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

[magny7]

Dear Everyone,

We have built a multi purpose vessel for remote jungle in Indonesia mainly used for everyday goods transport and sometimes water ambulance. We based on the previous design planing hull that proven achieving 30 knot easy with 2 x 500 hp cummins with HJ 322 waterjet.

But because we need a special compartment we add 0.7 m of length amidship (parallel middle body) and increase the horsepower to 2 x 602 hp cummins still with HJ 322 water jet. But as per operator request the boat is 5 ton heavier and using a gearbox (ZF 351 ratio 1 : 1). But now it could only reach 24 knot tops.

Specification :
L x B x D = 16.5 x 3.5 x 0.75
Displacement = 25 ton
material = Aluminum

Our target speed is at least 28 ton. Is this actually achievable ? Since out budget is small we couldn't afford doing test in water tank.

Your advice is greatly appreciated.

Thank you

Ari.

 

[JSL]

Your jets need to be larger..... Or install a third one

 

[gonzo]

The extra 5 tons and gearbox selection is probably the problem. Did you have a contract?

 

[magny7]

We don't have a contract since you can count this as "pro bono" project. The boat only 3.5 m wide, the space for two 600 hp cummins is barely fits, so an extra engine is not an options.

in theory, could the boat with specifications i mentioned earlier reach 28 knot ? should we adjust its dimensions ? Does the more powerful engine and jets is the only solution?

Thank you for your replies.

 

[Mr Efficiency]

Hardly surprising considering it was only extended by 0.7m, yet now has the requirement to be a whopping 5 tons heavier than before.

 

[magny7]

Any suggestion for future boat ?

 

[TANSL]

Without knowing the boat it is impossible to give a specific answer to your problem. You can always try something to improve the situation, for example, better weight distribution could facilitate planning. But again, without seeing the boat all we can say are mere speculations.

 

[baeckmo]

As expressed by TANSL; we need info on the hull shape and its trim in order to check resistance levels and take a step beyond speculation.

The displacement increase is not compensated by the length increase; the slenderness ratio (LWL/displ^0.33) is reduced, which increases drag/lift ratio in the speed range where this boat is operating. Note that the planing "hump" is close to the top speed of the revised vessel @Froude no 2.2 (using volume). Also note that Hamilton are recommending a maximum displacement of 15 to 20 tons for Twin hj322's; you are far over the limits!

This is indicating that there may be cavitation issues with the jets; what is the idea behind adding a transmission, using 1:1 gear ratio?

Another Point: what is the water depth where you have done the speed tests?

Btw, if you check the threads "Jet boat not performing" and "Replacing an outdrive with a jet" you will find info on similar issues.

 

[magny7]

I accept every speculation to consider and perhaps to be applied if possible.

I would gladly share the body plan here but I'm clueless on how to post a picture to a reply.

From your reply i just found out the HJ series had an engineering performance calculation online, as for now i'm studying this. thank you.

By to increase slenderness ratio, You mean we might need to make it longer still ? Wouldn't it increase it's frictional resistance? Since from what i've learn, wave making resistance contribution to total resistance is marginal than frictional resistance at above certain froude number ?

It is required to install the gearbox to make it able to "disconnect" the waterjet and to do a back flush, since the waterway there have quite a lot of debris.

The boat would normally operate as shallow as 5 meter of water.

Thank you.

 

[baeckmo]

When you click to post a reply, you scroll down and find "Additional options", then click "Manage Attachments" to attach file.

Then a question; what Cummins Engine do you use, and what is the rpm setting for 600 hp resp 500 hp?

 

[magny7]

Here I attached the lines plan. When launched the boat is evenly keeled at 0.7 m, so i guess its actual LCG is at the midship (around 8 m)

The engine is a brand new cummins qsm11 602 hp at 2400 rpm with ZF 351 1: 1 ratio gearbox.

 

[TANSL]

Just watching the water lines we can deduce that the center of buoyancy, to a depth of 0.7 m, is aft LWL / 2. Therefore, if the boat floats without trimming, the LCG should be less than 8 m, perhaps close to 7.5 m (the value may be less depending on where you place the AP)

 

[Mr Efficiency]

The performance as quoted seems very good for the dimensions, weight, and power, does it not ?

 

[magny7]

Ah yes you are right, silly me @TANSL

So, it has reach it maximum potential ? Some senior colleague of mine, while none of them a naval architect using a rule of thumb from previous lighter shorter boats, said it should reach at least 28 kn. It's really hard to said otherwise when there's lack of research data. @Mr Efficiency

 

[gonzo]

If lighter and shorter got 28 Kt, 27 is pretty good in a longer and heavier boat.