NACA profiles and hulls

[Ad Hoc]

This thread is useful, but what about slow (up to 3 knots) not-planning boat?

At such slow speeds, as Leo points out make the hull as simple as possible. The only factor to really have an inflection is then the length-displacement ratio. Make it as high as possible.

 

[daiquiri]

This thread is useful, but what about slow (up to 3 knots) not-planning boat?
E.g. half submerged NACA laminar profile 64A010 as a hulls of catamaran (lets forget about stability).

Most catamarans hulls looks like (A) - 'U' stern above the surface of water. But what do you thing about deply immesed, tapered stern like classic hulls of cargo ship? Cross-section (B) or (C) even.
It looks like a tail of NACA profiles. Becouse of this I initiated this threat.

The reason why you don't see that kind of hull form today is very simple - it has a high drag and is less maneuverable than modern shapes. Similar hull forms have been used through the past centuries. One notable example are pilot cutters:

which were sometimes so extremely deep and narrow that this type of hull was often refferred to as "plank-on-edge".

First of all, you need to understand that the ship drag can essentially be split (for convenience of analysis and calculations) into wave and viscous drag components. The viscous drag can then be split into friction drag and pressure drag.

Now, the wave drag depends essentially on the longitudinal distribution of volume and shape of section areas, and the viscous drag is mostly related to the amount of wetted area (friction drag) and to the streamlining of the hull (pressure drag).

For very slow boats (in terms of Froude number), viscous drag is the predominant resistance component. In particular, it is the friction drag that makes the biggest part of the overall resistance at low speeds. Since the friction drag depends essentially on the wetted area, it is clear that a hull shaped like vertical NACA foil is not the optimum one for the low-speed regime. That's because it has higher wetted area for the same displaced volume than more common and modern hull forms (for example, those based on Delft series).

Cheers

 

[daiquiri]

I just saw the attachment in your post #12.

What you have sketched vaguely resembles a "SWATH" concept, which is a type of hull with low resistance at HIGH speeds. where wave drag is important.
But your intention is to travel at LOW speeds, where friction drag predominates. Hence, the governing parameter for minimizing the resistance will be the ratio of the wetted area to the volume displacement. In other words, you need a minimum wetted area for a given displacement, everything else being equal.

Now, if we compare your proposed section to some other section shapes, we discover this:


Of the 5 different sections, the one you have envisioned has the maximum wetted area (sometimes also called "chain girth") per unit length and displacement. And plus it is certainly the most complicated shape from the structural and constructive point of view. Which means that it will be the one with the highest cost of design, construction and maintenance.

In conclusion, it is far from being a good shape for your requirements. As Leo and Ad Hoc said - forget it and just keep your vessel's hull sleek and simple.

Cheers

 

[laserek]

Manfred,

May I ask ab more information about the hull you showed in the picture?

Ml

 

[Manfred.pech]

Manfred,

May I ask ab more information about the hull you showed in the picture?

Ml

I will do my very best but I need some time. First I saw these hulls with an interesting rig at a boat show in Hamburg in the eighties and took lots of photos. The problem is to find the diapositives and digitalizing them. Here are some more pics: http://kleinanzeigen.ebay.de/anzeigen/s-anzeige/katamaran-kajak-schwimmer/226689602-211-3358

The main problem with the hull is the low displacement length ratio - in other words not enough "load carrying ability". And the distribution of the displacement is wrong.
Do you need something like this: ??? May be better for your houseboat.

 

[Manfred.pech]

Manfred,

May I ask ab more information about the hull you showed in the picture?

Ml

Dear Laserek, was a lot of work to find the old diapositives (Boat Show Hamburg 1984) and to scan them. I think the "Seafix" might be designed by an aircraft designer. The sail (leading edge slat <http://en.wikipedia.org/wiki/Leading-edge_slats> and wing), mast, cockpit and its location and the hulls are not common for a small sailboat like this. But I do not know it exactly what the designer intended. For a purpose like yours the hulls do not have enough displacement.