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Old 07-11-2010, 15:53   #1
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Speed Difference ?

Ok, so the theoretical speed of a hull is based on the hull length....there is not much of a difference in realized speed between boats ranging +/- 5'.....which is @ .5 nm/h. That is assuming you are compaing the same make/model's different lengths.

So, take out the variables that would affect things, like skipper experience, sail area,...etc. Is there really that much realized speed between different "popular" monohulls of the same length? Given you are at @ 5-7 nm/hr, does the hull design, make a noticeable difference in the speed? Or does the design impact more of the handling and the ride?
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Old 07-11-2010, 16:27   #2
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Good question but you lost me. Its not hard to lose me though.

Assuming the hull waterlength is the same then hull speed should be nearly equal. Acceleration of a lighter boat with the same windspeed will be greater as will be deceleration.

As the waterline lengths increase it takes more added feet to increase by a knot of speed. Work the forumula on 10 feet, 15 feet, 20 feet, 25 feet etc. and you can see what I'm speaking about.

I hope that answers some of your question.

kind regards,
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Old 07-11-2010, 16:41   #3
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Its more of a question to hull design. Is a amatuer racer that much faster than a cruiser/day sailer? Or is it more a pure weight issue? Would it be noticable when sailing them as a comparison?

Speed is good but having a bit of creature comforts is handy when you want it.
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Old 07-11-2010, 17:31   #4
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Yes, the hull design makes a significant difference as to how easily and under what conditions when (and if) theoretical hull speed can be reached (or exceeded). Obviously wind conditions, rig and sail area, etc., effect that as well as you noted.
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Old 07-11-2010, 17:37   #5
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Assuming that you are talking hulls that don't plane, the difference is found in lower winds. At higher wind speeds, the boats all end up at about the same speed. However, some boats can still point much higher which can lead to a better VMG. Also, waves play a big factor since some boats really get slowed down by them.

In all honesty, I think that the person sailing the boat has much more to do with it than the boat in a lot of circumstances. I am continually shocked by the much "faster" cruisers that I pass in my "slow" boat.
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Old 08-11-2010, 04:41   #6
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Originally Posted by houseboy View Post
, does the hull design, make a noticeable difference in the speed? Or does the design impact more of the handling and the ride?
This is a very interesting question! It would be more easy to answer if you were not writing in Swahili and added some extra details about you and your desires.

You initial post says you are looking for a boat to do weekly excursions at sea from San Fran Scisco (well some place in the USA?).

My 'thing' is very different. Long passages often well over 1,000 nms at a time.

Hull speed is partially about waterline length NOT overall length. LWL does not mean 'length of the water line' it means 'Load Water Line'. But basically for us its length in the straight line at the bow where it touches the water to the center of the stern where it touches the water.

in newer design boats the LWL is very close to the LOA but older boats the LWL is very much shorter that LOA.
Modern racing boats have a long LWL to LOA.

(Byt he way, remember that as you get closer to hull speed your boat becomes more and more unstable)

A modern 40 footer will sail much faster than a 1960's 40 footer because the LWL of the new one will be vastly longer.

If you are youngish and this is your first boat and you are going to be living in that area for a few years you might want to race you boat -either casually or seriously - at the local yacht club. No matter what the Brady Bunch taught you: Loosing Sux! You don't have to win, but coming rank last every Saturday afternoon smells bad, big time!

Racing imho teaches you more, quicker, than any other form of sailing. It doesn’t even need to be spinnakers and loads of cash, just casual racing is fine.

If that is an option then before you buy a boat pop down and visit some yacht clubs and see what sort of boats in your price range are racing. Good, small cruiser/racers can be excellent boats - good for the races and great for a weekend away. See what the divisions are at the yacht club, perhaps see if you can crew on someones boat in the size that interests you. Then if all that goes well have a look at the second hand market for boats that would fit the division - remembering to make sure you don't go less on LWL than the boats you are racing against!


Now, on the cruising bit of hull speed.
When folks are new, young, or ex racers they often want to make the boat go fast and they blather on about hull speed and fast passages and 'efficient passages' (WTF is that?). As one gets older, more boring, in Zen with nature () enjoys their passages, etc speed on passages becomes FAR less important. More important is getting there relaxed and with an unbroken boat!
My hull speed is about 8.5 knots but I always reef when we hit 8 knots no matter what.
Why?
Because my next leg is 2,680 mile sand I want to get there and not have to go pay money to the chandlery on my arrival!
The other day I was helping this German guy on an ex-charter Leopard 38 (or 40?) catamaran. He was complaining about the weather and how his **** sails let him down and if they were good he would have done 14 knots down here to the Canaries from Portugal.
14 knots in a 38 or 40 foot cat!!!!!!!
But, guess what I was helping him do?

Guess....

We were taking off his 3 sails as he had ripped them all. His main was shredded, his Genoa had a HUGE hole in it and his Genniker was mincemeat.

So what was the problem?
**** sails?
**** weather?
**** skipper?


So know your boat and what Hull speed is all about, but understand where and when you want to use it to go fast, or when you want to be well under it to go softly, steadily, safely and enjoyably


I hope this long diatribe isnt total gibberish


Mark
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Old 08-11-2010, 06:35   #7
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"theoretical speed of a hull is based on the hull length." The formula 1.34 x square root of LWL gives you the MAXIMUM speed a non planing hull can be driven through the water, no matter how much power is applied. The yacht designers aim is generally to design the hull to achieve that performance while applying an optimum amount of power either with sails or engine. The alternative is to design the hull to plane, i.e ride over the water rather than through it. Helpful???
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Old 08-11-2010, 07:21   #8
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Actually 1.34 is not a hard and fast rule. According to the reading I have done it's a reasonable compromise across various hull shapes but the absolute value can only be determined by tank testing. I don't believe the range is large (1.2 to 1.4 sticks in my mind) but it's not strictly correct to say 1.34 x the root of LWL equals the hull speed. And that refers back to the OP's question as I understand it. So the answer to that question would be "yes, there is a slight influence of design on the theoretical hull speed of different vessels with the same LWL".
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Old 08-11-2010, 07:33   #9
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That 1.34 is extremely variable for a "full displacement" boat. As stated the number is a good average number for general use. But getting to hull speed and pushing a little beyond is a function of power available which can be sail size/shape/style. Keel slippage/leeway is a major factor a high speed going the wrong way is not that desirable.
- - Some cat's are full displacement and some are not so they have different formulas since riding "on" the water is not so power demanding as riding "in" the water. Then again some "on the water" cat's get converted to "in" the water by the addition of a few thousand pounds of cruising "stuff" on board.
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Old 08-11-2010, 08:52   #10
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All I know is that if I'm going into the wind or reaching and the boat starts to get near hull speed.............I get scared, uncomfortable, and the boat starts to be a real handfull to hold the wheel! I've had the boat at 30 degrees heel lots of times, but only do it for a short time and the wind can not be gusting. So I adjust the traveler and/or reef to get back to 15 degrees heel and boat "slows down" by about .3-.5 knots and eveyone is a LOT happier and the boat isn't having a beating anymore.

Maybe if going faster was going to make money for me instead of costing me money I would look at it different!
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Old 08-11-2010, 10:14   #11
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While the 1.34x SQRT(LWL) is a good approximation, the big variables seem to me to be the LWL, hull shape, and comfort.

Can your vessel exceed the maximum theoretical speed? Yes.

As your vessel heels, the waterline length usually changes (mostly longer) and you can get a bit more speed, if that's what you're after.

A vessel who's hull shape allows the boat to power through the waves more efficiently or easier will over the long run give you a higher average speed. Also, a vessel with a deeper draft will generally track better so the distance covered over 24 hours can be significant. Leeway differences on short voyages probably won't make much difference.

Comfort is seldom mentioned but can make a big difference. It's always a thrill to "bury the rail" or set a new record on the knot log/GPS, but it can be uncomfortable. Crew may have less restful periods and moving around the boat can be less pleasant. Reefing or allowing the vessel to sail a more pleasant course may prove beneficial in that the crew are more rested and can keep the vessel on the speed edge, thereby giving you somewhat better average speed or distance traveled over 24 hours.

Around the buoys or racing is far different from cruising. When racing, speed is the primary concern, regardless of the comfort or stress to the vessel or crew. On a racing boat, you may be adjusting sail trim, ballast, and mast rake many times an hour. On a cruising boat, you may choose to adjust the sails or mast rake much less, preferring a more pleasant ride at the cost of 0.3-0.5kt.

However, when I was in sight of another sailboat, I tended to go into crase (cruising/race) mode, tweaking the sails a bit more, steering a tighter course, and trying to keep up/overtake the other vessel.
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Old 08-11-2010, 10:31   #12
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Maybe this is more to the point the OP was getting at, since boats rarely sail at hull speed.

When we sold our boat the GPS showed our average SOG to be 6 kts, which is 75% of the boat's calculated hull speed. On offshore passages we averaged about 85% of hull speed. Our boat was relatively heavy, full keel, and cutter rigged.

Anyone have similar stats for a lightweight racer-type boat, based on actual experience?
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Old 08-11-2010, 11:06   #13
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average SOG to be 6 kts, which is 75% of the boat's calculated hull speed. On offshore passages we averaged about 85% of hull speed.
I'm very surprised! Thats very fast! Averaging 75% and 85%. I would be ver interested to see if anyone has got averages on their gps.
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Old 08-11-2010, 13:30   #14
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Just to be clear. This is what I'm getting out of this thread so far and is what I think the OP wanted. Monohull displacement boat maximum hull speed is a function of the length of the water line (as Mark pointed out while in the water and loaded) not the hull length. 1.34 is a good average but not written in stone. Your actual hull speed will depend a little on design and weight vs sail area but you won't be exceeding hull speed (once it is determined) in flat water no matter how much power you put to it. If you exceed manufacturer's advertised hull speed in flat water then you need to up your maximum hull speed. Yes, you can exceed hull speed while surfing down big rollers but that's not flat water.
Is this about right?
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Old 08-11-2010, 13:47   #15
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Hi, John. He talked about hull speed, but also said "Is there really that much realized speed between different "popular" monohulls of the same length?" I took that to mean he was interested in actual experience when sailing, and sailing at hull speed is a rare experience (although we did so for 2+ days on an offshore passage with Force 9 winds off the quarter). So, in my post ^above^, I was assuming that if we can compare actual boatspeed data as a % of theoretical hull speed for different types of underbodies, we might learn something interesting.
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