Stepped Hull Resistance

[tkdchris]

Am working on a stepped hull design, LWL 11.7m with twin steps. I have a basis model without steps that when run through savitsky requires 776kW to reach the design speed, however the program give 725 for the stepped model.

My concerns are around the program i'm using - hullspeed - and the fact that the manual doesn't state that it can deal with stepped hulls. Not even convinced that Sovitsky is the correct method for stepped hull resistance.

Any help/advice would be greatly appreciated. Also pointers towards literature on the matter would be super

Chris

[yipster]

not of much practical help but did you read proboat magazine #127 page 56 to 71 with Michael Peters on (fast) powerboats?

[HJS]

Use this excellent paper by David Svahn. It can by extended to two steps.

js

[Kestrel]

Hi all,
I'm new in the forum, I've seen a quite interesting item ... , it's since a long time that I''m interested in. It could be interesting to give a look here:
http://www.idet.it/idra.html

bye

[tkdchris]

Originally Posted by HJS

Use this excellent paper by David Svahn. It can by extended to two steps.

js

Thank You very much, this is a most useful paper

[JorgenBeyer]

Hey Chris! I am pretty certain that the software Hullspeed does not consider stepped hulls. It also runs Savitsky and the difference you get must be input values...

Jurgen Sass: how do you suggest you improve svahn's model to include two steps? I feel some of his assumptions are only valid for a single step, and he said the same thing in a meeting i had with him yesterday.

Has anyone managed to model resistance on two steps? How would you predict the wake after the last step?

JBS

[Kestrel]

Originally Posted by JorgenBeyer

Hey Chris! I am pretty certain that the software Hullspeed does not consider stepped hulls. It also runs Savitsky and the difference you get must be input values...

Jurgen Sass: how do you suggest you improve svahn's model to include two steps? I feel some of his assumptions are only valid for a single step, and he said the same thing in a meeting i had with him yesterday.

Has anyone managed to model resistance on two steps? How would you predict the wake after the last step?

JBS

Hi,
virtually it's the same approach: You have to consider each planing surface, delimited by a step, running in the wake of the one foregoing, then compute the equilibrium condition (pitch and heave) for all the planing area together, and force distribution (i.e. 2 steps, 3 planing surfaces), given a Xcg, speed, displacement, it's normally an iterative procedure (in http://www.idet.it/idra.html, that program works up to 4 planing areas - 3 steps, by an iterative method using 3D geometry definition updated each iteration). More steps in the bottom, more local area calcutions must be done, and a bit more complicate to seek global equilibrium. The process must be done by software. Depending on error tollerance wanted, it involves thousands of iteration normally, even for a single speed data.
K.