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Old 02-02-2008, 04:37 PM   #1
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We were discussing twin v/s single screw relative to efficency when we were acused of being off topic by myself and some other person. I'm still not OK with this as we did'nt reach any conclusions. We just slung a bunch of opnions around ..as usual. My problem with this is that I think most people think SS is most efficent but I don't belive it .. and I'm only talking about counter rotating propellers. I sugested on the other thread that the turbulence from the keel was probably a negative for SS efficency. In an old Wooden Boat magazine I saw a boat design presentation by Tom Beard. The boat was 49'X10'9"X4'. Mr Beard said the boat should be able to make 9knots on less than 40hp. Whats interesting is that he incorporated in his design a feature that gives my theory of keel turbulence lots of credit. " The propeller is mounted off center where it will a less disturbed flow of water and the propeller shaft is angled in tward the centerline about 2 degrees to overcome the torque effect. This angle permits a nearly nutral rudder, with a further reduction in drag ". Note FF that this boat is only 15,300lbs disp and planked w 1 1/8 Mahogany. I don't think one could build a boat of this size in FG without resorting to exotic methods or materials. But not doing my case any good the boat could have been powered by 2 Perkins 4-107s instead of 1 Perkins 4-236.

Eric Henning
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Old 02-02-2008, 05:44 PM   #2
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Eric---

I would say you are correct in that the cleaner "bite" of water a propeller can get the more efficient it will be. The notion of angling the driveline relative to the axis of the boat to overcome some other force is not confined to boats. Most single-engine airplanes do exactly the same thing.

What would be interesting to know is how much the prop shaft and struts of a twin engine boat affect the waterflow into a propeller as compared to the interference of the keel to the waterflow into the prop of a single. Most of the time, the width of the keel at its end is not much greater than the diameter of the prop hub but of course it extends above and below the prop disk.

I have seen other examples of boat designs that use long, narrow hullforms to achieve good speeds at very low horsepower. In fact, even our short, relatively wide, square-stern Grand Banks 36 cruises a bit over 8 knots while using only about 60 of it's 120 horsepower.

Good examples of your premise can be seen in the so-called express cruisers of the 1920s and 30s. Some of these boats were very long--- 50, 60 feet----but they were very, very narrow. So not only were their hull speeds high because of their length, but they could be driven to hull speed with the relatively low power engines that were common in those days.

Fiberglass is very heavy for its strength as you noted. But there are carbon fiber composites these days that are very iight for their strength. Or you could use aluminum.

It will be interesting to see if the rising cost of fuel launches a move back to longer, narrower boats that can be driven at decent speeds with lower power. Two things will have to change if this is to happen. Longer slips need to be available in marinas, and people will have to be willing to live with considerably narrower accommodations inside the boat.
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Old 02-02-2008, 11:13 PM   #3
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RE: Design

Marin,
Who cares if the space is narrower ... it's much longer. Same space. You got a point about moorage though. I tend to forget about that up here where there ar'nt too many people. My moorage is a little over $500. a year. I think the keel offers much more drag and produces much more turbulence than shafts and struts. About your GB ... if you put two 55hp Isusu's in your boat and changed the shape of the stern so the bottom turned up at the stern to the water line or nearly so you would cruse at 7 1/2 knots burning 2gph. Just an opnion and just a thought. Speaking of thoughts check this out. 1943 Noorduyn Norseman 600hp 1340 PW 60K (about) 907 223 5282 or on e-bay. Thanks again for your comments on my pictures.
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Old 02-03-2008, 04:08 AM   #4
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No question the cleanest flow to the prop will gave best results , but an off center prop looses from pushing sideways as does a non flat shaft angle.

Lighter, slimmer and all that make the boat easier to push , but every boat will have some HP that is required at a specific speed.

The boat wont care if 50 Hp comes from 1 50hp engine , or 2-25 hp units , the speed will be the same.

BUT in terms of lowest fuel flow the single will be more efficient.

The second engine has internal friction that has to be overcome , the second tranny also has losses , as does the stuffing box.

Engines working the hardest are more efficient , so a 75Hp that is cranking out 50HP will use less fuel than a pair of 50hp engines putting out only 25 hp each.

And most boaters would be reluctant to install a 35HP engine (to raise the efficiency at 25HP) as the claim of twin engine reliability would be lost , as single engine operation would be quite slow.

ADD in the efficiency of less induced losses from a larger single propeller , perhaps even run at a lower speed with a bigger diameter .

Single screw will be the cheapest to operate , as well as requiring half the maint, oil changes, water pump repairs , rebuilt exhaust systems and re-cored heat exchangers.

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Old 02-03-2008, 03:20 PM   #5
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I would not want to eliminate keel-produced turbulance by losing keel-provided protection to the propeller.

In otherwords, one of the main purposes of having a keel infront of a propeller, is protection. Personally have over 45 years experience on single screw vessels without a dinged prop, and this in areas where there is considerable wood debris.

Knock-on-wood!
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Old 02-03-2008, 05:04 PM   #6
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Jay N,
I totally agree. I hope my Willard keeps the same record. I usually don't care if we go off subject but this time I do. I want to get to the bottom of this single twin efficency question. The thread is called Design so your'e not guilty at all but I'm having an awful time getting this question answered. At least FF has bellied up to the bar and made a claim. "But in terms of lowest fuel flow the single will be most efficent" Well now it may or may not be fact and it dosen't even need to be as long as you can convince me it is. I'll deal with reality when and if it comes in the near or distant future. Yes. With a given draft twins will have the advantage of larger propeller and I think the twin has it again with less turbulent inflow. I'm going to go out on a limb and say the twin also has advantage of larger disc area with a given draft. I still don't agree with FF or Marin about maintanance being double for a twin. Each engine will be half the size of the single so one would need filters half the size of a single, belts half the size, oil half as much ect but propeller maint will be much higher. I think maint costs for the twin should be about 120% of the single. Vinny, off the top of my head I think the offset propeller would reduce the prop walk going ahead too. How about putting the keel off center too ( a bit ) and ballance the whole thing out. I'm almost positive I hav'nt seen that done.
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Old 02-04-2008, 05:15 AM   #7
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"but propeller maint will be much higher."

As will shafts , struts and rudder repairs.

" so one would need filters half the size of a single, belts half the size, oil"

none of which are half price due to size.

The time for all the maint will be double , since everything will be done twice.

Worst of all the tiny diesels may be truck takeouts Yanmar , Perkins , ect, the engine with twice the rating might come from the next step up the reliability ladder Detroit, Mann , Mitsubishi (Deere) Deutz , a whole different world in durability and time in service.

Canting the engine a couple of inches to allow less rudder drag does work, but at mostly ONE speed .

The hassle is when repowering the prop rotation direction MUST be maintained , so the use of a more modern engine may not be doable.

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Old 02-04-2008, 10:00 AM   #8
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I know that in outboard applications multiple motors are always less efficient in terms of mpg than equivalent horsepower singles, and maintenance costs are closer to double than 120%. Don't forget that you're also doubling the drag from the running gear with two motors.

OTOH, getting a clean flow of water to the props is certainly an issue, as is shaft angle. The IPS systems, for example, achieve better efficiency by running in unobstructed water and at zero angle. As mentioned, though, there's the issue of prop protection.... But running twins doesn't necessarily mean clean water flow and no prop protection. The Great Harbour trawlers use twins with skegs for each prop.
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Old 02-04-2008, 02:20 PM   #9
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Eric---

If I am interpreting your comments about twin-engine vs. single-engine maintenance costs correctly, many twin engine boats use exactly the same engine as the single engine version of the same boat. For example, you could get the GB36 in twin or single. In the 1960s and 70s, the engine used most often in either version was the Ford Lehman 120. The only difference was how many FL120s you had.* For*a newer example, in the 1990s the GB36 could be had with one or two Cummins 210 hp engines. Again, the exact same engine in the single or twin GB36. So if it takes $800 dollars to service one Cummins 210, it will take $1,600 to service two of them.

It may be that there were-- or are-- production boats out there that in the single-engine version use a larger engine than the same boat would have in the twin-engine version. But with GB, at any rate, the models that were available in both single and twin engine configurations almost always used the same horsepower engines in the twin that were used in the single engine version. And I think the same held true for boats like CHBs, etc.

Also, I've not found that a small oil filter or smaller belt costs always costs less. In my experience, the same type of filter costs pretty much the same regardless of size. Same with belts unless there is a signficant size difference. Anyway, many larger engines use the same size filters and belts as smaller engines, so I don't think this cost is really going to be affected by the power of the engine.

The notion that a higher powered engine will automatically be more expensive to maintain than a lower power engine is not always the case. Take Carey's lobsterboat and our boat. Carey has a 425 hp Cat. We have two 120 hp Lehmans. But even if we had only one Lehman, Carey's maintance costs would still be way lower than ours if for no other reason than his engine has much longer service intervals than our engine. And servicing is much easier and faster on Carey's engine, so if the work is done professionally, the labor cost to service his engine will be considerably lower than the labor cost to service one of our engines.

So I don't think you can really tie maintenance costs to the power of an engine. The costs will depend on the engine's technology and its ease of accessibility in the boat-. In other words, things that affect how often it has to be maintained or serviced, and how much that service will cost in terms of materials (oil, belts, filters, etc), and labor time.

Where I do think your basic comparison cost is correct is in the fuel burn of a single vs. a twin. I don't believe that a GB36 with two FL120s will burn twice the fuel in cruise as a GB36 with one FL120. I'm not sure the twin will burn 120 percent of the fuel the single will burn--- it might be more like 150 percent--- but given the same kind of boat, same speed, same weight, etc., the twin's fuel consumption will not be double.

Also, I agree that the total volume of space inside a long, narrow boat can be the same as the space in a shorter, wider boat. What I meant in my earlier comment is that it seems a lot of modern "fat" boats use their width for house-like amenities like big tables, transverse settees, big, L-shaped galleys, and so on. This kind of interior layout would have to change if boats went back to being narrow, and some people might be reluctant to give up their big, wide "living rooms"

Interestingly enough, the almost flat afterbody of a GB's semi-planing hull does slope up toward the stern until at the transom the bottom of our boat is only a couple of inches below the surface at it's outboard edges and not much more in the middle. I assume this adds to its efficiency as well as the ability to be driven faster without a ridiculous amount of extra power. Even with our two FL120s, we can get about 11 or 12 knots at full throttle out of the boat (which has a theoretical hull speed of about 8 knots) but we need to run an extension hose to the marina's fuel dock in order to do it


-- Edited by Marin at 15:30, 2008-02-04
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Old 02-05-2008, 11:04 PM   #10
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RE: Design

Adam T,
You're like Marin and FF. You guys think everything on a twin is double. In numbers of parts yes but the size of the parts are usually half the size. Are you saying that a 50hp lower unit has the same drag as a 25hp LU?. I'm saying the drag of a 50hp LU is probably about exactly the same as 2 25hp LUs. Yes I see some trawlers have thier own skeg to protect the running gear. The big Krogen is another fine example.
FF
Why must a repower maintain the same rotation? I agree re a twin but I know you well enough to be quite sure your'e talking about a single.
Marin,
I agree now that your GB is a bit closer to a displacement hull that I have been thinking. However, the quarter beam buttock line is far too straight to consider the GB a displacement hull. Iv'e resisted saying this for a long time. Fourtunately for our relationship you own a twin. I think, from a design standpoint the practice of building boats with twice as much or half as much power is a very bad practice. The twin, like yours, has what I consider to be an appropriate amount of power but the single should never have been built. At 7.5hp per ton ( assuming about 16 tons displacement )
the GB single has the same power loading as a Krogen 42 ... a full displacement hull. Unless it has more power like the twin it should have a displacement hull. I look down on GB for marketing such a boat. That said, the worlds most perfect trawler could be a 36 GB with a properly scaled or sized hull the same as a Krogen 42. The perfect engine could be a Deere 72hp. One more criticism. I think it's inappropriate to use the expression " hull speed " relative to any hull except a full displacement hull.
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Old 02-05-2008, 11:39 PM   #11
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I really don't know the answer, nomadwilly, but my guess would be that a twin would have more drag than a single, but not twice as much.

I agree that it's a bit baffling to see semi-displacement boats being fitted out with insufficient power to take advantage of the design. OTOH, I've heard that GB has revised its hulls over the years to enhance planing performance, along with adding bigger engines.
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Old 02-06-2008, 12:20 AM   #12
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Eric---

I completely agree with you that the GB hull--- and the hulls of all "trawlers" using the GB's same basic hull shape--- are not full-displacement hulls. American Marine, the manufacturer of the Grand Banks line of boats, never called them displacement hulls. They called them semi-displacement or semi-planing depending on who at the company you were talking to.

I guess I need to understand more clearly why you make the statement about a twin not being double the single in terms of maintenance cost, power, etc. When you say the size of the parts on a twin are half the size of the parts on a single, what do you mean? In terms of the engine, each one of the engines on a twin engine trawler-type boat is the same size and power as the engine in the single-engine version of the same boat, because as I pointed out earlier, most production boats that are available in both twin-engine and single-engine versions use the same type of engine in each version.

The GB32 was only built as a single engine boat (with a tiny handful of very rare exceptions). The GB36 and GB42 were available as singles or twins. The larger GBs--- 46, 48, 49, 52--- are, to my knowledge, all twin-engine boats.

The main reason the GB36 and GB42 were offered as twins is many customers want twin engine boats for powertrain redundancy and ease of maneuverability and they're willing to pay higher fuel and maintenance costs to get it. People who want the single-engine version of the same boat put more emphasis on propeller protection, lower fuel and maintenance costs, and a less-crowded engine room.

The reason American Marine had Ken Smith design the hull he did is that they wanted a hull that could be driven relatively easily at speeds somewhat faster than hull speed. The reason they did this is because their customers wanted to be able to cruise very economically at hull speed but be able to run the boat several knots faster if they were in more of a hurry. You can do this with a semi-planing hull, but you can't with a full-displacement hull. Don't forget, this all happened back when fuel prices were not much of a consideration in boat operation costs.

Of course you need power in order to push a GB along at a semi-planing speed, which is why GBs have more powerful engines--- and in the newer ones, WAY more powerful engines--- than would have been needed if the maximum speed of the boat was hull speed.

At a cruise speed of about 8 knots, which is basically hull speed for our hull, each of our 120 hp engines is running at about 1700 rpm. At this throttle setting, according to the FL120 operating manual, each engine is putting out about 60 hp, or half it's maximum rated power. So both engines combined are putting out about 120 hp. Getting the same cruise speed in a GB36 with one FL120 requires about 1800 rpm, which I believe is equal to about 85 hp. I can only assume that the twin needs the additional 35 hp at the same speed to overcame the higher weight of the boat and the additional drag of two rudders, two sets of struts and shafts, and so on. Plus a propeller generates drag (turbulence) as well as thrust, so with two props you have twice as much prop drag.

So the GB36 single is a more efficient. less costly boat to operate than the twin.* But the twin offers engine and drivetrain redundancy and easier maneuverability.* Which one the buyer prefers is largely dependent on how they intend to use the boat.



-- Edited by Marin at 01:40, 2008-02-06
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Old 02-06-2008, 04:29 AM   #13
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"The main reason the GB36 and GB42 were offered as twins is many customers want twin engine boats for powertrain redundancy and ease of maneuverability and they're willing to pay higher fuel and maintenance costs to get it. People who want the single-engine version of the same boat put more emphasis on propeller protection, lower fuel and maintenance costs, and a less-crowded engine room."


For docking there is no question a single will only do 95% of what a twin can do, but with big buck GB customers most install a bow blower and the boats are about equal.

While 2 engines MIGHT seem safer , my yard experience has shown the opposite , shaft strut and rudder damage is FAR more commin in the exposed twins.

Its my belief the twins are mostly for a bit of stability at semi plaining speeds.

Most boats with "Grand Banks" style stern sections will roll somewhat less when pushed.

So get home when it blows up a bit is more comfortable.

I have never known GB owners to actually "Cruise " full time at 12 or 14K , most are far to cheap for 5X the fuel flow.

So for a bit of percieved better ride , at rare times. the boats loose efficiency from the semi plaining hull design and the engine choices.

A "pure" displacement boat might be a better all round choice ,perhaps *with Flopper Stoppers for the rare get homes?

FS are still cheaper than a high maint second engine , working underloaded most of its life!


And FS don't have the risk of extra shaft , strut and prop maint.

FF


-- Edited by FF at 05:32, 2008-02-06
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Old 02-06-2008, 10:45 AM   #14
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The people I know personally who have suffered prop damage or fouling have almost all been the owners of single engine boats, mostly sailboats. I only know one person personally who has has a problem with damaging a prop/shaft/struts on a twin. But that doesn't mean the twin is less vulnerable--- it isn't. Perhaps it just means that most twin operators in this area (PNW) keep a real good lookout for debris in the water because there is so much of it. I know we do. While at the same time the single-engine owners place more confidence in the protection of their prop than they should in these waters

Until the last few years, a number of the owners of newer GBs in our marina routinely cruised at 12 to 15 knots. Their boats have the power to do it.--- for example a GB42 with two 450 hp engines. The rapid rise in fuel prices has resulted in most of them slowing down although there are a few of them I see who are still moving out pretty fast.

When GBs first came out, bow thrusters were not available. Even when they became available, installing one in a woodie is tricky and is rarely done. So the twin engine versions did offer a significant maneuvering advantage over the single. But with the advent of fiberglass hulls and bow thrusters, a single is as easily maneuvered as a twin. As to the big buck owners, many if not most newer GBs are fitted with twin engines AND bow thrusters.

Based on our experience running both single and twin engine GB36s, the single is actually the more stable boat at speed because the keel is longer and the rudder is much larger.

-- Edited by Marin at 11:46, 2008-02-06
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Old 02-06-2008, 11:07 AM   #15
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Down here on the Gulf coast, it's pretty much the opposite. We run in skinny water all the time, and grounding is the major cause of prop damage. Almost all damaged props come on from twin screw vessels, while the singles are proteced by the keel.

I even saw one guy all torn up.... he got underway and forgot to raise his anchor! It ended up wrapped around one of the prop shafts... the anchor beat the bottom of the boat until it was finally ripped off by the force; I think it broke the chain! Boy, what a mess.... what was going through this guy's head!?
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Old 02-06-2008, 12:22 PM   #16
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A nice thing about the Grand Banks design, and perhaps others as well, is that on the twin-engine models, the props and rudders are well above the bottom of the keel. So if you run aground you hit the keel, not the props or rudders. In fact, if the boat is grounded in the mud and the tide goes out and lets the boat fall over at a severe angle, the props, shafts, struts, and rudders will still never hit the ground. All the running gear is pretty close in to the keel and the boat will come to rest on the chine before a prop or rudder hits the ground.

Fortunately I don't know this from experience but from talking to some folks who had this happen to their GB as well as seeing photos of a few GBs on their sides.
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Old 02-06-2008, 10:10 PM   #17
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Hello Marin,
The more I think about it the more I think you and Paul Graf are somewhat alike. I think what GB did was irresponsible. They needed to decide how much power the boat needed. Establish an acceptable range of hp required and choose a pair of engines and one single engine that matched that requirement. Lets say the requirement was 200 ( I think it should have been ). The twins would get two 100hp engines and the singles would get one 200hp engine. Both boats would go just about exactly the same speed and burn about exactly the same fuel but most importantly the boat would go foreward at the speed it was designed to go. A much better level of engine loading would also result. As to your hp/load calculations ... need to do it all over again. When you say your'e at 1700 rpm producing 60hp your'e not realiezing that that 60hp is measured at full throttle. All the points along a hp curve are maximum power numbers at that rpm. To get to 60hp at 1700rpm you would need to over prop your boat untill you had 1700rpm at WOT. Sounds like your'e only at a 30 to 40% load to me.
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PS Did you see that big seaplane? Beautiful high volume floats too.
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Old 02-06-2008, 11:51 PM   #18
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Eric---


I'm quoting the rpm/hp figures from the graph in the engine operations manual. Wide open throttle of the Ford Lehman 120 is 120 hp @ 2400 rpm. 1600 rpm is actually about 70 hp, not the 60 hp I had quoted before. I've attached the graph. I downloaded it from the GB site and sharpened it up as best I could in Photoshop. The numbers are not the easiest to make out but I think the key items are decipherable. I believe the RPM figures across the bottom are 800, 1200, 1600, 2000, and 2400. The brake horsepower figures up the side are 45, 65, 85, 105, and 125. But I'll check this against the same graph in our own engine manual on the boat this weekend.

I'm sorry, but I can't make any sense out of your statement that to get 60 hp out of the FL120 I have to turn it at 1700 rpm at wide-open throttle. I'm not saying your statement is incorrect, but it runs counter to every calculation of power I have ever encountered in vehicles, planes, and boats. Perhaps someone else on the forum can reinforce what you are saying and word it in a way I can understand.

I do agree with your theory that a twin-engine boat can have smaller engines than the same boat with a single engine. However, don't forget that the GB's semi-planing hull can be driven at considerably higher speeds than its calculated hull speed. But to do it, you need a fair amount of power. Putting in two of the same kind of engine that was used in the single gave the boat the power it needed to go faster than if it had just the one engine.

There are also some manufacturing and after-sales support issues that played a role. Using a common engine across the product line reduces costs, reduces parts inventory, simplifies some of the vessel systems, and so on.

And it's important to keep in mind when this was done. Fuel cost wasn't an issue in the 60s, 70s, 80s, and most of the 90s. Boats were not built with twin engines with any sort of economy in mind. They were built with twin engines so they would have more power and go faster than the same boat with one engine. Easier maneuverability was a bonus, but basically you put in more engines so you could go faster, or power a heavier boat, or whatever. And there is a sizable segment of the market that believes--- rightly or wrongly--- that a twin-engine boat will get them to where they want to go more reliably than a single-engine boat.

So in the context of the time and of what many of their customers wanted, I don't think American Marine (GB), or any of the other boat manufacturers who did the same thing, were irresponsible. They were simply producing what a large segment of the boating market wanted--- and still wants--- which was faster, more powerful boats. A good case in point is today, on the GB owners forum, there is a fellow who is contemplating the purchase of a 1985 GB42. This particular boat is fitted with two Cat 3208 diesels One of his stated requirements is that whatever trawler he buys, it has to be able to cruise at 16 knots. The hull speed of the GB42 is somewhere between 8 and 9 knots (I forget the HS formula). But with this amount of power, it will semi-plane along at 16 knots all day.* At 8 knots the boat burns 10 gph, at 16 knots it burns 30 gph. Now you and I can question whether what this guy wants to do makes any sense, but regardless. he is representative of what today is still a significant segment of the boat-buying market.

For a displacement boat, or a boat that is intended to be operated very economically, excess speed is no longer part of the equation. So if the desire is to have two engines for redundancy or maneuverability, then it could make sense to do exactly as you describe and either use one engine that produces x-power or two smaller engines that together produce the same x-power. I think one example of this philosophy is the Great Harbor 37. The CEO of the company makes no bones about his belief that twin engines is the only way to go, so he meets his economy objectives by using two fairly small diesels.




-- Edited by Marin at 02:11, 2008-02-07
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Old 02-07-2008, 03:50 AM   #19
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What you have posted is the max hp & max torque graph for the engine.

You will need to overlay a STD PROP HP , absorbing graph to see what percentage of the aviliable hp is used at any specific rpm.

The closer to full throttle at ANY rpm you are , usually the more efficient the engine is.

Usually proping for 2400 will be less than 50% power at 1800.

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Old 02-07-2008, 08:47 AM   #20
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Marin I think you'll find that 120 HP is actually at 2500 RPM not 2400. That info is found at the top of the graph just underneath the header info.

It says:

1 BS overload - max BHP 120@ 2500 RPM - Max torque 280 Lbs. Ft. @ 1600 RPM
2 BS rating - max BHP 108 @ 2500 RPM - Max torque 252 Lbs. Ft. @ 1600 RPM


Down at the bottom of the page it adds:

Deductions for a 4 blade 18" dia fan

Engine RPM 1500 1800 2000 2500
HP Deductions 1.5 2.5 3.5 7.0


Is this the STD Prop HP that FF is referring to?


Ken
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