Height of a wave

[Steph357]

Hello every one.

I was wondering something about a powerboat, does anyone know if it is possible to calculate the height of the wave creates by a powerboat plannig at a given speed ?

I've been looking on internet but all the works I found was about the height of the waves of the seas, and not of the boat itself. Of course we assume that the boat is running in a "no-wave" area like a lake.

[Ad Hoc]

Well, not so simple to answer, since the further away from the boat the lower the wave amplitude and it is also affected by the water depth/boat length ratio.

[CDK]

The main variables are:

-boat speed
-displacement (depends on weight, length and hull shape)
-distance from the observer

Water depth and bottom characteristics will also influence the wave height.
Actual wave height is only a rough indicator of the potential danger, the wave shape is also important. A hydrofoil at 50+ knots produces sinus-like waves, a heavy tug boat at 20 knots makes waves with an almost vertical flank.

[Guest625101138]

Originally Posted by Steph357

Hello every one.

I was wondering something about a powerboat, does anyone know if it is possible to calculate the height of the wave creates by a powerboat plannig at a given speed ?

I've been looking on internet but all the works I found was about the height of the waves of the seas, and not of the boat itself. Of course we assume that the boat is running in a "no-wave" area like a lake.

Michlet has the ability for determining wave height. The accuracy will depend on how well you can adjust for trim and lift as the program is intended for displacement craft.

The indicated result will be worst case for a planing hull if fully on the plane in 1000m deep water. It could be worse if the boat is badly trimmed with bow up. The attached wave patch (50m by 50m square) is for a 10m vessel displacing 4.6t doing 20kts. The trough at the very stern is minus .614m and the peak beside this is plus 0.614. As the wave fans out the peak and trough reduce somewhat.

If you provide some idea of why you want to know the wave height and the type and speed of the craft then it may be possible to home in on a useful result.

Rick W

[Guest625101138]

Just for interest the attached shows the wave patch for the same boat in 1m deep water. The wave height is less but the shape of the fan is different and the wave does not lose height as quickly.

Rick W

[Guest625101138]

You might find some interesting reading here:
http://www.cyberiad.net/wakepredict.htm

There are some examples here but none of planing craft as the analysis does not automatically compensate for lift/squat and trim:
http://www.cyberiad.net/wakeimages.htm

Rick W

[Ad Hoc]

Plots of "waves" are somewhat meaningless without a reference to depth. Since the wave shall be either
a) sub-critical
b) critical
or
3) super-critical.

Also there is a difference between the divergent and transverse waves in each category for varying reasons, all related to depth.

[Steph357]

Thank you for all your answer, I will have a check at every thing.

I want this information because I'm working on a project in a lake, probably involving small sport boat up to 8 meters running at full speed, i guess 30 or 35 knots. We want to know what could be the height of thewave to see what could happen to houses surouding the lake (it is a very small area, 4.5 sqkm). i don't need precise data, but to have an idea of what it could be.

[Guest625101138]

Originally Posted by Steph357

Thank you for all your answer, I will have a check at every thing.

I want this information because I'm working on a project in a lake, probably involving small sport boat up to 8 meters running at full speed, i guess 30 or 35 knots. We want to know what could be the height of thewave to see what could happen to houses surouding the lake (it is a very small area, 4.5 sqkm). i don't need precise data, but to have an idea of what it could be.

Do you have any idea of the total weight and beam of the hull.

A catamaran is likely to result in smaller wake. A monohull may make a very large wake during the lift-off stage depending on trim.

You can make a hull that does not make waves at a particular speed but it would be very long for 30 knots.

Rick W

[Ad Hoc]

Steph357

Ostensibly what you're talking about is energy, since amplitude is rather meaningless with the context of its energy and its potential destructive power to shore erosion, boats used for living on the lake etc etc.

What depth is the lake you're vessel will run in..since this is the major influence. Assuming it to run in the super-critical region. as you'll need to establish the froude depth number.

If a monohull, then you'll need to take into account that the long period waves will have approximately twice as much energy than a catamaran. As such in the super-critical region similar length and displacement cat's versus mono's, a catamaran will have significantly lower wash.

Such as, you'll need to distinguish between the long period waves (beaches and shallow banks) and the short steep waves (small pleasure crafts). However, If there is considerable fetch then there shall be some dispersion of the energy into the subsequent waves.

You may even get solitary waves too, low amplitude but potentially very damaging.

You should read the excellent paper "An Investigation of Fast Ferry Wash in Confined Waters", by T.Whittaker et al.

[daiquiri]

Originally Posted by Steph357

I want this information because I'm working on a project in a lake, probably involving small sport boat up to 8 meters running at full speed, i guess 30 or 35 knots. We want to know what could be the height of thewave to see what could happen to houses surouding the lake (it is a very small area, 4.5 sqkm). i don't need precise data, but to have an idea of what it could be.

A lake of 4.5 square kilometers would be, say, around 2.2-2.3 km from one end to another, right? It means that, running at 35 kts, you have crossed the whole lake in less than 2 minutes.
Well, I mean... What's the point of doing that? Unless it is a sort of race circuit, wouldn't it be more enjoyable to make a boat which will let you have a nice relaxing ride of, say, half an hour, or an hour? That would allow you to make a boat which would create a very small wave pattern.
Just my thought...

[Steph357]

Thank all of you.

I know it could sound surprizing, butI do that for chinese people, and they don't really what a boat is, and as you said, riding a boat at 30 knots during 2.5 km is pointless, but they ask me so ...

Aniway I think it is far beyong what they want me to do, but as a personnal interest I will try to read all of this.

Tahnk, again.

Cheers.