Let me start with a big thanks to ME Sail and Stu. Your posts are like having Nigel Calder and Frank Butler as friends. The pieces on stuffing boxes and compounding methods have been so valuable. Anyway my topic is laminar flow of water against the hull while underway. I am a cruiser/ fun racer. Dacron Sails and a fixed prop. It is Bottom paint time of year. I use a different color bottom paint each year so I know what to take off in the next fall and spring. I have a few dozen divots or depressions in the old lightly caked bottom paint after sanding. They range in diameter from a dime to a quarter or so. After sanding, the depth is about the thickness of a dime or thinner with feathered edges. We often race in light air. I usually roll on the next coat of Ablative bottom paint and that is it. Can I get a measurable benefit from filling the spots and sanding , yet again, after bottom paint application? What grit? What product to fill the chip spots? The boat sits at a mooring all season and gets cleaned once or twice before major races. Is this the point of diminishing returns? For my type of sailing I am trying to find the right place on the spectrum of bottom finishes between Lunar surface and mirror finish. Would I get a decent and appreciable hull speed benefit in the 2-6 kt range from still more work, or is the increment too miniscule to bother?
Is it worth It? Thin film expert wanted
- Thread starter DanNahant
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Not an expert, but bottom painting is almost as bad as watching political discussions. To be avoided at all costs!
If it was me I'd happily pay $2k to $3k for a folding prop with the potential for 1/2 knot of speed to avoid any bottom work with an unknown potential. Actually, if a smooth bottom was going to give me 1 knot increase I'd still pay the $ first for the prop.
sam
If it was me I'd happily pay $2k to $3k for a folding prop with the potential for 1/2 knot of speed to avoid any bottom work with an unknown potential. Actually, if a smooth bottom was going to give me 1 knot increase I'd still pay the $ first for the prop.
sam
Go sailing and don't worry about the bottom. One bad tack, a few second late to the start, or any other issues over the course of a race will cost you more time than a few small imperfections on the bottom. I used to have a J/24 and when I purchsed it the bottom half of the keel and rudder were painted with an ablative bottom paint. Planning to trailer sail, I stripped both and then had the keel and rudder painted with a hard, smooth paint, sanded to 1200+ grit. No noticable change in boat speed.
FWIW I redid the bottom of my C22 last year. Took it down to the gelcoat (about a 1/4 in of various bottom coats in a lunar lanscape motif) and put on 5 coats of VC Tar. The year before I did this it was a good day to hit 5 kts. Afterward, I was disappointed when I could not hit 5. Best speed previous to the work 6.4 kts - after 7.9.
It does help but a lot of elbow grease (or $$$ to someone else)
It does help but a lot of elbow grease (or $$$ to someone else)
Your bottom is costing you speed. If you want to be competitive it sounds like you need to strip the whole thing down to gel coat. If you do that, you may as well put some Interlux 2000 on for a barrier coat. The fixed prop is costing you big time as well. Assuming your in PHRF, the 6 seconds you get is not enough.
For filling small divots .... first lightly sand into the divot (only), get some 'low density fairing filler', some fresh ablative and mix the fairing filler and a wee bit of paint until to the consistency of mayonnaise, and with a polyethylene trowel 'fill' and 'fair' the divot, etc. You may need to do this several times until smooth.
If you have many divots/craters ... it usually means that you have too much accumulated ablative paint and the those 'losing adhesion layers' down deep will eventually have to be removed. Where I sail I can only get about 8 years of accumulated 'ablative' paint, then the 'early' or deep down layers lose adhesion (craters and divots) .... and I take it ALL off and start over, promising myself to put it on THINNER the next time.
For a 'FAST' hull when applying ablatives:
• Fair and smooth divots, etc. (as above).
• First coat - apply thin (about 6" wide) STRIPES (leaving an equal width stripe with NO paint) with WEST SYSTEM yellow roller - whole boat. Let almost (not quite) fully set (about 6-12 hours).
With WEST SYSTEM ''YELLOW" plastic 'trowel' and before the first thin coat 'sets' ---- take trowel and draw across (at a LOW angle) the painted 'stripe' to KNOCK DOWN the 'tops of the pimples' left by the roller.
• Immediately roll on fresh paint over previous painted stripe and immediately draw polyethylene 'trowel' across to 'level' and to fill the 'bottoms'/valleys of the roller-pimples. The 'drawing process' will fill the 'valleys'. Clean 'trowel' with 433 solvent to keep a 'clean edge'.
• Let cure to hard.
• After the first stripes are 'hard', Repeat rolling/filling/drawing in the unpainted 'stripes'.
• Let cure to hard.
• Repeat as necessary in any 'rough spots' and 'streaks/lines' from the previous coats.
• Very slight 'flat-sanding' to correct 'dribs'
Aim is to get the paint to 'baby ass smooth' - no divots, no 'roller pimples', no 'roughness' etc. The method is the same as how one applies gelcoat to a male plug mold .... when the last coat is the gelcoat. When I apply as such to my 'crab-crusher', I guesstimate that I typically shed 10+ seconds per mile (PHRF) !!!!!!!!!
The bottom paint will stay smooth, it will release growth FASTER (and you may only have to 'touch up' (with a roller & trowel) the following year ..... and the 'boundary layer' will be consistent and less turbulent (but not to the same smoothness and boundary layer performance as a sprayed-on job that is 'flat-sanded'). The trick is NOT to trowel over partly cured paint .... ruins the surface. Will be a definite gain in boatspeed, and 'Laminar flow conditions' will be greatly extended into the higher wind and boat speeds ---- FASSSSSST in the low-moderate wind speeds.
If you're really serious about boundary layers / super-slick hull .... consider a total 'fairing job' with 'long boards', etc. and use VC17 or "Baltoplate" and forget 'ablatives' altogether.
If you have many divots/craters ... it usually means that you have too much accumulated ablative paint and the those 'losing adhesion layers' down deep will eventually have to be removed. Where I sail I can only get about 8 years of accumulated 'ablative' paint, then the 'early' or deep down layers lose adhesion (craters and divots) .... and I take it ALL off and start over, promising myself to put it on THINNER the next time.
For a 'FAST' hull when applying ablatives:
• Fair and smooth divots, etc. (as above).
• First coat - apply thin (about 6" wide) STRIPES (leaving an equal width stripe with NO paint) with WEST SYSTEM yellow roller - whole boat. Let almost (not quite) fully set (about 6-12 hours).
With WEST SYSTEM ''YELLOW" plastic 'trowel' and before the first thin coat 'sets' ---- take trowel and draw across (at a LOW angle) the painted 'stripe' to KNOCK DOWN the 'tops of the pimples' left by the roller.
• Immediately roll on fresh paint over previous painted stripe and immediately draw polyethylene 'trowel' across to 'level' and to fill the 'bottoms'/valleys of the roller-pimples. The 'drawing process' will fill the 'valleys'. Clean 'trowel' with 433 solvent to keep a 'clean edge'.
• Let cure to hard.
• After the first stripes are 'hard', Repeat rolling/filling/drawing in the unpainted 'stripes'.
• Let cure to hard.
• Repeat as necessary in any 'rough spots' and 'streaks/lines' from the previous coats.
• Very slight 'flat-sanding' to correct 'dribs'
Aim is to get the paint to 'baby ass smooth' - no divots, no 'roller pimples', no 'roughness' etc. The method is the same as how one applies gelcoat to a male plug mold .... when the last coat is the gelcoat. When I apply as such to my 'crab-crusher', I guesstimate that I typically shed 10+ seconds per mile (PHRF) !!!!!!!!!
The bottom paint will stay smooth, it will release growth FASTER (and you may only have to 'touch up' (with a roller & trowel) the following year ..... and the 'boundary layer' will be consistent and less turbulent (but not to the same smoothness and boundary layer performance as a sprayed-on job that is 'flat-sanded'). The trick is NOT to trowel over partly cured paint .... ruins the surface. Will be a definite gain in boatspeed, and 'Laminar flow conditions' will be greatly extended into the higher wind and boat speeds ---- FASSSSSST in the low-moderate wind speeds.
If you're really serious about boundary layers / super-slick hull .... consider a total 'fairing job' with 'long boards', etc. and use VC17 or "Baltoplate" and forget 'ablatives' altogether.
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