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Author Topic:   Performance change from sanding bottom
acseatsri posted 08-01-2004 02:17 PM ET (US)   Profile for acseatsri   Send Email to acseatsri  
Just a quick note- after sanding bottom paint smooth with 400 grit paper, I picked up 300 rpm on the top end (from 4800 to 5100 with a 14.5 x 19 SST prop). I was bouncing around too much to read the GPS and see what the actual speed was. Hopefully someday someone will invent glasses with built-in gyroscopes like they have done with image-stabilizing binoculars!

On another note- thanks for the recent helpful discussion of deep-cycle batteries. I ended up having to limp home on my kicker yesterday- too many starts on short drifts, plus a weak back-up battery. Could the weak batteries be the cause of my electronics shutting off every time I start my outboard? I've always had this problem, even when batteries were new. Of particular note- my outboard has the miniscule 9 amp charging circuit.

raydent99 posted 08-01-2004 02:47 PM ET (US)     Profile for raydent99  Send Email to raydent99     
Check the battery terminal contacts for corrosion if your electronics keep shutting off when starting your motor.

jimh posted 08-01-2004 03:36 PM ET (US)     Profile for jimh  Send Email to jimh     
I have been wondering if that might make a difference on my boat. It has no bottom paint, but there is a rather rough surface of little pits of marine detritus left over from previous sailings. Other than those, it is a smooth, shining, gel coat surface.
jimh posted 08-01-2004 03:38 PM ET (US)     Profile for jimh  Send Email to jimh     
Some electronics shut down from OVER voltage, as well as under voltage. That may be happening on engine start due to transients. A weak battery will not effectively dampen voltage transients.
ryanwhaler posted 08-01-2004 04:11 PM ET (US)     Profile for ryanwhaler  Send Email to ryanwhaler     
I sanded the bottom paint off my little sport eleven and I don't notice any difference in preformance.
acseatsri posted 08-01-2004 04:13 PM ET (US)     Profile for acseatsri  Send Email to acseatsri     
Re sanding the bottom- I'm not sure if I would take sandpaper to a shiny gelcoat- my paint is now smooth, but in no way imaginable does it shine- I don't think it's possible to impart a shine on Micron CSC. And to clarify, my bottom paint was rolled on and was very bumpy- a far cry from a few dings in the gelcoat. If you were to fill in the dings and then go thru the laborious process of restoring the shine, perhaps you would pick up a little speed and efficiency, but probably not enough to justify the time and expense of doing so.

Re batteries and connections- I'm replacing the old battery with a deep cycle battery, and keeping the 1 year old Diehard Marine starting battery. I also plan on recharging the batteries after every outing from now on, especially if doing short drifts for fluke very near the launch ramp... the 9 amp charging circuit just doesn't provide enough boost to replenish the starting drain. I would guess that at my usual cruise of around 2500-3000 rpm that output is only about 5 or 6 amps at best. I checked it once on the trailer- at 1500 rpm on the earmuffs it put out less than 4 amps into a weak battery (measured with a digital volt/ohmmeter).

The terminals looked like they did when I installed the batteries this spring- no corrosion, still bright from the wire brush and sandpaper I used to clean them. But the wingnuts probably could have been tighter. I'm going to replace them with stainless hex nuts and washers.

acseatsri posted 08-01-2004 04:19 PM ET (US)     Profile for acseatsri  Send Email to acseatsri     
Ryanwhaler- at slower speeds, I doubt you would see any noticable difference in performance. The difference can be seen at speeds over 30 mph and especially over 40, where the horsepower required to gain a few mph goes up dramatically.
Sal DiMercurio posted 08-01-2004 10:36 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
If you gained 300 rpms by sanding the bottom, you had alot of marine growth besides paint.
There are pros & cons on bottom paint.
My boat gets the same speed it did before I painted the bottom.
I do believe if the bottom is painted with a stipple roller & the paint dries like your bathroom kinda ripply, your going top get more speed because your getting more air under the boat than if it were completly smooth.
Smooth water dosen't allow your boat to get it's maximum speed, ripply or 1'- 18" chop is optimum.
My buddy Dunk on the WMI board feels the same way.
I think this bottom paint thing where people are believing they will loose 5 mph by painting the bottom is a farse.
When I was racing professionally we would paint the bottoms of our flat bottoms with grafite paint to make it faster.
We ran these flat bottoms over 90 mph.
I think you had other problems with the boat [ marine growth ] or engine if you gained 300 rpms.
Did you by any chance raise the engine a set of holes & trim it farther than you ever did, did you have 1/16 of a tank of fuel with no one else aboard?
I'm not trying to be a smart ass but 300 rpms is just a whole lotta rpms to gain by just removing the bottom paint.
Chuck Tribolet posted 08-01-2004 11:58 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
Cranking is going to pull the voltage down, perhaps as low
as 6V. Humminbird used to make a box called the SureVolt.
Put anything from about 6V to about 20V in, 13.8V comes
out. It cured the shut down problems with my sonar and
GPS. It will only generate 1.5A, so it won't run your VHF.


Peter posted 08-02-2004 08:01 AM ET (US)     Profile for Peter  Send Email to Peter     
I'm surprised by the 300 RPM increase too. However, if the bottom was previous built up with many layers of paint which tend to flake off over time and leave craters, I suppose its possible.

If I recall correctly, you've got a mid 80s Evinrude or Johnson 150. Why didn't you just pull start it? It will run with a dead battery and if still warm it will start very easily. If cold it will still start but the effort is a little greater.

Bigshot posted 08-02-2004 12:31 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Bottom must have been a horror show. I have run the same boat with and without paint and noticed no difference.
acseatsri posted 08-02-2004 05:22 PM ET (US)     Profile for acseatsri  Send Email to acseatsri     
The bottom was pretty rough- I rolled it in midsummer in 85 degree heat- it was nearly drying on the roller. It had a sandy finish.

Re raising the engine- thanks for reminding me- I forgot I raised it one hole (3rd hole up) and removed the wedges when I had to nearly remove the engine to replace the trim pump. I didn't try any fully trimmed out speed runs (and probably never will!). The run I did I had it trimmed very modestly- probably just perpendicular to the keel.

Re pull-starting the engine- if I had farther to go back to the dock, I might have tried it. But seeing how I had put 0 hours on the kicker this season and last season, I thought it might be a good idea to at least try it out for the time when I really needed it- the 10 horse Honda moved the boat along OK, about 4.5 knots going against the tide. It took a long time to get past the Niantic RR bridge- I doubt we were moving .5 knot against the current- glad there were no boats coming as the current really screams thru there.

PS- now that you've mentioned it, I think I'll try pull-starting it just to see if I can do it. Thanx!

jstachowiak posted 08-06-2004 04:06 PM ET (US)     Profile for jstachowiak  Send Email to jstachowiak     
I raced Hobie cats and Prindle catarmarans and everyone always sanded their hulls with 400 or 600 paper. I believe it is called laminar flow.

The idea is the very fine grooves made by the sandpaper leaves a thin film of water on the surface of the hull and nothing is slipperyer than water on water.

Smooth glassy finishes actually sets up a swirling of water on the hull causing drag.

Dr T posted 08-07-2004 11:42 AM ET (US)     Profile for Dr T  Send Email to Dr T     
Back to Jimh's point on transients and Chuck's follow up:

Transients (essentially spikes in the current of very short duration) can cause harm to your electronics. Over time, the transients can cause the electronics to fail prematurely. The device Chuck referenced dampens these transients out.

I turn everything off while starting the engine (habit I picked up when flying). If you have a fairly extensive array of electronics, you may want to consider a master switch.

A note on wiring: even though your terminals may look OK, you should inspect the entire lenght of your cable runs. I have seen old marine grade wiring with a longitudinal crack in the insulation that was harboring corrosion INSIDE the wiring over several feet of the run. This will really cut down on your ability to start the engine.

Side note to those with older Diesel pickups: If your engine is getting harder to crank, particularly in cold weather, inspect the battery cables. I had the same problem on an old 89 Ford Diesel once.


JohnJ80 posted 08-07-2004 04:36 PM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
These dual subject threads are hard to follow.

On bottom sanding -
Being an avid yacht racer, I have spent more than my share of time wet sending hulls to get them fair. 300 grit is where you start and work your way down to 1200 grit for yacht racing. Then, if you need bottom paint, you put on something like V-17 which is an anti fouling paint that goes on like water and has teflon in it - very slippery. There is a version for powerboats.

If you gained 300 RPM after sanding with 300 grit, you had some other problems there first - like growth or a unfair spot in your hull.

On batteries -
any current technology motor needs a good battery - much more so that for carbed 2 strokes. With fuel injection and electronic ignition, if the the battery voltage sags, you can be S.O.L. The trend on motors is to provide lots more charging current - one reason for this is to make sure that you battery gets topped off ASAP after starting. For this reason, if you are on anything other than a small lake, I would strongly suggest dual batteries - one for cranking the other for accessories.

I know that the newer Evinrudes have two taps off the alternator to charge two batteries. Or you can use things like the Stealth One charger/isolator to charge both batteries seperately with priority to the cranking battery.

From my offshore days, its hard to have enough redundancy - like more than one battery. From my experience in small boats, I haven't found anything to contradict that advice.


fishgutz posted 08-08-2004 11:42 AM ET (US)     Profile for fishgutz  Send Email to fishgutz     
Wet Sanding.. What a bunch of Hooey. I raced Catamarans for years. Beat the whole fleet boat for boat (not even using portsmouth numbers) I never wet sanded. I never used all the add ons everyone else used. Boat handling and general good seamanship wins races. I can see if your boat has a finish problem, then I'd sand. Sometimes I'd wax my boat in the evening after a day of races just to make it shinier. or get the GreenBay scumline off. People would say,"That's gonna slow you down." I'd say, "We'll see." Then I'd wax their butt the next day.

Learn how to trim your boat and steer it properly. Get the right prop or change props for your different uses. I can honestly say I have none of the problems that everyone else has. My wife can even handle the boat better than most people.

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