A jetboat suddenly change the direction without any action on the helm and reverse bucket

We have a newly-built propeller-driven vessel. The length of the ship is approximately 5 meters. It is equipped with single engine and jet. The vessel performs well at low speeds. However, around the slip point of about 25 knots, without any steering or reversing operation of the helm and reversing bucket, the boat suddenly turned around. This situation is random, sometimes it turns left, and sometimes it turns right. As we know, for a jet propelled boat, if there is no helm and reversing bucket operation, according to normal circumstances, the boat should maintain its original direction. Could someone please help analyze this for us? Thank you very much!

 

Hi cwzsu, I'm wondering if the bow is down, making the stern to be riding high, not tracking well, and allowing the back end to 'step out' sideways, ending in a 180 spin; similar to a high powered car doing a straight line burnout, then spinning out sideways as the rear axle moves alignment sideways to the front wheels, and momentum plus drive pushing the back end out quicker than the steering correction can effectively deal with. You may need to lift the bow, and drop the stern, using weight adjustment, or jet nozzle trimming, or both. A deep bow can act like a pivot to the drive. That's my first guess.
Also check that the jet nozzle steering bushes (vertical) are not a very loose or sloppy fit, allowing too much nozzle movement despite no steering wheel movement.
Is it a petrol or diesel engine; what horsepower; is it a single or multi stage jet pump; how heavy is the entire boat with fuel etc. ?

 

Is it propeller driven or jet driven?

 

Hi seaquirt and gonzo, Thank you for your reply. Yes, it is single jet driven, with the specification as below: Boat LOA of 5m, Beam of 1.9m, displacement at sea trial is 2.8T. single engine power of 300hp, and with single jet, trim angle at static is 3 degree, and 4 degree at boat running (before boat turned around).

 

Pictures please

 

2800 kg's on a 5m boat.......you must be kidding, get rid of that designer!

Btw, what jet is it (inlet diameter)?

 

You describe a boat doing something undesirable. So there is a boat out there in reality. Pictures or at least good drawings, or this never happened.

 

I suspect as the boat is so short that as the speed increases the Center of lift moves sternward until it ventilates the intake, loses thrust and lift. The bow drops the steeper dead rise into green water and then trips either right or left

Alternatively the Center of gravity plus the combination of other forces moves ahead of the Center of lift and the bow drops
If this is the case, try to get the boat up to the 24 knot speed and hold it. If it begins to porpoise, you have lost the stabilizing couple and the bow drops.


Where is the fuel tank located.?

We had this happen with two boats, not ours,
The solution on one was to put a jetovator to keep the bow a bit more downward and the other was a much overly powered inflatable
that had a heavy when full fuel tank mounted right at the front of the boat.