Clean Bottoms

[John Bell]

I always figured the racer's obsession with keeping the bottom clean was a bit over the top. How bad could a little bit of fuzz be, anyway? Particularly on a powerboat. After yesteday, however, I'm a believer!

My boat had developed a bit of fine fuzz after wintering in the water,- nothing major, just a little algae. Running it this spring, I noticed that I'd lost about 150-200 rpms on the top end. When I finally checked things out with the GPS, the boat had lost 4 mph over what she'd run last summer: 19.9 mph vs. 24+ at WOT. What was worse, she would only pull 12.5 mph at 3000 rpm instead of the 15 I was used to. Speed isn't really my thing, but with marina gas at $2.50/gallon, I figured that needing more throttle to go the same speed wasn't helping my fuel economy.

So I spent yesterday afternoon freezing my butt off swimming under the boat with scrub brush in hand knocking the fuzz off. It took about an hour of moderate effort to finish, including cleaning the lower unit and prop on the outboard. Wonder of wonders, the missing 4 mph returned! Also, at lower power settings speeds were similarly higher. While we still don't get great gas mileage (about 3-4 mpg at cruise), I've got the minor satisfaction that it is better than it was.

[mmd]

;) The devil is in the details, eh?

From this I expect that you now realize how important it is to carfully fair in the depthsounder transducer and other thru-hull fittings, right? :D

[Figment]

Somehow, I suspect that if you'd stopped after cleaning the prop you still would've achieved similar results.

[John Bell]

Originally posted by Figment:
Somehow, I suspect that if you'd stopped after cleaning the prop you still would've achieved similar results.Unfortunately, that was the last thing I tackled. Next spring, maybe I'll experiment with doing first the prop, then the rest of the lower unit, and finally the hull. It's going to bum me out if I discover you are right. Well, for this year at least!
;)

[Wild Wassa]

Originally posted by John Bell:
"I always figured the racer's obsession with keeping the bottom clean was a bit over the top."

Some of us just don't like to lose and need more than just clean.

I only race in dinghies. The number of races lost by less than 4 seconds in handicap races, is enough to make you weep or polish. Only 4 seconds in races lasting between 1hr 30 mins to 2hrs 40 mins. I only need to find 0.008 of a knot and I'll not lose a handicap race. I look for that elusive 0.008 of a knot every time I touch a boat.

I had a visit from a Tuna boat Skipper about two years ago, who had his expensive boat painted in South Australia and he had lost 1/2 a knot after having the paint job done, so he said ... this was costing him dearly in lost time and increased the use of fuel. I sent him away to fair the paint.

I see there is a thread about 'Wooden Yacht Design' (by Larsen and Elliasen) on the Forum. It was after reading sections of this book, on drag and the laminar and non laminar flow boundary that makes me polish paint even more. If I can move this boundary back only half of 1% that elusive 0.008 of a knot will be shattered.

Not knowing the maths of it all just makes me fair even finer, lighter and smoother. Keeping the surface grain of the paint below 12-15 microns (I think L&E say) that's not too hard with a good cut and polish and you are getting there ... but to find that missing 0.008 of a knot, ARRHH!.

At Championships, the first thing that a novice will see and comments on, when the boats first turn up, is that the better performed sailors will polish their hulls for hours, before the racing starts.

The depth of a tram line in a brush stroke can be >50 microns ... this increase in drag will give you no chance of reaching a speed approaching the maximim possible hull speed for any boat.

I don't think that I'll ever find that elusive 0.008 of a knot increase in speed ... so I concentrate on not losing it.

Warren.

[ 06-07-2005, 08:57 PM: Message edited by: Wild Wassa ]