Boat design guy input needed.

Capt.Bill11 said:

Yes but the thing is a true semi-planing hull needs to be narrow and light. And fast majority of the boats people end up call "semi-planing" are neither.

For example, most people call a GB a semi-planing hull/boat. But it's not narrow, isn't light and can't run at semi-planing speeds with out a lot of HP.

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Depends how you choose to define semi-planing. Most definitions that I've seen key on the term dynamic lift. Grand Banks fits, but it's admittedly not an efficient example.

I've been wondering about adding a large plate that comes out basically as far as my swim platform and full beam would maybe increase efficiency even at displacement speed. Cheat my short beam to length ratio adding running surface.

Most boats you'll see here are semi-planing ..... Or semi-displacement. Same thing to most. To me SP is a hull that will sorta plane. Or go gracefully somewhat faster than a FD boat. To me a SD boat is a faster boat that won't go as fast as a true planing hull and is more eficient than same at slower speeds. "Semi" .. as in a bit like a displacement craft but only slightly (semi) different from a planing hull.

So SD and SP boats aren't pure full displacement or pure planing. They share some of the features and performance of a very high speed boat and a very slow speed boat. It can be easily seen that most boats here on TF are of the semi type. Very few FD or planing hulls. The vast majority are "SD".

But SD referring to a speed range means they can be very different. A boat almost a planing hull is way different than one almost a FD hull. Most boats in this middle ground are closer to the ends than the middle .. as in practically a FD boat or practically a planing hulled boat. Many more in the latter catergory. I'd like to see more boats in the center of this catergory. True SD hulls are not very common.

semi-planing said:

Depends how you choose to define semi-planing. Most definitions that I've seen key on the term dynamic lift. Grand Banks fits, but it's admittedly not an efficient example.

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The definitions I've seen speak to dynamic lift, lift from buoyancy, light weight and narrow beam. The vast majority of boats labeled semi-displacement around here do not fit those criteria.

And most of the boats labeled here as semi-displacement don't even run at semi-displacement speeds. And can't efficiently even if they had the HP because they are not true semi-displacement.

At least that's my take on it.

Capt.Bill11 said:

The definitions I've seen speak to dynamic lift, lift from buoyancy, light weight and narrow beam. The vast majority of boats labeled semi-displacement around here do not fit those criteria.

And most of the boats labeled here as semi-displacement don't even run at semi-displacement speeds. And can't efficiently even if they had the HP because they are not true semi-displacement.

At least that's my take on it.

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The design elements you noted would yield an efficient semi-displacement design. But I agree that there are some real dog SD hulls out there.

While the boat is climbing its bow wave ,,before the boat gets on top it can only be efficient if very skinny.

Thin 6-1 or 8-1 L/B ratio as multihulls do.

By creating a smaller mountain to climb you could say its more efficient.

When on top a true plaining boat will usually accelerate a good deal with no additional power.

The difference between plaining and wallowing, from pushing water with a huge wake , and skimming up on top.

Trim tabs (bigger is better), weight loss diet, propped correctly for WOT WITH the new tabs. Tabs better than wedges b/c can be retracted in following seas when you want bow up again and at displacement speeds when you don't need them.

Much can be learned by searching ....

Westlawn Institute of Marine Technology.

Then click on "News"

Then scroll down to:
"The Mashead - Volume 2. No. 2 - June 2008"

Then scroll down to "Speed and Powering Calculations and more"


Some define the FD, SD Planing hull definitions by:

FD
SLR of 1.5 or less
Buttock angle of 7degrees or more

SD
SLR of 1.5 to 2.5
Buttock angle of 3 to 6 degrees

Planing hulls
SLR of 2.5 or higher
Buttock angle of 2 degrees or less


SLR = Speed Length Ratio
Buttock angle is explained clearly in the link.

Yeah butt angle. Butt there is more to it than that.

Semiplaning boats can meet many needs | Soundings Online

Capt. Bill,
Eric Sorenson is a regular I believe on BoatDesign.net.
He paints a picture of a SP boat above (speed wise) the center of the grey area not PL or SD and used the expression semiplaning.

One of his very interesting and excellent points is the difference in best CG re the SP and planing hull. I'm a big fan of keeping the weight fwd in SP hulls .. and keeping it far enough aft in FD hulls.

Thanks for the link. After some study I'll get more out of it. And yes there's a lot more to it than wave length, LB ratio and all the other ratios, buttock angles, power to weight and WLL. Often people try to use one element to isolate one type from another. Talking to the general boating public I do as well trying to make it simple enough to be meaningful.

I love checking in on boatdesign.net. I learn something new almost everytime. Great site. And a great resource.