Quick cardboard model to show ends.

The bow looks a bit very blunt. The wave resistance will be enormous, and sailing becomes wet without fun. I think your ‘test tank’ produces no waves 😊
For example see the sketch of my 8 m ‘Kalapuna’. There I used also a fast and easy production principle. My L:B ratio is 18:1. If you eg. enlarge the vaka beam in cwl from 39 cm to 60 cm you may get a bunk width of around 65 to 70 cm, and the L:B ration changes only to useful 12:1.

Back to the drawing board…..... lots more odd idea’s to try out !!
Did try to create waves tho - but were more like river rapids : (

I remember reading about minimum useful bow entry angles, I think in a book by Derek kelsall on multihulls.  the conclusion was that anything finer than (was it 10 or 15?) degrees was just about pointless.  with very fine L/B ratios, you end up essentially straight sided pretty quick.

This was the same book that argued for finer ratios on catamaran hulls than tri main hulls because of all the wetted surface in light airs.  until a tri float is heavily depressed, it has a big advantage on wetted surface.  in heavy air, the cat hull is better adapted to carry the bulk of the displacement.

A pacific proa is more like a tri than a cat in light air, so I think there’s a bit more leeway for WL beam.  in heavier air, it has the cat’s advantage in lee hull suitability.

gotta love proas 😉

Tom

Hmmmm.

Found at Richard Woods:
“In simple terms the Half Angle of Entry is the angle that the waterlines make to the hull CL at the bow. If it is too low then the boat is wet to sail, and, in extremis, if it is hollow there is a pressure build up further aft which slows the boat. If too fat it is also wet to sail as the bow wave goes vertically up the sides of the boat. All sailors, no matter how skilful, sail slower if they can’t see where they are going because they are being blinded by spray! In both cases the correct Cp has to be maintained. So a 10 degree angle seems a good compromise.”

is 10 degrees the full angle or the 1/2 angle Othmar?

Tom

That would be half the bow angle since Mr. Woods is talking about the angle between the waterline and the CL at the bow or “half angle of entry”.

That’s a very good article, Othmar. I found it here: http://www.sailingcatamarans.com/hullshapes.htm

So what we seem to be coming to agree on here is as far as nice sailing qualities go for a multihull, bow angle entry matters a heck of a lot more than what the hull sides are doing (tapering or not) at the midline.

The pioneering naval architect Froude showed that a hull with a parallel midbody has the wavemaking resistance characteristics of a hull which is equivalent to that of the tapered ends removed and stuck together. The parallel midbody provides only frictional resistance. What this means is that the ‘hull speed’ of a hull with a parallel midbody is lower than that of a fully faired hull of the same length. For hulls with the same length to beam ratio, the hull with the parallel midbody will generally have less wetted surface than a fully faired hull.

What this means for hull resistance in general is that a hull with a parallel midbody will have less resistance at low speeds due to its lower wetted surface area. This is why we see this type of hull used for oil tankers where the speeds are very low relative to the size of the vessel, and where the cost of the vessel vs internal volume is an important factor. The photos of the large double canoes posted above are also examples of load carrying hulls that travel at relatively low speeds relative to their length. Barges are another example.

The charts attached below show resistance calculations for 16:1 L/B hulls of 12m waterline length. The parallel midbody hull has an 8m parallel midsection. Note the location of the hull speed hump in the resistance curve. Every dog has its day, and in this case according to the calculations and as shown in the first chart the parallel midbody hull has lower resistance as long as you are travelling at less than 2 knots.

Hi Mal,
this looks very interesting, particulary the gap at higher speed. There are apparent changes, if you calculate with other ratios or length? Or if you enlarge/shorten the parallel midsection? It’s possible to share the formula for playing with?
Othmar

Othmar,

To do the calculations I used my spreadsheet which can be downloaded from http://www.users.on.net/~malcolmandjane/HullCalc/HullCalc.xls . For the parallel midbody hull I had to find the resistance for a hull which was 4m long and the same volume as the tapered ends added together and then add the wetted surface for the 8m parallel midbody. On the spreadsheet, to add the wetted surface I used the appendage input. To make the volume calculation simple, I used a simple box section hull with a horizontal keel line for both the parallel midbody hull and the fully faired hull.

The spreadsheet performs only a simple calculation for the wavemaking resistance based on Newtonian fluid mechanics using a bare minimum of input values. It does not take into account variations in the resistance due to trim changes or even the effect of the wave profile on the sides of the hull. The results of the calculation show only the general trend of the resistance curve. In this case we can expect the trend of the parallel midbody hull having less resistance at low speeds and more resistance at high speeds to be true, but the actual percentage difference or the location of the crossover point may be different in reality.

While I believe the general trend as described will hold true in most cases, each case must still be examined individually to determine which is the best approach. Varying parameters other than just the volume distribution will obviously effect the outcome. Fortunately, using the spreadsheet as a tool, it is not too hard to examine a few cases to ballpark the best solution.

Aside from calculating hull resistance, the spreadsheet is also useful for quickly estimating the size of the hull you need for carrying the load you require and getting a feel for what the hull will look like. With regard to the resistance calculations, I wrote the formulas myself based on my own interpretation of a range of tank test data and full scale test results. The spreadsheet has not been ratified by any independent reviewers and I still haven’t gotten round to documenting it properly. However, I find that for any hull the results are at least in the right order of magnitude and generally within 20% of the actual values.

Cheers,

Mal.

Hi Mal,
first thank you for the download link. The EXCEL sheet is strong meet expectedly. Especially if you try to work with a symmetrical hull. So I try to reproduce your results with a more familar tool of mine. I have drawn similar simple hulls in Freeship with cwl lenght 12m, width 1.2m, displacement 3.0 t.
First recognitions:
Faired hull draft 32 cm, wetted surface 16,9 sqm
Parellel hull draft 26 cm, wetted surface 17,7 sqm
Your results for wetted surface are recerse.
Then I put the drafts into Michlet to check Wave Resistance, and Total Resistance. In Wave Resistance we have nearly equal diagrams. But as I saw the Total Resistance I was surprised. At last seems the parallel hull yet the better solution!? I am curious for the answers ... and questions 😊

Edit: Something runs wrong with the graphs! See correction below.

Othmar,

I think there may be an error in the total resistance curve from Michlet. If the wavemaking resistance curve is correct in showing that the parallel hull has more resistance, and your parallel hull has more wetted surface, then the total resistance for the parallel hull cannot be better than the faired hull because it is just a sum of the two components. If you compare the shape of the curves (the location of the peaks and troughs) in the total resistance chart to those in the wavemaking resisitance chart, it seems that the lables in the total resistance chart have been reversed, that is, the faired hull is really the parallel hull and vice versa.

Regarding the wetted surface, I don’t understand how you have more wetted surface for the parallel hull especially since the draft of your faired hull is at least 20% greater?

Cheers,

Mal.

I was wondering how the parallel hull had more wsa as it has less draft?
But this is one non performance related plus - for an equal draft it has more load capacity - that and ease of build.

Mal,
sorry for the confusion. As always the human is guilty. I can’t retrace what happened, but now I double-checked all.
First. I calculated the wetted surfaces (same length/width/displacement) with Freeship, Delftship and Michlet. Small differences within 2-3 percent. Averaged - the faired hull with 0.315 cm draft has 16.88 sqm, the parallel hull with 0.265 cm draft has 17.52 sqm.
Second. I generated a new graph, where I tried to merge the lines as correct as possible. The result is more plausibly. In addition to your chart at low speed until 2 knots, it’s interesting what happens between 5 and 10 knots. Sorry about the bad quality of the graph.