Fuel burn...

It´s way less a matter of wetted area than of wave resistance. For example Mapism´s boat is much faster. But mine makes others really go to sea when @wide open throttle, in fact everything above 16kts is unacceptable for others because of the ridiculous waves it generates. Was never designed for cruising speeds in excess of 16kts though and is a pretty solid sea boat with very deep V.

The fact is, you have NO WAY of measuring actual load at a given RPM and load (weight), nor do you have a way of measuring actual fuel consumption. Trying to measure an hour of fuel burn out of a 800 gallon tank through site glasses after only running an hour is like pissing in the wind and claiming to know your exact heading, due how much a little rounding error could change the results. NOW, if you ran at 1900 rpms for 5 hours started with a full tank, ended filling the tank with a fuel pump, you may have an actual idea of what your burn is, but still have no idea what load % it was. What the seatrial on the CABO shows, is ACTUAL fuel burn, RPM, load, etc. within 99.5% accuracy.

@ Mapish M ….if you came into this thread saying something like “ those Cabo figures are interesting because I have the same V8 800 MANs in a 17 M 50 % heavier etc hull ……and mine burns another 20 % -or what ever more .

Then no one would bat an eyelid .
This thread would not exist .Period !

Ie your numbers @ 1900 rpm ave cruise gen fitted into the norms and experience of other users .

Albeit at a significant lower speed like 23 knots compared to the Cabo’s 32 knots ( or what ever ?) …….because of your the bigger hull / weight etc but as we keep telling you unknowingly you could be on 89 % load while the Cabo we know is 79 %

Or worse still you push up further to 26-27 knots guessing in the mid nineties % load .
The consumption graph at the high end of the load graph is not linear , or I will say from experience looking at the screens on my boat it’s more like a ski jumping slope .A letter J .As you ascend it gets steeper .
Rockets up towards the end .
Repost of earlier pics to save flicking pages .

From 1 to pic 2 , see the 4% rise of rpm the 124 rpm increase and we go up 4 % load .But fuel rises 11 % …….it’s just at the beginning of that ski jump slope ( going upwards folks )

Indeed illustrates the “ sweet spot “ if such a thing exists….for my boat the balance of Hp , hull speed , around 30 knots and economy and arguably reliability/ longevity of the now 20 y old motors .It feel so right too FWIW .It positioned at the bottom flat end of that ski jump slope .



Pic 2 to pic 3 we ascend the steep slope to the top , climb up that slope in ski jumping language to where the guy sits before he pushes off .
Another 15 % rpm rise to the end and loads increased by 19 % but fuel burn in simple maths 37 - well near enough double the load rise for sake of argument .

So if the smaller lighter lower wetted area hull of the Cabo ( and mine compared to yours sacrificing 100 Hp and 4 litres of cubes and 2 jugs per side ) is set as Capt J says at 80 % load ……..then how do you know yours is not on that steeper incline of the ski jump incline for fuel burn ?

It’s slower sure .But you don’t know where the bottom area the area that flattens out is on the slope without load gauges for meaningfully comparable burn figures.

As said from what you quote those numbers how ever well intended dont add up , fit in to the numbers folks like myself, Capt J and probably many others see on there screens every day .

There’s an error a miscalculation, a miss calibration on your behalf.
Btw that’s does not stop me reading your posts or reply ing to them .

But you CAN'T. An hour running at cruise is less than 5% of your tanks capacity. Measuring it by the sight glass. If you are off 1/4" in measurement it changes your calculation greatly. It's impossible for you to measure fuel capacity accurately especially at sea. The other situation is with mechanical gauges you could easily be up or down 20-50 rpms from what you want to be at with mechanical gauges. I've seen mechanical RPM gauges off by 100 rpms on numerous occasions. Trim also effects load factor. While your measurements or GUESTIMATION are accurate for your needs and repeatable on your boat, they are not accurate enough to compare to other boats due to all of the various errors of measurement. You have no idea what your actual load is at a given RPM. You have no idea what your actual fuel burn is at a given rpm, since you're not sure you're even at that rpm unless you go to the engine room with a photo tachometer. You have no idea what your load is at a given moment due to trim. On many of the boats, I can go from 80% load without using trim tabs, down to 75% load by trimming the boat correctly and the fuel burn goes down as well. There are too many variables that your boat is simply not able to measure.

NOW, on an electronic diesel, load is very accurate on the display, it's repeatable, as is fuel burn. So you can easily compare it from boat to boat. On Caterpillar displays, you can record total fuel burned and can reset the display when you fuel. It IS over 99% accurate and repeatable. Hell on my Mercury 300r it records the fuel and is resetable and accurate running the boat all different rpms in a day. If it says I burned 38 gallons of fuel, that is all it is going to take. So what it's recording from the ECM is very accurate. I'm not stuffing 39 gallons in there.

@ MapishM .As per Capt J ^ .
In summary your spread sheets might be good for you own personal use , but useless when comparing figures out side with the guys with screens .
Any miss calibration ( my point again ! ) just carries through the entries in your spreadsheets .

My credibility point .

It’s only when you innocently pop your head up out from your ER from a sight glass reading and start communication about consumption with those with “ screens “ that eye brows are raised .

There’s not really much more that I can add or show from real time images on the subject .
Except arrive at the conclusion that your quotes fuel burn figures re the Cabo ( which is very close to mine ) are wrong ….too low .

Picking up on Capt J s point about even the slightest trim changes alters the screen numbers , load , fuel burn and EGT s .

A few years ago ….think Apollo mission think PTFE , the none slip anti friction coating the world was blessed with as a by product of putting Armstrong on the moon .
Was in a chandlery a few yrs back and they had a sale of antifoul ( reaching its best use by date ) with added PTFE .

It was 1/2 price of eq without .
Yeh right more slippy .Sure ?

Well I bought it at that price and applied 2 x coats in the usual way .
Yup ….it lowered all those numbers and gave me a knot or two more at like for like rpms .
Unfortunately I never saw it on sale again the following yrs .
Point is this it’s actually the screens in my case the aged MMDS ( buy they tell me enough) circa Boening 2003 ….that lead me to a greater understanding of marine diesel functionality.

With out them watching the Load , EGT + burn rates you really have no idea what changes effect what .

Or exactly where you are on the base of that J shaped slope or any graph for that matter .

You are just guessing and gut feeling , maybe hearing the engine sounds , maybe watching the stern wash , who know maybe miss calibrating tank sight marks extrapolating a tiny error up on a J shaped curve which results in a 20-30 % real world error .

No one’s doubts what you say just challenging where it’s sits with guys with screens .