SD Hulls and Fuel Consumption

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Gludy

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I thought i would relate my finding having completed my trails on Ocean Deep with an SD hull and having used Drumbeat with a planing hull.

On Drumbeat I spent most time cruising on the plane at 24 to 26 knost. Fule consumption varied between 0.5 and 0.6 mpg. I could not really travel comfortably at lower displacment speeds as the hull shape would ensure a lot of motion in any sort of heavy sea although that option was there in slight seas.

on the SD hull of about the same length etc - I found that crusing at 19 knots made the most sense - I was getting just 0.5 mpg. If i went at say 17 knots consumption was worse. If I went at 8.5 knots I could get over 2 mpg at this reduced to 1 mpg at 9.5 knots. All interesting stuff.

The ride at all speeds was better than the planing hull but the stabilisers were essential.

Basically I was losing about 6 Nm per hour for the same fuel consumption as the planing boat but getting a much better ride.

In this SD hull you either go at 8.5 knots or 19 knost - other speeds do not make much sense.

Assuming fuel cost of £2.25 per gallon and an engine service cost of £5 per hour - the costs per mile of the SD hull are:-
8.5 knots (2.2 mpg) £1.02 per mile fuel plus 0.58p service charge = £1.60p per mile.
19 knots (0.5 mpg) £4.50 per mile fuel plus 26p service = £4.76p per mile.
Planing boat at say 25 knots £4.50 per mile plus 20p service costs = £4.70p per mile.

Real long distance stuff through the night etc would be done at the 8.5 knots and hopefull use currents to boost that to 10 knots. At these speeds the ride, even in failry rough seas is very comfortable.

I hope these figures are of interest to some.
 
interesting reading

is there any way to tell how well you're doing MPG wise without having a guage on the dashboard that tells you?

i would like to find that more 'economy' speed on my boat. and likewise find the ones to keep away from

btw how was the passage round lands end a couple of sundays ago? had the sea settled down?
 
Well yes and no.
We got to Northney because there simply was not enough water on monday over the cill at Emsworth. The next day Trader collected her and she is now in Emsworth.

It was a great trip with the highlight being a pod of dolphins around Eddystone lighthouse. Cruised at 19 knots nearly all the way and decided I always needed the high speed option on a boat.
 
My figures are based on the cat computer and fule refill chacks. I am having a fuel flow meter system fitted.

We took at f5 sometimes f6 NW all tyhe way down the Bristol Channel to Lands end - Lands end itself was fine as it was once hidden from the wind at the Lizard.

The rest of the journey was easy except for container skipping and the unexploded mine near Nabs tower but the later was too far away to block entry into the river. /forums/images/graemlins/laugh.gif
 
The Navman 8120 has an input from diesel fuel sensors. This information is presented as a graph on the display unit. It processors the data from GPS and diesel sensors to give you the fuel consumption at different speeds and shows you the best cruising speed for the best consumption! Costs a bit for all the kit, but (apparently) you should recover the costs by being able to run more efficiently over time!

Garry
 
Thanks for sharing this, Paul. At the time you were considering the purchase of the Trader, I seem to remember that you were quite adamant that the fuel consumption of the SD hulled Trader was going to be better at displacement speeds than the Squadron. Do you have any consumption figures for Drumbeat at 8 or 9 knots and has it worked out this way?
 
Prior to purchase I had calculated that the Trader would give me the same fuel consumption at 18 knots that the squadron gave at 26 knots.

I do not recall going into the fuel consumption at displacement speed on the Squadron because basically the planing hull is not really suitable for low speed work in any sort of sea. I also never went anywhere at 8.5 knots in Drumbeat to work out any figures.

I am also comparing a 1996 Volvo to a 2005 cat engine - maybe the modern Drumbeat engine would provide better fuel consumption.

Basically at speed the SD hull is about 25% less efficient than the planing hull. The ride is very much better in the SD hull and you pay for it with that inefficiency.

At low speeds the planing hull is not really viable for most of the conditions I boat in so any consumption at those speeds is academic.

Basically - The SD hull offers me the option of low speed economic long distance cruising not on offer from the planing hull and a better high speed ride at the cost of a drop of 25% efficiency in that high speed ride.

The 72 foot marlow actually beats Ocean deep giving better fuel consumption at low speeds because it has a longer hull and higher displacment speed coupled with its lightweight and the new efficient c18 cats. So I have higher low speed crusing with even better fuel consumption and a 4000 mile range.

My recent experience has now ruled out the Nordhavn option - I really do want the option of higher speed cruising.
 
I Seem to remember some years ago now, Kim Hollamby of MBM comparing a displacement Pedro steel crusier to a Sealine 365, when they both ran at displacement speed on a MBM cruise. The Pedro returned about 3.5mpg and the Sealine over 5mpg, the larger engines doing little work at about 7-8knts.
Certainly in my own experience of lots of boats on the river, planing boats do seem to be more efficient than SD or D hulls at low speed. However as you say not very comfortable in any sort of sea.
 
Whilst, as you say, it is not really of great interest because planing hulls in any sort of sea other than calm are not comfortable at low speeds - in fact they depend on their speed for stability, what you are saying I cannot accept.

What really governs the fuel consumption with like for like is the waterline line length providing the maximum displacement speed - the longer the length the higher the very efficient displacement speed.

Normally on two boats with the same overall length - one SD and one planing, the planing with have a sharper rake back at the bow and will have the lower waterline length - hence it would have a lower displacement speed.

Im my example just one know from 8.5 mph to 9.5 mph more than doubles the fuel consumption and that is because the displacement speed has been exceeded and the boat is beginning to climb its own wave.

My example has two boats which are within an inch of each other with their loa one has twin 600hp engines and the other twin 700 hp engines - the SD with the latter.

What were the displacement speeds these boats run at? A do not know the WL length but at a guess the Sealine had a WL of say 30 feet that would give a displacement speed of about 7.6 knots and my guess would be that it may be nearer to 7 knots - is that the sort of speed the MBM boats cruise at?
 
Fascinating reading, thanks for posting.

Were you able to ascertain what a difference sea state made to economy ?

You clearly enjoy speed and I guess the only time you reduce to 8.5 knots is due to sea state ????????
if that is correct do you think the fast and slow stats could actually be even further apart if tested on calm days ?????





As I dont like it slow or rough my solution is to boat less and wait for calm seas. /forums/images/graemlins/cool.gif
 
At the risk of starting a long Gludy thread, but the facts speak for themselves, I'm not going to offer an exact scientific proof but as I understand the following was true. The 2 boats had waterline lengths of approx 33ft, the theoretical displacement speed therefore is about 7.5knts (Square root WLL x 1.3), this was the speed they were cruising at I believe. The Pedro was twin shaft drives approx 100hp each running at approx 60-75% revs consuming about 2gph, the shafts and hull shape are quite high drag. The Sealine would have been running at only 15-20%revs consuming only 1.5gph, it is of course sterndrive with no keel and less displacement, so is more easily driven. Perhaps the difference can be explained because the sterndrives are simply more efficient. But having read many boat reports over the years and compared fuel consumption, i'm always surpised at how uneconomical SD and displacement boats are compared to planning boats at low speed.
 
I agree with gludy. That may have something to do with waterline length. If I remember correctly, Kim's Pedro was 33ft long. Say it had a waterline length of 29ft, that gives a theoretical max hull speed of 7.2 knots. Say that the 365 had a waterline length of 33ft, the max hull speed would be 7.7 knots. So, the Pedro would probably have been at max hull speed at 7-8knots which would have made it a lot more inefficient than the Sealine which would not have been so close to max hull speed
I am sure there are other factors involved as well. Maybe as you say, the Pedro engines would have been working much closer to their max rpm, maybe the extra weight of the Pedro's steel hull affected consumption, maybe there were differences in hydrodynamic efficiency. Also, last but not least, it's actually quite difficult to measure fuel consumption over a single passage unless you have a flow meter because you can never be sure that the tanks are completely topped to the same level from one fill to the next
 
I actually enjoy going slow - the engines are in the background and its a very pleasant experience - just that I often do not have the time.

What i enjoy most is picking up a 2.5 knot current and doing 11 knots on 2.2 mpg - thats nice.

At 8.5 knots through the water I can hvae a range of 1500 miles call it 1200 miles for safety - that means I can go a long way and enjoy the trip.

Heavy weather does cost fuel - I think it can add 20% or so to the fuel bill.

I think that in calm weather at 8.5 knots I amy get 2.4 mpg instead of 2.2 or 2 mpg but in rough weather that can drop to 1.5 mpg.
 
I agree with all of that Mike.

There are many variables and It may well be that if you took a planing hull and an SD hull with its keel both the same WL length, both the same engines that the planing hull would be more efficient simply because of less drag but that is really not the point. To achieve those things the planing hull would have to be a bigger/longer boat and whilst low speeds may be Ok in calm weather it would not be the way to go on a long journey in rougher weather.

I am not, repeat not claiming that an SD hull is better than a planing hull - I am simply stating what appears to me to be the advantages and disadvantages of each as regards fuel consumption. LKets face it I have dropped 25% in fuel efficiency for extra comfort and have a slow speed option I did not really have before...... I also can go into a head sea fairly well and maybe achieve higher crusing speeds in head on sea ...... thats about it.

If I wanted to be based in the med and go out from a central base for weekends - I think a planing boat would be better for the job. Its horses for courses.

I can add that providing you have stabilisers working fine then the SD hull is OK in a following sea - I have experienced hours with a f6 , sometimes f7 behind me and was able to achieve 18 knots no problem and no tendency to broach. the speed itslf is of course a factor - it may have been different at 9 knots.
 
I have no experience of 50 foot boats but don't quite understand why you found it best to cruise at 24 to 26 kts in the planing boat.
Our 1991 Broom 37, which was to all intents a planing hull with a tiny keel, would top 28 knots but we usually cruised between 18 and 22 kts, a very comfortable and relaxed speed which clearly suited the hull as consumption never fell below 1 mpg, even with the old tech' VP 306hp engines.

One of the boaty rags did an excellent comparison of SD vs Planing hulls. It was something like a 40 foot Fairline vs a 40 foot Nelson, and although the Nelson was undoubtedly the better sea boat, in the sort of weather that most will go out in the modern planing hull is so good and the space so much more comfortable and accommadating, that there was not much point in the Nelson unless one particularly wanted one.
SD's at above full displacement speeds may be comfortable in a seaway but I do not believe they are efficient.
I think Deleted User said as much in your original postings on the subject, especially relating to Traders, one of which I believe he owned.
 
I found that there was no fuel saving doing 18 knots in Drumbeat - the whole boat seemed to like the 24 to 26 knots and so there was no point in going slower.


I am not claiming one is better than the other - it depends on your use of it.

I do not understand the point about space. I have about twice the space in the Trader than I had in my planing boat.

I also go out in f5 as a regular happening - it never dropped below f5 on my last trip to Lands End.

Also I was cruising in much more comfort with a smoother ride than the planing boat. So I really cannot go along very far with your points.
 
Miket, actually I did'nt own a Trader but a friend of mine did own a Trader 41 and we cruised in company many times. It was an interesting comparison because the 41 was fitted with TAMD61 engines (306hp) as your Broom and it was flat out at 18knots. At the time I had a Princess 435 which reached 28knots flat out with the result that my boat was significantly more economical than the Trader 41 and yet offered a similar amount of accomodation and was a similar price on the used market. My friend did try passage making at significantly slower speeds but he found the rolling motion to be unpleasant (no stabilizers). The Trader was also a very wet boat.
Overall at the time I could not see any advantage in having a SD type Trader boat over a good planing boat unless, as you say, you really want one which I can also understand because the Traders are distinctive and have huge deck space.
I think also you have to differentiate SD hulls and SD hulls. The type of SD hull that Trader use, basically a hard chine hull with a big keel is very different from the round bilge hulls used by the likes of Nelson. Then you get something like a Grand Banks which has elements of both. Not all SD hulls are going to be more comfortable at displacement speeds than a planing hull. I suspect a Nelson would roll very heavily in a beam sea
 
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