Small displacement boat recommendations.

[Davy_S]

I would have thought that at 40ft waterline, providing you had enough power, you would be capable of 9 knots (ish)

 

[castlevar]

yes it will do more than 6 knots but it is no longer displacment
maximum speed is 8.5 knots at 2500 revs fuel consumption 30 ltrs hr so thats why i work at displacment speeds and it is also noisy at high revs.

 

[whisper]

Surely at 8.5kts you are still just at displacemnt speed for your waterline length.
Mind you, I've forgotten how to do the calculation now - it's me age!!

With our waterline length of about 23 ft the theoretical max displacement speed is in the region of 7.5kts, I'm led to believe.

 

[castlevar]

Hi Whisper it is pye the waterline length equals displacment speed

 

[whisper]

Thanks. Presumably you mean waterline length divided by pye. Thus 7.3 knots for us. BUT 12.7 at 40ft ??

 

[Sundays_Child]

I believe it's considerably less than dividing by pi.

Formula I was told is square of the waterline length in feet multiplied by 1.34 = hull speed in knots.

Using that formula, max hull speed for a 40 footer is 8.5 knots.

 

[castlevar]

Hi c playa forget about 1.34 sales men use this figure to kid you that the boat is faster than it actually is but it uses more fuel .
for a 23 ft speed 4.79
for a 40 ft speed 6.32
See the book Voyaging under power

 

[Dave_Seager]

The formula should be the square root, not the square, of the waterline length in feet multiplied by 1.34 or possibly 1.51.

e.g. 25' waterline length, hull speed 6.5 (or 7.5) knots. Using the square would give about 834 knots and would break the sound barrier!

 

[whisper]

Sorry, but I can't believe that the displacement speed at 23ft waterline length is 4.8kts.
It is only after about 7kts that the bow tries to rise and the wake increases noticeably, on our boat.
Perhaps we have a different understanding of what displacement speed means.
Anyway, never mind eh!!

 

[Deleted User YDKXO]

[ QUOTE ]
Sorry, but I can't believe that the displacement speed at 23ft waterline length is 4.8kts.

[/ QUOTE ]

That's because it's not. The max hull speed in displacement mode is given by 1.34 x sq root of the waterline length which is 6.4kts. In practice you'll get a bit more which is why you're getting around 7kts

 

[whisper]

Yippee!!!! We've finally got there. Thanks very much.
Hope I don't forget the B formula again /forums/images/graemlins/wink.gif

 

[castlevar]

I am an ex merchant navy officer contact a merchant ships captain and ask him what speed does his ship travell at for economy.

 

[Deleted User YDKXO]

[ QUOTE ]
I am an ex merchant navy officer contact a merchant ships captain and ask him what speed does his ship travell at for economy.

[/ QUOTE ]

and your point is what?

 

[whisper]

As mentioned earlier, I think we are at cross purposes here.

Theoretical max. displacement speed is not the same as the most economical displacement speed, or is it?

 

[castlevar]

yes we are at cross purposes x1.34 on my boat is 8.5 knots consumption 30 ltrs hour
at 6 knots 6ltr hour
as we have been cruising the med during the last few years it would be impossible to cruise at the higher speed dont have the cash

 

[whisper]

Ah well, it's all clear now. That's taken up an hour or so!! /forums/images/graemlins/cool.gif
Suppose I'd better do some work before another similar thread comes along /forums/images/graemlins/grin.gif