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Old 09-18-2015, 11:36 AM   #21
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An aside....that prop size looks pretty "big" for the application. Just curious what gear ratio is in the transmissions. (Nothing wrong with overpropping, mind you....just curious).


PS I'd agree with others that the boat needs trim tabs if you want to travel at higher speeds. Our boat picks up 1-2 knots for the same power setting with tabs....and I don't have to stand at the wheel to see over the bow. Standard semi-planning behavior.
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Old 09-18-2015, 11:55 AM   #22
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OP's boat has almost enough power to plane, but not quite. For a planing boat, there are two efficient speeds: Hull speed and planed out. Since OP's does not have enough hp to plane, he's stuck with running hull speed. 8kts tops unless you want to plow and eat fuel.
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Old 09-18-2015, 12:21 PM   #23
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OP's boat has almost enough power to plane, but not quite. For a planing boat, there are two efficient speeds: Hull speed and planed out. Since OP's does not have enough hp to plane, he's stuck with running hull speed. 8kts tops unless you want to plow and eat fuel.
It should have enough power to "semi-plane", however. Our 32,000 pound semi-displacement Monk designed hull (44 OA) will make 13 knots from about 220 HP total....per measured RPM and prop charts....with trim tabs deployed. About 15 GPH (total for both engines). That's comfortably beyond the bow wave (the hump). No, the hull isn't completely up out of the water, but it's lifted considerably and at a fairly flat attitude....not plowing in the classic sense. The power required curve turns sharply upward beyond that speed until the maximum of 500 HP (total installed) nets 17.5 knots at a fuel burn of roughly 30gph (total). Semi-planning.
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Old 09-18-2015, 01:40 PM   #24
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you are not getting max power from the engines because they are not turning max rpms. If that would be enough to plane I don't know but adding tabs would probably raise the speed enough that the rpms would go up.


Personally I would tank to Bennett and consider adding tabe before repitching props. or just live with the idea that you have a slow boat. Didn't you intend to buy a "trawler".
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Old 09-18-2015, 01:40 PM   #25
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Semi-planning.
Whats "semi-planing"?

Seems to me you either are or you're not. In between is just plowing, throwing a big wake and wasting fuel.
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Old 09-18-2015, 01:54 PM   #26
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An aside....that prop size looks pretty "big" for the application. Just curious what gear ratio is in the transmissions. (Nothing wrong with overpropping, mind you....just curious).


PS I'd agree with others that the boat needs trim tabs if you want to travel at higher speeds. Our boat picks up 1-2 knots for the same power setting with tabs....and I don't have to stand at the wheel to see over the bow. Standard semi-planning behavior.
Trannys are BW Velvet Drives 7000 series ( not to many around) rated at 550HP each. Ratio is 2.66/1
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Old 09-18-2015, 02:02 PM   #27
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Thanks for all the information TF. I just wanted to know why a boat does what it does. I guess I didn't understand that the hull was trying to climb over the bow wake. I thought maybe the stern was being sucked down. I don't plan on changing anything. We are very comfortable cruising at 8 kts, 1400 rpm and 3.4-4 gal an hour. Having the ability to drive the bow up has come in handy a few time on Lake Michigan last month.
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Old 09-18-2015, 03:49 PM   #28
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Dave,
All ahead 1/3.
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Old 09-18-2015, 04:19 PM   #29
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Whats "semi-planing"?

Seems to me you either are or you're not. In between is just plowing, throwing a big wake and wasting fuel.
Depends on how you choose to define planing, I suppose. These SD hulls with wide and deep forefoot plus keel have a lot of drag and won't realistically get "on top" and skim the surface in the classic sense of the term plane. But they do produce considerable lift to reduce wetted surface (drag), and they do get beyond the bow wave. If you look back at old discussions, you will find the term used quite often, as some find it more descriptive than semi-displacement. If done right, these semi-planing hulls can be reasonably efficient and not just plow. Many (most) of them were never optimized for the transition through the hump and they do indeed just plow...but it doesn't have to be that way.
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Old 09-18-2015, 06:42 PM   #30
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Depends on how you choose to define planing, I suppose. These SD hulls with wide and deep forefoot plus keel have a lot of drag and won't realistically get "on top" and skim the surface in the classic sense of the term plane. But they do produce considerable lift to reduce wetted surface (drag), and they do get beyond the bow wave. If you look back at old discussions, you will find the term used quite often, as some find it more descriptive than semi-displacement. If done right, these semi-planing hulls can be reasonably efficient and not just plow. Many (most) of them were never optimized for the transition through the hump and they do indeed just plow...but it doesn't have to be that way.
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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Old 09-18-2015, 07:53 PM   #31
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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.
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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Old 09-18-2015, 08:01 PM   #32
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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.
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Old 09-18-2015, 09:03 PM   #33
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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.
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Old 09-18-2015, 09:30 PM   #34
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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.
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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Old 09-18-2015, 10:35 PM   #35
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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. :-)
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.
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Old 09-19-2015, 06:50 AM   #36
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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.
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Old 09-19-2015, 10:15 AM   #37
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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.
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Old 09-19-2015, 12:16 PM   #38
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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.
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Old 09-19-2015, 01:32 PM   #39
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Yeah butt angle. Butt there is more to it than that.

Semiplaning boats can meet many needs | Soundings Online
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Old 09-19-2015, 02:54 PM   #40
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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.
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