Double ended vs transom for fuel efficiency ?

Double ended vs transom for fuel efficiency ?


For fuel efficiency some architects choose double ended : Cap Horn 102 ( George Buehler ), Marguerite (Joubert ),
Other prefer small transom : Wildhorse, PBR 64’(Dashew), Less 82 (Saillard), Longcours 62 (Joubert),
And also some have ‘normal’ transom and good ‘fuel efficiency’ like the PL56 ( Tad Robert) or Range Boat ( Nigel Irens)
With transom we still have «*fun*» with nice ‘surfs’ but how much it cost in fuel consumption compared with double ended ?
We pass one winter in Norway on a river with good current (3, 4 knots, it’s why it doesn’t freeze !) and we could see the water ‘glued ‘ at our transom , you drop a small wooden part and it stay here ! At contrary at the stern of double ended Norwegian boat : nothing ! The water continue his way freely .
How much cost a transom(if cost something !) at the fuel efficiency ?
How much cost in terms of roll the research of the best fuel efficiency ?
Best regards
yannick

long-cours.62.over-blog.com/

 

Are you limiting the transom boat to NON- planing speeds ?

 

I think the transom allows better lines to the separation point at the stern. A good full displacement transom boat will look and be just about the same as a double end boat at the water line and below. You're experiment w the piece of wood must have been done with a semi-displacement boat with submerged transom. The semi-disp boat stern will be a little less to considerably less efficient dependent on the lines to the transom. However the full displacement Kady Krogen 42 (a transom boat) will have less drag aft than a double ended Willard at hull speed. Show us some hull lines for better answers but without more information I'd say double ended will be more efficient at slow speeds ...generally 15% or more below hull speed. If you want to run 15% over hull speed many transom sterns will be better.

 

It makes no difference at all. A well designed and efficient transom hull form will leave a clean, undisturbed wake, just like a well designed and efficient double ender. Using the name Buehler with the idea of efficient seems an oxymoron to me.

You have to talk apples to apples here. The underwater shapes of an efficient hull form will be similar between both transom and double ended craft. The ends will show some balance and well shaped waterlines without hard or abrupt direction changes, which can be a challenge in some double ended shapes.

If you want to plane off, well this is quite different as you'll need bearing area aft, which gets dragged around at off plane speeds in most cases, because of angle of incidence changes between speed modes.

Cape Horn 102 is Georges attempt at a near 6:1 ratio boat that will be efficient. Compared to other 102' vessels it'll do well, though most will want for elbow room, as this feels like a 50' boat with a few extra hanging lockers. It's pretty easy to make a 102' long vessel fairly efficient, relatively, but where are you going to park it.

Tad's PL56 uses good hull form shapes (unlike Buelher) and careful weight arrangement to achieve efficiency, plus the boat actually has elbow room. The two I list are completely different approaches and one clearly is superior.

 

It's clear we prefer the PL56 less weight, less draft, less mooring fee etc
I will try to joint the ligne of our actual boat , she his not bad in fuel efficiency :
- one engine running at 1400 t/mn : 6,6 nds , consomption de 0,61 lt per NM
- " at 1500 t/mn : 7 nds , consomption de 0,81 lt per NM
- " at 1600 t/mn : 7,4 nds , consomption de 0,92 lt per NM
- " at 8,3 nds , consomption de 1,46 lt per NM.
-two engines 9,80 nds , consomption de 1,78 lt per NM
But we are thinking : may be we make a mistake when we said to the designer to change his first proposal( double ended) to one with (small) transom .

We also noted than when the bow was 'trimed',say , around 6 inches higher than the normal wl our wake became 'nice blue flat water ' at normal trim the wake is like a Jaccuzi !!!

 

for have a look to the hull lines
http://long-cours.62.over-blog.com/photo-1911172-caracteristiquesection-1-_JPG.html

 

Longcourse62,
I'm just an amateur but I see this hull as optimized for speeds less than hull speed and see no advantage to a double end stern. This hull has a low PC at both ends and most powerboats are usually fuller aft. Looks like it was designed by a sail oriented person. The water line shows a very heavy displacement and in view of that and the low PC it would be hard to improve much on efficiency. Ther'es a little submerged transom but it's very small and adds to pitch stability w the very low PC. I'd say ...no need to think of a double ender at all.

 

A lot of transomed hulls are double enders with the stern cut off. At the waterline, they are the same.

 

Gonzo,
If the stern is cut off above the WL. My square sterned freight canoe is an 18' canoe made from a 20' canoe mould. They just put a dam in the stern 2' ahead and that canoe's transom is below the WL to be sure. It paddles better than I ever thought it would though.

 

This is true, but it makes me wonder if they can be still called "transom sterns", or are they just a special case of cruiser sterns.
For the resistance prediction (which is what the OP is essentially about) only the underwater part counts, so the case you have described is not considered as transom stern.

From the ITTC dictionary of Ship Hydrodynamics:

"Transom (stern), in which the buttocks and the waterlines, above and below the designed waterline, terminate abruptly in a transverse flat or convex surface or transom. The transom may be vertical or slightly raking aft."​

The key word here is "and" - which, as lawyers often teach us the hard way, is different than "or" .

Cheers

 

Most IOR boats were double enders at the waterline.

 

Transom sterns and double enders have some distinct differences. A transom permits the buttocks to remain flattened, even if the isn't immersed. A double ended hull usually requires buttocks to sweep up fairly quickly to met the stern post. This can be mitigated to some degree by dragging out the overhang and moving displacement distribution to provide "power" in her aft quarters.

Simply put, for a powerboat hull of the configuration the original poster is showing, he'll need these powerful quarters just to hold up the engines, tanks, and other gear, not to mention improve initial stability. It's up to him to decide how much quartering wave he wants to carry. The hull sections shown suggest a well immersed chine and a fair burden, so removing the transom drag may be just what he needs. There wasn't a profile that I found, so other suggestions are left wanting, but if it was me and I wanted a displacement hull form like this, I'd get that transom clear and settle on the preformance envelop this forces. On the other hand if he wants a semi displacement craft, I'd move some displacement aft and leave the transom slightly immersed, so she doesn't trim out weird underway.

 

Sorry but now we are on our boat and the profil is 1/20 and we can't scan it on board .
I have some dxf but I don't know exactly for joint to my message ...May be to old for playing with computer !
Now we are thinking lengtened (by ourself) our boat by arround 3 feets.

 

And then there are planning double enders such as the Bartender and IC canoe which have relatively flat and straight buttocks aft with a chine at the stern.

 

The Bartender, needs horizontal plates to maintain a stable plane. If you examine the lines of Fox's IC 14, then you'll see the displacement is dragged well aft as bearing area. In fact, you could that the original IC 14 lines and lop off the last 3 1/2 feet of hull (that which is aft of the rudder) and you have a craft that preformed exactly the same, likely with a preformance boost from the weight lose and cleaner exit. This said, you can get double enders to plane. Considering your current hull form and assuming the portions of the SOR that are forcing the development of these lines, you can just kiss off planning, unless you have pockets full of fuel money to burn.

I'd suggest you work up a solid SOR with your designer and let him sort out the details.