Friday, July 25, 2014

more & more &more = same as before

I spent one of the last "holiday" days sailing evolution, with a rigg, in conditions above the riggs range(10-15 knots). Hiding behind a dock, where the wave was relatively small.
The reason for sailing in in a windy but waveless spot was to observe,by eye, the behaviour of the boat under lots/excessive power.
By eye it seemed that upwind the boat would sail just as fast as it would with a well trimmed b rigg, perhaps there was more drift. But i think that the same boat with both riggs would perform identically.
Downwind the boat sails fast, but even at top speed the sails stayed tense, full. Delivering power that was lost because the boat would not accelerate more.

I was pushing the a rigg to the limit.
What is this "limit"?
This is a wavemaking drag barrier.
According to mr. Edmond Bruce, author of "design for fast sailing" by AYRS, the wavemaking resistance barrier, displacement boats face when sailing close to hullspeed, is less noticable when Lenght/beam ratio of a slender hull reaches 8. This "barrier", or a steep hill in the wavemaking resistance ploted against speed graph almost disappears when a hull reaches l/b=11.

I will use this post to go back to basics and come up with two ideas for future F5g boats.
Both ideas have been realised in the "big boat" world but have not been pushed to such extremes due to structural and seaworthiness limitations.

The goal is to make a fast boat.

In order to make a fast boat we need a force to move the boat.
Here the rule is simple, the more power(sailarea) a boat has, the faster it will go.

Some will argue that if the sails are more efficient a boat with less sail will sail just as fast as boat with more less efficient sailarea. Wing theory proves that, but in the real world a boat with more sail will be the first to start reacting to wind, if there is little wind. A boat with more sailarea has more tourque powering(moving is perhaps a better term) it through waves. The boat with more sail can take the wind from the boat with less sail etc.
The above are race proven facts that are true most of the time on a race course. But there are some drawbacks of carrying lots of sails.
Since we are talking about model boats that measure less than one metre in lenght, longitudinal stability is an issue. Our models tend to sink the bow, submarine so to say. Footys and IOM boat suffer from this greatly. In f5g i have succesfuly adressed this issue with a foil attached to the rudder, the foil is inverted, so it is pulling the stern into the water.
Yes this foil increases wetted surface and induced drag, but the area of sail i can carry with this rudder is much much bigger. Example: Fila has a5500cm2 a rigg, without the foil it can carry its rigg to about 8-9knots of wind(average wind) with the foil i sailed downwind succesfully in winds of 14-15konts! Evolution can sail with more than 6000cm2 up to 15knots of wind!(it has more bow-volume and a longer keel than fila)
The second issue is transverse stability. With no moving ballasts the most effective combination is a wide hull and a long keel. The combination of both gives the boat a satisfactory counter-heeling moment.
Big sailarea requires a wide hull and/or a hevy bulb. We can fine tune this combination to give good results with the chosen sailarea.
Until now i have never asked myself how wide can a monohull be? The idea of a 750mm 400mm wide F5g scow type yacht is interesting, but hullweight will probably be the downfall of this project.
The problem with wide hulls is that they all make a deep/high wave, causing a lot of distortion over a large area. Despite the consequental high allup weight such a boat could carry lots of sails and putt other f5g boats in shade. Planning potential downwind would be good, but upwind under heel it would not sail past hull speed leaving it just in front of the rest of the fleet.

Without moving ballast i can't see another way around this dragbarrier problem caused by normal displacement hulls of our model boats.

With an aerial canting keel/ballast arm and a narrow hull with a l/b ratio of 11, the boat could reach it's speed potential. The arm extending to windward would keep it right side up sailing upwind.
A rudder foil would keep it from digging in when running. Transverse stability downwind is a challenge. Sailing downwind the arm would be recracted to the middle of the boat so that the CG of the boat be about 10% behind the LCB, this gives good rudder response and keeps the bow from dragging too much. But the ballast would be placed relatively high, on the deck.
A gyro stabilisation for downwind sailing would be needed. Or capsizes would be inevitable unless some lead would be put into the keelfinn, which has to be present anyway(not really, a fore rudder could replace it, but for regata use a fore rudder is one channel to much for my concentration, and it takes away the good feel of a well balanced boat. Just like 4wheel drive takes away the livelyness of a 2wd car)
If only possible, putting led into the keel and having movable ballast at once should be avoided.
Because if you go through all the trouble of making a movable ballast sistem work, then you want all the ballast you have to be active, for maximum efficiency. The lead in the keel would be dead ballast just sitting there to save the boat from capsizing in extreme conditions.
Dispite the above practicallity along with reliability has to have the upper hand and a boat has to be desingned so that it will complete sailing the course when sailing as hard as required with no technical issues or capsize recoveries.

This was a thought about a fast monohull, to sail in a displacement or semidisplacement mode at planning speeds. For the first time i realized that it is almost a waste of time to work on "classic" type keelboats in an open class like F5g where nearly everything is allowed. IF we take a look at iom or marblehead where the rules are much more defined and refined, well that is another story.

Sunday, July 20, 2014

Evolution, stuff to improve

Salt water took its toll
 Jib sheet tube broke of the deck, it is fixed with bison, plasteline 5min epoxy, it is rock hard and great to sand! At the same time i reinforced the mast ram from beneath

 managed to tear the deck, when i was trying to put the iphone inside to measure speed and course..
..the tear is minor and will befixed when i get home

Below is a crack in spray fillar, many such cracks appeared where i haven't sanded it down yet, the cracks are due to the thickness of the layer  and is not structural, this was verified by sanding some down to bare glasfibre.
Sailing in good wind here in premantura is great fun, even with an unfinished boat :D

Friday, July 18, 2014

Getting close with Evolution

Still here in Premantura, Croatia.
The wind is ok, up to 8 or 10 knots everyday, the wave is unnoticable.
The conditions are ideal for A-rigg. Thiis are the conditios that f5g boats love.
Since these are very light boats with lots of sail-area they don't really like waves and riggs have to be change quickly.
But I desing and build my boats to suit in these prevailing conditions here on the croatian an slovene coast.
After three days of focusing on sail trimm Evolution handles like Fila only better. It has slight weather helm and points high, just like Fila. Unlike fila it's cg is far, hydrodinamicaly too far back.
Meaning that the bow likes to clear the waves, the cg position combined with a short distance between the rudder and the keel finn makes the boat happy to tack, and lively and very fun to sail.
It really feels almost skiff-like. The funnest(most fun) rc sailboat i have sailed so far!
The force from the sails seems to put the boat on the predicted waterlines as soon as there is some wind. I light airs this boat Will be probably slow, because the stern is about 7mm under when the boat is static. -as soon as you can feel the wind on your neck and the boat heels it sails beautifully.
It seems that cg far aft is a helpful tool when dealing with a boat whos' waterline is short and caries lots of sails.
to the pictures..
this is the max reasonable heel angle for this boat, above that the deck gets flooded.
That must be more than 30deg, lots of heel for such a wide-ass boat to sail normally

it points really high, with less nose down than fila!

but the wave crest is quite high, this boat definatelly won't exceed the hullspeed by much when saing upwind

the max "resonable" heel angle again, the rudder is getting some air, which offcourse is not good, but even worse the keel is already starting to show. No bubbles seem to travel down the no ventilation it seems...
the waterline is very narrow and the hull is pretty deep when sailing like this

the boat reaches speeds of up to 5km/h in such conditions with no wave, this seems to be a limit.
the rudder foil keeps the stern down, but if you look closely there is very little water unnderneath it

in a gust it just keeps going, if i don't pull the sheet in the main sail trimm goes shit, like here

too much heel, the back 15cm of the deck is getting washed.. this is a common site on most rc sailboats

when really powered up and pointing high the top of the jb depowers and interestingly the square top of the main starts fluttering so you can hear it.

bearing away...

..foil kicks in when the speed is up, and bow goes up in the next moment

Tuesday, July 15, 2014

The sea and the salt featuring evolution

At the moment i am in Premantura, Croatia near Pula.
Evolution passed it's first hours sailing in saltwater with no problems.
Today the wind was up at 11am gusting just above 15 knots with about 20cm waves.
Perfect conditions for f5g b-rigg.
I was sailing evolution, with a b-rig borrowed from Fila, and a new slightly bigger rudder with a foil.
The boat is very fast and ready for racing in such conditions.
upwind is the usual; hit hullspeed, point high, work the waves..
Here are some pictures:
downwind is a blast, the boat actually starts planing(yes it doubles its hull speed which is offically considered planning)
in hectic conditions the foil keeps the stern planted, so far i haven't ever stalled the rudder or lost control downwind!
picasa is shit software for editing photos..
main sail trimm for downwind is something to improove, the spreader is in the way of the sail, and i have no clue how to solve that problem, yet
this is still happening when bearing away..
no big deal though.
this is the new rudder, it is larger than standard, the foil attached is a hq3.5_10% thickness low renolds airfoil originally for model gliders 

previous experimets showed that an angle of attack of 1-2 deg is enough to keep the stern planted, so this 100x60mm foil is tilted 1.5deg downward.
So far so fun, thw boat is showing some cracks around the mast ram, these will have to be fixed before painting.
I am happy with the boat.
I will keep collecting speed and course measurments with the mobile phone inside the boat for the next week or so and come back with another post with a more numerical approach to the evolution of F5g sailing yachts.