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Old 21 November 2018, 23:15   #1
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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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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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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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Old 22 November 2018, 17:59   #11
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Yes I bought the boat 2nd hand, without an engine,



Propeller is a factory Suzuki 10.25 x 12 aluminum.
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Old 24 November 2018, 14:50   #12
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Great video! The big splash from the transom when it drops is a curious clue.
I say check your pressure. Perhaps go a but higher as an experiment.
How do you fill your tubes with air? Foot or electric pump? Electric is the way to go.
It looks like a big roll of fabric rolling under the boat caused from under pressure. (Like a big flag in the wind).
I bet you wish you could borrow a clear plexiglass floor to see what’s going on under the floor! I know I do.
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Old 25 November 2018, 11:54   #13
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I use a 12 volt pump for the tubes auto set to 3-3.5 Psi and the keel at 5-5.5 psi. I talked to my local inflatable boat shop and they recommended this pressure as an older boat.
At the moment I am thinking of installing trim tabs, smart tabs or something like that.

I don’t like the fact that I had to install a HydroFoil and possibly trim tabs, but that may just be the way it is for now.
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Old 25 November 2018, 13:38   #14
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Found this vid of a 3.40 doing the same thing I suspect on flat water



https://youtu.be/z42xO4_1mgU
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Old 26 November 2018, 00:56   #15
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Jeff great video. I’m not an expert but after watching it I think David’s boat is creating a wave or roll under the boat that is created by the water interacting with the fabric. It moves back slowly until it causes the bow to rise more and more then the higher bow releases the wave/ roll. Perhaps causing the transom splash in David’s video.
Now I could be wrong but here is a test to see.
If you made a board that is about 3/4” thick cut to be placed on top of the keel tube / under the floor.
Then take the boat out when you would expect smooth water.
Fill the boat to full pressure and run it as normal stock design. Time the duration between bow drops. Use a smart phone stopwatch that does laps. You can average 10 cycles.
Then drop the keel pressure 1 or 2 psi and time the same as the first test.
Then remove floor and place the board on the keel tube and replace the floor boards. Fill the keel to full pressure. Then the side tubes. This board should make the bottom fabric tighter, I expect. Test the cycles as before.
Average the time between bow drops of each configuration and compare.
I would expect the tighter the floor fabric the less problem you will have.
Now if I’m right I have no idea what the perminant fix is.
Also on your older boat the tighter skin could possibly come unglued?
Short of this test only other test is a video of your boat from another boat side and front from a low angle. This might show something?
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Old 26 November 2018, 05:47   #16
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hi pagick

your right with the slack hull senario hooked, concave whatever the terminology air gets trapped and compressed lifts the boat then releases at the stern causing cavitation to the prop if davids hull isnt bar tight that will be the main cause, the other thing is because its an old ish boat maybe the keel isn't original so not filling the hull out enough, a quick fix if so would be to get some closed cell foam to make up the space/form needed.
the other thing is the tubes seem to wobble a bit in his video which i think is under pressure i would certainly try more pressure yes it's an old boat but these were built to last.
in the video i posted its a classic porpoise on a mirror flat sea a bit of weight forward would have solved that surmising that the engine is trimmed properly i always think of boat outboards as being like a stiletto heel rest all your weight on it uneven and it can go anywhere.
so weight distribution is key
engine set up height/ trim in this case HP & prop ok as it is.[not sure on transom height on these short 15 inches or long 20 inches]
hull and tube pressures well up i dont use a gauge anymore i just go by feel at ambient temp.
one other thing all stringers in place on the main boards and boards locked together.
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Old 26 November 2018, 21:34   #17
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It’s a 20” transom and I am using a long shaft (20”) engine. Interesting ideas from the last few posts about placing something under the floor.
For now, I have ordered a set of self leveling trim tabs and I will report back after I get them installed. I appreciate all the ideas and advice!
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Old 27 November 2018, 05:07   #18
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never used trim tabs on a sib but did on a quicksilver 500 pilothouse which had a list to starboard for some reason they worked perfectly bennett auto tabs just a gas ram, hope it works for you
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Old 09 December 2018, 19:29   #19
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You need a Hydrofoil on the outboard to keep the bow down, here is what I have.

https://www.permatrims.com/default.asp
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Old 11 December 2018, 17:36   #20
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Hi. I installed a hydrofoil last month and that really helped, but did not solve it. I am going to install automatic trim tabs next. I ordered the Nautica self leveling tabs. I was a bit disappointed that they will mount slightly above the running surface of the bottom. So I hope that will work. I will report back after I install them next month.
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