Why semi-displacement (or semi-planing)?

Walt, Marin, Sunchaser,

Well .. as I see it Guru status is earned 100 % by quanity, 0 %* from quality. The Lehmans are'nt fuel hogs .. Marin is just running them hard enough to almost get out of the underloading zone. Sunchaser, there must be a missing link in your numbers as that seems to be a very big boat to go 8 knots on less that 5 gph unless it is a FD hull which it is not.

Eric Henning
Willard 30
Thorne Bay AK

Defever is full displacement.

RickB wrote:


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

That is pretty much nonsense.

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I dunno....* That line, although not the exact wording as I don't have the manual with me here in Xiamen, is right out of the operating manual for the EMD GP-30.** Granted, the GP-30 is an older second-generation diesel-electric locomotive.* The current generation locomotives like the GE Dash-9*utilize much more advanced systems--- pre-heat, etc.--- that make long warm-ups unnecessary.* Perhaps the difference between the warmup requirements in earlier*locomotive applications and marine applications was due to the loading the engines will be subjected to?

And I agree with the other gurus--- the primary determinant between the speed and efficiency characteristics of displacement and semi-planing hulls is in fact the color of the hull and the type of anti-freeze in the coolant.* Hull shape, power systems, etc. are irrelevant.



-- Edited by Marin at 03:36, 2008-12-18

Sunchaser is indeed Semi D. Running modern diesels, well matched props, at hull speed, clean bottom are the answers. Many boats show this kind of efficiency. Suggest you read Dashew's website re Windhorse which is the best chronicled expample of theis subject. Unless my hour meters are off (they are not) I'm a believer.

Okay, a Defever 44/49 is a full displacement with a square ass and sharp chines.

sloboat wrote:

The 44/49 hull looks like a half breed.* Full keel, chines are slightly rounded, and the aft sections of the hull are quite flat.* And there's that big square a$$.* Don't believe Nomadwilly would allow it into the club.*

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And a Nordhavn???* Square assed....hard chined....

Sloboat, Baker,

Never seen a Defever with a soft chine and as far as I know they are all SD. John, Iv'e been worried someone would mention the Nordhavn's in our duscussions about SD/FD. The Nord 46 is clearly a FD with 100hp and a disp of 48k .. about double that of a GB36. That should show how much TOTAL power would be required for a GB36 IF .. IF it was a FD hull. The N46 is the only Nordy I'm really famillar with .. Don't care to learn about the others as their so ugly. But they don't LOOK like a FD boat. I think the 35' Nordy is SD as I seem to recall pics of them at speed greater than FD. As for the rest .. if they have 5hp per ton or less and achieve hull speed they must be FD.

Eric Henning

I hope it is attached - a picture from my recent haulout of Defever 48 year 2003 model. *It is almost a round(ed) bottom but has a hard chine. No flat sections though. Again, a wonderful efficient Art Defever design.

Hey Guys,

That Defever is beyond a doubt SD. See that big flat transom extend way below the waterline ? That causes a tremendous amount of boiling and frothing astern at less than 10 knots .. big time drag. The quarter beam buttock lines are fine ahead of the transom for SD and if you made this boat light*enough for the transom to come completly out of the water it would indeed become a FD hull. I see the slightly rounded chines in the pics .. not round enough for any FD or SD benifits and not sharp enough for a planing hull. Defever probably made them that way for layup, strength and lightness benifits not to mention velnerability ( to damage ) and repairability. The roundness will reduce some of the tendency of a sharp chine to impeed rolling, however.*
Slowboat,
I see you never did get any feedback on the double chine. I have had a boat ( semi fast ) that had very soft ( rounded ) chines and when at speed that boat would over bank something fierce. Doing figure 8s I thought I was going to fall out of the boat it banked so much. This is a guess ( I think a good one ) but I think the doubble chine is to break the lateral flow of water so it dosn't suck the inbd side of the boat down so much .. flatter turns. Also in a straght line that same boat would never level out .. the bow was high no matter how fast one went. In a planing hull the chine is sharp to allow the water moving side ways to break free without sucking the stern down. AS I recall there was a more or less standard modification to the Bayliner 32' Explorer that suffered from this in addition to the tendency for the tunnel prop cavities to suck the stern down. In the doubble chine design ( I still think we should have a " design " thread ) being broad side to a breaking sea would have a significant advantage over a simple hard chine design ( that word again ) in that GB and other sharp chined boats do tend to trip on thier chines and thus .. capsize. One last thing .. getting the flat surfaces above the water line may make the boat quieter at anchor. Thats all I've got on double chines.

Eric Henning

-- Edited by nomadwilly at 17:15, 2008-12-18

In looking at the picture of 49 Defever stern section on the 2 above post, it is very different than my 48* stern section pictured above. By adding the 5' extension to the 44, they flattened out the aft sections and left the rudders forward. Note how on my 48 the BIG rudders are where they need to be - far aft! My guess is Art DF would not like this 5' extension - I'll ask him. Art's lament is too many yards used his plans and bastardized them. Many DFs he will not lay credit to for these reasons. In general, most*hull extensions are poorly conceived, especially with props and rudders left forward. PIcture* paddling a canoe from midship.

Monk insisted that they move the props and rudders to the back of the boat.

Sounds like a proper solution , BUT in the NYC dinner cruise business there was a big hassle with finding boat drivers with over a 100T ticket.

Since the Hooligan Navy considers a boat to end at the rudders (true in the 1800's) to document a boat small enough fora cheap hires ticket , the rudders were simply moved forward till the boat was "under" 100T.'

With both bow and stern thrusters they could (not easily) be docked in the NYC 4k-5k tidal flows.

FF

*What about these square assed Nordhavns with chines...what
exactly are they?* Why would the acknowledged king of the blue water trawlers put a square stern on their blue water boats?***


-- Edited by sloboat at 16:34, 2008-12-19

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internal volume easier acess to engine steering gear ect
more interesting is why they are sticking 5 foot cockpit extensions on some models not moving the prop or keel and moving the rudder back 18 inches I await the sea trials with interest

This is one of my favorite subjects but even I can get too much of it at times. I have had several opnions on how to define the difference. Some have been rather black and white and others have been a bit inspecific. One of the latest, best and quite specific is to look at the stern and observe that the water can flow smoothly from amidships back to the level of water at the stern. The transom on a typical sail boat is above the water for this purpose. Even at a SL ratio of 1 to 1.2 the wake at the stern will be quite active but there will be no frothing or boiling in the wake. The ugly mess astern of a SD or planing hull at 7 to 8 knots will not be present behiend a FD hull. The wake is more structured and the following wake is well defined close to the transom. My rule was that if any part of the transom was below the static WL at rest the hull was not FD. By this definition DFs, GBs, NTs, all TTs ( that I know of ) are all not FD. One should take into consideration the quarter beam buttock line. the steper the angle the more the hull is FD and the flatter the QBB the more SD or Planing. The QBB line is a line drawn along the aft end of the hull bottom
halfway between the chine and the keel. There are some boats, vessels that are clearly FD in every other way like many Alaska State ferries and some very heavy trawlers like some of John Seatons designs. I liked that definition because it was difinitive and something I could comunicate to others. A long time ago I thought a boat was FD if when, while increasing speed there came a time ( speed ) where one could observe the bow beginning to rise ( I pronounced this hull speed ) and if the boat couldn't go much faster and was not under powered it was FD. All my definitions have been performance or hull form related. Summer before last I ment two Naval Architects on the ferry and they explained it very well. I wish I had taped the conversation as I can't remember the def exactly. The center of the vessel would rise or sink when it reached hull speed .... or something like that. Perhaps someone on the site knows something about this ???. A black and white definition may not exist. One can observe the performance of various hulls and come much closer the truth on this issue.* I have done this for years and the hp per ton opens doors to firmly placing this hull nor that hull firmly in it's propper catergory. The submerged transom formula will get you very close. It takes 2 to 3 times as much power to drive a typical SD hull than a FD. The GB36 and the Willard 40 comparison is easy to buy. The GB takes 50 hp to make 7 knots and the W 40 takes 23. And if thats not strong enough numbers consider the fact that the W 40 is much bigger and heavier. One does not need to be a NA to say if you can drive a 17 ton boat 7 knots with 23 hp it must be a FD hull or an extreem design. Summary; comparissions can get you very close and there is some grey area and overlap .. like a lot of the Nordhavn's.

Eric Henning

what exactly distinguishes a full displacement hull from a semi displacement hull.

Look at most any cruising sailboat , and you find a FD hull.

The "racing" rules demand unseaworthy shapes , to gain a bit on ratings.

A slightly immersed transome (about a foot wide for every K ) seems to do no harm , provided it is barely immersed.

Why would the acknowledged "king of the blue water trawlers" put a square stern on their blue water boats?

Perhaps "king" of marketing an image , but would hardly be my choice , unless the GREENS (ex reds) were coming to put boaters in the Gulag , and I were in a big hurry.

Tho probably a motorsailor would be first choice ,,,

A wide stern can be used to increase cabin sizes , or if a cockpit boat EZ access to the dink.

Today, well this century ay least, one of the better sites to read carefully is Dashew's setsail.com. In building the MV Windhorse, he posted design basics and details. I'd say Windhorse, derived from very carefully CAD and finite element analysis programs,*has set the bar. Dashew is a sailboat hull designer of note, and this carried over to his power boat designs. The debate in this thread should rise above SD vs FD as that is missing the point of efficient hull design and seaworthiness*- except of course for those who have a very narrow and low $$$ FD (read canoe) mindset as a mantra.

Sloboat,
Staticly it's a combination of the quarter beam buttock line and amount of emersed transom. Dynamicly it's a matter of objective comparisons like the GB at 7 knots at 4gph and the W40 at 7 knots at 2gph with the same engine. Chines are not that important unless you want to plane. As for wide*square sterns they are always a big negative in following seas so I am amazed that they exist on the Nordys. The lobster types were much narrower 50 years ago and it can be observed that they require a very large rudder to control them. You'll notice Dashew's boats have narrow sterns. You often mention a " prop curve ". Is it a graphical presentation of how much hp is required to turn a specific propeller specific speeds on a specific boat? If so how do they arrive at thier numbers .. sea trials?. Not likely .. that would take a lot of time. I did have a Sabre Ford 120 ( same engine as Lehman ) that had a fuel burn curve that I think was derived from typical experience but a curve for a FD hull would be much different than a SD. I think your propeller curve may be a series of approximations. Enlighten me.

Eric Henning

-- Edited by nomadwilly at 13:24, 2008-12-20

-- Edited by nomadwilly at 13:25, 2008-12-20

I don't mean to be critical ( I am a critical person, however ) but I think you may have too many variables. We talk about speed over water issues and use speed over ground. We talk about hours under way at cruise speed and use total hours including lots of time at well under cruise speed or at no speed at all. It seems like fly stuff that shouldn't make any difference but all stacked up it can lead one from the truth. Nobody should say they burn X gph. They should all say they burn " about " X gallons per hour. There is , of course, one exception .. one who has a really good flometer .. but even then the dominos could stack up. Full tanks, lots of provisions, spare outboard, winds and seas and people on board .. or even all sitting on one side of the boat ect ect.
Does your prop curve come from the engine manufacturer or the marinizer? Your curve is for a SD boat. A 34 CHB and a GB are both SD boats but, quite different. The CHB is almost a FD boat that should be good at 6 or 7 knots but poor at 9. If it was a FD it may not even go 9. The GB do 9 very well but is harder to drive at 6 or 7. All we need to know about fuel consumption is basically " can I get to point B" and " *can I afford fuel for this boat or the one I want to buy".
Your'e right. These discussions are full of opnion, anecdote's and rules of thumb. Many are very useful like 20 hp per gallon. There is hard and fast numbers like 1.34 X sr*WLL but even here there is the " effective WLL*" that has to do with fullness at the ends of a vessel. There are a lot of specifics and numbers in NA but much intuition, aproximations, tendencies and creativity also. Engineers go for the numbers ( perhaps you are one ) and the rest of us live in the vauge world. The best NAs have split personalites of course. Unlike cars I think there are bad designs in yachts. With an airplane lots of standards must be dealt but with a yacht almost any kind of boat could be built and sold. It's not much of a problem except for stupid stuff like bulsa core construction.*
About the Nordhavn .. they don't look like they would be good in a following sea, they don't look like they would be good in head seas and they don't look like they would be very good in calm seas*either. I like the way they shape the trailing edge of the keel for smoother flow into the propeller though. Seriously .. they cross oceans regularly and have a fine reputation so you may be right ..*perhaps they have tricks. I'll look in my old PMMs and see if I can learn something.
Sunchaser. No. This thread is to discover why most trawlers are SD. If you want to talk about design efficency and seaworthyness, hit Baker up for a design thread. Otherwise be an outlaw let it all hang out here.

Eric Henning


***

-- Edited by nomadwilly at 00:20, 2008-12-21

An important downside of a Nordhavn is their inability to move "fast" while being efficient. The upside on the efficiency scale for a Dashew design is their ability to move "fast." As FF noted many posts ago, narrow is good. Fast is the extra 150 miles per day*one can make on a bluewater trip. The world's weather systems can safely be avoided with that advantage. Nordhavn logs are replete with the big weather they encountered. MV Windhorse logs are replete with the big weather they avoided.

But most would not like the narrow Dashew design as it does fit with coastal cruising. For the coastal cruising 95% of which we all do, beam is good ie spacious and fits in normal slips. So why does a wide Selene not equal a wide Nordhavn? Too big an engine and too small a tank(s). I'd of thought Howard Chen et al would have that figured out by now.

" As FF noted many posts ago, narrow is good. Fast is the extra 150 miles per day one can make on a bluewater trip."

GOOD is as always subjective, if speed has value skinny & light is the winner.

But "efficency" can also mean miving the most goodies with the least fuel.

At low speeds , under SL 1 skin friction is most of the resistance , the less the better.

A beach ball would have the least skin resistance for any given weight , but it looks too like a Bayliner for my taste.

For most folks the biggest concern is cost per mile cruised , with speed a lesser concern.

The interesting part is when one stats to play with the numbers .

Would it be more fun to travel at 18K at $10.00 an hour in a comfortable minimal boat , or wallow along for $10.00 an hour in a party barge at 5K ?

NO question the party barge will be more acomidating at dockside , but the vast volume has little use , and could be dangerous underway.

You pays your money and,,,,,

FF

Sloboat,

Seems I have a nose for engineers eh ? I'll take back the too many variables issue ( mostly ) but I still am sure FD is at least two times as efficent as SD and probably more. A 500K home is a world different than one for 1 million. I agree with your page 4 post that there is much room for improvment of efficency in present SD hulls. The majority of improvement will probably not be rocket stuff but simply reducing beam at the WL and reducing displacement. Looking at the boats of the teens and twenties will be great food for developement.*
My boat is so frozen in the ice it won't budge at half*throttle!

Eric Henning*

Also, Dashew's boats have little to do with what's out there in the "production" ranks.

Dashew is concerned with VOYAGING , not 1 in 10,000 or so is looking to do real ocean work.

Production boats are to be MARKETED , if its great as a condo , with the bride , 90% of the job is done.

Efficency is how fast the bucket gets SOLD , hardly how it performs.

FF

Knight and Carver in San Diego built a demonstrator for the M-hull and they run it around the bay from time to time. It is amazing to see it scream along while producing virtually no wake.*

Here's a link to*one pretty good article, there are many more for the searching.*

http://www.imarest.org/mer/MERSamp8.pdf

Oh gosh, is this Bat Boat a FD or SD? There is on off-shore Swedish racing design that is loosely similar and has enjoyed great success over the past decade. The interesting question for the Bat Boat is rough water ( +3" seas) ride. Catamarans and the old Chrysler tri-hull tend to*pound and bang*in heavy weather as compared to a mono-hull. Once the wave height reaches the Cat deck, some interesting noises and pounding occurs. Would this issue limit the Bat Boat's popularity? With government money paying for this study it will be fun to watch. By looking at the Bat Boat's*soft chine it must be FD.

just discovered this subject thread and had to make a posting so I remember to return and read thru it

FF said:

Also, Dashew's boats have little to do with what's out there in the "production" ranks.



Production boats are to be MARKETED , if its great as a condo , with the bride , 90% of the job is done.


FF

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Thanks, great line. I will use it in the future.

Marty