Boat using Waterjet troubles

[Mr Efficiency]

What is engine rpm at your current top speed ?

 

[ephraimjosuah]

The top speed (at 13 knots) is 2400 rpm,
we have just weigh the boat at load cell, and it shows 17,8 tons,
we do not put the draftmark correctly some months ago.

 

[devudegoa]

....A planing resistance spreadsheet can be found in the download section of this website...
View attachment 104956

Sir, can someone guide me to the download section where this spreadsheet is posted.

 

[groper]

You cannot fix a weight problem by looking at the water jets- it's like asking why there are waves on the ocean and ignoring the wind...

If you cannot reduce weight, then you have no choice but to increase engine power. Clearly the boat was built overweight, the only way to compensate is to increase horsepower. I know it costs money, but what choice do you have- write off the entire boat?

18 tonnes from 1000hp.... Sounds a bit unrealistic... With large diameter propellers instead of jets, you may find enough thrust to get over the hump resistance ?

 

[magny7]

Observing your plimsol mark, are you perhaps an Indonesian ?

This once happened to the ship i was designed. At rest it was even keeled, when we run it, it's trim changed only slightly. It was designed for 25 knots but could only reached 19 - 20 kt. What the engineers done was cut its weight for around 500 kg and offsets its center of gravity more to aft until practicality limits. And voila, 26 kt reached.

Lesson learned, It was stupid of me to design a hull without costumer being specific on what they want to put aboard.

 

[butchnz]

Hi
I am not a designer or boat builder, however I have had 30+ years experience with commercial jets and unfortunately was an owner of 2 x 340HC castoldi jets. 1st you need to trust any advice Baeckmo has to offer, he helped me with my jet problems and was 100% correct with his conclusions. A test I found useful was to do a static test. i.e tie the boat to a wharf, one jet at a time with the steering nozzle straight ahead increase the rpm, you should be able to reach max RPM of the engines. as you increase the revs you may hear a rattling sound (like stones in a tin can) in my case it started at about 1500 - 1700 rpm if this sound starts turn the steering nozzle hard port or starboard, if the sound goes then like I had, you have a serious cavitation problem no amount of moving weight will fix that. good luck. p.s your boat is very heavy for the amount of HP

 

[ephraimjosuah]

We have reduced the equipment to 16,5 tonnes, and on sea trial we get 23.5 knots. We use trim tab 600 mm x 500 mm at the left, right, center of both waterjet.
how is the normal tonnage for this boat to reach 28 - 30 knot?.

sorry Mr. butchnz, can you explain this sentences to me you have a serious cavitation problem no amount of moving weight will fix that. Does it mean no use to reduce weight ?

Dear Mr.baeckmo, what must we do to clear up the cavitation on the waterjet?
thanks

 

[magny7]

Perhaps, by using trim tabs you force the boat to broach less. Planning hull needs a bit more trim so that it could "jump" and "ride" the wake it generates. Have you try its performance without/with reduced trim tab ?

Please correct me if I'm wrong.

 

[baeckmo]

Hi Jusak, there are some contradictions in the numbers you quote. I take it that the engine is a QSC 8.3 with setting 500 hp @ 2600 rpm. In the runs with 18 t displacement you clearly operated below the cavitation limit (see attached diagram). Yet the full power rpms were lower than the nominal (2600 rpm), in spite of a gearing that would let the engine run on the regulator limit, particularly when cavitating.

With the present ratio (53/43), the jet will absorb about 370 hp @ 2400 rpm, 420 hp @ 2500 rpm and 460 hp @ 2600 rpm. The engine is running “light” with this gear ratio and should have overspeeded to slightly above 2700 rpm both in the first runs and the later with 16,5 t displacement. We must get clarity on this before looking into anything else. Since the pump power varies with rpm^3, it is very important that the tacho readings are correct. I suggest you start with the following:

• Check tacho calibration carefully. Perform readings without parallax faults.
• Check that the throttle lever can reach full travel.
• Check engine inlet pressure after intercooler (connection and value: see engine manual).
• Check exhaust temperature (connection and value: see manual).
• Check fuel flow (this engine is electronically controlled; connect scanning computer).
• Check speed at 2200 rpm, 2300, 2400, 2500 and max (2600?); two way runs.

The Qsc engine has a completely flat power curve on top, and I would have selected the gearing 52/44 instead. It would have been better for the engine (lower rpms, no risk for overspeeding, better fuel economy). But this is not the basic problem.

With the uncertainties regarding test values in this case, there are three possible scenarios:

A. The engine is producing full power (368 kW). I use a regression analysis (ref. see below) to find resistance in this speed range. Your operating readings (13 kn/18t; 23,5 kn/16,5t) combined with the available thrust are used to adjust the regression constants. You find the resulting drag curves in black on the attached thrust diagram. The propulsive efficiency is 44%, and the thrust is 2*13600 N.

Note that the drag and thrust curves are very close from 16 kn and upwards. Any minor increase in drag (weight increase, sea-state et c.) will cause a dramatic speed loss. If this is the case at hand, you would need about 2*600 hp to reach 28 kn with 16,5 ton.


B. The engine is lacking power (not completely run-in, hot fuel, low-energy fuel, slightly low regulator setting et c). In this case we might have a maximum power of ~440 hp (323 kW), which is shown in red in the diagram. Now it would be possible to reach about 27-28 knot with 2*500 hp.

C. The pump is not performing according to specification. I have seen Castoldi impellers with bad castings resulting in bad blade shape. In the end, the thrust was reduced due to increased cavitation.

The truth probably lies somewhere in between. When you have performed the checks in my list, we have better basis to judge from, but the main issue; the weight, still remains.

When trouble-shooting you have to be systematic and meticulously careful with details. In this case, we first check the Engine performance, then the jet performance before we focus on the hull. The golden rules for troubleshooters are:

** All calculations are wrong!
** All measurements are wrong!
** All Components are faulty!
** Everybody involved is lying, either trying to hide a mistake or because they do not understand the importance of their observations!

Ref: “Resistance Prediction for Hard Chine Hulls in the Pre-Planing Regime”, by Radojcic, Zgradic, Kalajdzic and Simic; Polish Maritime Research 2(82) 2014 Vol 21, pp 9-26.

 

[ephraimjosuah]

thank you very much Mr. Baeckmo,
trying your suggestion and check the engine

just to clarify Mr. Baeckmo, we have 43/53 gearbox ratio, not 53/43,
is that similar in this term?
i am beginner in gearing ratio term.

we have intermittent QSC 8.3 Engine,
do it also influence the output horsepower ?

to magny 7: we have done it but at 18 ton is no speed increased, but in 16,5 still at 600 x 500 trim tab, no reduced trim tab.

 

[baeckmo]

thank you very much Mr. Baeckmo,
trying your suggestion and check the engine

just to clarify Mr. Baeckmo, we have 43/53 gearbox ratio, not 53/43,
is that similar in this term?
i am beginner in gearing ratio term.

we have intermittent QSC 8.3 Engine,
do it also influence the output horsepower ?

to magny 7: we have done it but at 18 ton is no speed increased, but in 16,5 still at 600 x 500 trim tab, no reduced trim tab.

The gear ratio you have means that the engine will rotate 53/43 turns (ie 1.233 turn) for one turn on the jet shaft. When the engine makes 2600 rpm, the jet is doing 2109 rpm.

Btw, also check the temperature in the engine room, and that the ventilation areas are adequate and according to the engine suppliers specifications.