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Old 21 November 2018, 23:15   #1
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Country: USA
Town: Portland Oregon
Make: Avon
Length: 4m +
Engine: outboard 30 HP
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Issue with Avon 4.60 Supersport on plane

Hello
I have a 1987 AVON Supersport 4.60 meter (15 feet) SIB that I am in the process of getting dialed in. It has remote steering / control box and a bench seat mid-ship.
I have been doing river runs lately getting engine mounting height set with a new Suzuki DF30ATL , but I still have one problem I cannot solve.
The problem that I have is that when up on plane just cruising somewhere usually between 2/3 and ¾ throttle the bow will rise up without warning then slam/drop back down. This has been happening on all of my sea trials while looking for the proper engine height. Its very un-nerving as it happens without warning, so I definitely want to solve this problem. Does anyone have any ideas here?

Fast facts:
--Boat is Hypalon bottom, with inflatable keel and wood floorboards that lock into aluminum channels.
--Boat is rated for 55hp and 8 passengers, currently running 30hp and 1-4 passengers.
--Tubes at 3-3.5 psi and the keel inflated 5-5.5 psi
--Engine cavitation plate is 1" (2.5cm) below keel.
--Installed a HydroFoil (SE200) that helped a bit..
--This issue happens with a load of 1 or 4 people
--Adding 40 lbs (18 kilos) at the bow did not help.
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Old 22 November 2018, 04:20   #2
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Country: UK - England
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i would start by lifting the engine an inch and setting the trim pin to force the bow down
mine is set second pin up from the transom cav plate level with the keel
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Old 22 November 2018, 11:13   #3
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In my testing, I have tried raising the engine mounting up so the cavitation plate is even with the keel. The result was that the propeller was blowing out at anything above idle, so I lowered it one hole to where it is positioned now.



I could try replicating the trim pin adjustment by setting the power trim in that position…....
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Old 22 November 2018, 11:18   #4
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Trying to post a picture of the boat here...
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Old 22 November 2018, 12:20   #5
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I made a short video from a river trial. Kind of hard to tell, but the boat rises up then drops back down about 3-4 times while on plane.
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Old 22 November 2018, 13:14   #6
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Country: UK - England
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I've only just noticed you have a 15 hp on a 4.6 m boat that's a big boat for 15 hp my guess is that you are on the balance with even a small wave lifting the bow in short not enough power to keep the boat flat on the plane so allowing air to build up under the hull hence the slam when it exits the stern allowing the bow to drop hence the blow out to as the air hits the prop
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Old 22 November 2018, 13:23   #7
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Hello. Yes the boat is 4.6 meters. It has a Suzuki 30hp 4 stroke engine. The issue happens even on flat water as well. out of no where the bow rises up.....then drops back down.
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Old 22 November 2018, 13:48   #8
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God knows where I got 15 from the 30 should be plenty I run a 25 Suzuki they have plenty of grunt with the 3 cylinder engine. So it's power trim do you have a trim guage to show engine position.
I had a similar problem with a 5 m boat and a 40 hp it turned out the keel tube was off to one side allowing air to build up and blow out from the indented hull. You say your pressures are OK is the hull fabric tight.
What size prop are you running?
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Old 22 November 2018, 14:09   #9
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Problems with Avon 4.60 Supersport on plane

Difficult to tell from video...helpful though.

Could it be just porpoising? Often evident at planing cruising speeds with motor trimmed out a little too far. Simple cure, trim in a little.

Hope that’s the cause as simple fix. If not, a potentially more difficult issue to resolve could be a ‘hooked hull’. This typically pulls the bow down until it’s inherent buoyancy causes release, bounces up, then drops back down. The cycle then repeats.

I suppose first thing is to ensure tubes and keel are running at max pressure.
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Old 22 November 2018, 14:33   #10
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Quote:
Originally Posted by chipko View Post
Difficult to tell from video...helpful though.

Could it be just porpoising? Often evident at planing cruising speeds with motor trimmed out a little too far. Simple cure, trim in a little.

Hope that’s the cause as simple fix. If not, a potentially more difficult issue to resolve could be a ‘hooked hull’. This typically pulls the bow down until it’s inherent buoyancy causes release, bounces up, then drops back down. The cycle then repeats.

I suppose first thing is to ensure tubes and keel are running at max pressure.
I agree with the porpoising chipko and it could be a very fine line on adjustment that's why I ask if he has a gauge at least you could mark it off on trials for the best position.
If it were a secondhand boat does it have the correct keel that's why I ask is the hull tight.
Also the prop pitch could play a part the Df 30 & 25 are the same engine the 30 runs at 5300-6300 300 more the the 25 I run 10 1/4 x 10,11&12 without issue at 20-22 knots WOT in good conditions for info.
Weight distribution, too much weight forward & on one side will cause the prop to cavitate and slip causing rise and fall of the bow.
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