How important are shiny-smooth foils?

Greenheart

Greenheart

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I was sanding excess gelcoat off my repaired rudder blade and trying to detect improvement, when it occurred to me that the fine-grit sandpaper is probably smoother than the rudder.

It still works, steers satisfactorily (although I don't know how much better this boat might have steered), but how important a factor is the enviable glossiness of new foils/hulls, in getting the best performance possible?
 
Foils and bottom are the last thing I would do after crew work is spot on and rig and sails are tuned to perfection. Not important except for high level racing in other words, and you'll only get annoyed if you scratch / chip perfect foils.
 
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10% reduction of drag coming from the friction of the foils, perhaps (depandant of boat) a total drag reduction of 2-5%
 
europe172 said:
10% reduction of drag coming from the friction of the foils, perhaps (depandant of boat) a total drag reduction of 2-5%
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The thing is what finish compared with what finish???

The smoother the better but I would not bother my arse to polish them...

If they are wooden a light sand down and varnish/ gloss would do would do, the biggest difference on the rudder may well be steering and efficiency at steering. A few scratches are not going to be significant but if it feels like sand paper....

Then again given a choice between acceptable days sailing and a rough rudder I would still be sailing...
 
If the foils are super-glossy, weed slides off easier.
The down side is sailors trying to right the boat also slide off easier.
 
Thanks for that. I won't stress. My neighbour looked askance at the blade as I sanded, but he's a scooter-restorer, and a total perfectionist...so his suggestion that I paint the rudder will be ignored.
 
Get your rudder perfect, and treat it like a glass sculpture. Get it smooth, for light air speed. Get the rake right, spend time on getting the stop and purchase right so it cannot move 1mm fore and aft in any conditions. If there is any play, side to side movement between the blade and stock, fix with spacers and bolt tension, so you can only just get it up and down. Check for sloppiness in pintles/gudgeons, replace if necessary. For high winds, get the shape of leading edge right, will help prevent spin outs, broaches and death rolls, get the trailing edge squared off SHARP, to prevent the annoying buzz. And make sure your hold down clip is effective and in good condition, or you will loose the lot.

Once you have done all that, keep it in a foam bag all the time when not sailing, rinse it with fresh every use, don't drag it over the seabed, or up the slipway, and keep it safe in your bedroom when not in use.

I wet n dry to 1200, as I enjoy the emerging beauty, but all accounts say anything over 400 is a waste of effort.

If you don't care about the colour of repairs, or just need a good solid base to a bigger ding, I use epoxy metal from B&Q, you just cut a slice off the sausage, kneed it until even colour, and press it into the shape you want. Wait an hour or 2, then file and sand. Easier than mixing small quantities of filler or gel, strong, hard, and sticks well.

I like a good rudder. I can enjoy a boat even with shot sails, and blocks coming away in your hands, but a bad rudder ruins it for me.

Anything else?
 
I understood that polishing increases surface tension - which isn't desirable. Next time you polish the car then wash it, look at the water which gathers in droplets and refuses to run off. Without polish, it does run off.
So smooth is good - but smooth and polished isn't... I could be wrong though!!!!!!
 
Iain C said:
...from the man who leaves the main cleated and fitted rowlocks... ;-)
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Bally cheek! :D

ProMariner said:
Get your rudder perfect, and treat it like a glass sculpture. Get it smooth, for light air speed. Get the rake right, spend time on getting the stop and purchase right so it cannot move 1mm fore and aft in any conditions. If there is any play, side to side movement between the blade and stock, fix with spacers and bolt tension, so you can only just get it up and down. Check for sloppiness in pintles/gudgeons, replace if necessary. For high winds, get the shape of leading edge right, will help prevent spin outs, broaches and death rolls, get the trailing edge squared off SHARP, to prevent the annoying buzz. And make sure your hold down clip is effective and in good condition, or you will loose the lot.

Once you have done all that, keep it in a foam bag all the time when not sailing, rinse it with fresh every use, don't drag it over the seabed, or up the slipway, and keep it safe in your bedroom when not in use.

I wet n dry to 1200, as I enjoy the emerging beauty, but all accounts say anything over 400 is a waste of effort.

If you don't care about the colour of repairs, or just need a good solid base to a bigger ding, I use epoxy metal from B&Q, you just cut a slice off the sausage, kneed it until even colour, and press it into the shape you want. Wait an hour or 2, then file and sand. Easier than mixing small quantities of filler or gel, strong, hard, and sticks well.

I like a good rudder. I can enjoy a boat even with shot sails, and blocks coming away in your hands, but a bad rudder ruins it for me.

Anything else?
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Good grief, ProMariner!! I had put the thought to bed, content that an iffy rudder wasn't going to make much difference.

Assuming I develop even a fraction of your passion for hydrodynamic perfection, I'll still have a stack of questions.

How will I KNOW, whether the rudder blade's rake is spot on, or other than correct? Presumably fixed rudders are designed with this perfection in mind, rather than the compromise which allows the blade to kick up on grounding?

If my rudder wasn't perfectly symmetrical in section when it was new, presumably nothing I can do with filler and sandpaper can put it right?

I'd been advised hereabouts, that a really fine trailing edge isn't recommended. Presumably because it'll be vulnerable to damage. I think my trailing tip is about 2mm thick...I doubt the Osprey (in my hands) will ever top out at much over ten or twelve knots...how critical are imperfections at these speeds?

My leading edge ain't great. Realistically, do you think it absolutely needs to be as good as possible, or might you concede that in truth, your approach is...a tad monomaniacal?

No offence intended, just a question! I really don't know, so your advice is gratefully received, thanks. :)
 
Yep Dan. Forget the esoteric questions.
Do you have a security lanyard on your rudder? Will it leave the boat and sink if it comes off the pintles?
These are the things that should be occupying your mind.
 
Sailing boats is hard enough, why make it harder with bad gear? Ok, I probably put the same effort into my rudder as I do the rest of the underwater boat put together. Because this is the bit I have the most tactile contact with? I don't honestly know. But, with any boat, once you know it works kind of like a sailing boat, it floats, the sails look a bit like sails, and it seems to get blown along by the wind just fine, next you have to start somewhere in making her a happy boat, as she will repay you ten fold.

Everybody starts in different ways, some on upholstery and curtains, some on safety kit, some on cup holders, some on sail wardrobe. I start on rudders. It is quite cheap to make a useable rudder into a good rudder. If you care for it after, you only have to do it once, and it will look after you from then on.

I start by filling the biggest, most obvious damage, and building up the inevitable wear from slipway dragging. Easier at this point to remove the blade from the stock. I then hit repairs with files and 80 grit used wet, with a block, trying to get high spots to match original shape without altering the good surrounding surface, or letting my repairs 'grow'. I then hit the whole blade with 120, this will highlight any low areas, gelcoat these and 120 again before continuing. Be very gentle at the leading edge, use your bear hands to find lumps/hollows here, and never cheat by sanding without the block. On the trailing edge, never sand round the corner, it is like sharpening the back of the chisel, always with the chisel flat to the stone. You do the trailing edge last. By this point, you have a smooth surface all over, work up the range of wet and dry, one step at a time, you will only need one sheet of each, used in 1/4's, on your blade, each fineness of paper will take quarter of an hour. Repeat till 400 grade, or finer if you are bored. Use lots of water, warm is nice, a drop of fairy helps. To do trailing edge, clamp blade leading edge down, and run sanding block or board perpendicular to the long axis, until you have a nice sharp pair of 90 degree edges. If the flat back edge ends up anywhere less than 5mm wide, it will be fine. Razor sharp knifelike trailing edges last 10 seconds, squared off lasts for ever. Cut blade with rag and cutting paste if you like, wax with any old stuff if you like (will make it easier to keep nice).

Reassemble, sail. If the steering is not finger light, and you have the rig in the right place, angle the rudder a touch forward. If the steering is dead and too light, angle board aft a little. When you have a weight of helm (with boat flat, really flat) that pleases you, make a stop on the stock / rudder that stops the rudder angling too far forward. If you have a hold down line lead to the tiller, this needs to be good condition, low stretch 5 or 6mm line, the cleat needs to be grippy, and you need to pull this line as hard as you can after you launch. Most boats use a 2:1 for this, to get the rudder downhaul tight enough. If the rudder comes even a few degrees up, either by speed or weed, the steering becomes very heavy, sometimes resulting in swimming.
 
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Wow! Can I send my rudder blade to you for a service, ProMariner? ;) Maybe at the end of the season, as a winter project?

I'm excited by the idea of being so handy with the tools and techniques...but I know that my best efforts tend to look terribly home-made, and not in a good way.

I'm also aware that while getting to know a new boat which is likely to overpower me often, damage is likely, even to parts which aren't honed to delicate perfection. The top of the blade took a deep, fingernail-sized chip just sailing the other day in a very low-stress situation...

...so, with sincere thanks for the detailed advice, I reckon I'll happily rough it, for the time being. If I find the boat unaccountably unbalanced, or if the tiller stirs unhappily at speed, I'll look again at smoothing the rudder down.

Thanks again. :)

PS, yes, the rudder is very secure on its pintles and gudgeons...but I may add a cord to keep it safe.
 
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Without wishing to rain on promariner's parade, super shiny foils are actually a bad idea. Both my 49er and Moth have matt finished foils. Fair, yes, shiny, no.

Moths are right on the limits of foil technology due to the speeds involved. Put it like this, a mate of mine sails on freshwater, and sailed through the winter, and was having massive cavitation issues on his new boat that he wasn't getting during the summer. His foils were too shiny...he wet and dry'd them...problem solved.

However Dan, with respect you are asking whether a Landrover with knackered suspension towing a trailer would handle better with 35.8 psi or 36 psi in the tyres. You have many more ways of getting the boat to go faster than worrying about the foils right now. Once you have got a trapeze harness and spinnaker and sail the boat bolt upright and nail every manoeuvre and you're good enough that decking the foils by accident will simply never happen, and it's the BOAT holding you back, come and ask again!

Just go sailing!
 
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Iain C said:
Moths are right on the limits of foil technology due to the speeds involved. Put it like this, a mate of mine sails on freshwater, and sailed through the winter, and was having massive cavitation issues on his new boat that he wasn't getting during the summer. His foils were too shiny...he wet and dry'd them...problem solved............
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I think the water tends to stick to shiny surfaces, wasn't there some thinking that golf ball type dimpling was best?
 
savageseadog said:
I think the water tends to stick to shiny surfaces, wasn't there some thinking that golf ball type dimpling was best?
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That is to do with aerodynamics.

A ball is about the least aerodynamic shape you can imagine (second only to a brick, probably). On the trailing edge of a flying ball (strictly a ball doesn't have an edge, but you know what I mean), vortices swirl about and that produces drag.

The addition of dimples does much to improve the airflow around the trailing edge. The same is true of that join on a tennis ball and even (although to a lesser extent).

But, what we are talking about here (rudders/centreboards) are much better shapes, so the golf ball analogy isn't really relevant.

Ah, what the h*ll. I'm with Armchair Sailor and Lakey. Just get out on the water and have fun.
 
Dan you need to spend time perfecting the finish and the edges, anything less will impact the current levels of performance. Only you know it makes sense, so, do it again until its correct. Please post pictures.
 
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