johnson 4str 225 to 250?? ideas

signum · 18 March 2006, 18:05

need the extra power of the 250 4str of johnsons.... the pair of 225 goes up to 45knots with 16x23. a friend who has the 250's suzuki which are the same reaches easily 52-53knots with total of 4.5tonnes...mine on other hand is 3.5tonnes.
thinking of bying the "brains" of the 250's...better ideas?

Petrov · 18 March 2006, 19:12

I dont really believe that 25hp would give you 7-8 knots, wich brand (boat) does your friend drive.
I would guess that you need the new 300hp (coming next year, the Suzuki is the same engine as the Johnson) to have 7-8 knots.
Offcourse if the 45 knots today is with the right prop, revs and height of engine.

signum · 18 March 2006, 20:26

petrov thanks for the reply.my boat is greek manufacture...www.xtrim.gr the open-cabin 31.i;ve posted also photos in other thread about the boat with photos.
with the previous props 16x21,5 top speed 42.5-43 at 6000rpms..
now 16x23 top 45 at 5600rpms.
my next move is to put the motors 1 hole down to check....
but i'm thinking of "replacing" the ECU since IMHO the only diference is that....they have 3614cc...so i thing this is the only diference..ECu changes the mapping ... just like in tuning up the cars...

Petrov · 18 March 2006, 20:36

What prop are you using (brand) 5,600 revs is 400 to low, with the right prop you will have a couple of extra knots.
How high/low is the engine, do you know how many cm from bottom the cavitation plate is?
It sounds like your problem today is prop and height of engine, if you are loking for speed you should raise the engine and try to find a prop that dont cavitate, the risc of raising the engine is that the engine dont get enough cooling but there is seperate pickups for that.
Everything is about what you want to do with the boat, on my boat (with 250 Suz) i have the engines raised 10,5cm wich is A LOT but it gives plenty of speed and thats what i want but its offcours sometimes bad when i have load in bow for example.
I drive with a hydromotive 26 pitch (4 blade) prop.

signum · 19 March 2006, 13:22

petrov i used the johnsons props 16x21,5 and top rpms 6000.
the thing is that i bought the boat used 69 hours..the thing is that on the first and second ride i made on august (belonged to previous owner)i saw in gps with that props 47-48knots.
then when i took it in my hands the manufacturer told me that he raised the motors one hole up and put a wedge negative to give more trim but propable that "killed" the boats top speed...also yesterday that i runned for test with the new 16x23 i noticed a shine walking which it didn't had on the first time.
notice...on august that i first ride it (same boat) the previous owner was leaving for vacation and the boat was full loads...imagine:
350 ltrs water,450 ltrs fuel,6 persons.should weighed about 4tonnes and the motors where brand new with 10 hours....and reached 47-48knots!!
now only 44-45 with bigger props and just 200 litrs fuel?
propable the raising and the wedge made faulse...
on a 2 stepped hull?