Cavitation issues----------------HELP

[Roadie]

This has got me thinking. I have got a Mariner 15hp 4 stroke 2007 which has 5 trim holes for a pin. When it in it on the lowest trim position the motor locks onto the pin but I get ventilation or cavitation; sounds like the motor is running in thin air. So I run the motor without a pin in at all, so the engine is not locked down. Is this correct or should the engine be locked in position?

[slate1234]

I get exactly the same problem with a 3.2m airfloor and 9.8hp twatso, it only happens if I have a passenger, on my own it flys, I started by shimming the engine up but with no luck, all the pressures are spot on, I ended up cutting 25mm out of the transom and droping the engine down for when I have a passenger and it flys two up, and shim itback up when alone

[dave3235]

Originally Posted by Roadie

This has got me thinking. I have got a Mariner 15hp 4 stroke 2007 which has 5 trim holes for a pin. When it in it on the lowest trim position the motor locks onto the pin but I get ventilation or cavitation; sounds like the motor is running in thin air. So I run the motor without a pin in at all, so the engine is not locked down. Is this correct or should the engine be locked in position?

Hi

If you check the trim angle with it in or out it is probably no different apart from you can lock the engine. That is what my engine does.The unlocked way would be fine for running low water situations so your engine can move up if you reach the bottom, if you need to put it in to reverse the engine will lift up.

[Projectile]

I have had the same issue with my large SIB: great performance at a slow plane, but ventilation at wide open throttle.

I have found that I need to *over* inflate my keel to eliminate the ventilation. Manufacturer suggests 3.5 psi but I go to 6, and the problem is largely non-existent.

I just got a 4 blade prop that I will try this weekend. I'l let you know how it goes.

My AV plate was also 3 inches below the bottom of the transom. I had no problems with ventilation until it was raised to 1-1.5 inches below the bottom of the transom. Your engine may be sitting too low, but a low engine, IMO does not cause ventilation. Something else is causing the ventilation, and once that's fixed, then you can consider raising the engine to get less splash or better performance. If your first priority is fixing the ventilation, I don't see how raising the motor could help.

[philpot]

Originally Posted by Projectile

I have had the same issue with my large SIB: great performance at a slow plane, but ventilation at wide open throttle.

I have found that I need to *over* inflate my keel to eliminate the ventilation. Manufacturer suggests 3.5 psi but I go to 6, and the problem is largely non-existent.

I just got a 4 blade prop that I will try this weekend. I'l let you know how it goes.

My AV plate was also 3 inches below the bottom of the transom. I had no problems with ventilation until it was raised to 1-1.5 inches below the bottom of the transom. Your engine may be sitting too low, but a low engine, IMO does not cause ventilation. Something else is causing the ventilation, and once that's fixed, then you can consider raising the engine to get less splash or better performance. If your first priority is fixing the ventilation, I don't see how raising the motor could help.

I do confess that I thought by raising the engine would make things worse but having never had this problem with any of my previous SIBs, I am somewhat at a loss.
I am going to try the raised position because it will be part of a process of elimination coupled with the pressures.
I really like the Excel boats but I do wish I had bought an aluminium floor and put up with the extra weight.

Phil

[dave3235]

Hi projectile.

I remember your old thread.Seem there are a few of us that are suffering with this problem, the only thing I have noticed different from my previos engine witch was a mariner 15c short shaft with a 9.5 prop lots of space between and ran well.The size of the prop on my 20HP is 10 1/8 I only have a very small gap between prop and cav plate may be 5 mm.So if I was to get the engine hight right it would be running at the surface and back to square 1.May be a 9.5 inch prop and a higher pitch than 13 witch is what is being used on my engine now its all very baffling hopefully we can all hit that sweet spot soon and the head ache gone.

[philpot]

It is a very complicated issue to my mind and I do feel that the air floor SIBs seem to suffer far more than ali floor models which perhaps is why I have never had the problem before.

I am also really concerned about the pressures as again it seems to be very critical, I had even wondered about placing a sheet of 6mm ply between the air floor and the outer skin to provide a secondary membrane to help keep the shape required but as the boat is new, I am being a little cautious. To be honest if I could buy the ali floor and use instead of the air floor, I would be very tempted but they don't supply them separately nor do they fit by all accounts.

Phil

[Peter_C]

Phil, what happens if you take the boat out alone? Have you tried shifting passenger weight fore and aft, and if so what was the result? You could bring a couple of shims out on the water with you and experiment with raising the motor too.

[slate1234]

Peter I did as as you have suggested, I put it down to a poor quailty infltable but there seems to be quite a few people with the same problem a local guy who has been useing ribs for a long time suggested I get a 3m-3.1m rib and all my troubles will be gone, I for one will never have another air floor as long as live, I may go for aluminium or wood floor, I have now cut a piece of ply that goes from the transom 12" towards the front and the change is fantastic.

[philpot]

Interestingly enough Peter, I have not taken the Excel out alone, always been with someone else on board either fishing or pleasure but it is a test I will be doing if I have the chance this week before I go away on holiday.

Your piece of plywood sounds a productive move Slate, I wonder if others have tried inserting ply below the air floor and if so what results did they get.

Phil

[Chopperbill]

Having the same problem myself. Solo it runs great, add passenger and I have to get creative on weight placement. Even a difference of 10# of the passenger make it a whole new situation. I am running at 5000' above sea level so I do need a lower prop pith to adjust to this hieght. I suspect an under inflated keel as the tubes and floor feel hard when correct on pressure settings. However, the keel still feels soft when even above the recommended specs. How much over pressure specs will these thing take?

[philpot]

Just returned from holiday and preparing for some tests to address this issue. Listing the tests, I have the following
1 try the boat at speed solo to see if there is any difference.

2 check pressures and again after leaving it on the water for about 20 mins

3 take a couple of wood shims at 1 cm thick to see if the height improves things both solo and with a passanger

4 try a piece of 6mm ply full width of the floor and about 12 - 18'' towads the bow. This is to be placed below the air floor.

5 mark the prob and hub/shaft to check for spun hub

6 try different trim positions when I am happy with the pressures

7 take a stove to keep me going with loads of coffee me thinks

Now is there anything else I have over looked guys and I will add it to the list.

I would like to say thank you to you all for your input so far.

Phil

[Fenlander]

If it's driving you mad Phil I hope you find a solution with your outfit. However my experience with air floor sibs (the flat type like yours not the V Aerotec type) is that the slightest thing can trip them into cavitation/ventilation/prop slip (whatever you would call it). A certain combination of boat make, load weight/distribution, outboard make and even prop type/condition can cause it to be an issue... then even if you find a perfect setup certain sea states can induce it.

So for me I accept it as a possible/occasional part of the air floor character offset by their great portability and ease of inflation.

[martin_smith]

I had an aluminium floored Seapro 340hd and also suffered with cavitation issues. So perhaps the floor/boat type isn't particularly the issue. Funnily enough I was using a Parsun 15 like yourself. Perhaps it's the engine? The only thing I found helpful was to trim it right in as close to the transom as it would go. This improved matters re the cavitation but I was left fighting with the tiller as it seemed to then be greatly affected by the torque from the prop. I did find the Parsun 15 to be quite a heavy motor for that size of boat.

[Peter_C]

Originally Posted by philpot

Now is there anything else I have over looked guys and I will add it to the list.

8 Try moving the passenger forward and backwards to see what changes.

[philpot]

I am inclined to agree with you Fenlander and wish to god I had gone for the aluminium floor but having said all that, I do love the ease of the air floor so a compromise is needed.

The Parsun is a 15hp 2 stroke weighing in at 37 kgs Martin so I would have thought that not to be too heavy and I have bought it now so fingers crossed, having said that I am going to buy a little 2.3 - 4 hp motor for my river/lake fishing which will be a fair percentage of the use, scanning ebay as we speak.

I forgot to put that on the list Peter so as I cannot edit the list on here, so a note has been made to test that out.

Phil

[martin_smith]

Mine was a 4 stroke Phil and much heavier than that so just ignore me !! Hope you get it sorted

[philpot]

Well I have been playing on the sib today trying various things to improve the cavitation / ventilation issues. I set the pressures and checked them with a separate gauge. After launching, I moored up on the river, had lunch and fished for a little while, actually 1 hour. Pressure test--------------fine so on with the test run.

No signs of the cavitation whatsoever, same with two on board so pressure was to blame I guess BUT wait for it, we now have a Mexican wave going on under the floor. Difficult to really describe but the water or air is passing under the floor with a steady movement lifting the floor by what seemed a couple of inches as it worked it's way under the hull.

This is driving me MMMMMAD

Phil

[Poly]

Originally Posted by philpot

No signs of the cavitation whatsoever, same with two on board so pressure was to blame I guess BUT wait for it, we now have a Mexican wave going on under the floor. Difficult to really describe but the water or air is passing under the floor with a steady movement lifting the floor by what seemed a couple of inches as it worked it's way under the hull.

This is driving me MMMMMAD

Phil

that phenomenon has been described here by others in the past...

[Fenlander]

Yes indeed... only last night, having just bought an air floor Zodiac we hope to get on the water this Sunday, self and Mrs were pondering if it would have the air floor character... as if you passed over the back of a whale now and again.

Part of the experience!