Maximun Boat Speed Calculations

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Paul

I have been told that a quick way to find the max speed of your boat is to divide the waterline length by 3. My boat has a waterline (LWL) of 31' 1", therefor the max speed should be about 10kts. Is this calculation true? Can a boat exceed this so called calculation and are these speed ratings based on up-wind or down-wind. Comments...
 
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james rohr

hull speed calc

hull speed is the theoretical max speed a displacement hull will attain in the water. it can be exceeded if enough power is in the sails to make it surf as a dingy or small non disp. sail boat. the calc is 1.34xsq rt of water line length.(note this is not the static length at the dock but length under sail. these numbers can be quite diff depending on the hull design).most 30 footers weigh in at about 27 0r28feet, even though they list example.o'day 30 at 25.5'. while under sail it is about28' capn jim sv "Que Pasa?"
 
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Paul

Beam width and waterline

Waterline 31'1" and beam of 12'6". Does a wide beam cause a boat to go slower? Is there more to it for this calculation? Thanks...
 
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Gord May

Nearly useless numbers

A theoretical displacement Hull-Speed formula, that is based exclusively on waterline length and a “magic” multiplier is only a very rough approximation, hence any calculation beyond whole numbers (/w perhaps whole fractions) is self-delusional, and mostly meaningless (except to say longer boats are faster). The precision suggested by a factor to two decimal places (1.34) is spurious, and the choice of a number to represent LWL is also somewhat arbitrary given the variations in hull design, and sailing dynamics. The praqctical application of formula assumes that there is no current, wave, or wind resistance and the hull is clean and free of objects that could cause excess drag. In reality many other factors including weight, slenderness, displacement/length ratio and the fullness of the bow and stern (fineness of entry), prismatic & block coefficients are also involved. It is not a fixed number for all hulls. Notwithstanding: IF Hull Speed = (1.34) x Square Root of (LWL) Root 28.0 x 1.34 = 5.2915 x 1.34 = 7.0906 Root 25.5 x 1.34 = 5.0498 x 1.34 = 6.7667 Hence, the difference (0.3239 Knots) in theoretical hull-speed between a 28 foot heeled waterline (*) and a 25.5 foot static waterline exceeds the presumptive accuracy of the basic formula, and your ability to measure speed, hence is absolutely inconsequential. (*) I also presume you will be estimating the actual heeled waterline, at any given time, beased upon angle of heel. Your hull-speed is “something under” 7 knots. Here’s Ted Brewer on “SPEED/LENGTH RATIO (V/ L.5)”: QUOTE <i>This is the speed in knots divided by the square root of the LWL. For example, a 25 foot waterline sailboat moving at 5.5 knots would be at a V/ L.5 of 1.1. while a 400 foot LWL destroyer travelling at 22 knots also has a V/ L.5 of 1.1. Both vessels would develop about the same resistance per ton of displacement as they are both running at the same V/ L.5. The limiting speed for a pure displacement hull is a V/ L.5 of 1.34. Above this speed the stern wave moves aft so that the stern loses bouyancy, the hull squats, and great additional power is necessary for a small gain in speed. In truth, the typical cruising sailboat probably averages a V/ L.5 of about .9 - 1.0 and only gets close to 1.3 when reaching in a stiff breeze. Tender boats may never get above 1.2 as the crew has to ease sheets when the rail buries! The modern beamy, super light ocean racer can have a stern wide enough to resist squatting and the stability to stand up to a breeze so often achieves speeds well above 1.4, but that is semi-planing and the boat is getting lift aft due to its speed. My BOC 60 design exceeded 20 knots at times, a V/ L.5 ratio of over 2.6, but those are very specialised yachts. </i>END QUOTE There are some other calculations that may pertain to your fundamental examination.  Horsepower Required for Hull Speed: Determines the theoretical horsepower required for a displacement hull shape to reach hull speed. Doubling the speed requires 4 times the horsepower so, if you set the speed down one half of hull speed, the energy required will be reduced to just a little more than 3/4 horsepower. Maximum Boat Speed for a Propeller As a propeller turns, it's pitch determines how far it travels through the water. No propeller is 100% efficient so that in reality it "slips" by moving water aside rather than straight backward. For a planing hull shape, this propeller slip can be as low as 10%. For a displacement hull shape with a high speed propeller, this slip is usually around 45%. This means that the maximum vessel speed is a function of the shaft speed in revolutions per minute, the pitch of the propeller, and the amount of propeller slip.  BTW: it would require a waterline length in excess of 55' to achieve a 10 Kt hull-speed. Gord
 
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David Foster

Boatspeed

First, with enough power, a sailboat (like a power boat) can plane. Planing means riding on the bow wave, more or less on top of the water. Most dinghies will plane in a good breeze. A keel boat may plane moving down a wave. Modern racing boats, starting with the Cal 40 and through today's "sleds" plane much of the time that they are not beating into the wind. Length and beam are not very important to planing speed which is determined more by power and hull shape (flatter is better.) There is no fundamental limit to planing speed if enough power is available. Most of the time, a cruising keel boat is sailing in (instead of planing on) the water, as a "displacement" vessel. For a displacement vessel, the length of the wake wave(s) around the hull is the fundamental limit. The speed of any wave is a fixed function of its length. When a boat is moving slowly, the wake forms waves along the hull. As it moves faster, there are fewer waves, then just one and a fraction waves. At the "hull speed" calculated by James (although he is right that the waterline length ("LWL") for most boats lengthens while sailing, we usually use the design LWL to compute hull speed) There is just one wave along the hull with one crest at the bow, and one crest at the stern. Once the wave is the length of the boat, the boat is trapped in the wave. (You can feel a slowly accelerated power boat "sit down" at this speed.) More power results in very little more speed. (Unless there is enough power to move on top of the bow wave, or plane.) Our '77 h27 has a hull speed of 6.2 knots. We have sustained speeds of 7.5 knots according to our GPS in Lake Erie with minimal current, reaching with good sails in a 20 knot wind. This happens because our waterline does lengthen with moderate heel, and as the wave forms. We are also pushing the wave a few tenths of a knot. But length is the controlling factor if we are not planing. A few planing bursts on the face of a wave give us short periods at up to 8.5 knots, but our 7,000 pound displacement quickly drags us back into the wake. Even on our glorious reach from Ashtabula to Erie last summer, lulls in the wind dropped us below hull speed some of the time. And in a week long cruise, we were below hull speed 90+ percent of the time. Below hull speed, wetted surface area and efficiency of the hull shape moving through the water have an impact on speed. Wetted surface is driven by displacement (heavier boats ride lower in the water) and underwater shape (that's why fin keels and rudders are faster.) Beam has little impact on wetted surface, and may be a slight penalty in efficiency moving through the water. But beam increases the stability of the hull, allowing you to carry more sail upright without excessive heel, so beam is seldom a penalty for a sailboat. Most sailboats experience much more drag when heeled beyond 15 to 20 degrees, slowing the boat, despite other factors. But for the 90 percent of the time below hull speed, sail shape, trim, tuning, ballast, and helming are critical to speed. David Lady Lillie
 
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Dan McGuire

Not Exact

A 7.5k hull speed would require a LWL of almost 42 feet. The reason has to be something else other than a lengthening of the LWL. I have exceeded the hull speed of my Mac 23. The hull speed for the Mac 23 is about 6k. I have had it up to about 8k for a few seconds per the GPS. There is virtually no current where I sail. I can't account for it unless somehow the Mac 23 is more willing to plane when heeled. In general I found the 6k limit to be pretty accurate. I wonder also, if it is possible, that the boat exceeds the hull speed for a few seconds while the bow and stern wave are established. To answer Paul's question, the beam has no effect on the hull speed. A wide beam will slow the boat down, but the boat should still reach hull speed with more power. It just takes more sail to do it. But like I said, it is not an exact science.
 
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Jim Quibell

Don't bring GPS into discussion -

Speed over ground is not what we are talking about here. Hull speed can only be measured by an accurate knot meter on your boat. The 7.5 knots you read on your GPS may correct out at 5 knots. Cheers,
 
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Dan McGuire

Why Jim?

A GPS is extremely accurate, much more so than the best knot meter you can buy. The only problem with the GPS is that it measures speed over the earth and therefore, current or tide must be taken into account. I am not sure how a GPS speed of 7.5 corrects down to 5k. If there is no current or tide, which is true in my case, there is no correction. Also a GPS measures speed by calculating the time it takes to move between two points. This is done very rapidly and is not a factor for the relatively slow acceleration of boats. This is pointed out just to head off a future argument.
 
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ben310

polars

Get the Polars on your boat. They should be available from the builder or designer.{see link}
 
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Jim Quibell

Dan - details can be found in Archives -

Look up "Real Speed in the Forum Archives". There you will find detailed explanations as to why GPS knot speed readings are not the true hull speeds as indicated by a good knotmeter. Tim especially gives a dead on explanation. Far better than I can try to explain to you. Cheers.
 
Dec 2, 2003
4,245
- - Seabeck WA
Jim, I couldn't understand where Dan could be

miss-informed. He says he sails in 'no current or tide'. GPS should be accurate. I finally found the archived article and read Tims explanation. It says nothing to counter Dans understanding. In Dans environment, the GPS is unequaled for accuracy.
 
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Clyde

Theoretical Hull Speed

The "Theoretical Hull Speed" equation, 1.34 times the square root of LWL, or length of waterline is just a simplified mathematical model of a heavy displacement hull. The equation was derived by Sir Anthony Deane in 1670 to estimate the speed of a British "Man-O'-War". Most, if not all, modern recreational sailboats are light displacement fin keel, wing keel, swing keel or centerboard sailboat that should be classified as semi-displacement hull. This means that their hull design will always exceed the classic hull speed equation. Dave Gerr, a Naval Architect from New York City, came up with another "Theoretical Hull Speed" equation to approximate a recreational sailboat's hull speed. He wrote the books, the "Nature of Boats" and the "Propeller Handbook" (the industry-standard reference on propellers). He uses the sailboat's displacement, as well as LWL in his simplified equations. His equations are: D/L=(weight/2240)/(0.01 * LWL)^3 S/L = 8.26/(D/L)^0.311 Modern Semi-Displacement Hull Speed = S/L * LWL^0.5 You can do the math or use Carl's Sail Calculator if your sailboat is listed in his calculator. At the bottom of the page, Carl does both the Classical hull speed and Gerr's hull speed when calculating the propeller size. If you surf down waves you will exceed Gerr's hull speed equation. It's just a mathematical model of a typical recreational sailboat. As with most mathematical models, its just an approximation and should be used as "Rule of Thumb" to get you in the "Ballpark" range for real world boat speed. If you have a full keel heavy displacement sailboat, then the Classical hull speed equation should be closer to your actual boat speed. Carl's Sail Calculator http://www.image-ination.com/sailcalc.html Fair Winds, Clyde
 
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Dan McGuire

Talked to Garmin

About five years ago, the military got rid of the random error which was included in the civilian GPS. My Garmin 12 would not give a reading below 1k because of this random error. I called Garmin to see if there is a way to change the software in my Garmin so that it would read below one knot. I was told that there was no way to change the software, but the speed was accurate to almost .01 knot at speeds above one knot. No knotmeter will be that accurate because it disturbs the water as it moves through it. A GPS is one of the few instruments which has ever been invented which does not cause a disturbance in the environment it is designed to measure.
 
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Jim Quibell

You are right - GPS good over land

but not for measuring boat speed on the water surface. GPS gives you the speed over the bottom. Come on you guys - everybody knows that! Cheers.
 
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Dan McGuire

Everyone does not Know That

The reason everyone does not know that is that it is not true. IF THERE IS NO TIDE OR CURRENT, THE SPEED IN RELATION TO THE WATER, THE EARTH AND THE WATER IS ALL THE SAME. If there is a tide or current then the speed over the water and over the bottom will be different. Am I missing something? If I am, please explain.
 
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Paul

10 kts

Thanks for your elaborate response. How did we get our Beneteau to sustain 10 kts for 2 minutes then. Is our knotmeter incorrect? I really don't care how fast the boat goes but I really cannot understand the math that you provided and the speed that was attained last summer on Lake Ontario. Regards
 
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Paul

Here are the spec of this vessel..you do the math

1996 Beneteau 36s7 LOA-35.9 LWL-31.1 Beam-12'5" Keel 6'1"--3650lbs steel Displacement-11,684lbs Sail Area-660 sq ft. Mast above waterline-53'10" Sailing area-Lake Ontario-No tides, no currents Please do the math Thanks
 
Jun 5, 1997
659
Coleman scanoe Irwin (ID)
Paul, s u r f i n g is the easiest explanation...

In the Pacific, running dead down wind (DDW) under wing-on-wing jibs "Rivendel II" -- our Legend 43 -- would often exceed hull speed quite significantly if winds around 20-25 knots would enable us to start matching up our boat speed more closely with the speed of the tradewind swells. The feeling of surfing on big ocean waves is one of undescribable elation and was easily the high-point of our downwind passages!! (BTW; this does not violate the laws of physics, as the bow wave effectively drops away while surfing). Have fun! Flying Dutchman
 
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Clyde

Predicted Hull Speed

LWL = 31.1 feet Displacement = 11,684 lb. Classical theoretical hull speed = 7.47 knots. Dave Gerr's theoretical hull speed = 9.27 knots. Fair Winds, Clyde
 
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Jim Quibell

No currents on our inland sea???

Hey Paul, Come on down to the west end of the lake - we will show you currents and tides (we like to call them swales) that will knock your socks off. There is one hell of a current out from Hamilton/Bronte/Oakville. Don't forget Niagara Falls continues to work 24/7. That's the reason you cannot use GPS to give you true boat speed in our little corner of the world. Cheers
 
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