Very high speed prediction

[fede]

It seems that savitsky rules tend to overestimate resistence as speeds get very high (Fn>4)
Any suggestion on how to get a realistic idea of performance for very high speed small craft ?

[rxcomposite]

Try Leo Lazauskas Michlet program or download some of his papers.Leo is a member of this forum.

[tspeer]

Michlet is useless for this problem - it deals with wave drag at displacement speeds and cannot handle dynamic lift. The wrong end of the spectrum completely.

[fede]

I gave a look to Michlet and I do agree with tspeer
Thanks anyway Rxcomposite

[Leo Lazauskas]

Originally Posted by fede

It seems that savitsky rules tend to overestimate resistence as speeds get very high (Fn>4)
Any suggestion on how to get a realistic idea of performance for very high speed small craft ?

As others have suggested, Michlet is not appropriate for this problem.

Exactly what are you looking for? Another, supposedly better empirical formula than Savitsky? Or do you want a physics-based, 3D planing code?

Regards,
Leo.

[fede]

Hi Leo,I'm open to anything here:

1)do you guys agree when I say that for high Fn Savitsky gets a bit unrealistic?

2) Could you suggest a way to better use savitsky at high Fn ?

3) any other solution is welcome be it some formulas or a software,I'd just love to be able to get some realistic predictions for high speed crafts.

[DMacPherson]

First, I'm presuming that you are referring to the volumetric Froude number when you say "Fn>4". Is this correct?

With respect to the Savitsky drag prediction methodology, it's important to recognize that a Savitsky analysis is not the whole story. All that the Savitsky method can do is evaluate the hull drag (and lift) under a particular prescribed trim angle. So, to reasonably use the Savitsky analysis for a full boat, you need to:

a. Embed the basic Savitsky analysis into an iterative solution solving for the trim, so that forces and moments equals zero (i.e., the equilibrium state). In other words, each trim angle gives a different position of the bottom planing area and the center of lift (or center of pressure, CP). This needs to be solved so that the CP correlates to the LCG of the boat. (This is the "simple method" as described in Savitsky's paper where all forces are simplified to go through the CG.)

b. The above needs to be extended so that the effect of the thrust line vector (i.e., shaft line) is part of the above solution. A thrust vector that points below the CG, for example, will apply a moment providing bow lift. This moment in turn changes trim, then lift and drag and CP, so you'll be iterating this until you reach a solution. This become the "general case" method from Savitsky's paper.

c. This is still inadequate for high-speed analysis, as windage and appendage drag impart other forces and moments on the boat, requiring more components to the solution. As speed increases, the relative proportion of windage and appendage drag typically becomes greater. (One caveat - many of the appendage and windage prediction models were never tested at extremely high speeds, so their magnitudes may be suspect.) If you have stern wedges or trim tabs, these too need to be part of the prediction model.

d. You cannot forget the effect of propeller lift. This typically creates a bow down moment that should be part of the solution.

So, each implementation of the Savitsky analysis is not necessarily the same, nor is each a complete planing hull drag prediction. We have had pretty good success with our implementation of Savitsky (as developed for NavCad and SwiftCraft) for very high-speed craft. In one example that I can think of immediately, one of our users asked us to run a comparison to one of their model tests for a planing monohull (a variant of the Gentry Eagle) at speeds up to Fn>7 (volumetric Fn). This was for hull only (no appendages) and the results throughout the speed range were well within 5% drag (a bit over-predicted) and one degree of trim. (Trim prediction was actually better at the higher speeds.)

Regards,

Don MacPherson
HydroComp, Inc.

[fede]

Dear Mr MacPherson, thanks for your post.
I'm talking of FnV of 11 and more.
(specific case :7.5 mt racing boat 1.4 tons,70 or more knots[36 m/s circa])
I usually take into consideration points A B and D of your post while doing calcs.
What I was not taking into consideration is the possibility to trim the leg and change moment when using trimmable
legs.
Tried playing around with prop angle and of course the smaller tha angle the smaller the friction due to the bow up moment.

[yipster]

did you try comparing savistki, bailey, keith, crouch, and imperical results?
keep myself posted for better free info

[fede]

Hey Ypster, thanks for the Dingo XLS,I had that,I've done one myself sometime ago and more or less results are similar to Dingo's.
I'm not confident with bailey, keith and crouch.
Anyhow, for all of us high speed crafts enthusiasts it is coming out a book in early 2006 on high speed hydrodinamics, finally 680 pages 100% sails free!!!
I've been waiting for years...
http://www.cambridge.org/catalogue/c...1845688&ss=exc

[yipster]

took a quik peek, book looks interesting!
here is the ( said to be fairly acurat ) Crouch calc http://www.geocities.com/howardsteph...speedcalc.html

[rxcomposite]

Originally Posted by fede

It seems that savitsky rules tend to overestimate resistence as speeds get very high (Fn>4)
Any suggestion on how to get a realistic idea of performance for very high speed small craft ?

The savitsky program i am using has a tendency to show a wetted keel length longer that LOA at very high speed.The instructions says it degrades the calculation.

It is corrected by using a longer and narrower hull input. Also there is the M factor or the hull drying out at high speed. The results tells me to use a deeper V.

This suggest a Very Slim Hulled Design or the cigarrete type variety. Always, my hull would arrive that way at high speed prediction.

Always check the behavior of the program you are using as it may be telling you something.

I have used NavCad by hydrocomp and it is great. Dingo has done also an excellent job. Try to compare the results of the programs by feeding the same parameters.

Also, at very high froude numbers, have you considered tunnel hull design? JimBoat has a lot of introductory design guide and has a very good software for the design.

Regards,
Rx

[fede]

Hi Rx,tunnell is out of question since I'm working for a racing class where the only choice is a conventional monohull.

[Alik]

We just did number of power/speed predictions for 68' powerboat for design study. Crouch gives good argeement with prototypes. Bailey's seems to overestimate horsepower for high speeds (45-50 kts), but gives good matching with staticstics in range of 30-35 kts. For Bailey, it is not understandable where 'trim' comes from. Also, deadrise is supposed to have more influence on drag. Savitstky method is trim-sensitive, so one can get almost any results at preliminary design stage
Actually, the owner wants 50 kts (40 kts is more reasonable) on 68', so expect he needs plenty of horsepower... just deciding boat weight/horsepower now.

[Yellowjacket]

After looking at this thread and a bunch of others I do agree that Savitsky is overestimating drag at speeds where some small very light monohulls are operating. I was looking specifically at small light racing runabouts and in the speed range up to about 45 or 50 kts, the results were pretty accurate. But as you go faster than that, the real world boats seem to be doing a good bit better than the predictions. Specifically the Class D boats were running over 70-75 kts and Savitsky would say that can't be done.

One explanation that did occur to me is that, if you look at the Savitsky results at the 45 to 50 kt regime is that the running depth is getting very small. That is, the hull is running at a very low trim angle and a low depth (about an inch or less). Coupled with this is a low surface loading the question is, could the hull be starting to ride on the top of the surface ripples (which are at least an inch high or more)? If so is the hull trapping air under it and seeing a significant drag reduction at these speeds due to this type of effect. I did hear a long time ago that some hulls tended to "break out" at high speed, and could this be what is happening here?

Also, if the beam width were to be narrowed, Savitsky says the drag would be significantly reduced, but if you increase the surface loading, might the above effect be countered, and you would be now saddled with the Savitsky calcs, and might the result be slower than what these boats are running now with a wider beam?