Fairing hull to reduce drag

[float6536]

Hi all,

We have an aluminium sailboat with a 60 tone displacement that is beautifully faired above the water line, however below the waterline is far from so.

I'm contemplating filling and fairing the entire underside to reduce the coefficient drag and ultimately improve speed and efficiency. I understand this will vary depending on how bad it is now v's how fair it ends up being.
My main question is for anyone with personal experience that would have some feedback or for anyone who has come across any case studies related to this topic.

I figure I better weigh up the potential gains before I commit to spending a heap of time and money to the project.

Cheers in advance

[Fore and Aft]

You will definitely notice the difference. I surveyed an Island Gypsy 44 power boat before and after the hull was repaired for osmosis. The hull had been totally glassed and faired. Then a year later the boat was slipped for antifoul and check up and the owner happened to mention to me that they he was making decent fuel savings just from a fairer bottom. I was amazed as there did not appear to be much difference in the below water shape.
I then mentioned this to another sailor who has a Flying 15 sail boat and he had completely faired his hull and now was in the front of the fleet and not in the middle. All from a fairer hull. Surprising really?
Another bonus with a fairer bottom in your case would be better resale value I would think?
Cheers

[belizesailor]

Fairing the hull is pretty common on serious racing boats...makes a noticeable difference.

How does one fair aluminium?

[JPA Cate]

How does one fair aluminum? Well, it's tricky, because it wants to stretch. So most people just lay in bog. Unfortunately, the bog tends to want to come off. If I owned the boat and cared enough, I'd try to find someone who worked with aluminum body work on cars, (like old Porsches, because you want a good skill level), and I would be prepared to pay a lot. Then fair and etc. Otherwise leave as is. Some projects are just too $$ intensive.

Ann

[Fore and Aft]

Your not wrong Ann. Upside down with a torture board (Sanding board about a meter long with sandpaper stapled to it) is bloody hard slow work. If you fair the bottom make sure whoever applies the filler has some experience. As the better the filler is applied the less sanding is needed. I would suggest using slow epoxy hardener when applying the bog. That's one job you want to have plenty of time for.
Cheers

[GordMay]

Greetings and welcome aboard the CF, Float.

[trifan]

Something I found while surfing the web:

DRAG REDUCTION VIA LUBRICATING NON STICK AND SLOW RELEASE ANTOFOULING HULL COATINGS

KISS-COTE v PTFE

Self-Bonding Polymers (KSBP*) are claimed to provide a non-stick drag-reducing finish so boats will go faster on less fuel.

Conventional marine coatings have two principal mechanisms of action. The anti-fouling character is from soluble heavy metal salts which poison the marine life that grow on the surface. This is combined with a "self-polishing" or "ablative" character. The toxins are held in a slowly dissolving matrix which has two purposes:

1. Its continuous loss of surface layer allows exposure of fresh toxins as the toxins at the surface become depleted;

2. Marine organisms will still grow on the surface despite the presence of toxins, so the coating also "sheds" it surface layer, shedding the marine growth with the eroding surface.

The majority of KISS-COTE formulations are "fouling-release" because of their inert non-stick finish. Since they are non-toxic they are not anti-fouling. However, marine organisms do have trouble colonizing KISS-COTE treated surfaces since they need an adherence mechanism to colonize. As an example, it is claimed that Zebra Mussels will selectively colonize any other surface, including Teflon® (PTFE), rather than growing on KISS-COTE.

Products containing PTFE as the active ingredient are not anti-fouling. However, some anti-fouling coatings are available with low levels of PTFE, the most common being VC-17 by Courtalds. Please note that barnacles will grow on and penetrate a PTFE coating and Zebra Mussels readily colonize PTFE. Barnacles do not stick to or penetrate KISS-COTE and Zebra Mussels avoid substrates coated with this material.

KISS-COTE's MegaGuard Marine LiquiCote combines the benefits of KISS-COTE Polymers with the anti-fouling effect of conventional bottom paints. The LiquiCote is applied simply by painting over a standard anti-fouling paint, softening the surface layer of the bottom paint. It dries in a few minutes, with the bottom-paint re-hardening into a non-stick anti-fouling finish that exhibits reduced drag. The life-expectancy of the anti- fouling and the non-stick character is determined by the quality of the bottom paint. We are finding a 9-12 month life in our harsh salt-water environment. At the end of this time, one is left with a conventional bottom-paint that can easily be pressure washed and re-coated with the LiquiCote. Meanwhile, the bottom paint lasts years longer and does not leach the same high levels of toxic biocides into the water. In addition, there is much less contamination than one would find using a standard maintenance protocol on a regular bottom paint, including frequent sanding and re-painting.

(KISS-COTE, KSBP and MegaGuard are trademarks of KISS Polymers LLC. US Patent #4,839,456. Copyright 1993, KISS Polymers LLC. all rights reserved.)

[Suijin]

I think we need more information about your boat to comment on the performance gains you might see from fairing. First of all, aluminum hulls are usually not optimal shapes due to the limitation of aluminum shape fabrication in the first place. Second, a 60 ton boat is a big boat. What does the keel look like? How much do you draw?

If you have a picture of the hull below the waterline that would be very helpful. It's possible that extensive and costly fairing might amount to putting lipstick on a pig. Or it might be worthwhile. Can't say without looking at what you're dealing with.

A more cost effective approach might be to fair the keel, ensuring that it fits a NACA profile to ensure greatest potential lift and hydrodynamics and leave it at that. Another option is to just figure out how to enjoy the extra day or two it takes you to cross an ocean.

[Elie]

Many years ago, I was avid racer. I faired the fiberglass bottom, of my French(Yatchting France Jouet 1140) to improve my results. It was tough job that took me 2 seasons to complete. It was worth it racing wise. Compared to another similar boat I gained 10 minutes on a 14 mile downwind race in moderate winds. But for cruising... not sure it would mean anything.

[Jim Cate]

If you have a picture of the hull below the waterline that would be very helpful. It's possible that extensive and costly fairing might amount to putting lipstick on a pig. Or it might be worthwhile. Can't say without looking at what you're dealing with.

I remember reading an article by Bruce Bingham relating to doing this sort of work on his traditional timber schooner. He did the fairing, installed flush skin fittings, reshaped the rudder and did a nice paint job. IIrC he claimed very noticeable improvement in sailing qualities... but it was a long time ago and my memory is not exactly perfect. A search might recover the info, and it might be relevant to the OP's query.

Jim

[HankOnthewater]

Hi Float, fairing a 60 ton boat,..... would translate in many weeks of fairing. I agree with the previous posters. Fairing is hard work.
A guess, the reason for fairing is that the hull is angled on the frames ?

Apart from the amount of work (and money), there is one other reason why one should not: that is if you have keel cooling. I know of one aluminium powerboat that was overheating at times. It had its hull beautifully faired...... and yes, the fairing (up to 15-20 mm thick in places) acted as a thermal insulator. the fairing (on the outside) was chipped off where the cooling channels were that run on the inside, and overheating problems were resolved.

The speed gain if you were to fair the boat.... depends on how bad the hull is now. I did fair my previous boat, and I believed it was worth it, but have no hard data to back that up. Hehehe, painting a car red also makes it go faster.

If you have no success here to get a more definite answer, post your question on Boat Design forum, where there are very technical contributors.
https://www.boatdesign.net/forums/

[robert sailor]

You couldn't convince me to spend the money, talk about diminishing returns, your money would be much better spent on good sails. I too have slogged away helping to fair a boat in my racing days and sure it helped but the extra miles it would give You on a long passage would be minimal. In a race if you pick up a couple of minutes on your competitors you are in the money but that isn't your goal...There are just so many other really important things on a cruising boat that will add to your pleasure, I don't think this is one of them..that's my opinion.

[Kim Klaka]

Working overhead to fair the hull would be a soul-destroying job. I suggest focussing on fairing the keel and rudder, then getting the hull surface roughness smooth. The following article explains the influence of smoothness on friction drag. https://www.fsc.com.au/wp-content/up...h-a-bottom.pdf
The effect of fairing a hull on drag is a slightly different matter, if anyone has a reference to relevant experimental data I would be interested to hear. The article cutandpasted below sheds some light. I wrote it for the Australian Naval Architect journal a few years ago. Hope it helps.
Kim


The yacht that turned to port
Kim Klaka, Curtin University
Have you ever wondered how accurate the section of a keel or rudder has to be? I have, so I did some background reading to find out. It all started a couple of years ago when my 10m sailing yacht showed a tendency to turn to port and had an appalling tacking angle of nearly 100 degrees. That’s the subject of a long and slightly different story, suffice to say I had narrowed it down to a hydrodynamic cause. So I set to making templates for the fin keel and spade rudder to rectify the problem. Before doing this I had to satisfy myself that I could fair the keel to the required accuracy, which begs the question: what is the required accuracy? I am not talking about surface roughness (that is well understood and widely known), but larger scale undulations – waviness, lumps and hollows.
As is so often the case with foils, the best experimental work is usually to be found in the archives during the period 1930-1960. This instantly leads to three reference points – Abbott and von Doenhoff (originally 1949), Hoerner (originally 1958) and the suite of NACA technical bulletins from that era. Sure enough, they did not disappoint.
Abbott and von Doenhoff comment mostly on surface roughness and transition, but in part 7c of their 1959 edition they do provide the following delightfully simple and practical advice about waviness:
“For the types of waves usually found on practical-construction wings, the test of rocking a straightedge over the surface in a chordwise direction is a fairly satisfactory criterion. The straightedge test should rock smoothly without jarring or clicking”.
Similar advice is often given by good shipwrights. To find out what happens to performance if you don’t achieve this level of smoothness, I had to refer to NACA technical notes.
Powell (1954) tested helicopter rotor blades with and without fairing filler. The maximum variation in thickness was 0.13%, which resulted in 6% more power required to rotate the blades. However, the test conditions were rather different from the flow round a yacht keel.
Ward (1931) compared foils that were not exactly wavy, but they differed slightly in local thickness. For example, he tested a NACA 0021 section against a subtle variant, the NACA 100. The greatest thickness difference between the two sections at any point along the chord was 0.35% chord i.e. for a typical keel of chord 650mm, this is a section thickness variation of 2.3mm. The wind tunnel tests showed, with all the usual caveats about two-dimensionality, Reynolds number and pressure gradients, that this modest difference caused a 9.4% drop in maximum lift and 2.3% drop in max lift/drag ratio. So we now have a rule of thumb:
Every 1mm of waviness decreases lift-drag by 1%.
Is it correct? I didn’t have a wind tunnel handy so I reverted to very coarse full scale measurements. I measured the profiles of my keel and rudder, and found out two things:
• Firstly, the maximum thickness port and starboard differed on average by about 5mm over a chord length of 1400mm i.e. 0.35% of chord.
• Secondly, the thickness deviations from a NACA 630-series section were about 2mm i.e. 0.07% chord.
After two weeks of sanding and fairing, I had reduced the asymmetry and unfairness to about 1mm worst-case (0.007%). The boat now tracks in a straight line and tacking angles are back below 90 degrees, saving me about 5 minutes in a 2-hour race. So I conclude that waviness of more than 1mm makes a significant difference in performance.
References
Abbott I.H. & von Doenhoff A.E. (1959) Theory of wing sections. Dover, New York.
Hoerner S.F. (1965) Fluid dynamic drag. Hoerner , New York
Hoerner S.F. (1975) Fluid dynamic lift. Hoerner , New York
Powell R.D. (1954) Hovering performance of a helicopter rotor using NACA 8-H-12 airfoil sections. NACA Technical Note 3237, August 1954.
Ward K.E. (1931). The effect of small variations in profile of airfoils. NACA Technical Note 361, January 1931.

[Jim Cate]

Kim, thanks for that informative post. Actually, I kinda hate you, 'cause I suspect that if I was to do the same sort of measurement I might find similar asymmetry, and the work to rectify that defect would be awfully expensive, even DIY. I don't have obvious performance differences between tacks, but the suspicion lurks nonetheless.

At any rate, it is illuminating to have even one set of numbers to think about, so well done, both the job and the reporting.

Jim

[skipgundlach]

The bottom of Flying Pig was a mess when we hired a supposed fiberglass pro to remove the paint.

He took off way more than paint, but it gave us an opportunity (need, really) to do it up right.

Many blister/incomplete catalyzation repairs later, we faired it.

We gained at least a knot in our typicial-5-knot boat.

The gory details can be seen in this gallery:

Pictures: Flying Pig Refit 2011-2012/Bottom Job