Speed calculation please
#1
Team Dysfunctional
![](https://www.offshoreonly.com/forums/images/icons/charter_member_star.gif)
Thread Starter
Join Date: Jan 2001
Location: Bolton Landing NY
Posts: 1,420
Likes: 0
Received 0 Likes
on
0 Posts
![Default](/forums/images/icons/icon1.gif)
I found someone with the same boat as mine. I was looking to see what kind of speeds to expect out of the same boat with 575HP . This is what the other guy has
1988 Scarab 28' Excel 4600 lbs dry, Holds 80 gal. of fuel
MerCruiser 454 Mag 365hp
Bravo One outdrive 1:50 gears with 23p Mirage Stainless Prop Max Speed 62 mph @ 4700 RPM
Thanks
Paul
1988 Scarab 28' Excel 4600 lbs dry, Holds 80 gal. of fuel
MerCruiser 454 Mag 365hp
Bravo One outdrive 1:50 gears with 23p Mirage Stainless Prop Max Speed 62 mph @ 4700 RPM
Thanks
Paul
#7
Registered
Join Date: Oct 2000
Location: Simpsonville, SC 29680
Posts: 653
Likes: 0
Received 0 Likes
on
0 Posts
![Default](/forums/images/icons/icon1.gif)
rpm x pittch
__________ = speed with no slip
gear x 1056
108100
_______ =68.24 - slip ??13% = 59.37 mph
1584
15 hp to go 1 mph????
62 mph would be a 4.4% slip I DON'T THINK SO.
365 was at the crank
if you are 575 at the crank and not the prop it will be apples to apples.
575 - 365 = 210 / 15 = 14 more mph + 59.4 = 73.4
How did I do???????????
__________ = speed with no slip
gear x 1056
108100
_______ =68.24 - slip ??13% = 59.37 mph
1584
15 hp to go 1 mph????
62 mph would be a 4.4% slip I DON'T THINK SO.
365 was at the crank
if you are 575 at the crank and not the prop it will be apples to apples.
575 - 365 = 210 / 15 = 14 more mph + 59.4 = 73.4
How did I do???????????
![Big Grin](/forums/images/smilies/biggrin.gif)
![Big Grin](/forums/images/smilies/biggrin.gif)
![Big Grin](/forums/images/smilies/biggrin.gif)
#8
Offshoreonly Advertiser
![](https://www.offshoreonly.com/forums/images/icons/charter_member_star.gif)
![Default](/forums/images/icons/icon1.gif)
__________________
Marc
www.mercruiserparts.com
www.go-fast.com
www.bammarine.com
www.cyborgtransmissions.com
It's not alive -www.BoatStuffExpress.com - temporarily retired
Marc
www.mercruiserparts.com
www.go-fast.com
www.bammarine.com
www.cyborgtransmissions.com
It's not alive -www.BoatStuffExpress.com - temporarily retired
#9
Team Dysfunctional
![](https://www.offshoreonly.com/forums/images/icons/charter_member_star.gif)
Thread Starter
Join Date: Jan 2001
Location: Bolton Landing NY
Posts: 1,420
Likes: 0
Received 0 Likes
on
0 Posts
![Default](/forums/images/icons/icon1.gif)
Thanks for all the help here. I am going to go with what MIKEHTMSR24 said. It seems well thought out and close to what the base speed of the boat should be. I didn't check out the other sights, Mike seems close enough to me. What I need to find is some honest GPS speeds. Its hard to find someone that will tell the troth on how fast there boat goes unless it's something to brag about. Thanks
Paul
Paul
#10
Guest
Posts: n/a
![Default](/forums/images/icons/icon1.gif)
rule of thumb that also works well
Sqrt(new HP)
___________ x old max speed = new speed
Sqrt(old HP)
so:
Sqrt(575)
________ x 62mph =77.8mph
Sqrt(365)
If you start with 59mph which as MIKEHTMSR24 pointed out seems more reasonable then the new speed will be 74mph
And y'all told your 6th grade math teacher when will I ever use square roots in real life
Sqrt(new HP)
___________ x old max speed = new speed
Sqrt(old HP)
so:
Sqrt(575)
________ x 62mph =77.8mph
Sqrt(365)
If you start with 59mph which as MIKEHTMSR24 pointed out seems more reasonable then the new speed will be 74mph
And y'all told your 6th grade math teacher when will I ever use square roots in real life
![Big Grin](/forums/images/smilies/biggrin.gif)
![Big Grin](/forums/images/smilies/biggrin.gif)
![Big Grin](/forums/images/smilies/biggrin.gif)