Critical speeds for Semi-Planing

I'm still confused by the term "semi-planing" that is
sometimes bandied around on this board.

I have calculated the dynamic sinkage and trim of a small (18.7t)
catamaran with fairly standard wave-piercing demihulls as shown in
an attached figure.

The demihulls are each 19.1m long, with 1.1m beam, draught 0.77m
and displacement volume 9.375 cubic metres.

The other figures show the draught, displacement volume, squat (given
as the vertical change of the bow and stern from their static values)
and sinkage force as functions of speed in knots.

Can the catamaran be said to be semi-planing above any particular
speed based on these calculations?

Above 10 knots, the bow starts to rise and the stern dips, but
that's certainly not an indication that the boat is starting to plane.

I can make a case for three "critical" speeds that are better
indicators.

* Above 13 knots, the displacement volume is less than the static
value.

* Above 14 knots, the sinkage force, which has so far been negative
(i.e. downwards and increasing displacement), becomes positive, i.e
acting upwards to decrease displacement.

* Above 18 knots, the draught is less than the static value. At the
same time both the bow and stern are above their static values as can
be seen in the squat graph.

So which of these, if any, do you guys and dolls think are the best
indicators of "semi-planing"?

Regards,
Leo.

 

We are getting into a touchy mess of conflicting definitions anytime we start talking about that "grey area" between a full-displacement hull and something that is fully on plane.

I would argue that for a boat to be considered to be fully on plane, the following criteria should be met:
- Both bow and stern reference points should be riding higher than their static values;
- Overall draught should be less than the static value (sort of a given, if the first point is met);
- A separate bow wake does not exist, as the boat has "climbed" over it to produce a single diverging wake system;
- No significant transverse wake system is being produced astern of the craft

From a technical standpoint, I know this is a bit sparse (where does dynamic lift come in, for instance?) but these are the observable effects that seem to be shared by virtually all craft said to be "on plane".

Now, this messy "semidisplacement" or "semiplaning" phase:

Boats that are said to be in this condition seem to share a few traits:
- The bow rides higher than in the static condition;
- The transverse wake system often remains;
- The stern does not necessarily rise (sometimes it even sinks a bit) and the overall draught does not necessarily decrease;

Basically, we seem to figure "OK, so she's not on plane, but she's clearly lifting somewhat, therefore she must be a semidisplacement / semiplaning boat."

How do you interpret that from the graphs? Well, the boat above shows a definite "hump" at 10-14 knots in which one would likely not want to operate for long; above 18 knots where the stern is lifting above its static position, I'd say she's trying to plane. So I'd say she's in full displacement mode below 10 knots, in an inefficient transition state from 10-14 knots, in a useable semidisplacement / semiplaning state from 14-18 knots, and climbing to plane once you get past 18.

Is that enough conjecture and black magic for you?

 

Leo,

As Matt said, you've opened that proverbial can of worms, for all the reasons (and a few more) that both you and Matt mentioned. The fact that hull design experts use terms like "semi" means that we all know it's imprecise. My guess is that the reason is that the transition phase from displacement to planing mode is relatively long, meaning the boat is in transition over a wider range of speeds.

I would add, from an operators viewpoint, that there is a point of transition observed at the transom, when the water begins to "break" cleanly away from the transom. In other words, although the bottom of the transom is submerged, a void opens up, and the bottom of the transom is visible. This is the effect of the transition from plowing to the beginning of planing. The bow may still be high and the stern deep, but the transition has been made, dynamic lift is supporting, and small throttle increments will produce larger speed increses.

My experience is all with monohulls, and I'm not a designer, but I thought it might be helpful to add that observation to what Matt has described. Semi-displacement/semi-planing hulls, in my experience, have been able to achieve a plane (observable by the clean break at the transom), but are not able to achieve the more efficient "full plane", in which the bow comes down, stern rises a bit, and higher speeds are possible. There have been a few designs able to operate efficiently in this "barely on plane" mode, and they might be the closest to a true semi-planing design.

Matt's analysis of your figures seems accurate. I agree that the 10-14 knot range is probably plowing. Above 14 knots the hulls begin to plane.

 

Do a google for 'planing hull shape' or something to that effect. I got a very nice article explaing the different shapes and their pro's and conn's.

The best hull shape was as your's but stepped upwards. Find the article, it's quite interesting.

 

Thanks for the thoughtful replies!

I very much doubt that my results show anything like true planing.
For me that would be when the majority of the vessel's weight
is being supported by dynamic forces. My calculations for the
wave-piercer show that, at the highest speeds, far less than 50% of
the weight is being supported. That's not too bad for a displacement
hull I suppose.

Do any designers take these semi-planing effects into account when
they are estimating catamaran performance? They probably aren't all
that important for sailing vessels which can't get over the resistance
hump, but they seem significant for powered types.

All the best,
Leo.

 

I don't agree that a void appearing at the transom suggests planing, simply that the speed is sufficient to not give the water time to come back.

I have a semi displacement cat and it does come up about 6 inches and then the rear comes up. She rides very slightly up from normal static position.

If full power is applied it will try to come up more, I presume that if I had more power to apply then there would be a point when the boat would be skimming.

The bow wave is always there as the bulbous bows remain well under water. There is also a second bow wave at the cheek,-- 24KTS

I think semi displacement is a wrong word and semi planing is a better description of whats happening.

 

Leo
Are you working on a Michlet upgrade?

While you are lurking - have you ever considered the limitations of Michell's thin ship theory with respect to wide, rather than slender, hulls. Under what conditions of L/B does the theory fall down?

I have found that Michlet and Savitsky give surprisingly good correlation around 20kts for wide planing hulls.

Rick W.

 

My books suggest that semi-planing starts when the suction force is less than the lift force - which is 14kts in your plots. Deciding when you are fully planing is another matter...

 

on that L/B and speed i doubt the warp is still effective

 

Hi Rick.

1. No, I'm not working on extending Michlet. I've been working on
my other codes Flotilla and Flotsm.

I'm nearly ready to release a demo version of Flotilla.
I've put up three reports that I will use as examples at:
http://www.cyberiad.net/flotilla.htm

One report is for a Surface Effect Ship with similar proportions
to the Royal Norwegian Navy's "Skjold".

Another report looks at a destroyer with similar proportions
to the Australian Navy's Hobart Class Air Warfare destroyer.

The third report compares predictions of a model air cushion
vessel with experiments in infinite depth and finite depth water.

I am still working on examples for catamarans and several exotic
vessels that use combinations of air cushions and displacement hulls
and others that have non-constant pressure distributions.

The calculation of squat and other near-field effects are far too
slow for Godzilla. The graphs I attached previously took about
30 minutes on a dual core PC. The main problem is that the hull
attitude must be iterated until forces and moments are in equilibrium.
At high Froude numbers that can take about 2 minutes for just one speed.

2. I am not sure where Michell's theory breaks down.

It used to be said that L/B=10 was about as thin as one should go,
but I suspect that comment was based on comparisons of poor
calculations with suspect experiments. At a workshop on
wave resistance calculations in 1979, calculations of the wave
resistance of a Wigley hull varied by about 15%. This is bloody
awful given that 3 figure accuracy is easily achieved with proper
attention to the numerical quadratures. Comparing poor quality
calculations with experiments that could have errors as large as 50%
is hardly convincing evidence one way or another.

Don't quote me, but I've heard of pretty good agreement for hulls
with L/B=4.

What happens as the draft is reduced? I haven't really pushed Michell's
integral very far that way. That is more properly the domain of "flat-ship
theory" which is extremely difficult. The equations are well-known, it's
just that they are not very helpful for practical calculations. Hence the
popularity of Savitsky's method for all its faults and limitations.

All the best,
Leo.

 

I have not been able to do my own comparisons with hulls having L/B around 4 but the performance for power requirements from boats I have looked at on this forum indicates that Michlet gives useful results in this range. So nice to see that there are others with the same view.

One thing I have realised is that an efficient hull will not have large wave drag so the wave component becomes less significant. In this case you are relying predominantly on the viscous drag and the ITTC 1957 formula fits well with what I have determined.

 

Thanks, Frosty. I've heard that condition quoted as well. I agree that it is no indication of planing whatsoever. A dry transom might be necessary, but it isn't sufficient to define planing.

Leo.

 

Thanks, PI.

The condition that the suction force is less than the lift force seems sensible, however I think it might be better if the displacement is also less than its static value. The trim of the hull could be such that the displacement is still greater than its static value even though the lift force is greater than the suction. The condition that the displacement is less than its static value is not enough because that can occur (very slightly, admittedly) at low speeds.

Regards,
Leo.

 

Have you assessed any cases where the theory gives very poor predictions for small L/B? If not, then you are likely to be accepting results that fit your ideas and rejecting those that don't. That's not science!

All the best,
Leo.

 

I would concur, although many more seasoned folk tend not to... my runabout is most certainly "fully planing", but some of its weight is supported by displacement even at 30 mph.

Also fair, in my view. Using my runabout again: when fully loaded, clean breakaway of water from the transom occurs before the bow even begins to rise, before the bow and stern wakes merge, before any lift is noted.

It can, yes... but it can't do so reliably, on an arbitrary shape, is my understanding. The simplifying assumptions built into the mathematical theory tend to be reasonably good for slender hulls; in general, though, they are not always valid for wider, bulkier hulls. I see no problem with using Michlet on fatter boats for the purposes of better understanding the theory, or for comparing against other predictive methods. I would not use it for design on wide hulls, because without a cross-check against a theory or test known to be valid in this case, you can't trust your results.