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Old 24-02-2021, 09:54   #1
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Can a 30-40 foot cruiser reach speeds of 18 knots w/ engine?

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.
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Old 24-02-2021, 09:58   #2
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Re: Can a 30-40 foot cruiser reach speeds of 18 knots w/ engine?

No......you need a big power boat
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Old 24-02-2021, 10:02   #3
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Re: Can a 30-40 foot cruiser reach speeds of 18 knots w/ engine?

No.

Based on the forum you posted this in and your mention of a Valiant 40, I'm assuming you're talking about sailboats, and the answer is no. You need to research hull speed. Basically a displacement hull can only go a certain speed, which is dictated by it's waterline length, because it will create a wave trough that it can't climb out of. A 40ft boat probably has a hull speed of 7 knots tops.

As for piracy, the best defense against piracy is staying out of high risk areas. I will leave it to you to dive down the contentious rabbit hole of carrying guns onboard.
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Old 24-02-2021, 10:04   #4
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Re: Can a 30-40 foot cruiser reach speeds of 18 knots w/ engine?

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Originally Posted by Scraphound View Post
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.
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....
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Old 24-02-2021, 10:36   #5
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Re: Can a 30-40 foot cruiser reach speeds of 18 knots w/ engine?

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Originally Posted by Scraphound View Post
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.
I would look at the dragonfly 120. It can get up and go 18 knots with the 50HP motor as well as sail fast
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Old 24-02-2021, 10:37   #6
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Re: Can a 30-40 foot cruiser reach speeds of 18 knots w/ engine?

Besides the answer being, NO. I don't think doing 18 knots would be much of a help against pirates.
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Old 24-02-2021, 10:50   #7
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pirate Re: Can a 30-40 foot cruiser reach speeds of 18 knots w/ engine?

Hull speed dictates your maximum speed. There are few sailboats that will reach speeds in excess of 8 to 10 knots unless they are the modern racing hulls with foils.
I have never felt unsafe while in anchorages worldwide, however I stay away from any countries or anchorages deemed unsafe regarding theft. Most theft occurs while the occupant are at anchor and away from their boat. A good alarm system would be a deterrent but being in a large community anchorage where neighbors can watch each others boats even better.
If you want a foiling sailboat that does 18 knots or better you are trading speed for comfort.
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Old 24-02-2021, 10:55   #8
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Re: Can a 30-40 foot cruiser reach speeds of 18 knots w/ engine?

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If you want a foiling sailboat that does 18 knots or better you are trading speed for comfort.
And possibly safety....
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Old 24-02-2021, 10:56   #9
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Re: Can a 30-40 foot cruiser reach speeds of 18 knots w/ engine?

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.
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Old 24-02-2021, 10:59   #10
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Re: Can a 30-40 foot cruiser reach speeds of 18 knots w/ engine?

Quote:
Originally Posted by sv_pelagia View Post
[...]

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

[...]
Theoretical hull speed (LWL in feet):
1.34 * (LWL)^1/2
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Old 24-02-2021, 11:12   #11
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Re: Can a 30-40 foot cruiser reach speeds of 18 knots w/ engine?

Not with a heavy mono like the Valiant 40.


yes, with a lighter catamaran (ex. Outremer 40) or lighter Trimaran. Not sure a Dragonfly 1200 with 60hp can do 18knts, but it certainly would be up in the teens.
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Old 24-02-2021, 11:13   #12
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Re: Can a 30-40 foot cruiser reach speeds of 18 knots w/ engine?

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Theoretical hull speed (LWL in feet):
1.34 * (LWL)^1/2
I know... should have put in ^

I asumed that because "*" was put in for multiplication, the 1/2 would be recognized as an exponent. But yes, best to put in "^".
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Old 24-02-2021, 11:15   #13
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Re: Can a 30-40 foot cruiser reach speeds of 18 knots w/ engine?

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Originally Posted by Shrew View Post
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.
A traditional cruising sailboat with a rounded hull profile I doubt would ever plane no matter how much horsepower in the engine. The hull form would have to be flat enough in the aft sections to support planing.

There are reports of clipper ships actually sailing into the water and sinking when hit by a sudden squall and the crew didn't have time to furl the sails.
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Old 24-02-2021, 11:22   #14
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Re: Can a 30-40 foot cruiser reach speeds of 18 knots w/ engine?

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Originally Posted by Shrew View Post
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.
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.
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Old 24-02-2021, 11:29   #15
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pirate Re: Can a 30-40 foot cruiser reach speeds of 18 knots w/ engine?

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..
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