Marine Forum

BOAT TOP SPEED

Anonymous user

08 July 2008 07:10:31

Joined: Unknown | Posts: Unknown

Hi. I have a 2001 Skeeter sx190 with a 150hp vmax ox66 efi. With a full tank of gas and three people and fishing gear my top speed is about 46 per gps. I have a 23 pitch prop. What should my top speed be and if I switch to a 21 pitch four blade will i lose top speed or gain due to higher wot rpm. Any suggestions will be appreciated as I am new to powerboating..Thanks.

Re: BOAT TOP SPEED

Anonymous user

25 July 2008 11:12:02

Joined: Unknown | Posts: Unknown

You can do all sorts of things to improve the performance of your boat.

Cleaning the hull on a regular basis and new props and servicing all help to squeeze every bit of speed of her.

One thing to appreciate is some boats, mainly early sailing boats, were designed with a particular “Hull Speed” in mind.

Hull speed, sometimes referred to as displacement speed, was a common rule of thumb used to provide the approximate maximum speed possible of the hull.

It is generally described as the speed corresponding to a Speed-Length Ratio of between 1.34 and 1.51 depending on which of the limited sources one refers to.

Hull speed is typically not a term used by naval architects (they use, instead, a specific speed/length ratio or Froude number for the hull in question) but is often used by amateur builders of displacement hulls, such as small sailboats and rowboats.

Wave making resistance begins to increase dramatically in full-formed hulls at a Froude number of about 0.35 which corresponds to a Speed Length ratio of slightly less than 1.20. This is due to a rapid increase of Wave making resistance due to the transverse wave train.

At a Froude Number of 0.40 (Speed-Length Ratio about 1.35) the Wave making resistance increases further due to the increase of the resistance caused by the divergent wave train which is added to the transverse wave train resistance.

This rapid increase in wave making resistance continues up to a Froude Number of about 0.45 (Speed-Length Ratio about 1.50) and does not reach its maxima until a Froude Number of about 0.50 (Speed-Length Ratio about 1.70).
This very sharp rise in resistance at around a Speed-Length Ratio of 1.3 to 1.50 was probably insurmountable in early sailing ships and became an apparent barrier.

It must be pointed out that these values will change dramatically as the general proportions and shape of the hull is changed. Modern displacement vessels can easily exceed their Hull Speed.