Can a 30-40 foot cruiser reach speeds of 18 knots w/ engine?

[mitiempo]

Depends on the boat.

Definitely not a displacement boat though.

[Capt. Lace]

no only a planing hull will reach that speed. to calculate any boat speed get the square root at the waterline

hull speed in knots equals 1.34 times the square root of the waterline length in feet (HS = 1.34 x √LWL). 36' boat square root is 6 times that by 1.34=8.04 knots

[Montanan]

FYI:

Reference: https://www.boatinternational.com/ya...-piracy--28281


Snipet copied there from:

"Another issue is how fast the yacht travels – some superyachts only reach a maximum speed of 17 knots, while the pirates' skiffs can run at 35 to 40 knots."

[RiverRat37]

Originally Posted by captlloyd

Physics theory. Hmmmmmmm. Don’t believe everything you read when it comes to physics. I remember physics scientist, probably in the 60’s, declaring drag racers would never do the quarter mile in less than 9 seconds. Too much weight, not enough horsepower, not enough traction, yeah, they had it all figured out. The drag racers wasn’t listening. When they got down into the 8 second range, 7 seconds became the next limit. You see where I’m going with this. Lots of folks said human powered flight was impossible. Tell that to the guy who flew across the English Channel. Hell, If they were alive you could tell it to the Wright brothers.

It's a bit dangerous to take other examples and apply their physics here. Ask any designer of displacement hulls if they can ever exceed their "hull speed" by over 15% safely. They know.
But here's the real proof. You got a displacement hull sailboat? Try having it towed by a powerful power boat and watch what happens. It's gonna sink if it gets too far beyond it's hull speed. You can watch the wake behind it grow and grow and grow as the tow picks up speed. Too fast ... it sinks.

The US Coast Guard can tell you. They send out a cutter (a biggish one) to tow back a displacement disabled sailboat and ask them how fast they can tow it. They'll tell you. That's why when you read the stories they seem to take forever to get back to port towing a disabled displacement sailboat. Cutter can go 20-30 knots but they don't. They go 7 or 8 or 9 knots because physics demand they do (they're supposed to save it, not sink it).

Or ask a tugboat captain how fast he can tow a displacement hull sailboat. He'll tell you.
There are past stories where the captain didn't know (long ago) and sunk his tow.

In fact, take an ocean going tug and see how fast it can go (they don't plane well!) Answer: Square root of the water line times 1.34 -- give or take a knot or two. The fact they have a few thousand horsepower is not the issue.

The tug Edward J Moran is 98' with 6,500hp and draws 16 feet. Top speed 13 knots. Figure it out: square root of 98 = 9.9 times 1.34 = 13.27 She's not likely to make 15 knots safely. This workhorse has so much power she can go from 13 knots (full speed ahead) to 13 knots in reverse in 15 SECONDS! Power is not her problem. Waterline length is.

As to your drag racers. I'm pretty sure it's the tires that got the speed up. You know how they spin the tires to smokin' before running the race? They want 'em hot and sticky. It works but Goodyear and Firestone, etc. figured out how to make them even more sticky. And then even more sticky. Chemistry did it. HP wasn't the issue. They already have more HP than they can use.

Now maybe it's possible for displacement boats to go faster -- thing is we haven't found it yet. (Well actually we did -- they put planes on the keels to lift the boat and they go like a rocket under wind power alone.)

I've often wondered if we will ever figure out how huge whales go so fast underwater. They go too fast for their size. Submarine designers would love to know!! Their Los Angles class subs can almost equal the speed of a blue whale -- around 35mph.

Best you check out how the waterline length limit works before thinking it's baloney, eh?

[captlloyd]

Riverrat37, all conjecture. Seems to me a big difference between driving a boat under it’s own power, and towing it. We will never know until it is tried, and that is something no one would want to attempt.

[Searles]

No matter how you put it the answer is still no,if you put 250hp in a displacement sailing hull and a big enough shaft and prop ,you would just dig a large hole about a midships and the bow and stern wave would sink the vsl it will not try to climb out of the hole ,surfing for a short period is a whole different matter. Maybe a shallow and narrow 100fter .⛵️⚓️

[mvweebles]

The Caribbean coast off Nicaragua and Honduras has been a Hotspot for piracy on/off over the last several years. The pirates run in fast pangas and are comfortable running 50-60 nms offshore to attack. Guessing 40 kts is not unheard of for them.

18 kts would get you through their area quickly, hopefully before they could mobilize and get you in their gunsights. But you would not outrun them. Not even close.

My Willard 36 trawler is a round bottom, full keel displacement trawler similar to a Westsail. The only flat sections to support planing is the bottom of the keel. She wouldnt plane if she had a pair of Pratt & Whitney's bolted to her deck. The only way she'd exceed 8.5 kts is if the hoisting straps broke while being loaded onto a ship

Good URL for current piracy events in Caribbean

https://safetyandsecuritynet.org/tag/attempted-piracy/

Peter

[RiverRat37]

Originally Posted by captlloyd

Riverrat37, all conjecture. Seems to me a big difference between driving a boat under it’s own power, and towing it. We will never know until it is tried, and that is something no one would want to attempt.

Not conjecture, but thank you for your input. Please note that the waterline rule doesn't really care how the speed of the hull got there -- towing or propulsion, the law still applies.
As to trying it? Happens every day with Coast Guard tows, Tugboat tows and even "helpful" power boat owners offering "assistance". Many's the big power boat owner that was shocked when being helpful, at the resistance of the tow and size of the wake as the sailboat goes lower and lower and lower in the water. Shocking enough that they tend to throttle back realizing "something is not right".

Carry on.

[hamburking]

Piracy is likely the very last thing you should worry about.

There are so many other "real" dangers that are much more likely to occur...big wind, big waves, engine and equipment failures, injuries. These are the things to think about.

Just like on land, don't flash your rolex in a sketchy neighbourhood.

[Corvidae]

Originally Posted by RiverRat37

Nope, won't work. Nor will 12,000hp. Physics gets in the way. You cannot power a displacement hull more than wee bits beyond it's hull speed. Instead of speeding up, the boat goes lower in the water until it sinks (technically it's climbing it's own (larger and larger) bow wave and theoretically it could end up doing a backflip, but I suggest it would ship water first and sink). And fuel consumption goes to positively dramatic heights.

See: https://www.boatbuilding.xyz/science...ull-speed.html

All bets are off when putting "wings" on the keel for lift.

I can disprove this with any smooth stone. It skips when thrown.

Nothing pushes a boat down other than gravity. Going faster doesn't increase a boats mass until you near light speed, nothing any of us will ever have to worry about.

Also I happen to live on a displacement hull that does 150% of hull speed with the original engines. It's not a magical barrier, it's just the point where power requirements tend go up exponentially.

[Montanan]

The discussion of displacement and hull speed is all rather irrelevant. The cruiser is not going to out run a pirate's skiff that will typically operate at 35 to 40 knots.

Forget about pirates, the possibility of an encounter is minimal and only occurs in places that are sketchy / places you likely would not wish to go to or near. You are probably much more likely to have your car hijacked than have your boat pirated. Just go enjoy your voyages. Recalling what FDR said: The only thing we have to fear is, fear itself.

[Westcliffe01]

https://youtu.be/kJ_S5QPSPg8

[Scorpius]

Originally Posted by Corvidae

I can disprove this with any smooth stone. It skips when thrown.

Nothing pushes a boat down other than gravity. Going faster doesn't increase a boats mass until you near light speed, nothing any of us will ever have to worry about.

Also I happen to live on a displacement hull that does 150% of hull speed with the original engines. It's not a magical barrier, it's just the point where power requirements tend go up exponentially.

It seems to me some of the tugs on the BC coast - 40 footers with maybe 1500 hp - could and would (on occasion when running without a tow) exceed their theoretical hull speeds. The wash they created was HUGE, sometimes over 6' of cresting, breaking waves.

They don't seem to do it so often anymore. Maybe they've been sued too many times.

(By the way, a skipping stone is, I believe, totally irrelevant. It's in planing, not displacement, mode).

[Hamish_ct]

OK YES. Going up the east river in NYC with an 8 knot current, 20knotts of wind behind you, with the spinnaker up, no weight in the boat and the hull gets up and plains.

Otherwise, yeah no. Not possible, and most likely in the example I gave you would kill yourself hitting a commercial boat because you would be totally out of control.

But boy what a great article in the NY Post that crash would be!!!!!!!

[friz]

Originally Posted by sv_pelagia

NO! A Valiant 40 (and most other displacement hull sailboats) is limited to its hull speed (and brief higher speeds when surfing down waves etc).

Theoretical hull speed (LWL in feet):
1.34 * (LWL)1/2

Valiant 40 = 7.8kn

Wanna go faster:
You'll have to get into planing or foils....

Does the formula hold true on planning hulls (power boats)?