HELP! SIB Bending causing Aeration/Cavitation

[mtlg]

Hi all,

I'm having some issues with an Achilles LSI 360E I recently purchased, and I'm hoping someone could confirm this is an actual issue and how I could fix it or what I should do about it.

Setup:
- 3.6m Air deck sport tender with a 15 HP 2-stroke Mercury (36 kg)
- The outboard is way below the maximum specification and is the correct shaft lenght.
- The outboard has been correctly trimmed and height adjusted (different settings have already been attempted)
(Achilles Inflatable Crafts | Boats & Parts)

Issue:
- The boat floor flexes with what is apparently an air or water bubble, that travels from the front to the back of the boat. As this "bubble" escapes the transom the engine over revs and sometimes become aerated, requiring further acceleration to get a grip on the water again.
- This happens consistently when accelerating from a standstill before planing, at medium speeds (light plane) and sometimes at full speed while planing.
- Have confirmed this happens with 2 or tree people on the boat (boat is rated for 6).
- Have already checked the boat pressure is to specification in multiple trips, both before and after getting into the water.
- I have attached a video, where you can clearly hear this happening twice at the start of it, and towards the end you see me accelerating a bit trying to get rid of the bubble and again the outboard revving.

I'm currently in a situation where the boat seller is telling me this is bad for the outboard (over revving) while the boat seller tells this is normal behavior...

I feel like I can't do much at the moment due to lack of knowledge on my part. So to all SIB owners, is this normal behaviour? Has anyone had similar problems in the past? If so, how did you fix it?

Open to all advice on the matter.

Cheers

[jonp]

Hey mate for some reason the video won't play.

As for the problem this is a new one on me so I look forward to hearing how you get on. My initial thoughts were just that you were under inflated, then maybe the engine mounted to low. As you know we use lots of these smaller sibs as rescue boats normally with more power over here.

Have you asked the manufacturer?

By the way are you able to return the boat if you aren't happy with it?

[beerbelly]

this never happened with my honwave 3.2 airfloor have you tried more pressure in the floor do you have an accurate electric pump you wont get the correct pressure with a manual pump

[mtlg]

@ Jon

I'm still waiting on a reply from the manufacturer. I've been dealing with the distributor, who hasn't helped much. I believe the only I could return it is via some troublesome consumer affairs claim that the boat is faulty.


@ beerbelly

I have so far tried to fill to specifications (top end of rating) without success, and using a foot pump with the Achilles recommended pressure gauge. So I have some confidence on that part.

That being said, after some talks to the distributor today they asked me to attempt to over-inflate the whole thing by 0.5-1 psi, which does not sound right to me. But then I have everything in an email if things go south. Might try it tomorrow if I have the chance.

[Max...]

99.9% of the time this will be due to a lack of floor pressure issue.

Check with a decent gauge.

Remember the cold water will lower the pressure effectively so you need to compensate.

You may well still experience a slight wallowing effect - par for the course with some longer airfloor SIBs.

Also try distributing weight (people) differently - important in a low power, lightweight SIB.

[jeffstevens763@g]

hi mate

i had this issue with a ali floor sib in that case it was under inflation. you say your floor is flexing which is what is causing your problem so its definately under inflation i take it this one is a flat floor with a sausage inflatable keel so two compartments to inflate have you tried pumping up some more when its on the water firstly, secondly you have room with inflation to pump up a tad more the guage might not be accurate so try both of those,
also you might have a slight leak we had that happen on one of these at work it never held pressure from day one we had to get a replacement sent out worth pumping up and leave for 24 hours or do a soap test to trace a leak if you have one.

[Fenlander]

*Your video plays OK for me and sounds exactly how I'd expect when this issues happens.


>>>an air or water bubble, that travels from the front to the back of the boat. As this "bubble" escapes the transom the engine over revs and sometimes become aerated, requiring further acceleration to get a grip on the water again. This happens consistently when accelerating from a standstill before planing, at medium speeds (light plane) and sometimes at full speed while planing.


I've owned several flat (flat as opposed to those that have the high pressure floor panels in a downwards V) air floor SIBs and they are all prone to this to varying degrees.

Once you get above 3.2/3.4m or above 10hp this is far more likely to happen. It is also more likely to happen on the types such as yours with a low pressure "sausage" keel compared to the Zodiac Acti-V range with a deeper high pressure keel.

I have experienced some odd changes to the degree this is experienced... for example on a 3.6m air floor with an occasional but acceptable amount of this flexing I changed the prop on my 15hp outboard to a budget replacement with a slightly different blade shape (same pitch though) the flexing and slipping was terrible. Bought a new genuine maker's prop and it was back to ok.

If you have checked the floor is up to pressure once on the water with a gauge you trust and tried various loading/trim changes then it is likely this is a character of your boat.

Another factor is this bothers some folks more than others... I don't like the effect at all so have moved on to a V-shape air floor type.

If you are interested there is a scientific paper that describes just such issues in a flexible hull boat... section 5.1 here...

http://eprints.soton.ac.uk/183099/2/...r_Halswell.pdf

[office888]

Recheck all pressures after 15 min in the water with a gauge.

Sounds like an underinflated keel and underinflated floor to me.

[dave3235]

Hi all

+1

[xchsurfers]

Originally Posted by mtlg

Hi all,

I'm having some issues with an Achilles LSI 360E I recently purchased, and I'm hoping someone could confirm this is an actual issue and how I could fix it or what I should do about it.

Setup:
- 3.6m Air deck sport tender with a 15 HP 2-stroke Mercury (36 kg)
- The outboard is way below the maximum specification and is the correct shaft lenght.
- The outboard has been correctly trimmed and height adjusted (different settings have already been attempted)
(Achilles Inflatable Crafts | Boats & Parts)

Issue:
- The boat floor flexes with what is apparently an air or water bubble, that travels from the front to the back of the boat. As this "bubble" escapes the transom the engine over revs and sometimes become aerated, requiring further acceleration to get a grip on the water again.
- This happens consistently when accelerating from a standstill before planing, at medium speeds (light plane) and sometimes at full speed while planing.
- Have confirmed this happens with 2 or tree people on the boat (boat is rated for 6).
- Have already checked the boat pressure is to specification in multiple trips, both before and after getting into the water.
- I have attached a video, where you can clearly hear this happening twice at the start of it, and towards the end you see me accelerating a bit trying to get rid of the bubble and again the outboard revving.

I'm currently in a situation where the boat seller is telling me this is bad for the outboard (over revving) while the boat seller tells this is normal behavior...

I feel like I can't do much at the moment due to lack of knowledge on my part. So to all SIB owners, is this normal behaviour? Has anyone had similar problems in the past? If so, how did you fix it?

Open to all advice on the matter.

Cheers

Hi Had a similar problem with a Honwave 3.8 the floor used to lift from front to back like a banana, pressures were correct. had a honda 20hp on the back
The only way I could stop it was by sitting on the floor, sold it in the end, just got fed up with it.

[xchsurfers]

Hi
Had a similar problem with a Honwave 3.8 the floor used to lift from front to back like a banana, pressures were correct.had a honda 20hp on the back.
The only way I could stop it was by sitting on the floor, Sold it in the end, just got fed up with it

[jeffstevens763@g]

Just had a thought we have a resicraft 5 m built by SIT it's a sausage keel with a convex air floor which is stitched in by rope to the tube sides. As said usual problems if under pressure, but something we did find is that the sausage keel can roll to one side creating a slack ish side allowing an air bubble to build up but when it's central the hull skin is as any other inflateable tight as a drum skin.

[mtlg]

Originally Posted by Fenlander

*Your video plays OK for me and sounds exactly how I'd expect when this issues happens.


>>>an air or water bubble, that travels from the front to the back of the boat. As this "bubble" escapes the transom the engine over revs and sometimes become aerated, requiring further acceleration to get a grip on the water again. This happens consistently when accelerating from a standstill before planing, at medium speeds (light plane) and sometimes at full speed while planing.


I've owned several flat (flat as opposed to those that have the high pressure floor panels in a downwards V) air floor SIBs and they are all prone to this to varying degrees.

Once you get above 3.2/3.4m or above 10hp this is far more likely to happen. It is also more likely to happen on the types such as yours with a low pressure "sausage" keel compared to the Zodiac Acti-V range with a deeper high pressure keel.

I have experienced some odd changes to the degree this is experienced... for example on a 3.6m air floor with an occasional but acceptable amount of this flexing I changed the prop on my 15hp outboard to a budget replacement with a slightly different blade shape (same pitch though) the flexing and slipping was terrible. Bought a new genuine maker's prop and it was back to ok.

If you have checked the floor is up to pressure once on the water with a gauge you trust and tried various loading/trim changes then it is likely this is a character of your boat.

Another factor is this bothers some folks more than others... I don't like the effect at all so have moved on to a V-shape air floor type.

If you are interested there is a scientific paper that describes just such issues in a flexible hull boat... section 5.1 here...

http://eprints.soton.ac.uk/183099/2/...r_Halswell.pdf

I have checked the pressure before getting into the water, and straight after leaving the water. The water I go to is rather warm, and I noticed 0 pressure drops. That being said, I'll make sure I measure while ON the water next time.

It could be that the gauge is wrong... that being said they did test it when I purchased. They have agreed to re-check the gauge to be on the safe side. Follow up to come.

From my point of view and after reading up on it, I also believe it to be an under-inflated floor/keel, but the gauge has always been telling me otherwise.

Regardless, next time I'm on the water (probably Saturday) I'll be over-inflating the boat to see how things go.

Originally Posted by xchsurfers

Hi
Had a similar problem with a Honwave 3.8 the floor used to lift from front to back like a banana, pressures were correct.had a honda 20hp on the back.
The only way I could stop it was by sitting on the floor, Sold it in the end, just got fed up with it

Thats disappointing. I'm so hoping this is a one off issue with the boat and that I can either fix or replace it.

Originally Posted by jeffstevens763@g

Just had a thought we have a resicraft 5 m built by SIT it's a sausage keel with a convex air floor which is stitched in by rope to the tube sides. As said usual problems if under pressure, but something we did find is that the sausage keel can roll to one side creating a slack ish side allowing an air bubble to build up but when it's central the hull skin is as any other inflateable tight as a drum skin.

I have noticed the front of the keel can be moved around, but I always check it is nicely lined up with the floor. I'll pay better attention to the whole keel next time to ensure it is straight throughout.

On a second note... my hull is not tight at all. Isn't that the norm?!

I can clearly see and feel that the hull is not tight all over. I sometimes place a fabric sheet on the floor to avoid damage, and it keeps getting blown out/sucked in at the floor gap (where you fill the keel from). This happens even when I'm idling in gear, indicating to me the hull is constantly flapping around.

If the hull is indeed meant to be "as tight as a drum skin" then I believe this could be a major reason from my problem...


And by the way, thank you everyone for the help.
I'll investigate all points made and will come back with results. In the meantime any extra advice is welcome.

[jeffstevens763@g]

if the hull skin is not tight then you have a lot of drag for starters and areas for air to get trapped more than is expected.
the keel sausage is the key getting that up to pressure and tightening the hull skin along with the floor.

[Fenlander]

I know on another forum a guy had performance problems with a hard floor sausage keel SIB where the maker realised the sausage keel was too small and didn't stretch the fabric totally. They replaced the keel with a fatter one.

Problem with a flat air floor even when up to pressure they allow the keel to push the air floor up a little wasting some of the stretch that the keel would make on the outer floor had it been a wood or alloy floor.

[breezeblock]

I ended up not using the air floor and replacing it with a sheet of 6mm ply and it transformed the way the boat moves on the water cheap as chips solution for a wobbly floor

[jeffstevens763@g]

Fenlander makes a good point it would be easy I suspect for a 320 keel put in a 360 boat and worth asking, I can see there can be some floppyness with the design but constant cavitation isn't right to me because the boat isn't fit for purpose IMO.
Breezeblock has the answer but that shouldn't have to be again IMO.

[sameh]

Cannot see the video but Had similar problem with my 3.8 m zebec with 15 hp mercury (tohatsu like)..but here it is wooden floor.
What solved my problem was mainly trimming the outboard to lowest point! Although been advices that usually second trim is usually recommended.

Secondary thing was better inflating with manual oump by the feel with no pressure gauge..

Although yours is air deck it may be same solution.

[mtlg]

Hi again everyone, it took me a while to get things tested!

Here's the latest:

- My pressure gauge suffered complete failure on my weekend trip, hence the lack of updates.
- Replaced the pressure gauge this week and went out on the water again.
- Over-inflated the boat by about 1 psi.

Surprisingly, the issues got much better. So it could be that the boat was indeed under-inflated and the pressure gauge was faulty all along...

I did find that I had to lower the outboard too get things going well, with the anti-cavitation plate about 2 inches from the bottom of the boat. If I raise it to the standard 1 inch below the boat the aeration issues become constant. Can anyone confirm their outboard also needs to sit this low?

On inspection, the hull skin seemed tight, but it is still somewhat flexible. How tight should the hull be? Is there anything I could compare to?

I'll be going on the water again sometime over the next few days, and I'll attempt to use the recommended inflation to see how it goes.

[Fenlander]

It's good there seems hope for at least some improvement. Engine height isn't an exact science and it's just best to find the ideal height for your setup.