Bow Dropping & Prop Ventilation

[Excelent]

Originally Posted by jeffstevens763@g

It’s a bubble of air escaping hitting the prop it loses grip and drops off the plain floor needs to be taught and weight distributed so the boat travels level it doesn’t need much to throw it out of balance.

Thanks. I have read this somewhere before. This is essentially what it feels like so I will get my pressures spot on next time and check after been in the water.
My explanation of the prop popping out the water was only an assumption based on what it felt like. Obviously I didn’t have a visual on this event happening but your explanation sounds like it could be the issue.
Was only my first time out so will no doubt be some time until I find the sweet spot on set up.

[jeffstevens763@g]

The way I work it is I’m 20 stone (all muscle) I put my fuel tank opposite me at the stern next I would put one crew opposite at the bow now we have just about got level weight port and starboard. kit in the bow if I carried a child he/she would sit on the floor in the middle. Don’t forget that your balancing on the stern underway so the boat is on a fulcrum from there forward so 10 kg could be 20 kg in the bow. To be honest have a play day with the max weight you will carry get that right then you know the limitations having an empty plastic drum you can fill with seawater will allow you to add ballast where it you need it and move people, kit to suit.

[Excelent]

Originally Posted by jeffstevens763@g

The way I work it is I’m 20 stone (all muscle) I put my fuel tank opposite me at the stern next I would put one crew opposite at the bow now we have just about got level weight port and starboard. kit in the bow if I carried a child he/she would sit on the floor in the middle. Don’t forget that your balancing on the stern underway so the boat is on a fulcrum from there forward so 10 kg could be 20 kg in the bow. To be honest have a play day with the max weight you will carry get that right then you know the limitations having an empty plastic drum you can fill with seawater will allow you to add ballast where it you need it and move people, kit to suit.

I’m guessing something like this and a good quality automotive tyre pressure gauge is the best way to get accurate pressures?

https://supinflatables.co.uk/media/catalog/product/cache/027f34773b3a6fc5c4f4166895fda8ae/r/e/red-paddle-co-schrader.jpg

[jeffstevens763@g]

I would look at the paddle board pumps the pressures for them are more crucial than a SIB

[Excelent]

Originally Posted by jeffstevens763@g

I would look at the paddle board pumps the pressures for them are more crucial than a SIB

That’s what I used as it was the only pump I had.
I’m going to rig up as above with my tyre pressure gauge I used to use for setting slick tyres pressures for motorbike racing. Should be fairly accurate

Interestingly, I’ve just found this on another thread regarding the air pocket / ventilation problem;

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.

[jeffstevens763@g]

Ok if it works for you just be careful. Couple of things if you have a sausage keel under the floor make sure it’s central sometimes they roll when pumped up. Second I have always set my outboard with the cavitation plate level with the keel this boat I have now cavitation was bad when I set it up on a block took the block out cav plate now 2 inches below keel now no
make sure the sausage keel has been installed / glued in correctly

[TmMorris]

I wonder if it is just the usual air pocket cavitation and the bow is dipping after rather than before?

I had the cavitation happen during one outing where the water was very flat and I hadn't bothered to pit any extra air in the sausage after launching on a very hot day

[Excelent]

Originally Posted by jeffstevens763@g

Ok if it works for you just be careful. Couple of things if you have a sausage keel under the floor make sure it’s central sometimes they roll when pumped up. Second I have always set my outboard with the cavitation plate level with the keel this boat I have now cavitation was bad when I set it up on a block took the block out cav plate now 2 inches below keel now no
make sure the sausage keel has been installed / glued in correctly

Was looking more at the pressures of the keel etc that the member in the post was referring to.
TBH, I was absolutely pouring with sweat on the manual pump and with the keel being the last thing to be inflated, I might have been a bit slack on the pressure.

[Excelent]

I think my sausage was a little flaccid upon reflection

[Excelent]

Originally Posted by TmMorris

I wonder if it is just the usual air pocket cavitation and the bow is dipping after rather than before?

I had the cavitation happen during one outing where the water was very flat and I hadn't bothered to pit any extra air in the sausage after launching on a very hot day

I think my sausage was a little flaccid upon reflection

[Steve509926]

Nothing worse than a flaccid sausage!

Interesting that Tim mentioned cavitation in flat water. The only time I suffered very bad cavitation was on Ulleswater when the water was so flat it was like a mirror and I also dropped off the plain.

[Dydigital]

Have you tried raising the engine slightly? This has helped others with a similar problem.

I run my Volante 390 with s old Suzuki that's nearly a standard shaft (but offically a short shaft)

Didn't really have a problem you have but managed to create it by messing with my pressures, then resolved it by lifting the engine 10-20mm on the transom with a bit of wood.

[909]

IMO a 9.9 is underpowered for a 13 ft boat.

Your problems will go away with a 15 or 20

[TmMorris]

It should be fine. A Dell Quay will plane with a 10hp. But also, the issue does appear to be one of set up as the boat is getting on the plane.

[909]

Then in that case OP should try lowering the outboard to parallel and pumping fast and hard until his floors and tubes are erect and firm