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Old 12-08-2008, 11:31 AM   #41
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RE: Why semi-displacement (or semi-planing)?

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nomadwilly wrote:

Hi Walt,

I took another look at the Shannon and have a different take on it now. I think the hull is as wonderful as ever but Shannon being a sail boat manufacturer .. *thier minds are stuck in the rag boat box. After thinking about it I wouldn't even want a Shannon. Like a sail boat if your'e not on the bridge or in the aft cockpit your'e down in a hole barely able to see out. Not for me! The stern looks more like a hot tub than a boat and visibility is probably a bit shy under way. I'd go for a 23' version with a Sea Dory style cabin. About the ports and vents I think thats like bootin Goldie Hawn out of bed with a wart on her toe. Bet you wonder how I know that huh.

Eric Henning
That is the exact reason I went from sail to trawler.....the "cave effect"!
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Old 12-08-2008, 06:53 PM   #42
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Why semi-displacement (or semi-planing)?

I had an express boat before SeaHorse and although I could make an easy 30 knts (The sea allowing) as you say, we spent a good share of our time down in a hole. SeaHorse has the stateroom, shower and head down below. Everything else, dining, galley, etc. has a view! we love it!

Walt

-- Edited by SeaHorse II at 19:56, 2008-12-08
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Old 12-14-2008, 02:56 AM   #43
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RE: Why semi-displacement (or semi-planing)?

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nomadwilly wrote:

Marin,
Re: your message at the bottom of pg 3. Why do you think your power is actually 60 to 65 hp?
Because that's what the Lehman experts at American Diesel told me it would be at the rpm we use in the boat we have.* Also it's what the table in the original GB owners manual on our boat says it is (not the table in the engine manual, the table in our American Marine GB owners manual).* It's also what a friend in the marine diesel industry told me it would be.* I'm not saying they're all correct--*I don't have the expertise to know.* I'm*just answering your question as to why I made the statement that at our cruising rpm each engine is generating about 60 to 65 hp.

Our GB36 weighs about 26,000 pound (empty).

(currently in Xiamen, China)
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Old 12-14-2008, 11:13 PM   #44
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RE: Why semi-displacement (or semi-planing)?

Hi Marin,
Well, if the Willard 40 was 36' and 28K# it would take less than 20 hp to make 7 knots. Am I right in saying the GB 36 would need about*80 ? Would seem to be right if you take 120 hp to make 8 knots. That would mean it takes 4 times the power to drive a SD hull than a FD. I would have thought the difference would have been a bit less .. like 3 times.
Sloboat,
Re:*The GB 42. They*usually get these optimistic numbers by computing thier fuel burn by*using thier hour meter and the amount of fuel put in the tank. People spend a lot of time warming the engines up, going through "*no wake " zones, entering and leaving port at various low throttle settings and other less than cruise throttle engine time. It seems everyone can tell you how many GPH they burn but how do you suppose yhey compute it ?

Eric Henning**
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Old 12-15-2008, 01:54 AM   #45
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Why semi-displacement (or semi-planing)?

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sloboat wrote:

Marin, That number seems high for 7-8 knots. The prop curve for our semi displacement hull shows around 30-35 horsepower per engine at that speed range. ......Also just saw an ad on T&T for a '78 42 GB classic where the owner states 4 gph @ 8.5 kts from twin Lehmans.
You may well be correct with regards to the hp used at cruise settings.* But I was given a basic fuel burn formula by the folks at Northern Lights/Lugger that states (if I'm remembering it correctly) a "typical" marine diesel uses 1 gph for every 20 hp developed.* When we do the math for the average amount of*fuel*each of our engines burns at our cruise rpm, we get about 60 hp.

I would agree with you that 4gph @ 8.5 knots for a GB42 with Lehmans is a bit optimistic.* We burn about*6 gph (total) for an average through-the-water speed of 8 knots.* I assume the ad meant Lehman 120s.* I believe Lehman 135s have a higher fuel burn than the 120.

-- Edited by Marin at 02:55, 2008-12-15
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Old 12-15-2008, 03:32 AM   #46
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Why semi-displacement (or semi-planing)?

A ton is 2240 lbs of displacement , NOT the "tons (volume) listed on the USCG papers.

2 hp per ton will usually hive the longest range SL .9

3 Hp per ton or so will give best cruising SL 1.15

5 hp per ton will give a healthy SL 1.4 (hull speed with a big wake).

20 HP per gallon is ONLY for a well loaded engine at an efficient rpm.

For most boats with out a cruising prop that can pull MFG recomended top rpm, slowing down to a nice cruise seldom loads the engine to even 15 hp per gal.Many are at 12hp/g.

20 hp/g is fine for a fish boat that operates efficiently (80%load /90% max rpm),(peobably mechanical injection and turbo) does not have an oversized engine from the advertising dept , and is in good condition.

22 Is possible from a electric modern common rail unit with computer and multiple injections per stroke.

More hp/g only comes from the commercial 2 strokes , with cylinders the size of a bus and a speed of 100 rpm or so.Tankers.

YRMV,


-- Edited by FF at 04:36, 2008-12-15
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Old 12-15-2008, 03:51 AM   #47
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RE: Why semi-displacement (or semi-planing)?

"People spend a lot of time warming the engines up,"

This is sure death for an engine, no more than a min or so (as soon as it hits on all cylinders) is time to get underway with a modest load.

The engine has a different SHAPE while cool , so wears far faster .

Modest 1200rpm? will barely load most engines to 20 hp and is fine to get the water temp over 160F , where an extra few hundred will be OK, till warm.
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Old 12-15-2008, 02:28 PM   #48
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RE: Why semi-displacement (or semi-planing)?

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FF wrote:

"People spend a lot of time warming the engines up,"

This is sure death for an engine, no more than a min or so (as soon as it hits on all cylinders) is time to get underway with a modest load.

If this is true, then why do*a lot*of the trucks I see at truckstops idle their engines all night long? Wouldn't this theory hold for them too?
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Old 12-15-2008, 04:22 PM   #49
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RE: Why semi-displacement (or semi-planing)?

To stay warm?
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Old 12-15-2008, 04:38 PM   #50
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RE: Why semi-displacement (or semi-planing)?

Regarding fuel consumption - my Defever 48 at 8 to 8.2 knots burns 4.7 gph both engines and genset combined. Engines are 225 HP Perkins Sabres runnning at 1725 or so RPMs. Every fall when*I refuel I go by tach clock hours and gallons added. Now my diesel heater is another matter. It will burn about 2-3 gallons per day in the PNW cool climes.*Those Lehmans you've mentioned are sure fuel hogs, old and inefficient I guess.
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Old 12-15-2008, 07:27 PM   #51
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RE: Why semi-displacement (or semi-planing)?

Yes to stay warm but a lot of truck drivers arn't too bright either and a very high percentage of them don't own the truck they drive. And then there are millions of pickup truck drivers that emulate the commercial guys and leave the diesels running. And then there's the boaters emulating the fishermen. Why do they call them fishermen? Frequently they are not shipwrights, boat repairmen or even boat operators.

Eric Henning
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Old 12-16-2008, 02:00 AM   #52
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Why semi-displacement (or semi-planing)?

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Gulf Comanche wrote: ....why do*a lot*of the trucks I see at truckstops idle their engines all night long?
In very cold weather it can be better for a diesel to idle for long periods than to shut it off with the possibility that it may not start again.* For example, the trucks that hauled oilfield equipment made in Texas to the North Slope of Alaska in the winter would not shut their engines off from the time they left Texas to the time they got back to Texas.* Once in northern Canada, the drivers also had to get up every two hours at night and run the trucks back and forth to keep the lube oil in the tranmissions and differentials from getting so thick the trucks would be unable to move in the morning.

However all the semis I notice in the rest stops we use have their engines shut off if the driver is sleeping or taking a break.* The small diesels powering the refrigerator trailers are, of course, always running.

Railroad locomotives were usually left idling (actually, it was a fairly high idle) if they were scheduled to be used again within a couple of days because of the long amount of time it takes for their huge engine blocks to come up to heat.* Loading the engines when they are too cool will wipe out the engine pretty much right away.* However, this practice has been diminished somewhat with the escalating price of fuel.* It's become cheaper to pay the engine crew to wait around for the several hours it takes the big V-12s and V-16s to heat up from dead cold*than the cost of the fuel to let them idle continuously for several days.* But if a locomotive is not going to be used for several hours or even a day, the engine is generally left idling.* BNSF has a small sorting yard in Bellingham next to our marina and the locomotive used for switching is generally always running even if it's not being used during the night.

We were advised with regards to the warm up idle time for the FL120s in our GB to start the engines prior to the last steps in getting ready to get underway.* By the time the shorepower cord is disconnected and stowed, the electronics are up and running, and the lines are cast off, the engines will be ready for departing the marina.* Once out of the marina the temps are getting close to their nomal cruise readings and we gradually*add power in three stages.


-- Edited by Marin at 03:02, 2008-12-16
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Old 12-16-2008, 01:29 PM   #53
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RE: Why semi-displacement (or semi-planing)?

Here are a list of links you might find useful on this topic.
<a href="http://www.google.com/search?hl=en&q=site%3Awww.passagemaker.com+semi-displacement+&btnG=Search">
http://www.google.com/search?hl=en&q=site%3Awww.passagemaker.com+semi-displacement+&btnG=Search</a>
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Old 12-16-2008, 07:28 PM   #54
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Why semi-displacement (or semi-planing)?

"Railroad locomotives were usually left idling (actually, it was a fairly high idle) if they were scheduled to be used again within a couple of days because of the long amount of time it takes for their huge engine blocks to come up to heat."

RR industry practice was to leave them idling if the*temperature was going to drop to 40F or lower since they did*not use antifreeze in the coolant.*Habit is the main reason they still idle so much. This is changing with emissions laws and installation of aux power units to keep batteries charged and jacket water warm.

"Loading the engines when they are too cool will wipe out the engine pretty much right away."

That is pretty much nonsense. Tugboats use the same engines for propulsion and they don't sit around idling and not all of them use block heaters. Those engines can fire up and take a load as soon as oil temperature is showing and pressure has stabilized. We used locomotive engines in DE submarines and fired them up as soon as the overboard exhaust valves were above the water and ran up to full power from ice cold in moments. When snorkeling we abused them terribly from ice cold to overspeed with back pressure that would scare you to death and while they spat flame out of cylinder relief valves when starting*they ran like tops for thousands of hours.

-- Edited by RickB at 20:30, 2008-12-16
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Old 12-17-2008, 03:49 AM   #55
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RE: Why semi-displacement (or semi-planing)?

"RR industry practice was to leave them idling if the temperature was going to drop to 40F or lower since they did not use antifreeze in the coolant."

For folks with close to overheating problems , there is something to be learned here.

Pure water is the best at caring off heat , most anti-freeze has only 5/8 the heat transfer ability.

So even with only 35% anti-freeze , the usual minimum , about 10% of heat removing ability is lost.

FF
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Old 12-17-2008, 05:24 AM   #56
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RE: Why semi-displacement (or semi-planing)?

"Pure water is the best at caring off heat ..."

Before anyone decides to*cool their*engines with pure water, be advised that pure water will destroy the engine in very short order. A coolant treatment is absolutely required to reduce corrosion and prevent cavitation on liners.
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Old 12-17-2008, 05:26 AM   #57
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RE: Why semi-displacement (or semi-planing)?

During the past many decades my experience with very large diesels with radiators is they all used antifreeze and additives as recommended to keep system clean and* free from scale buildup. Scale buildup quickly negates any SG and thermal conductivity "gains " FF mentions. *Big earthmoving equipment I've been around always shuts down when not in use - up to 3000 hp anyway.*There is about a 5 minute warmup as the opeator checks fluids etc after a cold start.*Large standby gensets (size not necessarily Kw) we always kept in preheated circulating coolant state.*The physical size of a 1950s*era 2000 Kw* GM/DD diesel genset is impressive.*
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Old 12-17-2008, 09:34 AM   #58
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RE: Why semi-displacement (or semi-planing)?

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FF wrote:



"Pure water is the best at caring off heat , most anti-freeze has only 5/8 the heat transfer ability."


FF
Geez Fred, where do you get this stuff? You're Guru status may have to be reviewed!

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Old 12-17-2008, 02:15 PM   #59
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RE: Why semi-displacement (or semi-planing)?

"Geez Fred, where do you get this stuff?"

Been talking to Phil I think. FF is correct in saying pure water has better heat transfer characteristics than antifreeze but his figure is way off for the mixtures of water and glycol commonly used as engine coolant. The antifreeze mixture*transfers heat (thermal conductivity) at a rate of*around 85 to 95 percent that of water.

The thermal conductivity of pure water is .67 Watts per meter per degree K. Pure glycol is around .25 W/mK so a mixture of 50 percent glycol or less transfers heat at a rate pretty close to that of pure water. The dirt, *rust, and crud normally found in a poorly maintained cooling system will reduce heat transfer way more than glycol.
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Old 12-17-2008, 02:42 PM   #60
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RE: Why semi-displacement (or semi-planing)?

" ...the relative merits of antifreeze in a semi-displacement hull versus a full displacement hull...."

Well now, personally I believe the hull color has more impact.
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