Large-Scale Propulsion

Looking good

Interesting hull profile...i thought you may want to follow the Series 64 type, with approx 50% draft at the transom. Our own research has shown that at higher Fn's the Stern type you ahve drawn has slightly higher resistance.

Not easy fitting a waterjet with zero immersion at the Stern too. Waterjets operate best with zero head, ie the CL of the jet is coincident with the DWL.

She looks like she might be stern heavy. Not much buoy aft, but a lot of "stuff" aft. Have you done an LCG/LCB check yet?

An LCG chase is essential in a vessel like this too...but not sure if your software program can do this or even give reasonable results to use as 'design'.

One thing to bear in mind with GT's is all the aux stuff required. From our experience the weight savings of GTs are not fully realised owing to the massive G/boxes, the huge air filtration and air intake systems to maintain air purity etc etc.

2 types of fuel....have i missed something?

Philippines etc your assumption is correct. These countries are good for repairs of diesels, all low tech. Anything high tech requires time and specialized support which is few and far between in such countries.

 

In all honesty, this is just a parametric transformation of a hull I found in Maxsurf. I've distorted the ratios of length, beam and height to serve my needs and I flaired the hull a bit to help it better reflect radar.

I wish we had a larger library of designs, but we don't. Do you use Maxsurf, by chance? If so, do you have any hulls for public distribution? I'd totally understand it if you said "no".

Unexpected, but not surprising.

Hopefully any alterations I might make to the stern won't severely damage it's performance.

We use a program called Hydromax, which uses our Maxsurf design for stablility and strength calculations.



This only includes the hull steel (which I assumed to be 1" throughout to overcompensate) and weapon systems (which were the only things I could get concrete numbers for). It does not include the weight of the superstructure. I have to determine just how big it is, first.

I left almost 140 feet in front of the bridge so I could move it forward some if neccessary. Also, that platform amidships (with the VLS) is 40 feet long, which is bigger then it needs to be, so I can also calibrate my ship's trim that way.

My (very) quick weight estimation puts the ship at 2712.39 LT. This includes the superstructure, deck plating, weapons, power and propulsion systems, 2 RHIBS and 2 SH-60s. I obviously made some assumptions, but always tried to err on the side of caution. For example, I made deck steel area equal to 5 times my DWL waterplane area and 1" thick. I figure the 1411.11 LT it came out to is a little excessive...

My next task is to determine how much fuel I'll need and what it weighs.

Gas turbines don't run on HFO... do they?

I hadn't actually thought of that, but it's another great reason !

 

Abraxas

I don't use maxsurf. We've been lucky over the years to have someone do the lines for us from our hand sketches (well, that was their job!). However, I now have paramarine, which is seriously seriously sophisticated for hull modelling. Makes maxsurf etc look like a chisel and stone! (You can get free downloads for a trail version now). So just starting to use this for lines.

But hull shape doesn't really make that much difference. When looking at computer plots etc it seems like there are real differences and advantages to be made. Not so. The length displacement ratio is the key to the hull 'design'.

It would be worth doing a "quick" midship section. Use DNV or LR, or at worse ABS. Establish the SWBMs and do a spot check on worse case, midships, for slamming. You can then derive some scantlings. Since Global and local loads will determine the minimum thickness required.

When you have completed your weights, total weights, add a margin. For now i would recommend 10%, since so much is unknown.

I assumed you would be using marinised GTs!!

BTW
I assume you are using the standard 8 weight groups, for your weight estimate??

 

First of all, you don't need two types of fuel. Turbines burn JP8 or diesel just fine. If you are fueling with sludge that's a problem, but the diesels won't burn it either. Anything you can burn in a diesel you can burn in a turbine.

I was thinking that 2mw is a pretty small amount of power for something this large at that speed. I know we are talking about displacement speeds here, but this seems low to me. You are still going to need about 4 mw for shipboard power, so your total power requirement is going to be at least 6 mw. That is too small for a LM2500 for sure. You need to be comparing much smaller turbines to the diesels. Two three megawatt turbines would weigh about 33 long tons less EACH than the diesel. A high speed (15,000 rpm) alternator for 4 mw will weigh less than 1,000 pounds.

The weight you are quoting (88 long tons) is for an LM2500 with an electric generator system. The engine alone weighs 21 long tons. There is no reason to couple the gas turbine to an electric generator that large since you can drive the waterjets directly or use a single reduction gearbox, which won't weigh anything near what you are talking about here. Remember if you generate the electricty, you need a motor of equal size as well as a control system to run it. Figure at least 30 more long tons per big engine to reconvert from electricty to shaft power. You are paying 80 tons per waterjet for nothing by trying to drive them electrically. Turbines have maximum torque at lower speed, so there is no reason to use an electric drive with a turbine, even on prop system. With a diesel you have a problem accelerating the boat since the power available is a function of speed. What that means is that at low speed, even with a diesel throttle open, you still don't have big power until you get up to speed. With a turbine you accelerate the gas generator and the power turbine puts out more torque. You don't need multi-speed gearboxes or electric drive, it just adds more weight.

The diesel system you are looking at is running at 1100 rpm and appears to be an all electric output system. That is why it is so heavy. A gear drive to the waterjets would be a lot lighter. You could still put a smaller generator on the shaft and make electrical power, but any system running at such a low speed is going to be heavy. You would be better off running an 1800 rpm engine of about 2000 hp directly to the waterjets with a shaft speed alternator (of up to one mw) piggybacked into the drive system, and then distributing 2 or 3 one mw generators around the ship to provide additional electrical power. With what you have now, you are going to need an additional 20+ tons of drive motors to drive the waterjets and that is unnecessary.

You are also underassuming what your payload will have to be in terms of what you carry in the weapons system bay. The LCS has room for 5 small craft and they have found out that the payload bays aren't big enough for what they want to do. The small craft are limited to about 11m and 22,000 pounds. The problem is that the craft for minesweeping and ASW are heavier than that, as well as the need for more crew protection in the 11m ribs. This is making life very difficult for the teams developing the MEP's. Room for larger and heavier MEP's is going to be imperative and you need to have room for more like 5 or maybe even 6 small craft. Figure out how to get any one them in and out of the ship without launching any others first. This is why the LCS is limited to 5 small craft.

Finally, I'm not a NA, but I would have thought that you need more lift aft if you are going to push this hull anywhere near 45 knots. I would have thought that it needs to look more like a semi-planing hull or you will hit the wall at 25 knots.

 

With respect to the waterjets, the largest diameter waterjet to use is the best and as such the RPM is to be low as possible. Gearboxes are required.
One cannot drive a waterjet directly from a GT, the ratio is out by a factor of 15~20.

The power delivered to the jet follows the cube law. So it matters not if more power is available lower down the RPM range, the jet cannot absorb it anyway.

As for speed, being some 130m long (I don't understand imperial units, sorry), at 25 knots this equates to Fn = 0.35, even at max speed this is a tad over 0.5. This is a displacement hull, not a planning hull.

The Fn is the important figure to understand, not the speed.

Issues such as:
"...The LCS has room for 5 small craft and they have found out that the payload bays aren't big enough for what they want to do..."
sadly are all too common. The initial design will have accounted for them. But during the life cycle of the project, the tenders will have changed, the cost overruns will ahve required modifications to the parent ship, to ensure the main principals are not affected, time for delivery etc etc. All these play a part, in slowly eating away at the original design intent. Not to mention politicians who were elected to bring the "ship to their backyard" for employment, pushing to get the job finished in their tenure....all looks good on their CVs!

 

I was looking at 1.3 x the sq rt of the waterline length and thinking that hull speed would be about 26kts. Pushing it to 50 kts appeared to be about 2.5 x the square root of the WL lenght and I was thinking that was a bit much, that's all.

I didn't mean to imply that a gas turbine wouldn't require some type of gear reduction. It is just some gas turbine engines already have a built in reduction box, so a separate one isn't going to be needed.

One thing that is changing is that the speeds for water jets that are driven by gas turbines are being increased. The previous designs were optimized around diesel systems and were larger than were necessary because of the low speed of the diesel. The specific speed of water jets is increasing since the resultant jets are smaller, lighter and take up less room in the vessel. If you look at the latest Kamewa product line the flow volume has been increased one step for the same frame size. This trend will continue as designs that are more axial in nature, as compared to the current mixed flow designs, are brought along. I've seen some of these newer designs and the reduction in size is pretty impressive. As a result, while a reduction gear is going to be required, in the future the overall ratio for a large system (Like an LM 2500) is going to be closer to 6 or 7 to one, as opposed to 15:1. Moreover, since the input speed is higher, the overall gearbox is a lot lighter and smaller than if you were coupling a diesel engine to it.

I was only mentioning the low speed torque capability in regard to prop systems, as you noted, most water jets have an issue with cavitation a high power and low speeds.

 

Unfortunately, the suction specific speed (based on flow, rpm and NPSE at impeller inlet) is difficult to increase, even if the axial pumps are inherently slightly better than the diagonal type. The design challenge is to reduce inlet losses AND obtain an even velocitydistribution into the impeller. This is the real limiting factor for the low speed high power operation of water jets, and will boil down to a limiting relative velocity at the blade inlet tip. One danger with these applications is that there is an increased risk for early cavitation damage before there is a noticeable performance loss.

 

baeckmo has hit the nail on the head, with regards to waterjets.

In a nut shell, one selects the largest diameter waterjet one can use, regardless of its weight. There are so many waterjet boats that have serious cavitation issues or have their top speed cut-back owing to incorrect jet size selection in the design and/or vibration problems. More often than not being attracted by the lighter weight and top end sales figures for it speed, but accidentally on purpose omitting the obvious from the sales blurrb.

The velocity of the flow of the water though the duct is to be as close to the velocity of the water at the ship side....in other words, no loss. In reality this is not possible, but it is the 'design intent'. The closer to this 'ideal' the higher the efficiency. One cannot do this with small diameter ducts, regardless what their performance figures look like. Why, the escape velocity of the water is proportionally higher, hence the max efficiency is lower.

As for:
"..I was looking at 1.3 x the sq rt of the waterline length.."

You noted you are not a naval architect, so there is not point me criticising this assumption. This type of "generalisation" formula, some work some don't and most have their "limits", just like any computer program. Hence understanding the 'limits' of generalised statements and formulae is just as important as understand what ones software can or cannot do.

When comparing hulls and what is or is not possible, one only really needs to know the Froude number [V/sq.root(L.g)] and the length displacement ratio. These two ratios can inform a naval architect immediately of the hulls potential, or otherwise. Other form factors, such as L/B ratio also help, but are ostensibly just a derivative of the aforementioned.

 

Just a detail, but since naval architecture is so full of "dimensional constants", I would like to wave a flag in favour of the nondimensional slenderness ratio; LWL/(displ.)^(1/3)!

 

I see your flag waving from here...jolly nice it is too

 

Sorry for my absence. I've had a busy few weeks capped off with the 3-day Annual SNAME Conference.

Anybody here go?

My experience is with boilers exclusively, so that was news to me when I found it out.

My power scheme has changed slightly:
- 2x FM 251F 16V 1000 RPM Diesel GenSet (2.4 MW each)
- 2x LM2500+ Gas Turbine GenSet (29MW each)
- 1x LM500 Emergency Gas Turbine GenSet (4.2 MW)

I disagree a ship of this size needs 4MW for ship service systems. Our training ship, I admit, is not a warship, but it services all our needs, including laptops, TVs, video game consoles and iPod chargers plus lighting, SW, FW, LO, DO and FO pumps. Throw in FO heaters, air and reefer compressors, LO purifiers and various HVAC fan rooms and we still don't come close to consuming the 1.5MW we have available. I can't remember off the top of my head, but I doubt all this is more then 1MW.

Assuming 2MW for ship service systems and 10MW for ESW, ECW and weapons systems, I figure 2 diesels and an LM2500+ provide enough power (29MW) for standard operations. The other LM2500+ GT GenSet allows for much higher speeds around 40+ knots (even against the exponential rise in the power v. speed diagram).

I mean, the RN Type 45 Destroyer has a max output of 40MW... so I doubt I'll be underpowered.

I admit that right now, my only data is from Hullspeed, which isn't the most accurate program. I'm working on a NavCad Pro file now, which is much better.

I'm reconsidering my propulsion methods in light of some research done with various propulsion combinations for the Joint High Speed Sealift (JHSS) ship. It actually has some pretty interesting data on how more axial-flow waterjets change power requirments at certain speeds when compared to more conventional mixxed-flow waterjets (which have a combination of radial and axial flow).

I'm going to read the report and find out what their conclusion is and report back. Hold tight.

I never viewed the LCS as an assualt ship. That's what the LPD-17 is for and it's silly to try and get a 4,000 LT frigate to even try and compete.

IMO, 2 RHIBs provide enough resources to conduct searches, support commando operations and patrol the vessel in potentially hazardous waters. Fortunately, the 2 rectangles amidships are actually 40' long alcoves for a 36' Zodiac RHIB. I think this is enough for what it needs to do.

Correct... but unfortunately I have no data sheets on planing hull dimensions, so I can't accurately design one.

But a displacement hull can still reach high speeds... provided it has enough power. Aircraft carriers can reach speeds over 30 knots (closer to 40 knots) by virtue of its atomic power plant and even the JHSS, which has a relatively standard displacement hull, is expected to reach 39 knots and beyond given 150MW of available power.

"Exponential" does not mean "impossible"... just "frickn' hard" .



More to follow soon... but now it's bed time. I have WAY too much sleep to catch up on.