Ok, try again…
You should be able to achieve a good plane well below 20kn – depending on how much you are willing to play around with and/or spend on your motor ‘setup’. Given that 150hp is the max hp, and a 4-stroke (i.e. at the very heavy end of the motor transom weight), you may need a lot of tweaking if you are looking to reduce your minimum planning speed by as much as possible.
Do a search here on motor height, which will reveal a lot of threads discussing this, and the numerous options of achieving a lower planning speed (motor height + ; 4 blade props, foils, trim tabs… all of these etc), and the significant benefits people have achieved from doing this.
Start with the motor height, as this is the easiest (and most likely the cheapest) to rectify – if required. Given by your 20kn remark above, I would ‘guess’ that it might be mounted a hole too low (i.e.) the motor vertical height position is a hole too low. A motor mounted too low definitely gives you a bow-high attitude, and a faster ‘minimum planning speed’ than otherwise could be achieved.
Do not lower your motor height, it will make it worse!
Depending on the interpretations of this terminology, it could mean the same thing.
For a motor with 4 mounting bolt holes as below…
Transom top
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|o| << is your bolt in this hole?
|o|
For the above motor installation, the “bolt in 3rd hole down” also means the same as the “motor in the 2nd hole down”, meaning the “motor is in the second hole down from the very top position”.
When people say “2nd hole down”, a lot of them are talking about “the motor position lowered down one hole from the top/highest motor position”, not where the bolt is.
Do a search on here and look at the pics/vids on A/V (cav) plate location in the water stream, when at a good cruising speed, and compare to yours and go from there. The dealer could have buried the motor ‘just to be safe’ (well in his mind at least).
Also, if you are only getting 5500rpm max, I would definitely be looking at a smaller pitch prop, especially if you regularly to big trips with a heavy load. Given this, go straight to a 4 blade that provides good transom lift. Most 4 blades provide good transom lift, but only some 3 blades will, and, ‘normally’ a 4 blade will provide better cruising speed economy than a 3 blade. If the dealer can’t help you, give him his prop back & get your money back, and call SOLAS. Give them some rpm/GPS speed data of your existing prop, and they will probably have 2 or 3 four blade props to try… but… sort your motor height out first, as it could change the performance dramatically.
Cheers
Brendon