Semi Displacement - best hull shape fuel efficiency

[Bajansailor]

Paul, you had noted that :
"Those who own a planing boat at these low speeds would probably do even better because there is less drag - no deep hull, no keel to drag through the water".

For the same length hull there probably would be less skin friction drag (which is basically a function of wetted surface area), however a planing hull will usually have a lot more form drag than a displacement hull at displacement speeds, as form drag is more dependent on the hull prismatic coefficient. Planing hulls have relatively high prismatic coefficients in order to have planing surface aft, whereas displacement hulls usually are designed for optimum prismatic at typical displacement cruising speeds.

The bottom line is that for displacement boats at displacement speeds a large diameter, slow revving propeller will always be more efficient than a smaller diameter propeller that is wizzing round much faster.
Stoaty's Lister is a classic example of this.

Another example is an old Norwegian Colin Archer fishing boat I knew - she is 55' x 17', weighs probably 45+ tonnes, and she used to have a big Wichmann single cylinder diesel (that developed all of about 60 hp) driving a 4'+ diameter controllable pitch prop, going round at probably 120 rpm. And at cruising speed, using maybe 40 hp, she could happily do 6 knots.
Oh, and her flywheel weighed 1,500 kg.....

 

[Gludy]

Thanks - I find all that interesting and I am learning.

#I do not dispute anything you say but would like you to comment on the published figures for boats like the Nordhavn and many others as reviewed in the mags - these do not seem to share the same super efficiency as Stoaty's boat.

I am in open minded learning mode here - just trying to make sense of those figures that i see published all the time.

 

[Bajansailor]

Re the published figures.... the only explanation I can think of is that the Nordhavn's have relatively larger engines (running at higher speed, and driving relatively smaller propellers) than Stoaty's Lister or Lista Light's Wichmann, and maybe this is why their relative efficiencies are less.

Taking this philosophy to the extreme, a super tanker might have a controllable pitch propeller 30' in diameter rotating at perhaps 60 rpm, yet it is quite capable of propelling maybe 100,000 tonnes through the ocean at 12 knots.
(This is just a very rough generalisation)

 

[Gludy]

But itsn ot just Nordhavn's - if you look at published figures for many d hull boasts yo tend to get the same result yet i ttoally accept what you are telling me as well - so I am confused.

 

[Stoaty]

My boats previous owner told me that she would plod along quite happily at 6 knts on a 20hp auxillary engine that used to be fitted.

You did say "at slow speeds" If I knock my boat into gear at tickover 450-500 rpm I will get aound 3 to 3½ Knts that's slow in my book, and I would not be surprised if I used a pint and a half an hour.

I have seen some big, 100ft plus displacement boats running quite happily on engines under 100 hp.

When they say the comsumption for the Nordhavn is 2½ gallons an hour at 8Knts I think they are referring to US Gallons which equates to 1.87 Imp gallons or 4¼ Nmpg. Its a shame they didn't publish the figures for 6Knts I would expect it to be much more economical. It has a very nice looking hull shape, they must have nicked the design off a cygnus.

What I do know is this. If you try to push your displacement boat above its hull speed, then large amounts of fuel is spent into producing wake and a hole in the sea.

 

[Stoaty]

Reading the specs on the Nordhavn 43 it has a Lugger L1066T engine that produces 165hp and 8.5Knt max speed

It also says this.

[ QUOTE ]
Although the reliability of single-engine installations has been proven by scores of successful circumnavigations, some owners may wish to opt for an auxiliary "wing" engine. This "get home" package features a totally separate 27 hp diesel engine with its own independent electrical system, separate l0 gallon day tank, transmission, shaft and propeller. This engine will drive the vessel at 6 knots in calm weather (3 to 4 knots in difficult conditions) and will maintain an electrical charge to the house system.

[/ QUOTE ]

I would imagine that running the main engine at low speed it would sip fuel at 6knts.

 

[gavinharris]

Hey - thanks for all the feed back and in depth comments. I am new to using forums and realised it must seem rude not to reply. I now have a bettera understanding of what I need to be looking at. There is no 'ideal' boat - semi-dipsacement/planning/displacement is all going to vary. Depends on your lifestyle/location/budget/etc. Great reading other comments on related threads.
Cheers

 

[holderone]

Hi,

I don't think anyone has talked about IPS and hull shape, I have a Flyer 12 on IPS designed semi displacement hull and I am amazed at the sea handling in all sea conditions and fuel usage. You get all the benifits and non of the down sides as far as I can tell although I haven't had too many boats to compare with. Certainly worth looking at.