Engine Revs Low

Never had a fouled prop on my boat so can't help with regard to that query.

But I do have a prop I can adjust the pitch on. If I set the pitch too course, than the maximum engine revs is limited. My boat can still get to about 6knts put it has to be in "no wave/swell" conditions. If I adjust the prop pitch a bit finer then the engine revs a bit higher and I need these extra revs to get the boat to 6 knts. But I can now get to 6knts in a moderate sea. And if I adjust the pitch even finer I can push the boat into a bigger swell. My engine needs to be revving at about 2,700 to produce its rated 30hp. But as revs go up so does fuel consumption. Therefore I do not set my prop pitch for max power but a compromise between power and fuel efficiency.

1 Possibility is the prop on the boat you are looking at has been sized/changed to give a compromise between fuel efficiency and power. Meaning it will never get to 3K.

I only tried forward and reverse

Yep that is my fear that they aren't matched

The boat unfortunately is 300kms away from me

Hump de bump.

There are a number of factors that may cause the situation.

1. Propeller balance and equal pitch of blades.

2. Alignment of stern bearing and P or A bracket bearings.

3. Incorrect pitching of propeller. (Not likely if you are getting different results in forward and reverse.) However likely if the gearing is a different ratio between forward and reverse.

4. Shaft to engine alignment.

5. Straightness of propeller shaft.

6. Worn out engine???

I don't think you will find a definitive answer here on the forum. If you have not bought the yacht yet, it is the vendor's problem to fix.

If you already own the yacht, you are going to have to suck the puss somewhere along the line.

Don't overlook the obvious.
Check that the throttle cable/ linkage and bits are actually going to the full throttle stop on the injector pump.