Catamaran Speed

Thanks Yipster! The app works great!

A few insights from playing with it:

- High prismatic coefficient is good - cuts down on both wave and frictional resistance.

- I was surprised that the frictional resistance drops off at higher speeds (>15 kts). I would have expected it to continue to increase with speed.

- I was also surprised that wave resistance does increase with speed and becomes the dominant form of resistance. I had thought that frictional resistance becomes the limit. Backwards from other reading I've done.

- It would be nice if the app allowed for the setting of add'l parameters to fine-tune the hull shape. But it does give some feedback.

- And finally, length matters - a lot!

 

I would recommend to study physics behind the app and its limitations. Sometimes ignorance of those leads to wrong conclusions.

 

Alik, thanks, that's helpful.

On your graph, right about at Fn 1.2, for all of the hulls the slope of the curve becomes less steep, like the hull is cresting a 'hump'. If I'm interpreting your graph correctly, that hump is generally about where fatter monohulls get stuck.

Which lines (red/blue) represent the chine hulls? Quite a difference.

 

Yes, I was a little suspicious about the inconsistency of the results from my previous research. Unfortunately, not much background was provided regarding limitations.

 

Red line is round bilge.
Hump on cats is greatly influenced by demihull spacing. Cat with same demihulls but smaller spacing will have higher hump compared to wider cat. That hump actually happens at almost same FnV or FnL as hump for monohulls, but its height is affected by demihulls spacing.

Note that FnV volumetric Froude number, FnV=v/(g*V^0.333), all in metric system.

 

Interesting. So if you modeled just one of those demihulls individually you wouldn't see that hump?

 

You would, but not the same height as for two.

 

as allway's; thank you Alik !
of freds calculator i uploaded what i still had but do remember the multihull site i got it from
had a warning going with it that alltough made for multihulls it only gave rough estimates

 

Without getting bogged down in details, the main reason a catamaran reason does not "get stuck on its bow wave," or more accurately put: does not generate high wave resistance even at hump speed, is due to hull shape. As Alik points out above the slenderness/ length displacement ratio is the most important parameter affecting wave-making. Second most important is the sectional area curve (how the cross sectional area of the under water parts change along the length of the boat). Third is the tunnel width if the operational length Froude number is less than 0.8. If FnL > 0.8 then the demi-hull separation stops playing a significant role. Other factors such as hard-chine, round bilge play less important roles.

 

I would say that relative length and demihull spacing are primary factors. With unfavourable demihull spacing one can get extra 40% of resistance. Other factors as area curve/CP, etc. ar of second order.

 

Any guidance on what constitutes unfavorable hull spacing at lower Fn's? Is it a function of hull length or hull beam?

 

See:
"Optimum spacing of a family of multihulls",
E.O. Tuck and L. Lazauskas, Schiffstechnik,
Vol. 45, No. 4, Oct 1998, pp. 180-195.

http://www.cyberiad.net/library/pdf/tl98.pdf

Also, don't confuse the surface elevations made close to the hull,
and the waves behind the vessel which contribute to the wave
resistance.

Good luck!
Leo.

 

Hmmm..you're starting to mix up hydrodynamic behaviours and cobbling them into one size fits all.

First, as already touched upon here, the 2 principal factors that affects speed are the length displacement ratio (L/D) and the underwater hull shape.

1) L/D

As can be seen here, the residuary resistance falls dramatically with ever increasing L/D ratio.



2) Shape.
A box is a box is a box. A box cannot go fast. A long slender plank of wood on end, is a long slender shape, that has a high L/D ratio.

Why does shape matter?

There are basically 3 realms that a hull shall be in, when running, viz:-
1. Displacement
2. Semi-displacement
3. Planing.

The long slender hull, plank of wood, may not be planing, in the true sense, but the Fn is extremely high. This is because of its L/D ratio.

Going back to your wave thingy.....a box, when running closer and closer to the speed at which a single wave generated from the hull length is approached, as you say there exists a crest and trough. Without going into too much depth (for clarity), the box, whilst running in displacement mode is fine, yet with increasing speed finds itself unable to go faster. The forces on the hull change from static (hydrostatic) to hydrodynamic.

The hull shape must change to accommodate the change and allow the waves to pass under and push the boat up, so to speak. In broad general terms. The hull is supported by dynamic forces, rather than static forces.

The combination of L/D ratio and hull shape (in realms 1,2 or 3) will dictate whether the vessel can go 'fast' or not. And to define 'fast' means a function of the waterline length, and hence its Fn (Froude number, by length)

After that you can address what you mean by efficiently

 

I would say it depends on speed. For speed around the hump I would agree with your statement. At planing speeds the inverse is even true- narrowly separated planing surfaces enjoy some beneficial intereference on the lift coefficient.

Savitsky, D. and Dingee, A., "Some Interference Effects Between
Two Flat Surfaces Planing Parallel to Each Other at High Speed,"
Journal of the Aeronautical Sciences, VoI. 21, June 1954, pp. 419-
. 420.

 

Yes, agree; but I assume we are talking about hump speeds in this topic.