When motoring, I need to hold the tiller hard with both hands as the ruder wants to continually shift from one side to another.
That's fairly common. We have it with our Pearson P30.
Hint: There is an amount of throttle beyond which you'll find you gain little speed but the engine is working much harder and so are you. For us, in relatively flat seas and with little-to-no headwind, that is right around 6 kts. With no
headwind and very
flat seas she'll do 6.5 knts comfortably. Abracadabra's hull speed is 6.7 kts, IIRC.
It also has a two blade folding prop wich I don't like and I intend to replace with a fixed blade prop during the winter. Will a different prop change anything?
First of all, Daniel, your boat is a "she," not an "it."
As far as having to keep a firm grip on the rudder: No, a fixed prop won't do much, if anything, for you. You'll get a bit more forward speed and better reverse, at the cost of losing speed under sail. In light air: Quite a bit of speed lost. When our folding prop became ill, and we temporarily swapped it out for the fixed two-blade prop we had, we noticed a significant
loss under sail.
I had the same issue on my Islander 30 II.
I eliminated this problem on my boat by removing
the inboard Atomic 4 and installing an outboard on
the transom. The boat performs much nicer under
power or sail now.
I find it exceedingly
difficult to believe that any 30' boat performs "well" under power with an outboard motor hanging off the transom.
Coincidentally: Just brought to the top by a comment: Pearson 31 inboard or outboard