# Bottom growth and its effects on speed



## TSOJOURNER (Dec 16, 1999)

Hi everyone,

I know I'm supposed to have the bottom of my sailboat scrubbed every so often to prevent the growth of barnacles and slime. I'm also supposed to defragment my hard drive, go to the dentist every 6 months, eat broccoli, and do all kinds of short term pain / long term benefit activities. Point being, I'm trying to gauge how lazy one can be with this.

I understand the effects of all this, including the fact that growth slows down the boat. Question is, by how much? Obviously this depends on many things (e.g. I'd think the speed differential should be much larger in stronger winds.) But let's just take an average case: say I'm going 5 knots or so in 10 knots of wind. I don't go beyond the slime stage. Would growth cause .25 knots of speed differential? Or 2 knots? I don't know what a normal number should be.

In a similar manner, I think my speedo is showing slower speeds than actual compared to before - at least that's what my GPS shows. I know about tides can affect this differential and have already accounted for it. Is it common for growth to cause extra friction (of sorts) to the speedo and cause e.g. 6 knots to register as 5 knots?

(I've mostly been on the asking side of forums recently, but one of my previous questions resulted in such a crazy response that many of us got entertainment value out of it - search for the word "pressure" in
http://www.sailnet.com/forums/seamanship/17999-slightly-unorthodox-anchoring-technique.html)

Thanks!
Iraklis


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## Sasha_V (Feb 28, 2004)

It is not just your speed that is effected.

If you have growth then the wind still pushes your sails just as hard, but your hull isn't slippy enough to convert that energy to speed...so you heel a lot more. A LOT more.

I once had the embarassing experience of having a gunnel firmly plowing through the water, waves washing up over the saloon window...and travelling at 1.8 knots in forwards! Not comfortable or cool.
It also stresses the rig and rudder system more.

Look at it this way, go swimming with a mitten made out of some old carpet, give the boat hull a bit of a scrub and then you do not have to worry about it.



Sasha


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## catamount (Sep 8, 2002)

Iraklis said:


> I'd think the speed differential should be much larger in stronger winds.


In strong winds you have more power available to help overcome the extra drag, so the RELATIVE speed differential is not as great. It is in light winds where the extra drag will be a more significant problem.



Iraklis said:


> In a similar manner, I think my speedo is showing slower speeds than actual compared to before - at least that's what my GPS shows. I know about tides can affect this differential and have already accounted for it. Is it common for growth to cause extra friction (of sorts) to the speedo and cause e.g. 6 knots to register as 5 knots?


Most speedo's are simply little paddlewheels sticking below the hull. It is not un-common for growth to foul the paddlewheel so much that it doesn't turn at all and your speedo doesn't read ANY speed. For this reason, the paddlewheel transducer is usually designed to be removable from the hull, even while the boat is still in the water.

Regards,

Tim


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## TSOJOURNER (Dec 16, 1999)

Hi Tim,

I wrote the exact opposite of what I was thinking - the speed differential should be higher in *less* strong winds; when the wind is strong, you'll probably be going close to hull speed regardless. I totally agree with you on that. 

Also, this is exactly what's happening to my speedo; in the beginning of any sail it fails to register any speed; afterwards, it shows me some speed, but not my true speed.

As far as removing the transducer while in the water, my local West Marine manager told me that he's only seen it done twice by professionals and that "it wasn't pretty". So I'll admit I don't have the balls to do it...

Not to look a gift horse in the mouth - I appreciate the responses, but I still didn't get a clear understanding of how much exactly the growth will slow me down in knots, at least in some average case.

Thanks,
Iraklis


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## sailingdog (Mar 19, 2006)

Removing the speed transducer isn't all that big a deal, but alot of it depends on the boat and where the transducer is located. The deeper it in the water it is, the bigger a deal it becomes. Transducers with easy access are less of a problem to swap out, than ones with limited access. 

Growth on the hull not only slows your boat and causes it to heel more, it also can damage the hull. The slime allows barnacles to get ahold of the hull, and the barnacles will attack the laminate. The adhesives they use to attach to the boat are among the strongest ever created by man or nature. 

As for how much slower it will make your boat, that really depends on a few things. 

1) how much growth in on your boat. Are you talking a thin coating of slime, or clusters of barnacles. Big Difference. 

2) How strong the wind is, the stronger the wind, the less you'll notice the effects of the growth, to a point...provided the growth isn't clusters of barnacles. 

3) the size and shape of your boat. A small boat, with alot of growth on it is probably going to be affected more than a larger boat with the same amount of growth. This is due to the larger boats larger sails and a larger underwater profile to begin with. 

Just my $0.02 worth...hope this helps.


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## lgherb (Jul 2, 2006)

Iraklis said:


> Also, this is exactly what's happening to my speedo; in the beginning of any sail it fails to register any speed; afterwards, it shows me some speed, but not my true speed.
> 
> As far as removing the transducer while in the water, my local West Marine manager told me that he's only seen it done twice by professionals and that "it wasn't pretty". So I'll admit I don't have the balls to do it...
> 
> ...


Cleaning the growth off of the transducer unit on your knot meter should be pretty easy to do. There is most likely a thru-hill that the sensor unit fits snugly inside and is held in place with a cotter pin.

You want to have a thru-hull plug handy in order to quickly plug up the thru-hull once you remove the transducer - some models actually come with a plug that is identical in size to the speed sensor that you use to plug the hole. You want to work fast as water will start gushing into your boat at the rate of about 15 gallons per minute (depending on the diameter of the thru hull.)

Remove the cotter pin and pull straight up on the transducer. It does not thread in to the thru-hull, but has an O ring or two to prevent water from leaking into your bilge.

Once the transducer is out and the plug is in (there will be a small amount of water still coming in, most likely), use a toothbrush and pocket knife to remove and scrape the growth off of the paddle wheel on the transducer. You may or may not need to use the pocket knife.

When the paddle wheel on the transducer is clean and able to spin easily, note the arrow on the top of the transducer - this not only points to the bow but usually is aligned with the cotter pin hole so that you can get everything back in as quickly as possible.

That is basically it...you don't need to be a professional to accomplish this, you just need to work fast when removing the speed sensor and putting the dummy plug or thru hull plug iin place. So here is a recap:

1. Find transducer thru-hull (usually somewhere in the v-berth) and have thru-hull plug or "dummy plug" from manufacturer ready.
2. Quickly remove transducer/speed sensor from thru-hull and replace with suitable plug.
3. Clean fouling off of paddle wheel with old toothbrush and/or pocket knife.
4. Quickly replace transducer/speed sensor back into thru hull ensuring that reference arrow is pointing forward.

This will probably take a maximum fo 15-20 minutes to do.


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## metrognomicon (Aug 15, 2006)

It might increase the comfort level to keep an extra tapered plug or two in the locker where the thru-hull is located....just in case.


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## SteveCox (Jul 12, 2006)

On the one boat I have done it on, cleaning the wheel was no big deal. It did cause a lot of trepidation the first time I did it though! On mine there is a cap over the speedo proper that threads onto the fitting. There is also an identical plug that is kept right next to the through-hull. It is simply a matter of unscrewing the cap, lifting it out, and screwing in the blank fitting. It takes about 10 seconds tops and really doesn't let in very much water. HOWEVER, this is a 26' San Juan so I'm only about 12" below the waterline. If your speedo is located lower than this the amount of water pressure will increase dramatically but it still isn't any big deal as long as you have a plan and have thought through the process. BTW, keep a wooden plug next to you while doing this in addition to the speedo blank just in case


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## Gene T (May 23, 2006)

Cleaning the spedo is easy, just freaky the first time. I like to use silicon grease on the o-rings when they are out to make them easy to slide in and out. Cleaning the bottom is easy with a dry-diver (at least I think that is what it is called). I have used them with great success from the finger piers. Won't get the keel, prop or rudder but cleans the hull nicely. I just cleaned the rudder from my dingy or dock by using a boat brush with a long handle. I would hire a diver to clean the bottom and check the zincs every few months. A clean bottom makes a big difference in the joy of sailing.


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## Fstbttms (Feb 25, 2003)

A dirty hull will reduce boat speed by as much as 20% or more. It negatively affects boat handling and fuel consumption as well. Having a clean bottom is as important as proper sail trim, maybe even more so. You can correct a poorly trimmed sail, but with a dirty bottom your boat speed is being reduced every foot of every mile, every second of every hour you are on the water. Not only that, but a dirty hull looks unseamanlike. 

And there is no substitute for have a set of eyes and hands go over your boat's bottom on a regular basis. Dockside cleaning tools like "Dri-Diver" are all well and good, but as mentioned before, they will not clean the keel, running gear, through hulls or transducers. They can't report on blisters or zincs, much less less replace them. And I guarantee that even the most well-intentioned "Dri-Diver" user is missing spots on the hull. 

Don't kid yourself. Having your boat's hull cleaned on a regular basis by a diver (either yourself or someone else) is as important a part of boat maintenance as just about anything else you can name. Not that I'm biased or anything.


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## btrayfors (Aug 25, 2006)

One other consideration. In certain conditions, the stronger the wind blows the MORE a dirty bottom will slow you down, sometimes dramatically. This is when you are powering to windward in a chop or sizable seas. A dirty bottom may then prevent you from ever achieving a reasonable hull speed, i.e., enough to overcome the stopping motion caused by head seas.

In the Drake Channel in the BVI's, for example, I've noticed that a dirty bottom can make the difference between 3 knots and 5-6 knots when banging into headseas.

The moral is simple: keep your bottom clean!

Bill


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## catamount (Sep 8, 2002)

Fstbttms said:


> Don't kid yourself. Having your boat's hull cleaned on a regular basis by a diver (either yourself or someone else) is as important a part of boat maintenance as just about anything else you can name. Not that I'm biased or anything.


When I bought my boat in the Chesapeake last year, the existing anti-fouling (VC Offshore) was at least a year old. The boat was on the hard, and we splashed it for a sea-trial and to complete the survey. After closing the deal, I debated having it hauled back out to renew the bottom paint (the boat was going to be stored in the water through the next winter, before being sailed up to New England). Ultimately, I decided to forgo the fresh bottom paint and instead have a diver come by every month or so (except in the dead of winter) to scrub the bottom. In the end, this was less expensive than the the haulout and bottom job would have been, and even if I had done that I probably would have had to have a diver come by anyways! I also very much appreciated getting the brief monthly reports on the condition of the boat's bottom, zincs, and underwater hardware that were submitted each month with the diver's invoice. Now the boat is on the hard again, and I would have to say that the bottom cleaning service really did pay off.

Regards,

Tim


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## mrkeith (Jan 25, 2006)

*bottom cleaning*

how do you clean the bottom w/out removing the antifouling. everytime i even touch the bottom lightly, my hand comes away w/ antifouling. so by rubbing the hull to make it clean, are nt we removing the antifouling? sorry for asking such a silly question


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## Fstbttms (Feb 25, 2003)

mrkeith said:


> how do you clean the bottom w/out removing the antifouling. everytime i even touch the bottom lightly, my hand comes away w/ antifouling. so by rubbing the hull to make it clean, are nt we removing the antifouling? sorry for asking such a silly question


It sounds like you have an ablative anti fouling paint. These are fairly soft paints and do not stand up particularly well to in-water cleaning although they generally need it every bit as often as harder paints.

I clean ablative paints with the same kind of scrubber I use on epoxies, a 3M "Doodlebug" pad. Hopefully your paint is in good condition and you can clean it with the least abrasive pad, the white one. Even if you have to go with the next level up, the blue pad, if you are careful you won't damage the paint excessively. Keep in mind that _*any*_ cleaning of an ablative bottom is going to result in a plume of paint in the water. That is the nature of the beast.

Use the least abrasive scrubber you can that will do the job, be gentle, take your time and you will be able to clean you ablative bottom on a regular basis and get a reasonable lifespan out of the paint.


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## Sailormon6 (May 9, 2002)

A dirty bottom doesn't affect only your speed. It affects everything about your boat's performance. As sailboats move to windward, they generate higher apparent windspeeds, which generates more power in your sails and thus more boatspeed. More boatspeed enables your boat to sail closer to the wind, which means you can sail a shorter distance to get to a destination to windward. When the water moves faster and smoother over the rudder and keel surfaces, the boat performs livelier. A boat with a foul bottom behaves sluggishly when you tack. When you're going in to a tack, the boat decelerates faster, and when you come out of a tack, the boat accelerates more slowly. A boat with a foul bottom won't coast as far. Thus, when the wind dies, your boat will be the first to come to a stop, and the slowest to start when the new wind arrives. A dirty bottom takes the a lot of the fun out of sailing and makes it frustrating.


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## CoralReefer (Jul 20, 2006)

To answer your original question on the speed, I just bought a 26 footer and it was kept in brackish water and it had a lot of black slime with a few barnicles. I'm 35 miles from an inlet now and it would take 6 hrs to make the trip down the ditch (going app 5.2 kts maxed). After scrubbing the bottom with a dri-diver 3 times and power washing the rudder the trip now takes me about 3.5 to 4 hrs and my speed ranges between 7.5 and 8 kts. I'm waiting for a blast of warm water to get under and do the keel. Speed measured with a GPS


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