Freeship waterjet Savitsky

[aeol2000]

Hello
I'm doing my university degree project. I used Freeship to the hull shape. Now I need to make the hidrojet prediction power. Using the Savitsky method I find that there are outside domain (400bhp). The program calculates 1700kW per unit, but the manufacturer tells me they are 1300kW per unit.
There any way to perform the calculation for that domain?
Any other ideas?

Other data of my proyect.
Lpp 30m
Lwater 26m
Beam 6m
Bwaterline 4.8
Displ. 60tons
V 35knots

[daiquiri]

Based on the above dimensional data and without knowing details about your hull's shape, as a first estimate I'd say that you'll need between 1300 and 1500 kW effective power for that speed - which translates to around 2400-2800 kW engine brake power, or 3200-3800 BHP. That's in line with the declared power of some existing yachts of similar size, though the data is pretty scattered. How does it compare to your calculations?
And can you post some pics with a hullshape of your yacht?

[aeol2000]

ok i try

[aeol2000]

de savitsky power prediction

[aeol2000]

daikiri first of all thanks for your reply.
The efective power matches with your estimation, 1300kW, but the brake power predicted by freeship is about 1700kw per unit, and I supouse to that It should be aprox 1300kW per unit as you said.
I don´t known the way to macht the efective power with brake power.

[daiquiri]

1300 kW is the effective power, not the brake power. the brake power will be the effective power divided by waterjet efficiency and by the total efficiency of mechanical transmission between the impeller and the engine. I have assumed a conservative values of 0.55 waterjet efficiency (could have been 0.60 as well, now that I rethink of it...) and 0.96 shaft&transmission efficiency, which give 0.53 total efficiency.
If you have more precise technical data from your WJ supplier, you can perform a better estimate.

[baeckmo]

Slavi, I think it is wise to use a conservative figure, say 55-56 % for jet efficiency. The reason is that the inlet is very sensitive to disturbances, and I have found a number of jet units are "underperforming" due to this. It is not uncommon to find that total thrust and efficiency values are calculated with a fixed inlet loss. In reality it varies widely with the IVR ("inlet velocity ratio", Vadvance/Vthroat).

It should be remembered that the performance figures for jets are always based on calculated values regarding the various losses in the system, only the pump efficiency is tested with any accuracy.

The hull has a high deadrise, and "aeol" must not forget to check the hump resistance, since it is not necessarily the thrust at max speed that is the limiting factor. Normally the non-cavitating thrust line for the jet is rising with reduced speed, but if the jet is not selected carefully, cavitation may occur around hump speed, leading to disaster.

[aeol2000]

ok, thanks, I note the contrinutions.
I guess several improvements to the hull design can be made to make it more efficient, but only really what I'm looking for is an efficient selection water for this hull.
Does anybody knows a program that predict it, based on waterjets series curves?
i thank you this estimate coeficients.
Thanks agains for helping me, I continue learning.

[daiquiri]

Originally Posted by baeckmo

Slavi, I think it is wise to use a conservative figure, say 55-56 % for jet efficiency. The reason is that the inlet is very sensitive to disturbances, and I have found a number of jet units are "underperforming" due to this.

Thanks for that note, Bodo. In fact, 55% is what I usually use for these preliminary calcs. I had mentioned the possibility of up to 60% due to the fact that this is a high-power beast, so perhaps it is safe assume somewhat higher efficiency due to scale effects - what do you think?

Originally Posted by baeckmo

The hull has a high deadrise, and "aeol" must not forget to check the hump resistance, since it is not necessarily the thrust at max speed that is the limiting factor.

I agree, though it is in fact a rather high length-to-displacement ratio hull (close to 8), so the hump might not be so severe.
Cheers!

[Olav]

aeol,

find attached two papers that you might find useful for your project. Also attached is a little Excel spreadsheet based on the work by S. B. Denny and A. R. Feller that is cited by Blount and Bartee in the appendix; there was some discussion about this which you can find here.

Real-world waterjet performance curves can be found at UltraDynamics' website, see the Designer's Manuals (free registration required).

Hope this helps.

[Alik]

I won't use jet on such a high deadrise; I also believe that such deadrise is not required for 60t displacement.

As to jet efficiency, I would take 0.53...0.55 for first estimates.

[baeckmo]

Olav, I would be a vee bit sceptical about that excel sheet. Without going into the details of the algorithms used, I find that it gives wrong (too low in this case) thrust and there is nothing warning the novice user for out-of-shape proportions for the pump, or cavitation limits. Also, the pump efficiency asf of size (=Reynolds number), specific speed aso are "out of control".

As for the "McPearsson"-paper, it uses the nozzle area as primary selector, and although the basic principles of this method has merit for selection in the early design stage, the real limitations regarding jet performance are to be found in the inlet dimensioning. In this respect, I would rather have seen the impeller inlet as primary selector.

And yes Slavi, you are of course right on the slenderness ratio of the hull (I use LWL/(displ)^0.33 for comparisons), which means that there is very little of hump to worry about. My comment there was more to remind the student to be careful about the operating envelope as a whole. BTW: has air resistance and resistance increase due to weather/wave conditions been catered for??

[aeol2000]

Are these coeficients based in any scientific study referred?

[baeckmo]

......which coefficients do you refer to?

[aeol2000]

55-56% jet efficiency