Tank testing (effective power) VS Real life

[MechaNik]

Hi, I am helping with a performance upgrade on a jet driven boat. The vessel turned out over weight (nothing new) and never achieved the commissioned speed.

It just happens that the new lightship displacement is exactly what the operating displacement should have been. Hence we have a spread sheet of tank testing data for the current lightship displacement from the Wolfson unit in South Hampton. LCG and trim seem correct for this displacement too.

Is it good enough to work out an efficiency rating between the predicted effective power and true power to determine the hull and drive line efficiency? Then use this value to determine the necessary power upgrade needed at the speed vs effective power prediction.

At first look it appears like there is only a 14% increase in power needed which can be made by an approved engine tweak so physically nothing would change.

What are the pitfalls and cautions? Can we rely on the tank data to get this right?

[Tad]

The critical thing with jets is having enough thrust at hump speed.......I would first correlate your tank data with a decent Savitsky spreadsheet (several available on this site). Then I would calculate the drag predictions at the real weight and LCG and create a thrust vs drag map as shown below......the jet supplier will have the thrust/vs speed and HP stuff for you......Be careful to use the actual HP at the shaft.........

[MechaNik]

Thank you Tad. Talking to the suppliers it appears the installation is well match and performing well. There is just not enough thrust to make the contract speed and perhaps do to slightly lower than predicted propulsion efficiency.
The engine supplier can only offer a 15% improvement in power whilst retaining the classification and warranty on the engine. This is by way of an rpm and power increase to the next rating level. The jet supplier says this will give the required thrust (from the tank testing data for the displacement for speed required). But it is close and the builder is unsure if he should trust the tank testing data.
The builder can opt to pay a penalty, or he can have the cheaper upgrade done, but is worried of having to do both if unsuccessful.

[Tad]

Well.....in theory (and only to a point) pump efficiency should increase with speed....so I would go with the power increase (cheapest solution) first......have you tried a speed trial at lightship? (or as close to as possible)

If that doesn't work it's lighten the boat, increase power and/or thrust, or change the bottom........

[MechaNik]

Good point, when comparing power applied to the theoretical power the efficiency is increasing all the time to max rpm, here we get a figure of 62%. We were told the pump was tailored to achieve 65% at max speed but we assume some extra drag from appendages and defects in the custom made intake ducts.

[Ad Hoc]

Originally Posted by Tad

The critical thing with jets is having enough thrust at hump speed......
Attachment 53607

Absolutely!

Check you ahve enough at this hump esepically to ensure you're jet is nowhere near the cavitation zone. Most designers/builders also opt for the lighter cheaper smaller units. This is usually a mistake. Fit the largest units you can get away with, that are in the working range for your boat. This helps to avoid problems at hump speed as Tad noted too.

Yes, you can trust the data. Especially if you ahve now corroloated the data with real sea trials.

[Jenny Giles]

Originally Posted by Tad

The critical thing with jets is having enough thrust at hump speed.......I would first correlate your tank data with a decent Savitsky spreadsheet (several available on this site). Then I would calculate the drag predictions at the real weight and LCG and create a thrust vs drag map as shown below......the jet supplier will have the thrust/vs speed and HP stuff for you......Be careful to use the actual HP at the shaft.........

Attachment 53607

Thank you, Tad.
Do you also apply some kind of safety margin after correlating the data, and then re-iterate the calculation?

TIA

[Tad]

Jenny...

Safety or fudge factors would depend on your perception of the input data's accuracy......and the requirements of the output....that is, if there's a huge performance penalty attached you're going to be extra specially careful and conservative.....

This is another example of why I believe in the apprenticeship system of training competent naval architects/designers......If you have worked closely under someone with experience, they can pass that real experience (and the proprietary data) along.......thus you have something concrete in the way of comparables to work from......

[Jenny Giles]

Originally Posted by Tad

Jenny...

Safety or fudge factors would depend on your perception of the input data's accuracy......and the requirements of the output....that is, if there's a huge performance penalty attached you're going to be extra specially careful and conservative.....

This is another example of why I believe in the apprenticeship system of training competent naval architects/designers......If you have worked closely under someone with experience, they can pass that real experience (and the proprietary data) along.......thus you have something concrete in the way of comparables to work from......

Thanks. I understand.
Sometimes reality works in our favor too. If the test data says you need a 120hp outboard but you can only buy 100hp or 150hp, the margin has been taken care of without fudging