Police are urging boat owners to be alert following evidence that the theft of high value engines by professional gangs has increased "significantly".
More than £60,000 worth of boat engines have been reported stolen since November in St Mawes and Malpas alone, according to police.
This is the sharpest rise over the winter months to date, according to Truro CID.
PC Sophie Curtis said: "Something has changed. It is fair to say that there is a professional outlook on it. We think that whoever is doing it is giving it some consideration – this is no longer spontaneous theft."
PCSO Jim Eagles said there were 17 reported incidents of marine thefts on the Roseland last year.
He added: "We have been hit hard over the summer for marine crime. Boats have been targeted where we have lost outboards from them in a professional way.
"Some of the equipment has been moved very well. It is people who know what they are doing, have the knowledge and tools."
Even the Truro harbour master's office, one of the agencies working to prevent marine theft, has fallen victim.
On New Year's Day one of its two patrol boats was stolen from Malpas and moved to a quiet location before having its 80-horse power engine ripped out. The boat was later found with "significant" damage.
Mark Killingback, harbour master for Truro and Penryn, said: "It's not just the replacement of the engine, which ran into thousands of pounds, but the damage caused to the boat."
Thieves had to cut through the steering and wiring which connects the engine to the control levers to remove the engine.
"It is expensive, there is an insurance claim, it takes time and effort," Mr Killingback said.
Even extra security measures advised by the insurance company, like bespoke bolts, did not deter the thieves. He added: "It seems that whatever security measures the industry comes up with the thieves are one step ahead."
Truro CID is calling on all boat owners to step up crime prevention measures and be extra vigilant.
The measures include joining a Boat Watch scheme, fitting CCTV systems and adding extra lighting. Boat owners are also advised to make a record of serial numbers on engines and even take pictures to allow the equipment to be tracked if it is recovered. For advice call the CID number 101.