Why can't most catamarans get over the hump ?

Discussion in 'Multihulls' started by tommymonza, May 4, 2014.

1. Joined: Oct 2008
Posts: 7,276
Likes: 1,165, Points: 113, Legacy Rep: 2488
Location: Japan

Being a "warship" makes no difference, that's a misnomer.

The systematic changing of the parameter(s) of a known hull is the point, the point I suspect you are failing to grasp. Since keeping everything the same, and change 1 parameter,..what happens? It could be used for a cruiser it could be used to transport coal it could be for anything, the end usage of the hull is irrelevant.

There is no such thing as a typical catamaran hull. All that is required is 2 hulls, simple. If you change endless other parameters, but 2 hulls still remain it is still a catamaran, irrespective of its shape and usage!

2. Joined: Oct 2004
Posts: 261
Likes: 14, Points: 18, Legacy Rep: 58
Location: Brittany, France

hump101Senior Member

I suspect that I am actually grasping that point quite well. I was sitting next to Patrick doing my own PhD when he was doing these experiments, and we had several discussions about the validity of the hullform he was using, since I had catamaran design experience at the time.

The NPL hullform is that of a typical monohull warship, so flat, v'ed bottom, round bilge, immersed transom, bow flare, etc., as is required in a monohull which derives its stability from its form. A catamaran derives its stability from the spacing of the two hulls, so the individual hulls are not required to be stable in their own right, hence they can then be optimised for resistance with minimum wavemaking, minimum wetted surface area, etc., resulting in a very different hullform.

This is relevant because the sensitivity of an NPL hullform to, say, a change in B/D, is different to the sensitivity of a different hullform. Patrick used the NPL series hullform because his work was a continuation of a significant body of earlier work, and changing from this hullform would have prevented any comparative analysis on his new results.

Notwithstanding any of the above, if you take a standard hullform and change only one parameter in a systematic manner, then you are determining the sensitivity of that hullform to that one parameter. You are not determining the sensitivity of a different hullform to that one parameter, and you are certainly not showing anything about the difference between two hullforms, precisely why Patrick stuck with the NPL form.

If you need any further clarification I can ask Patrick to chime in, as he is far more knowledgeable than me in this area.

3. Joined: Oct 2008
Posts: 7,276
Likes: 1,165, Points: 113, Legacy Rep: 2488
Location: Japan

I’m sorry but you’re missing the point again. All boats have a Vee of sorts, all non-super high speed boats have round bilges and bow flare, well….has nothing to do with it..its above the waterline. A hull is just a 3dimensional shape that moves through water. What you do with it and why, its SOR, is important.

Now you’re comparing apples with pears. If you wish to cite catamarans as being optimised for stability, then you need to look at the parameters of said hull form of one against another, Not against a totally different hull form. Why..as already noted here.

And that’s the point if you’re going to compare two totally unrelated hulls. It’s comparing apples and pears again.

I beg to differ.

Correct.

But that’s what you’re doing…by suggesting a hull form that is used as a warship and not a suitable as a catamaran etc.

None needed thank you.

If you have not already, may I suggest you read the various posts again, as it appears your replies are simply repeating previously made statements. Especially the post here.

4. Joined: Oct 2004
Posts: 261
Likes: 14, Points: 18, Legacy Rep: 58
Location: Brittany, France

hump101Senior Member

Unfortunately your responses don't seem to relate to my comments, as none of my posts are suggesting any hullform is more or less suited than another, I was just trying to show the limitations of the data previously presented, since I had direct experience of that data being generated.

I may not have communicated quite as clearly as I thought. I have read the previous posts, and it was not evident to me that the limitations of this data were being understood. It still isn't.

5. Joined: Oct 2008
Posts: 7,276
Likes: 1,165, Points: 113, Legacy Rep: 2488
Location: Japan

Well, I’m curious why you made this statement then:

As that is exactly what you’re doing.

The limitations are always written in the reports. Anyone who reads them should understand their range of applicability. Or is that you’re point, read the report? (I have by the way and many more like them…)

It sure appears that they are not, since many still think shape plays a significant role, when it doesn’t. As already noted in the previously cited research by Warren, et al and Blount et al., and of course not forgetting Yeh et al, to name several of many stating the same. Perhaps your views may be limited by reading just Patricks paper alone and not the many others that do such sensitivity analysis?

6. Joined: May 2008
Posts: 2,097
Likes: 41, Points: 48, Legacy Rep: 436
Location: Sydney Australia

oldsailor7Senior Member

Quote:
Originally Posted by hump101 View Post
All Patrick's models used the same hullform (actually of a warship form, NPL round bilge series, so not a typical catamaran hull)..Quote:

That is very interesting, as Lock Crowther told me that after his stay in the UK, where he spent time at the Naval test tank facility, he based the shape of Bagatelle's hulls on the results of tank tests of the RNs fast destroyer designs. This was a complete break from the hull shapes of his Spindrift designs.
Bagatelle, with her bulb bows and destroyer like hull sections, then became the prototype for Locks very successful Catana line of catamarans.

7. Joined: Nov 2007
Posts: 2,931
Likes: 66, Points: 48, Legacy Rep: 719
Location: Melbourne/Singapore/Italy

I notice when you tow a beach cat it sucks down and down, due to the rocker no doubt

8. Joined: May 2008
Posts: 2,097
Likes: 41, Points: 48, Legacy Rep: 436
Location: Sydney Australia

oldsailor7Senior Member

9. Joined: Nov 2007
Posts: 2,931
Likes: 66, Points: 48, Legacy Rep: 719
Location: Melbourne/Singapore/Italy

On a modern f16/f18 that are flat at the rear, they will tow fast if you get all the weight aft to lift the the board cases and forward out of the water but as the saying goes, you can ski on anything

10. Joined: Oct 2004
Posts: 261
Likes: 14, Points: 18, Legacy Rep: 58
Location: Brittany, France

hump101Senior Member

I'm not interested in getting into a pedantic discussion, but saying that a monohull ship form is not a typical catamaran form is not to suggest that you can't make a catamaran out of two monohull forms, simply that this is not typical.

Like you I have also read most of the papers in this area, and I also do model and full scale testing as a part of my job. Most model tests will not be able to adequately quantify the effect of small changes in shape, not because the small changes aren't important, but because the scaling issues make such effects very difficult to quantify. At full scale the effects can be clearly measured, and are significant. This is one reason why we are moving slowly over to CFD rather than model testing, as scaling effects can be eliminated (and replaced with other modelling issues, but that is for another thread).

If you want examples of the importance of shape refinement, just look at any of the many racing development classes, power and sail. Many are constrained to length and weight, so length/displacement is constant, but the difference in performance between different shapes is very significant.

DCockey likes this.
11. Joined: Oct 2010
Posts: 10,401
Likes: 1,033, Points: 113, Legacy Rep: 702
Location: Australia

Mr EfficiencySenior Member

Jeeeezz....I'm sick of hearing how the L/D ratio is "God" in catamaran resistance, is this for a given demi-hull beam or not ?

12. Joined: Oct 2008
Posts: 7,276
Likes: 1,165, Points: 113, Legacy Rep: 2488
Location: Japan

Then I'm at a loss what you are actually trying to say, it is very muddled.

Good luck with your CFD....you'll need it when verifying and validating!

That just suggest you're ignoring all the other factors that influence these vessels. Just ask Team Oracle!

13. Joined: Oct 2008
Posts: 7,276
Likes: 1,165, Points: 113, Legacy Rep: 2488
Location: Japan

Who said just catamarans??....not me.

Any hull.

14. Joined: May 2008
Posts: 2,097
Likes: 41, Points: 48, Legacy Rep: 436
Location: Sydney Australia

oldsailor7Senior Member

Originally Posted by Mr Efficiency View Post
Jeeeezz....I'm sick of hearing how the L/D ratio is "God" in catamaran resistance, is this for a given demi-hull beam or not ?

Did you mean L/B ratio?

15. Joined: Oct 2010
Posts: 10,401
Likes: 1,033, Points: 113, Legacy Rep: 702
Location: Australia

Mr EfficiencySenior Member

No, "length/displacement ratio", which Ad Hoc insists is what determines resistance almost exclusively. So if I have a 25' hull that is 6' wide displacing 2 tons, I will have the same resistance if I go to 15 feet wide with the other parameters the same. Who'd have thought it !

Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.