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Old 17-08-2005, 15:26   #1
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Small Engines and Barnacles

New to the Forum. New to Sailing. Excuse me if this is known and irrelevant!

My current boat - an S2 9.2A has a small Yanmar engine - 2GM putting out a whopping 12HP I think. It's a bit of a problem cos the S2 at 9800lbs is therefore rather underpowered.

I had 2 problems occur recently - the old Yanmar decided that it wouldn't go fast anymore - so instead of a max rpm of about 3500, it was now only willing to get to 2500.

The second problem was that my prop was getting covered in Narragannssett bay barnacles (prolific little buggers).

I was getting a double whammy therefore and my motoring speed was down from nearly 5kts to just under 3kts - difficult to deal with any significant current.

So I tackled the easy one first and went below and knocked off the barnacles. (Incidentally I did this with a ball-peen hammer - crack the shells and they drop like flies - is this the best method?). It took a while cos the prop was pretty well infested - and becasue I only have little lungs and couldn't stay down long.

When I'd finished and started her up, lo and behold, the engine's back to getting to 3500rpm (thank the Lord I didn't try to fix it first!)

I was really surprised that my lowly little Yanmar would be affected so greatly by the barnacle build up. Does this phenomena only happen to little engines or is it evident on bigger ones too?

Just curious...

Bill Balme
s/v Toodle-oo!
Outbound 44 #27
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Old 17-08-2005, 18:12   #2
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When overloaded - any diesel engine (larger or smaller) will emit black smoke*, overheat, and fail to reach fully rated RPM at WOT.
A foul bottom & prop’ may have a greater effect (in overloading) in a boat-engine combination that is slightly ‘undepowered’**, such as yours. A given load is represents a greater percentage of a smaller engine’s power.

* Black smoke may also indicate a restricted air supply, air or dirt in fuel, or a malfunctioning fuel injector.

** A good rule of thumb (and there are several) to determine the correct power for your boat is as follows: Pleasure use - two horsepower per one thousand pounds of hull displacement. Long range/world cruising - two and one half horsepower per one thousand pounds of hull displacement.
Eg: 9.8 x 2 = 19.6 HP ~or~ 9.8 x 2.5 = 24.5 HP (suggesting you might prefer between 20 & 25 HP vs the older 2GM’s 13 HP continuous rating.


Gord May
"If you didn't have the time or money to do it right in the first place, when will you get the time/$ to fix it?"

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Old 18-08-2005, 13:16   #3
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Bill- Welcome! In my experience I have found that barnicles are not very picky. They like just about anything, especially the prop and prop shaft on a 32' Pearson sailboat. When we laid her up last fall, the barnicles had built a highrise on the prop(also leading to less propulsion). So, yes, it happens to big and small boats. My particular cove does not have a strong tidal flush, leading to great conditions for the barnicles. We used a scraper and some sandpaper to get the prop and shaft back.

Fair Winds!
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Old 18-08-2005, 17:04   #4
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Whenever you clean the prop..

Clean also the inside of the raw water through-hull. I use an old flat-blade screwdriver to get the barnacle growth out of the thru-hull fittings where they can restrict the flow of water (do the ones for the head, etc. as well).

I use a 3-inch "putty knife" or scraper having thick gauge steel so that it is stiff and you can really whomp on those barnacle attachment points (the most difficult part to remove). The main body of the barnacle is easy to get off the rest must follow else new barnacle larvae more quickly attach and grow.

Follow the prop cleaning process with a rough 3-M pad to make it really smooth. The smoother the blades are the longer it takes for growth to become a problem.

The good news about barnacles: They cannot attach and stay when the prop is not protected by a zinc and if the metal is being eroded away by electrolysis. If you find a bright clean prop you have trouble!
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Old 18-08-2005, 23:11   #5
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Barnacles and Bottom Growth

My first keel boat was a Southerly 23 that had a 6hp outboard engine. When I bought her she hadn't been out of the water for some time and she had quite a bit of growth on the bottom. I sailed her like this for a couple of months.

I had the bottom cleaned and new anti-foul applied and I was so surprised at the difference this made to speed both under sail and power.

Regular cleaning is an important part of maintenance, and I have a broom with stiffish bristles and I use this to clean the hull from the deck or jetty.

Fair winds


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Old 19-08-2005, 03:24   #6
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All of the above...
Hey Gord, where did you get the info on power to weight. I was taught years ago, and have been going on the understanding, that the formula was 3 hp per ton for cruising vessels. This would make his 2GM only slightly underpowered, unless of course, it is the 2GM20, in which case, it would be fine for cruising.
Steve, another factor is RPM. That engine will only develope about 60% of it's rated hp at 2500 rpm. The sea growth not only slows the boat, but if the prop is too overgrown, the engine can not reach it's peak horspower. Anyway, you get the point. Good guess and good fix.
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Old 19-08-2005, 14:42   #7
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Hi Bill,
As you guessed and discovered and other posts have confirmed, barnacle growth on a prop really cuts down the engine RPM's and efficiency of the prop. That growth coupled with a barely adequate auxillary engine will ensure that you don't go anywhere fast and that if the prop is badly fouled you may have trouble maintaining control under power, particularly in reverse gear. My 37' sloop weighing in at about 14,000 lbs. has a 3GM30 Yanmar which is also barely adequate for the job. The builders of many boats apparently try to economize by installing the smallest engine they can get away with. Until you get your boat hauled out for a new bottom paint job, you will have to resort to frequent cleaning (perhaps monthly, depending on how fast barnacles grow in your area and how often you use the boat. When ever you take your boat out for a sail be sure to run the engine for a few minutes. Seems like frequent spinning of the prop will retard the growth or attachment of barnacles. Also, as pointed out previously be sure to clean out the thru hulls, although this may be quite difficult without diving gear. As for a tool to help with the scrapping job. There is a tool sold at hardware stores called a "painters tool" . It is about 6 or 8 inches long, with a putty knife style handle and has 4 or 5 facets on the blade that are useful in scrapping different shaped surfaces. I have found it to be quite handy. And the next time you haul out for a bottom job be sure to give special attention to priming and painting the shaft and prop. The paint won't last as long on a spinning metal surface as on fiberglass but a good paint job will lessen the number of times you have to go underwater. Interlux paints puts out a brochure that may be very helpful in that regard.They have a website at Hope this is helpful.
S/V Sun Dog
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Old 19-08-2005, 15:52   #8
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After leaving my sailboat for the first time. I returned after 6 weeks. I quickly checked all my systems and fired up the motor, started to prepare to shove off for my first solo voyage on my new home. Untied the dock lines, shoved here back put it into gear and nothing, by this time the current grab me and I was headed for the mango trees. I knew as soon as it happened what was going on. After my heart started racing and securing my boat to a mango tree I went below to find I had my own eco system growing on the prop and drive shaft. I found even though before I left weeks before and had just takin her out of the boat yard with fresh anti fouling on the drive train that I forgot to run a wire from the motor to the bolt of the strut I replaced. I think what happened is all the copper was destroyed because of electroylisis. I also added rule number 4 to my list, "always check to see if I have power before I untie my lines". My boat is in Ruskin Florida and from my understanding the warmer the water the more growth you get. I was also told not to put deep scratches into the bronze because that leads to pink brass.

There my .02 cents, and please excuse my spelling.

Good Luck

Matt Hager
SV Vagans
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Old 19-08-2005, 20:00   #9
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First of all, lets get one thing clear. It doesn't matter what HP you have, be it underpowered, correctly powered or way over powered, if the propellor is fouled, you ain't goin anywhere. So power has nothing to do with the movement part.

Matt, the problem you have noted as "Pink Brass" is called de-zinkafication. The Zink is being leached out of the bronze due to the electrolosys. For this to happen to the point of failure in only six weeks, you need to check you don't have an electrical problem somewhere. Ensure that ALL metal components are bonded together, or your anodes will not be protecting everything.
And yes, always check for drive in both forward and reverse before letting the line go. Why both forward AND reverse" well I know of a situation where the owner check for forward, Yep no problem and motor away. He was away from the dock, had to place it in reverse and suddenly a clunk was heard and no reverse. His prop had pulled the entire shaft clean out of the boat. So not only did he have no motion, but he was taking on water. OK, so maybe that is an extreme and hopefully rare case, but I have always checked both directions.

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Old 19-08-2005, 23:42   #10
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I could not resist...

I could not resist referring to Gerr's Propellor Handbook.
His tables and formulae suggest that a slip/length ratio for a 14000lb sailboat with a 24hp (18shp?) engine would be 1.15(not the 1.34 to 1.5 commonly assumed).
Estimating that a 37' sailboat would have a waterline length of 31' we get 1.15 * sq. root of 31 = 6.4 kts.
Further reading and calculation suggests that a propellor with a diameter of 16" and a pitch of 9.6" would be apropriate for 1200 rpm shaft speed.
My calculating ability can be higly suspect at times but if your propellor is a long way from these figures it may be worthwhile to get an engineer to check the propellor specifications.
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Old 22-08-2005, 03:44   #11
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Hey wheels, all the checking in the world will be trumped by human error. How long do you think it took me to diagnose my crew mate (Our yacht club comodore coincidentaly) forgeting one of the bow lines. Just can't take those authority types anywhere.

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