tunnel slot design?

See the "Type C" in this page:
http://www.multihullsmag.com/magazine/articles/cat hulls/performance_of_cat hulls.htm

 

daiquiri, thats the article i was looking for but could not find ;-) hmm, type D really thunderclaps?
and yes great boats, even like those parted windscreens (when well done as here)

Vulkyn, as we know specially smaller (speed)boats have a very low "hullspeed" (sort of sound barrier for planes)
a cat with a lower lenght beam ratio makes with less power it easy'r to penetrate and pass that hump
cats ride smooth and more stable, altho having a higher wsa than a monohull they'r still having a better fuel economy
cats are more complicated to build, ussually use a dual engines, have other layouts and have many variations in design


Boathull Speed=1.34 x sqrt WL in feet and knots. Or 4.5 x sqrt W=Speed (metric)
Easier is using the Froude Nr. V/(g x L)^0.5 where V=velocity in feet or meter per second, L=length, g=acceleration.
The longer a boat, the longer and faster the wave she rides in. S for a 6m boat= 9 km, S for 16m=18 km, S for 50m=31 km etc.
Very small catamaran hulls also cut wave drag and are a good example of configurations cheating the rule
So calculating a boats power it rises steeply at about Fn 1.34 and thats why displacement hulls operate at a Fn of 1.34 or less.
Semi-planning hulls operate at a Fn of 1.0 to 3.0 and planning hulls operate at a Fn of 2.3 or more.
A fairly accurate power or speed predicting formula for planning and semi planning hulls was made by Grouch
HP=W/(V/C)2 and C=V/(P/W)0.5 wherein V=Speed in MPH, W=Weight of boat in pounds, P=Shaft horsepower
V=C, W/P, C=Crouch constant. C=140 for small runabouts to C=270 for larger cruisers.
Planing boats operate in reduced wave resistance and being most out of the water friction also drops in this mode.


here are some some links with articles:
http://www.powercatplanet.com/2009/...ent-powercats-the-malcolm-tennant-comparison/
http://www.powermultihulls.com/magazine/articles/displacement or plane.htm
http://www.boaterslife.com/displacement-power-catamarans.389.15.66.htm
http://www.powercatmarine.com.au/page/Why_PowerCat/
and this guy who way back went from PT boats to powercats
http://www.powercatboat.com/history/History.htm

 

There is a tiny little mistake in that calculation Yipster.

it should read: S= 2,43 x sqrt WL in metric.

Regards
Richard

 

Yeah, a confusion is always behind the angle when there are mixtures of SI and Imperial units.

Various, most frequently used cases:
V in knots, LWL in feet: V = 1.34 sqrt LWL (all Imperial)
V in knots, LWL in meters: V = 2.43 sqrt LWL (mixed Imperial-SI)
V in m/s, LWL in feet: V = 0.689 sqrt LWL (another mix of Imperial-SI)
V in m/s, LWL in meters: V = 1.25 sqrt LWL (all SI)

Cheers

 

thought the story was sort of relevant to subject
i'm not using these formulas daily and i'm a sloppy guy to start with
so have my thanks for the correction Richard and samples daiquiri

now do any of you guy's see a reason why a split hull with small tunnel
should thunderclap any harder than his similar deadrise uncut monohull?

instead of checking what i'm saying i've been thinking about that..

 

Right, I still calculate knots only, never m/s.

 

Thunderclap noise is due to water and spray rushing up through the tunnel and then smashing against a flat hull above. The sound is more annoying because it is produced right beneath the cabin or cockpit.
The difference between type C and type D is the depth of the tunnel. Type C, having a deeper tunnel, simply slaps less frequently and with less energy, for a given wave height and encounter frequency.
A vee monohull doesn't have such a flat surface (apart chines) against which water could impinge in that manner. Also, in the case of monohulls the sound is produced in a point on the hull which is more distant from the cockpit or cabin with persons.

 

right, that must be it, didnt think a low small slot would pound that much
btw, not saying i wasnt way off in mathemathics, however accurat the figgers, it are approximations depending on various monohull design

 

My mathematics skills are very poor at best !!

Still awesome links and information !!! Thx folks truly this forum is a gold mine !

Just a quick question daiquiri, as you mentioned the thunderclap noise issue.
If one changes the flat tunnel area into a wide V angle (maybe with lines running backwards) just enough to disperse the water, would that decrease the noise?

 

This is how some catamaran designs deal with that issue, which is also related to seakindliness:



However, I believe it is not a necessary addition in case of such narrow and deep tunnels. In your case a cheaper mean (imho) to disperse the wave and spray energy before it smashes against the hull could be a coarse net spread inside the tunnel from the bow to the stern, running parallel to the cabin bottom at a distance of some 20-30 centimeters from it. How coarse and how distant from the hull bottom would depend on the boat size and speed, of course.

But we have some really experienced cat designers, builders and owners here in the forum, so I'd rather invite them to share their opinion on that issue.

 

Oh wow great !! I'm just glad im starting to make sense ..... took me 20 years but meh ..

Thx for the feedback and great pics ! i am reading A Comparison of Power Catamaran Hull Types link .... thx

OK i like the V part !! looks cooler lol ...
But yah i understand your point of view .... im still reading through the wealth of knowledge post it ... just give me some more time to make heads or tails on the mathematical calculations

 

the sound that air entrapped in water slaming makes is indeed comparable with a thunderclap i now read
...dont think i've ever heard that and didnt know thunderclap is a common nautic fenomena

 

Does that phenomena cause discomfort when the boat "slaps" on a wave (not the sound but the motion)? it would be uncomfortable slapping against the wave (if the boat tunnel area is low).

Edit: ok i read through the link and i guess i answered myself on that (Type - D)

 

http://www.youtube.com/watch?v=qtgpUEw4Vvw&feature=related
boat is rising, no waves nor thunderclapping in the slot
as in between cat C and D she looks very good actually

 

she does but thats very calm water, doesnt that phenomena occur in rougher weather when the boat is slapping after passing a wave?

The boat looks too high in my untrained eye, i would feel the CG is too high for comfort, but maybe the tunnel design is offsetting that ....