flatout

I have an 84 formula 272 with the twin chevy 5.7's, right now trimmed out I am at 47 mph gps at 4900-5000 rpm's. thats with just enough fuel and three adults and a child on board with just the essential equipment. I am running a set of cleavers 21p, not sure they are cleavers but they are 21p.both right hand drive. I have alpha 1 drives. I have heard that thses old v's like the 3 blade round ears props.
I would like more top speed (like everyone else). I am not a drag racer so hole shot isn't too important.
any input?

throttleup

If your gears are a 1.47 then you have some serious slip issues. I am coming up with 30%. This is unbelieveably high and means that your efficiency is only at 70%. You should be 1/2 of that at 15% worst case scenario. I would try to find some 21 Mirage Plus propellers and test them.
If you can reduce the slip then the speed will follow.

Julie

flatout

Thanks Julie

bobl

You sure you don't mean 57 on GPS??? I have a customer with that exact boat and props. I built some modified 350's for it and we were getting 64 on GPS with those same props and gears at around 5300 RPM.

flatout

positive it was 47 and my engines are bone stock so if you got 64 with built engines I am not too far off of where I should be I would think. I am sure with more practice and ideal conditions i could get mid 50's. I plan on heads cam and intake over the winter.estimated 400 ponies per, then I will probably bust my drives. all in good fun though. The only other thing i can think of is my props aren't at all polished anymore they look like brushed aluminum now, I don't know if that makes a difference. I will post a pic and let you see.

flatout

here is a pic of the props, the only markings on them is 21p and a couple numbers.

Superboat Guy

My friend has the same boat, stock power, his is counter rotating. I dont think you gain speed with that, just handing. He runs Mirage 21 and sometimes will switch to 23. His are not Mirage plus. He has runs a best top speed of 59 mph GPS and with the 21 somewhere around 55.

throttleup

Flatout,

Those are Mercury canned cleavers. Old style props which will produce decent top end but performance can leave alot to be desired.

Julie

flatout

thanks julie

flatout

I did some work and some testing today, I adjusted my throttle cables so now I am getting full throttle. they were off before thats why I had a hard time matching rpm's. I got a best of 52.8 on the gps this time at a solid 4800 rpms. now as soon as I trimmed down the rpms dropped off like nobodies business. trimmed all the way down I was only getting abot 4100-4200 rpm's. I would like to see 60-65 mph which is what everyone else with this boat has done.
My drives are Alpha ones does that limit me? is there any shorty lowers or any performance upgrades for them?