Hi All,

i post this update after changes to the trimming of the boat.

I said the boat is poorly trimmed and seem to "sit" too much with the
stern in the water.

So i put around 3/4 of tons of water (750 liters) in the anchor room
(the small cabin in the bow which contains the chain - is quite large
and sealed).

This had the effect to sink the bow around 8 inches and lift the stern
around 3 inches. The boat is still NOT level but at least is getting
closer.

Now the effect on travelling speed:

I could reach 5.5 knots at 900 rpm instead of 1100 rpm ! And my fuel
consumption went down 40% at once (for the same speed). The benefits
would be less and less trying to drive the boat faster and faster -
basically the speed improvement at full throttle would be
insignificant. Also waves were greatly reduced at 5.5 knots (almost
reduced ripples).

My theory: with the bow "lifted" too much the physical boat lenght
starts quite some distance from the actual entry point in the water.
Adding weight at the bow would push it down and effectively lenghten
the WL.

Now i have a long-range tank in the bow (1.5 ton) which is empty now
(i normally use other two tanks, in the middle of the boat, each 1/2
ton). Next week i will bunker diesel and fill ALL the 3 tanks to the
top - this should push down the boat another bit and eventually set it
level - then make new trials.

I also looked for green slime and the like on the hull - not much
there.

I will post the results in another 2 weeks or so.

Matteo

(Matteo) wrote in message om...
This is the situation: My 40' LWL boat (15 ton displacement) has a 150
PS engine. From the formula for speed I calculated a hull speed of

sqrt(40)*1.34 = 8.47 knots

*but*: when i did trials last week (absolutely calm water, almost no
wind) those are the results:

800 rpm 5 knots no noticeable waves generated
1100 rpm 5.5 knots small waves
1800 rpm 6.5 knots (flat out) - huge waves generated, stern deep in
the water, boat "running uphill".

1100 rpm is around 50/60 PS (from the engine rpm/PS table).

Question: what could be the cause of the "slowness" of the boat ? I do
not pretend to reach 8.4 knots cruising but at least 7 knots should be
in.

I'm thinging of dirty hull (green slime), incorrect weight
distribution (bow tends to "point" upwards even when crossing small
waves).

Any experience ?

Thanks
Matteo