Hull design for a small displacement boat

[daiquiri]

Originally Posted by mwatts

Daiquiri, one of the most problematic surfaces are the sides of the central hull. I actually tried to design these to be straight verticaly, however, Rhino creates bulging surfaces instead. I'll have to fiddle with this to get is right. It would by far be best, I think, if each side can become one developable surface. Do you have any suggestions?

It is strange that Rhino creates sweep surfaces of that form, because flat vertical sweeps are extremely easy to create. Of course, the intermediate sections need to have a simple form, straight lines and no composite curves.
I can also suggest you to try with Freeship or with Delftship. Both of them have reliable modules for the creation of developed sheets from the curved hull surfaces. I know Rhino does that job too, but I don't know if you have installed the software module for sheet unfolding.

[mwatts]

Originally Posted by daiquiri

It is strange that Rhino creates sweep surfaces of that form, because flat vertical sweeps are extremely easy to create. Of course, the intermediate sections need to have a simple form, straight lines and no composite curves.
I can also suggest you to try with Freeship or with Delftship. Both of them have reliable modules for the creation of developed sheets from the curved hull surfaces. I know Rhino does that job too, but I don't know if you have installed the software module for sheet unfolding.

I will try making the sides of the center hull out of two halves tonight. Maybe that will create simpler surfaces, which I later can join. Or maybe it would be easier to build if it where two physical halves too?

Rhino has a "Unrol developable surface" command out-of-the-box. That's what I have previously used to create a (very small) model (print it out on a sheet of paper, and cut it out of balsa or cardboard).

I have tried Freeship. Somehow, I don't get along with it though. It doesn't give me the control I want. For instance, I had a hard time trying to create some hard chines. But that probably is just me not knowing the program very well of course.

Originally Posted by FAST FRED

Compared to the origional model Atkin , the pods (Amas) might only increase wetted surface and drag. They would be far more complex to create , yet the only reason would seem to be extra stability when stopped.

Have you visited, www.atkinboatplans.com/ and actually looked at some of his box keel designs? They were refined for a couple of decades and might be worth a look.

Start with "Rescue Minor"

Well, the purpose of the pods was two fold: (1) to create a tunnel of water towards the prop. (2) to provide bouyancy (115 Kgs displacement each at the moment)

I have looked at the Atkins boats. The problem with the inverted V shape, for my purpose, is that it raises the bottom of the boat aft, which causes the CB to go further to the front. The Atkins boats compensate by placing the engine (inboard) forward, but I want to use an outboard in my design (positioned near the end of the pods). This would cause the boat to lean back even more than, for instance, Rescue Minor already does. Another problem is that I want to have the bottom of the boat, where the engine is placed, submerged at all times, because I want to have as little noise from the engine as possible.

I considered creating a form simular to the Atkins hulls by lowernig the sides of the transom, instead of raising the center of the transom, and folding the bottom sheets in that shape, but I figured that would cause unwanted lift at the aft of the boat at higher speeds.

Just my two cents, I could be wrong.

[Guest625101138]

Martin
You will get significant stiffness from just 3mm plate if you use a gentle curve in all plates. Flat surfaces will require more stiffeners to hold the shape. The unsupported spans are all very small.

Rick W

[daiquiri]

Originally Posted by Rick Willoughby

You will get significant stiffness from just 3mm plate if you use a gentle curve in all plates. Flat surfaces will require more stiffeners to hold the shape. The unsupported spans are all very small.

That is correct. In Delft or Freeship it is really easy to achieve and indeed I always do include a small sheet curvature.
But I don't use Rhino normally (I use Freeship for hull modeling plus UGS for the rest of the work) so I don't know what kind of developability control it has and how handy it is. Hence the advice about flat vertical sides.

[Guest625101138]

Shifting some displacement to the outrigger hulls reduces the drag. I had to model them as symmetrical hulls of the same volume. This gives power requirement of 8kW for 10kts.

The flow off the stern of the main hull is higher than before. I looked at it in two ways and it shows higher than normal level right behind the central hull.

Rick W

[mwatts]

Originally Posted by Rick Willoughby

Shifting some displacement to the outrigger hulls reduces the drag. I had to model them as symmetrical hulls of the same volume. This gives power requirement of 8kW for 10kts.

So I should slim down the central hull and increase the size of the outer ones?

Originally Posted by Rick Willoughby

The flow off the stern of the main hull is higher than before. I looked at it in two ways and it shows higher than normal level right behind the central hull.

That's a bad thing or a good thing?

[Guest625101138]

Originally Posted by mwatts

So I should slim down the central hull and increase the size of the outer ones?


That's a bad thing or a good thing?

You may be able to do a little better with lowering drag using longer outriggers but it is marginal benefit. What you have now is midway between the best catamaran and the first single hull.

The outrigger hulls force flow toward the main hull so the centrally mounted outboard motor would be much less likely to ventilate than with the original single hull. Overall the peak wave height is almost halved in the latest arrangement. It is better than the catamaran in this regard.

As noted earlier curving all the plates could avoid the need for stiffeners depending on how it is decked in.

The control points seem much more complicated than what would be required in Delftship to achieve the same result.

Rick W

[Guest625101138]

Martin
There is some discussion of the stiffness of curved plate on this thread:
http://www.boatdesign.net/forums/boa...tml#post293039

You are proposing much heavier plate and unsupported spans are not much larger.

An advantage of longer side hulls would be taking torsional stiffness further forward.

If you have difficulty envisioning this it can help to build a simple model.

Rick W

[mwatts]

Rick,

Yesterday evening I started with a copy of the latest design, and tried to get all the curves simpler, with quite good results.

I changed the size of the outriggers too. In the new design they are a lot longer, they follow the curve of the boat, and they are symetrical. As a result, they now have a displacement of about 320 Kgs each. That's about 2.8 times more than the old version.

This means the central hull becomes a lot shorter, and a bit narrower. The question is of course if this will be better. Surface friction will possibly be more. Hopefully I can complete the design tonight so we can see.

In any way, it is a lot easier to develop.

If you can provide an example of what you mean by curving the plates, that would help. Thanks.

[Guest625101138]

Originally Posted by mwatts

..
If you can provide an example of what you mean by curving the plates, that would help. Thanks.

Martin
If you have a look at the photos of the steel canoe shown in the photos at the start of this thread you should get an idea of what I mean.
http://www.boatdesign.net/forums/boa...ect-28485.html

Not one of the plates is installed flat. There is even slight rocker to ensure the bottom plate has some curve.

If you set a curve into a thin plate it dramatically increase its 2nd moment of area looking onto the arc. Hence it becomes much more resistant to bending. You can check this with a piece of cardboard spanning two supports. If it is flat it supports nothing. If it has a slight curve it will support a load. You will not need much curvature in 3mm plate to span the 600 to 700mm spans that you have with most plates.

A significant advantage is that the plates are less inclined to buckle uncontrollable when stitched initially. It also means that the only stiffener likely to be needed is along the gunwale.

Even the keel plates on each hull could have a slight curve - a little rocker. Something like 100mm lift either end. Or more at the bow and less at the stern.

It sounds like you already have curve on the side plates of the three hulls now. It will not be difficult to curve the side plate toward the bow.

I suggest that once you have a design you are happy with, scale to about 1/10th scale and make a cardboard version. It will highlight any obvious weakness.

Rick W

[mwatts]

Rick,
I see now what you mean with the curving. I'll have a look where I can apply it. I get the feeling I will be redoing the hulls for the 4th or 5th time tonight (probably tomorrow morning for you)

[masalai]

look at this image.. http://www.boatdesign.net/gallery/sh...4/ppuser/22903

[mwatts]

Masalai,
Thank you for the pictures. Although I will be making a tri-hull with the main hull in the water, and not a Cat, I do like the pod shapes you have used. They seem to be close to what I should try to achieve.

[mwatts]

This weekend I had some time to do some more designing. Here's the result:





This design is actually fully developable. All surfaces where redesigned to have only single curvatures. Almost all surfaces are curved, to give the hull more strength. The back of the ship has been tapered a little, for beter flow.

LOA=590cm. Displacement approx 950Kgs.

Comment welcome as usual of course.

[masalai]

quite an interesting exercise, hope you have fun with it, and do not try to overpower it....