How slow does weed make a yacht go?

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Alcyone

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Noticed at the weekend that our 28ft Cobra seemed slower than usual, under power and sail. I estimate 1/2knot to 1knot. No change in trim, or weight, but I did notice more weed than I would have expected on the hull.

Can a growth of weed make that amount of difference?

Any thoughts?

Usually getting 4-5 knots, over the weekend, 3-4 knots, power and sail, upwind, downwind, just generally.
 
My mate leaves his boat on its mooring for 2 years, and the motor wont even drive against the tide to get it lifted out.
 
Right. OK. Ours is in Fresh water most of the time, moored in a river last summer and now in salt water. First time I've noticed growth. Need to look at some better antifoul for next year then.
 
Weed or Barney Kulls will easily make that sort of difference. Move back to your river location now & again during the season. Stuff that lives in sea water often can't cope with fresh & vice versa.

I am moored in a river/creek, mostly fresh water at low tide & salt at HW. Very little growth of weed or Barney Kulls - to the point where I only have one coat of A/F every other year.
 
Boat just gone back in, with totally clean prop and bottom.

Speed under power has increased by 500 revs, that is to say I can achieve 5 knots at 2000 rather than the 2500 I needed in April.(caught me a bit by surprise from lift to mooring.)

Wind today was about 6 knots and I was quite happily plodding along when last year I would have been struggling.

It also made the boat more responsive.

Cheap ways to get your boat to go faster;

Clean bottom (grid and scotch pads, costs peanuts)

Take out as much weight as you can from the boat.

Practise.
 
Yes; even moderate weed/slime will loose you up to a knot; heavy growth much more. Look at the trouble the racers go to keeping the bottom clean !
 
Alcyone
I nipped into you 'Profile' --- Funny!
Before we go into your question
I think the Searider is a great RIB!
Yep
Weed will make a great difference and 'Barney Kulls' (I like that Rushie!) these are my observations/ experiences.
Planing Hull, now a serious lack of perfomance will suffer from a 'beard'
Semi Displacement?
Now I have that type of mobo right now.
She cruises at 14 kts no prob with a clean Bum.
Any bit of growth and we are down to 10 for the same amount of throttle.
I had an old lifeboat once
8 kts cruising
She sat one Summer for 3 months without use at a place that encouraged growth under Her bottom.
Didn't look too bad at the time.
But believe me, 2 knots or more were knocked off the perfomnace.
We are talking mobos here.
Must make a heck off a difference to a 'Yogurt Raggie Whatsit' !!
 
klingons

nothing like a few klingons on the bottom to slow you down!..If your a bilge keel a tide on a sand bank with a sharpened garden hoe only take 30/40 mins to scrape
 
Searush said:
Weed or Barney Kulls will easily make that sort of difference. Move back to your river location now & again during the season. Stuff that lives in sea water often can't cope with fresh & vice versa.

I am moored in a river/creek, mostly fresh water at low tide & salt at HW. Very little growth of weed or Barney Kulls - to the point where I only have one coat of A/F every other year.
Click to expand...

I think therein lies the problem. In Cardiff bay, it's fresh water, with periods out in the channel. Last year, it was a river mooring, with periods in the sea. This year, it's all salt water.

Thanks for all of your replies. Missis and I are both divers. I spent two weekends layiing our mooring at Dale, so am I right in thinking it's her turn with the scraper......?

:D
 
Kawasaki said:
Alcyone
I nipped into you 'Profile' --- Funny!
Before we go into your question
I think the Searider is a great RIB!
Yep
Weed will make a great difference and 'Barney Kulls' (I like that Rushie!) these are my observations/ experiences.
Planing Hull, now a serious lack of perfomance will suffer from a 'beard'
Semi Displacement?
Now I have that type of mobo right now.
She cruises at 14 kts no prob with a clean Bum.
Any bit of growth and we are down to 10 for the same amount of throttle.
I had an old lifeboat once
8 kts cruising
She sat one Summer for 3 months without use at a place that encouraged growth under Her bottom.
Didn't look too bad at the time.
But believe me, 2 knots or more were knocked off the perfomnace.
We are talking mobos here.
Must make a heck off a difference to a 'Yogurt Raggie Whatsit' !!
Click to expand...

Re: Avon. I've been on a few, but keep coming back to the old searider. Mine is a dirty old grey, patched one with a scratched hull. Heavy. I've tried to abuse her at times, tried to bury the bow surfing down a wave etc, and she won't have it. Great boat, and just the ticket for diving, and carrying chunks of barnacle encrusted brass.

:D
 
Searush said:
Weed or Barney Kulls will easily make that sort of difference. Move back to your river location now & again during the season. Stuff that lives in sea water often can't cope with fresh & vice versa.
Click to expand...

One of my boats - I'm not rich, just rather bad at selling 'em - is in the river at Kirkcudbright. The salinity changes constantly there, from near-sea at high tide to near-fresh at low tide with the river running fast after rain. I dried her out a few weeks ago after five years afloat: a bit hairy round the water line, a wee bit of slime below and a few small patches of barnacles.

So seconded - changing salinity is a jolly good and cheap way of keeping bottoms clean. In the days when the boat used to winter in the Crinan Canal and summer in the sea she simply didn't ever need antifouling ... just a stiff brush round the waterline every month or two.
 
One of the few things that stayed stuck in my noggin from Ship Resistance classes at college many moons ago was the fact that even just a layer of slime on the hull bottom can knock 10% off your speed.
So you can imagine how this would be compounded by the addition of a few Barney Kulls (I love that name! :)) and Weeds.......
 
Last edited:
Drag

Presumably your Cobra 850 is a sailing boat.
Drag on a hull is what stops it going fast.At low speeds like under 5 knots drag comes from surface area of the hull. You might construe that the surface area is hugely increased by weed and barnacles while the shape causes more turbulence at the skin. Now as you increase speed to over the "hull speed" towards 7 knots, while the drag from the fouling will increase the wave making drag of the displacement hull becomes exponentially huge making the fouling drag insignificant.
So at lower speeds fouling is far more devastating to resultant speed. A new a/f paint job will show you just how much. A good clean while diving will be nearly as good. See my comments below on diving. olewill
 
Ours gained just over 1 Knot after jetwash.



Alcyone said:
Noticed at the weekend that our 28ft Cobra seemed slower than usual, under power and sail. I estimate 1/2knot to 1knot. No change in trim, or weight, but I did notice more weed than I would have expected on the hull.

Can a growth of weed make that amount of difference?

Any thoughts?

Usually getting 4-5 knots, over the weekend, 3-4 knots, power and sail, upwind, downwind, just generally.
Click to expand...
 
You're doing quite well; I did my Stag 28 over the weekend - 13hp motor gave me 4kts to scrubbing posts (diesel pouring black smoke on full throtle) with bad fouling and 6.5 kts on the way back to the mooring with the diesel revving cleanly afterwards...

.
 
I would say that a light coat of slime will knock 20% off your speed.I dive evrey two weeks or so to clean the bottom.I've even developed a technique using a push broom.Whenever there are barnacles a short piece of wood is used to scrape them off.
 
New anti fowl (:-) this spring but still managed considerable growth. Took her out and sailed briskly for 300 miles and pouf ... all gone.

On the return journey, we made 9.2 on primary GPS and 9.1 on the cockpit unit. ISO 7.5-8 kts before trip. Usage helps keep her clean. Next weekend is the waterline cleanup.

Now pleased again.
 
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