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Old 10-30-2012, 02:01 AM   #1
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Location: Perth, Western Australia
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Default Slime = Less RPM

Just thought I would share an experience with you all, for the last few weeks I have been trying to solve an RPM issue with my 3500 SCR, I used to get 4200RPM@WOT out of my twin 7.4 MPI's fully loaded, this would give me about 27kts, I am more than happy with this speed, however over the last few weeks I have had a progressive but noticable drop in RPM until last week I could not get her over 3500RPM and struggled to get on the plane, I did all the usual checks, compression, changed spark plugs, fuel filters, checked timing, took weight out of the boat, I was at my whits end, I had taken a swim under the boat to check the "growth" on a rescent outing and was under the belief that all was well.

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This was a photo taken of the underside at this time, I noted that I had little to no growth at all, however there was some slime.

Well low and behold the weekend after taking this photo I started to take on water from the drive shaft coupler (look under "sinking - not on my watch"), since I had to take the boat out the water I decided not to waste the expence and blasted and painted the bottom.

Well what a difference!, I now get 4300 RPM and 29.5Kts and am on the plane in less than a few seconds not only that but to maintain the plane I am at less than 1/2 throttle, I cannot fully explain the complete transformation the boat has taken after completing this work, other than its amazing!

I still struggle to see how slime and only slime was causing all the drag and slowing me down but I am fully converted to making sure I have a clean bottom .......I mean that the bottom of my boat is regualry cleaned and the anit foul paint is well maintained.

SO if you are lossing RPM and think it could be the engines, perhaps have her pulled and pressure washed before you spend the big $$ on engine parts

Below are the pics after the job was done, at this point I need to send a MASSIVE thankyou to my brother in-law who took the day off work to come help me paint the bottom, as this was done at short notice it was done during the week and all my other mates had comittments they could not break...SAM your a champ thanks heaps!


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Old 10-30-2012, 01:28 PM   #2
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Nice, I'm glad it worked out for you. Thanks for the report out. A clean, smooth bottom is definitely key to maximizing performance and fuel economy. Even those that keep a clean bottom, but poor maintenance on the overall paint condition will see a decrease in performance. When you see the guys with paint that has broken off in chunks, and they scrub it, sand it and slap another coat over the 'pot hole'. This creates drag. The paint job should be smooth, like what you have in your pics.
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