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

[argonauta1]

Well, it is obvious a yacht cannot expect to outrun the bad guys if they are operating high powered fiberglass speed boats or ribs. Slower fishing boats maybe. When we came through the Gulf of Aden, our precautionary action was first to keep a good lookout and at the first sign of a suspicious action, turn to put that vessel on the stern and maintain best speed. We could expect to make 7 K and while a fishing boat might make 9 K, seen at say 3 miles, overtake would need about 2 hours. Meantime we would have got on the radio calling mayday. Never happened.

[Nepidae]

There are 70' sailboats that can exceed the speed you are looking for.

You probably can't afford 1.

Check out the sailboats that are competing in the America's Cup.

The best idea is to avoid those areas, which seem to be spreading to more and more areas of the world.

Unfortunately, with our politically correct world, we won't put an altered 50' boat in the area that with the approach of pirates, and the push of a button, would open up into a 50 caliber killing machine and take them out. Privacy would end very quickly if those who left on a raid never came back.

My own personal view.

[mstrebe]

Short answer: No.

Long answer: Displacement boats are limited in speed by drag through the water. You can calculate the maximum speed of any non-planing hull form using the traditional hull speed formula:

(HS = 1.34 x √LWL)

Valiant 40s have an LWL of 34 feet, so the maximum speed through the water is 7.8 knots (8 knots) regardless of how it's powered or what you do. Going above this speed not only requires orders of magnitude more power, but it also lifts the hull out of the water and onto a plane, which is unstable for non-planing hull forms and all boats with keels. If forced, a sailboat will both plow and capsize. It's simply not possible. Now, the 1.34 number was empirically derived from observation 150 years ago, and in truth it varies from about 1.2 to 1.5 based on the hull form of the boat. 1.34 is an average.

Many of the of the new ultralight modern sailboats are semi-planing and can get a few knots above their displacement speed safely. For those boats you can use a 1.5 multiplier in the displacement formula to account for their semi-planing hull-form, but that still won't get you over 10 knots in a sailboat.

Now, it is length-dependent and longer boats go faster. You can reverse the formula to determine the necessary length of boat to reach a speed: square(18 knots / 1.34) = 180 feet. A 70 foot boat can reach maybe 12 knots with a little leniency unless it's semi-planing, in which case it might get to 13.

To go faster than this formula speed, a boat must have a planing or at least semi-planing hull and enough power to get the boat up and over it's own bow-wave. This requires enough power to literally hoist the boat up and out of the water. Once on a plane, the boat is freed from the drag formula and can go as fast as the surface of the water allows based on sea-state.

Now, there are planing sailboats, and there are models of sailboats that are designed with semi-planing hulls that can pull up their keel and plane on a large outboard motor up to 18 knots, but those boats are not rated for offshore sailing (E.g., MacGregor 26M). These boats are extremely unstable while transitioning from displacement to true-planing, and should only be helmed straight through the transition from 8 through 13 knots. Once planing, they're again stable. There are complicated reasons for this instability zone, but suffice it to say that while in a turn in displacement, the hull lists away from the inside radius of the turn, while in planing mode the hull lists towards it. In the instability zone, the hull wobbles as it goes over and back off the bow wave, creating hysteresis. If you leave the keel down in these boats, the hysteresis mode remains, and the boat will capsize (and that has happened).

[valhalla360]

Originally Posted by Shrew

A few slight misconceptions. Theoretical Hull Speed is the fastest speed the hull can run while still operating at full displacement. Any faster and the hull begins to climb out of the hole and start to plane. The vessel is not limited to that speed as a result of math or physics. Manufacturers simply install engines and gearboxes to reach (and not exceed) that speed. As stated, a full displacement can exceed theoretical hull speed when surfing.

You could install a large enough engine to exceed hull speed and even plane. However, the engine would need to be large with high horsepower. You'd also need a significantly larger fuel tank. The shape of the hull would not be optimum for planing and I suspect the vessels handling characteristics would be horrible.

So the answer is still NO, but with caveats.

If you have a planing boat, that is true.

For a displacement hull design, no, you just plow a bigger bow and stern wave. They are not designed to lift the hull up out of the water.

Hull speed is not a specific as the formula suggests. If you throw a lot of HP at it, you might do an extra 0.5kt but for every 0.1kt beyond hull speed, the HP requirement goes up exponentially. Eventually the forces would tear the boat apart trying to throw enough power. Narrow hulls do tend to have higher hull speeds for the same length (this is why cats can often exceed the formula).

Hull speed drives the engine size, not the other way around.

Probably the biggest issue is range at high speed. Used to have a 31ft fly bridge power boat. It got around 1.5mpg. With a 200gal tank, it had a range of about 300miles. At 40ft, many report 1.0mpg...so even tougher to get long range motoring.

OP: You could get something like a Gun Boat (brand name) catamaran. They can successfully sail at speeds in the teens. I think motoring is limited to 10-12kts.

But those are very expensive boats and not for the feint of heart. Realistically, avoid pirate areas is a far better approach.

[Gingersnap]

How Captain Fatty sailed through piracy waters

https://www.cruisingworld.com/destinations/tiptoe-through-pirates/

10 tips to avoid piracy

https://www.yachtingworld.com/blogs/elaine-bunting/10-tips-for-avoiding-pirates-10074

10 common piracy areas -2019

https://www.marineinsight.com/marine-piracy-marine/10-maritime-piracy-affected-areas-around-the-world/amp/

Excellent advice

https://sailingacatamaran.com/do-you-need-to-worry-about-pirates-when-sailing/

[pbmaise]

My former vessel could easily exceed maximum hull velocity owing to the hull shape. The center hull was mostly flat from midship to stern. The vessel could easily climb up onto this large flat surface and achieve speeds of 25 knots when it had it's original 79 foot tall rig.

However, there is a big downside to this type of hull design. Noise!

Waves traveling in line with the vessel would pick up the vessel and then drop it. BOOM! BOOM! BOOM! This would occur especially when going downwind and when the bay was rough on anchor. I had to be careful around the vessel if I was in the water when this was happening. It could pound on my head too.

The noise was disturbing enough that I refused to live in the owner's cabin despite the fact it had panoramic 270 degree port windows and a king size bed. Instead I lived 11 years in the v-berth with nothing but a hatch cover to provide light and no view.

[David Mathis]

A full displacement hull has a maximum efficient speed which is approximately the square root of the hull length in feet times about 1 1/3 which means your 40' sailboat will cruise at about 8 knots. Can it go faster? Yes, of course. However, the force required to double the speed of a full displacement hull, unlike a planing hull, is significantly exceeds the four times the power that would be required to double the speed when planing. Let's go far beyond the practical application of power, get into the area of absurdity, and install one of the new 2400 hp Caterpillars in your 40' sailboat. How fast would it go? You would have a calculated high top speed which could never be reached because trying to push a full displacement hull to that velocity would result in severe stability issues causing your boat to be absolutely uncontrollable. You would have a submarine. If you feel that you need a vessel with sufficient speed to outrun the bad guys, get a planing hull powerboat with big engines. A few of the 40-50' powerboats can cruise all day at mid 20s or higher and a few have top speeds of over 30. The disadvantage is fuel costs and range. Although even at hull speed and with 2 mpg being attainable, circumnavigation doesn't sound reasonable with a planing hull powerboat of this size.

[Wireless1]

I had a 24' Trimaran that could regularly sail over 20 knots, but that's a planing hull. And to be quite honest that boat scared me. But I sure as heck wouldn't go cruising on that little thing! Ha!
As others have said; pick your cruising spots and don't lose any sleep over this.

[col50]

Originally Posted by RiverRat37

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?

Submarines and Fish under water are not limited by hull speed because they do not try to climb the waves they make, as a displacement hull will do, on the surface.

[Popeye46]

Originally Posted by Scraphound

I'm dreaming of a circumnavigation in the distant future. One thing that worries me is piracy. It's scary today and I can only imagine it will be worse 10 years from now.

I've read that one of the best defenses against piracy is maintaining a speed of 18 or more knots.

Is that possible to achieve with a cruiser? Could a Valiant 40 (or modern equivalent) be refitted with a new engine and be capable of reaching those speeds? Not for prolonged travel, but in an emergency.

Any boat can do 18 knots if you provide a big enough engine but it will only create other problems.

[Grith]

I have a 28 foot cruiser ( Imexus 28 from Poland) that can do over 22 knots fully laden for extended cruising but I wouldn’t even contemplate ocean crossings in it but I am considering the notorious Bass Straight between mainland Australia and Tasmania as feasible in the right weather window. It is a hybrid Trailable yacht /powerboat with a 180hp inboard diesel and has reached 30 knots empty with 3 people on board during initial engine run it testing. Just thought I would comment here as there are some unconventional solutions being manufactured out there but it’s not a world cruising yacht! I do intend multi month cruising in the Challenging Kimberley’s in North Western Australia however but need to work my lovely new (previously non sailing but loving it) first mate up to this adventure with some tamer ones first! Perfectly capable extended islands and inshore sailing cruiser with power planing ability but not for ocean crossings with any margin for safety. Still have heard and read of people doing outrageous things in tiny boats but not for me!

[Martkimwat]

You could buy a Magregor 27 but I wouldn't recommend it for sailing around the World and as a sailboat they are pretty sh*te. I think it's capable of 30knots with a 70hp outboard on the back) It's basically a semi powerboat hull with water ballast, drop keel and a sailing rig, don't think it is particularly good in either mode. It's what you get when you try to make a boat that is a jack of all trades.

[Martkimwat]

Originally Posted by Popeye46

Any boat can do 18 knots if you provide a big enough engine but it will only create other problems.

Not if it's a displacement hull. read other posts on here. You need to understand the physics involved.

[Grith]

Originally Posted by Martkimwat

You could buy a Magregor 27 but I wouldn't recommend it for sailing around the World and as a sailboat they are pretty sh*te. I think it's capable of 30knots with a 70hp outboard on the back) It's basically a semi powerboat hull with water ballast, drop keel and a sailing rig, don't think it is particularly good in either mode. It's what you get when you try to make a boat that is a jack of all trades.

I am going to call out that speed in a Macgregor 27 as rubbish! 70 hp outboard and 30 knots! I think you need to check the log or gps for serious errors! LOL A friend has a Macgregors 26 with a 90hp and completely empty and at wide open throttle in perfect conditions barely cracks 20 knots. LOL

[Grith]

By the way I think the premise of this whole thread in trying to get a big ocean crossing cruising yacht to power at those speeds for avoiding piracy is a bit ridiculous but if you want those abilities I suggest looking at fitting a ocean crossing capable cruising catamaran with a pair of similar big powerful inboard powerboat relatively light turbo diesels like my Imexus 28. A pair of these in a moderate weight cruising catamaran could I feel get it up on the plane and really move.🙂