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Old 27 August 2014, 12:35   #21
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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?
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Old 27 August 2014, 12:51   #22
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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
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Old 27 August 2014, 12:54   #23
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Originally Posted by Roadie View Post
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.
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Old 28 August 2014, 23:38   #24
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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.
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Old 29 August 2014, 05:44   #25
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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
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Old 29 August 2014, 06:41   #26
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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.
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Old 29 August 2014, 07:29   #27
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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
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Old 29 August 2014, 12:53   #28
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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.
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Old 29 August 2014, 14:27   #29
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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.
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Old 29 August 2014, 14:49   #30
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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
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