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

[Narfi]

I read a good supply of carpet tacks helps......

[Muaddib1116]

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..

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

[AJ_n_Audrey]

The only time I've consistently exceeded hull speed (7.1 knots) was traveling north in the Gulf Stream for 40 hours. It was cool doing 10 to 11 knots for hour after hour....

But yes, 1.34 * Square root of LWL is the max speed in the real world. And pirates are way less of an issue than you think, except maybe around Somalia.

[Muaddib1116]

Originally Posted by AJ_n_Audrey

The only time I've consistently exceeded hull speed (7.1 knots) was traveling north in the Gulf Stream for 40 hours. It was cool doing 10 to 11 knots for hour after hour....

But yes, 1.34 * Square root of LWL is the max speed in the real world. And pirates are way less of an issue than you think, except maybe around Somalia.

That sounds like SOG, not boat speed through the water.

[joelhemington]

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.

Probably not. You'd need one hell of a big engine to reach those speeds and even then the boat would be damned near impossible to maneuver given you're way over the hull speed. The impact such a speed would have on your true wind and apparent wind would be an interesting hurdle as well. Seems to me like a power skiff could easily exceed 18 knots if it really wanted to (sounds like a guide for cargo vessels which are much more likely to be the target of pirate attacks). It'd be pretty hard to board a big ship doing 18 knots. Areas known for piracy often provide military convoys for sail boats transiting trouble zones. Pirates are looking for juicy targets which usually won't include your average boat scum (sailorboy 1's opinion of us ). Just don't prance around the village flashing your Rolodex and HH gear and you shouldn't have a problem

[SteveSadler]

We see performance sailboats sailing 18kts consistently. Especially foilers. Buy one of those and put a 300hp engine in it. No problem!

[Corvidae]

You need about 600 to 800hp to get a Valiant 40 to 18 knots. Which will require not only much larger engines (it comes with a 45), but some way to attach them to a hull that was in no way made for them. Probably many times the fuel capacity as well.

There's also the trade off that the modifications needed to add those engines would essentially end it's life as a sailboat, and make it the worst powerboat possible.

Not to pick on the Valiant btw, I'm sure it's a great boat. But getting any displacement hull up above hull speed takes ridiculous amounts of power. Catamarans kind of cheat, but even then you're not going to maintain 18 knots in a cruising cat outside of weather conditions you don't want to be in.

[sv_pelagia]

Originally Posted by joelhemington

Just don't prance around the village flashing your Rolodex and HH gear and you shouldn't have a problem

No, you certainly wouldn't want to be flashing your personal organizer (Rolodex) in those villages.... [emoji6]

https://en.m.wikipedia.org/wiki/Rolodex

[Belezar]

I’ve heard plenty of people bragging about their big Rolodex. I didn’t know it was a safety risk.

[Apollo366]

Yeah, my aunt Maggie always said a big Rolodex on your wrist was a great status symbol!

[Fore and Aft]

If you want to do 18 knots then find the power boat that can and put a rig on it! It would probably have to be a ketch since the flybridge will get in the way of the main boom. You will need water ballast or outriggers since weight is the enemy of speed.
Cheers

[RiverRat37]

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.

No matter how powerful the engine/motor is you will NEVER get a deep displacement hull (like a Valiant 40) to plane. It's very interesting what happens when one tries to exceed the theoretical hull speed ... the boat does not go faster, it goes deeper. Yep. it goes down until it goes so low that water comes aboard and it sinks. It's a fact. And the wake it would put up would be awesome!

Some displacement sailboats have engines a good bit larger than necessary and at full RPM they simply use up more fuel and put out a larger wake, but they don't go (much) faster. Say .... maybe 1/2 a knot but the fuel is draining out the fuel tank at exciting speeds. One case where extra power could be beneficial is in a storm. One could better overcome headwinds with more power since the boat is not going thru the water at hull speeds. Adding power going downwind in strong winds can be a poor idea.

[RiverRat37]

Originally Posted by Corvidae

You need about 600 to 800hp to get a Valiant 40 to 18 knots.

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.

[captlloyd]

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.

Exactly. You can make a shipping container plane if you put a nose on it and big enough engine. ( big, big engine)

[RiverRat37]

Originally Posted by captlloyd

Exactly. You can make a shipping container plane if you put a nose on it and big enough engine. ( big, big engine)

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