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

[Rohan]

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.

Some racing sailboats (mostly trimarans) can reach that speed but that would depend on your luck with wind speed and direction at that time. A powerboat can hit that speed more consistently but they won't have the range to circumnavigate the world.

However, the issue of piracy is probably not as bad as you think. There are a few hotspots in the world, so just avoid those areas. I believe they have also been significantly cleaned up in the past 10 years. I haven't crossed the globe (yet) but I would think weather and breakdowns would be much more of a concern.

[Aurora38]

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.

This is by far the best answer. Theoretical hull speed only applies within certain conditions. Most people don’t talk about (or know about) that though. I commonly hit 9 kt when surfing down waves whereas my “theoretical hull speed” is no more than 8 kt. And a big enough engine could certainly make it go faster, but it would not likely be something you would want to try.

[Shrew]

Originally Posted by Muaddib1116

The first part is technically true, but it's sort of a pedantic argument. I don't think you could get a powerful enough engine inside a ballasted keel displacement hull that would be able to get it up on plane. Maybe if you stripped out the entire interior in order to fit it in. In all practicality, it is not possible.

I agree. My only point was that previous posts alluded to it being a 'limit' which it is not. It is a calculation of the threshold where displacement changes to semi-displacement. It is absolutely possible to exceed Theoretical Hull Speed. As I mentioned, not practical with hull shape and design (and as you mentioned...ballast).

[belizesailor]

Originally Posted by boatman61

When I have on the rare occasions exceeded hull speed I have found the bow begins trying to bury itself into the water.. Likely what causes pitch poling in monos..

Yes, years ago racing we were pushing a boat really hard under spinnaker...and then we noticed the fore deck was only inches out of the water! The whole crew went and stood on the stern transom!

Afterall, we had a race to win so slowing down or dosing the chute was not one of thd options.

To some of the other comments on the thread:

That was not surfing, nothing but chop on an inland lake...and 30 knots of breeze!

We were WELL above theoretical hull speed, but nowhere near 18 knots...more like 10.5 as I recall on a Bene 38 First.

Only way to get a Valiant 40 to 18 knots is to drop it off the boat lift!

Ive got a Hobie 33 that will do 18 knots, but there is a big sacrafice in comfort for speed unless you go really big.

I used to race on Corsair Tri's...if we werent doing at least 18 knots then something was wrong!

[AiniA]

Going 18 knots is still a problem if the pirates can go 20 knots. The best defence for pirates is not to go where they are problem. Instances of piracy of small boats like we sail is well-known and pretty avoidable. There are many other, more important dangers to worry about.

[Sailorichiban]

A power cat with large enough power plants can achieve that type of speed.
Typically nearly flat stern sections, with each hull having a relatively narrow beam. Possibly eliminate the fly bridge and add a rig to create a motor sailer.
Like most motor sailers it would be a compromise.

So , yes it could be done

[MIRELOS]

there are multihulls with big motors that can get high motoring speeds, I recall a cat from Gold Coast Yachts that could motor at 20kts...monohulls being displacement hulls are limited by their waterline length

[boatman61]

Originally Posted by Muaddib1116

I'm guessing that's when you were surfing down a wave?

Keeping pace... the water was maybe 6" from washing down the fore deck.. got a bit nervous over the next hour till I got behind the LA Corunna breakwater.

[chrisr]

firstly, there are plenty of race boats out there that will exceed 18k under sail - i've owned several

there are also plenty of race boats out there that will exceed 18k under sail for long periods - i've owned 1

(look at the speed some of these guys have AVERAGED around the globe !)

but these are planning hulls - not displacement. as many have said earlier : it's pointless to try to get a displacement yacht up to such speeds

secondly, i think this speed (of 18k) is fine for avoiding pirates if you are a ship. it get's seriously hard for them to board a ship at such speeds. the pirate skiffs i've seen can go faster than this and i don't think 18k is a magic number for a small boat (that being said i also don't think pirates are going to spend a lot of time / fuel chasing if the speed diff is small...)

finally, piracy in most of the world is not the risk that it use to be - esp around horn of africa / somalia. i'd still stay well away from gulf of guinea / nigeria and a few other spots, but that's not hard.

cheers,

[Chris Cringle]

No. And, btw, what makes you think a pirate skiff can't do 20 knots?

[Stu Jackson]

Originally Posted by Scraphound

>>>>>
One thing that worries me is piracy.

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

The best defense against piracy is to avoid the pirates.

[billgewater]

You have a problem here. The assumption is that the pirate threat is off the Somalian coast or nearby. (Or Halmahera off the southern Philippines)

No. Piracy is more prevalent and inescapable when dealing with; some insurance companies, some brokers, some boat yards, some registration systems, most chandlers. And they don't carry guns but smile like a Cheshire cat as they drain you of your hard earned assets.

You don't need a sailboat that can do 18 knots as they have speedboats that go faster.
What you do need to avoid the real pirates is experience, competence and the ability to sift the dross from reality.

For what it is worth, my understanding re the Somalian piracy is that when the country had internal problems, the European and Med. fishing fleets scoured the Somalian coast depleting the local fish food stock. So the locals took to piracy. What goes around comes around. However, I understand that the area is now well patrolled by various naval boats. Russian? Protecting oil ships? Maybe someone can enlighten us.

[Kd9truck]

I have herd a full sail plan up and working and the motor running is more intimidating to them, as most pirates are poor and we all know that.... sailing is for the rich.

I would never Cruze where there is the potential, I worked the neighborhoods of NYC as a firefighter and have seen enough people in desperate conditions, they do desperate things.

To me my safety and my family’s safety is paramount.

[rondelais]

Another problem in trying to get a monohull going much faster than hull speed is that seen when trying to tow a sailing dinghy too fast with the board down - it will usually capsize if towed too fast. I suspect this is because the center board (or keel if a keel boat) will develop lift (it is a foil) and as soon as it heels slightly due to wave action, the lift will roll the boat. Ive seen this happen a number of times when small sailing dinghies have been towed by power boats (usually kids in a whaler). I suspect the same might happen if a keelboat were pushed by an engine capable of sufficient thrust. So, to get high speed from a monohull sailboat hull, you probably need shape to allow planing, and then figure a way to retract the keel.

[captlloyd]

Originally Posted by RiverRat37

A shipping container has a flat bottom, so of course it could plane. A deep displacement hull? Nope. It will never, ever plane (except for surfing down the face of a big wave). It will sink first. You gotta know the physics involved. Too deep to go into here. Try this:

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

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.