how to work out the speed of your boat.?

[dunmor]

can anyone tell me how to work out the max speed of a displacement boat from the water line length. is the water line both sides and the transom, or just the waterline on one side?

 

[sailorman]

can anyone tell me how to work out the max speed of a displacement boat from the water line length. is the water line both sides and the transom, or just the waterline on one side?

http://www.boatsafe.com/kids/speed.htm

 

[dunmor]

thanks for that. answered my question perfectly.

 

[Nostrodamus]

thanks for that. answered my question perfectly.

We have a transom like many boats that when at rest is out of the water. When the boat is under way or healed then the transom is down to the waterline and presumably this is the waterline length you should use to measure Hull speed, not the length quoted as the waterline length?

 

[Mistroma]

Can't remember where I read it but calculation I've always used for displacement boats is:

1.3 to 1.5 x sq. root of waterline length in feet gives speed in knots.

The waterline length will change when a boat heels and so usually increases with sailing boats (as mentioned previously).

N.B.
Factor just indicates that some hull designs are "slippier" than others and that this is the sort of range you'd usually expect.

Calculation just gives the speed expected with a modest amount of power. i.e. You can go faster but law of "diminishing returns" kicks in.

This calculation only applies to hulls with length:beam ratio less than 7:1 (approx.).
Long thin hulls (catamaran, canoe etc. can go much faster than fatter designs without soaking up so much power).

 

[BlueSkyNick]

http://www.boatsafe.com/kids/speed.htm

A shame the author of an article for kids can't actually spell properly, as in

the length of the water line does effect the speed of some boats. In particular displacement type boats

 

[Lakesailor]

Can't remember where I read it but calculation I've always used for displacement boats is:

1.3 to 1.5 x sq. root of waterline length in feet gives speed in knots.

The waterline length will change when a boat heels and so usually increases with sailing boats (as mentioned previously).

N.B.
Factor just indicates that some hull designs are "slippier" than others and that this is the sort of range you'd usually expect.

Calculation just gives the speed expected with a modest amount of power. i.e. You can go faster but law of "diminishing returns" kicks in.

This calculation only applies to hulls with length:beam ratio less than 7:1 (approx.).
Long thin hulls (catamaran, canoe etc. can go much faster than fatter designs without soaking up so much power).

Just use this http://www.marinesurveysr.us/calc_Hull_Speed/calc_Hull_Speed.aspx

Steamboat launches often have very long, narrow hulls with fine entries to cheat the rule and get high speeds.
(Yes, I know it's not a real rule)

 

[sailorman]

A shame the author of an article for kids can't actually spell properly, as in

it was the first i found with google

 

[John Barry]

It's about the speed at which waves move and their wavelength, which are related.
As you travel through the water you create waves that move at that speed, and as the speed increases the wavelength (distance between crests) increases. Once the wavelength reaches the boat's waterline length you're perched on a crest at the bow and a crest at the stern (with a trough amidships), going faster means that the aft crest drops behind the boat, and the stern starts to settle into the trough - and you're going up hill, which takes significantly more power.
Hull speed is not a limit, it's a tipping point on a graph where the power needed to travel faster increases significantly.

The formula associating wave speed and length is

Speed (kn) = sqrt( 9.8066wavelength/2pi x tanh(2piDepth/wavelength) ) x 3600/1852

For wavelength you can read waterline length
Wavelength and depth in metres
The tanh function is in radians
9.8066 is gravity
3600 changes m/s into m/h
1852 changes m/h into Kn

You can leave out the tanh function if the depth is more than half the wavelength. At this depth the function resolves to .996, Deeper and it increases towards 1, shallower and it makes an increasing difference.

This formula gives a result (depth >5m) for a 10m waterline length of 7.7Kn.

Or you can do 1.35 x sqrt(waterline length in feet) = 1.35*sqrt(10/.3048))= 7.7Kn.

It's just that the proper formula (rather than the approximation) is more accurate, and tells you than if you reduce the depth to 1m your hull speed drops from 7.7Kn to 5.7Kn, and at 0.1m (!) it's 1.9Kn.

OK, nerdy bit over at last, other than to say that if you want really accurate figures you need to allow for the relative densities of air and water - you can find your own formula for that.