Estimate on effect of minor fouling on boat speed.

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Oscarpop

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We have a 40ft medium displacment yacht.

I did not have the time to get a pre season lift out and wash this year before going off on our summer cruise.

I dived under the boat and found it to be initially, visually clean.

however running my hands over the hull there were the occasional small ( less than 5mm) barnacle and a a very very light green algal growth all over.

Now I am not talking about kew gardens here, more like the sort of downy fuzz that certain women seem to have. A bit like a peach.

anyway, metaphors aside.
Would this have any noticeable effect on hull speed? if so what would be the guestimate of how much we lost?

Cheers
 
If it's anything like my boat, about 1 to 1.5 knots is the difference between clean and as you descibed. I also see a big difference when motoring: 1300 RPM = 6 knots when clean. 1600 RPM = 5.5 knots when needing a scrub.
 
Agree, on my 28ft boat, just a light coat of slime is enough to knock a knot off the speed. It just feels much more sluggish than you know it should. Managed a scrub a couple of weeks back and now back to normal for a while.
 
After my scrub and AF of my 28ft 3 ton longkeel yacht this spring I could get 6.7 knts on a windless tideless Cardiff Bay motoring flat out at 1600 revs. Now 4 months down the line with a slighty slimey hull, same revs and conditions I get 6.2 if lucky.
 
After drying out for a scrub, from slime only (no weed or barnacles), 34 ft boat full throttle speed went from 7.5 to 8.25 knots in flat water with no tide. Only time I have actually measured it, but have noticed the difference many times.
 
Most horrid sailing or motoring to windward.

You can't tell anyone any fink on here. I will not tell ya to buy a Brizzo and get some gentle regular exercise, ahem
 
I used the wrong antifoul last December, Cruiser Uno as opposed to a stronger one and I had what looked like a little weed round the bows and it felt a little rough when I put my hand under the waterline. I could only motor at my normal cruising revs at about 4.5 knots. I got lifted on slings for a high pressure blast off. It didn't look that bad with slime and a little algae covering about 1/3 of the hull. After a clean I was motoring at cruising revs at about 5.8 to 6.2 knots. Guess who's using a stronger antifoul next year?
 
On a Rival 34 (waterline length 25 ft) with a 40 hp diesel those were the flat out log speeds (pre-GPS) and in general EPs based on lower 5 to 5.5 knot cruising speed log readings seemed to work out. Dragging a big quarter wave flat out. Might not have been 8 knots: no way of telling unless I'd bothered to go and find a measured mile. The point anyway is the % difference after scrubbing off a little slime.

My present 35 ft AWB (waterline 32 ft) when just launched with clean hull and prop will do 7.9 knots flat out with 27 hp. Three months later it's 7.5 knots. Even a trace of slime slows you down at least 5%, as does crusty patchy antifoul, which is why racers have smooth clean bottoms.
 
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