Resistance factors, planing hull at low speed

That is scientific enough Sparky.....an observed difference with the transom clear of the water. I can only assume the pressure exerted on the flat transom immersed underwater underway at low speed is less than atmospheric pressure. Weird science indeed, I'm sure that will whip up further debate.

 

Just how heavy is this "planing" boat and what is the waterplane area? L/B?

 

I did have in mind a power cat about 26 feet, hard chine sponsons, waterplane area would be roughly around 90 square feet and weight around 2 tons. Twin outboard powered. The idea entered my head that it may be possible to convert to ( and from ) an efficient displacement mode vessel by the use of detachable fairings to the sponsons, the outboards tilted out of the way, and power supplied by a smaller, high thrust, extra long legged outboard mounted in the middle, for pottering around in protected waterways. Unless it is going to be a lot more economical, and importantly give an extended range, it obviously isn't worth it.

 

I own 26'/2.1t planing powercat myself; using 2x140HP Suzuki it is very economical cruising at 17-22 kts. On seaway, one doesn't want to run slower.

 

Yes, but I have under consideration part of the course being protected waterways, but a long way from fuel supplies, so extending the range is attractive. I would say running a twin-engine planing cat at displacement "hull speed" using one engine, with today's 4 stroke or direct injected 2 strokes, would get better mpg ( assuming wind and tide weren't a big issue ) than the full planing twin engine mode, by a considerable margin. The question is can a further considerable improvement be achieved by the methods described above. I think it is a "yes", but it has to be substantial to be worth the bother. Hence the original question, and whether the hard chines shape is a drag inducer. I can only modify the transom, albeit not optimally. The boat will need to be brought into protected shallows to attach/detach the fairings, but that is OK.

 

Why would the pressure on the transom need to be less than atmospheric?

The pressure on the immersed transom with the boat underway is presumably lower than elsewhere on the immersed poriton of the hull surface, but it would still be greater than atmospheric.

Trying to explain drag changes by looking only at local areas frequently leads to mistakes. Drag equals the integral of the components in the direction of travel of the pressure and shear stresses on the hull surface. Change the trim of the boat and the integral changes because the direction of the surface normals change.

 

Hi D

Interesting paper. I’ve seen these type of papers authored by them before, I have a nice collection

Trouble is, not all the data is there, for real comparative analysis, but since many do not wish to publish their hull data, not surprising.

You note fig.1
So looking at the hulls, 100B and 100Z, these are the NPL.
In the data supplied, the Cb is 0.4.

Using this, the draft for each of these models, at the given “fixed” displacement is 2.51m and 3.51m respectively. This leads to B/T ratios of 3.82 and 1.97 respectively.

This is the problem with “fixing” the displacement. All designs are, or should be, designed for a certain draft. Increasing the draft simply increases resistance. As such these hulls may well be running outside their “ideal” range of displacements, simply to be fixed at 500tonne for this analysis.

This is bourn out in the B/T ratios. One being 2 times the other. It is well known that such a large change in B/T ratio affects resistance, however small or large, depending upon the Fn.

So hull 100Z is sitting very deep in the water 3.51m, but fortunately the B/T ratio saves it, so to speak, and counters the effect.

For this work to be ground breaking, it really need a comparison of like with like. And this paper does not give you that. However, I suspect that was not the intention. It is what it is, a general guide. As naval architects that is all we are after, trends, or guides, not absolutes.

As for location of weight and trim etc, sparky et al. This is all easily explained in fig.7.
It is very very closely related to the LD ratio of the hull, no surprises there. But it is also dependent upon Fn too.

As with everything in design, you establish an SOR and design to THAT. Not some world record fuel efficient hull to run at only one speed in one sea state etc that would never meet the clients SOR. We can all play with numbers and state ephemeral figures upon time-transient conditions. But the boat must satisfy the clients SOR, period. Knowing trends and the effects of “what if” assists in the goal. Looking at exact absolutes in one condition, does not.

As they note in their conclusions too:

“Low resistance is only a component of the overall design process as a holistic solution needs to be found at the end of the design spiral…”

 

Thanks DCockey, it is a can of worms, and staring at it makes me giddy..

 

It can be expained scientifically. If the the transom is lifted the waterplane changes from a triangle to more or less symmetrical airfoil shape. The airfoil shape has less resistance.

 

The hydrostatic pressure on a wet transom acts in the direction of motion, i.e it can be viewed as acting to reduce resistance. A fully dry transom (i.e. vented to atmospheric pressure) lacks this pressure and so many researchers add a component sometimes referred to as the "hydrostatic resistance".

Leo.

 

Pls note that for planing cat hull spacing is not optimum for displacement speeds, the hulls are too close together. Say, You are looking at transom terms now, BUT non-optimum hulls spacing can give You up to 40% of extra resistance. Is it worth discussing transom?

 

Depends on the spacing ? Overall beam is 10 feet, I suspect that will be in your "problem" zone. This wake interaction thing is beyond my reckoning.

 

I assume the half-angle at the sponson bows will come in to play here, with the wave interaction ?

 

From first glance it is far from optimum. What is beam of hull at WL and distance between hulls?

 

Beam WL around 9 feet and 7 feet approx (sponson centrelines) apart.