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Old 02-05-2019, 07:03 PM   #61
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Fish wrote;
“Up here in Maine there's tons of SD hulled lobsterboats that do quite well in the planning department, I have one friend with a 42' lobster boat that does over 40kts and race boats do up to 70kts with an SD hull.”

Those are actually planing hulls. The soft/rounded chine causes more drag but those boats have enough power to overcome it. If they had hard chines the’yd be faster but the’re planing hulls. Reason being the majority ofthe bottom is flat and there’s a clean break at the transom/bottom edge. Given a little speed the water breaks free at the end of the transom. And as you go faster the water comming up around the chine curve has lesstime to rise up so the soft chine restricts speed even less.
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Old 02-05-2019, 07:07 PM   #62
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There are several sentences that I take issue with but some may be me not getting your point exactly. But this one "The bastard child is the SD hull, not as efficient at either mode, typically has characteristics of planing hull but is over weight and underpowered for that mission. Nevertheless, it is very popular for other reasons.". Up here in Maine there's tons of SD hulled lobsterboats that do quite well in the planning department, I have one friend with a 42' lobster boat that does over 40kts and race boats do up to 70kts with an SD hull. I 've had a couple one was a BHM 32 with a 215hp Isuzu that did 18kts WOT and the other a Sisu 26 with a 200hp TAMD41a Volvo that did 25kts.
In other words, you know of planing boats that you prefer calling semi-displacement.

Already posted, but a good description of the thtee types of hulls is here:.

https://www.outerreefyachts.com/hull-types
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Old 02-05-2019, 07:08 PM   #63
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IMO......
Does She love a following sea.....
If so... full displacement....
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Old 02-05-2019, 07:11 PM   #64
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Fish wrote;
“Up here in Maine there's tons of SD hulled lobsterboats that do quite well in the planning department, I have one friend with a 42' lobster boat that does over 40kts and race boats do up to 70kts with an SD hull.”

Those are actually planing hulls. The soft/rounded chine causes more drag but those boats have enough power to overcome it. If they had hard chines the’yd be faster but the’re planing hulls. Reason being the majority ofthe bottom is flat and there’s a clean break at the transom/bottom edge. Given a little speed the water breaks free at the end of the transom. And as you go faster the water comming up around the chine curve has lesstime to rise up so the soft chine restricts speed even less.
. I think it's fair to say that a semi displacement boat that lacks the power to plane becomes a planing hull if you increase the power sufficiently. Unlike a displacement hull, which, as Outer Reef notes, "can never plane."
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Old 02-05-2019, 07:23 PM   #65
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Fish writes the SD hull is the bastard child in hull design. Indeed it is. Kind of a “morfidite” but important as it fills the gap between planing and FD. A planing hull does SD speeds but very inefficiently and w/o enough speed to be in solid control with her small rudders and even smaller keel. The SD hull offers some speed w significantly better economy than the planing hull. Without the SD boat rec trawlers would be stuck with 6-8 knots or a planing hull. Rec Trawlers probably owe their existance to the SD hull. Note that SD boats designed for the upper range of SD speed are very close to planing hulls and frequently called the wrong hull type. Often on TF (not so much as in the past) people would call SD boats FD. If we tried to label some boats half of us would say it’s FD and the other half would say SD. And since there’s no clear decisive description of both types the boat is just in a grey zone. Probably 20% (or more) of the trawlers here are in this grey zone. All one can do is hear and evaluate some differing opinions on the same hull. And then draw you’re own conclusion.
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Old 02-05-2019, 07:48 PM   #66
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Delfin wrote;
“I think it’s fair to say that a semidisplacement boat that lacks power to plane becones a planing hull if you increase power sufficiently”

Not so at all. The hull type is entirely due to it’s form or shape. For example many have tried to define the three hull types by tieing wave length to group them in their various types. But the definition falls appart in argument.
Other attempts have been made but one can’t apply them to some hull types, especially very unusual designs.

The best (IMO) way to separate the three hull types is to assign approprate QBBL angles ... basically the angle that the bottom of the hull is to horizontal. Horizontal would be planing. A slight angle to a bit more = SD and anything more is FD. Of course arbitrairly assigning numbers makes grey zones and one could argue it’s up to sombody’s whim.
This has been done though with a group like a bunch of advanced students at a well recognized school of yacht design. As an example with a source representing so much accumilative knowledge one could assume some fairly high degree of credibility and/or accuracy.
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Old 02-05-2019, 08:00 PM   #67
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Originally Posted by Nomad Willy View Post
Delfin wrote;
“I think it’s fair to say that a semidisplacement boat that lacks power to plane becones a planing hull if you increase power sufficiently”

Not so at all. The hull type is entirely due to it’s form or shape. For example many have tried to define the three hull types by tieing wave length to group them in their various types. But the definition falls appart in argument.
Other attempts have been made but one can’t apply them to some hull types, especially very unusual designs.

The best (IMO) way to separate the three hull types is to assign approprate QBBL angles ... basically the angle that the bottom of the hull is to horizontal. Horizontal would be planing. A slight angle to a bit more = SD and anything more is FD. Of course arbitrairly assigning numbers makes grey zones and one could argue it’s up to sombody’s whim.
This has been done though with a group like a bunch of advanced students at a well recognized school of yacht design. As an example with a source representing so much accumilative knowledge one could assume some fairly high degree of credibility and/or accuracy.
Not so sure about that, Eric. You could drop 10,000 hp in Delfin and she still wouldn't plane, but if you increased the hp on a Fleming to, say 2,000, I am pretty sure she would because while not horizontal in her stern sections like a navy destroyer, it's not far off. In my mind, you really only have two hull types - full displacement where drag increases faster than lift and everything else where lift can exceed drag if the propulsive power is sufficient. Under that scheme, a semi-displacement boat is just an underpowered planing boat.

I do like the description of the three hull types in the Outer Reef link above.
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Old 02-05-2019, 08:22 PM   #68
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Well Mr Vizier,
OK good. We have an opinion.
But you’re assigning a type by it’s speed capability. Perhaps you’re trying to explain to a better degree common trawlers like the IG. Take the keel mostly off and put it on a serious diet .. weight wise. Then nobody would doubt it was a planing hull/boat type.
But if that’s your yardstick put my 37hp engine in an IG32 and call it a FD boat.

I put in the link as listed and got everything outer reef but hull types. ???

I actually like four hull types.
1. Planing
2. Semi displacement
3. Semi planing
4. Displacement

1. The planing hull operates at 2 to 3 times it ‘s hull speed retaining good seakeeping abilities as in control and the ability to carry medium to light loads.
2. Semi displacement hulls operate at slower planing speeds. They displace more water therefore not “on top” to the degree the full planing hull does. Thus they sit a little bit down in the water.
3. Semi planing hulls plane to some limited degree. Usually at a higher angle of attack that all the other types. Not much faster than a FD boat and with not much speed advantage. But some degree of dynamic stability at their speed at and above displacement speed. Good examples are older NT and GB boats.
4. Full displacement boats are the most efficient and seaworthy of boats. Sensible speeds .75 to 1 knot below hull speed deliver great fuel economy, easy motion and excellent low speed handling abilities.
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Old 02-05-2019, 09:06 PM   #69
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4. Full displacement boats are the most efficient and seaworthy of boats. Sensible speeds .75 to 1 knot below hull speed deliver great fuel economy, easy motion and excellent low speed handling abilities.
So, by that definition, regardless of speed, a SD or even planing boat, becomes a FD when is is run hull down 0.75 or 1 knot below where is would try to start coming out of the hole?
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Old 02-05-2019, 09:31 PM   #70
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No the def is assuming a boat on her lines.
Running hull down is not part of it. Challenging thought though.
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Old 02-05-2019, 10:04 PM   #71
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So maybe that boat is "running at full displacement?"

Since the hull is fully in the water it is displacing at its maximum displacement value.
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Old 02-05-2019, 10:16 PM   #72
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Seventy posts to answer the question so far....you guys are making this hard.


Full Displacement: You know it when you see it!
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Old 02-05-2019, 10:30 PM   #73
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Seventy posts to answer the question so far....you guys are making this hard.


Full Displacement: You know it when you see it!
How many hp do you need to waterski behind her?
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Old 02-05-2019, 10:33 PM   #74
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How many hp do you need to waterski behind her?
Depends upon how flat bottomed the ski is.....
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Old 02-05-2019, 10:34 PM   #75
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menzies,
Play on words there I’m think’in.
Not an NA but I think you just described a boat “on her lines”.
Assumably that would be at max working displacement.
And of course would not require any motion or speed.
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Old 02-05-2019, 11:03 PM   #76
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Well Mr Vizier,
OK good. We have an opinion.
But you’re assigning a type by it’s speed capability. Perhaps you’re trying to explain to a better degree common trawlers like the IG. Take the keel mostly off and put it on a serious diet .. weight wise. Then nobody would doubt it was a planing hull/boat type.
But if that’s your yardstick put my 37hp engine in an IG32 and call it a FD boat.

I put in the link as listed and got everything outer reef but hull types. ???

I actually like four hull types.
1. Planing
2. Semi displacement
3. Semi planing
4. Displacement

1. The planing hull operates at 2 to 3 times it ‘s hull speed retaining good seakeeping abilities as in control and the ability to carry medium to light loads.
2. Semi displacement hulls operate at slower planing speeds. They displace more water therefore not “on top” to the degree the full planing hull does. Thus they sit a little bit down in the water.
3. Semi planing hulls plane to some limited degree. Usually at a higher angle of attack that all the other types. Not much faster than a FD boat and with not much speed advantage. But some degree of dynamic stability at their speed at and above displacement speed. Good examples are older NT and GB boats.
4. Full displacement boats are the most efficient and seaworthy of boats. Sensible speeds .75 to 1 knot below hull speed deliver great fuel economy, easy motion and excellent low speed handling abilities.
Agreed, Eric! That said... I like having planing hull capabilities for accomplishing inner waterway and close-coastal cruising. Such as the majority of pleasure powerboat owners enjoy doing.
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Old 02-06-2019, 01:09 AM   #77
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A displacement boat under 45-50 foot in length (or more?) doesn't need more than an 80 horsepower engine.
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Old 02-06-2019, 01:37 AM   #78
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A displacement boat under 45-50 foot in length (or more?) doesn't need more than an 80 horsepower engine.
That is true.

However, quite slow.

Fairly inexpensive... comparatively.
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Old 02-06-2019, 02:51 AM   #79
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That is true.

However, quite slow.

Fairly inexpensive... comparatively.
Not slow at all.
It should make hull speed as designed.
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Old 02-06-2019, 06:52 AM   #80
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Originally Posted by Nomad Willy View Post
Fish wrote;
“Up here in Maine there's tons of SD hulled lobsterboats that do quite well in the planning department, I have one friend with a 42' lobster boat that does over 40kts and race boats do up to 70kts with an SD hull.”

Those are actually planing hulls. The soft/rounded chine causes more drag but those boats have enough power to overcome it. If they had hard chines the’yd be faster but the’re planing hulls. Reason being the majority ofthe bottom is flat and there’s a clean break at the transom/bottom edge. Given a little speed the water breaks free at the end of the transom. And as you go faster the water comming up around the chine curve has lesstime to rise up so the soft chine restricts speed even less.
The builders and designers don't think they're planning hulls...….https://ellisboat.com/ellis-downeast...lacement-hull/
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