resistance for long B/T ratio

[pavel915]

I am designing a passenger vessel whose B/T(beam/draft) ratio is about 10,
can anyone tell me which method will be ok for the resistance calculation of my ship? I used Holtrop and mennen method, is it ok? otherwise what can i do?
can anyone suggest me a good software for powerprediction? I want a software or spreadsheet which is valid for a long range of B/T ratio.
plz suggest me about my problem.
thanx.

[Guest625101138]

Download Michlet from here:
http://www.cyberiad.net/michlet.htm
It gives very good results.

If you want some help then post your intended displacement and desired speed and I will get you started with using Michlet.

Rick W.

[pavel915]

my Lwl is 67.43 , displacement :634 m^3 , Beam wl 12.74 draft: 1.28
I can use michlet. when i calculated my resistance with michlet. i found a verrry much larger value of reistance than other methods like holtrop and mennen. Is michlet applicable for my ship?
plz tell me in detail; which method for wave making resistance should i follow.
thanx!

[Guest625101138]

What speed are you designing for?

I have found Michlet to be very accurate for slender hulls at displacement speeds. I believe the basic version in common use cannot automatically accommodate changes in hull pitch and sinkage as it starts to develop lift.

Rick W.

[Leo Lazauskas]

Originally Posted by pavel915

my Lwl is 67.43 , displacement :634 m^3 , Beam wl 12.74 draft: 1.28
I can use michlet. when i calculated my resistance with michlet. i found a verrry much larger value of reistance than other methods like holtrop and mennen. Is michlet applicable for my ship?
plz tell me in detail; which method for wave making resistance should i follow.
thanx!

The L/B ratio of about 5 is a liitle bit low for Michlet. The high B/T ratio of 10 means that the hull is fairly flat which suggests that it might plane at a high enough speed, in which case Michlet is again not suitable.

Holtrop and Mennen's method is a blunt object. It is not a physics based method so you could get something reasonable at some speeds and junk at other times. In any case, it will not help you with planing or semi-planing modes.

Good luck,
Leo.

[pavel915]

my ship is designed to operate in 10 knots.
now would you plz tell me , my ship is suitable for michlet or not?
if holtrop and mennen is not ok for my ship , then for which method should i go?
plz explain.
thanx!

[Guest625101138]

I approximated your hull and did a run with Michlet. I have attached the output file. (You need to view the file without the text wrapping for the headings to line up) It indicates hull power to do 5m/s (say 10kts) is 114kW.

This power seems reasonable for this size boat and speed. In practice you should get a propulsion efficiency around 70% so installed power would be 170kW. This is lower than my experience tells me for a craft of this size but the calculation assumes the hull is free of fouling or damage while this is hard to maintain in the conditions I am familiar with.

How does this compare with your result? What power are you predicting?

My view is that Michlet is producing a reliable result. Wave resistance is significant but you are below planing speed.

Rick W.

[pavel915]

thanx, your result really seems to reasonable.
I have attached my intup file here. would you plz tell me why it does not give me resonable result? is there any problem in making my input file?
plz chek my input file.
thanx you very much.
-pavel

[pavel915]

Rick, can you plz attach the input file?
thanx again!

[Guest625101138]

I checked your hull and I think what Michlet is telling you is meaningful. The calculated hull power of 685kW seems extraordinarily high though. I feel the stern is not very good for a displacement boat. There is no energy recover.

I have taken the liberty of doing a FreeShip model based on the mlt file you sent. I added the sides and deck. An image is attached.

There are some odd bumps about midship but the stern is the killer I believe.

I ran Godzilla for an optimum boat of 634Cu.m with the draft constrained and it can get hull power down to 75kW at 10kts. By the way your in.mlt file is showing displacement of 667Cu.m.

I would be guided by what Michlet is telling you.

[Guest625101138]

I have attached an image of what Godzilla comes up with. I allowed the draft to go to 1.4m and beam down to 11m. I have the original displacement of 634Cu.m

The Godzilla offset file is also attached. Energy recovery is important so this design looks sensible. If you do not like these shapes then you can constrain Godzilla in other ways to get something that is more suitable. Also you do not need to have the deck constrained by the underwater portion.

I only design and build low displacement boats but I chase every ounce of efficiency. I normally do a run with Godzilla with only displacement set and the target speed. Then I start adding constraints to determine what each costs in the way of power.

Rick W.

[PI Design]

Well that's quite unlike any passenger vessel I've seen Rick!
Seriously though, are those hollows in the forward and aft parts of the waterlines? I'm suprised Godzilla calculates that as optimal?

[pavel915]

Thanx Rick for your effort.
Actually i dont used Godzila before. only used michlet.
I cant understand what is the problem. Can resistance vary in soooo much large range(685kW -75kw) just by changing the hull shape keeping the principal particulars almost same?
Actually i made the input file from freeship i modelled my hull in freeship.
Really struglling with michlet!
thank you all!

[Guest625101138]

Originally Posted by PI Design

Well that's quite unlike any passenger vessel I've seen Rick!
Seriously though, are those hollows in the forward and aft parts of the waterlines? I'm suprised Godzilla calculates that as optimal?

I constrained Godzilla to beam >11m and draft<1.4m for the earlier hull.

If you want the optimum hull for 10kts and 634t it looks like the attached.

I looked around for something that might be similar. The old Sydney steam ferries are not too far off it. The Kanangra is 295t, 47m long and 9.5m beam. It is a double ender. The Godzilla creation for 634t is 53m, 8.7m beam. So not too much difference if you do the scaling - more slender. This optimum hull has a draft of 3.4m. Hull power to do 10kts is down to 57kW with this one. (Kanangra was originally steam powered so unlikely to have heaps of power hence hull would need to be efficient - I could not find any data on speed or power)

I have enough confidence in Godzilla that this would be the one I would build if my sole interest was economy at 10kts for 634t.

Sometimes it helps to change perspective. I try to envisage a hull sitting still in a flowing stream and think of what shape would cause the least disturbance. What Godzilla produces passes this test.

I have reviewed more than 10 hulls I have performance data for and Michlet is always close providing the hull is in displacement mode.

Rick W.

[Leo Lazauskas]

Originally Posted by Rick Willoughby

I have attached an image of what Godzilla comes up with. I allowed the draft to go to 1.4m and beam down to 11m. I have the original displacement of 634Cu.m

The Godzilla offset file is also attached. Energy recovery is important so this design looks sensible. If you do not like these shapes then you can constrain Godzilla in other ways to get something that is more suitable. Also you do not need to have the deck constrained by the underwater portion.

I only design and build low displacement boats but I chase every ounce of efficiency. I normally do a run with Godzilla with only displacement set and the target speed. Then I start adding constraints to determine what each costs in the way of power.

Rick W.

As PI Design said, the hollows in the lines seem a bit odd. I'd try it again with a constraint on the offsets so that hollows are excluded. Another trick is to optimise over a small range of speeds rather than at one speed only.

But you are absolutley correct in suggesting that the very large transom stern is the problem, particularly at low speed. I wouldn't think that such a large stern could run dry at anything but high speed.

Regards,
Leo.