Savitsky correction for waterjets?

Hi,
I'm dealing with a waterjet powered boat and I'm close to make power/speed prediction. But how can I take into count the "hole" of the jet inlet into Savitsky method? I am thinking about adding a negative force into the overall equilibrium, which would be the dynamic pressure on the correspondent inlet area.

 

Noting again Stavitsky's fine work, however unless your working with anything but a prismatic hull form you will be lucky to get within 10% of the true resistance. Treat is as what it is, a preliminary tool and if nothing else is available add a margin.

 

(1) Measure the floating waterline area
(2) Measure the surface of the two holes
(3) Subtract the holes area from the floating area
(4) Divide the new surface by the waterline length and you will obtain the new breadth for Savitsky.
(5) The problem are not the holes but how estimate the dynamic trim angle?


RANCHI Otto

The method of Savitsky is too optimistic and I have tested several high speed craft in Model Basin. The differences btw. the figures of Savitsky and the recorded values are very important.
For this reason I have programmed my own method and I'm very satisfied...

 

(5) The problem are not the holes but how estimate the dynamic trim angle?

Exactly, the most difficult task is to estimate the new trim.. I was thinking about adding a new negative force in the hole's centroid in order to subtract a force moment from the overall. But now I think it's not worth, I also noticed that Savitsky's is too optimistic, but unfortunately it's the only method for planing crafts I studied at University.
Do you sell your method?

Esatto, il problema più difficile è stimare il nuovo assetto.. pensavo di aggiungere una forza negativa, di valore uguale alla pressione dinamica moltiplicata per l'area dell'inlet, e applicata nel baricentro del "buco", ma poi ho lasciato perdere perchè anche io mi sono accorto che Savitsky è troppo ottimistico, ma è l'unico che ho studiato per barche plananti all'università.
Lei vende il Suo metodo?

 

Dear Nemo, I don't sell methods but I share my knowledges in the field of hydrodynamic for high speed boat.
How to do in order to transmit you my method for estimate performances ?

You think really that an estimated trim angle corresponds to the trial recorded one?

Saluti RANCHI OTTO

Caro Nemo, io non vendo sistemi di calcolo ma li condivido per quanto concerne le imbarcazioni ad alta velocità.
Come fare per trasmetterti il metodo?
Pensi veramente che esista un sistema per calcolare l'assetto dinamico?

saluti RANCHI OTTO

www.ranchidesign.ch

 

Dear Nemo,

First I wish to welcome Otto as a new member of the forum. I am sure that his knowledge will bring a lot (personal: we have not worked together for a long time, Otto !)

Second, some waterjet manufacturers (KaMeWa, I think) are claiming that the waterjet is generating a positive vertical thrust on the waterjet inlet which is conter-acting the weight of water enclosed in the inlet and the loss of lift due to the inlet penetration in the bottom. If this is true, it is useless to substract 'holes' surface from planing area.

Best regards

Vincent Guilbault

 

Dear Vincent,
I'm glad to read you again !!
Concerning waterjet intake openings, the vertical positive thrust has a benefit only at high speed where the trim angle for a waterjet craft is very stable.
At low speed the waterjet is an handicap because the speed of the water in the duct doesn't give practically vertical thrust.
I have compared speed vs power performances of the same boat with waterjet and screw on line.
The waterjet version needs a lot of power up to 30/35 knots after....
Best solution is a hull shape ASD (Arrow System design) + piercing propeller where there is the possibility to change the propeller thrust at low/intermediate speeds.

Ciao Vincent, contact me at my e-mail....

Otto RANCHI

 

Thanks for all your answers.
I would be glad to see your method, if it's not too big you could send it to
stefano@delledesign.tk

of course I don't think that an estimated trim angle is corresponding to the real one, especially on small craft where the LCG can change a lot.
I think the best way to estimate power/trim is still the model basin, even if it's expensive.
Stefano

 

Dear NEMO,

My experience with tank test is that only with craft over 20m you are able to have enough valid estimation of bare hull resistance.
Normally the model scale is to large because the chariot speed for small craft with high speed is limited. The laminar flow on bottom and the air resistance have to important effect in recording the resistance.
I have carried out a lot of model tank tests but for craft from to 18m to....
I have tested in Vienna model basin a craft having 26 m length in resistance and propulsion for speeds up to 50 knots.
How to correct for scale effect the appendage resistance? (the craft had 2 brackets and rudders). The hull and the appendages have two different Reynold's number and the resistance of the appendages is overestimated.
I had calculate separately the resistance of each item and added to the measured bare hull resistance. The scale effect for this craft at 45 knots was of 60%!
I will collect the material and I will see how to send it...

Ciao Otto

 

Vincent, I noticed In your welcoming post that a few waterjet manufacturers stated there is a POSITIVE vertical force at the inlet. Rubbish! We call jets " vacuum cleaners " for a real reason. The suction effect is what allows these jet sprint boats to turn 180 degrees in 2 boat lengths at 50 knots. IF the driver does not get a blood clot from the G force. It is a actual impossibitity to lose control of a jet boat in calm water. + vertical force my foot. Those people are YES people who have NEVER ridden in a "sprint jet boat". They probably look at the hull raise slightly when the jet is shifted into neutral orrr raced in neutral and see the stern also rise higher. What a bunch of Simpletons. ALL the reaction thrust is now down. What did they expect, downward or no change.Jets ARE vacuum cleaners at the inlet. Period.

 

They are saying that the higher the downward suction, the more positive the column lift it generates. Put a duct around and over a helicopter blade at full lift power then duct it 180 degrees to the ground so it is sucking the air from the ground UP. See if you can convince any engineer that is going to increase the + vertical thrust. Sure it does.