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Jul 30, 2009
Vessel Name
Skinny Dippin'
Vessel Make
Navigator 4200 Classic
Can slime on the bottom slow you down?

I have lost a couple-hundred rpms and thought it was the fuel filter... It appears not to be the case. I know the bottom is getting some growth since the haul-out in December. Could that be the culpret?


In our marina I use a diver that does the boat every other month in the winter and every month in the June -July - August*summer months.* I have had*no problem with slime or growth.* Also used Trinidad SR as the bottom paint and got over two *and one half years out of it.* So either pay the diver or do bottom paint or some form of both.* The diver also keeps the running gear clean.

We will see how the new boat does once we get her up to New Bern in a couple of weeks.

Sorry didn't answer the question.* The answer is yes with out a doubt.

-- Edited by JD on Monday 30th of August 2010 05:04:44 PM
Every month? Do you need to do it that often? Well, I can imagine that if you have a diver on it every month, sure, you will never have a slime problem. That's like saying, I bath every 30 minutes so I have never had a problem smelling bad :)

We painted her bottom in December. I guess I'll get Bud to put a diver on her this month or I'll do it.

-- Edited by GonzoF1 on Monday 30th of August 2010 05:09:30 PM
Anything that causes drag will reduce speed. A thin coating of slime will reduce speed but the amount may be so insignificant to not be worth bothering about, particularly in a 28,000 pound, eight-knot boat like ours.

We have our boat's bottom paint renewed every two years (we're going six or eight months longer this year because of my work and travel schedules but hope to get it done in October or so).

Divers can no longer wipe down boat bottoms in the water in our marina (and perhaps other marinas in Puget Sound, I don't know). The reasons are environmental. We use a so-called Slime Resistant (or Reducing) ablative paint and it seems to do a pretty good job. When we haul we'll have barnacles on the rudders and shaft struts becaus the prop blast takes the paint off the rudders faster than it wears off the hull, but the hull itself is pretty slime-free. Or at least there's not enough on there to worry about. We also use the boat year round so we're "hosing it down" with the water movement past the hull every time we go out

I would imagine warmer water is more condusive to slime formation than colder water, so that may be one reason we don't see a lot of it up here.
Gonzo:* Check your propeller and keel.* It's coming toward the end of summer and I bet the water is pretty warm where you are.* We have to clean ours monthly.* The water temperature runs from the mid 70's to almost 90.* Plus some areas are very rich in nutrients.

Let us know what Bud finds out

Hobo KK42
La Paz, BCS, MX
Marin's situation is another example of the law of unintended consequences. Since they can't clean the hull in the marina, he uses an ablative paint that sloughs off on it's own.
*Here in the Galveston bay area, I use Petit's Trinidad SR (slime resistant) and I get 4 years out of us usually, with no slime buildup at all. I quick haul the boat annually and it's usually as clean as a baby's bottom, except for the metal strainers. I have the yard pressure wash it mostly just to clear those out. I have Propspeed on the prop, so just running the boat slings off anything that has tried to attach.

I think in your case JD, you may have too much cleaning going on, wearing the bottom paint off too early.

Gonzo, I'd look elsewhere unless your bottom or prop is really fouled up.
Keith wrote:

Marin's situation is another example of the law of unintended consequences. Since they can't clean the hull in the marina, he uses an ablative paint that sloughs off on it's own.
Sort of.* We don't use an ablative paint because the divers can't wipe it down.* The no-wipe-down*reguation just went into effect a couple three years ago.* We've been using ablative paint since we bought the*boat twelve years ago, mainly because it greatly reduces the buildup of paint on the bottom.** And of course*it doesn't slough off with the boat just sitting in its slip, which is why it's probably not the best choice for bottom paint on a boat that doesn't get used all that much.

The no-wipe-down lawin our marina*is to reduce pollutants in the harbor itself.* But it wouldn't surprise me if someday the notion of ablative paint is abolished altogehter.* If it is, I would expect to see it enacted in California first so at least*the rest of us will have fair warning that it's coming.

To answer your question, yes, theoretically slime can slow you down. BUT, I do not believe it would cause a loss of 200 RPMs. It is either fouled with something more than slime, or you have something else going on.
Here is a picture of Willy last week w a bottom w over 2 yrs time on it. Looks rather bad but there was no barnacles on the hull or prop. I get as much Armorall on the prop it as I can just prior to launch. The hull had 3 coats of Interlux Micron Extra. I experienced no detectable speed loss. I cruise at 2300 rpm and 6.15 knots. Electronic tach and Garmin 2010 GPS. The Micron Extra was a good antifoulant but I had to scrape it off. I've got Pettit "Sea Mate" on now at $83 gal and won't be reporting on it for some time (I hope).
Eric Henning


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Props and rudder are clean. I just looked at them two weeks ago.

I guess I need to figure out what else could be going on.

*Side Note* We used CPP paint on the bottom in December and Trinidad on the prop and rudder. It's done quite well. Virtually no growth (short of the slime) on the bottom. It's the middle-of-the-road priced paint and it works very well.
Gonzo,I'm not sure but I don't think putting anti-foul on the prop is a good idea. Copper is the active ingredient in most AF coatings and even though both metals are nonferrous there must be at least some interaction between the prop and the copper. Usually if one runs a boat fairly often fouling of the prop isn't a problem. I use Armorall on my prop and haven't had any fouling since (several years).
Armor All?

To answer your concern, this is a "system" that includes a primer that separates them.
GonzoF1 wrote:I have lost a couple-hundred rpms and thought it was the fuel filter... It appears not to be the case.
Just to be clear, do you mean that it takes 200 more rpm to achieve a given speed now?

If this is the case, and if your engine is running fine, not smoking, not doing anything obviously different, is it possible the error is in your knotmeter?* Has the paddlewheel sender (if that's what you have) built up some friction (which could be bottom growth up in it)?

Or, in the opposite direction, could it be your engine tach is off by that amount?

WoT used to be 2750 a few months ago. It now only maxes out at 2600. Everything else seems normal.
Do you have an EGT (exhaust gas temperature gauge)? Good thing to have if you don't. Also known as a pyrometer. They show you how hard your engine is working. If you were getting less rpm at WOT with a higher EGT, then additional drag from some source could be the problem.

But as Baker said, I can't imagine some slime on the hull would cut your power back by that much, if any. It could cut your speed back a bit (maybe) but I don't know that I'd expect that much difference-- or any difference--- in your WOT rpm.
I have a hand held unit I have wanted to use for boat diagnostics, but don't know exactly where to test the temp.
A true EGT uses a probe that sits in the exhaust stream itself. On our boat they are mounted in the water-cooled exhaust elbow upstream from the point where the cooling water is injected.

An infrared temp sensor outside the engine will sense the temperature of the outside of the engine, but to get the true value of knowing the exhaust temperature the sensor needs to be physically in the exhaust stream. This also give you an instantaneous indication if the load on the engint goes up or down, where measuring the temperature of the outside of the exhaust components will have quite a lag to it since the metal has to heat up first. And you may never get the same temperature reading on the outside of the exhaust components as you will inside since the metal parts are losing heat to the surrounding air all the time.

-- Edited by Marin on Tuesday 31st of August 2010 05:01:13 PM
Gonzo, when I take my boat out after it's been sitting for a few weeks, it always takes a while to get "up to speed". I baby my 37 yr old Lehman at around 2000 RPM where it's happy, no smoke or excess temps.

When I first leave the dock I'll be going 6.8 kts or so but within ~5 miles I'm up to 7.4-7.4 kts. (GPS measurement) I attribute it to accumulated slime and the water "scraping" it down as I run the boat.

The concept of "optimizing performance" in a '73 trawler seems a little oxymoronic., and I'm just glad she runs reliably and well. If I wanted those extra kts I'd have bought a Searay.
ARoss wrote:When I first leave the dock I'll be going 6.8 kts or so but within ~5 miles I'm up to 7.4-7.4 kts. (GPS measurement) I attribute it to accumulated slime and the water "scraping" it down as I run the boat.
Wow!* Don't you think that tide, wind or current is helping the knot meter to elevate?

ARoss wrote:*

When I first leave the dock I'll be going 6.8 kts or so but within ~5 miles I'm up to 7.4-7.4 kts. (GPS measurement)
A GPS is relatively worthless as far as telling you what speed you're really going because it's measuring speed over the bottom, not speed through the water.* The only time GPS means anything relative to a boat's speed through the water--- which is what you want to know with regards to drag, etc.--- is if there is no current or wind whatsoever.

A knotmeter--- if it's accurate--- is more useful in determining how a boat's speed is being affected by bottom fouling, prop performance, etc.* Over even a one mile stretch our GPS-measured speed can vary as much as three and even four knots around here, depending on what currents we happen to wander into.* But our boat speed through the water remains constant over this same stretch*at any given rpm.


For what its worth, in our area, I am noticing this last month especially, enough growth in 2-3 weeks to slow boats down. Even that black triiple engine arneson driven 1,320hp beast down the dock is getting slowed down in that amount of time. While I would expect the growth to affect the overall performance less on a slower boat, even a little growth (or a line?) on the prop will slow you down. Unless you cleaned it yourself last time I would double check to make sure the prop is smooth as a new born baby's behind. Also, if the diver scratches the paint on your prop, it gives the growth even more places to take hold faster.

Next time you haul, I would spend the money for Prop Speed. We hauled our boat 12 months after putting it on. Even with minimal use the first 12 months due to all the other work going on, we only put a diver under the boat once, and because we ran it once a month, had very little growth on the prop. The paint was in good enough shape after 12 months to leave it alone, and we still have had very little growth on it this summer. While it is marketed more to the high performance boats, I feel it is worth the money on cruising boats like ours too.....
ARoss,I'm wondering if maybe your tach needs warming up. Is it non-electronic? Perhaps a cable drive? If you go 6 knots at 90 degrees and 270 degrees delivers 8 knots your speed is about 7 knots and the tidal current is 1 knot. Thats about as close as one can get to your real speed through the water.
Hi Tim... I didn't know you lurked around here. YAY!

Like I said, the prop seem clean. I dove on it when we were in Beaufort two weeks ago. However, I didn't put a magnifying glass on it. :) We're probably going to pull her out sometime this Fall and do a few little projects. We'll see then what is going on under there. But we are also thinking about putting a diver on her to de-slime her before the Summer ends. Just to disprove the theory.

I suppose I'm still wondering where those 150 rpm's went. I wonder if it's just a throttle cable adjustment. You get kid of tangled up in it whenever you go into the engine bay, having to push a pull it to get past it. I wonder if I have knocked it out of adjustment. Are there "stops" that mark the high limit of the throttle like there are for the idle speed?

What is Prop Speed?

Skinny Dippin'
New Bern, NC

-- Edited by GonzoF1 on Friday 3rd of September 2010 04:58:48 AM*I've had it on my boat for about a year and a half and it works great! Tricky to apply, although it can be done but takes two people, since you have to etch then coat immediately.
Interesting stuff, but I'm skeptical to its comparative effectiveness. I used a similar system when we were out in December that included a primer coat with Petit hard bottom paint applied to it and we have virtually no growth on our gear either (after 9 months). I talked to a few "old salts" about it and what they said was that they have tried about everything they could think of to stop it and none of it worked and better than just leaving it bare and running the boat often. But I applied it anyway since the process was so cheap to do. Literally was about $30. I don't know what Pro Speed costs, but I would venture that it's FAR more expensive than that. Does it work better than what I did? I certainly am not the one to answer that, but what I can tell you is that what we did, including the regular usage, has worked swimmingly.
Several have commented that I should factor in wind, current, heading in figuring out actual speed thru the water. From our marina, there's only one place to go - a straight shot 120 deg down the Pamlico River. Took the boat our Sunday to a raftup ~ 6 miles downstream, and during the run (no wind, very little current) the GPS speed gradually climbed from 6.6 to 7.3 kts @ 2000 rpm. On the way back, 4 hours later, I ran 7.3 kts the whole way, same throttle setting. To me, this seems to remove most or all of the wind/current issue, and I attribute it to reduced drag as the slime worked itself off the boat.
Top speed is not the issue (although, I haven't measured, or noted, any loss of speed at cruising rpm's), I lost 150 rpm's at WOT and was wondering if more drag on the bottom created by slime could be partially at fault.
Gonzo,Your problem is not slime. It's mechanical.
GonzoF1 wrote:

Top speed is not the issue (although, I haven't measured, or noted, any loss of speed at cruising rpm's), I lost 150 rpm's at WOT and was wondering if more drag on the bottom created by slime could be partially at fault.

You mentioned getting in and past the engine you may have to*move the cable.* Check that you didn't move the outside of the cable at the end points. On a Bowden cable if the end points are not clamped down tight enough it will change the throw of the lever.

Will do.

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