Hi everybody,
just a short return after some months…
from one removal to another I’m still getting some spare time to “rest” !!!
About Nose Down :
I would like modelers to bring their opinions about this subject analysing the “causes” and “remedies”
Among others I started with the attached sketch :
Here the same sail area is depicted with different masts. CE exact height not calculated yet, I suspect to be higher then depicted (pendulum tool still in the boxes)
ClaudioD
Claudio,
Are you asking for reasons for nose diving?
A gust hits our little boats - the light weight rig quickly accelerates, but the very heavy lead weight at the bottom of a long keel takes a longer time to get moving (inertia)
Solved by making the rig heavier and the lead weight lighter and the keel shorter (not really desirable - so accept some nose diving in gusty conditions)
Jon
some thoughts
the low aspect ratio rig (the small right one):
[ul]
[li]the main looks more like the ideal elliptical wing, so less drag and less torque
[/li][li]the more tilted jib luff should give more lift at the bow
[/li][/ul]
but concerning nose dives, both come only into play while bearing away at the top mark
Some interesting thoughts by Robert May:
‘The top sailors know that accelerating through the turn is the secret to reducing submarining and they are ‘sailing’ the submarine out of their race. This is achieved by careful rudder/ sail balance; not turning too quickly allows a boat to accelerate, turning quickly puts the brakes on and increases the submarining moment.’
Hi,
I do not believe that it is mainly inertia of the lead bulb but more the drag of hull and appendices. There are many boats around here where it depends on little deviatons if they are nose diving or surfing. If the boat is able to accelerate more quickly, it will get the bow up and will start to surf in the gust. It it is accelerating to slow it will start nose diving. Slim boats often have advantages because they usually accelerate better even if they carry heavy lead bulbs. During the last years boats have become more narrow and lead weight has increased simlutaneously from about 500g to 600-700g which helps a lot in a breeze. Nose diving has not become worse. In the early days of RG65 in Germany, nose diving was a much bigger issue than today.
Of course, a low aspect rigg lowers the center of effort which will help reduce nose diving. Thus, a conventional rigg has some advantages in a gust because the jib produces more lift. But remember, sailing a small rc-boat is like sailing a big one in honey -from a hydrodynamical point of view water becomes viscous for our boats. This is much more important …
Hi Jon,
not exactly, just to propose the readers to expose their solutions to the phenomenon if any, unless it is a military secret ! .
Personally I would suggest to start with the following points and considerations :
1 - within the class, the heavy boat will suffer more then a light boat
2 - the bow could be of tulip form to increase volume of the upper part above water
3 - a slim form hull could produce less drag then a large one but care shall be taken to avoid deep wave formation
4 - inertia ca be reduced by lowering the bulb weight
5 - The CE could be lowered as depicted in my previous sketches still keeping the same area
6 - it is assumed that the wind speed at 45cm above water is not much different from the one at 55cm above water
7 - comparative tests are needed
Interesting to see some sectional forms !
ClaudioD
Claudio,
I think it is the sudden increase in wind pressure that obviously that induces nose diving.
As Tuppesw has stated above, the top sailors try to manage this increase as much as possible.
From a hull design point of view, there are many factors that could help reduce the tendency to nose dive, but in the end, any sudden increase in pressure of a sufficient magnitude is going to cause problems.
Haegar’s comment about narrow hulls is valid. When they do nose dive a little, the drag has not increased as much as a “v” shaped hull, hence they are more likely to safely increase speed and avoid a loss of speed as they nose dive.
I would prefer forward section that was “U” shaped to promote planing and does not increase drag too much as it nose dives, rather than a “V” shaped that does not produce as much lift and increases drag as it nose dives. If the narrow deck section does go under, it often is still controllable, where the wide deck version mostly becomes uncontrollable.
I tend to agree with you Jon !
The istantaneous acceleration is the cause of nose down and all models suffer .
One should consider the various parameters :
Excluding for the moment the rig and Fin/bulb assemblies the Hull Drag, that contribute to reduce the acceleration, is proportional to the wet surface but also to the draft that create the lateral wave. Lift up/planning cannot, in my opinion considered, because the speed reached by our models is far too low.
I suggest as a starting point the choice of the Hull Main Section Area .
Here below some sketches :
fig. 1
The linear distance is the contour of the section that determine the wet area.
The 25cm² of all sections promote a hul displacement of 950cm3 about if the Prismatic Coeficient is 0.58
950cm3 is on the lower side of an RG65 displacement
This is the starting point: reduction of hull drag
Of course a compromise is needed.
ClaudioD
Hi, Claudio
from my experience hull drag due to the lateral wave (what is the English word for that?) is of lower importance for a RG65. Wetted surface seems to dominate (which is quite in line with theory (I believe). I read somewhere that in our Re-number area drag is mainly caused by friction which is more or less given by the wetted surface).
Reality shows that narrow hulls (fig. 4) have a big advantage especially in the low wind regime where again friction/wetted surface dominates. Wide shallow hull can compensate for this partially by reducing the fin area. It makes the boats faster on a run, but causes some problems when pointing. Fin area cannot be neglected - is is a significant amount of the wetted surface!
Shallow hulls perform better in strong winds - hull speed is reached anyway and wide, shallow hulls start more easily to surf (or sometimes even plane!)
All new boats here in Germany have narrow hulls now …
Hi Claudio,
I’m no engineer so forget the simple solution but in RC multi we have been adding extra volume in the forward sections of the hulls to reduce nose dive which is more an issue with multihulls and we don’t use bulbs so none of this inertia thingy !
In the IOM the trend has been to add chine, reduce beam and move more volume forward to reduce nose dive which result in loosing the control of the boat at worse. The other element that surprises me with the RG65 is that I still see a good number of them with a relatively flat deck which I don’t believe in as though the forward volume will reduce and not eliminate the nose dive, the round foredeck will help the boat to get back in its normal position horizontally quicker by reducing resistance to the water once the boat has its bow under water.
In the IOM class we can’t play very much with the rig and sails so we are limited in our options to reduce nose diving but I believe the RG65 class rules are more open in that respect and I would think that as the wind increase using a lower rig with same sail area should help, and using a shallower fin keel too but not sure if the class rules allow for that.
I’m sure there must be others but that’s my 2 cents
Atb
During my ‘flue time’ I got reading the book of Norman L. Skene “Elements of Yacht Design”
At page 90 of my edition it is reported that most of the time the Sail Area for a given Design is simply based upon what has been used on similar boats. Jut simply an empirical approach.
Skene work is dated in the years 1930 when it is notorious that racing Yacht carried a lot of Sail Area, see J Class.
One of the practices is to determine the Sail Area as function of the Wetted Area and / or Midship Section Area.
The first Ratio is varying from 2 to 4, most of the time being 3 . Ratio of 4 for used for very low Wind conditions.
The second Ratio is varying from 45 to 55, 52 being a common figure.
Now if I use the ratio Sail Area/Wet Area of 4, being the maximum suggested and applies these criteria to one of mine RG65, I get:
580cm² (wet area) x 4 = 2320cm² of Sail Area
The RG65 Rules call for 2250cm² maximum Sail Area
This is suggesting that the actual RG65 Sail Area of 2250cm² it is really adapted to very low wind conditions.
A ratio of 3.2 would be a better choice to avoid loss of stability, including reduction of ‘nose down’ phenomenon !
i.e. 580cm² x 3.2 = 1856cm² instead of 2250cm² as per Rules
ClaudioD
By my previous message I was only suggesting to give attention to excessive sail area.
Rarely the wind will blow below 1 o 2 kt and surely is not under these conditions that ‘nose down’ will occurs.
My understanding is that the actual RG65 Rules are not suggesting anything about the Wind Range vs Sail Area as it is done on some readings about Marblehead with 6 diferent Rigs.
My conclusion is that 2250cm² of Sail Area shall be handled with care !
I would probably doing the same things for other models that in general are using too much sail area !!
ClaudioD
We had a big discussion the other day on when to switch to B rig. (talking RG65’s) We all agreed that more sail area means faster boats. But at some point you have to make the switch. We had two different lines of thought:
We did not reach any concensus… Would like to hear your thougts on this, please…
Generally speaking someone wrote, I forgot the name, saling models shall not heel more then 30° for more of 10" under puffing to avoid loss of waterlines form and drag.
Over that limit, better to change the Rig.
All other being equal !
ClaudioD
As another consideration about ratio : Sail Area vs Weight .
I assume that the heavy boat would requires the max Sail surface. Being heavy, will have larger inertia under gusting conditions.
The second one being lighter, the sail area could be reduced, the CE lowered as well the inertia.
I would be interested to see what can be gained or lost between to RG65 , one weighting 900g with 1850cm² of sail area and another weighting 1100g with 2250cm² of sail area.
Both sailing in the range from 3Kt to 8kt.
ClaudioD
about nose down…
there is 2 boats from joysway - dragon force and explorer.
this is a perfect example for comparison about nose dive, because those boats are 100% equal.
(same parts, same rig, same keel etc…)
with one difference - explorer has a wider hull. (not only stern part but front as well)
the result is that the explorer almost slides on the water (plannig?) while dragon force immersed as submarine…
aXon
(sry for bad eng.)
aXon,
what are the respective weights , sail area and bulb weight ?
ClaudioD