Cat hull resistance change with heel?

I don't see how you can build hulls without knowing where the daggerboards go, how big the rudders are, nor how big a rig you will have? Normally one adds the necessary reinforcing as you build, not as an afterthought. Of course it also implies you don't have an accurate weight or CofG yet.

RW

 

A place for everything

Hi Richard

It seems like my questioning is giving the wrong impression.

The dagger board cases are indeed built into the completed hulls. Many years have gone into drawing and analysing every detail, first with Maxsurf then with Rhino. The stresses and displacements have been analysed with a finite element analysis software called Scan and Solve.

As the building progresses, every assembly is weighed and the results go into an excel trim table. The trim table holds everything imaginable. The completed hulls have been weighed (2870 kg each) and the CoG verified by physically balancing each of them on a fulcrum. All the coefficients such as prismatic, LCB and LCF are derived from Rhino and Maxsurf. By pitching the hulls fwd and sinking the lee hull I can measure the change in the wetted surface areas and estimate the longitudinal and lateral stabilities.

However in spite of all these efforts I have many doubts.

Apart from the performance of the hulls out in the Pacific, there are plenty of other unknowns: the D shaped rotating wing mast, the electric propulsion and the rudder linkage. All of these present difficult design judgements.

Here is a picture that sort of shows the fin cases forming a T structure across the front of the mast beam bulkhead. The arrays of red boxes in the engine rooms are Lithium Iron Phosphate cells for the 25 kW DC motors - more about that in another thread soon. The other picture shows CNC cut temporary hull frames.

Thanks for your feedback

Michael

 

Michael. Your boat is fascinating and I have to commend you for doing it. One day I plan to also do what you are doing. But honestly, those daggerboards will be too far forward in my humble non professional opinion. Even with my 2.4m screecher pole and 70m2 screecher, 69m2 square top main and no daggerboards down I still have maintain 5 deg of weather helm to steer straight. You say your bows are just skimming the surface. Then do this thought experiment. Look at your sail and appendage plan and lop off the front of your hulls in a visualization as you said yourself they are not attributing much to the displacement. Now how far forward do the daggerboards look? They look even more extremely forward.

I think your project requires a thread of its own. I hope you find the time to make a thread for it. I have many questions to ask, like does it have 4 rudders? Then what is that item which looks like a rudder drum with a dagger rudder? Where are you located? What is the target weight? I am sure the boat in its finished form will be awesome. Please start a thread.

 

Adhoc thaks again or the reply. I maxed rep on you ages ago.

I wrongly though of the 2 hulls sharing the displacement which would then change the DLR. Because there are 2, the resistance of each needs to be added so its wrong to half it. In the same way as moving displacement from one hull to the other ends up with one hull having much more resistance than the other. When you add them together the difference is minimal. As the "overriding" factor that determines the resistance is still the same, the DLR. Am I getting it?

This brings me to what I wanted to know in the first place. Determining the DLR of a proa. This is not the same as a cat as each hull has different length. Assume this 12m 2000kg proa to have 50/50 weight distribution. But one hull is twice the length of the other. I may have another flaw in my thinking. But it seems to me when flying the smaller hull the DLR will be twice as high compared to if it would theoretically fly the long hull. If this is correct it would mean the total DLR when each hull shares equal displacement should be somewhere between the two. So unlike a cat, when this proa heels due to sailing forces the DLR will change?

Thoughts?

 

Denis

Technically yes. But that would mean the total displacement (i.e of the two hulls) is always and only ever being supported by the one. In other words one hull in the water and the other is always ‘flying’….which of course makes no sense in terms of requiring a datum baseline for preliminary design.

What one would do, is to ascertain the DLR correctly…that being with the two hulls and both hulls in the water at level trim. Then perform sensitivity analysis of the type you are suggesting. Since this is more about the changes than the absolutes, per se. As two principal parameters are changing that being the DLR and WSA from two hulls to one (out of several others not mentioned). It would require a significant amount of analysis, thus begs the questions what is the objective and would any outcome drive the design when in ‘normal’ mode, that of two hulls waterborne? Thus the datum is the DLR in the usual way, the total displacement over the length; otherwise far too many variables influence the DLR, albeit in a minor or major way, to the point the comparisons/assumptions no longer remain valid.

As for a proa, it would be the same, total displacement, but over the dominant hull.

 

The objective is just to understand trends in total hull resistance of various hull configurations. IE cat, vs pacific proa vs harryproa. Assuming similar material lists. The cat will be the shortest. Assume the HP and PP to have the same hull lengths as each other, but the HP carries say 60% displacement on the short hull and the PP carries 30% displacement on the short hull (at rest or motoring). If The HP and PP have the same length hulls and displacement, do they also have the same DLR, even though one carries more of its weight on the longer hull? I would assume the PP would have less resistance (in this scenario at least).

Once again this is just isolating one parameter which is not overly useful in comparing designs. But IMO the more you know about individual parameters the more insight you may have into understanding the whole design, and designs in general. EG, the PP may require water ballast to stay upright in stronger winds, and may be scary to cruise etc, have less usable space. Depending on rig height the HP may shift enough of its displacement to the longer hull when it is needed anyway, IE stronger winds that would allow a very fast ride if the DLR was high. In light winds the short and heaviest hull may not be much of a hindrance as the speed potential would not be there anyway.

I am not going to perform a sensitivity analysis. I would have no idea what to do. But noticing a trend may be possible without it, going by what we know about the most important parameter when it comes to resistance. DLR.

Just trying to get my head around it.

 

It is a worthwhile exercise, to understand what factors influence design. However it must apples v apples. You have noted cat v proa etc…each design is a finished design. As such identifying , for example, the DLR of a cat and then comparing it to say a proa may not yield any meaningful insights. Why?...Because the raison d’etre of the cat, its SOR, is different to that of the proa, in terms of the final finished design. Thus the compromises required to satisfy the SOR of a proa may not be applicable to that of say the cat. Hence you would not be comparing apples with apples.

Every design is different.

It would be a more valuable exercise to compare the variables within each class of design.

 

Yes it is kind of an apples to orange comparison. To find out if one might prefer an apple over the orange based on performance. Then if an apple has better performance for similar cost vs accommodation, is the granny smith better than a red apple?

The SORs of advertised fast cruising proa designs are supposed to be similar to cruising cats so I think it fair to compare them. The claim is they do the same thing in terms of live aboard cruising, but with a similar materials list the proa is much longer therefore faster and more comfortable.

Given the fact the proa has different length hulls, I was trying to understand if its valid to go by the length of the long hull when comparing it to a cat to judge potential performance, and how the displacement is distributed may effect performance. Why not compare the proa by using the smaller hull? I'm not sure that is valid either. I still am not sure what is valid. Either way I am a little wiser after starting this thread.

 

Denis

When trying to explain...especially the more technical and finer points of design...i often use the car industry as a good analogy.

What you are attempting is to compare a Ferrari with a Prius. When you should be comparing the Ferrari with a Lamborghini or Aston Marin etc…all in the same Class.

And if sales person says their Car…a Nissan Skyline 280Z is as fast if not faster than the Ferrari…(as it is)…and is much cheaper, seems a fair comparison. But what the Nissan offers and the Ferrari offer, apart from that one stat is usually very different indeed….since the unit of measure…now has more than one variable.

It all comes down to what YOU as the owner/client want. Not the sales figures per se.

No two designs are the same, simply because the SORs are different!

 

What I, as the client wants, is the one that is faster and more comfortable for similar cost accommodation and safety. And to be able to figure out how to best do a performance estimation. The advertisements claim the longer hull means much better performance, but ignore that there is also a smaller hull which often carries the most displacement. That is what I am trying to resolve.

Its fairly easy to compare 2 catamarans. How much do they weigh? How long are they? How much sail area is there? But the choice is between a proa and a catamaran. Choices do exist between classes when the costs are similar. How much does the proa weigh? How long is it? The same boat is 6m and also 12m long!

I'm sorry if I am being frustrating.

 

Not at all. This is the problem with advertising...especially untrustworthy and stretching the truth. But that is the nature of advertising…to get you hooked and convince of their product...and it’s better than the competition!!

End of the day...if you can actually go see feel touch and hence try one out...that will be your only tactile and palpable measure. Since numbers are…well...just that...numbers! Just like getting into a Ferrari...either you’ll like the Ferrari...or you’ll prefer the Lambo or the Aston...Despite each claiming similar. It is how you feel in each and how you feel it suits your needs. No one car nor one design is perfect….but there is one car and hence one design that is best for you…only way to know…try it.

 

I would prefer the Ferrari over the Skyline, but not if I had to pay for it myself and I don't need to even drive one to know for sure. Because sometime aesthetics is everything and performance is known. But where do I even get to drive a Ferrari? I actually did own a R31 skyline, but fitted it with a motor out of a supra. I did not know how it would perform, but I estimated performance based on calculations without trying it first, and when I got my 10.8 second quarter mile run I was finally happy Was the car perfect, not at all. But I was happy that it cost me very little and I done the engineering myself. SOR achieved on that one. As for getting the same results for a boat? I hope maybe one day.

 

QED.

It is more than just about simple 'numbers'.

 

Damn straight, but if the Ferrari didn't also have good numbers I might get the Lambo.

 

Interesting question. In terms of comfort, the long hull (and the high prismatic) is responsible for the reduced pitching and likelihood of burying the bow.
However, the difference in hull lengths is what makes the harryproa different to catamarans in the upwind comfort stakes. Because the bows are not perpendicular to the centreline of the boat, they both meet the waves at the same time so there is not the corkscrewing motion you get with cats. The harry motion is more like a mono, but without the heeling.

In terms of speed, I think it is more about which hull is to leeward than which is longer. I have never tried it but I am pretty certain that a harryproa with the rig and short hull to leeward would be slower than with the rig and long hull to leeward. This is reinforced as the windward hull becomes less weighted and the lee hull more so. At slow speeds it doesn't matter, as speed then is about wetted surface which is about weight and hull shape.

There are more variables which will come into play if you dig deeper. Such as the hull shape (harrys all have beam:length ratios of 11:1 or higher), overall weight (harrys are considerably lighter than cats and somewhat lighter than weight to lee proas. There are many reasons for this, not all of which are to do with the harryproa type), draft and appendages.

Hope this helps.

rob