HullSpeed Overall Efficiency for Patrol Boat

Using the Maxsurf Hull Speed, what is the value of Overall Efficiency do you use for 15m Patrol Boat?

 

No specific values ​​for use in Hull Speed. That depends on many factors which have nothing to do with the software used, but with the overall performance of the power transmission line.
I was gonna say you used 60% but, without knowing the specifics of your boat, one can not venture a number.

 

Depends on many factors such as type of drive, parameters of drive and its installation, speed range, etc.

We used to take some margin on speed prediction at design stage, say 5% for inaccuracy of resistance methods, plus engine derating factor for tropics (can be 10%). Plus added resistance on SS1-SS2 for conditions of sea trials as delivery trials never happen on flat water. There might be overweight factor also built into horsepower requirement.

Thus, in every case talking about estimate of OPC we need consider how these factors are included in resistance predictions. If those factors are not considered separately, I would take 0.50-0.55 as good starting point.

 

Are you looking for Overall Propulsive Coefficient of the boat? I think you are putting the cart before the horse so to speak.

You need to define first your target trial speed then find Pe (power effective). Then decide what type of propulsion you want. Propeller, Waterjet, SP has increasing efficiency over a given speed range. (To find Pe or EHP, you can use any trusted power prediction program suitable to your hull design.)

That will determine your hull appendages such as struts, skegs, rudder,ect. Each propulsion type has advantages/disadvantages.

From the propeller, to the engine, there are mechanical losses. 1.5% for every bearing used, 1.8-2% for marine gear, another 1.0% if there is a thrust bearing, plus the derating factor for the engine if the engine room is hotter than rated.

For starters for a fixed pitched prop, 50% efficiency is used (double the Pe), then gradually move up to 55% as Alik has noted. 60% is possible if you get all things right the first time. Big wheels, over square, deep immersion, shaft angles,ect.

You have to define your design so we can be specific in our answers.

Attached some notes from Naval Architecture from Tupper.

 

I'm using three (3) outboard engines.

 

As you screen name offers an inaccurate suggestion, based on this question, about how far along are you toward your degree?

 

That would be a poor choice for a patrol craft! This petrol powered oversized "speedboat" is a floating bomb during engagement.

 

What could be the OPC using outboard engines? Do you need more info?

 

"Internation Council of Marine Industry Associations

Standard No. 28-83

Power measurements and declarations for
marine propulsion engines and propulsion systems"

"3.3

Declared (rated) power of an engine model or propulsion system shall be the full throttle power at the declared (rated) speed at the final output shaft of the engine or propulsion system as offered for sale by the manufacturer, based on corrected power of one or more engines or propulsion systems

3.3.1--Power shall be declared as Propeller Shaft Power at the propeller shaft of engines sold with complete propulsion units, and at the couple to the propeller shaft of engines sold with reduction or reversing gears.

3.3.2--Power shall be declared as Crankshaft Power at the engine output shaft of engines sold without reduction or reversing gears, stern drive or sail drive units. In such cases, the power declaration shall be accompanied by a statement that usable power will be reduced by gear losses."

"For outboard motor enthusiasts, this standard clarifies several frequent questions. First, the "rated power" is to be the corrected power after careful measurements at the "rated speed" which is to be the mid-point of the full throttle range recommended (3.1.1). Thus, if a manufacturer declares the full throttle range to be 5000-5500 RPM, then the rated power should be derived from measurements made at 5250 RPM.

The "rated power" is to be measured at the propeller shaft. (3.3.1)"

 

But Arabs will buy it.

 

Your kidding right? Do you honestly think someone should hire a "naval architect" asking questions as broad, primitive and basic as this? So, again, how far along are you, on your studies and what school?

As will the Nigerians, but get your money up front.

 

From my experience, for Nigeria it should be inboard.
Otherwise it will be stolen before the boat touches the water

 

Do you know know your Pe and Ps? (Physical education and Post script)

 

Over here, the pirates have three outboards fitted. If the hulls gets riddled with bullets, they unbolt the outboards, commander another boat and bolt the usable outboards. Easy.

 

The pirates used to have inboards and they guard the boat with automatic weapons. But one day, when they started the engine, the engine went whhhhrrr. THE PROPELLER WAS STOLEN! That is why they switched to outboards, as the story goes around.