Resistance factors, planing hull at low speed

It is quite remarkable lack of professional curiosity... someone has increased the speed of certain class of boats 2-3 times, and professional do not care.

 

Perhaps somebody could lead us into this.

Six years ago, I was using a powering software for predicting performance of semi displacement craft. Two of the inputs are transom immersed area and maximum cross sectional area called At and Acx. one of the output is At/Acx ratio and has some effect on resistance. At low speed, low transom immersion (or none at all) seems to reduce resistance. At high speed SL greater than 1.0, transom immersion or area seemed not to matter much.

To date, I have not found the theory or application of this At/Acx ratio. Anybody has used this or similar program before?

 

I think You are talking about Holtrop method? There is known problem with transom terms there.

 

Why should I care about this??? (2-3 times is exaggeration again).
These small boats are just toys without any practical application...

 

Don't have a clue about what software did you use but, as a general info, try (for example) to see the paper linked in the post #57 of this thread, and also the post #38 of the thread http://www.boatdesign.net/forums/mu...ms-give-higher-speeds-22541-3.html#post233161

 

This is comprehensive study, but problem is: Series 62 is valid in range of Froude numbers FnV=1...6 (see PNA Vol.2 p.101). You are using extrapolation beyond the limits of series (even for lighter models FnL=0.3 is about FnV=0.85), so the comparison You made for FnL=0.3 is not correct.

In no case systematic series should be use beyond their limits.

Pls also note that Delft yacht hull series gives much lower resistance around FnL=0.3 compared with other displacement boat series, say for full shapes (trawlers, tugs, etc.) resistance would be much higher compared to sailboat hull.

And believe me, in my calculations, the hull forms were significantly different

 

Thanks Daiquiri for the link. I have downloaded the paper and trying to read it whenever I have time. The link to Leo's post says transom immersion is still largely misunderstood.

My old program is in still intact in a hard disc. Unfortunately, it is infected with virus so I dare not open it.

Thanks also Alik but it is not Holtrop. But you already hit it. It is Mercier Savitsky (1973) you have mentioned in your post no. 3. The best I have is a mention of the formula and the notes At/Ax. Do you have a copy of Mercier and Savitsky? I am still stuck with my encircled notes on that At/Ax and have not progressed beyond button punching.

 

Yes, I got a copy of this paper from Dr.Savitsky.

 

No exaggeration. hull speed of 18ft hull is ~5.7 knots. Australian 18ft skiffs sail to windward at up to 14knots. 14/5.7=2.45. (Still exaggeration? )

At the beginning of XX century aeroplanes and planing motorboats were just toys.
Some really big companies make huge profits from producing and marketing toys with no practical application.
Some NA make decent living designing impractical boats.

In any case, if someone do something out of range for current state of the art, it is well worth the attention of professional.

I think so. I do it whenever I can.

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By the way, is recently posted comparison between 62 Series and Yacht at Fn=0.3 scientific enough? Do prove, that at low Fn displacement hull could have 2 times less resistance than pure planing hull?

 

It would prove if series 62 is used correctly. Now it is not

 

Perm Stress's second attachment (from Professional BoatBuilder #128, p 22) includes a series 62 resistance plot for FnL which goes down to 0.25. Blount wrote the surrounding text so I assume the data is valid. In the main article which is an interview with Blount he says the tests started at displacement speeds though "The planing part of the spectrum was our focus when we wrote the paper."

 

What is 'hull speed'? 'Hull speed' is definition suitable for heavy displacement hulls with narrow sterns. For planing hull (like 18' skiff) it has no sense.

Show me any real NA who makes living on designing skiffs; we will laugh together

 

Did you check the attachment with resistance graphs of 62 series hulls?
I use the data, as published. No extrapolation.
In my practical experience, including sailboat speed calculation with various hull resistance calculation methods, yacht hulls indeed show excellent performance at low Froude Numbers.

I did not say hull forms were not different. I did say hull forms were designed for similar and overlapping speed ranges.

And finally, what was a point to prove: hull, optimised for displacement sailing could have 2-3 times less resistance below hull speed, if compared to hull, optimized for high speed planing.

 

The problem for PermStress is that he is using PBM and FreeShip as main sources of knowledge

P.S. In the paper daiquiri posted here, Bount himself is using his 62 series starting from FnL=0.4.

 

Are You joking? Those data are not intended for any calculations, refer to real source of engineering data! The series is for FnV=1 to 6, and Your calculations are well beyond the series.

Nothing has been proved. Study how to use series 62 first.

P.S. My Ph.D. is in sailboat hydro-aero-dynamics and performance/controllability predictions. Bet You didn't know