Need help : 16.5 m multi purpose boat, speed not achieved 25 knot

That's 5 ton lighter and 0.7 m shorter. And the decrease is from 32 *edit* kn to currently 24 knot.

So, could this boat, reach 26 or at least 25 knot through conventional way ? Does rearrange the load so it's slightly aft trim will help ? Should we increase B/T or make it longer still (increase L/B) ?

Thank you.

 

Change the trim can help. If you can test the boat in the the water, check how it works with various trims.
Could we see the shapes of your boat?

 

Post #11 has the lines drawing

 

My brain is flawed. Indeed, I had seen and had commented.

 

Although should be considered with much prevention, I attached a calculation I made for your boat, which I hope will be helpful.
In the list of hydrostatic values you can read that, at the draft of 0.7 m, the boat is 20.8 tons displacement.

 

TANSL: you present identical diagrams for 20.8 t and 28 tons; there must be a mistake. Further, the original 20 ton boat is 0.7 m shorter than the hull you seem to have modeled.

What version of the Savitsky algorithms do you use? Are the Blount/Fox corrections for speeds below Fnd ~2 included? Any air resistance or added wave correction? What jet propulsion efficiency have you used to get the hp values cited? Generally speaking, particular care should be excercised with Savitsky in the low Fnd speed region.

Mr Efficiency: The weight/Power ratio is 20.8 and 20 kg/hp for the long and short versions respectively. The 24 knots achieved with the long one is slightly low in that perspective, particularly if the short version actually reached 30 knots with 20 tons displacement.

I fully concur with TANSL that a few running trim angles would be useful to validate the calcs, and I would also like to see a real measurement of the freeboard heights fore and aft at the normal operating displacement. The statement on the lines drawing that the boat was floating "between 0.7 and 0.8m draft" gives a span of roughly 4.5 tons with the wp area demonstrated by TANSL, which is far too imprecise.

As for the jets: the 322 can be delivered with at least three nozzle sizes; 165, 170 and 175 mm if my memory is correct. In order to see how the jets were laid out performancewise, I'd like to know which nozzle is used for the 25 ton boat?

Btw magny: What is your role here; are you representing the owner/operator or the designer/builder?

 

Baeckmo, thank you for your expert clarifications.
I have created the model according to the body lines the OP has shown in his pdf file.
As I said in my post, my numbers should be treated with great prevention, that is, I can't say to what extent are they are good or bad. Simply compares the situation when the ship displaces 20 or 28 tons.
I used Hullspeed to determine the curves power/speed, with an efficiency of 50%. I do not pretend to tell the OP the power his boat needs to reach 25 knots, but allow him to compare two different displacements of the same hull.
By the way, do not need to perform any calculations to know that a flotation can be 0.7 or 0.8 m deep, on a boat with 1.8 m in depth, it is very imprecise.

 

Thank you all so much for all your insight and contribution.

@Baeckmo. Our task is to design the layout to optimize space usability, refined the structure (recalculate plate thickness and appropriate profile) so it would be approved by classification society. This is my first project as a team leader (does two people considered a team?), so I'm really nervous with this results.

They use the largest impeller available, so I assume 175 mm.

@TANSL. From pictures the builders just now sent me (geographic and monetary restriction didn't allow me to attend the launching), it was 0.72 m draft at test run (load condition is not available).

Again, thank you all.

 

magny7, although you have not asked, I would like to comment regarding the data the builders sent you:
Launching condition is likely to have missing many weights and would be very important to know what were the weights that were not on board and, also, what weights aboard are not really of the ship. On the other hand, it is almost impossible for the boat, in that condition, to float without any trimming. The displacement can change quite with the trim. You should ask to clarify these circumstances.
If you think I can help with anything, do not hesitate to contact me, if you prefer by e.mail.

 

They've sent us photos of the boat during launching and test run, some of which shows water line condition is equal on aft and forward. But you are right, we should have ask them to record the load condition.

Many thanks TANSL.