Mercury outboard auxillary

am using a 2012 15 hp 2 stroke Mercury on my 22foot trailer sailer and am wondering if i should be using a finer pitch prop i see Yamaha sell a hi thrust prop for there 8-15 hp engines when used as auxillaries, would like to run the engine at higher revs to hit that (sweet spot) any thoughts? or any one got what i need ? cheers

High thrust prop will give more push at lower revs, and cavitate less.
IIRC the Yamaha also have a "reversing" high-thrust prop, with a concentric flange allowing exhaust to exit to the front of the prop when you're in reverse.
15HP seems overkill for a 22 foot TY. 8hp should be fine - much lighter than most 10/15hp.

I think you will find that it's recommended to have a finer pitch prop for that sort of use. It prevents the revs fluctuating so wildly if/when the yatch pitches and the prop tries or does come out of the water. It makes for more relaxed motoring haha.

And I agree about the size. 8hp should do the job quite easily. If you were planing on doing long hours of motoring then a slightly bigger one would be better perhaps. but it is a yatch. Smaller ones are easier to whip off the back when you're sailing too.

What 15 mercury is this one? Is it the american one or the Jap one? What Trailer sailer?
While the smaller outboards will do the job fine, nothing wrong with the larger outboard. You would be lucky to save 10 kgs with a smaller twin cylinder one. Gives you a bit more power when going against sea / wind.
As others have said High thrust props are available. Probably in your situation the main benefit will likely be in reverse only, as I don??

The yatch has a low hull speed, compared to a planing hull. It also pitches a lot which makes them prone to lifting the prop.
Because they only have a slow hull speed the low or high pitch prop will keep the thrust on. If though it cavitates or comes out of the water, the high pitch's load coming off makes the engine race at normal throttle settings. If you are using the low pitch prop, you still get up to hull speed, still have revs to spare, but if the prop comes out the engine doesn't rev up much at all.

Low pitch is recommended and far better for that application.

Yes, go to a finer pitched prop. The standard prop is probably pitched for about 20 knots on a light planing hull. Your 22 footer will have an optimal waterline speed of a bit over 6 knots, and obviously would not require 15 hp to propel it at that speed. Only guessing, but the 15 hp Merc would have enough power at lower revs than its "optimal rpm range", so I see no need to go so fine in pitch, it is reving at, say 4500 rpm to achieve the 6 knots. If the motor is "sweet" at 3000 rpm, I would be quite happy with the prop, as you have power to burn.

As others have said, a "high thrust" prop, possibly a 4-blade would be good, particularly when manovering.

IIRC the OP has a Bonito Aquarius - with outboard in a well. Lifting a 15hp in and out of that would give me the sh*ts.
10/15hp from most makers used to have the same block/engine, re-jetted down to 9.9 to meet US and Euro regulations limiting outboard power on lakes etc. So you'd buy a 9.9, then re-jet them to 15hp, still has the 9.9 covers, an inspector isn't going to know. From that perspective in NZ, there was often no reason to buy a 10 rather than a 15 - unless say on dinghy with was rated to 10hp max.
If the OP's Mercury is one of the one with gearchange/reverse in the throttle (ie not separate gear lever), then sell the thing. My experience is that this system solved a problem which didn't need to be solved, the linkages under the cover to gearbox, throttle, choke are made of nasty plastic which distorts - damned unreliable, inaccessible, and hard to set up.
There is a hell of a lot of weight difference between motors. 9.9/15 Honda or Yamaha 4 stroke are not much less than 50kg. Tohatsu make a 9.8, also available with thruster prop, weight is only 23kg. This based on the ~155cc block used for the 6hp, "tuned up" to 9.8. (their 9.9 is a 200cc block, shared with the 15hp as above) But it's a 2 stroke - so stinky in a cockpit well. Parsun make a knock-off of the Tohatsu, same weight, but only a couple of hundred $$ less - so not worth the saving for Chinese made IMO, even if Parsun offer 3 year warranty. Tohatsu also make a 4 stroke 9.9, which at about 35kg, is about the lightest 10hp/4 stroke.
I don't own a Tohatsu. Most people who do use them as auxiliaries swear by them.
A smaller motor may have another advantage in a TY with cockpit well - you may be able to steer with it to some extent, but a larger motor may not be able to be turned in the confines of the well.

If you are going far or it's rough it's better to have the motor right out of the way isn't it. in a locker somewhere, tied down.

Small motors are lighter and smaller for that. I reckon the idea for yachts is to get the smallest motor that comes with a long enough shaft for you boat.

twenty foot yacht will move reasonably well with four horsepower. as long as there is not too much wind. Once you get into a sea though you need a prop that's deep enough and can get plunged down a fair way without the engine getting submerged, so you need a long shaft. Eights are pretty good compromise for boats this size and type.

thanks for the thoughts, its a Bonito 22 and is been used as a floating caravan on the Rotorua lakes not to skilled in the sailing dept,, seemed a cheap way of a large volume boat on the lakes, was not wanting to lug engine at low rev,s not sure if that makes sense cheers

So you have no concerns now with the way it performs? If you are having no issues, e.g. plugs fouling up etc. If the motor responses readily to throttle at your cruising speed, it probably isn??