Power/Resistance Calculation for Planing V-Hull & Fuel/Engine Required

[Kapitan_Raider]

Hi All,

I'm a new member and a student and im making a boat. I would like any help on the following :-

The Boat:

V- Planing Monohull
Length: 140ft (waterline)
Beam: 30ft
Draft: 8ft
Displacement(As calculated in Excel using Simpson's Rule): ~660,000lbs
Dead Rise: 16.594deg
Planing Trim Angle: 3.27deg (while static) Probably be ~ 4 - 4.25deg while planing
See attached Excel Hull Plot to understand better:

Now the problem is that I need to know the propulsion requirements.

The displacement speed: 16knots
Intended Cruising Speed: 35knots
Intended Cruisng Range Radius @ 35kn: 6,000 miles (so 12,000 total)
"Sprint: Speed for atleast 10 hrs: 55knots
Engines: 2 X Diesel Engines for Cruise + 1 Turbine Engine to get to Sprint Speed; Will rely on 2 X Waterjets for Cruise & 1 Another Waterjet for Sprint


I will be using 2 Lift Stakes also, 1 knuckle but I need to know how and where to place them . I dont know how to get the planing/lifting resistance, wave making resistance nor the skin friction drag. How do i calculate those ? The skin is to be made out of carbon fiber composite. i know that planing lift will be atleast >50% of the total displacement. Also I need the draft at which the boat will start to plane at, at different speeds . The other problem is which engines will I need and how much horsepower for them . Also very important is how do i calculate their fuel consumption to attain my desired range? Once this has been tackled as u may imagine i will also require the weights & dimensions of the engines, their shafts and also the same for the waterjets .

I intend to do all calculations by hand, but any simple software that you know that will help me will also help.
Any and all help and advice will be thoroughly appreciated .

Thankyou & Regards,

Kapitan Raider

[marshmat]

Hi Kapitan,

Are you familiar with the work of one Mr. Savitsky from way back in the mid-20th-century?

There's a fair bit on this forum about his papers, including a few Excel sheets and macros to actually evaluate some of the formulae.

Basically, the Savitsky method is a relatively quick, relatively straightforward way to estimate many of the key operating parameters of a planing hull. The equations are nasty, yes. But do a search on the forum for it... you might find some of what you're looking for.

[Rick Willoughby]

Here is a link to a Java version of Savitsky:
http://illustrations.marin.ntnu.no/h...ing/index.html

The results you get for drag are likely to be very optimistic unless you know what you are doing. If there are any appendages like rudders, planing strakes and jet intakes these will all add drag. Then of course you have to allow for the efficiency of the drive units when it comes to actually calculating power. With jet units on a vessel like this you could use 50% efficiency unless you know what you are doing.

Rick W

[BarendGrobler]

Here's a link to a discussion on savitskey, there is a very good spreadsheet on the post written by DINGO, I use it often and it works verry well!
Savitsky Power Prediction

[Kapitan_Raider]

ohh these are nice... No I wasnt aware of Savitsky. Though the excel sheet posted on the other thread, Im unable to understand what the different symbols. Whats VCG. if its vertical CG location, then I dont the position of CG cause i havent done my weights yet. I dont know the values for which engine,shaft, waterjet I will use and their weights and dimensions. Also the calculation is with propellers, wont waterjets change that? Typically whats a good number for propeller efficiency? - 70%?

But on the other hand, just from you engineering instinct how do the numbers for my boat look like. Would it work?

[daiquiri]

Do you realize that you are designing a floating eco-disaster?

[Kapitan_Raider]

Why is that?

[Joakim]

Originally Posted by Kapitan_Raider

ohh these are nice... No I wasnt aware of Savitsky. Though the excel sheet posted on the other thread, Im unable to understand what the different symbols. Whats VCG. if its vertical CG location, then I dont the position of CG cause i havent done my weights yet. I dont know the values for which engine,shaft, waterjet I will use and their weights and dimensions. Also the calculation is with propellers, wont waterjets change that? Typically whats a good number for propeller efficiency? - 70%?

But on the other hand, just from you engineering instinct how do the numbers for my boat look like. Would it work?

Just give somewhat reasonable values for VCG and LCG, put propeller shaft angle to zero and it's distance from VCG to zero or whatever.

Just after a quick check, with given information you need ~10 MW (under 5 MW is certainly not feasible) of effective thrust to reach 35 kn, thus ~20 MW of engine power. That will consume about 4000 kg/h of diesel. Your 12 000 M trip will take 340 hours, thus you need 1400 tons of diesel. The given displ of your boat is 300 tons, thus at full tanks it would be 1700 tons and would need 80 MW engines to reach that 35 kn and those would consume four times more fuel.

It is impossible to make a planning vessel that would have a range of 12 000 nautical miles using diesel in the tanks.

Joakim

[daiquiri]

And let me add that it is really strange that you didn't try to do some statistic research of the existing yachts of similar size, before defining your design goals. If you try to do it, you'll discover that nearly all of them are either displacement or semi-displacement vessels, with the top speed of something under 20 kts and around 15 kts cruise. There must be a reason for that, right?

Assuming that Joakim's calcs for the power required are correct (and the numbers do seem huge enough at first glance), 20 MW engines (around 27000 HP) would consume an enormous ammount of diesel fuel. Joakim says it would be around 4000 kg/h or 1056 gal/h though I believe it would be something around 4500 l/h or 1200 gal/h.

So, at 35 kts that yacht would consume something between 129 liters (34 gal) and 115 liters (30 gal) of fuel for every mile.
Or, putting it in another way, it would be able to make some 180 to 200 feet on 1 gallon of fuel (14 to 16 meters on 1 liter).

That's what I call an eco-disaster.

[Kapitan_Raider]

Thanks Joakim for the reply. i managed to do what you said at it shows that I need 15000 ehp for 35kn. I have attached the excel file for you to check. Did I make any mistakes? Also for the example you did, did you use the excel or some other formulas? If some other formulas i would be glad if you can share them with me. Also how do you calculate fuel consumption? i also used this:
http://web.archive.org/web/200504060....com/speed.htm and put hull rating: 235, 660000lbs and 35kn and it gave me ~19500 shaft hp? Where am i going wrong?

[gwboats]

It would be well worth looking at this reference:

http://www.proboat-digital.com/proboat/20071011/

There is a lot of background information on the design and operation of 'Destriero' - the transatlantic Blue Riband contender.

From my own personnal experience in the design, trials and operation of chine boats I can tell you that I was aboard a 90' waterline boat with 12,750HP running at 95 tons which achieved 60 knots in calm conditions.

At that performance our 25 tons of diesel fuel lasted just 8 hours (which I think reinforces daiquiri's eco comment!)

Please do some more searches on this site and the many others around and you will get a good grounding in chine boat design.

Best Regards,
Graham Westbrook
Naval Architect

[Kapitan_Raider]

Planing.xls
http://i33.tinypic.com/5cyfsy.jpg

Thats true daiquiri... I did do research and i didnt find any planing boat...
this design of mine is suppose to be a theoretical design. sorry for not stating the purpose, the reason for the great speed is that it would be a long range raiding craft with a payload of 100klbs, i thought i could fit enough fuel and a skeleton crew. i certainly didnt know the fuel consumption rates, nor how to estimate them. i had guestimated 400 l/hr and so the range came out fine.

But at this point a total range of 7000 miles and perhaps a slower speed would be acceptable. But the question again is that how do I get the fuel consumption, lets say taking a good quality generic diesel engine and find an optimal speed?

[daiquiri]

There are few planing boats in 130-140 ft range, but very few indeed. There is an AB Yachts model (AB 140), which arrives at 50+ kts with 12000 HP, but with half the weight of your yacht. It's 19 tonnes of fuel will probably give him some 550 NM of range at max speed.

As about the fuel consumption, it is 0.05 gal/h per HP (0.18 to 0.20 l/h per HP) for most commercial engines, where HP is engine's max HP. It might be somewhat less for bigger engines, as they tend to be more efficient.

[Kapitan_Raider]

So to attain an 8000 mile total range what speed and effective horsepower do require?

this is bad, i feel really bad that it cant go that range at that speed. Assuming 15000 hp for .18l/hr per HP using 35% of my displacement for fuel at 35kn i get only 1837miles.

[Kapitan_Raider]

assuming 100 waterjet efficiency i forgot to mention. Whats the best that waterjets can convert ?