Humpless Planing

I have been reading through some old threads and in the middle of one on I14/moth design was an interesting discussion on 'humpless' transition from displacement sailing to planing. For this to be possible my initial thoughts were that the minimum planing speed of a hull must be at, or less than, hull speed (circa 1.32*sqrt L). For a boat like the 49er hull speed is roughly 5.3 knots. Is this really fast enough to get planing? I'm not sure that it is, even with it's big rig, light weight and large, flat sections. I would guess 7kts+ is needed - does anyone know?
If the humpless transition needs a displacement hull speed of 7knots, either the hull must be at least 28 feet (but no heavier) or the 1.32 factor must increase to 1.75 or more (for the 49er which is 16ft). The value of 1.32 is often quoted because it equates to successive bow waves being the length of the hull apart - thus any further seperation would cause the stern to sink into the wave trough forcing the boat 'uphill'. Clearly though, this is not an absolute limit to displacement sailing speed as multihulls routinely prove. Presumably, once the boat starts travelling at greater than 1.5 *sqrt L the stern is no longer in a bow wave trough and so experiences a dip in drag and a corresponding boost in speed. Most of the literature available show graphs with this hump at 1.32 -1.5 and then attribute the dip in drag at 1.5+ to the fact that the craft is planing, but this is not necessarily the case. A displacement boat will also see a dip in drag above 1.5 as its trim levels off. Indeed it would be a coincidence if planing did occur immediately after max hull speed.
Anyway, I have digressed a little. If humpless planing is to be achieved and if (as I suspect) speeds of 5-6 knots are insufficient to get on the plane, then the only solution is for the hull to be unresponsive to the effects of the bow wave. What factors affect this? A fine bow would appear to have a number of benefits. Firstly a smaller bow wave should be generated, creating less of a 'hill' to climb. Secondly there would be less buoyancy in the bow, forcing it to sit lower in the water (again maintaining more of a level trim). Thirdly, if the bow is fine and highly polished then the stagnation point will be further aft, and so therefore will the bow wave. Coupling this with the crew moving their weight forward would eliminate the bow up moment caused by the bow wave. Finally a broad, submerged, transom would provide buoyancy at the stern which would also help prevent the bow rising. Are there any other factors? If these are the factors required for humpless planing, do they compromise the boats ability in other areas e.g. top end speed? Am I barking up the wrong tree?

 

I cannot completely follow you, but if you are talking about sailing boats of considerable measure, then their displacement plays a prominent role.
The formula you mention is totally unknown to me, also you cannot plane a boat from paper only.

A general displacement sailingboat will never plane; she might under very specific circumstances surf and that differs from planing.
Planing is the final stage of the speed process and surfing is the result of being in specific circumstances, mostly running with the wind where the speed of the waves are contributing to the boat's speed and together they push the boat beyond her hull speed's limit.

Such an occasion will not happen below 10 knots.

The planing situation, where a sailing yacht of minimum displacement, e.g. an ULDB, can come into a situation of continuous plane, that will only happen at speeds far exceeding 10 knots.

Powerboats that are designed for planing speeds, are doing so at speeds starting at 17 - 19 knots. Bigger boats require even highe speeds.

Resistance plays a big issue in this process and certain parameters are required before any sailing boat will plane.

Furthermore: the design of the boats plays a major role: A VOR boat might be brought easily into a planing situation; an '70er IOR will never go in plane, but will surf when the conditions are right. Often those two are mixed up.

 

Sorry, D'artois my first post was a bit confusing to follow. I was really referring to high perfomance dinghies like the 49er which claim (and I have no reason to doubt) that they progress from displacement sailing to planing without a nasty 'forced' mode in between. This forced mode has very high drag as the boat has exceeded its 'hull speed' but not yet reached planing speed. It is a condition that affects most dinghies, even high perfomance ones, but some designs claim to have cicumvented this phenomonum. My post was just me thinking out loud for an explanation.

The formula is well established. Basically it limits the maximum speed of any length of hull, whilst in displacement mode. Thus, in theory a 16 foot boat has a top speed of 5.3 knots, a 25 ft boat peaks at 6.6kts and a 36 ft boat can do 7.9kts. It is similar to the Froude number – which should be used instead as it is dimensionless.

 

High Performance Sailing

In Bethwaites "High Performance Sailing" he discusses the "Dynamically Humpless" Hull or as he prefers it the "Free Flowing" hull.He shows the drag curve of three boats: an IC, a Tasar and the B18 . The B18 has a completely smooth curve showing no "hump" whatsoever.
He discovered this ,apparently, by accident. He had observed Sydney 18's with average speeds of 7-10 knots in winds of 5-7knots."There was no way in which an 18' waterline boat with a hull speed of 5.6 knots and forced 'hump' in it's drag curve which would be steepest at about 7 to 8 knots could sail at these speeds in light wind". He says:"The Eighteens, in the cut and thrust of fierce competitive developement, have a achieved a design revolution".
The 49er was designed specifically to take advantage of this phenomenon and compared to other hulls back then appears to have a very fine entry and very litle rocker.

 

I am not familiar with open mini's and in my plain and simple language a 49er is a 49 foot boat. Therefore my not fitting to measure reply.

Since this is an international forum not all memebers are aware of boattypes that are singularily used in specifi countries.

My apologies for that.

 

Toby, I think you are misreading the data charts. In the first post, it appears that you are trying to relate Resistance/weight ratio vs Volummetric Froude number to Wigley wavemaking theroy. You can't, apples and oranges. In the second, you shift back to Wigley theroy alone. While there are "humps and hollows" in the wave making resistance, for most hull forms including most high performance monohull dinghies the curve of TOTAL resistance has no major humps. There are profound problems in trying to compare low L/B planing designs which are designed to produce hull lift (such as the 49er) with high L/B hullforms (such as a Moth or 10M canoe) that power over the wavemaking hump.

 

There's a discussion of this topic on the thread So, Are They Planing?.

 

Actually on some modern boats you don't really get any kind of bow wave at all. Its really quite bizarre and I don't understand what's going on at all.

 

Jehardiman - I was a bit careless with my nomenclature (and thinking!). There may not be a drop in total resistance but Bethwaite (for example) does show that the rate of change of drag drops after the 'forced' mode and clearly annotates this flatter region as planing. My contention here was that whilst planing may occur at this point, it may not. The graph would still flatten out, even if still in displacement mode (providing you had enough power to reach this point). Admittedly, it would flatten out more for planing than displacing.

Thanks Skippy, I was unaware of Gerr's formula that you show on the other thread. Is there a paper? If it is applicable, this formula suggests the 49er displacement hull speed in excess of 9 knots - which certainly is greater than the speed needed to plane. This corresponds to a speed/length factor of 2.17 as opposed to the more traditional 1.34 (not 1.32 as per my first post!) and the calculated requirement of 1.75. Unfortunately , I doubt that the formula ia applicable to dinghies as I calculate that a Tasar should be able to do 8.4kts, which it can't (whilst in displacement mode).

gggGuest - Which boats? Any theories?

I suppose what I'm getting at is that to achieve humpless planing requires a surprisingly efficient displacement hull shape and that the ability to plane is of secondary importance (but still importatnt). It is not just a question of getting the hull to plane at the lowest possible speed. Combining an efficient displacement hull with one that planes readily would appear to be a grand design feat indeed.

 

Toby P: I was unaware of Gerr's formula that you show on the other thread. Is there a paper?

David Gerr: Nature of Boats 2nd ed., McGraw-Hill (1996?)
or Offshore, Dec. 94, pp 29-33


Toby P: I suppose what I'm getting at is that to achieve humpless planing requires a surprisingly efficient displacement hull shape

Oddly enough, I have some suspicions in the opposite direction. Also, I have a feeling the hull speed is not even relevant to whether or not there's a hump. The hump occurs because the partially planing mode is often unstable. Once a hull starts getting dynamic lift, it starts to come out of the water, its drag drops, it speeds up, and lift increases, until it's operating at its maximum efficiency of dynamic lift. Usually the lifting surface operates best when the hull is partially out of the water, so it lifts better and better up until that point. Whenever the boat hits a bad wave and loses its plane, drag increases, the boat slows down, lift decreases, and the boat sinks back into displacement mode if it's not still going fast enough to get beyond the hump again.

This is an interesting but difficult question (for me), but if I had to guess now, my vote would be that theoretically speaking, it may very possibly NOT be possible to get rid of the hump, only from a practical standpoint to make it smaller (at lower speed) by lightening the boat. To eliminate the hump, you need to somehow get the hull PARTIALLY planing very early, but somehow not transitioning to fully planing until it's going much faster. That may require that the hull's fully-planing drag actually be higher at moderate speeds than the drag of partial planing. Or that the hull loses lift efficiency as it comes out of the water. I think both of those phenomena are unusual, and maybe even impossible?

 

If your are graphing rate of change in drag vs speed then of course the faster you go the less the rate of change occurs for ANY well designed early planing hull and most well designed displacing hulls after the first hump (some high performance displacement hulls actually get a hollow in total drag after the first hump). This is because of the relationships between surface friction, surface area, lift, induced drag, and speed. The only way the rate of change would be any thing else would be to ADD weight as the hull speeds up. There is always going to be a hump...either you tackle it early with skin friction or later with wave making. It could be possible to design a hull that the TOTAL resistance/weight is a linear function over speed, but why? Most high performance dinghies are overpowered anyway so it would make sense if you wanted to achieve some constant power....which IS NOT the case with any sailing craft...you get what the wind gives you so you design for maximum speed at an "average condition" which means you want to mimimize total resistance/weight at that point.

 

Moving on slightly

I have been used to thinking in terms of drag increasing proportional to the speed squared, and had not really considered what happens past 'hull speed' before. Frank Bethwaite attributes this slow down in drag increase to the fact that his hulls are planing, but this is not necessary in general (although might be true for his particular hulls). I suppose this is obvious, but it hadn't occured to me before.

Anyway, this has got me thinking. Hull speed is usually given as:

p * LWL^0.5

where traditonally p = 1.34 has been used as it matches the bow wave length to LWL.

Realising that it is not a constant value, David Gerr has derived an empirical formula for calculating p based on the displacement/length ratio (his book is on its way from Amazon, thanks Skippy!), which presumably is a more accurate indication of true hull speed. Clearly, however, hull form also affects a boat's maximum hull speed.

My thinking is that a formula along the lines of the one below could be used to establish whether a hull is limited to a hull speed or whether it is able to break free and continue accelerating in displacement mode e.g. catamarans.

k = (D:L* tan(a))/(L/B)

D:L is the displacement:length ratio (non dimensional),
a is the bow angle of entry,
L is waterline length,
B is waterline max beam.
The waterplane coefficient, Cw, could be used in place of angle of entry of bow, ideally just Cw of forward half of hull would be used.

Some critical value of k splits those hulls that can readily progress through the normal p = 1.3 - 1.5 barrier e.g. cats, from those that can't. Clearly some coefficients and indices need to be added to the terms in this equation to ensure that the influence of each factor is weighted properly.

For hulls that do not exceed the critical k value, factors other than bow wave formation dominate drag, so the concept of relating the hull's maximum speed to the square root of its length is of little practical importance. However, for hulls that do exceed the critical k value the hull speed coefficient, p, could be estimated on a revised Gerr formula that incorporates coefficients of form such as L/B, D:L and a. Has anyone done this? Does it sound like a stupid idea?

Alternatively you could take the opinion that there is no such thing as hull speed, hence trying to calculate it is pointless. Whilst this is technically correct (supply enough power and you WILL go faster) I believe that the notion of hull speed has proven it's worth over the years, so trying to establish it more accurately (without the need for CFD/VPP software) has to be a worth while exercise.

P.S I have started a new thread based on this post in the boat design section of the forum as it seems to be relevant to non sail powered craft as well. To avoid duplication of comments it is probably best to respond in the new thread.

 

beam to length ratio

At some point I learned a theory that a high beam to length ratio hull like a cat has a theoretical "hull speed" of 4 times the square root of the water line. And in recent times I've read about 10/1 beam to length non-foiling Moths begining to plane at about 15 knots.
It's always interested me to get into the effects of beam to length ratio on "delaying"planing but I haven't made the time. Fascinating subject ,though.

 

I've heard such things too, but... Long, skinny boats are simply easier to push in forced mode. Planing begins when there's enough lift. I think these things could be unrelated.

 

You guys all seem to be thinking of planing as a binary state: it is or it isn't. 'Tain't so.Whatever planing is, and I'm not sure I could write a watertight definition, it is on modern designs a gradual process. There's no magic, now I'm planing now I'm not point. The boat is partially supported by form lift instead of buoyancy well below "wave system length = hull length" speed, and partially supported by buoyancy well beyond it.