Centreboard optimisation

[abindoff]

Hi all, I'm newly registered here but have been browsing for a longer time.

I have a Jim Young designed "Rocket 5.5" trailer sailer which I bought from the previous owner and builder. He has moved and I can't get in touch with him to ask some specific questions so I thought I'd try to figure a few things out. I can't locate plans or even a sail plan.

The hull is very light displacement, and has a high freeboard. It bites in a little better when heeled, but not much. I am particularly fond of this boat, though. It is fast and surprisingly sea-kindly for it's type.

The builder used an old Hartley rig which had a CE further forward than the designed sail plan. He moved the centreboard case further forward about 3" to compensate, which he got spot-on as it's very well balanced.

However, the centre-board looks too short to my eye and we make too much lee-way. I don't race, but it appears that we're not pointing as high as others but I'm not sure that a boat of this type will ever point as high as a larger, longer boat? (We're about the smallest on the river, so all boats I can compare to are larger and longer!)

Due to the centreboard case adjustments, there is plenty of room to install a deeper centreboard of the same width. Would there be any issues I should be aware of if I followed this path? It could only be a good thing at the end of the day, surely?

The boat is trailered so it's a bit hard to see, but the glassing on the centreboard appears to be starved of epoxy. If the whole centreboard were rough textured, might this affect my pointing ability? Or would it just slow us down a little?

If I were to build a deeper centreboard, am I right in assuming a NACA 0012 section would be a good choice?

The rudder is also a bit odd looking but it works quite well. It's a Hartley rudder cut in half lengthways and sanded to a knife point on the leading and trailing edges. It only vaguely resembles a foil and is frighteningly shallow. Despite this it has no major vices.

I was wondering what happens if you increase the centreboard area, do you also need to put a corresponding increase in the rudder area?

Thanks in advance for any assistance. I hope to be able to contribute something some day! I've designed a sea kayak which I love; despite it's insistence never to change course. I just point it where I want to go before I launch and we get there without breaking a sweat.


Regards,
Aidan

[tspeer]

I'd go for the deeper centerboard. You could keep the same area and make the chord smaller, or just make it longer and have more area. You haven't mentioned that it's stalling out on tacks, etc., so it sounds like it has enough area already. But a longer board will reduce drag and help with pointing.

The NACA 0012 is a pretty good choice. It's a good all-round section.

It's much the same story with the rudder. If it's deeper, it will be more efficient.

[abindoff]

Hi Tom, thanks for your help. If I make the centreboard larger in area, does the rudder need to be proportionately larger?

So will a longer centreboard improve pointing even if it is the same area as the original? If so, why is that?

Just in case anyone is wondering, more draught will not be a problem with this boat where I sail. Unless I forget to go about...


Regards,
Aidan

[frosh]

Centreboard and rudder area both contribute to lateral resistance and so by altering the area of one foil only will change the position of "Centre of Lateral Resistance" CLR. However depth of foil change without change of area will not change the CLR but generally higher aspect foils are more efficient and a centreboard so altered to be longer in relation to chord will allow the yacht to point higher. It will also lead to greater heel force generated by the rig for a given wind strength as the distance between CLR and CE of the sails will now be larger. Rudder foils need to be well foiled with a fairly blunt rounded leading edge. Chord thickness percentage should be larger than for a centreboard as the angle of attack of the rudder will nearly always be greater than that of the centreboard. Thin, or poorly foiled rudder blade sections, or with a sharp leading edge will still steer OK but drag force will be high slowing the craft down. At higher boat speeds the turbulence created by the rudder of poor section can make steering feel heavy and less controlling.

[Tim B]

Be careful not to confuse yourself on rudder characteristics, frosh.

Rudders often operate at (a minimum) of 1/2 Leeway angle. This can be a couple of degrees before you start to manouvre. The keel operates at more like 7/8 Leeway angle. (these are rough numbers based on one hull-shape!!!!)

The keel can actually be quite a thin section (perhaps 9% thick), but the rudder would probably benefit from being thicker. The limiting factor is the load case to right the boat. The important thing for the rudder is to find a section that will work well at high angles of attack. You can do this with X-FOIL (but remember it doesn't do cavitation) or CFD (some solvers will handle cavitation).

It is quite a tricky question, because you can't really seperate it if you want to answer the whole question. You also need to model the rig to get the centre of pressure, and whilst it is quite possible to do the whole problem, it's a pretty big job.

So to simplify it, we make assumtions of the operating conditions (as I did at the top of this post), then use 2D theory and the results of lifting line theory to come up with an answer. Because we can consider it to be a basically steady problem the results are actually quite good. What we can't do by that method (or at least, only geometrically) is to balance the boat.

Anyway, generally an increase in centre-board area will bring the CLR forwards, so the boat may have more weather helm. It will also lead to a reduction in leeway angle, so the rudder (if central) would encounter a smaller angle of attack. So you'd have weather helm to counter that. Alternatively, a camber-changing flap on the trailing edge of the rudder would be a very slick way to overcome the problem.

There's a lot of answers to a simple question,

Tim B.

[abindoff]

Hi Tim and Frosh, thanks for your replies. I'm definitely getting a deeper understanding of the physics of this problem.

Taking stock here; the boat is currently well balanced, and the centreboard is at least an efficient profile (made to Jim Young's spec).

So, if I fabricate a longer centreboard of the same area I go some way to increasing my pointing ability but move my CLR forward.

If I fabricate a longer rudder of the same area with an improved profile I go some way to moving my CLR aft and improving rudder efficiency.

One thing I didn't mention is that the centreboard case is disproportionately longer than the distance it was moved forward (as if it were designed for a longer foil!), so I could, if absolutely necessary, move the centreboard aft (but not forward).

I could also build the rudder a little longer, test it, then start lopping the end off until the balance felt right, would that be a valid approach?

Regards,
Aidan

[Raggi_Thor]

One important issue that many forget is that it's no good to change the planform shap if that disturb the profiles. I mean, if you choose naca 0009 for the keel and naca 0012 for the rudder, each and every section should be perfect. Many sailers have tried to make a keel or rudder "elliptic" when that was a fashion, and in the process ruined the shape. Hpoe you see what I mean :-)

[abindoff]

Hi Ragnar, I think I know what you mean. I like a rectangular planform with a rounded tip so that the foils will kick up more easily if I hit something.

If I chop it up into sections, each section should have the correct profile regardless of where it is on the planform. Is that correct?

What I SHOULDN'T do is make a rectangular planform, shape it, then make it elliptical in planform. Yes?

As for the bottom tip, do we cut these square now, or do they get rounded under?


Regards,
Aidan

[Raggi_Thor]

I think you got it :-)
If you round off the tip, check the profiles with some templates.

I think square bottom tips is OK, but it's a lot of opinions...

[tspeer]

Originally Posted by abindoff

...If I make the centreboard larger in area, does the rudder need to be proportionately larger?

Best way to find out is to try it. Put in the big board, and see how the boat balances and maneuvers.

So will a longer centreboard improve pointing even if it is the same area as the original? If so, why is that?

Same lift (that's determined by the sail trim), less drag. There are two main sources of drag for a board. One is the profile drag - mostly skin friction - so if you keep the same board area, that doesn't change much. The other source of drag is induced drag. It is proportional to the square of the lift on the board, but it's inversely proportional to the square of the board's length. And doesn't depend on the area. So, since lift is the same, the longer board will have less drag.

[abindoff]

Thanks everyone, this is fascinating stuff!

Sounds like a worthwhile winter project. Building the rudder too long and chopping it down seems the best approach. I guess there is no need to round the tip off, it's going to kick up either way.


Regards,
Aidan

[frosh]

You will be on the right track starting with a new rudder blade that you believe will be too long. I have been through the same exercise. First time I went sailing with my new long rudder blade my boat kept trying to go off the wind. After shortening my rudder blade and sailing again a found that the balance was about right. If you want to glass the blade hold off until you get the area right. You can just put 2 coats of resin in the wooden blade while testing.