Horsepower/ Speed Formula
#1
Horsepower/ Speed Formula
Hey Guys,
I recently came across a formula for calculating top speed/ hp and it’s been pretty accurate from running the numbers on some of my friends boats. I don’t think it’s perfect but so far is seems to get close.
Here it is:
Horsepower/Weight of Boat = X
Square Root of X = Y
Y x 250 = Hypothetical MPH.
Hypothetical MPH x 90% = True MPH (accounts for prop slip).
I’m curious to see if this holds up, especially considering the many variations in Hull Design. Post your numbers from the formula vs what your boat will actually do if you’d like.
I recently came across a formula for calculating top speed/ hp and it’s been pretty accurate from running the numbers on some of my friends boats. I don’t think it’s perfect but so far is seems to get close.
Here it is:
Horsepower/Weight of Boat = X
Square Root of X = Y
Y x 250 = Hypothetical MPH.
Hypothetical MPH x 90% = True MPH (accounts for prop slip).
I’m curious to see if this holds up, especially considering the many variations in Hull Design. Post your numbers from the formula vs what your boat will actually do if you’d like.
#3
Offshoreonly Advertiser
Offshoreonly Advertiser
Take a look here
Speed Calculator
The prop slip is actually not relevant in this case. The key is having the correct constant for the boat. If you know the existing speed, weight and power you can calculate the constant. Once you have that this calculator is surprisingly accurate.
Speed Calculator
The prop slip is actually not relevant in this case. The key is having the correct constant for the boat. If you know the existing speed, weight and power you can calculate the constant. Once you have that this calculator is surprisingly accurate.
__________________
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
#4
Take a look here
Speed Calculator
The prop slip is actually not relevant in this case. The key is having the correct constant for the boat. If you know the existing speed, weight and power you can calculate the constant. Once you have that this calculator is surprisingly accurate.
Speed Calculator
The prop slip is actually not relevant in this case. The key is having the correct constant for the boat. If you know the existing speed, weight and power you can calculate the constant. Once you have that this calculator is surprisingly accurate.
Very good, though prop slip is still relevant. This table gives us a Constant to use based on Hull Type as well as provides more accurate prop slip numbers.
So instead of generic Yx250, we can get more specific with this table and choose the proper constant for out equation as well as adjust the prop slip portion of the formula.
Last edited by Romeomikehotel; 02-15-2019 at 05:58 PM.
#5
Nfw
doesn’t work for me.
Interested in my next config, testing the formula with a previous known config:
9600 lb boat (not factoring in fuel or passengers)
1200 hp
top speed actual was 68.9 mph
using the formula...
Horsepower/Weight of Boat = 0.125
Square Root of 0.125 = 0.35355
0.35355 x 250 = Hypothetical MPH of 88.38
= 22% error
Interested in my next config, testing the formula with a previous known config:
9600 lb boat (not factoring in fuel or passengers)
1200 hp
top speed actual was 68.9 mph
using the formula...
Horsepower/Weight of Boat = 0.125
Square Root of 0.125 = 0.35355
0.35355 x 250 = Hypothetical MPH of 88.38
= 22% error
#6
doesn’t work for me.
Interested in my next config, testing the formula with a previous known config:
9600 lb boat (not factoring in fuel or passengers)
1200 hp
top speed actual was 68.9 mph
using the formula...
Horsepower/Weight of Boat = 0.125
Square Root of 0.125 = 0.35355
0.35355 x 250 = Hypothetical MPH of 88.38
= 22% error
Interested in my next config, testing the formula with a previous known config:
9600 lb boat (not factoring in fuel or passengers)
1200 hp
top speed actual was 68.9 mph
using the formula...
Horsepower/Weight of Boat = 0.125
Square Root of 0.125 = 0.35355
0.35355 x 250 = Hypothetical MPH of 88.38
= 22% error
Thats how I I initially ran the formula as well.
Take a look at the little paragraph on the Black Table above, it states that you need to use full loaded weight in the calculation. Also, you forgot to multiply by .9 to account for prop slip. I wonder if that would get you closer.
If I take your info, add 1k lbs for passenger and fuel, subtract 10% from your hp to account for drivetrain losses and then remove the 9% for prop slip, we end up at 73mph.
Still not perfect, but much closer to reality.
Last edited by Romeomikehotel; 02-15-2019 at 06:19 PM.
#7
Offshoreonly Advertiser
Offshoreonly Advertiser
doesn’t work for me.
Interested in my next config, testing the formula with a previous known config:
9600 lb boat (not factoring in fuel or passengers)
1200 hp
top speed actual was 68.9 mph
using the formula...
Horsepower/Weight of Boat = 0.125
Square Root of 0.125 = 0.35355
0.35355 x 250 = Hypothetical MPH of 88.38
= 22% error
Interested in my next config, testing the formula with a previous known config:
9600 lb boat (not factoring in fuel or passengers)
1200 hp
top speed actual was 68.9 mph
using the formula...
Horsepower/Weight of Boat = 0.125
Square Root of 0.125 = 0.35355
0.35355 x 250 = Hypothetical MPH of 88.38
= 22% error
__________________
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
#8
Offshoreonly Advertiser
Offshoreonly Advertiser
Thats how I I totally ran the formula as well.
Take a look at the little paragraph on the Black Table above, it states that you need to use full loaded weight in the calculation. Also, you forgot to multiply by .9 to account for prop slip. I wonder if that would get you closer.
The variable is just that. It changes according to hull characteristics and setup. If you change to labbed props and gain 3 mph your variable will be different.
The best way to use this is to establish the variable for your individual setup. Then you can see what a change to weight or power will accomplish.
__________________
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