Convert 15HP outboard to High Thrust

[rodbolt17]

and by posting some drivel.
we now know the poster zeno sumptin. aint goota clue.\
don't matter if its a speed boat or a (*&%^%$#@ blow boat.
the rules still apply.

\but the zeno poster is now on the ignore any advice list.

[boscoe99]

I get somewhat annoyed when it is suggested the "right" prop is one that allows the engine to reach max revs! This is merely a convenient mantra. Not according to Yamaha it is not. They recommend propping the motor so that it can run at or near its red line when the boat is loaded.
The OP has stated he is not interested in revving these at such a high speed. I read the OP's posts as him wanting maximum engine thrust when he is docking the boat. If it takes 5500 RPM to get maximum thrust then so be it. No harm no foul to the motor.Neither would I want to rev these motors at full 5500 rpm for docking manouvers - certainly would attract onlookers who would probably worry and question the operators skill.
There is a reasonable revs to do this at and the prop must be maximised for this task! The negative is that the engines may be overrevved because of the subsequent lack of resistance. If the motors are propped to run at or near red line when the boat is loaded then the motors cannot over rev. If it were possible that they could the rev limiter will take over and prevent motor damage. I suspect that there is nil chance of a T9.9 over revving when trying to push a sailboat around in wind or currects.
My comments a based on the assumption that these engines are not the prime source of propulsion and are used for slow speed taxiing etc.

I would venture to bet that the motors are over propped and this is why the OP is asking for help. Quite common in the situation that he is in. I doubt that he will be able to find any propeller that is going to let the motors run in their best HP RPM range. He needs that HP to translate it into thrust. Max HP comes at 5500 RPM.

[zenoahphobic]

Thanks Boscoe, I did intimate that the OP may have trouble finding a right prop. And I also said he needs an 11 inch prop with a large blade area.
We don't know the size of his boat (actually it is the mass that is important here).
And somewhat in answer to Rodbolt, sometimes you need to be a bit provocative in order to get people to think and actually help the OP.

I was feeling that if the OP gets the same prop with a reduced pitch, that that may not help him. We don't know enough about the OP's boat, his prop(s) and what is actually happening. Boscoe suggests he may be overpropped, we don't know. He may well be underpropped, depending on what is meant by underpropping. E.g. the situation might be that a reduced pitch (all else being equal) will just make the engine rev faster with NO ADDITIONAL THRUST; that is it will not bite or grab anymore water to push! Standard prop diameter is 9 inch and high trust is about 11 inch. Reducing pitch on the standard prop will do little to help in this situation. What is important is that the prop needs to push a lot of water slowly, and not have the situation where little water is pushed very fast. (Prop efficiency or actual slip needs to be improved)

My "drivel" as might be suggested is:
Maximum thrust can only occur at maximum engine torque which invariably is at less revs than "redline".
Maximum thrust can only occur at maximum blade diameter and blade area (give or take all sorts of efficiency looses in prop design, and gearbox ratios can come into play here).
A larger output motor will not produce any more thrust than a smaller motor with the same propeller.
OP needs 11 prop with greatest blade area (largest diameter that will fit) at appropriate pitch.
Granted, if it takes 5500 rpm so be it.

Unless the OP's boat mass is huge, one would think 10 HP(X2) should be able to push it back and forwards without too much resistance by inertia. We need much more information and then I would hope someone can make an informed suggestion on what prop (that produces the least slip for the boat combination) and where to get it.