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Old 06-03-2011, 09:27 AM   #1
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Calculating hull speed.

We all know the calculation for a displacement hull.
<ul>[*]Velocity = (g * Length / 2 pi) ^ [*]g = acceleration of gravity (32.15 ft/sec ^2)[*]Convert ft./sec. to Knots and the result is 1.34 * L^[*]Metric V(m/s) = 1.25* LWL^1/2.[/list]I have several friends who operate fast trawlers or other semi displacement hulls.

All are trying to slow down to save on fuel.

One *question is how to calculate the buttock angle for a semi displacement hull.

Then othe numbers come into play like the wave length vs LWL. And wetted surface.

There are boats operating on the Bering sea in the salmon fisherie. A law requires a max hull length of 32 ft. So some of these boats are 20*+ feet wide.

Anyone even fooling with this kind of stuff to determine the best speed/cost to operate the boat.

When I travel at hull speed 7.46 knts I am barley pushing 1600 rpm with a WOT of 2800 am I loading the emgine enough?

At 2000 rpm ( roughly 75% of WOT) I am running at about 8 knts. and burning more fuel to the tune of an x-tra 1.5 GPH*

SD

*

*

*


-- Edited by skipperdude on Friday 3rd of June 2011 09:49:22 AM
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Old 06-03-2011, 11:53 PM   #2
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Calculating hull speed.

Hi Dude,

1.34 X the square root of the WLL is the usual route to hull speed.

"One *question is how to calculate the buttock angle for a semi displacement hull." I don't think it's calculated as such but marine architects do observe (on paper) the Quarter Beam Buttock line angle and and consider it's flatness or steepness like references to a rule of thumb. They know from experience what is appropriate for the design goals they have in mind for a particular design. 1600 rpm for you is 1200 down and that would be 1800 for me and I almost always run at at least 2300. I'd consider that too low but the underloading issue is still a bit hazy. If I had your boat I'd at least run it about 2200 two or three times a day on a long days run for about 10 minutes. Percentage of load is "calculated" by comparing your fuel burn to what the engine burns at WOT. If your engine burns 4 gph and at WOT it burns 8 you are at 50% load. Percentage of rpm is not even very close to percentage of load because the propeller absorbs the load and it's loading on a boat is not not lineal at all. But like you say everybody's running slow to conserve fuel and if people are having problems with doing that they are keeping it to themselves. But a marine engineer is the one to answer the question of what engine should run at what load and under what circumstances.* In the Sabre/Lehman manual it says the 120hp Lehman engine is rated for continuous operation at a 114.5 hp at 2500 rpm. To do that one would need to be very slightly underpropped. I'd say the question can't be answered without a panel of a number of Marine Propulsion Engineers. Mechanics are good at what they do but they are not engineers and not qualified to predict levels of performance and limits of operation. Many mechanics know a lot more than how to fix or repair engines but are subject to the limitations of their knowledge, old wives tales and opinions far and wide. Does this help w what you were looking for Dude?







-- Edited by nomadwilly on Friday 3rd of June 2011 11:58:47 PM
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Old 06-04-2011, 03:57 AM   #3
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RE: Calculating hull speed.

"Anyone even fooling with this kind of stuff to determine the best speed/cost to operate the boat.

Very few do chose to motor at "hull speed" as throttling back can cut fuel burn by 2/3 or so.

For a rag bagger hull speed or better is a fine goal, as wind -power- is FREE!

The NA term SL is the sq rt of the lwl, and the most economical cruising is found at somewhere between SL times .9 up tp SL times 1.15.

Hull shape is important in terms of fuel burn at any of these speeds , the more like a displacement sail boat the cheaper to operate.

The bigger the transom dragged thru the water the poorer the burn.

Engine efficiency also determines the fuel flow / speed relationship. A higher load is always cheaper.

A larger engine is usually poorer the further it is throttled back , in terms of HP produced per gallon of fuel burn.

That is why there is a band , .9 to 1.15 of speed , it is possible that the engine can make better use of the fuel loaded slightly better.

In small gen set sized engines the difference between hp/gal at full throttle and light use can be 300%.

Its usually not that bad (300% worse) on larger engines , but loading the engine at the RPM cruised at is important.

THe euros will almost always spec a CPP on long range fish boats , but there is a cost involved that a cruiser might not recover.

That's why many will select a "cruising prop" and learn how to operate with a better loaded engine.

A cruising prop does not change the speed , it changes the engine efficiency at lower speeds.
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Old 06-04-2011, 06:31 AM   #4
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RE: Calculating hull speed.

so 400 rpm picks up just over a half kt at the cost of 1.5 gph? Seems to say the boat is happy at 1600 rpm I have a 120hp lehman and at 1650 rpm the motor just sounds happy and burns just over 2gph At 1900rpm speed picks up just less then 1mph
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Old 06-04-2011, 11:48 AM   #5
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RE: Calculating hull speed.

Motion,

"motor just sounds happy and burns just over 2gph At 1900rpm speed picks up just less then 1mph". How do you know it's happy? Do you take it's temperature? You should as that is the problem w running too light ** .....engine dos'nt get warm/hot enough. And I'm not talking about the coolant either. Valve guides, rings and many other parts of the engine that heat the oil enough to have proper lubrication keeping the bad stuff in suspension so it dos'nt gum up the works. One can measure lube oil temperature but that is subject to variables like oil coolers, amount of oil in the sump, where you measure it's temp and other stuff. Somebody correct me if I'm wrong but I think the main potential problem is stuck rings. So one should be able to test for the possible problem by checking compression.*Most of the time we think our engine is "happy" when it makes less noise or causes less vibration but I don't think the* engine gives a rat about what we think or feel. But it seems there may be very few engines that are subject to damage from running too light as there are many that have run light for years and don't seem to suffer. I'd say don't worry about it much but even if your engine goes tits up your boat is probably overpowered and will benefit from a smaller engine that can be run harder and burn even less fuel. Fred brings up a very good point. Running at a SLR (speed length ratio) of less than 1 will produce much more economy. As I recall a SLR of 1 is hull speed. This is all just opinion, my opinion and others so it would be nice if we could have some engineers on here to settle this thing. When fuel gets to $10 a gallon I may want to underload too.
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Old 06-04-2011, 12:09 PM   #6
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RE: Calculating hull speed.

A lot of truth to what you're saying Eric. For me, the empirical evidence says that a Lehman 120 can run at 1600 or 1650 for thousands of hours without problems. I've shot temps on the oil filter, transmission, water header tank, each cylinder at both block and head, and just about everywhere else one can shoot with an infrared thermometer and there's not much difference between 1650 and 1850. I know it's burning more fuel and must be creating more heat, but the cooling system seems to have no difficulty getting rid of it.

I presume if gummy rings was a problem on this particular engine, Lehman 120's, people would be rebuilding the engines at 5000 hours. The people I heard about overhauling were exclusively heat related, too hot, not running too cold.

Ken
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Old 06-04-2011, 01:56 PM   #7
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Calculating hull speed.

Quote:
nomadwilly wrote:
Motion,

"motor just sounds happy and burns just over 2gph At 1900rpm speed picks up just less then 1mph". How do you know it's happy? Do you take it's temperature? You should as that is the problem w running too light ** .....engine dos'nt get warm/hot enough. And I'm not talking about the coolant either. Valve guides, rings and many other parts of the engine that heat the oil enough to have proper lubrication keeping the bad stuff in suspension so it dos'nt gum up the works. One can measure lube oil temperature but that is subject to variables like oil coolers, amount of oil in the sump, where you measure it's temp and other stuff. Somebody correct me if I'm wrong but I think the main potential problem is stuck rings. So one should be able to test for the possible problem by checking compression.*Most of the time we think our engine is "happy" when it makes less noise or causes less vibration but I don't think the* engine gives a rat about what we think or feel. But it seems there may be very few engines that are subject to damage from running too light as there are many that have run light for years and don't seem to suffer. I'd say don't worry about it much but even if your engine goes tits up your boat is probably overpowered and will benefit from a smaller engine that can be run harder and burn even less fuel. Fred brings up a very good point. Running at a SLR (speed length ratio) of less than 1 will produce much more economy. As I recall a SLR of 1 is hull speed. This is all just opinion, my opinion and others so it would be nice if we could have some engineers on here to settle this thing. When fuel gets to $10 a gallon I may want to underload too.
* So you are saying my engine is just like my girl friend? Cause she dont care a rats ass*how I feel or if I am happy either** Thought for sure my diesel loved me as it has never let me down


-- Edited by motion30 on Saturday 4th of June 2011 01:57:23 PM
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Old 06-04-2011, 02:00 PM   #8
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RE: Calculating hull speed.

so ken at what rpm do you run your 120 ? My boat is old,1973 I dont know how many hrs are on the motor but I have put 1500hrs on it over the last 3 years and she starts in one turn and runs good
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Old 06-04-2011, 04:57 PM   #9
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RE: Calculating hull speed.

My standard cruise is 1700-1750. If it's a nice sunny day and I really don't want to be anywhere 1600-1650. I'm not quite to 6000 hours on my 1980.
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Old 06-04-2011, 10:14 PM   #10
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RE: Calculating hull speed.

I just sold my boat and it's Lehman120 that I put 2000hrs on over the last 5 years. I cruised her at 1700RPM the first year and 1500RPM the final four years. I did not run her up during the cruises to burn off the bad stuff. The buyer had a full blown engine survey done with blow-by and exhaust gas analysis and the whole nine yards. The engine guy stayed with her for 6 hours including running her at WOT for 20 minutes. She passed with flying colors, everything was perfect. I run a cruising prop that will only allow 2100RPM at WOT.

Bottom line is if you want to conserve fuel dollars then slow down and get a cruise prop to properly load your engine at those slower RPMs. You won't hurt the engine only help it.
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Old 06-05-2011, 12:16 AM   #11
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RE: Calculating hull speed.

Daddyo,

Your "cruise prop" made it such that 1400rpm was about the same load as 1800 with a properly propped boat. You were essentially in high gear and since you never ran it at a high enough rpm to overload it the engine did'nt suffer. Also at 1500 the water pump and oil pump was running slower and that may have made the engine run a bit warmer. So you did the "cruise prop" stunt and got away w it but if you had run it 1800or 2000 it may/ probably would have been overloaded. Running a boat is like climbing a hill in a car and the "cruise prop" is like shifting into overdrive while on the hill. But it works if you never extract more than about 60hp from your Lehman. Glad you finally sold your boat as I know it was for sale for a very long time.
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Old 06-05-2011, 05:06 AM   #12
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RE: Calculating hull speed.

"Running a boat is like climbing a hill in a car and the "cruise prop" is like shifting into overdrive while on the hill."

Yes and no, most diesels are far happier at ANY rpm with a proper (heavy) load for that particular rpm.

Matching the cruise speed to the low rpm where the engine is "sweet" is the challenge.

The modern gas car will always be in the highest gear it can be in , load is computer monitored.

All the new OTR trucks (diesels) attempt to cruise at about 1200 rpm , with 80,000 lbs of load.

The problem with matching the load and prop for greatest efficiency , is the possible need for reserve power in a blow.

Or the desire for a higher inshore cruise speed , where fuel is easy to find.

This trade off is usually made by not going "all out" for efficiency , and allowing an overhead of say 500rpm the engine can pull, on its new cruising prop WOT.

Then most cruising will be done 500 rpm down but the ability to increase 200rpm (or 300 on a heavier duty engine) solves the headwind/speed problem.

There is no "best" prop , only a compromise to allow the owners desires to be realized.
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Old 06-05-2011, 09:23 AM   #13
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RE: Calculating hull speed.

There is definitely only one way to properly prop a boat. And there is a narrow range of proper power loading too. People buy big heavy old trawlers and unlike the owners that had the boat when it was new they are seriously lacking in money for fuel so so they resort to desperate measures to save enough fuel so they can operate their boat. That is the only defensible reason for underloading or "cruise propping". There is just no other possible explanation for it. If everybody here had a credit card that someone else was paying for there would be no underloading or overpropping. And the new boat buyers establish what the boat will be... what it will look like and how it will perform and everything else. For most of us we have hand me downs that we can't afford to use they were intended to be used. You don't see Walt or Mark over propping and underloading because they have plenty of money for fuel. If I had enough money I'd have a lobster yacht w a DD 6-71 or a Uniflite 36 w 2 6-71s. And I'd be running 18 or 20 knots. You don't match a prop to a boat for greater efficiency. You match an engine to a boat first ( hp actually) and then you run it as it should be run. But it's back to money again. Solving the underloading issue would be to repower w much less hp to go the speed you want/need to go but that takes lots of that money stuff again. And slow is'nt the speed you really want to go so if you had the money you'd be going faster anyway. So if you're desperate to save money on fuel you can't afford to repower and the only option is to limit time underway and reduce fuel burn as much as possible with what you've got. So you're right Fred, the "cruise prop" and underloading is a questionable but viable way to make an old trawler work but it's absolutely not the right way to run a boat and the're is a possibility the engine won't like it but it seems the're are some like the Ford Lehman that will take it. And yes there is a "best prop" and that is a prop that will allow the engine to be loaded at rated rpm and WOT. That is the best prop and anything else involves sacrifice and limitations that should'nt half to be made. And lastly you don't need extra speed "in a blow". You're just use to lots of extra power in your cars. Anybody with any sense slows down in a blow. But if you need "all out" fuel efficiency, with the right engine, and realistically realizing the limitations of "cruise propping" it can be done and it can save fuel****** ......... but it's NOT the right way. The're IS only one right way to power a boat.
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Old 06-06-2011, 04:05 AM   #14
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RE: Calculating hull speed.

"And yes there is a "best prop" and that is a prop that will allow the engine to be loaded at rated rpm and WOT. That is the best prop and anything else involves sacrifice and limitations that should'nt half to be made. "

Setting a cruiser to operate at WOT is (unless you have a CPP) is a guarantee the vessel will be inefficient at any speed that is not close to WOT.

A 135 HP engine that is working 95% of the time at 2gph or 3 gph is using 1/3 or less of rated power .NO way it can be efficient!

The cure is simple understanding.

And then adjusting the vessels load (the propeller) to the 95% operating condition.

The savings in noise , engine life and fuel will easily pay for the cost of a prop.

You are correct , a plaininbg or semi wallowing boat needs to have a prop that will allow WOT.

A true displacement vessel is better tuned to its actual use.

Since most owners would consider dumping a big engine and installing a more rational engine a radical step, simply installing a better loading prop is less costly and can be reversed should the next owner like to make BIG wakes.
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Old 06-06-2011, 10:55 AM   #15
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RE: Calculating hull speed.

Fred says,

"Setting a cruiser to operate at WOT is (unless you have a CPP) is a guarantee the vessel will be inefficient at any speed that is not close to WOT."

I'll use your words Fred. Yes and no. There are times when if you're climbing a hill in high gear and you downshift you'll be burning less fuel. WOT and less than half rpm would be hard to achieve in a boat though. It's a function of load v/s fuel burn. I think most of the time you're right though Fred but you should have used the words less efficient instead of just plain inefficient. The difference may be very small and if it is the overpropping may not be worth the risk ov damaging one's engine. But if it's a big difference it would be worth doing if one understood the dangers of overpropping fully and if fuel consumption was of paramount importance. If it's a desperate measure to save fuel it can be done safely if one never overloads the engine at higher throttle settings. The problem is that nobody knows exactly where the overloading (lugging) begins on the rpm scale. So it can work exactly as you say Fred except no one knows the amount of savings that can/will be made and overloading and engine damage can happen. So it's not a good idea but it can work and perhaps work well as long as the throttle is set low enough to keep from overloading. Have you ever talked to an engineer about this Fred? By the way I like that you used the word "cruiser" instead of trawler. PS.....how many "semi wallowing" boats do we have in the forum?
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Old 06-06-2011, 12:04 PM   #16
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RE: Calculating hull speed.

Has anybody ever tried running a displacement vessel with a 2-speed gearbox? MB Sport built a wakeboard boat with a 2-speed gearbox for a while - they may still do that. You could think of it like an overdrive where you'd be propped for WOT in direct but you could drop to 2/3 of WOT with a .6 OD. I'm just curious if somebody has already tried this and rejected the idea.
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Old 06-06-2011, 01:49 PM   #17
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RE: Calculating hull speed.

So if you have a*boat with a hull speed of say 8 knts.

*And you had an engine that would*push *the boat at 8 kts using 75% of the available hp.

(With the parasitic load calculated in)

This would be the ideal scenario for a trawler.*

I can't run my boat at hull speed because that only gives me about 1650 rpm when the Cat is set for WOT at 2800 rpm.

The reason for the tread.**What other vriables are to be considered when calculating hull speed.

Wetted surface area.

How clean is the bottom.

how much weight on board.

Exhaust back pressure.

Mechanical condition of the motor (i.e.) valve adjustment, injectors,clean oil,etc

SD*
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Old 06-06-2011, 10:02 PM   #18
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RE: Calculating hull speed.

Northbob,

Sure. That would work. Don't know if anybody has tried it but I've heard of guys putting automotive gear boxes with a clutch peddle under the helm. I can imagine some gear crunching unless one had a propeller shaft brake. One could be a bit busy at controls in close quarters and I don't even want to think of twin screws. But of course one could install two transmissions like a 2 speed gear and a BWVD.

Skipper,

Surface area and displacement to be sure and many boats have underwater appendages like keel coolers, propeller struts, keels, rudders ect ect. Some trawlers have workboat rudders (very dirty) and others have very low drag rudders that look more like an airplane wing. But the most influential element of resistance is prismatic coefficient. A high number is a high PC and the lower numbers represent lower PCs. What this means is that the PC indicates what percentage of disp is carried into the ends of the hull. A canal barge has an extremely high PC and a sailboat has a low PC with it's pointy ends and wide beam amidships. If you were* interested in a trawler that was VERY efficient you might consider converting a sail boat into a trawler. I was thinking about that when I was shopping for Willy. I'd cut half the keel off and get rid of all the rigging and sails. I'd cut off the whole deck and gut the hull. Then I'd build a trawler cabin on it much like a Nordic Tug. I'd put the engine and fuel amidships along w other heavy stuff. I could power a 40' boat of this type w the same engine I have in Willy (40hp). Or if I converted a 34' sail boat I could use a 27hp Yanmar or simillar. The sail boat is optimized for low resistance while the trawler is optimized for space and stability while paying a high price in efficiency. Prismatic coefficient sounds complicated basically it's not. It's just an expression of how pointed a boat is. For the minimum PC think of 2 arrow heads back to back or an elongated diamond shape pointed at both ends. Many canoes are diamond shaped to optimize low resistance and most all sailboats have much lower PCs (at both ends) compared to trawlers. I'd guess 2 hp per ton or less would do for a sailboat and trawlers need 3 to 5 hp per ton of displacement. That is full disp trawlers. Compute you're own power loading and don't be too shocked at the high number as most trawlers are over powered to way overpowered and have their sterns submerged deep in the water creating lots of drag. And they have a high PC. Observe that sailboats don't have slab sides. A quarter of the way fwd and aft of center the sailboat has a narrow beam and amidships a very wide beam. If you really want efficiency you need the diamond shape. But more normally shaped power boats CAN also be efficient too.If you want to see an efficient trawler search TAD Roberts NA and find "Yellow Cedar". Yellow Cedar is 38' long, 10' wide and makes 9 knots w a 28 hp engine. Look at other stuff on TAD's site as he is responsible for many modern and fuel efficient boats. So Skipperdude the elements of efficiency are PC, disp, narrowness and cleanliness. Wetted surface gets in there too but it's not dominate at hull speed and less.
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Old 06-07-2011, 03:46 AM   #19
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RE: Calculating hull speed.

"The reason for the tread. What other vriables are to be considered when calculating hull speed.

Wetted surface area.

How clean is the bottom.

how much weight on board.

Exhaust back pressure.

Mechanical condition of the motor (i.e.) valve adjustment, injectors,clean oil,etc"

None of these would change Displacement hull speed , although the amount of POWER (and fuel burn) required to get to SL x 1.34 might be 200% or 300% over what would be normal or rational.

You have described things, like a foul bottom ,or excess weight that will slow a boat , but are overcome with throttle , till you run out.

The wallowing boats are the ones that carry 50 or 100% more power than displacement cruising requires , going over "hull speed" but far less than climbing up on a plane ( usually SL x 3+) and can get a 9K boat up to 12K or 14K , with boggieing fuel flow and wake.

These are boats that would enjoy light weight and L/B ratio over 6-1 but seldom are built that way.

Yes ZF makes a marine two speed , but it is at ZF prices , for purchase and repair.

WE planned on a CPP and ZF in our sadly unbuilt "box boat" (fits inside a shipping container) , the efficiency would have been high enough (sez da computer) to get lots of free space in marine mags and trade publications.18K@ 4-5 GPH ,12K @2 gph.

$20K in added machinery costs should have been worth perhaps $200K in publicity , free advertising , a fair trade.
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Old 06-07-2011, 03:51 AM   #20
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RE: Calculating hull speed.

Folks planning on an offshore style cruising would do well to purchase Bebbe's PASSAGEMAKER book (first ed only).

THere are graphs that will allow a very fine estimation of displacement cruising speeds vs HP required.

With lighter hulls , more efficient engines ,and the use of a CPP his numbers can be bettered by a modest percent.
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