# Hull design for a small displacement boat

Discussion in 'Boat Design' started by mwatts, Jul 31, 2009.

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Think you'll have a direction stability problem....too much lateral area fwd

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### mwattsMartin

Masalai: thanks. It is a lot simpeler than what you are building though... I hope this is coming near (or is) something a hobbiest could build him/herself.

Ad Hoc: do you mean she'll suffer from cross-currents? I don't know how I would fix that though. I need the displacement forward, don't I?

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Well, before i fully answer, do you understand the term, directional stability?

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### mwattsMartin

Somewhat. It indicates wether a vehicle is inclined to keep aligned with the direction it's heading (like car wheels lining up straight when you let go of the steering wheel).

But in the case of this boat, I wouldn't expect it to start spinning while the rudder is held straight . So the only thing I can imagine is it being vulnerable to wind and currents (mainly the latter).

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Well, generally speaking the car analogy is correct. However there is a bit more to it.

There are basically 4 cases that can describe directional stability, that being a boat on an initially straight course, and then encounters a 'disturbance' (of whatever nature), the effects are thus:
1) The path, or course, after the disturbance is still a straight course, but no in the same direction. This is straight line stability
2) The path after the disturbance retains not only the straight line, but also its direction. The path though is in a different location, ie parallel in some manner. Between the disturbance and the final path the boat oscillates after the disturbance before finally heading off on the final path. This is termed Directional Stability
3) Same as case #2, except that the boat does not oscillate after the disturbance, just passes smoothly to its final path, again, parallel in some manner.
4) In this case, the direction AND path are the same as that prior to the disturbance, ie the transverse position is the same, not at some parallel location as in #2 and #3. This is termed Positional Motion Stability.

All of this is 'controlled' by the location of the neutral point on the boat which is the centre of where all applied forces do not cause the boat to deviate from a heading.

The exact location of this point varies from boat to boat, it depends upon many factors, but generally the hull shape and its lateral underwater area in profile. The distance from the LCG/CoG of the boat determines how "stable" the boat is. Since a couple is introduced which could be either 'positive' or negative', between the natural point and the LCG.

In the absence of expensive tank testing one generally looks at the underwater profile, there 'should' be more lateral area aft than fwd, otherwise you may encounter case #1) or #2).

This is why placing a rudder aft improves the direction stability (it introduces yaw), but at the expense of turning. Too large a rudder can make the rate of turn very difficult, the boat is considered "too stiff". Hence if the rudder is then placed with its 'hydrodynamic centre' at the neutral point, the rudder would produce no yaw motion.

Skegs are another way of introducing lateral area 'aft' without affecting the design of the rudder and all the mechanics that go with it.

So, if you have too much lateral area up fwd, you may well have a directional stability problem, ie find it hard to keep the same path or course...as such you'll have to have a skeg or larger rudder to move the location of the neutral point.

If you consider a dart, as the simple analogy. A dart has fins at the tail section. If you throw the dart is flies easily. If you simply throw the dart "back to front", what happens, the opposite effect. Because the tendency to force the dart in a particular direction has moved from the back to the front of the dart, and more importantly the distance from the darts CoG. Since there is no other fin or 'area' on the dart to restore the dart to its original path, the restoring moment has changed magnitude (ie +ve or -ve).

So as a general guide boats with large skegs aft and a rounded forefoot will tend to be more directionally stable than one without these features. Also long slender boats (which is why L/D ratio is so important to understand, but these PlayStation non-designers cannot fathom) are also more directionally stable than short fat ones, ie low L/D ratios.

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### mwattsMartin

John, thanks for the explanation.

I am working on a new design at the moment. It will have more V in the main hull, and different proportions for the hulls. When it's done I'll post it here of course. It's profile will be different to the current design, so it will probably behave differently.

Is there a section on the forum for rudder design? That's the next thing I will be looking at after the hull has been done.

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Designing a rudder is very simple...well it is if you know what your doing. Again, don't need endless computer data....just simple pen paper and a few charts.

But you need to draw something first, before 'designing' it....as always

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### masalaimasalai

Jeees, just make a small model (about 1:10) and use model engines/sails or whatever with remote controls from your hobby shop, then you can see and feel what happens if you learn to look and understand... - JUST DO IT.... - for me, - that is enough -

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Can't argue with that. Good instructive tools..We almost always make models of our boats...usually for tank testing and then play with them afterwards.

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### mwattsMartin

I probably will make a model. The remote controls aren't a problem. I have an rc airplane here that has all the controls I'll need (8 channels.. ). You aren't supposed to operate an RC boat on airplane frequencies but for just a testing model.. who cares.

Bob, what do you use for making a model. Plywood?

You can measure the thrust the electric motor you put in the model generates. Do you use the performance of the model to get figures on expected performance of the real thing? I suppose it should be possible to calculate hp en speed using the models thrust and speed... Or do you just look at the behavior and handling of the boat (intuition)?

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