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 ?

Alway come here first.... if your not sure, ask us...

anything in writing? hwta was it reoresented as?? who was the shop? what specifically were you told, whats the build sheet say??

The boat on trailer with 1/2 tank of fuel (50 gal) weighed 7400 lbs on certified scale, Trailer weighed 1700. So boat weight was 5700 with fuel and gear, no passengers.

So adding 200 for operator...

https://cimg4.ibsrv.net/gimg/www.off...d062dfc867.jpg

I think you are being nice to the boat using 250. :-) that hull isn't that good. That boat with around 600 HP ran 72

That helps..since any input can be the variable, if we put in 5900# 600 hp and 72 mph we get a constant of 225, no remove the hp and plug in 80 mph he is going to need 739 hp and if he is puching a 6000 single with 740 hp, he is going to need a new bravo real soon....

i appreciate everyones time regarding my situation im leaning towards a procharger m3sc kit as i do not want to cut up the hatch what do u all think as i am learning to ask the professionals lol

Whats the compression of your motor?? whats the valve train/cam? Do you have someone that can tune it? Thats a decent size boat with a bravo. are you ready to upgrade the drive and or replace it? Do you have the knowledge to properly setup the fuel system? is the oil system up to the task? Is your bank account comfortable with it??

At 9.8:1 I'm not sure how much boost I would want to throw at it. And you get up over 700 hp, xr or not a bravo will not be your friend . You have 2 motors, leave one in the boat and enjoy it while you do a proper build on the other

What exhaust are you running?

stainless marine 4 '' nickerson said that would be fine and spoke to me about past experiences that have worked fine i understand it takes alot of hp to move this 271 but in life a mans word is his word and thats all u got sometimes i was completely mislead and am excepting this regarding my dealings w this builder

Have you considered what running a thick head gasket will do to the quench?

please dont take this the wrong way...I want your build to be succesful and your goals reached,,, just trying to save you more headaches.... Im not an engine builder by a long shot, just finished my first motor, but as I found doing that its not as simple as just bolting parts together you think will work. I dont know what you want for quench, but to much can actually lead to detonation even with lower compression if you have too much.... those non blower piston and rings which had enough gap NA can butt ends and shatter the top pf the pistons under boost.... Your going to need some major carb work to go with the procharger and probably need larger diameter from the tank back and it just keeps going........

THats why I suggest keeping the other motor, building one over time while enjoying the boat with the other motor, while you use the right pistons, rings, the right cam etc.... Plus that gives you a backup motor should your new blower motor go boom. It keeps you from getting discouraged as you keep spending money on more parts while your 27 foot yard ornament collects dust and your friends are out on their boats.