Catamaran Speed

Thank you, Leo. That was a nice piece of work. Even an ignoramus such as myself could follow much of it.

Here are the basics that I didn't follow on your paper:

1. I thought that 'all' divergent waves come off a hull at an angle of 19 degrees. I think it was Kelvin who identified that. So wouldn't you want to concentrate hull spacings to cancel waves at that angle? If the 19 degrees is not actually the case, what hull parameters minimize divergent wave heights?

2. I have read elsewhere that divergent waves have little effect on hull efficiency, and it's the transverse wave that we have to deal with. But your paper suggests otherwise. I'm confused by that. Do we have to worry about wave resistance from both wave types? I recognize that your research was aimed at minimizing wake effects, so maybe that's where my confusion comes from.

Again, thank you for the link. I've run across other articles of yours in my research and they're invariable accessible for moderately technical people like me. Much appreciated.

 

No. 19.5 degrees is the deep-water Kelvin angle. It is the line made through
the intersection of the diverging and transverse wave patterns.

In linear theory, you can think of waves being created at all angles, but weighted differently for each angle.

Transverse waves are more important at low Froude numbers. Diverging
waves become important at high Fr.
See Figure 1 in the paper for some (dimensional) values of the wave
resistance of a monohull.

To minimise the wave resistance, you have to choose a method
appropriate for the speed (or range) in question. There is little to gain by
trying to minimise transverse waves at high speeds if there is little energy
in that part of the wave spectrum. (In fact, as shown in the paper, you
can''t reduce transverse waves by choosing the lateral hull spacing of a cat).

Spacing the demihulls so that the centres lie close to the Kelvin angle is
a reasonable first cut at reducing the diverging waves. A better way is to
look at the free-wave spectrum of the demihull to see at what wave angles
(not Kelvin angle!) where most energy is being shed and to choose the
spacing to reduce the largest peaks (while not creating any other peaks).
Of course, practical considerations might render that choice useless.

The method is in the paper, but it wouldn't surprise if you missed it on a first
reading.

Leo.

 

Ad Hoc, I've been spending a lot of time the past couple days pondering that graph. Because a lot of the terms & coefficients are unfamiliar to me I've had to grind my way through them. But it's starting to gel in my head.

What I find most striking about that graph is that, in the hull sizes, displacements, and speeds I'm interested in, a fairly small increase in length (10%) can produce a large decrease (>25%) in resistance. The added cost of hull length could have a large return in fuel economy.

In that graph, is the Rt the total resistance or just the residuary resistance?

 

Alik, how did you calculate the resistance differences between round bilge and hard chine? Do you have any data on how the two hulls compare in waves? For monohulls there is plenty of evidence that hard chine hulls have less added resistance in waves than the round bilge ones even for displacement and semi-displacement speeds and therefore keep their speed better. Do you know of any similar data for catamarans?

 

Thanks for the response, Leo. I did look at Figure 1...pretty striking difference.

I'm having a hard time visualizing why that's the case. I can understand the transverse wave resistance with the hull climbing over the wave and all that, but what about the divergent wave is pushing back against the hull's forward motion?

It appears from figure 8 that at 8.25 m/s yields a peak wave amplitude at about 45 degrees for cats. I'll have to work my way back through the trigonometry to figure out what the optimum spacing would be. It also appears that the peak is pretty broad, so there isn't one single spacing that will eliminate most of the waves. Or in other words, almost any (practical) spacing will yield similar results for a small cat.

Is there a model out there that can provide wave properties for other non-Wigley hulls?

I plan to read through your paper a few more times to fully digest it. Lots to mine in there.

Many thanks again!

 

Any hull, even a box, with sufficient power can go faster than hump speed. (Whether it will tend to travel upright in a straight line is another question.) How large the resistance "hump" is depends on a number of factors which others have already discussed. Long, narrow, light hulls tend to have minimal humps.

 

That's not the way to look at it. For a start, the hull is not trying to climb over the wave.

As mentioned before, Michlet can give valuable insights into wave cancellation.

 

Exactly. A simple increase, in length only, has a beneficial effect all round, not just powering, seakeeping too. Thus, if you can add a bit more length, do it.

Rt is is total, and W is weight of boat, thus the speeds are quoted in Fv (volume Froude number). Not all graphs are presented in this way, but the basic trend is the same.

Depends upon the Fns. A low Fn the difference in resistance of mono's is minimal, small variations in local shape can have minor beneficial effects. Only at higher Fn (> 1.0) is this true: hard v round.

All our data over large Fn ranges shows that hard chine to be slightly greater resistance. Published data of individual hulls, often look rather good: hard v round shape. But when you collate a large number of the papers and also compare against actual tanks tests of many different other parameters (often excluded from such papers to indicate bias), round bilge is slightly better.

That's not correct.
A box cannot exceed its prismataic hump speed. The trim would be excessive and stability a serious issue. Assuming one ignored the powering rising at a high expontential value, as it does.

 

I use two different series - Molland-Insel and MullerGraf; actually might be not the exactly right way but the results for both series are anchored to our sea trials data so I believe they are correct.

Added resistance in waves is kind of grey area as it depends on wave spectrum; one might get opposite results in different conditions.

 

The message I posted was truncated/edited by Ad Hoc. It is very unfortunate when a single sentence from an earlier post is quoted out of context with the claim that it is "incorrect". His stability and power objections to the first sentence in my post were addresed in the two sentences which followed the one he chose to quote by itself. The entire post I made was:

Tow a box with a powerful enough vessel and the box will exceed its prismatic hump speed. As I said in the earlier post whether it will tend to travel upright in a straight line is another question. If the box has a sufficiently high Length/Beam ratio then the hump resistance may not be very large.

Also, prismatic boxes have been used in tow tank studies of planing surfaces. Given sufficient power, not too much displacement, and an appropriately located CG a prismatic box will plane.

 

Hmm..it seems in your zeal to be pedantic you’ve, as always, thrown the baby out with the bath water.

Any hull, really??...so a box of say 1x1x1 dimensions, is any hull.. Unless you disagree?

A 1 x 1 x 1 box is not a long slender box thus it is not any hull.

Linking your two sentences “as if the same thing” (which is what you’re suggesting) just highlights a lack of understanding in hull forms and the parameters that effect the resistance. You need to clearly separate the issues that affect the resistance. It is not a one size fits all.

This is again, highlighted here:

Firstly, towing only refers to “a hull” that is being towed in a test tank for powering estimation. A boat, real boats on the water moving from A to B, have engines (or sails if a yacht) and do not need towing (unless no engines! Or break down!).

Secondly, if it is no longer upright, how does it propel itself, if it is no longer upright?... it is clearly no longer moving as its means for propelling itself are now in the air not the water!...unless it is a for the sole purpose of being towed and its stability trim and huge drag and wash it creates is never an issue. I’ve yet to come across a hull that has that SOR!

Thus it is not “another question”. If the hull is no longer upright, it has no purpose nor function and is about as useful as saying steel is acceptable for making aircraft; just wont fly very far nor under controlled conditions once released from another aircraft that is “towing” it. Other than creating plenty of wash down stream for those that like to surf behind moving objects or headline stories of kids on beaches being pulled away from the shoreline by their parents for fear of being swept away and drowned by wash created by a box moving out of control while being towed, please enlighten me on the purpose of a 1x1x1 box hull that is attempting to approach its prismatic hump speed.

Here you are probably, I am assuming, referring to landing craft that have these characteristics. These are not true boxes per se. Most pre-hump landing craft of low L/B ratio have powering that goes beyond the 7th power when approaching their prismatic humps. Hence they don't even bother trying to. Those that do plan, have changes in their hull shape up fwd and have a greater L/B ratio than a box of 1x1x1. We have several successful designs like this that do indeed plan, but they are not boxes of any shape. They do 30knots and carry troops..or general cargo.

To link these designs a box style landing craft with long slender box type hull of fast ferries we have designed, with a box of any shape (as being equal) is misunderstanding the concept of hull forms parameters and their effects. One size does not fit all and each requires its own definitions and associated behaviour.

 

I'm not sure what to make of the post above, # 41. It has little to do with my posts if they are read in their entirety and not split into separate phrases. Any further response would be pointless.