Planing Hull at Disp Speeds

Will,
The 40' Willard can go 7 knots w less power than my 30' Willard. Length is golden at slower speeds. What's interesting about this is that the GBs requiring the same power to go at the same speed (but different S/L ratio's) shows that the GBs are showing performance characteristic's that should be unique to displacement hulls. I don't think that this proves that they are displacement hulls but it's indeed very interesting. I used to cringe when the guys on Trawler Forum talked about cruising at "hull speed". I thought "hull speed" didn't apply to straight run square assed boats like the GB but perhaps I was wrong.

Easy

 

CC - I don't want to get into an argument with you about GB's - you've almost certainly seen more of them than me. I described the early GB's as hard-chined displacement hulls, albeit with straighter buttocks than you might find on what I would describe as a true full-displacement hull like the early Nordhavns. One could argue, I guess that they are approaching semi-displacement hulls... these things are not black and white...
However, I've attached the only pic I could find of the GB 42. This is not a planing hull.

 

Will,
Your thumnail says it all. That GB is semi-displacement .. no doubt in my mind. Clearly the forefoot is much deeper than the stern and the QBBL rises a lot at the keel but is almost flat at the chine .. a lot like the CHB. But just as clearly (to me) it;s not a displacement hull. Actually I'd love to have a 20' version of that 42 GB for an outboard to go 12 knots!
I did think of a way to see the difference (in drag) between a planing hull and a full disp hull. Go row your boat. Row a nice pulling boat (or other proper FD row boat w narrow wineglass stern and only the keel in the water aft. Then row a typical straight run aft planing boat like a Lund aluminum skiff and the difference in resistance and handling characteristics will be profound! It will seem like the skiff has 3 times the resistance. Go do it and I'm sure you will say the difference is huge. But this is only a seat of the pants experience (perhaps I should write a song "Pants on the Water") and I'm still looking for numbers, not pages of engineering but ballpark numbers.

Easy

 

Actually the difference in resistance due to transom immersion at speeds close to "hull speed" (S/L ratio 1.34) is not that significant. Win Willard of C. Raymond Hunt used to say that their deep vees were not that bad at displacement speeds. Much of this was due to light weight. At lower speeds wetted surface (friction drag) is a larger part of total resistance than wave-making which is mostly due to form.

The bare hull resistance of a 17,000 lb Willard 30 at 7 knots is about 167 pounds. That of a 33,000 lb W40 at 7 knots is 191 pounds....within the margin of error.

The newer version of the GB 36' Motoryacht at 31,000 lbs requires about 4gph at 7.5 knots, and about 5gph at 8 knots. This data from a 1996 review. The new GB 41 (planing) at 37,000 lbs requires 3.1 gph at 6.8 knots and 13gph at 10.8 knots. The 31,000 lb Camano 41 (semi-displacement) requires 1.8gph at 7 knots and 4gph at 9.4 knots. The American Tug 41 requires 5.9 gph at 9 knots and 2.7 gph at 8 knots.

I enclose a drawing by Ken Smith of the prototype GB 36, "Spray". Note that the rabbet line is that of a typical skeg built lobster boat.

 

I am not an expert on this but there is a number called the Immersed Transom Area Ratio or ITAR for short. It is the ratio of the immersed transom area to the maximum section area. Displacement boats, with no immersion will have a number of 0. A high speed planing boat will have a number approaching 1. Semi displacement and semi planing boats fall somewhere in between. From discussions I have had with NAs and hydrodynamicists it is important to get this number right.

 

Tad,
I know this brings up the old forum urban legend bubaboo but isn't that Camano keelform hull a variant of some sort of the box keel popularized by Atkin? By my rough estimation that Camano looks like it's using 36 hp @ 7 knots and 80 hp @ 9.4 knots.

It looks like that box keel boat (if that is what it is) has a modest fuel burn compared to the other boats you mentioned. How does the magic work?

 

Tad,
Great. Here are some numbers. Using the rule of thumb 20hp = 1 gallon the newer GB is applying about 80hp for 7.5 knots. The newer GB 41 requires about 90hp at 7.5 knots. The Camano and the American Tug numbers seen too good to be true. I suspect you got them from magazine boat tests and of course they get the numbers from the manufacturers. But allowing things to be too good to be true to be true .. lets guess 50hp for the Camano and 50hp for the American Tug. The Krogen 42 and the Willard 40 require about 35hp. That makes the required amount of power to push the average semi-displacement (leaning toward planing) hull at 7.5 knots is 200% of whats required for a FD boat. That's twice as much power as FD and I'd guess the full planing hull would require very close to 3 times as much. Will, you said 110%. I think it's more like 220%.

Easy

 

One has to be very careful trying to compare boats...it's easy to attribute performance to some factor while forgetting another. We can't just compare two boats and claim the performance differences are due to transom shape. We can only do this if we had two identical boats (same weight, waterline, CP, LCB, LCG, etc). That never happens...except occasionally in the test tank.

So we can try to reduce various factors to non-dimensional ratios. Speed/length (properly square root of waterline length) and pounds (disp)/HP are two ratios easily derived. The Camano 41 is 40' LWL and 31,220 lbs displ, while the GB41 is 38'LWL and 36,000 pounds. They are different sizes operating at different S/L's.

Comparing data published in Power & Motoryacht, RPM is from the engine guages, speed is a two-way average measured with a Stalker radar gun, GPH is from the engine manufactures fuel monitoring system or published fuel map.

At S/L 1.1 the Camano 41 requires one HP to push 834 lbs of boat, while the GB 41 requires one HP to push 562 pounds of boat. At a S/L of 1.75 the Camano requires one HP to push 202 pounds of boat, while the GB 41 requires one HP for every 373 pounds of displacement. Unfortunatly there is no data between these points (our area of interest) but see the attached graph. They come out pretty close......

Another factor in a given boat's performance is her propulsion system and it's efficiency....The Camano and GB have very different systems, the Camano a traditional propeller and rudder behind a large keel (Keelform hull), and the GB has twin Zeus (Cummins) pod strut drives.

View attachment FuelComparison.xls

 

TAD,
So many variables. Yes. I like your boats a lot and I like your commentary as well. I'm going to assume that I was mistaken in that the wide, deep and square ended boats only require about twice as much power or a bit more. I thought they needed 3 to 4 times as much. I may have been making judgments based on their wakes. That American Tug makes a wake that puts fear into the 14' boater. I guess that answers my question .. people design and buy boats w big wide sterns because thy are not as bad as I thought they were. Come to think about it, that's the only possible answer.

Easy

 

Sorry to have left the party - been away cruising for a few days....
As Tad has (eloquently as always) stated, its important to compare apples with apples - at least to the extent that it's possible. And also to be careful about the conclusions that you draw.
The rough 10% that I indicated was on the basis of comparing "similar" boats - length, WL beam, displacement etc. Sure there will be examples that are wildly different from this.
In the GB / Camano comparison (neither of which are displacement boats of course) it's important to remember that these are two quite different boats... the GB weighs almost twice as much and is two feet beamier for instance...(the specs I looked at quoted 28,000# for the Camano & 40,200# for the GB, which would narrow the difference beween them somewhat...).
Which brings up another interesting point... one should be careful about basing calcs on published data.... Sadly, it is generally all we have

 

Adding qualifiers.....

On the Camano weight....

Power & Motoryacht stated 28,000 lbs dry...
To which I added the weights they stated in their test results.
Fuel 130usg = 970 lbs
Water 90usg = 750 lbs
1,000 lbs of gear
two people = 500 lbs (creep )
Test total of 31,220 lbs.

Power & Motoryacht stated 37,000 lbs as half-load displacement for the GB 41
They also stated 190usg fuel (500 capacity) (< 1/4 load)
100 usg water (200usg capacity) ( 1/2 load)
four people (Full load?)
250 lbs gear(< 1/4 load)
So I took test total at 36,000 lbs.

The GB is 15" wider than the Camano, has twice the HP, costs $370k more, and achieves an additional 4.5 knots, approximately $82k per additional knot......

 

Tolly,
Are you saying I could put the 37hp engine in my 30' Willard in a 41' Camano w double the displacement and much increased wetted surface and attain a speed well above my cruising speed? Even w you being one of my favorite people here thats a really hard one to swallow.
TAD and Will,
I sorta thought we had a problem w number sources. Sorta like listening to Fox or CNN news (or what ever it is). People that have magazines or boats to sell hardly ever give objective information. By the numbers I saw the Camano would burn half as much fuel as I know the 36 GB burns to go 8 knots. Full displacement boats are easy to predict. The Willard 40 and the Krogen 42 (and probably the 39 as well) are very comparable boats. Power, speed, displacement and length. But once you enter semi-disp no two boats seem to be alike. Not even if they have the same power, speed, disp and and length. In regard to the thread question can anybody think of two boats that are typical of the type that have comparable power and are on the upper end of SD or a planing hull? But then what's the point if we don't have any way of telling the fuel burn and hence the load on the engine and thus the power required to do the trick.

Easy

 

Easy,
Ye of so little faith
I am extrapolating from Tad's fuel burn numbers. A diesel provides about 20 hp per gallon per hour, a gas motor roughly half that.

"The 31,000 lb Camano 41 (semi-displacement) requires 1.8gph at 7 knots and 4gph at 9.4 knots"

1.8 gph @ 7 knots = 1.8 * 20 = 36 hp @ 7 knots
4gph @9.4 knots = 4 * 20 = 80 hp @ 9.4 knots

34% more speed takes a little over 120% more power. The horsepower estimates are only as accurate as the reported fuel burn.


"But then what's the point if we don't have any way of telling the fuel burn and hence the load on the engine and thus the power required to do the trick. "

Personally I have a flow meter on my boat and I always know what I am burning. It's been the best investment I ever made for my boat. It's my go to gauge. I even track the sweet spots in currents and back eddies by watching the far right side of the decimel point.

I'm fascinated with hull dynamics and powertrain optimization. It's sort of like sailboat racing back in the day with many more creature comforts. It is my belief that except for racing craft the vast majority of small boats are not designed for optimized performance.

The designers shoot for a fat sloppy middle ground. And who can blame them. The target is too fuzzy. Most of their customers either don't understand or won't accept the compromises neccessary for optimization. The easist thing to do is make sure there is a big enough motor to push that bloated floating condo up to some reasonable level of speed. They want it all. Space, all the comforts of home, speed, economy. People complain about fuel economy but they scream about speed. LOL until they don't. By then it is too late. The boat is built and they wouldn't have bought it anyway if it had been too slow. Stepping down from my soapbox.

 

Hi Will,
I'm not interested in faith. I'm looking for fact and I don't believe a Camano 41 at 7 knots will burn less that 2gph. I'm chokin on the hook Tolly. And since I'm already skeptical of flow meters, scales, speedometers and many other instruments because they frequently aren't accurate at the lower end of their scale. I know I burn less than a gallon an hour so a flow meter is not even on my list. Here's a picture of my Willard.

 

A mast and a hayrack and you'd have a little pocket troller. What do you heat her with?

I'm pretty happy with my flowmeter for several reasons but it is my obsession I suppose. It has smoothing and calibration features and the first 6 or 8 tanks of fuel I kept tuning on it. Tracking distance traveled via gps, fuel burned, and fuel to fill the tanks I got to within 5% quite consistantly. That's close enough for me. It seems to work well at low speeds, I should do some experimenting. I usually don't go past 3/4 throttle. I run between 6-15 knots or so. It tops out at 22-24.

At a gallon an hour a flowmeter is certainly not going to do as much for you as it does for me! When you're slaking the thirst of a healthy V8 priorities may differ.

I don't have an opinion on the accuracy of the burn numbers Tad posted. I used them straight up. I generally take the claims in a brochure with a grain of salt, but Tad has forgotten more than I will ever know so I trust he knows how to translate a brochure.