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Old 10-10-2010, 11:08 AM   #21
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RE: Low Powered Cruiser

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nomadwilly wrote:

Tom,
The hull won't allow you to speed up anyway so what's the point? You can't use anymore than 5 hp per ton so anything in excess of that is not only a waste but always produces an underloaded engine.


You were posting while I was. FD hulls DO NOT NEED RESERVE POWER. Most people think they need reserve power to buck head winds but in practice one needs to back off instead.*If you have a huge engine in a FD hull and want to go faster you can get 10 - 15% more speed but it hardly alters you're situation at all. The real reason you NEED more power is FEAR. You are afraid you will be lacking some day for some reason but it's not true. I have never ever needed more power and I'm right at 5 hp per ton. Fear that you're engine is prematurely wearing out. Diesel engines run at 80% loads for at least 10,000 hrs and 99% of us will never see more than a small fraction of that. Another reason people want more power is so that they can run at low engine speeds. Less noise to be sure but mostly guys just want to be more like a Harley Davidson and loaf along rather than sing along at 2 or 3 times the rpm like other motorcycles. With a smaller engine at higher revs it sounds like you're struggling to keep up and while lugging it looks and sounds like it's easy * *....like you're loafing along. So It's partly a hair on the chest thing.
But as the 46 Nordhavn shows us * * *..there is no rational need.


And as Rick points out running an engine at low revs is less efficient.


-- Edited by nomadwilly on Sunday 10th of October 2010 10:52:08 AM
*
Willy*i couldn't agree more in reference to displacement boats. Size your engine to hull speed or maybe 5% more.
The only application in which all the extra power really makes sense is a semi displacement/ planing hull. The only reason I don't include planing hulls is that I don't like the way they handle at slow speeds. My lobster boat (with it's long keel) handles like a dream at low speeds. However, with it's low freeboard and round chine, it's not so hot *in three to four foot seas at eight knots. It wallows and rolls a bit too much (which could probably be remedied with ballast in the keel). But, that's when the throttle goes down and we get up to full cruise.


Carey

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Old 10-10-2010, 11:54 AM   #22
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RE: Low Powered Cruiser

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nomadwilly wrote:And as Rick points out running an engine at low revs is less efficient.
In the interest of intellectual honesty and to avoid the type of engine buffoonery posted by FF, when you quote me, please at least make a minimum effort to keep it in context.

A small recreational diesel is microscopically less fuel efficient at lower power and most operators probably use or waste more fuel by poor power management practices and bad boathandling than they would from operating at reduced power the rest of their lives.

And there are many reasons "reserve power" is useful. A fouled hull or prop, weight increases over the period of ownership, wind from most any direction and poor steering technique.

I don't think anyone is talking about doubling or tripling the HP required to move a displacement hull so the argument is right up there with angels and pinheads but I have met more boat owners who said they wish they had more power than said they had too much. They all have a throttle.

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Old 10-10-2010, 12:24 PM   #23
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Low Powered Cruiser

Rick,
I stand fully corrected on engine loading per your last post.


However I soundly disagree w you about "reserve power". *Weight increases should be taken into account at the beginning, steering should be learned and propellers should be cleaned.


Carey,



Yes but a full displacement hull should not be run at "hull speed" (as in 1.34Xthe square root of the WLL). *Hull speed can be maximum but FD boats should be run at 1.1 to 1.2 SLR * * *....not 1.34. !.34 is an expression of wave length not something to attain. I don't even know if my Willard can attain 1.34 (7 knots)







-- Edited by nomadwilly on Sunday 10th of October 2010 12:33:20 PM
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Old 10-10-2010, 01:00 PM   #24
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Low Powered Cruiser

The designer of the Coot, George Buehler, agrees with*Eric*about operating at 1.2 SLR:


"Id tend to cruise at about 1.2 or so speed/length ratio. 6 1/2 to 7 knots is a good clip, and the engine will easily do it. Notice the difference between 7.6 knots and 9; that translates to 1.2 gallons an hour to over 70 an hour.....

*
<center><table border="3" cellpadding="0" cellspacing="5" style="width:562px;height:223px;"><tbody><tr><td height="206" width="100%">

<center>*S/L Ratio..... Knots..... HP
1 ...... ..........5.63........ 3.9
1.1............... 6.19....... 6.0
1.2 ...............6.75....... 9.5
1.25............. 7.03...... 12.8
1.3............... 7.32...... 17.3
1.35.............. 7.6........ 23.5
1.6................ 9.0 ...... 149.3</center></td></tr></tbody></table></center>

So that's less than*9.5 horsepower for moving the 14-ton, 31.7-foot-waterline Coot on flat seas with still winds.* But that means even a 40 hp diesel is operating at only 25%.* Nevertheless, the designer specified an 85 hp engine.* I find this hard to reconcile with the recommendation to operate at 75%.* *What's up?* Inflated hp figures from engine manufacturers?* Lost horsepower at the transmission?* Or are we talking revolutions versus horsepower output where there might not be a linear relationship as in three-quarters maximum revolutions produces 25 percent of available horsepower?


-- Edited by markpierce on Sunday 10th of October 2010 01:06:54 PM
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Old 10-10-2010, 01:12 PM   #25
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The notion of putting a big engine in a displacement boat-- by which I mean an engine capable of producing significantly more power than the hull can put to use---- makes little sense unless one just likes big engines

However I think sizing an engine at least a bit more powerful than necessary is a good idea for a displacement boat. I am of what is probably the pretty old school of engine operation adhering to what I was told years ago by one of the most respected mechanics (and pilots) in aviation (who also had and maintained himself a deFever with smaller Cat engines) who in answer to my question of what could I do to ensure long engine life said, "A piston is only going to go up and down so many times. So the easier you make life for the engine, the more times the piston will go up and down." By which he did NOT mean to run it at settings that were too low.

So if that philosophy still holds any water, I would think a somewhat more powerful engine in a displacement boat would yield a comfortable margin between running wide open all the time to achieve cruising speed and running at a more conservative throttle setting which would help keep heat down, etc. Maybe this is not true with today's generation of diesels, I don't know.

As has been noted by others, a displacement hull cannot be powered significantly faster through the water to counter a strong opposing current or to outrun an approaching front. But I think there may be times--- very rough water, for example--- where having a bit of a reserve of power might be useful. Not to go faster, just to help the boat handle the slop. But I've never run a displacement boat other than a 20-ton narrowboat in the UK where the roughest water one encounters are the wakes from the swans paddling past. So I don't know if having a reserve of power would be helpful in a displacement boat under rough conditions.

The other thing to consider is the auxiliary loads on the engine. In addition to moving the boat is it going to have to power an oversize alternator or a hydraulic pump? Is there a likelihood of significant marine growth on the bottom and/or running gear that could generate additional drag between hull-cleanings?

Plus I would imagine that selecting power for a recreational displacement boat also comes down to what's available. There are only so many marinized engines to choose from. If the hull specs on a boat indicate that 35 hp is needed to move hull-x through the water at or near its displacement speed but the only engines available near that rating that will properly fit the boat are a 30 hp and a 75 hp, which should be chosen? To my way of thinking the best choice would be the 75 hp engine.

-- Edited by Marin on Sunday 10th of October 2010 01:15:00 PM
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Old 10-10-2010, 04:57 PM   #26
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RE: Low Powered Cruiser

A very*bulletproof engine in the size range you are considering*is the Cummins 4B rated at 85Hp. Trans Atalntic sells them re-conditioned with the* transmission as a package. On the west coast Seaboard marine does the same. I was just looking at the RPM and prop curves and they*look pretty good for your intended task. The 4B will go*all day at 1600 RPM burning less than 1.2 gph.
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Old 10-10-2010, 06:26 PM   #27
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RE: Low Powered Cruiser

Mark,If those numbers on the Coot are accurate I'd be look'in at the Isuzu 54 hp or Yanmar 4JH at 55 hp. Then cruising at 1.34 will still set you up at less than a 50% load on the engine.
Not Ideal but better than 35 to 40%. I think I'm running my Mitsu at 60 to 65%. Seems to be a good balance between noise and under loading. 23 hp would consume about 5 quarts an hour and even w 54 hp the engine would still be under loaded. Ask George Buehler if he would recommend 54hp.
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Old 10-10-2010, 06:59 PM   #28
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RE: Low Powered Cruiser

I recall chatting to the owner of Bagan, the now famous Nordhavn 57, a few years ago when he was visiting New Zealand. He had just arrived from Fiji and made the journey at an average speed of 11 kn* since it was "only 1100 miles".

Ths is an S/L of 1.5 which could only be achieved with quite a few more*hp than a "correctly sized engine" could deliver. I believe it was a Lugger 325.

Our 53 ft lobster boat with 540 hp runs happily at 9.2 Kn at only 1250 revs. I calculate this to be using 90 to 100 hp. The previous owner also did this for days at a time - interspersed with hours of idling tending nets and a mussel farm. The engine had run for 18,000 hrs before*its rebuild.**Tony Athens,**moderator on boatdiesel.com says he has yet to see an engine damaged from underloading, they just tend to last longer.

I believe there is a good case for a bit of additional hp for both displacement and semi-displacement hulls, which most of us have.

I sure like to get her up on the plane at 12 kn when the weather cuts up, she definitely handles the sea*better. It's also fun going WOT after relaunching*with new antifouling and cracking 18kn - admittedly with a knot of tide under her!*
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Old 10-10-2010, 07:27 PM   #29
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RE: Low Powered Cruiser

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Marin wrote:I was told years ago by one of the most respected mechanics ..."A piston is only going to go up and down so many times. So the easier you make life for the engine, the more times the piston will go up and down."
Marin, you sure can find the whacko experts!

That statement not only contradicts itself, it is just flat out wrong. An engine that lasts a few thousand hours more than another one of the same because it was operated at lower power has pistons that went up and down quite a few more times its short lived higher power cousin.

If that guy had said an engine will only burn so much fuel before it wears out then he might be worth listening to. Many engine manufacturers are now listing TBO as pounds of fuel burned because that is directly related to the power produced and an engine will only produce so much power before it wears out.
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Old 10-10-2010, 08:24 PM   #30
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He was saying this not as a literal fact having to do with how many times a piston actually goes up and down but simply as his favorite way of expressing his view that an engine will eventually wear out to the point where it needs an overhaul, and the "kinder" you are to your engine, the longer it will be before it wears out.

I'm not sure I'd characterize Bob Munro, one of the three founders of Kenmore Air Harbor and the pilot/mechanic who ran it for the next 54 years until his death in 2000 as "whacko" Kenmore Air, if you don't know, is one of the the world's most successful and long-lived seaplane operators/aircraft rebuilders. I suspect Bob knew that the pistons in an engine that ran for 10,000 hours before needing an overhaul went up and down a few more times than an engine that ran for 8,000 hours.

Anyway, I don't see how your statement "An engine that lasts a few thousand hours more than another one of the same [kind] because it was operated at a lower power has pistons that went up and down quite a few more times than its short-lived, higher power cousin" is any different than what Bob said--- "the easier you make life for the engine the more times the piston will go up and down." Unless I misinterpreted your statement.




-- Edited by Marin on Sunday 10th of October 2010 08:31:46 PM
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Old 10-10-2010, 09:23 PM   #31
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RE: Low Powered Cruiser

The way I read it was that he was saying there are only so many "piston miles" available before an engine wears out. But then he said that lower power output will return more "piston miles."

The first satement is wrong, the second is true, that's why I said it was both wrong and contradicted itself.
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Old 10-10-2010, 10:21 PM   #32
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RE: Low Powered Cruiser

I can see why you said what you said and I don't disagree with you. But Bob's statement was not intended to be a technically correct statement but simply his way of expressing the notion that the easier you make life on an engine (while still operating it correctly) the better and longer service you will get out of it. The phrase "A piston is only going to go up and down so many times" is something his pilots tended to remember in the context of "don't abuse the engines in the planes." I obviously did

I suspect that in a more technical discussion he may have agreed with your fuel burn or power produced method of talking about potential engine life.
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Old 10-11-2010, 01:09 AM   #33
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RE: Low Powered Cruiser

Seems that we're not adequately considering the need to produce electrical power to recharge batteries and power the refrigerator, running lights, air-horn compressor, GPS, cabin lights, depth-finder, radio,*electrical pumps, search light, and radar (while omitting non-essentials like electrical stoves/oven, air-conditioning, and electrical heat).* So, how much horsepower is going to be used to generate power in the typical medium-sized trawler in addition to moving the boat?
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Old 10-11-2010, 04:21 AM   #34
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RE: Low Powered Cruiser

how much horsepower is going to be used to generate power in the typical medium-sized trawler in addition to moving the boat?

Most small boats have a hard time using the full output of a 6KW noisemaker.

Engine sizing to create a noisemaker is 2 HP per KW.

So the actual load will be less.

Efficiency does play a part in HOW you chose to create the energy.

My preference would be a 300A 24V alternator and a 4KW inverter and load sharing.
Bigger boats can parallel the inverter is needed.
Advantage , the ability to charge a massive batt set quickly in coastal cruises.
Inverter sine wave electric will operate big morors with efficiency.

Offshore choice for the longer passage folks would be a Hyd power pump and a 4KW or 6KW power pack. This could be driven by the noisemaker / get home as needed.
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Old 10-11-2010, 04:27 AM   #35
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RE: Low Powered Cruiser

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markpierce wrote:So, how much horsepower is going to be used to generate power in the typical medium-sized trawler in addition to moving the boat?
Read the dataplates and add the power requirements. Figure out how much you want to consume at any given time and for how long. Figure out how long and by what means you want to power those consumers. Think about how, and for how long, you want to charge batteries.

You need to convert it all to a common unit, Watt, BTU, hp, whatever you feel comfortable with and go from there. Remember that everyone of those units comes on board at the fuel dock unless you have a long extension cord.
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Old 10-11-2010, 08:02 AM   #36
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RE: Low Powered Cruiser

Mark - Consider two alternators, a Cummins is easily set up for this. That way you can easily split the load to batteries on one and other stuff on the other. Two 120 amp units would do it nicely - as rickB and FF note, add up your* loads plus assume you will be (through the inverter) cooking your potroast in the sink, coffee pot on, microwave doing bagels and hairdryer running etc*while underway.
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Old 10-11-2010, 11:50 AM   #37
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RE: Low Powered Cruiser

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FF wrote:

Most small boats have a hard time using the full output of a 6KW noisemaker.

Engine sizing to create a noisemaker is 2 HP per KW.

So the actual load will be less.

Efficiency does play a part in HOW you chose to create the energy.

My preference would be a 300A 24V alternator and a 4KW inverter and load sharing.
Bigger boats can parallel the inverter is needed.
Advantage , the ability to charge a massive batt set quickly in coastal cruises.
Inverter sine wave electric will operate big morors with efficiency.


My boat will have a 24-volt system with an 8KW toroidal isolation transformer.* Don't know what the amperage of the alternator is.

So, with an 8KW transformer, as much as 16 horsepower would be needed from the engine, representing 18% of the maximum horsepower output of an 85 hp Deere??

*
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Old 10-11-2010, 01:12 PM   #38
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RE: Low Powered Cruiser

Do you realize*that your last post made absolutely no sense at all?
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Old 10-11-2010, 02:37 PM   #39
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RE: Low Powered Cruiser

An isolation transformer is only needed when the engine is off and you are plugged into shore power. I suggest you source and engage a GOOD proven*marine electrician to assist you in this effort and ignore all of us.
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Old 10-11-2010, 03:26 PM   #40
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RE: Low Powered Cruiser

Yeah.* I'm going to find out the alternator's capacity.
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