling -- as mentioned above.

Deric

Edited by - on

Prospector
Master Marine Consultant

Members Avatar

Canada
3159 Posts

Response Posted - 01/30/2009 : 08:14:31

I thing the OP was asking how much wind do I need to reach hull speed... The answer to that depends on the sailor. I don't know of a "formula" that will give you the answer, but depending on point of sail, sail configuration, trim, crew/ballast placement, the condition of your bottom, the tension of your rig, the variability of the wind (direction and speed), wave action, tall/short rig, and whether or not you have on red underwear, your answer will vary.

There have been times when out in a blow I could hardly get any forward speed up (over canvassed, and a bad reef), and ther ehave been times when I was tooling along on barely a breath of air (5.5 kts one night in a calm before a thunderstorm).

The sign of a really good sailor is thta they can make amazing headway on no wind. I think the amount of wind it takes to reach hull speed is mostly dependant on the sailor's ability. Some day I'll get it all figured out and be able to consistently get good forward speed.

Edited by - on

Steve Milby
Past Commodore

Members Avatar

USA
5902 Posts

Response Posted - 01/30/2009 : 08:43:30

I generally agree with the others, but disagree that a C25 can only surf on a steep wave for short periods of time. Last year I sailed my C&C 35 Landfall (the cruising version, which has a similar underbody and rig as the C25) on about a beam reach in small craft warnings on the Ches Bay, and surfed for hours at nearly 2 knots above hull speed. Hull speed seems to only be a limiting factor when sailing to windward. Moreover, your sails can generate far more power off the wind than your motor. As you bear off downwind, the amount of power generated by your sails increases dramatically, and the size of waves downwind increase. The waves don't have to be really big, but they have to be just big enough to provide a little boost to your boat. I think what happens is that, as the sail power increases, and the waves start to add a little boost, the boat's bow wave moves far aft. As the bow wave moves aft, it gradually becomes less of an impediment to the boat. At some point, it only takes a little more power (such as a brief gust of wind) to drive the boat up over the bow wave, and the boat starts to surf, and the combination of all those forces enable the boat to surf for long periods of time. My theory is that the critical point is when the bow wave moves aft of the boat's center of gravity. At that point, the bow tips down, and the boat begins to surf down the face of the wave. It lasts for as long as all those forces remain in harmony.

Edited by - Steve Milby on 01/30/2009 09:28:20

Dave Bristle
Master Marine Consultant

Members Avatar

Djibouti
10005 Posts

Response Posted - 01/30/2009 : 09:54:42

I too have measured speeds well above my C-25's theoretical hull-speed of 6.3 knots (David means "square root" above)--like Steve, sailing off the wind on a broad reach. As I understand it, you reach "hull speed" when your bow wave angles back such that its wavelength, measured parallel to the hull, puts your boat in the center of the wave's trough. Then the only way to go faster is to apply enough power to "climb out" of that trough. The narrow displacement hull won't help as much as a broad planing hull in doing so, but it can be done. A pretty good indication is spray (not just waves) flying out from about amidships. Take a close look at a picture of a sailboat that's really hauling to windward, and you can see how it's centered in its own trough. (Makes me want to go sailing--right now!)

Waves interact... You have your bow wave, and you have the seas you're sailing in. If the seas are bigger than the bow wave (and they often are around here), they will affect your boat more than the bow wave does. If the seas are following, as you surf down the face of a wave, you can easily exceed hull-speed--your bow wave is not a factor. As the wave passes you, your momentum will hold for a while. So on a broad reach, you might see 7.5, 6.8, 7.5, 6.5, etc. on the GPS. Upwind the waves generally work against you, and on a beam reach they're neutral--it's you and your bow wave.

Surfing on waves can be a little tricky, however... As a wave moves through the water, it momentarily carries some water with it--not very far, but there has to be some motion. That motion can be such that your rudder momentarily loses steerage--it's like it's sitting still in the water. This is what can cause a boat to round up abeam to the sea and broach when running downwind in big seas. You can feel the rudder lose its control--the tiller sort of wobbles in your hand as you begin to surf until you start to round up, and then the pressure becomes enormous as the water moves in the opposite direction to form the next wave. That motion can also fake out a knotmeter, but the GPS knows.

Edited by - Dave Bristle on 01/30/2009 09:56:05

Steve Milby
Past Commodore

Members Avatar

USA
5902 Posts

Response Posted - 01/30/2009 : 11:12:19

In about 2006, I also crewed on an old Coronado 30 (which was, I believe, the predecessor to the Catalina 30), racing downwind in 18-26 knots of wind, with twin headsails, and we surfed most of the night, with our max speed a fraction shy of 9 kts., which was well above it's hull speed.

It's hard to believe until you've seen it.

Next time I'll try to remember to get some video. In 2006 it was at night, and last year I was so thrilled by it that I didn't think to get video.

Edited by - on

Arlyn Stewart
Master Marine Consultant

Members Avatar

USA
2980 Posts

Response Posted - 01/30/2009 : 11:29:48

Two points... David, on the cat I'm not sure that the problem was lack of buoyancy when going fast... I'd rather think it is the couple between increasing amounts of hull drag and greater amounts of apparent wind moving the force of the sail plan further forward. That couple leverages the bows down.

Steve has it right, the sails can generate far more power than the engine but I'd slightly disagree that only downwind can see hull speed plus speeds. The beam reach can in my opinion provide more sail power than can be had in a given amount of wind for a downwind run.

With the beam reach, as the boat builds to hull speed, hardening a little more sometimes produces a very powerful configuration where the foresail and main interaction supercharges. There are some arguments about whether the jib causes increased flow of air over the main, whether the main drafts increased flow over the jib or whether they simply form one very large foil eliminating two of the four prominent drag areas considering individually each sail has a turbulent drag area fore and aft.

I tend to go with the latter theory but wouldn't bet the back forty on it in part because it seems that when this condition exist, excess heeling seems reduced somewhat, causing me to think it is sailplan drag that is reduced. What I do know from experience is that the supercharging does take place and can kick a boat another knot or better above hull speed.

Edited by - on

DaveR
Master Marine Consultant

Members Avatar

USA
2015 Posts

Response Posted - 01/30/2009 : 11:48:07

I did a down wind/following sea thing in late 06 and [url="http://www.youtube.com/watch?v=ejXeRHsWXAQ"]did get video of it.[/url] It was a pretty constant speed too of 7.4 knots, but we were surfing with a 15 to 20 at our back. And what DaveB says is so true, there's a millisecond where you loose helm and you have to be on your toes and correct quickly before the boat's momentum carries you too far off course and you have to apply ALOT of pressure on the tiller to bring it back. Kind of scary knowing that our(C-25) tiller is a weak point, but also pretty fun when managed correctly.

Edited by - on

JimB517
Past Commodore

Members Avatar

USA
3285 Posts

Response Posted - 01/30/2009 : 11:54:13

Indiscipline at or near hull speed. Note the bow wave and how the boat is sitting in her own trough.

This is cracked off just a little from max pointing, pretty hard on the wind, in about 10 - 12 knots of breeze.

Edited by - on

Dave Bristle
Master Marine Consultant

Members Avatar

Djibouti
10005 Posts

Response Posted - 01/30/2009 : 12:21:48

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Arlyn Stewart
 David, on the cat I'm not sure that the problem was lack of buoyancy when going fast... I'd rather think it is the couple between increasing amounts of hull drag and greater amounts of apparent wind moving the force of the sail plan further forward. That couple leverages the bows down.<hr height="1" noshade id="quote"></blockquote id="quote">That's what I think, too--particularly on the leeward hull. I've experienced it--happy I didn't pitch-pole the thing! Hobie hulls seem better designed to prevent that than Nacras.

I also agree that you get greater power on a beam reach than downwind, since apparent wind is greater than true wind on the beam reach, but not when heading down. But downwind in, let's say 3' seas, you can get the surfing effect, which can really break you through the hull-speed barrier. Seas directly abeam don't do that.

Edited by - on

jerlim
Master Marine Consultant

Members Avatar

USA
1484 Posts

Response Posted - 01/30/2009 : 13:15:06

Walt - we've gotten to 6 and change with the wind at about 15 knots...clearly, as explained in detail above, there are way many variables and influencers.

Edited by - jerlim on 01/30/2009 13:15:53

delliottg
Former Mainsheet C250 Tech Editor

Members Avatar

USA
4479 Posts

Response Posted - 01/30/2009 : 13:53:36

<blockquote id="quote">quote:<hr height="1" noshade id="quote">David means "square root" above<hr height="1" noshade id="quote"></blockquote id="quote">
It's weird, when I first posted the formula, the square root symbol showed up as well as in the preview, but now it shows up as #8730;, I wonder what causes the difference in the rendering?

Paul?

Edited by - on

Deric
Captain

Members Avatar

USA
408 Posts

Response Posted - 01/30/2009 : 16:04:05

Jim,

Nice picture and explanation.

Deric

Edited by - on

Walt Oler
1st Mate

Members Avatar

USA
44 Posts

Response Posted - 01/30/2009 : 16:30:52

Here is the basis for my original question that started this thread:

We want to develop our cruising skills and are reading up on navigation. Initially, we want to be proficient with dead reckoning and not become dependent on gps too soon. So, my thinking was that when you are projecting along your course and need your speed to estimate a future position, can you often just assume that you have enough wind to be at or near your hull speed?

Edited by - Walt Oler on 01/30/2009 16:32:54

John Russell
Master Marine Consultant

Members Avatar

USA
3444 Posts

Response Posted - 01/30/2009 : 17:22:28

I think you have to estimate your future position based on actual speed of the boat, not potential speed of the boat. Your dead reckoning position will need to be adjusted as your speed changes. Make a best guesstimate on departure based on what you think your speed will be. Then adjust once the sails have been properly trimmed and the course set. Dead reckoning is a very active method of navigation requiring frequent updating.

Edited by - on

Arlyn Stewart
Master Marine Consultant

Members Avatar

USA
2980 Posts

Response Posted - 01/30/2009 : 17:33:36

Walt... I don't think one can expect winds to generally push the cruiser to hull speed. It certainly is not true for cruising the Great Lakes.

My rule of thumb is, if I can't make three knots under sail, I motor but that is because I generally have an itinerary with only a couple of no-go days for a 10-14 day cruise and the leg distances are usually 35-90 miles.

I've had cruises where I burned as little as three gallons of gas and enjoyed some really fine winds. I remember one cruise however that provided only one really good sailing day... though it was a sixty five mile twelve hour run with a quartering sea of 5-6 ft swell and 20-30 mph winds.

Edited by - on

JimB517
Past Commodore

Members Avatar

USA
3285 Posts

Response Posted - 01/30/2009 : 17:51:13

For long term cruise planning, estimate you'll average 4.5 knots under sail or power. Also if you have a nice 4 stroke motor estimate a combined sail/power nautical mile MPG of 15 nmpg.

These numbers have been proven over several thousand miles of cruising in my C25.

You'll rarely see hull speed of 6.3 knots. It uses too much fuel to hit this under power and sailing it will be rare to go this fast, especially TOWARDS your destination (as opposed to through the water).

For DR (Dead Reckoning) navigation you have to project positions using current average speeds.

Edited by - on

Dave Bristle
Master Marine Consultant

Members Avatar

Djibouti
10005 Posts

Response Posted - 01/30/2009 : 18:08:28

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by JimB517
 ...and sailing it will be rare to go this fast, especially TOWARDS your destination (as opposed to through the water).
<hr height="1" noshade id="quote"></blockquote id="quote">Good point. If your destination is dead upwind, your "velocity made good" (rate at which you are getting closer to the distination) as you tack toward it is roughly 2/3 of your boat speed.

Edited by - on

Prospector
Master Marine Consultant

Members Avatar

Canada
3159 Posts

Response Posted - 01/30/2009 : 18:23:10

BUT...

If you are doing a DR plot, don't you do a DR before each tack, so that your run follows a zig-zag course with each tack as a marked position? I ask because I have never actually done this although I suspect I will be on this summer's vacation.

Also...
If you are on a DR course, especially to windward, and especially in strong winds, be sure you aren't over-canvassed. If you are heavily over canvassed, the boat can be pointed to wards your objective but making so much leeway that your plot will be all shot to h-e-(double hockeysticks). This did happen to me once last summer. (The first half of the summer I never sailed with anything smaller than my 155%, and then realized that bigger isn't always better)

Edited by - on

Voyager
Master Marine Consultant

Members Avatar

USA
5377 Posts

Response Posted - 01/30/2009 : 21:55:03

The process of performing Dead Reckoning is not trivial, and one of the best ways to learn how to do it is to join the US Power Squadron and take the Piloting and Advanced Piloting courses.

You need to estimate both speed and position, as both are affected by wind and current. Taking fixes by sightings at regular times and plotting these fixes on your chart [noting times] provides you with a means for predicting your current position. You should learn to use Current charts like NOAA or Eldridge to estimate leeway.

IMHO, it is difficult to learn DR by yourself, and best to take a course or two to reinforce the techniques.

Edited by - on

Arlyn Stewart
Master Marine Consultant

Members Avatar

USA
2980 Posts

Response Posted - 01/31/2009 : 03:46:34

Dave, 2/3 boat speed for vmg to an upwind mark seems optimistic at least it is for the 250 in the sailing venue I'm used to on the Great Lakes. I'm thinking less than half from my experience.

I recall one slog with winds 18-22 mph and boat speed 5.5 mph yielding a vmg of around 1.3 mph.

We reached this leg around lunch time with only eight miles to destination and my initial thought was that we'd plenty of time and that it would be a good sail. Then reality came... it would take over six hours to get there by very hard sailing and an hour and fifteen minutes with the outboard.

In that case, we needed to reach harbor before the HM left for the day as a pump out and fuel were needed.

Edited by - on

britinusa
Web Editor

Members Avatar

USA
5404 Posts

Response Posted - 01/31/2009 : 04:35:18

square root (radic;)= √
If you edit the post, you have to retype the & radic; (no spaces)

Paul

Edited by - britinusa on 01/31/2009 04:37:33

Steve Milby
Past Commodore

Members Avatar

USA
5902 Posts

Response Posted - 01/31/2009 : 09:06:02

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Arlyn Stewart
 Dave, 2/3 boat speed for vmg to an upwind mark seems optimistic at least it is for the 250 in the sailing venue I'm used to on the Great Lakes. I'm thinking less than half from my experience.<hr height="1" noshade id="quote"></blockquote id="quote">These formulas are theoretical, and their accuracy can be greatly affected by variations in such things as sail selection, sail trim and helmsmanship and the size and shape of waves. As some have pointed out, if you're flying too much sail area, the boat will heel excessively, and the amount of drag will increase, and VMG will decrease. If you steer off the wind by just 2-3 degrees, it can drastically reduce VMG.

For example, last year, on a friend's boat, we were beating into about 15-18 mph winds with a single reef. One crewman was steering the boat 2-3 deg. too far off the wind, and the boat was laboring heavily, i.e., heeling too much, and there was so much strain on the jibsheet that I heard a loud cracking sound, as if something was about to let go. Each wave knocked the bow off to leeward, and showered the cockpit with spray, and we were making very slow progress to windward. Then the owner took the helm and steered the boat 2-3 deg closer to the wind. The strain on the rigging decreased, the heeling decreased, and the boat began punching through waves, instead of being pounded by them. The difference between one hand on the helm and another, was marked. Some of these formulas are only guesstimates, and are likely to vary from person to person. DR is likewise a process for making an educated guesstimate of your position. You can't expect a formula that is accurate for all people.

Moreover, I was cruising once, and keeping track of my progress on a chart, every hour. At the start, the wind was more abeam, and the marks were far apart, but, as the day wore on, the wind shifted onto the bow, and the distance between the marks on the chart shortened rapidly. There is no formula that enables you to predict such effects in advance, so that you can use them in a DR plot. You have to wait until you observe the shift, and then adjust your calculations to take them into consideration. My point is that some things are predictable in advance with reasonable accuracy and some are not.

Edited by - on

Dave Bristle
Master Marine Consultant

Members Avatar

Djibouti
10005 Posts

Response Posted - 01/31/2009 : 09:23:25

Steve, it sounds like the first helmsman sailing a few degrees low should have called for easing the sheets accordingly. I've often used that combination and found that boat speed and comfort make up for the increase in distance sailed, and leeway was probably reduced a little by boat speed (better lift from the keel)--I probably wasn't actually moving as much lower as it looked. I didn't measure it, but VMG probably suffered very little. Harder is not always faster. These boats are not like AC boats--they sometimes do better sailing slightly lower than the max (IMVHO)--especially if we don't have a new set of laminated sails.

Edited by - Dave Bristle on 01/31/2009 09:52:58

Page: of 2

Topic

New Topic

Topic Locked

Printer Friendly

Jump To:

Association Forum

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.