Reply
 
Thread Tools Display Modes
 
Old 02-12-2016, 05:40 AM   #21
TF Site Team
 
Bay Pelican's Avatar
 
City: Chicago, IL
Country: USA
Vessel Name: Bay Pelican
Vessel Model: Krogen 42
Join Date: Oct 2007
Posts: 2,796
Quote:
Originally Posted by chicagoq View Post
So the "overwhelming factor" here is actually the hull shape, not the weight, Woo, this is an eye opening point. I never would have thought that!
While not the sole factor hull shape is extremely important. The closer to a traditional sailboat hull you are the more efficient. Designers have been working on the efficiency of those hulls for centuries.

Our Krogen 42 with a full displacement hull reportedly takes only 40 hp to move at hull speed. When using our main engine (135hp) at 1600 RPM we cruise at 6.5kts. Our 27 hp wing engine running at mid level gives us 4.5/5 kts.
__________________
Advertisement

__________________
Marty
Bay Pelican is offline   Reply With Quote
Old 02-12-2016, 07:31 AM   #22
FF
Guru
 
FF's Avatar
 
Join Date: Oct 2007
Posts: 16,537
To separate the BS from reality the old tome from Beede has great tables that can be used.

They do expect the hull form,scantlings, and engines to be for an ocean voyager .

Find a 1st edition , the second and rest were hokum for a boat shlocking company.
__________________

FF is offline   Reply With Quote
Old 02-12-2016, 09:46 AM   #23
Guru
 
TDunn's Avatar
 
City: Maine Coast
Country: USA
Vessel Name: Tortuga
Vessel Model: Nunes Brothers Raised Deck Cruiser
Join Date: Feb 2015
Posts: 672
Both boats are below nominal hull speed. The critical numbers are displacement and waterline length. ASSUMING that both boats are running in pure displacement mode at those speeds, it is easy to calculate approximate horsepower needed to push the boats to those speeds. I did that using the Gerr, improved Gerr, Keith and Wyman formulae and took the average of the results. Thoe results are:

KK 58 (lwl= 52.3') = 61 hp

BTFT (lwl = 43.9') = 30 hp

If the engine-drive train combinations in both boats produce 20 hp per gallon of diesel burned per hour that works out to just over 3 gph for the KK58 and 1.5 GPH for the BTFT. Using these simple formulae, the difference is attributable to the differences in displacement and the speeds relative to nominal hull speed. Note that the Beneteau with its shorter waterline length is running at a higher percentage of hull speed than the Kady Krogan. So basically the results posted above are pretty much what would be expected.
TDunn is offline   Reply With Quote
Old 02-12-2016, 10:29 AM   #24
Guru
 
BandB's Avatar
 
City: Fort Lauderdale
Country: USA
Join Date: Jan 2014
Posts: 13,188
Quote:
Originally Posted by Britannia View Post
All the references I can find show the ST50 LWL as 43'9". LOA is 49'2".
I saw LOA at 49'2" but thought the 43'9" was length without pulpit and platform, so hull length, but not LWL. I didn't see it indicated anywhere as LWL, so perhaps you found a different source. Their brochure calls it "Hull Length". Now it may be the waterline length, just not my interpretation of it.
BandB is online now   Reply With Quote
Old 02-12-2016, 10:33 AM   #25
Guru
 
BandB's Avatar
 
City: Fort Lauderdale
Country: USA
Join Date: Jan 2014
Posts: 13,188
Quote:
Originally Posted by BandB View Post
I saw LOA at 49'2" but thought the 43'9" was length without pulpit and platform, so hull length, but not LWL. I didn't see it indicated anywhere as LWL, so perhaps you found a different source. Their brochure calls it "Hull Length". Now it may be the waterline length, just not my interpretation of it.
LWL on that boat is 40'2" per the article referenced in an above post from Passagemaker.

LOA 43′ 9″

LWL 40′ 2″

So LOA with pulpit and platform 49'2", without 43'9". LWL 40'2".
BandB is online now   Reply With Quote
Old 02-12-2016, 10:37 AM   #26
Guru
 
BandB's Avatar
 
City: Fort Lauderdale
Country: USA
Join Date: Jan 2014
Posts: 13,188
Quote:
Originally Posted by Insequent View Post
Dave Gerr published a very good article in the Westlawn Masthead in 2008. Refer to pages 12-17. He advocates a better 'hull speed' calculation that includes displacement. I have not played around with his formulae to see how much effect weight has at slow speeds, I just know that my sea trial data fits his predictions quite well.
The reality is that all hull speed calculations are just formulas and estimates. Every even slight difference in hull design or shape, in how it sits in the water, would cause a variation. We change the multiplier for some designs of have over the years. The formula isn't exact science by any means.
BandB is online now   Reply With Quote
Old 02-12-2016, 11:18 AM   #27
Wannabe
 
Britannia's Avatar
 
City: SF Bay Area
Country: USA
Vessel Name: Stillwater
Vessel Model: Kadey-Krogen 54
Join Date: Dec 2014
Posts: 766
Quote:
Originally Posted by TDunn View Post
Both boats are below nominal hull speed. The critical numbers are displacement and waterline length. ASSUMING that both boats are running in pure displacement mode at those speeds, it is easy to calculate approximate horsepower needed to push the boats to those speeds. I did that using the Gerr, improved Gerr, Keith and Wyman formulae and took the average of the results. Thoe results are:

KK 58 (lwl= 52.3') = 61 hp

BTFT (lwl = 43.9') = 30 hp

If the engine-drive train combinations in both boats produce 20 hp per gallon of diesel burned per hour that works out to just over 3 gph for the KK58 and 1.5 GPH for the BTFT. Using these simple formulae, the difference is attributable to the differences in displacement and the speeds relative to nominal hull speed. Note that the Beneteau with its shorter waterline length is running at a higher percentage of hull speed than the Kady Krogan. So basically the results posted above are pretty much what would be expected.
Except that the results posted are opposite to what you indicate. They show the FT 50 using almost twice the gph of the KK58.

Richard
Britannia is offline   Reply With Quote
Old 02-12-2016, 11:19 AM   #28
Guru
 
City: gulf coast
Country: pinellas
Join Date: Jun 2014
Posts: 2,199
I don't believe the conclusion, something inconsistent probably operating at different pints on speed vs length curve
bayview is online now   Reply With Quote
Old 02-12-2016, 12:08 PM   #29
Guru
 
Join Date: Apr 2014
Posts: 1,291
Apples and oranges yes they are different and if you try to make comparisons accurate identification and quantification of differences is needed and that is what OP is missing. Some of the posts point in the right direction. The boats are different the propulsion is different multiple factors involved not easy to pick the % each factor contributes to different performance. I would think waterline and drive system certainly big players. I have noticed on many IPS boats with published speed fuel curves some relatively poor performance in the mid range where many trawler types would travel. When I researched IPS as possible drive system for my custom build the engineers that build the units advised against based on mid range speeds I expressed as my desired travelling mode. However,I was told how much fuel I could save if I wanted to travel in mid to high 20k range or above.
eyschulman is offline   Reply With Quote
Old 02-12-2016, 12:37 PM   #30
Guru
 
BandB's Avatar
 
City: Fort Lauderdale
Country: USA
Join Date: Jan 2014
Posts: 13,188
Quote:
Originally Posted by eyschulman View Post
Apples and oranges yes they are different and if you try to make comparisons accurate identification and quantification of differences is needed and that is what OP is missing. Some of the posts point in the right direction. The boats are different the propulsion is different multiple factors involved not easy to pick the % each factor contributes to different performance. I would think waterline and drive system certainly big players. I have noticed on many IPS boats with published speed fuel curves some relatively poor performance in the mid range where many trawler types would travel. When I researched IPS as possible drive system for my custom build the engineers that build the units advised against based on mid range speeds I expressed as my desired travelling mode. However,I was told how much fuel I could save if I wanted to travel in mid to high 20k range or above.
Mid range is definitely not the strength of Pods. Planing ability isn't. I'll give you a comparison of pods vs. straight on the same boat.

Boat: Sunseeker Manhattan 65

Engines and Drives
Twin Man 1200 Shafts with Sidepower stabilizer fins
Twin Volvo IPS 1200

WOT: Mans 2400 RPM, 31 knots, 117 gph, 0.26 nmpg.
IPS 2380 RPM, 32 knots, 81 gph, 0.4 nmpg

At the other end:
Mans 900 RPM, 9.8 knots, 9.5 gph., 1.03 nmpg
IPS 1000 RPM, 9.5 knots, 10.83 gph, 0.88 nmpg

In between:
Mans 1400 RPM, 15 knots, 33 gph, 0.45 nmpg
IPS 1400 RPM, 13.5 knots, 27.74 gph, 0.49 nmpg

Mans 1950 RPM, 25 knots, 79.3 gph, 0.32 nmpg
IPS 2000 RPM, 25.6 knots, 55 gph, .46 nmpg

Now load is also interesting in that cruising speed at 80% load for the MANS is around 26-27 knots and for the IPS around 22-23.

I don't have it handy at the moment but time to plane is longer on the PODS. At 1600 RPM the MANS are running 19.1 knots and the IPS only 16.8 knots.

So very different performance curves between the two. There are some interesting comparisons done on other boats. I will say one thing in defense of the pods. Although this model Sunseeker is sold very frequently with IPS, that's not what it was originally designed for. The standard engine previously was 1000 hp MAN's. A sidepoint. At nearly all speeds on this boat the 1200's are more efficient than the 1000's.
BandB is online now   Reply With Quote
Old 02-12-2016, 01:37 PM   #31
Guru
 
TDunn's Avatar
 
City: Maine Coast
Country: USA
Vessel Name: Tortuga
Vessel Model: Nunes Brothers Raised Deck Cruiser
Join Date: Feb 2015
Posts: 672
Quote:
Originally Posted by Britannia View Post
Except that the results posted are opposite to what you indicate. They show the FT 50 using almost twice the gph of the KK58.

Richard
You are right. I was thinking GPH. I never use mpg.
TDunn is offline   Reply With Quote
Old 02-12-2016, 02:12 PM   #32
Guru
 
City: gulf coast
Country: pinellas
Join Date: Jun 2014
Posts: 2,199
Boats don't operate well or efficiently at mid range speeds. Either well on plane or bow down well off plane are the only reasonable choices, Mid range is only for the "hull speed" advocates who don't know better.
bayview is online now   Reply With Quote
Old 02-12-2016, 02:29 PM   #33
Guru
 
Ski in NC's Avatar
 
City: Wilmington, NC
Country: USA
Vessel Name: Louisa
Vessel Model: Custom Built 38
Join Date: Jan 2014
Posts: 3,892
I'm skeptical that the KK can make 7.7mph on a total burn of 2.5gph with twins. Could be true, but "show me" comes to mind.

Kind of like a kid saying his f150 truck gets 25mpg. BS flags pop up.
Ski in NC is online now   Reply With Quote
Old 02-12-2016, 03:01 PM   #34
Guru
 
Join Date: Apr 2014
Posts: 1,291
Quote:
Originally Posted by bayview View Post
Boats don't operate well or efficiently at mid range speeds. Either well on plane or bow down well off plane are the only reasonable choices, Mid range is only for the "hull speed" advocates who don't know better.
True that the best economy is one to two k below Hull speed but I personally prefer to travel at or just above hull speed valuing my vacation time more than the difference in $s. Considering the overall cost of my boat and maintenance the difference in cost between 7K and 10K is hardly significant. If my boat were a commercial boat or one that put on thousands of hours a year or had to have the range to cross an ocean I might do differently. I think many cursing people know the difference in fuel burn and opt for the mid range for other reasons.
eyschulman is offline   Reply With Quote
Old 02-12-2016, 03:12 PM   #35
Guru
 
City: Carefree, Arizona
Country: usa
Vessel Name: sunchaser V
Vessel Model: DeFever 48
Join Date: Apr 2008
Posts: 6,380
Quote:
Originally Posted by Insequent View Post
Dave Gerr published a very good article in the Westlawn Masthead in 2008. Refer to pages 12-17. He advocates a better 'hull speed' calculation that includes displacement. I have not played around with his formulae to see how much effect weight has at slow speeds, I just know that my sea trial data fits his predictions quite well.
Brian

It seems that many of us with near 50 footers are in the same ballpark as your sea trial findings. What gph are you at around 8 knots?
sunchaser is offline   Reply With Quote
Old 02-12-2016, 03:53 PM   #36
Guru
 
Join Date: Apr 2014
Posts: 1,291
when we see a boat with a long WL narrow beam sharp entry big prop and relatively light build you are looking at efficiency. The modern trend in Trawler cottage types is in the opposite direction and no surprise it takes more power to move the cottage. Back when engines were considerably less powerful long low and narrow was the norm and efficiency came with the package.
eyschulman is offline   Reply With Quote
Old 02-12-2016, 06:35 PM   #37
Guru
 
Insequent's Avatar
 
City: Brisbane
Country: Australia
Vessel Name: Insequent
Vessel Model: Ocean Alexander 50 Mk I
Join Date: Mar 2012
Posts: 1,427
Quote:
Originally Posted by sunchaser View Post
Brian

It seems that many of us with near 50 footers are in the same ballpark as your sea trial findings. What gph are you at around 8 knots?
Closest data point from sea trial immediately after refit is below. Displ. at trials was 60,000lb, bottom paint had just been done etc. Note that I had Naiad's installed also, their drag will have taken some fraction of a knot off the performance.

1500 rpm, 4.0 gph (total) 7.9 kn.
__________________
Brian
Insequent is offline   Reply With Quote
Old 02-12-2016, 06:42 PM   #38
Guru
 
Insequent's Avatar
 
City: Brisbane
Country: Australia
Vessel Name: Insequent
Vessel Model: Ocean Alexander 50 Mk I
Join Date: Mar 2012
Posts: 1,427
Quote:
Originally Posted by Ski in NC View Post
I'm skeptical that the KK can make 7.7mph on a total burn of 2.5gph with twins. Could be true, but "show me" comes to mind.

Kind of like a kid saying his f150 truck gets 25mpg. BS flags pop up.
But Ski, it's only 6.7 kn and the boat has 52.3" LWL !

The JD's aren't that far above idle. That's the fuel consumption I have with higher rated versions of the same engine at that rpm.

It's like the truck is coasting downhill.
__________________
Brian
Insequent is offline   Reply With Quote
Old 02-13-2016, 08:04 AM   #39
FF
Guru
 
FF's Avatar
 
Join Date: Oct 2007
Posts: 16,537
But Ski, it's only 6.7 kn and the boat has 52.3" LWL !

The Sq RT of 52.3 is 7.23.

Efficient cruising is done at the sq rt times .9 to 1.15 depending on the hull.

7.23 times .9 is 6.5K , drag is mostly skin friction , little from wave making,

Seems about right.
FF is offline   Reply With Quote
Old 02-13-2016, 09:14 AM   #40
Senior Member
 
City: Portage
Country: USA
Vessel Model: Temporary between boats
Join Date: Aug 2014
Posts: 105
Once at a boat show I asked a Beneteau sales rep why fuel consumption of the ST50 is higher than that of ST52 according to the Boat Tests, which Beneteau was quoting in their marketing materials. The sales rep appeared to be surprised, started checking with his iPad, but could not find the answer. Took my business card and promised to get back to me. That was 3 years ago and I never heard from him since.

Major efficiency factors affecting fuel economy are, in descending order of importance, propeller efficiency, hull efficiency, and engine/transmission efficiency. Since hull and engine efficiencies should be relatively close between ST50 and ST52, the IPS propellers of the 50 must be responsible for high fuel consumption. If the Boat Tests numbers are correct.
__________________

Lost Horizons is offline   Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off





All times are GMT -5. The time now is 05:09 PM.


Powered by vBulletin® Version 3.8.8 Beta 4
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2006 - 2012