The Physics of Boats

[@rickmunn7356]

Very interesting. My father in law had a double ended trawler with a small engine. When the engine failed he replaced it with a much more powerful engine but he couldn’t go faster. All that happened is he used a lot more fuel. He was stuck at his hull speed and that hull was no capable of planning no matter how much power you put to it. Your explanation is great. Thanks

[@Bryn0x]

Very good explanation, but it misses the part "wavemaking resistance" or "pressure resistance". It's not so much the frictional resistance which causes the resistance to rise sharply around "hull speed", it's the wavemaking resistance. The frictional resistance increases more or less linearly with speed. Many ships (e.g. naval ships, patrol vessels, motoryachts) are too heavy to plane, but they "beat hull speed" with a slender hull form and a lof of power, often using features to reduce wavemaking, such as a bulbous bow on the front and a Hull Vane on the back. We call this "fast displacement" when the ship is not lifted out of the water, or "semi-displacement" if they are reasonably light and are lifted somewhat out of the water due to hydrodynamic forces on the hull.

[@River_Living02]

Well produced, very informative. Thank you.

[@joehopfield]

Nicely done. Also the paddling demo was really helpful and the best low key product placement I've seen.

[@Dilan-qj1em]

The formula you've provided, Hull Speed = 0.4 × √(g*L), is close but needs a bit of adjustment to match the conventional formula used to calculate hull speed. The correct formula for hull speed (in knots) is: Hull Speed (knots)=1.34×√(LWL) (in feet) · LWL: Length of the Waterline (in feet). · The constant 1.34 comes from empirical studies related to the behavior of waves created by boats moving through the water. However In your formula, √(g * L), g is not typically used in the calculation of hull speed. Instead, the square root of LWL alone is what's used, along with the constant 1.34 to convert it to knots. If you were trying to calculate speed using gravitational acceleration g (9.8 m/s²), the formula might be more complex and applicable to other dynamics, but it's not used for the classic hull speed formula.

[@spelunkerd]

Yes I agree. For my displacement sailboat, with a waterline length of 30.5 ft, my max hull speed by your calculation is 7.4 knots, which accurately describes practical observation. Using the mixed metric formula in the video gets nowhere close to reality (3.8 knots). I don't know if that's an effect of the metric conversion or whether the freud number is inaccurate for small sailboats.

[@rustysailor4672]

The empirical studies are mentioned in the video. They resulted in the number 0.4. Since g is a constant you can describe: Hull speed = constant x √L , where the constant is 0.4 x √g If you use metric values you will get the speed as m/second. With imperial values it's feet/second. Convert this to knots and you will get the same hull speed as with your short formula.

[@deerfootnz]

​@@spelunkerdAmerica centric much? It's the metric formula. It used to be said that three countries in the world have not gone metric, but both Liberia and Myanmar are now on the path to metrication leaving just the US.

[@Kleeblatt2304]

Thank you for this video which has such an astoundingly high information density. It is a great contrast to those thousands of videos of other channels which take just a tiny piece of thought and elongate it to a 10 minutes video.

[@danielhaynes9262]

In the example calculation of the 20 meter boat, the left side of the equation should be multiplied by a factor of 1.94384 knots/meter/second. Otherwise the result will be in meters/second not knots.

[@deerfootnz]

You should also note that very long narrow hulls with light loads can go much faster than any theoretical hull speed. Hence why multuhulls are faster.

[@josephlai9759]

There is another formula which I find more handy. The hull speed is calculated directly in knots. It is 1.34 multiply by the square root of LWL (in feet).

[@Waterlust]

That formula comes from Froude number experiments, just a more simplified way to express it (and easier to remember).

[@SV_OceanGirl]

@@Waterlust also worth noting that the way you expressed the Froude number formula is dimensionless and it works for any (self consistent) set of units whether inches, feet, meters, nautical miles, or astronomical units !

[@artsmith103]

​@@Waterlust Does the 1.34 shortcut work for all boat types or just sailboats with their pretty consistent length/width ratios?

[@Waterlust]

@@artsmith103 in general will work for most boats, however very slender hulls can operate at higher Froude numbers without as dramatic a penalty like kayaks and catamarans.

[@voornaam3191]

​@@Waterlust Yes, competition kayaks are great. There still is development in hull shape. I've often wondered if such a kayak could get faster, if it would foil. But most people say, I think it takes too much power to make a foil going faster than those kayaks do today. Still, I wonder if a foil designer can prove them wrong.

[@CharlesLampman]

Really good production value. I will share with fellow nerds

[@Waterlust]

Nerds unite!!!!

[@chrisslaunwhite9097]

This was awesome, and will help me in a huge way. Subbed my man!

[@EngRMP]

Very cool! I often heard hydrodynamicists talking about "transverse waves".... I always thought they were the V-shape waves... now I finally have the true picture. And, I've felt that extra push that is needed to get "over hump"... now I understand the physics better.

[@Waterlust]

Glad it was helpful! More ocean science videos like this coming, stay tuned!

[@gefginn3699]

Great post my friend. I have plans to build my own Trimaran and information like this will be crucial to my dimensions. 🤩⛵

[@SailingSVPipedream]

Well hopefully you built the tri. However for anyone designing, multihulls are designed to beat the hull speed calculation by having very thin hulls in relation to length. This means they can exceed traditional hull speed calculations.

[@BefriendWater]

What is fantastically informational video! On to watch more of your stuff, guys 😊

[@Waterlust]

new videos like this coming soon!

[@mikecrockett3669]

Excellent! Thank you.

[@rachidtayeb1465]

Thank you for better video, can you but a video for a planing boat

[@leorutherford-m1v]

what a great vieo! Also shoutout mini simmons haha!

[@糖糖飛天糖果]

05:30 estimating their length Should length refer to the observation wavelength and be able to calculate the ship's speed?

[@alans172]

Great video, (I'm also fascinated by the transverse waves, but thatr's a separate video). I would really like to see the discussion relating to catamarans. I don't think the effectively doubled waterline contributes to the calculated waterline in the Froude equation, but I am also pretty sure cats (of the sailing variety) are much faster than monohulls with the same sail area.

[@deerfootnz]

That's because they are lightly loaded long narrow hulls. The waterline length doesn't count double because you have two hulls, though the load sharing between two hulls does mean the effective displacement length ratios is halved.

[@ccserfas4629]

Good stuff

[@crackedemerald4930]

4:07 if the froude number is the highest efficiency, wouldn't it be the valley before the big ramp up? in this graf, it looks like the least efficient speed, because at least, you get faster for the other ones.

[@myparadiseonbantayanisland9030]

How does that formula work on a multi hull boat?

[@Gizmos_and_stuff]

You can plane in sailboats, you just need good wind and good sail trim, and a decently fast boat (like a laser)

[@jpx1508]

Water to Air Density Ratio - 830:1 Not 1,000:1 Jus' say'n

[@xxskippernate409xx6]

Good man. Waters density can vary a bit, depending on salinity.

[@ratandmonkey2982]

your resistance vs Froude # graph did not make sense with your explanation. Why the dip (less resistance) with a higher Froude #?

[@Waterlust]

The “hump” in the plot at 04:07 is caused by the dramatic increase in resistance when a boat tries to overcome its primary transverse wave. If it’s able to, some boats will experience a brief region of reduced resistance. Note, every boat has a different resistance curve, so the size and shape of the hump varies a lot. One way to prove this phenomenon to yourself is by going in a small planning boat and setting the throttle so you’re stuck at that awkward hull speed with the bow up. Then have somebody carefully move to the bow to help the boat initiate planning, but don’t change the throttle. Works really well in small inflatables. What can happen is the boat starts planning and going dramatically faster despite the engine producing the same thrust. The only way this acceleration can happen is if the overall resistance on the hull were to drop, which supports the idea of the resistance dip at speeds slightly above the critical Froude number. Great question!

[@ohnousagain]

what music was used in this video?

[@糖糖飛天糖果]

05:07 At 950 feet long, the cruising speed of a Panama container ship is around 25 knots. In the film, a Panamanian container is described with a captain of 950 feet and a cruising speed of approximately 25 knots. But when the hull speed is calculated to be 40 knots, why do most container ships only have 25 knots? Does not match the hull speed.

[@williamstrachan]

Fuel efficiency most likely. MAN Energy Solutions has a paper out titled "Propulsion of 14,000 teu container vessels" and shows (amongst other things) that identical ships are vastly more efficient at 21.5kts instead of 23.5kts. Eg. At 23.5kts their example ship consumes 197.9 tonnes of fuel/24 hours, but at 21.5kts it consumes 145.4 tonnes of fuel/24 hours. I can't imagine the fuel burn at 40 😬😬

[@cowchin8]

@@williamstrachanslow speed is definitely burning less fuel as it travels less distance. There is a relationship between fuel oil consumption & ship speed & power. It is not a linear relationship. Therefore, all the vessels go slow-steaming in the poor economy period.

[@crackedemerald4930]

didn't Froude also make a number where you can slow the footage of model ships by to get accurate boat simulations?

[@jimbo_davis]

how can you estimate the hull speed?

[@Waterlust]

Use the equation in the video. By estimating the boat length, you can calculate the hull speed using the Froude number 👍

[@糖糖飛天糖果]

@@Waterlust Should length refer to the observation wavelength and be able to calculate the ship's speed?

[@deerfootnz]

​@@糖糖飛天糖果no. The waterline length of the boat.

[@francoistelemaque6429]

Good information however modern racing sailboats from small to large do plane and without to much difficulty

[@TheJmills39]

Couldn't understand anything he was saying at 3:23

[@thedolphin5428]

I was once told that the angle between divergent waves OF ALL BOATS AT ANY SPEED is exactly the same. True? I was also told that anti-smuggling Customs satellites can pick out boats approaching the coast from searching for that universal diverse wave angle disturbance left on the ocean. True?

[@brianlittle717]

If the Estonia had have done that when the bow broke it might not have sunk if it went real fast with the bow out of the water.

[@Grejegando]

I don't think I'm convinced that the boat creates waves that hinder its speed. The only waves I see are to the sides and travel to the rear which are visible in this video and anytime you look at moving boats in the water. That "secret" hidden wave isn't possible given how boats are shaped. If you're talking about sea waves then that's something else but boats create one type of waves only and they are visible and travel in the same direction all the time. Anyone with eyes and anyone who has kayaked, paddle-boarded, or floated in an inflatable object around moving boats can also confirm this. The wake is always the same and depends on the size of the moving boat and its speed only.

[@1320fastback]

Wind resistance is the same for automobiles. It takes 4X the power to double your speed.

[@brianlittle717]

Did you know that boat and float and goat all rhyme?

[@bubbleobill267]

You forgot to add the equation of your wallet and sanity being constantly challenged when owning a boat.

[@rtopalovich]

Tone down the background music 'noise'.

[@agoogleuser2507]

i don't think you went into enough detail about planing. You also didn't mention how planing allows you to use less fuel for the same distance with greater speed.

[@olddog57]

The formula for max speed (that in displacement mode of sailing) is improper, and leads to speed underestimation: using it I got 5.6kn, not 10.8!

[@humbleguy9908]

My first thought too. Then I realized that 5.6 m/s is 10.8 knots.

[@olddog57]

@humbleguy9908 i thought i was clever than that.. 😞

[@francus7227]

When will "fishing" kayak companies realize a 10' × 40" yak is better than a 12' × 34" yak. We need the STABILITY a 40" wide yak provides MORE than the minimal extra top speed a 12' yak provides over a 10' yak..... We fish at slow speeds and travel minimal distances from the original launch point. It doesn't take a scientific equation to figure this out..... DUH ! Life is a series of trade-offs. Stability is better than speed in a FISHING kayak.

[@vicariouswitness]

Hydrofoils

[@brianlittle717]

Don’t those stupid carp jump up and hit you the faster you go?

[@learnwsail]

.there's more distracting noise than information