High Speed Rudder issue....

[CT249]

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I'm sure you're right, Marmoset. The foil shapes are tailored to the boat's characteristics and use. But if high aspect rudders (as in the F18) were "more draggy", as claimed, they wouldn't be used in high performance racing boats.

A while ago we had a line-up of foils at a cat regatta - my 1993 ones (from a boat of similar size, weight and speed to an F18), a set from a Hobie Tiger F18, and a set from the latest Capricorn. The increase in aspect ratio over 20 years was remarkable, and I cannot believe that people kept on going for higher aspect rudders if 1:1 would have been just as good.

 

[powerabout]

F18 foils would have gotten longer except the class finally capped the over all length

 

[oldsailor7]

Chris.
The difference there, is the use of high aspect ratio rudders, (or boards), in high performance boats.
Of course they are more efficient, as in long winged thermal gliders. But they have to be carefully handled at high speed in order not to stall them. The point I was trying to make was that the lower aspect ratio shapes were more forgiving, for instance, in the hands of a pleasure sailor, where the extra drag of large tiller movements is not a terribly great concern.
It's a simple fact that even you cannot deny, that higher aspect ratio foils stall at a smaller angle of attack than lower aspect ratio foils of the same area.

 

[CT249]

Chris.
The difference there, is the use of high aspect ratio rudders, (or boards), in high performance boats.
Of course they are more efficient, as in long winged thermal gliders. But they have to be carefully handled at high speed in order not to stall them. The point I was trying to make was that the lower aspect ratio shapes were more forgiving, for instance, in the hands of a pleasure sailor, where the extra drag of large tiller movements is not a terribly great concern.
It's a simple fact that even you cannot deny, that higher aspect ratio foils stall at a smaller angle of attack than lower aspect ratio foils of the same area.

I would never deny that low aspect rudders have wider stall angles, but you were claiming that high aspect foils are "more draggy" which is not the case. Even 14' surfcats have moved away from 1/1 aspect ratio rudders.

 

[powerabout]

I thought I had to be careful at low speed with my high aspect foils to prevent stalling
high speed is a cake walk

 

[Marmoset]

I don't think I was right about anything! Lol I was throwing out what came to mind *** examples to help me grasp the concepts. So many times we get something that's the most power best this or that, but context has everything to do with application. Example: someone says "this new engine has 1000 more pounds torque than any other, so it's the best" but maybe it's diesel and would pull more hay than anything on the planet, but it wouldn't make a light dragster do a 5 second 1/4 mile.




Barry

 

[oldsailor7]

Quote: but it wouldn't make a light dragster do a 5 second 1/4 mile. Quote:

But an electric motor would.

 

[Marmoset]

an electric motor would pull the hey and run the 1/4! lol but lets not get into what batteries it would take to do it! haha


Barry

 

[paxfish]

an electric motor would pull the hey and run the 1/4! lol but lets not get into what batteries it would take to do it! haha


Barry

 

[Marmoset]

ahhh haha! ya made me spit my coffee!


Barry

 

[brian eiland]

I've not seen anyone suggest raking the mast aft a wee bit.

Something I learned from Randy Smyth (Olympic silver medalist) when I was dealing with beach cats, ....rake the mast aft and kick the rudders forward to obtain a 'powersteering effect' that cuts down on the weather helm you introduced by raking the rig aft.

I suspect you might first rake the mast aft a wee-bit, since you already have the rudders kicked forward.

You might also consider adding a shim piece under the mounting of your lower gudgeon-to-hull connection, effectively removing some of that rudder rake (if you don't care to rake the rig).

Fences, etc, I don't think are your answer. KISS

 

[Erwan]

What is a "Powersteering effect" please ?

 

[redreuben]

Erwan.
Balanced rudder, that is, the area of the rudder ahead of the shaft "assists" the rudder to turn, the greater the area the more assist until it starts to snatch control away from you, too little area and you labour harder to turn the rudder.

 

[oldsailor7]

Well I think this thread has thrashed out the subject pretty well.
It seems high AR rudders are indeed more efficient for higher speed multis but are subject to stalling and structural problems under heavy side loads.
Lower aspect rudders however are more forgiving, work well, and are less likely to sustain impact damage.
IE:- "Yer pays yer money , & takes yer choice", as the old saying goes.

 

[Erwan]

Thanks Redreuben