99 271 fastech trying reach 80mph please help
 

installed all the proper steering tabs and xr drive then built a 675 565 cu inch motor and it just wont get going greater then 20 percent drag cant spin a 28 bravo 4 blade to 5400 only 5k and 73 mph tried a 30 and same thing tried a 26 and 71mph but hit 5400-5500 has anyone reached 80 plus and what did u do please help this is getting expensive do i need a bigger motor i love the boat its in great shape runs well just not producing the speed im trying to get any setup ideas tried and true methods would be greatly appreciated thanks guys eddie 631 4873557 the prop shaft center is 5 inches below pad and and x demension is 18 '' from bottom of pad to crankshaft thanks guys I have an XR. drive 150 ratio

Another piece of the equation will be your drive gear ratio. Do you know that? That should help some of the guru's figure this out.

It took 750hp for my 28 Advantage to hit 80mph, and it had a couple of small steps in the hull.

are you sure you are making 675 HP? I thought those hulls go 72 with 500 HP. I had a 242 that I added 100 HP and gained 5 mph...so your hull should be going about 80 if you have 675 HP.....

50Viking - to get this out of the way too, what elevation do you run at ?

That Merc 525SC supercharger is too small for your 565 Viking...if that's what you where inquiring about over in the for sale section.

Well... The motor Im finishing should make about 625...Im praying to kiss 80 and that is a 22 foot boat thats a LOT lighter than yours.....

Dry weight best I can find is about 5750,,,given the fact that every mfg's dry weight number is more optimistic than it pitot speedo, with gear people fuel, beer cooler etc your probably looking at 7000 pounds. so when we plug 7000 pounds.+ a constant of 240 ( resonabley good shape single step bottom), 675 hp and an optimistic 8-9% slip. into the bam speed calculator (which is pretty good when given realistic infornation) you get a top speed of 75 mph, per the calculator to hit 80 you need at least 778 hp. blueprint the bottom, put the boat on a diet, you can get close with 675 hp Also when doing the math with your max 71 speed, your motor (if everything else is good) is making about 100 hp less than claimed....

http://www.go-fast.com/boat_speed_predictions.htm

https://www.offshoreonly.com/forums/...stech-mph.html


https://www.offshoreonly.com/forums/...top-speed.html

Viking, any chance in heck you are in Texas ?
There is a guy that has a prop dynbo in TX that is a member here. He is retired but maybe interested in making some money ?