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Old 03-11-2011, 07:20 PM   #41
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Quote:
markpierce wrote:


Moonstruck wrote:

*The formula for hull speed is 5/7 times the square root of the water line length.*
I've always known hull speed described by the formula "1.34 times the square root of the waterline length."



http://www.psychosnail.com/BoatSpeedCalculator.aspx*
You are right, Mark.* I reversed 7/5 which is the way I was first taught.* 1.34 was the way I was later taught.*

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Old 03-13-2011, 06:38 AM   #42
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While "hull speed" might be the sq rt of the lwl (SL) x 1.34 , it is only the speed that moat sailboats attempt , as the wind is FREE!

Most power boats will prefer SL x .9 to 1.15 as that will usually take half or on a better hull 1/3 the fuel burn of "hull speed".

Usually that extra knot is far too expensive for most cruisers.

The difference between 6K and 7.25K is only an extra hour of put puting.

The difference between 10 hours at 2 GPH and 6GPH can be impressive at the fuel stop.
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Old 03-13-2011, 10:50 AM   #43
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Hull speed is an issue that only has to do w full disp boats. The dynamics of hull speed and the math that applies to it just is'nt meaningful w semi-disp hulls unless they are VERY close to being FD hulls and on this forum almost none are in that category. Maybe none. On any boat there is it's bow wave and all the smaller waves that come after it. The trick is to run your'e FD boat right in the middle of the bow wave and the 1st following wave. In the pic below see that the boat is basically w her bow on the bow wave and her stern on the 1st following wave. This boat is at WOT and it looks like the crest of the 1st following wave (at her stern) is just a bit aft of the stern. This is perfect for a FD boat. With a planing hull or a semi-disp hull the boat can go fast enough so the 1st following wave comes up some distance aft of the stern. But a submersed transom and a rather flat stern w rather straight lines fore and aft are required to allow the hull to go that fast. Also much more power is required. FF makes a very important point that FD hulls should run well under hull speed. My own Willard runs at 6 to 6.5 knots while cruising (6.15 mostly) and has a hull speed of 7 knots. Most GBs (say a 36) run about 8 knots and would not be able to do that gracefully if they were a FD design. Only a few trawlers are FD.
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Old 03-14-2011, 04:59 AM   #44
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To my mind the Semi Plaining boats are not great to cruise.

The under body shape required to SP is far from ideal at displacement (normal) speeds , so the cruising fuel burn is always higher.

And over sized engines must be maintained and fed , for the chance of a few K.

The SP boat never actually gets up on a plane so the "over the hump" speed is never reached , causing higher fuel burn at lower speeds than a true plaining boat would get.

One nautical mile per gallon would be a gift from heaven as some of these SP get all the way to 15K.

Anyone wanting to go cruising at 18K plus , and do it with reasonable fuel burn MUST get a boat designed to go these speeds.

What happens to engine life and fuel burn and handeling at slow displacement speeds is up to the experience of the boats NA.

Some can actually do it!
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Old 03-14-2011, 12:25 PM   #45
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RE: New Member

thanks guys, guess it really makes sense when talking FD hulls.

how about larger freight ships, i have been onboard some and chatting with the Cap i was astounded to hear they were cruising at 25+ KTS.
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Old 03-14-2011, 01:52 PM   #46
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RE: New Member

Quote:
Per wrote:

thanks guys, guess it really makes sense when talking FD hulls.

how about larger freight ships, i have been onboard some and chatting with the Cap i was astounded to hear they were cruising at 25+ KTS.
Typical freighters go below*displacement-hull speeds, typically in the teens.

Cruise ships also travel well below DH speed.* For instance the approx. 900-foot LWL of the Norwegian Gem indicates a max. DH speed of about 40 knots but the ship is capable of less than 25 knots.* The former*ocean liner QEII was much faster at over 30 knots but still not over DH speed.

On the other hand, catamaran ferries in the SF Bay are capable of over 37 knots, several times more than DH speed.
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Old 03-14-2011, 06:50 PM   #47
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Hi Fred,Seems odd you'd say "To my mind the Semi Planing boats are not great to cruise." since you have one and almost all boats on TF are SP/D boats. But since you have one I guess you'd be qualified to criticize it. Marin made a good point about the SD hard chine boat's suitability for trawler cruising in that stability is maximized and in view of the fact that trawler people or other power boat people are dead set against tippy or roll prone boats the typical trawler seems quite well suited to cruising. All that room afforded a wide square assed hard chine boat delivers another extremely desirable trait for a good cruising boat * ....space * ..as in how do you say it FF? ....roomarian? Sorry Fred but I think they make good cruisers. And cruisers is what they are * * .......heavy cruisers. And nobody can argue that weight is good for range and low fuel burn.


Mark,
Good points. Rob's 40' Willard requires less power to go 7 knots than my 30' Willard.
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Old 03-14-2011, 09:01 PM   #48
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EricI too was confused by FFs statement, for the same reason. I have a Semi planing *hull, and find it great at all speeds. Mine is however soft chined, and can be rolly at slower speeds in big water (3 ft and above), so I simply kick up the speed and it's much better. At fifteen knots I do get 1 mpg, and when calm waters allow me to cruise seven or eight, I am getting over 2 mpg. How about a little more explanation FF.


-- Edited by Carey on Monday 14th of March 2011 09:03:26 PM
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Old 03-14-2011, 09:13 PM   #49
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Quote:
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Hi Fred,
Seems odd you'd say "To my mind the Semi Planing boats are not great to cruise."

And cruisers is what they are * * .......heavy cruisers. And nobody can argue that weight is good for range and low fuel burn.


Eric, you had to know that as soon as you said "nobody can argue" that someone would argue....

Heavy displacement boats have always been preferred as ocean passage makers because of a kindlier motion.* So I guess it depends on your definition of a "cruiser".* I loved my SD Albin, and felt it could shoulder its way through in shore slop very comfortably, but I harbored no delusions that it would be the hull form to take offshore, unless I was towing a fuel barge behind me.* At slow speeds in slop, SD hulls are noticeably less comfy, at least in my experience.* Coming into Barkely Sound from the Pacific in a near gale comes to mind as a particularly uncomfortable ride that would have been enhanced with seat belts.

Delfin weighs 130,000#+, has a round hull with 7.5' of draft and doesn't leave a wake at 6 knots.* At 7.2 knots, she burns about 2.5 gph so I am going to have to argue that heavy when packaged in the right hull form with the right prop is good for range and fuel burn.

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Old 03-15-2011, 01:02 PM   #50
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As you point out weight can be good

BUT

not for better range or lower fuel burn.

I reread your post and see your point that weight (assumably along w strength and otherwise good design) can allow a boat to go across the water far enough and at any time to make it possible to have great range. And at certain speeds a bulbous bow can reduce fuel burn and one can't have one without added weight. So I do stand corrected. But if you added weight to the boat without adding very carefully chosen shape changes mpg would decrease.












-- Edited by nomadwilly on Tuesday 15th of March 2011 01:20:26 PM
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Old 03-15-2011, 02:36 PM   #51
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Eric, I had no idea you were such a damned handsome young guy instead of some old fart from Alaska.

I think we agree on the main point.* Adding weight doesn't add efficiency, however, it isn't automatic that heavy means less efficient.
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Old 03-15-2011, 02:48 PM   #52
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RE: New Member

Quote:
nomadwilly wrote:

As you point out weight can be good

BUT

not for better range or lower fuel burn.

I reread your post and see your point that weight (assumably along w strength and otherwise good design) can allow a boat to go across the water far enough and at any time to make it possible to have great range. And at certain speeds a bulbous bow can reduce fuel burn and one can't have one without added weight. So I do stand corrected. But if you added weight to the boat without adding very carefully chosen shape changes mpg would decrease.


I was reading that a bulbous bow does not help with boats under a certain length.

I wonder why??

SD

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Old 03-15-2011, 06:09 PM   #53
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Skip,
Don't know. But I doubt if it's true. BBs may be of little or no value on long vessels that run at slow speeds. And it looks to me that just extending the bow has more advantages than installing the Bulbus Bow. It seems silly to me to have a BB that dosn't extend to the top of the stem when one could just have a more vertical stem.
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Old 03-15-2011, 08:11 PM   #54
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My understanding is that there are many benefits from a bulbous bow.* One is to extend the LWL, which increases hull speed and hence drag at a given speed.* The BB also disturbs the bow wave, reducing its drag on the vessel.* And, there is supposedly some reduction in pitching, which stabilizes trim and also reduces drag, all resulting in increased fuel efficiency.* They aren't used on vessels shorter than 50 feet, and increase in effectiveness as speed and length of vessel increase.* I don't understand the physics of why shorter vessels don't benefit to the same degree as londer ones.* FF is an NA, as I recall, and may be able to explain it better.
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Old 03-16-2011, 04:32 AM   #55
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I'm No NA , but the problem with the BB is it can not just be stuck on with hopes that it will work.

Even the big boys like Carnival Cruise MUST do tank testing to get it optimized.

The other hassle is the BB is only really great at ONE speed.

For a Cruise ship or an box boat or oil can this is easier than for a yacht.

With the slower cruise speeds of the economic slowdown (keeps more boats required) the ships are frequently being modified to a different bulb shape to optimise a lower cruise speed.

One advantage on smaller boats as noted ,is a lower pitch moment , if it was done right , but it takes larger seas than most will choose to operate in to notice.
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Old 03-16-2011, 09:13 AM   #56
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The actual WL or WLL is not increased but the BB effects the dynamics as though it was.
True, FF. Only good at one speed but that speed is where the boat or ship w the BB will be run most all the time. My Willard cruises at 6.1 to 6.4 knots as is. With* a BB added it may/probably will cruise at 6.5 to 6.75 knots w the same power (approx) that it took to cruise at 6.1 to 6.4 knots before but if I was to slow down to 5.8 knots it would take more power than before. And at 5 knots ect on down but with the BB maximum speed will be increased. As for added benefits reduced pitching and plunging is it as far as I know. Carl, perhaps you know of more. And in order to be able to withstand pitching and plunging in big head seas the BB must be really strong and that means lots of extra weight. To achieve the speed benefits on my Willard I'd rather extend the stern.
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Old 03-16-2011, 01:02 PM   #57
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Off topic but,* Eric I like your new avatar,* displacement hulls are beautiful to look at!

Sorry, Back to the BBs

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Old 03-16-2011, 06:18 PM   #58
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Kinda sorry about the topic. We have a big problem w that. When I scan the threads to see what's up and what I may want to read or talk about most of the time after a page or two the discussion going on has absolutely nothing to do w the listed thread. John was a good nag for about a year, at which point (w his head rather bloody) he more or less gave up. Don't blame him. If I thought something could be done about it:
1.* I wouldn't be off topic now and
2.* I'd be starting a thread about it.
I don't see what can be done but if a thread appears I'll participate.
Now I've forgotten the thread topic.
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Old 03-16-2011, 09:55 PM   #59
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Quote:
nomadwilly wrote:

When I scan the threads to see what's up and what I may want to read or talk about most of the time after a page or two the discussion going on has absolutely nothing to do w the listed thread.
Frequently that's a good thing.

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