what is the tinest engine i could use to plane this boat off?

[racing fan]

yea sorry about that Pierre R i wasn't reading that carefully so on your opinion what hp would i need to do 70 at 3/4 throttle or like just before full throttle. (with 4 people) I was thinking like 135-150 what do you think of that?

[Pierre R]

Originally Posted by racing fan

yea sorry about that Pierre R i wasn't reading that carefully so on your opinion what hp would i need to do 70 at 3/4 throttle or like just before full throttle. (with 4 people) I was thinking like 135-150 what do you think of that?

3/4 throttle on a 140hp outboard should get you about 55 mph. Fast enough that I won't be in your boat.

[racing fan]

ok that works for me so with a 140 i would get like 60-65 maybe 68 with a good prop set up and full throttle right? judging by you opinion.

[Pierre R]

Originally Posted by racing fan

ok that works for me so with a 140 i would get like 60-65 maybe 68 with a good prop set up and full throttle right? judging by you opinion.

Maybe

[racing fan]

OK thanks well i at least want to go 60 so i guess ill go with a 140 and what year like old 70 mercury or like a 80 johnny etc.

[Pierre R]

Originally Posted by racing fan

OK thanks well i at least want to go 60 so i guess ill go with a 140 and what year like old 70 mercury or like a 80 johnny etc.

There were quite a few options for modifying props and such for those years of Merc's so I can't fault your thinking, only your judgement .

[pistnbroke]

perhaps I missed something here but the max speed is determined by the prop pitch ......so if you take a 12 pitch( 25 hp ) this gives a max speed with no slip of 34 mph ...so you may get 25 mph ....rough guide speed is 80% of 3 x the prop pitch in inches assuming a 2:1 gearbox and 6000 rpm engine revs ....so you need a motor big enough to drive a ( 70 div .8 div 3 ) 29 pitch....so we are into serious power ..special surface piercing props etc etc .

[racing fan]

note hydroplanes are not like regular boats they can go faster and this is a big boat so no 25hp try 125hp an 2 stroke about 7000rpm 1:1 gearing racing lower unit and then you half to add drag from the boat,wind weight,ect

[pistnbroke]

true racing fan but no boat can go further in one rev of the prop than the pitch in inchces......and of course to drive a big pitch prop you need a big motor

[racing fan]

yea you are right about that but less boat in the water the more prop you can run until a point