CNC Plans not Included

Jorge - what are you doing!? Those drawing above look absolutely ugly, please tell me you are not going to have a hull which looks like that!?

If you want more volume or width in the hull, just increase the waterline beam and get rid of the chine to keep the lines smooth and sexy...

 

Actually Groper it was this portion of his testing that attracted my attention in particular,...

I was seeking to reduce the rather deep forefoot of Tenannt's designs for use on a sailing version (motorsailer actually) of that hull form.

Really :?:

So I would ask if your 'facts' about fuel consumption and HP requirements is based on actual measurements of some sort,..or just general observations, or some other competent study. I actually don't know many owners of vessels who have ever accurately measured their fuel flow rates. Yes they may have some 'approx usage rates' over some periods of time and/or distance, but no firm numbers.

Ar least Noah took some considerable time to evaluate the differences between various hull shapes utilizing the same computer models,...more in depth study than many other designers have bothered to do.

As I said, I was not really interested in the small differences in fuel consumption that these long thin hulls might exhibit, but rather how the would handle a seaway

 

What has been documented in countless papers is the DLR ratio. You won't find any study paper which compares similar hulls in terms of DLR and such a huge difference in powering requirements as shown in the figures by Noah Thompson. It just cannot be true unless there were other differences effecting the predictions - you won't know unless you had a detailed look at his prediction methods or apparatus

I chose not to use a CS for my design as my prediction software predicted a higher resistance due to higher wetted surface area for a given displacement compared with transom stern design. real world results show that similar DLR = similar fuel burn for catamarans...

 

Back to the version of the 2 chines. Also here is a view of the taper up to the chine which hopefully minimizes slapping. The actual chine is 1.5 inches wide and 6" above the water line.

I have developed my own method for drawing, it's very accurate and quick.

The only thing is I don't know how to patch, blend, smooth the joints and I don't know how to use cutting planes so I can trim the transom. Anyway a bit more experimenting and I will be able to draw the whole boat.

 

I don't really think you will see the chine, it's down on the waterline. I can lose the top one, that is not a big deal.

 

If the Chine is on the waterline- why have it at all if you won't see it? It's an extra complication that doesn't need to be there!?

 

Groper, the Tennant power hull efficiency is due to more than just the hull shape apparently.

From an interview with Malcolm...

"Most of the hulls low resistance may be attributable to the basic shape. But we have also considerably reduced appendage drag. There is no exposed propeller shaft, nor any supporting strut. We also maximised propeller efficiency by having a horizontal tail shaft. The propeller is at right angles to the water flow but still protected from damage. Another attribute of this hull form is its ability to take the bottom and being a catamaran it remains level. This hull form, and its many variations, have proven to have one of the lowest resistances’s of any hull so far developed. This has been demonstrated many times in the test tank and full size in the ocean. Slip factors of from 12 to 15% are commonplace."

From http://www.webring.org/l/rd?ring=mu...catamarans.com/news/2006/04/catcomparison.asp

 

Well none of that matters Barra- the schionning prowler designs that use the CS hull and powered by outboard motors still have a similar fuel burn as my full transom stern boat at speeds above 16kts.

The CS hull does better at displacement speeds as there is not as much transom drag , but the Noah Thompson article refers fuel figures at 26kts and I would argue that the cs hull is worse at these speeds - due to a higher wetted area for equal displacement....

 

Sorry, meant right down by the waterline. Paint a contrasting bootstripe and I think it looks good but whatever, I will try again with no chines.

The version I posted earlier with no chines earlier is probably as narrow as the SIG, that is not going to fly.

Now that I have my new technique, I will try again. Does anyone know how much drag or speed is affected as the ratios change, ie... 1:12 1:13 1:14

 

Ok, hull tapers out a bit, didn't want to lose waterline ratio, a std queen isn't going to fit sideways, will have to be short by an inch or two, or can use a double which is what I would do anyway. I will render this one when I get back, got a 6 hour road trip today!!

0 back of boat
250 start of stairs down to back of boat
500 start of aft cabin
1848 waterline starts to narrow to bow
3000 end

Going to try to do the reverse bows again. This time I think I can make it work better.

 

Jorge- your missing a fundamental issue by only considering the static waterline. When the boat sails, the Lee hull is pressed into the water and your chine will be immersed both in calm water and as it moves through waves. You won't see any advantage to a wine glass shape in anything but very light airs where the Chine may not become immersed at all.

So the waterline beam will effectively be whatever your total width is including chines. My point is why create a building headache when you can simply increase the curvature of the bilge radius to gain the total width your looking for?

Going from a length to beam ratio of 15:1 to 14:1 is probably immeasurable in terms of performance. Remember as I've been trying to make a point to Brian earlier- the number 1 factor on performance is the displacement to length ratio. Slight changes to length to beam and beam to draft ratios are almost immeasurable in comparison.

 

I picked that shape after reading Shuttleworth's white paper. I didn't like the lightbulb like hull though.

Here is a pic of what I think you are talking about but, I think, if that waterline gets any fatter, on this spartan boat, it's going to be like a popping cork on top of the surface

Also about the leeward hull digging in, on those images I posted at 15° heel, the chine barely touches. If you are heeling that much in a cat you are zooming. Look at the pic that chine is virtually gone by the time it reaches the bow, what is left should help with buoyancy up front.

I do agree the upper chine is ugly!!!!.

I will keep working on it though, here is the pic, not too bad, you can still fit a double bed sideways. It is a bit egg shaped though.

 

yes a bit like that.

If you want it to be smaller/lighter, get rid of the lower drawers so it can all be squeezed together some more.

Towards the aft of the hull, the bottom of the bilge can be flattened out quite a bit also, so it could have an almost flat bottom with rounded chines to the topsides as many modern designs are currently using.

heres a pic of my friends 40ft cat which uses the wineglass chine... he said it has been an absolute PITA to get this right, hundreds of hours fairing. It has to be perfect as its the most visible part of the boat done in high gloss...


Theres always trade offs with boat design. You want your bed sideways in the hull (i think your mad - with 2 in the bed 1 person has to climb over the other) - that means your dimensions are restricted by that if you dont accommodate it some other way. My suggestion was to incorporate a cut out on the inboard side of the hull so the dimesion can be expanded towards the bridgedeck. This will allow you to fit your bed sideways and lower the false floor to reduce overall depth of the hull throughout.

Remeber if you use a symetrical hull, the entire thing doesnt have to be symetrical, you can cut away the inboard side to encroach on the bridgedeck a bit and create this hull to deck union as part of the bridgedeck module/piece/molding etc

 

Ok, I am going to devote some time to that shape and see if I can keep it tighter. I just hate cutup spaces and steps.

 

There isn't much point rehashing jorge... aren't you going to pay for a design? I believe I would have an easier job selling mine if it were a known design or of a known designer...