Problem!

[babakshabani]

A 10 ton planning hull with design speed about 55 knot with two inboard engine( 800 hp at 2600 rpm) have been tested. It has a trimax surface drive with propeller about 21 inches. Max rpm that I reached is about 2300 rpm, and the speed was below 50 knot.
How can I solve this Problem!

[RANCHI OTTO]

Which is the displacement? Who is the props manufacturer?

[PAR]

Sounds like you need new props to get your engines at their target WOT RPM.

[RANCHI OTTO]

sorry, the displacement was indicated in your thread...
with 2 x 800 hp and 10t displacement, 55 knots are attainable..
As suggested by Par, it seams that the propellers have a pitch problem.
Have both the propellers the same rpm at full speed?

[babakshabani]

Dear sirs,

Thank you for your answers.
That’s correct, the propellers have different rpms!
The Pitch / diameter ratio was 1.5, In spite of this I have tested another propellers with the greeter diameter and smaller pitch, but the result is not satisfactory.
Actually I can’t find a common approach for choosing new ones.
Can you explain more details about pitch effect.

[RANCHI OTTO]

In order to give you my opinion concerning your boat I would need a lot of informations...
If the 2 propellers have not the same rpm it is good practice to take measurements of the torque at shaft, so doing it is possible to have the real delivered power of the engines.

The differences btw estimated speed and reached speed is huge...try to contact the best surface propeller manufacturer:

Ing. Franco Gotta

ROLLA SP PROPELLERS SA

Tel +41 91 6952000

Fax +41 91 6952001

E-Mail franco.gotta@rolla-propellers.ch

are you sure that the 2 trimax have the same inclinations at speed?
Does the boat have some porpoising at over +45 knots?

[babakshabani]

I'm not sure about the trimax inclination at speed, but some porpoising was seen about 48 knot. What kind of information do you need? I will send you by email.

[RANCHI OTTO]

Check if there are discrepancies btw what the Trimax trim instrument on panel indicate and the real inclination of the shaft axis.(to be done out of water)

Check with engines consumption and exhaust gas temperature the real engines power.(contact the manufacturer)

Check the tabs position, they have to be 2/3 cm abt over the bottom and if they work well together with the same inclination.

Who is the propeller manufacturer?
Do you have noted some rolling at 48 knots?
Where is located the l.c.g.?

With 10 t and 2 x 800 hp the estimated speed would be 58 knots abt.......

[babakshabani]

I'll be checked them as soon as possible.
Propeller manufacturer is Eliche Redice.
Do you have any practical paper or somthing like that about SP propellers or surface driver. I want to see some refrence related to such problems.

Babak Shabani
[email="b_babakshabani@yahoo.com"]

[babakshabani]

No any noticeable rolling found.L.C.G! i don't remeber now. I can tell you later.
I think at 58 knot speed , EHP for 10 ton planingn hull is about 1100 hp. so the drive system must be work with efficiency 1100/1600 or 68/7 %. Is this attainable?

[RANCHI OTTO]

Have a look on my diagram based on sea tri
als of several boats with SP propellers

For your boat KR = (2x800)^0.551/(10)^0.476 = 19.5

According to my diagram Vk = 58(min) to 60 (max)

[nosigma]

What sea condition is the excel chart (KR to speed) relationship good for? I have been looking for exactly this type of chart (sea trial based) for quite a while for sizing to sea state 3 and sea state 4 conditions.

Thanks

[RANCHI OTTO]

standard sea trial conditions.
not far from B0-2, clean prop and hull.

[babakshabani]

What is the form of the boat in trials, and which surface drive has been test?

[RANCHI OTTO]

B. each point corresponds to a different boat, this is only an indication how fast would a boat run...for a quick check it's very good for me but for a more detailed speed vs power forecast I use other systems...