Cat Length question
 

Theoretically, shouldn't a 38' Cat be able to pack more air underneath than a 37' 9" Cat assuming everything thing else is equal? I realize we're talking about a minuscule difference. But is there any reason the boat that's 3" shorter would be faster if everything else was equal?

Looking for technical responses please. No BS, No polotics.

Please.

Re: Cat Length question
 

Okay, I'll play. Theoretically a 37'9 1/2" boat could have less wetted surface in the water and be faster-overcoming the decrease in possible lift. Remember that most boats only have a fraction of the hull wetted-esp cats. Assuming a 2 foot wetted surface at speed the 2.5 less inches would have a much greater effect than 2.5 inches of a lift area that is 30 plus feet long. So there ya are...thanks for playing.

Re: Cat Length question
 

The width, height and funneling of the tunnel will also effect the amount/velocity of the air underneath the cat.

Re: Cat Length question
 

the extra 3" inches wouldn't be necessarily wetted surface on a cat, the sponson could be 3 inches longer and would have negligible effect on the running attitude of the boat, although 3 more inches of tunnel could effect the running attitude greatly....

but to answer the question, tunnel length and boat length aren't mutually exclusive...a longer cat won't pack more air unless it has a longer tunnel....and if a longer cat has a longer tunnel, it also has more weight to lift now... kind of a viscous circle...

Re: Cat Length question
 

I don't know where the 2.5" is coming from, whether it's the tips of the sponsons or the transom which would affect the tunnel and airfoil, but either way we're talking about a difference of 0.5%. Say it is in the tunnel area, lift and drag behave linearly with surface area as defined by L (or D)=1/2 * rho * V^2 * S * Cl (or Cd), where rho=density, V=velocity in fps, S=planer area in sqft, and Cl=coefficient of lift (or drag). Again, 0.5% differences...maybe.

After running a quick CFD comparison, I see no appreciable difference.

Re: Cat Length question
 

the 2.5 inches in question is overall boat length from tip of sponson to the top of the transom... there is no set length for tunnels...

Re: Cat Length question
 

Is being 3" short going to give ANY advantage? Does it have ANY disadvantage?

PS - Hello to my old friends Uncle Teddy :evilb: and Nick :drink:

Re: Cat Length question
 

Gotcha! So, given two boats exactly 38' in overall length, their tunnels can have literally an infinite combination of characteristics (within their width limitations) pretty much indepentant of OAL. Therefore, differences in lift and drag are a moot point.

Re: Cat Length question
 

Back at ya! How's it going? Hope all's well. :cool: :drink:

Re: Cat Length question
 

Life is good.

Re: Cat Length question
 

I was told that water is something like 800 times denser than air, so a boat with 2-1/2" of less wetted surface should have a speed advantage in flat water over a longer boat with the same OAL to tunnel length and attack angle/compression ratio.









I think.... :D

Re: Cat Length question
 

after reading all your "rocket science" replies, I've concluded that most of you should be more worried about your 2/12' shortages in another place... :drink:

Re: Cat Length question
 

put it on a tread mill :D

Re: Cat Length question
 

I'll take it !!

Say the boats are running 100 MPH
Race lasts 1/2 hour
I win by 1/4 mile.... :D

Re: Cat Length question
 

thats why a 36 nor tech is 37.8 perfect :D

Re: Cat Length question
 

Regardless of one's current stature, the addition of 2.5" is always good. :drink:

Re: Cat Length question
 

you wouldn't be adding 2.5" of wetted surface though... think of a beak on a fountain... :D does that really effect the running surface?

and you are right, dry air at sea level is 1/800 the density of water (approx)......

Re: Cat Length question
 

swim platforms don't count... :drink:

Re: Cat Length question
 

Weeellllll, not exactly.....even if this 2.5" was directly centered on the apparent C.G. (combined C.G. and C.L.), shorter means:
Less aero drag = good
Less aero lift = bad
less direct hydrodynamic surface drag = good
less lift = less dynamic buoyancy = more drag = bad

It's a wash. Net result = approximately zero.

Let's put it this way, those 2.5 inches will amount to whether or not you emptied the change in your pockets.

:D :D

Re: Cat Length question
 

i doubt if percentage of lift directly corresponds to percentage of velocity.... but your right, if you were 0.5% faster you would win by that distance, but just having 0.5% more lift doesn't mean anything by itself... you could have too much lift and have to slow down... :D

Re: Cat Length question
 

Sean,

I know what you're saying, and I'd thought of that.
It's obvious that an unwetted beak changes nothing.

It might be in the way that we each interpret the question.

Would I not be correct if both boats were produced from
the same mold and shorter one was built using as 2-1/2"
dam at the transom?

9 Lives

Re: Cat Length question
 

Yes Mark.

The disadvantage of not being long enough to meet the spec class minimum length.

Whether there is an advantage or not, if boats are under the minimum length, then they are not to be allowed to run. Period.

I always thought SKATER Cats had this issue in the past prior to the OSS organization... :confused:

Re: Cat Length question
 

in that case yes, but if they moved everything to keep the cg correct (steps, cockpit, engines), then, no... just damning the mold probably would make a ill handling and riding boat.... but if you were to add length (especially just inches) to a cat, the sponson would be the easiest and best place to do so.... but the rules say you can't have hull extensions to meet length requirements either, so it should be interesting to see how this is resolved.

and the 388 skater has a transom notch, so adding 2.5" to the transom might not increase running surface anyway... as long as the boat was rebalanced....

Re: Cat Length question
 

Wouldn't it just be much easier to have all of the owners vote to have the minimum length reduced to 37' 6" and open up the racing to a much wider range of boats? :confused:

Re: Cat Length question
 

I was assuming for both boats to be balanced correctly and
agree that the importance of correct balancing to be immense.

Now that you mentioned it, I didn't see any thing in the OSSC rules
regarding notches and wonder if they'll allow Tony to add the required
amount to the transom above the running surface as in a small notch.
Of course, re-balancing of the boat would be required for this,
unlike a simple sponson extension at the front. (Beak)

As to Nick's response. I thought that your 0.5% calculation was
in reduced wetted surface and do agree that 0.5% of added lift
would show little, if any, performance increase.

Re: Cat Length question
 

Got it. Sorry for the confusion. :)

Re: Cat Length question
 

I'm sure it would, Tim. I'm with you. I'm also sure that if it were put to a vote, the outcome would have been different. After all, who in the world would go out of their way to eliminate a fellow year-long racer over 2.5" - that wouldn't be in the spirit of competition.

:cool:

Re: Cat Length question
 

Hey you all are veering into a discussion of actual events, be careful or posts will start to disappear and alter egos will show up :eek:

Right back at ya, Round Shady :cool: :D

Re: Cat Length question
 

Rules are rules; not recommendations. :D

Re: Cat Length question
 

Shark, I agree with Uncle Teddy, let's not veer off to actual events.

So far I've seen a lot of good info, but no conclusive evidence.

I know in stock class, the longer boats are forced to carry more weight as thier able to pack more air under the tunnel. That's the main reason I ask the initial question.

Re: Cat Length question
 

What does all this have to do with a treadmill???? :D

Re: Cat Length question
 

:D
If boat A is 2.5" shorter than boat B and they're traveling in exactly opposite directions on a head-on collision course while on a section of the Mississippi River with a 10 mph current and both of their GPS's indicate 100 mph, what is their closing speed with regards to one another? What would each water pitot speedometer read. assuming they're accurate? :evilb:

Re: Cat Length question
 

I must of missed the part where they said you were in 1 boat and BS was in the other...... :confused:
:eek: :evilb: :evilb: :D :drink:

Re: Cat Length question
 

BS is definitely the one in the wrong lane..... :eek: :rolleyes: