
03302014, 12:06 PM

#1

Senior Member
City: Florida
Vessel Name: Lady Di
Vessel Model: 2012 Beneteau Swift Trawler 44
Join Date: Mar 2014
Posts: 432

Newbie Questions: Hull / Planing Speeds, etc...
Hello All,
Newbie here. I'm confused about exactly what is meant by "hull speed", "planing speed", "cruising speed", etc...
I'm "all about efficiency", unless faced with extenuating circumstances.
I know, I can hear some of you now, "then WHY on earth did you ever buy that boat"??! Nonetheless, we did, we love her and have ZERO regrets. Remember, "A man convinced against his will is of that opinion still". I warned you that I was a "newbie"!
Here is some background for you:
Hull: Semidisplacement
LWL: 37' 6"
Beam waterline: 12' 8"
Displacement (full): 30,000 lbs.
Power: 2 Volvo Penta D4 turbos, 300 hp ea.
Here are a few of the actual stats I recorded last summer:
3,500 rpm's, 24 kn., 30 gph (WOT)
2,800 rpm's, 15 kn., 18 gph
2,000 rpm's, 11 kn., 8 gph
Has trim tabs, which have no effect under 2,000 rpm. When hull is planing(?) the "bow down" attitude results in an increase of about 1 kn.
As usual, we're interested in any thoughts or recommendations you may have.
Thank you!
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03302014, 12:23 PM

#2

Senior Member
City: Florida USA and Ontario Canada
Vessel Name: anytime
Vessel Model: 2007 Chaparral 270 Signature LOA 29'
Join Date: Jan 2014
Posts: 301

Trim tabs are not only to increase the hull speed while planing. Here is a short video about trim tab functions:
The hull speed is a theoretical mathematical formula defining displacement hull/mode speed for a particular length of boat's waterline: Hull speed  Wikipedia, the free encyclopedia
The planning speed is the speed at which particular planing hull gets on plane and/or minimum speed it stays on plane. It is somehow murky in case of semi displacement (or semi planning) hulls ... IMO.
The cruising speed is more of a subjective value one prefers based on one's priorities ... speed/time to get there vs fuel efficiency.
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Best ... Richard



03302014, 12:27 PM

#3

Technical Guru
City: Wilmington, NC
Vessel Name: Louisa
Vessel Model: Custom Built 38
Join Date: Jan 2014
Posts: 5,986

Hull speed is about 1.3x sqrt of water line length, so on your boat it is about 8kts. Planing speed is where boat rides over the bow wave and bow drops slightly, usually around 1315kts minimum.
When increasing speed, once you get to hull speed any increase in speed takes lots of increased power for little change in speed.
Most boats of this type end up with two choices in speed: A touch below hull speed, say 7.7kts, or planed out at say 15kts or above. The burn rate per mile planed out is usually 2.5 to 3 times more than hull speed, so that speed is very expensive fuel wise.
Most consider the speed range between hull speed and planing speed a "dead spot" to be avoided.
Run 7.7 or 15up.



03302014, 12:44 PM

#4

Valued Technical Contributor
City: Litchfield, Ct
Join Date: Aug 2012
Posts: 5,679

Well, I'll bite and tackle your questions:
Cruising speed is anything you want to make it. Some people cruise at or below displacement speed, others cruise at wot all day.
The gurus over at boatdiesel believe that to cruise long term, ie all day long, day after day you should operate your engine at no more than 3035 hp per liter. For your Volvo D4s, that is about 140 hp each or about 1213 kts based on the data you gave.
You can cruise faster but at some effect on engine life.
Hull speed is a theoretical and practical concept. Theoretically it is 1.34 * sqrt(LWL) or 8.2 kts for your boat. Practically its when you have to start climbing over the bow wave to go any faster.
For displacement hulls this takes a huge amount of power because the aft hull is squatting in the stern trough which means it has a tough hill to climb. There isn't enough displacement at the aft end of the boat to keep it up. If you look at a displacement hull it is rounded and the rocker (the sweep of the hull up to the transom) is steep. All of which makes for little lift aft.
Semidisplacement hulls do have much more of the beam in the water aft that gives the hull significant lift. They also have flat planning sections aft to help lift the hull hydro dynamically as well.
But semidisplacement hulls never really get "over the hump" and "on plane" like planning hulls do. They sort of mush forward and up and the more horsepower you apply, the faster they will go. If you could measure instantaneous horsepower vs speed (some electronic engines can do this) and plot a curve you would see a smooth continuous curve.
Trim tabs help a semidisplacement hull because they provide hydro dynamic lift at the transom which makes for a better planning angle and the boat planes rather than mushes through the water.
A planning hull has enough planning surface area for its weight (actually the main difference between planning and semidisplacement hulls is the weight) so it can hydro dynamically lift up and get the hull out of the water so only maybe the aft third is touching.
If you were to do the same hp vs speed plot for a planning boat you would see the curve climb up and then level off for 5 kts or so and then continue to climb but at a reduced angle. Once hp per kt levels off you are "over the hump" and "on plane". The wake also smooths out and the boat feels "sweet" riding up high and skimming the water.
But, if you are all about efficiency, cruise at 7 kts and you will burn about 3 gph.
David



03302014, 01:43 PM

#5

Guru
Join Date: Oct 2007
Posts: 21,337

>Hull speed is a theoretical and practical concept. Theoretically it is 1.34 * sqrt(LWL)<
This is ONLY for fat boats 3x or 4x as long as they are wide.
Once you get into long and skinny , cat or try hulls that speed formula does not work.
Years ago the AYRS came up with a simple method of figuring "hull" speed for a variety of boats.
The old formula was created by looking at fat boats of the 1800 rea , and doesn't work well for skinny fast boats.
S = L/3b X SQRT (L) When run for some theoretical hulls it may be a bit fast , but seems to work.
Practically a 61 LB ratio gets the job done on a heavy boat , 81 to 161 do better as the weight decreases.
Your boat is a go fast ,created to plane underway.
Its ride comfort and much stability come from being ON the top of the water.
Sure it eats fuel, but what is currency for?



03302014, 02:11 PM

#6

Senior Member
City: Florida USA and Ontario Canada
Vessel Name: anytime
Vessel Model: 2007 Chaparral 270 Signature LOA 29'
Join Date: Jan 2014
Posts: 301

I have a skinny pocket cruiser with only 8.5' beam ... but it's within upper limit of the 3x or 4x as long as wide criterion. Just for kicks ... I thought I will calculate the hull speed based on both formulas:
LWL = 25'
BWL = 8'
S = speed in knots
S = 1.34 x SQRT(LWL) = 6.7 knot
S = LWL/(3 x BWL) x SQRT(LWL) = 5.2 knot
FWIW ... my hull clearly follows the first formula as it's efficient "coasting" or hull speed, as opposed to efficient "planning" speed, is 6.5 to 7.0 knots. Interesting enough, at the optimum throttle settings while coasting (1400 RPM) or planing (3400 RPM) I get the same 2.5 MPG on my gasoline powered VP 8.1L 375 HP engine (not talking about GPH here).
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Best ... Richard



04022014, 05:44 AM

#7

Guru
Join Date: Oct 2007
Posts: 21,337

ON a small light boat the results you get , the boat gets more efficient while up on the plane is common.



04022014, 10:09 AM

#8

Technical Guru
City: Wilmington, NC
Vessel Name: Louisa
Vessel Model: Custom Built 38
Join Date: Jan 2014
Posts: 5,986

Quote:
Originally Posted by Richard W
I have a skinny pocket cruiser with only 8.5' beam ... but it's within upper limit of the 3x or 4x as long as wide criterion. Just for kicks ... I thought I will calculate the hull speed based on both formulas:
LWL = 25'
BWL = 8'
S = speed in knots
S = 1.34 x SQRT(LWL) = 6.7 knot
S = LWL/(3 x BWL) x SQRT(LWL) = 5.2 knot
FWIW ... my hull clearly follows the first formula as it's efficient "coasting" or hull speed, as opposed to efficient "planning" speed, is 6.5 to 7.0 knots. Interesting enough, at the optimum throttle settings while coasting (1400 RPM) or planing (3400 RPM) I get the same 2.5 MPG on my gasoline powered VP 8.1L 375 HP engine (not talking about GPH here).

This can be explained in part by the nature of gasoline engines. At low power settings, the amount of hp per gph (bsfc) is very poor. At higher power settings, it improves greatly. On a diesel, the bsfc stays much more constant, so low speed running sees nmpg much better than when planing.



04022014, 10:51 AM

#9

Senior Member
City: Florida USA and Ontario Canada
Vessel Name: anytime
Vessel Model: 2007 Chaparral 270 Signature LOA 29'
Join Date: Jan 2014
Posts: 301

Got it ... makes sense. Thanks.
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Best ... Richard



04022014, 10:52 AM

#10

Guru
City: Concrete Washington State
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 17,637

Diesel and gas are almost the same re efficiency at full bore.
But nobody runs at full bore.
Re tabs I can't imagine them being anything but drag on most trawlers that are typical here on TF. That would include wedges under the stern too.
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Eric
North Western Washington State USA



04022014, 11:22 AM

#11

Senior Member
City: Florida USA and Ontario Canada
Vessel Name: anytime
Vessel Model: 2007 Chaparral 270 Signature LOA 29'
Join Date: Jan 2014
Posts: 301

The OP's boat has a semi planing hull ... the tabs are advantageous in his situation.
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Best ... Richard



04022014, 11:33 AM

#12

Technical Guru
City: Wilmington, NC
Vessel Name: Louisa
Vessel Model: Custom Built 38
Join Date: Jan 2014
Posts: 5,986

Quote:
Originally Posted by manyboats
Diesel and gas are almost the same re efficiency at full bore.

Not close at all.
370hp Mercruiser 8.1 @ full power 30.1 gph: 12.3hp/gph
380hp Cummins 5.9 @ full power 20.1 gph: 18.9hp/gph
Pretty substantial difference.
Using 2.7 exp load curve
Merc at cruise 197hp 19.6gph: 10.1hp/gph
Cummins at cruise 208hp 10.4gph: 20.0hp/gph
Merc data from a Sea Ray 380 test report, Cummins data from qsb5.9 performance curve.
You can see the hp/gph drop towards cruise with the gas engine, it improves a little with the diesel. I don't have any data at hand to look further into the lower rpm numbers.



04022014, 01:16 PM

#13

Guru
City: Holladay, UT
Vessel Name: Dream Catcher
Vessel Model: Nordic Tug 37065
Join Date: Oct 2007
Posts: 698

Quote:
Originally Posted by Richard W
I have a skinny pocket cruiser with only 8.5' beam ... but it's within upper limit of the 3x or 4x as long as wide criterion. Just for kicks ... I thought I will calculate the hull speed based on both formulas:
LWL = 25'
BWL = 8'
S = speed in knots
S = 1.34 x SQRT(LWL) = 6.7 knot
S = LWL/(3 x BWL) x SQRT(LWL) = 5.2 knot
FWIW ... my hull clearly follows the first formula as it's efficient "coasting" or hull speed, as opposed to efficient "planning" speed, is 6.5 to 7.0 knots. Interesting enough, at the optimum throttle settings while coasting (1400 RPM) or planing (3400 RPM) I get the same 2.5 MPG on my gasoline powered VP 8.1L 375 HP engine (not talking about GPH here).

Here's a similar sized example in diesel:
25'7" LOA, 23' LWL, 8.5' beam, 19 degrees deadrise at transom, 11,000 lb on the water loaded.
We cruise on plane at 18 knots, 1.75 nm/gal. At 6 knots, 4.5 nm/gal. We like 6 knots.
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Richard Cook
Dream Catcher (Nordic Tug 37065) Poulsbo WA
Previously: New Moon (Bounty 257), CDory 22 Cruiser
"Cruising in a Big Way"



04022014, 02:01 PM

#14

Senior Member
City: Florida USA and Ontario Canada
Vessel Name: anytime
Vessel Model: 2007 Chaparral 270 Signature LOA 29'
Join Date: Jan 2014
Posts: 301

Quote:
Originally Posted by RCook
Here's a similar sized example in diesel:
We cruise on plane at 18 knots, 1.75 nm/gal. At 6 knots, 4.5 nm/gal. We like 6 knots.

I like your 6 knots cruising characteristics too ...
My amended results, gasoline engine ...
my 5.3 MPH does not even come close to your 6 knots:
1000 RPM ... 05.3 MPH ... 03.4 MPG ... 01.6 GPH
1400 RPM ... 07.2 MPH ... 02.5 MPG ... 02.9 GPH
3400 RPM ... 30.2 MPH ... 02.5 MPG ... 12.1 GPH
(leading zeros added for alignment)
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Best ... Richard




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