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Old 09 October 2013, 11:22   #91
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Quote:
Originally Posted by Projectile View Post
Ok. I will post a video of the non-stop cavitation after supper.

I really appreciate all of the input from you guys. This is a big problem for me, and I feel like you guys are my only resource.
The F470 has an odd shaft length. 18" as I recall. You need to use a long shaft (20") engine on it. You haven't ended up with a short (15") by accident have you?
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Old 09 October 2013, 12:08   #92
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If you follow thread. Projectile has Zodiac Work boat 465. Similar to f470 but no speed tubes. Same 18 transom. Originally his motor was way too low.
Hang on there Projectile. You got great boat and great motor. Unfortunately they don't match up ideally.
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Old 10 October 2013, 03:38   #93
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Took my boat out this afternoon. Trimmed-in to #2 starts bow steering. Faster I go worse it gets. Didn't bother to try #1. Can't imagine using that setting. Trimmed-out to #4. Bit of cavitation rarely, lost a bit of top end. Didn't try #5 since again obviously pointless. #3 was best performance overall. Very stable. Topped out at wee bit over 25 mph in very light chop. Curiously riding glass smooth water I was loosing 1-2 mph. Seems like that light chop gave least resistance. Nice 40 mile trip. Heading back hit some standing waves/chop in main channel heading back. I was catching air half the time for good 20-30 minutes. Boat shrugged it off. No video. I was too busy holding on. Made me wish I had a jokey seat with steering for sure.
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Old 10 October 2013, 23:11   #94
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SIBRider: that's with a 25 hp motor on an FC-470?

Amazing! I'm jealous.
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Old 11 October 2013, 04:14   #95
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Thanks. Yes that's my Zuk 4 stroke. Yours should do better if you get right prop.
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Old 11 October 2013, 22:50   #96
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A hook in the hull will cause bow steer at speed. Are you sure the bottom of the hull is true?
I've been reading about this hull issue ("hook" in hull). It seems that if the hull is hooked, the more one accelerates, the more the bow gets pushed down. That is *exactly* what I experience.

It seems to be nicely explained here:
http://www.marathonmarine.com/boats_guide.pdf

I think that this is my problem. I'll go check out the bottom of the boat this weekend (it's in storage). My recollection is that when I was swimming around the boat this summer and feeling the bottom of the boat, there was a hook just fore of the junction of the transom and the hull: there was a fold in the material there, like a little cup that I could almost grab.

frankc, I think you've diagnosed the problem. I'll confirm it, and hopefully there will be an easy solution.
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Old 12 October 2013, 08:48   #97
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The transom and hull area immediately ahead of the transom are critical. Even the back edge of the transom plays a part and should have a sharp angle, not rounded.

When under power, the force of the motor would tend to produce bending at this point if the transom rotates forward under power, so you may not see a problem with the boat at rest. If you notice a problem with the motor not running, it will only increase under power.

When I replaced the transom on my fiberglass boat, the shrinkage of the glass when drying formed a very shallow cup forward of the transom. It is only noticeable above 23 - 26 MPH.
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Old 12 October 2013, 13:52   #98
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Can't imagine hook being an issue on inflatable with fabric hull. Rigid hull yes. Low pressure area will create suction effect pull bow down. Still we talk about speeds far in excess observed. 25-30+.

I firmly believe higher situated motor/prop underloads engine, creates excessive cavitation at correct (neutral) trim. That's all. With excessively trimmed in engine there might be all kinds of problems present but they are not worthy dealing with until engine is positioned correctly.
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Old 12 October 2013, 18:32   #99
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Cavitation and a hooked bottom are two different subjects. Until the bottom is clean, you are fighting an uphill battle. You cannot pick a propeller with a cavitating engine, as a tach would go wild during cavitation.

A hook is very much a factor with an inflatable bottom. Underinflate one and see how well it performs. It is easy to repair in a hard bottom, but not such an easy job with an inflatable.

How does running an engine without a load on it create cavitation? You can over rev it, but the prop is still pumping water.
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Old 12 October 2013, 19:15   #100
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This photo is taken from directly below the bottom of the boat where it meets the transom, as if you were lying in your back under the boat looking up. You can clearly see the indentations that I mentioned being able to feel while swimming around the bottom of the boat. You can grab those. More to come.
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