Thread: GIS sail tweaking

Hey Mik,
I'm going with the biradial design in dacron. I was tempted to try a laminate, but like durablility. It's in the mail, and my sewing machine is waiting! I will be glad to document the sewing and testing.

My current flat sail would not draw well in low wind last weekend compared to the racing fleet of Lido 14's. I want to smoke them, as usual. It will be interesting to see the difference in sail performance with my new Sailrite sail.

Another topic is my bottom paint. My boat has rough grey bottom paint. Not sure exactly what it is, but very rough to the touch, like an anti-fouling paint. It also absorbs water over a long sail, such that when I pull the boat, the paint is wet feeling and dark, then dries out over time. I've wondered how much speed I'm giving up, and have been thinking about repainting the whole hull. I'm partial to deep purple blue (like the gulf stream). So my questions are:
(1) how much drag is this stuff causing?
(2) and if I repaint the whole hull, is there any reason not to go wth Interlux Brightside?

Rough paint is always slow. Can you afford to extend to International Perfection? More expensive, but oh so awesome finish and diamond hard. I finished my foils with it and it's simply awesome straight off a foam roller. Should last years.

I'm going to paint my GIS with it one day.

Love the bi-radial cut! It'll be real interesting to see what you think of it.
(MIK wrote - see the sailmaking link here to read the story - posts have been moved - Sailmaking 101 for Goat Island Skiff, Beth)

A long time ago I painted a gelcoat trailer sailer with antifoul bottom paint and it felt like I lost almost a knot off the top end. I never made careful speed measurements before applying the bottom paint so can't back this up with real numbers but the difference was significant.

Simon

I used interlux brightsides for my GIS and Beverly. It is easy to use with roll and tip, and comes out epic. A little softer than international perfection, but sooooo easy with bangin' results. I am a huge fan, especially since that's as pricey as I will go and it's a stretch for me.

Thanks for the recommendations. Sounds like I can't go wrong with either one. My plan was to sand down what I have now to a pretty smooth finish, but not remove all the old paint, then coat over it all. The "roll and tip" - foam roller, and quick brush over right? What type of brush is good? I went back and read some older forum stuff on this, but didn't see a note about brush type.

Yep, cannot wait to hoist the new sail!

Roll vertically, tip horizontally, work in short sections, thin appropriately, and paint in reasonable temps. Always tip dry into wet, and think of an airplane doing a touch and go-- landing, runway, take off. It's like a dance, very meditative. You can go nuts on the brush, I went to my local hardware store and got a Wooster Yachtsman, I think 2.5", maybe 3". Run of the mill nice brush without going crazy. I got a mirror finish, though over time it lost the gloss just a bit. The paint job is the nicest part of the boat.

Originally Posted by callsign222

You can go nuts on the brush

In fact, going nuts with this one tool is not nearly as expensive as pouring lots of money into hardware or rigging or even the paint itself. I think I spent on the order of $15 for the brush I used. That's 200-400% over the price of a disposable chip brush. Except, I haven't disposed of it; it's hanging in my workshop ready for the next job. To me the important difference was the lack of loose bristles to pick out of the wet finish. I used cheapo brushed to tip off my primers coats (which is quite a bit thicker than the Brightsides by design). The shedding was expected and not a big issue because I knew I'd be sanding off much of each coat. When I switched to a quality brush for the top coat (tipping off only) I wondered why I skimped earlier.

I'll leave the discussion of fluid dynamics and boundary layers to others who are educated, but I would imagine your paint would have to approximate the roughness of barnacles to really affect performance, no? (You guys on the left coast DO have barnacles, right?)

$15 is reasonable, it's about what I spent on mine. I was referencing the $65-$70 brushes.

I found a great video of a guy "roll and tip" painting the bottom of a melonseed using Interlux Brightsides:

Rolling and Tipping - First Coat - YouTube
You can really see the how the process works, and how the paint looks lumpy after the rolling, but glosses over with the tipping. You can see also what is meant by "painting in sections". Very cool. I'm partial to "sapphire blue".

I read that you use the Interlux 333 thinner, but how do you know how much to use? Trial and error?

As to surface and speed, there is a lot of forum stuff out there, but what I noticed is that the small sail crowd are huge super glassy fanatics. If you read the laser forums, they think any paint is sacrosanct to a perfectly smooth gelcoat hull. There has got to be something to it. Bruce, would you think of racing a laser with rough anti-fouling paint on the bottom?

Originally Posted by warm beer

As to surface and speed, there is a lot of forum stuff out there, but what I noticed is that the small sail crowd are huge super glassy fanatics. If you read the laser forums, they think any paint is sacrosanct to a perfectly smooth gelcoat hull. There has got to be something to it. Bruce, would you think of racing a laser with rough anti-fouling paint on the bottom?

The guys on our Master's Laser circuit usually make sure the boat has a super smooth bottom, even if an old boat. Unless they are really old boats, they are usually not painted due to the weight of the paint. I have an el cheapo, older No.2 boat that I keep at the Harbour club I sail at on Friday nights. It was a nice stiff hull, but had an ordinary bottom, lots of scratches from being transported in containers and some minor osmosis from sweating under a cover all its life. Eventually I couldn't help myself and I filled all the scratches with gelcoat, finished the bottom with 2000 wet & dry paper and then gave it a polish to help make cleaning off the oil easier. I'm not sure a glass smooth bottom is that much faster, but sailing a boat with a rough bottom eventually did my head in!

I finished the foils with International Perfection, which is a 2-pot product. I tipped it off with a brush after rolling it on, but did an experiment where I omitted the tipping off on another set of foils. There is no difference in the finish which I think is due to the 9 hours it takes to become touch dry. It flattens itself out nicely to an incredibly glass-like finish that after a few days is harder than gelcoat. Ideal for Laser foils which must operate in a casing where no protection is allowed.

Originally Posted by warm beer

I am trying to tweak my sail tuning to improve pointing ability and shape in upper moderate winds (12-15 knots). It almost seems that I give up a bit of pointing as the wind gets over 10 knots, and the luff starts to break earlier on both tacks. The upper third of my leach starts to flap as well. I've got my downhaul tight enough to bend my boom a couple of inches, and my outhaul pretty darn tight (with a 3 to 1 outhaul rig). Has anyone noticed an improvement in these conditions when they switched to a stiffer boom? Any other ideas to try?

I'm beating up on all the day sailers, (it's fun rolling over the stayed gib, and main boats), but I want more.

I've also noticed that my better tack seems to be the port - (with my boom on the port side of the mast). In other words, I point better with the boom on the upwind side of the mast. Other GIS sailers, have you noticed the same?

I would try to put some depth in the foot of the sail. It will give you a bit of extra weather helm, increase the power and allow for sail twist. Also we found with Alex's lug rigged OzRacer - it had a floppy leach and adding 1 in 10 depth to the bottom of the sail solved it on launching day. Can't say it will happen with every sail ... but worth a shot.

Do you have the stopping knots in the traveller to limit the minimum angle of the boom to 10 degrees? Christophe's thread on sailing performance started off with exactly the same comments as you make above - MORE!

And these changes were ones that helped him. Brad Hickman in his OzRacer derived PDR that won their nationals last year also had similar initial problems. But more downhaul - I think he runs 8 to 1, more depth in the foot (it will be more critical in a blunter entry boat like the PDR), controlling the traveller angle for sheeting angle all helped him too.

With the lug..

One thing to be aware of is a subtle difference from tack to tack. The sheeting angle is changed as the pivot point of the downhaul is not in the middle of the boat. So sheeted to the same point equidistant from side to side at the traveller means that when the sail is on the lee side of the mast the angle is a bit closer in. Opposite when the sail is to windward. I think this is of some little importance because the "less efficient tack" (mast to leeward) is compensated for by a slightly wider sheeting angle - which might be what you are feeling?

It does show that the "loss of performance" because of the spars interfering with the sail on one tack" is a very subtle thing indeed - unlike the gross effect that lots of people still think it is - obviously nobody ever thought of really checking the difference in real boats. Develop the lug to a similar amount to the bermudan rig and it is not far off - as you are finding with the local boats - or even better allied to the efficient light hull of the goat and HAVING GOOD FOILS!!!

To get the really correct sheeting angles from side to side would take quite a bit of trial and error - but as you say - you are already scaring the daysailers and really only looking at a percent. Whereas everything I am mentioning here might be 5 or 10 percent.

Also don't forget that pumping the 1 in 10 upwind and shy reaching mainsail bottom depth up to 1 in 7 when you have attached flow crosswind and the beginnings of broad reaching increases power up to 40%.

Running and broad reaching when the sail is stalled the foot needs to be tight - it is a pure area game. But you can get more with the sailing by the lee trick. I think you have been trying that already. But main point is that you ease sail and it powers up - unlike having the sail on the normal side.
Sailing unstayed Cat Ketches and Cat Yawls safely and efficiently downwind in strong winds | Michael Storer Wooden Boat Plans

MIK

When it comes to the last bit of performance, I have a question also: In The Sharpie Book, it is saying that they made the bottom a bit rounded aft for getting less drag in later sharpies (the centerline of the bottom slightly deeper than at the chin logs). It would be quite easy to round sligthly the transom and the last bulkhead Would this be a bad idea? I think it is sure it would give a bit more performance in speeds less than the "hull speed". How is it in speeds above the "hull speed"?

Hey Mik,
I have been adding foot depth for my boom-on-the-lee tack, by loosing my outhaul a bit, over my boom-to-windward tack. I swear you're right, the sail performs better this way. Imagine that!

Originally Posted by engblom

When it comes to the last bit of performance, I have a question also: In The Sharpie Book, it is saying that they made the bottom a bit rounded aft for getting less drag in later sharpies (the centerline of the bottom slightly deeper than at the chin logs). It would be quite easy to round sligthly the transom and the last bulkhead Would this be a bad idea? I think it is sure it would give a bit more performance in speeds less than the "hull speed". How is it in speeds above the "hull speed"?

Howdy,

The main aim of such a shape is to prevent the transom corners from dragging at low speed if the boat is heeled. It makes the transom look more like a rounded hull boat.

There are two ways of doing it geometrically - you have to imagine the boat heeled

1/ you can round up the corners as suggested. The Penguin Dinghy by Rhodes does just this.
2/ you can narrow the stern relative to the max waterline beam of the boat - which is what happens with the goat. The transom is narrow enough not to drag.

The problem with the first method for boats with good speed potential is if you look at the boat from the side it means a sudden increase in rocker right at the back of the boat particularly as you take lines parallel with the keel (they are called buttocks technically). A little bit of rocker is good in the last third of the boat as it helps to keep the bow out of the water at higher speeds (as does the narrower transom method of the goat) - but too much will slow the boat down badly.

So I would say it is a bad method for boats that are expected to perform well in a range of conditions but great if you want to really max the boat out for light winds.

In effect the first method gives you two bottoms on the same boat. Along the keel it is all fair and straightish - good for speed reaching and running or planing upwind if the boat has a trapeze or leaning plank. But moving out parallel to that you have buttock lines that "put the brakes on". Might make the boat safer in big following wave at speed maybe - but my guess is the narrower goat stern does much the same thing.

So in a way you can also say ...
if the boat is fast it will spend less time at the speed where the rounded corners give an advantage
If the boat is fast a lot of time upwind you will have a clean breakaway of water from the transom
If the boat is being sailed fast it will be LEVEL so the corners of the transom won't start to drag anyhow (sharpies were traditionally too big to be sailed level by crew weight.

In a boat the size of an Oyster Sharpie - 35 - 40ft the amount of transom dragging from heel would be tiny. But it might help speed with half a load of oysters on. But I would think the difference would be VERY marginal and would require a lot of boat to boat racing to identify the advantage. If the racing wasn't done with boats of the same weight and crew skill, then it is just one of those things that has been said with little analysis - probably a long time back by the proud owner of the boat with the slightly rounded transom - "it makes it faster".

But i would say the statement appeals to those who think rounded hulls are faster - which is a bias - in reality there are very few faster dinghies without chines.

International Canoes, Cherubs, National 12s , Australian Skiffs, 49er (which completely outsailed a range of round hulled boats in the selections to become an international class - I think it didn't come below first in any race. National 12s are interesting showing that Chines and round bilge can compete equally, but fewer round bilge boats are being built.

So I don't think this semi rounded approach really makes much sense. Though I did get on one of my hobby horses there for a moment!

MIK