Resistance factors, planing hull at low speed

the hull at WL is likely to be 750mm wide?

 

Around 600mm, with 1500mm of tunnel width, so 2700mm overall WL beam.

 

Tunnel height will be high enough to be clear of hull generated waves.

 

Vertical clearance of tummel for displacement cat should be 4-6% L; for planing cat 2-3%L

 

Tasaki (1992) has calculated the wave interference effect of catamaran. there are two significant points where the wave interference has beneficial effects, at Fn 0.32 and 0.25 of a predetermined hull spacing.

 

What is the mechanism at work here ? Are we talking about the wash from one side creating a trough aft under the other sponson. Defeats me !

 

correct. wavemaking resistance an interference with the other hull. Both hull are creating transverse and diverging waves.

 

There are two major resistance factors of hulls interaction:
• interaction of wave systems of hulls, effecting mainly wave resistance;
• increase of water flow speed between hulls, effecting mainly frictional and viscous resistance.

 

You might get some insights into wave cancellation and re-inforcement from the (preprint) paper:
"OPTIMUM HULL SPACING OF A FAMILY OF MULTIHULLS"
http://www.cyberiad.net/library/pdf/tl98.pdf

Leo.

 

Thanks Leo. I was going to cite Principles of Naval Architecture and the paper from Cyberiad library but forgot the title and author. Was it from Doctors and Day?

Sorry. It was Tuck and You. How silly of me.

 

No worries.

Sandy Day co-authored a paper roughly validating some of the findings in:
Day, Sandy, Clelland, David and Nixon, Edd,
``Experimental and numerical investigation of `Arrow' trimarans",
Seventh Int. Conf. on Fast Sea Transportation (``FAST'03"),
Ischia, Italy,
pp. 23--30, 2003.

Leo.

 

Thanks, but way too mathematical for me. Am I right in thinking that theory largely discounts shape and emphasises spacing as the main factor? Can I make a judgement based on observed waves made by a scale model ?

 

You're doomed!

The two factors are inter-related, but the paper I cited is only concerned with spacing.

If the individual hulls make small waves, then spacing isn't all that important (there's not much to cancel!)

If the demihulls make large waves, then there might be scope for some cancellation using a judicious spacing, but that depends on the design Froude number range.

Catamarans cannot cancel pure "transverse" waves; the lateral spacing only acts to reduce (or reinforce) "diverging" waves. Hull shapes (well, standard ship shapes) have almost no effect on transverse waves, however, a judicious choice can reduce diverging waves.

Have another look at Figure 1 in the paper. (Converting the speed scale to a Froude number would be a VERY useful exercise for you). Note where transverse waves are the major component of the total wave resistance, and where diverging waves are dominant.

Ideally, what we would like to do is to choose the most appropriate method to reduce the dominant components at each Froude number of interest. That's not always possible with hulls of fixed shape and fixed spacing. What we then try to do is to find a compromise - i.e we want a vessel with demihull shapes AND spacing that minimises the drag at our design operating Froude numbers.

In one exercise I did a few years ago, the Navy supplied a frequency diagram showing how long the ships were expected to spend at each speed over their lifetime. You could try something similar with your design to gain some insights.

Of course, this is all assuming calm water. If you are operating in a real seaway you probably need the services of an interior decorator with engineering skills (or "naval architect" as they're sometimes called ).

It depends on the model size.
Small models will produce small waves that will be flattened by surface tension. I am very mistrustful of models that are less than 1.5m long.

Good luck!
Leo.

 

OK, thanks again, some helpful stuff there. This vessel will only have two modes of operation, full planing and displacement "hull speed". The dimensions are pretty well set, so I guess it is a matter of establishing whether there is an unfavourable "constellation" of factors surrounding the diverging wave pattern as regards resistance at that slow speed.

 

Here are the resistance data for different kind of hulls at displacement speeds.
Two completely different kinds of boats are compared:
1) 62 series hard-chine hulls, data sourced from "Professional Boat Builder", issue Dec 2010/Jan 2011;
2) Generic yacht hull, from Delftship default database, scaled to the same L/B and L/(D^1/3) and same displacement, as respective 62 series hull it is compared to.


Attachments:
1) "62 series lines.pdf" -page from PBB with 62 series hull shape drawings
2) "62 series resistance.pdf" -page from PNN with 62 series resistance graphs
3) "yacht parent hull.jpeg" -lines of generic yacht hull used
4) "62vYacht.pdf" -resistance comparison tables

The tables are structured the following way:
*on the left, there are tables with Resistance to Weight Ratios:
62 Series at Fn=0.3,
Generic Yacht at Fn=0.3
(62 series) / (yacht), demonstrating, how much less is resistance of hull, specifically designed for displacement sailing
*on the right, there are tables with key data of yacht hull, demonstrating comparability to respective 62 Series hulls.

As I have posted earlier, difference in resistance between "pure planer" and "pure displacer" is on the order of 2. If we take only residual resistance, without friction, differences will be on the order closer to 3.

To Alik: so small differences you find in your calculations, are caused by the fact, that much more similar hulls are compared, as here. They are actually intended for about the same operating speed, with some bias to higher or lower end of design space.
My understanding of tread starter's intention is to have a comparison between pure planing and pure displacement designs below hull speed.