Hi again Dave. I have pondered this all day today while doing other engineering jobs for freinds.
Firstly, forgive me if I ask any question that has already been asked. I read so much I forget who asked what when and where.
I guess one question I forgot(I think) to ask was, is the boat in or out of the water
? If out, then making some measurements is going to be easy.
This is what I have been thinking and I may be wrong, but.
First you need to ensure two measurements are correct. Use Dels diagramme above. We need to know The distance the prop is away from the shaft bearing is correct. This is "d" in the diagramme. It sould be between 1.25" and 1.8".
The other measurement is "a". This should be no less than 3.15".
Now trying to suggest anything else on a prop that is self adjusting is next to useless. Duh! I should have thought of that this morning, but I don't think well in the morning.
There are a few things I am thinking.
1:, the prop is too big for this engine
. Leave aside the fact that the boat maybe able to take a 21" prop, the issue is that I don't think the engine can spin it due to Hp, or lack of it and factored into that is the shaft speed. a 1.9:1 box is actually quite fast for a 21" prop to spin.
So I think there maybe an issue in that lot first off.
Next, even if the prop is wrong and too big, I can't see this ever bending the shaft to a point of seeing a bow in it. This is quite disconcerting. The effort to bend a 1.25" shaft from a fulcrum point of only an 1" or 2 from the bearing is huge. Humungusly huge. Plus the damage it must be doing to stern bearing and to transmission
bearings must be terrible.
But the only thing I can think of that would do something like this (within this context) is that the blades are not deploying properly. So I would get that prop inspected that all is well. Which then takes me to the next possibility then.
It may be possible it is one other problem. The prop lifts itself through the water
. This force is transmitted along the shaft and to the gearbox
. The gearobx is connected to the engine. The engien connected to the boat and so the boat is pushed along. Now if the shaft on the inside of the boat is too long between the stern bearing and the gearbox
, the "push" along the shaft will push the gearbox and motor
off centre. Especially if the motor is soft mounted. This will allow a bend to occur in the shaft and as the force increases, so will the bend. It is possible to get it to bend in either forward or reverse if that motor is pivoting on a broken or faulty mount. I imagine you would notice the bow on opposing sides of the shaft between fwd and rvs if that was happening.
So I hope that gives some addition info to check that maybe you haven't checked before.