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Old 09-15-2013, 08:18 PM   #221
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Tony wrote;

"The above info is my understanding of the whole thing. That don't necessarily make it right"

I should put that disclaimer on half (or more) of my posts. I get into trouble but not much that I know about.

And I think as I believe you do that too much importance is placed on hull speed. With a FD boat running at HS is something you'll do only briefly and being able to attain it is not important at all. What happens 1/2 to 1 knot below HS is very important and it's the speed you'll be going 99 to 98% of the time.

Bruce I see your post. Buttock angle is usually the imaginary line from amidships, half way between the chine and the keel that runs aft and terminates at the transom. A high (steep) buttock line will usually have the transom out of the water like most sailboats and FD craft. a flat and straight quarter beam buttock line w 0 degrees angle will be the B line of a planing hull.
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Old 09-15-2013, 08:36 PM   #222
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...
The disadvantage of FD hulls is not just reduced speed but greatly reduced interior usable space. ...
Please explain why a motorboat's FD hull results in "greatly reduced space."
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Old 09-15-2013, 08:52 PM   #223
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Please explain why a motorboat's FD hull results in "greatly reduced space."
Because i said so.
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Old 09-15-2013, 09:01 PM   #224
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Pretty sure it's a design "consideration" not an absolute that a FD vessel's interior space be limited.

A barge is usually designed to "maximize" interior space and I don't think I have ever seen a semi-displacement or planning barge hull.
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Old 09-15-2013, 09:04 PM   #225
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Please explain why a motorboat's FD hull results in "greatly reduced space."
Sorry Mark. I couldn't resist the previous post.

Seriously though, a motorboat's FD hull shouldn't be much different than a sailboat's FD hull. Most FD hulls are rounded of sorts and most SD hulls are squarish or rectangular(ish) athwartship. This curved shape is also usually shallower than the square or rectangular. This results in less cu. ft.. Most of the lost space is in the less visable areas. These areas are usuallly along the side walls.
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Old 09-15-2013, 09:59 PM   #226
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In a smaller boat the FD boat that is heavier (most are) sits deeper in the water. This leaves more room for the engine and of course the power required is about 1/2 the salon floor can be considerably lower. A tremendous advantage in a smaller boat. Compare the height of my Mitsu and Mark's JD .. both 4 cyl. Lower cabin. Less windage. Lower CG. And a full headroom engine compartment can be had (all other things being equal) in a smaller boat.

In pic #1 Dixie has full standing headroom under the wheelhouse.

In pic #2 My own Willy has her small engine low enough that a fairly high exhaust riser can be installed. And the propeller shaft can be horizontal. There's enough vertical space in Willy for a 71 series DD.

In pic #3 see how much space is below the WL in Dixie.

I'd say FD boats do NOT lack space in their hulls. They lack deck space aft though on many boats.
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Old 09-15-2013, 11:55 PM   #227
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The Resort 35 ( larger version of Auscan`s boat but without the sails) is seriously big inside, especially for 35ft, sleeps 10, though they should be good friends, initially at least, and has a huge semi circular cockpit. It would be the most common (? the only) FD production trawler in OZ, and features in charter fleets.
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Old 09-16-2013, 05:38 AM   #228
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While the semi circle of a true displacement hull will roll easily ,it is usually the easiest roll to live with.

How the boat checks , reverses the roll usually decides if it is a Vomit Comet underway in sloppy conditions .

The semicircle is usually the smoothest at roll reversal, and takes well to paravanes , flopper stoppers or the more complex and dangerous driven fins.
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Old 09-16-2013, 06:20 AM   #229
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Quote:
Originally Posted by Tony B View Post
T

The major advantage of a FD hull is fuel economy and that is a real biggie.
The disadvantage of FD hulls is not just reduced speed but greatly reduced interior usable space.

.

I have always thought the opposite, that a full displacement hull provided more interior space as the hull was consistently deeper from forward to aft.

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Old 09-16-2013, 06:39 AM   #230
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I have always thought the opposite, that a full displacement hull provided more interior space as the hull was consistently deeper from forward to aft.

If built as a flat bottomed barge , there could be more storage space in the bilges.

Perfect for a houseboat!
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Old 09-16-2013, 09:05 AM   #231
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Most sailboats hulls have somewhat of a spade shape and carry their widest beam for only a very short distance. Most SD hulls carry their beam most of the length.
The reason for the spade shape is for water flow around the hull. The water parts in front, like most boats but in a FD sailboat hull, the water moves along the hull smoothly and then closes back to itself with narry a ripple. This more or less helps push the boat forward. I Dont' know anything about FD powerboat hulls, I just assume all FD displacement hulls were alike. On a previous post I guess I was getting a bit narrow minded thinking that we were using the same hull. Typically, most sailboat FD hulls have very little space under the stern and bow. A powerboat usually has more freeboard so I guess technically, that is still part of the hull. In most power boats they have a good part of their living area above the waterline, in sailboats we live just about entirely below the water line. So then after re-thinking this I can say that some powerboats with FD hulls can have more interior space because generally they carry their beam for a greater distance and they usually have more freeboard.
I think Mark could probably answer this better than anyone since he went from sail to power and I think his Willie is also a FD hull.
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Old 09-16-2013, 09:15 AM   #232
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Sailboat hulls are even considerably different...full keel, canoe stern/full bow sailboats have a lot more room in them than say an offshore sled which is certainly more the "wedge shape" to an extreme...lots are somewhere in between.
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Old 09-16-2013, 09:16 AM   #233
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The combination of round sections forward with hard chine vee sections aft is nothing new at all in the semi-displacement world. Builders really like having unique key-words to define the product, thus the "Dual-mode" moniker.
Agreed...the basic shape looks a lot like our late '80s Monk designed OA...full rounded forward sections transitioning to flat surfaces aft and a 3/4 length keel...even has the flat section behind the keel (perhaps you could explain what that's about?). But Dixon appears to have incorporated/integrated all the little tricks...plumb bow, prop tunnels, and the slightly reversed chines (I think that's the term).

If the magazine testers are to be believed, the design elements in both the 43' and the 50' Magellano(tested a couple years ago) seem to work well in combination. I did disagree with the more recent write up on the 43' where the "test pilot" recommended the bigger (355 hp) optional engines to achieve 22 knots versus the standard power (305 hp) which achieves 18 knots. Defeats the intent of the original design as you point out.
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Old 09-16-2013, 11:24 AM   #234
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Thanks, you just explained why the hull of my Krogen 42 is like a barrel.

And in a beam sea we roll (sans stabilizers) like a barrel of monkeys.

Marty
Thanks Marty. You took the words right of my mouth.
Also, why our hull is so efficient
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Old 09-16-2013, 01:10 PM   #235
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Having followed this thread and muddled through some of the lopsided posts:

I can’t help but get a kick out of all the back and forth hub-bubb about assorted hull shapes/types superiority regarding their efficiency at slow speeds... or, for that matter, their efficiency (or lack thereof) at elevated or top speeds; as well as their comfort or handling capabilities at any speeds in assorted sea conditions... including at anchor!

When I keep my hard chine planing hull boats at or just under their mathematically calculated WLL hull speed the fuel efficiency is near or equal to FD, or SD, or SP hulls of similar WLL and weight. If I place my planing hull boats into a power range that squats the rear-end deep into water and does not provide ample power to climb over the bow wave then fuel use skyrockets as hull-travel-through-water efficiency therein becomes decimated (similar to FD hulls trying to go faster than their completely limited displacement hull speed... before proposing begins). But, when I apply enough power to climb over the bow wave and reach a clean plane with trim tabs correctly positioned the hard chine hull’s planned speed-efficiency-design then comes into play and we get places quickly; albeit at considerably reduced nmpg than traveling at hull speed or below.

That said - - > There are plenty of reasons to want to own and to greatly enjoy boats with different types of hull designs. Many of the reasons have to do with a boat owners’ planned boat use as well as capital available for fuel and maintenance costs. But, for FD, or SD, or SP, or FP hulled boat owners (in other words any boat owner) to simply discard the value and worth of other boat owners’ hull shapes is... well... short sited to say the least!

Personally I like every hull design type and appreciate each of their attributes. I simply choose full-planing hull boats cause they fit our current needs and lifestyle!

PS: We often cruise at 6.5 knots. About 1 knot below our 20K lb GW, 34’ boat’s mathematically calced WLL hull speed of 7.58 knots... at that reduced speed fuel efficiency is approx 2.75 nmpg, a fuel-use to speed ratio that approaches or reaches similar size and weight FD and SD hulls at same speed. I also at times cruise 16 to 17 knots on plane at approx 1 nmpg... costly, but we reach our destination in well less than 1/2 the time! And, I can easily push our Classic Tolly into the 20 + knot range to quickly get someplace or to git da heck out o’ da way – if items so require! I’ve only needed to utilize that higher speed in short durations twice over the last several years.

Happy Boating Daze!
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Old 09-16-2013, 03:14 PM   #236
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I've heard that one before Art but if it were even close to true sailboats would look like Tollies.

Tony,
You must mean "Coot" instead of Willy.
Yes the diamond shaped sailboats are optimized for light winds w their fat midships sections and skinny ends. And they are extremely efficient at 1.5 knots or less from hull speed. Newer canoes and kayaks are diamond shaped for the same reason ... very low propulsive power available. Willards have their maximum beam only at one point too but w much fuller ends than most sailboats. I think Willard's were designed by a mostly sailboat designer. For that reason I tie Willy to a float w her stern in close and her bow off a bit. Much easier to board that way. And I don't like "help" tying up as people always like to pull in the bow close. I usually just tell them to hold the line slack but usually they want to pull on it.
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Old 09-16-2013, 03:56 PM   #237
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I've heard that one before Art but if it were even close to true sailboats would look like Tollies.
Eric - I didn't feel necessary to mention that contents in my post # 235 were only to do with power boats. That is self evident. This is Trawler Forum... not Sail Boat Forum. Geeezzz!
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Old 09-16-2013, 04:15 PM   #238
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Old 09-16-2013, 05:37 PM   #239
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The disadvantage of FD hulls is not just reduced speed but greatly reduced interior usable space.
Does that mean the designers of freighters and tankers have been getting it wrong since the first cargo carrying hollow log?
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Old 09-16-2013, 05:53 PM   #240
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Eric - I didn't feel necessary to mention that contents in my post # 235 were only to do with power boats. That is self evident. This is Trawler Forum... not Sail Boat Forum. Geeezzz!
Sail boats, canoes, kayaks and freighters are all FD types all for the same reason. They are the most efficient. So "When I keep my hard chine planing hull boats at or just under their mathematically calculated WLL hull speed the fuel efficiency is near or equal to FD, or SD, or SP hulls of similar WLL and weight" apparently just isn't true. When efficiency is really important FD boats are universally chosen.
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