Friday, November 23, 2012

Sailing characteristics, now what?

So, should I do something to the boat as a consequence of what I've learned?

In light air, I can not see room for any improvements, except perhaps that a partially deployed lifting foil would improve helm balance under screacher. And keeping the weight low would probably be crucial.

Upwind, the boat tends to sit on it's stern.  This is, as I have seen it from postings on the forum, a well known characteristic of the Farriers and the remedy is to move crew forward, especially in lighter wind. I think that a longer float stern, with a bit less rocker would also improve this aspect.  All in all the boat behaves and performs very well upwind.

As I mentioned in my previous post, reaching is where improvements can be made. Again, the boat is very stable and safe as long as the speed is kept under 15 -17 knots or so, but when pushing hard, more longitudinal stability is desired.

1. Water ballast.
It should be room for about 50 litres of sea water under the swim step.  This would be equal or better than one extra crew on the aft beam, and I expect this will be of significance.  I've never used any water ballast system, and can not really imagine how to operate it.  Farrier says "Fairly simple to do, by just using dinghy bailers",

but this do not make me any wiser.  I don't want to disturb him more than necessary in this stage of his production set up either.   It seems to me that such a bailer would have to be operated from inside the tank, and then it do not fit my perception of fairly simple.  Any tips/suggestions/experience with such systems are very welcome!

2. Lifting foils.
This would obviously be a nice feature.  Downsides would be weight, cost, one more thing to operate. The advantage seems, from reports from the Dutch F-32SRC, obvious, and just in the conditions I want it.
Ian will make me foils and cases, but with shipping and Norwegian taxes this will be a considerable expense.  Designing lifting foils by my self is a very uncertain project with respect to efficiency, and with the demand of accuracy in the building process in such parts, the production will be time consuming and expensive.  Jury still out on this, but if I add foils, it will be Ian's.

3. Modification to float shape.
As I had these damages to the port float, with cracks/holes in the outer laminate at at least seven places, repairs and re-fairing/re-painting of this float has to be done anyway.  Also, last year I rebuilt the net support, and this will also be happy with a round of fairing/painting.

Which have led me to start playing around with the float shape.  To be able to put any useful volume in, I find it necessary to lengthen the floats to match the main hull.  Also, make plumb bows.  Here is one suggestion, also showing my bow art

And a bit refined, with more rounded top, and also planning to add a 40mm thick rounded deck forward of the forward horn cleat, and make sure the shape will easily get back up when submerged.

The challenge here is to make modifications to improve what I want changes to, but leave the things that work very well as is.  This means no volume added all the way down, as this is what is used in light airs.  Then, when moderately pressed on a reach add all the volume possible, but not necessarily so when pressed upwind.  So, obviously impossible to get totally right.  I might end up having to add a bit at the stern as well to keep the balance right. This will also give more longitudinal righting moment, but possibly lead to a bit adverse effect on wave handling.

Possible modification of stern keel line

Here are some ideas as to where to add volume:

The bow bulkhead cross section.  10mm of foam to be added to the shape on the original plan, to the left the suggested surface shape at that station. I have not yet been able to calculate how much volume added this will be.  Did some rough calculations this afternoon, and it seems that 20 litres in the front two feet is very realistic.


CrashGybe 22 said...

Hi Tor,

The bow mods look good and somewhat familiar, but I am not sure that the additional volume will produce the required force to stop the bow bury. One suggestion not on the list are bow fins. here is not a great example but you will get the point ( pardon the pun )

Question really then becomes can they be made in such a way that produces the required lift and looks the part without interfering with the bow down when going into wind?

I am still thinking foils but having put float to hull yesterday, the room to move is a real problem.


Tor Rabe said...

Thank you Andrew!

I'm afraid I'll have to disagree.

20 litres of buoyancy as far from the COG as possible will likely make a significant difference, adding another virtual crew at the aft beam.

Ideally, I'd like 50 - 100% increased volume in the floats, and float rudders, but then it would not be a folding boat.

Regarding bow fins, I've considered that. I'm sure Murphy is out there sailing as well, and sooner or later I'm afraid such fins will contribute to the ultimate nose dive.

The only viable foil option is C-foils, the way I see it.

The one thing not on my list is canting rig. This will reduce down force and hence make effective float buoyancy higher.

CrashGybe 22 said...

Hi Tor,

You have a great point about the additional buoyancy and distance from COG and yes the bow fins are also the final straw when it really turns to custard.

I agree on the foils but the C,L, I or T config is the question. I figured that I could make the C section in parts but Silas should be able to eat it with his CNC though. The L has some control advantages and the T is a proven design but they both have issues with trailer and folding.

I guess we are in the same boat, on opposite sides of the planet so to speak.


Tor Rabe said...

This time I totally agree! What I meant about C foils is the total of pros and cons for me. Essentially, that's the only solution to keep the boat foldable and trailerable, except I-foils which I consider less user friendly and almost certainly less effective