Speedboat transom height

I have,after a lot of searching,Identified my boat.
It's a late 70's Broom 14ft Gemini.(UK)
The distance between the top of the transom to the bottom of the transom at the hull, is 15'' (380mm)
I have a short shaft Evinrude 18hp,the cavity plate is 3'' (75mm)lower than the bottom of the transom.
Is there a fault somewhere that I am not understanding?
Will the difference in height cause a lot of drag,or is this standard?
I also have a 1999 25hp Johnson,which is a longshaft.
I have raised the transom 3'',will it be possible to use the 25hp Johnson on it?
I don't need speed because I use the boat for sea fishing.(inshore)
Any opinions will be greatly appreciated.''V''

 

Those old short shaft motors were typically about 17" from the transom ledge to the cav plate. Reason being, lots of old boats had a keel of sorts, 2 inches deep at least, that could disturb the water ahead of the prop, causing cavitation problems. I think you will need to go more than 3 inches, but has this boat got a keel/skeg ahead of the engine ?

 

Gooday Mr ''E''
There is no keel or skeg,it's a nicely designed planning hull, lateral chines at deck level,a pointed bow,and is almost flat at the stern.
It won't quite plane with the 18hp original prop on it(doesn't quite reach wot)9.25''X 11''prop.
I have changed the prop for a 9'' X 9'' which solved the problem.
I got the 25hp Johnson for a little more power(and it's more modern)It's got very low hours on it and looks fairly new.
Some owners are using 60hp motors on this boat,but the weight for me to handle is a bit too much because I need to take the engine off to store it.
Changing the subject slightly,have you tuned into ''Dangar Marine'' on Utube,he's on the Hawksworth river above Sydney.
Clever guy,great personality.Well worth a look.

 

I'd say you will need to go up the 5 inches, at least, V.

 

Just what I thought Mr E,back to the drawing board,Haha.
I'll have to change the helm/steering setup,and the controls and cables to suit the 25hp.