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Old 02 March 2021, 06:51   #1
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17" transom on aerotec for short shaft...

Hi all, we took our new (to us) Aerotec 420 out on Friday for the first time with our short-shaft 25 HP Mercury...it was great but suffered from cavitation (I guess we were quite a light load). The transom is already cut down to 17 inch...it looks, from googling it, as though we may need to cut another inch off to stop this. Any wisdom?

Thanks
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Old 02 March 2021, 16:26   #2
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Be very wary of this as we often have to raise the transom height on the 380 models (that take a short shaft) by 1-2" to stop splash into the boat and this added height can be done without any ventilation even in turns. So I'd make more checks before cutting more.

Have you checked all other sources of that prop slip ventilation feeling... floor and tubes up to pressure and rechecked after being in the water for a few mins... floor properly pulled down into hull by tightened straps... prop in good visual condition and the right profile for your use... the prop hub not spun... tried different person/weight loadings re trim... tried different outboard trim settings etc.
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Old 03 March 2021, 03:45   #3
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Thanks Fenlander. Yes indeed - we've still got our 380 and I have raised that transom by about 30mm for the same outboard...so the measurements end up pretty similar. I'll get the outboard on for a photo later in case that helps...seems odd that such a small difference in height would cause such a significant difference in outboard performance.
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Old 03 March 2021, 04:24   #4
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Here's the setup - anything look odd? Cavitation plate looks about right for height to me? I've got one more hole in the trim adjustment that I could lower to...

Thanks again for help
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Old 03 March 2021, 18:44   #5
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Quote:
Originally Posted by Jhoughton View Post
Here's the setup - anything look odd? Cavitation plate looks about right for height to me? I've got one more hole in the trim adjustment that I could lower to...

Thanks again for help
That looks a little low if anything unless I'm looking at it wrong. But I'm wondering one blade of the prop looks like its been sandblasted? Could there be an issue with the prop blades?
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Old 03 March 2021, 19:12   #6
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Yes unless it's the camera angle that looks low.

See this thread and my posts #69-73 and #81 for the lift I needed to do and where the AV plate ended up to stop splash on a Mariner 9.9 4-stroke... which also suited a Yamaha 15 2-stroke.
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Old 07 March 2021, 15:09   #7
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That looks a little low if anything unless I'm looking at it wrong. But I'm wondering one blade of the prop looks like its been sandblasted? Could there be an issue with the prop blades?
Thanks - I don't think so because I ran this exact motor on my 3.8 with no issues whatsoever...I think it must be down to very fine-tuning of trim and height...I'll take it out again with fewer kids on board and try a few things!
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Old 07 March 2021, 15:10   #8
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Yes unless it's the camera angle that looks low.

See this thread and my posts #69-73 and #81 for the lift I needed to do and where the AV plate ended up to stop splash on a Mariner 9.9 4-stroke... which also suited a Yamaha 15 2-stroke.
Thanks - sorry if I'm being slow but I can't see a link?
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Old 07 March 2021, 15:50   #9
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Ahh sorry posting after midnight... missed the link.

https://www.rib.net/forum/f50/aerote...e-71528-2.html
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Old 07 March 2021, 16:47   #10
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Hi Jhoughton, my experience tells me there should be zero cavitation with the positioning of the prop you have shown. Are you sure the prop isnít spinning on the hub when you accelerate?
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Old 08 March 2021, 03:55   #11
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Hi Jhoughton, my experience tells me there should be zero cavitation with the positioning of the prop you have shown. Are you sure the prop isnít spinning on the hub when you accelerate?
Thanks - interesting! How would I be able to diagnose it? Feels pretty solid when off the water...
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Old 08 March 2021, 16:59   #12
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Thanks - interesting! How would I be able to diagnose it? Feels pretty solid when off the water...


It happens when the bushing in the hub of the prop is perished. The prop canít grip the shaft of the engine. Under force, the shaft spins and the prob doesnít spin with it. This video on YouTube explains the problem and the solutions. A new prop could be the easiest solution if this is your problem. Give it a watch. https://youtu.be/0eI5HZXSTnM
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