Most efficient speeds

[Channel Ribs]

My instinct says that trim tabs cause drag which is less fuel efficient, however maybe a little trim and staying on the plane at a slower speed is more efficient i.e. better MPG.

Trimming down, so that the tabs are protruding slightly will bring the bow down, this can improve performance depending on the weight distribution of your boat.

The most noticeable influence trim tabs have is on ride comfort/handling, as has been said you can use them to alter the boat's attitude to suit the conditions.

 

[Deleted User YDKXO]

I'm also not convinced that trim tabs make much difference either.
I normally set them about mid way and then trim them to keep the boat level (that is the port and starboard sides of the boat at the same level).
Generally, I dont use the trim tabs for much else.

I used to think that but Ferrettis definitely go a knot or 2 faster for the same rpm if you fiddle about with the tabs. My old F46 used to go best with the tabs about 3/4 down; I always assumed that this was because the engines were located well aft so the hull benefited from some lift at the stern. The new one seems to cruise best with tabs about 1/3 down. You know yourself that if you can squeeze out a bit extra speed for the same rpm, that translates into a proportional improvement in mpg so it's worth having a fiddle with the tabs to find the optimum setting at your usual cruising speed

 

[tinkicker0]

According to the fuel burn specs for my boat, the figures for 5 knot displacement cruising and 75% RPMs (30 knots) the relative MPG is the same.

However fuel burn in GPH must be 6 times greater at 75% RPM, than at displacement speed.

Worst consumption is around 10 knots and with the size of the wake, I can well believe it.

 

[Deleted User YDKXO]

According to the fuel burn specs for my boat, the figures for 5 knot displacement cruising and 75% RPMs (30 knots) the relative MPG is the same.

However fuel burn in GPH must be 6 times greater at 75% RPM, than at displacement speed.

Worst consumption is around 10 knots and with the size of the wake, I can well believe it.

Umm, thats v unusual indeed. Normally the faster you go, the more fuel you use in terms of mpg although as you point out, there's usually a decrease in mpg around hump speed. What kind of boat is it and what are the actual figures? Would it use even less fuel at 40kts

 

[MapisM]

Umm, thats v unusual indeed

Unusual yes, but not impossible. See my post #22 in this thread.

PS - since you asked about what happens at 40kts, actually on my boat the mpg is still better at 60 than at 5... (1.8 vs. 1.5)

 

[tinkicker0]

Umm, thats v unusual indeed. Normally the faster you go, the more fuel you use in terms of mpg although as you point out, there's usually a decrease in mpg around hump speed. What kind of boat is it and what are the actual figures? Would it use even less fuel at 40kts

Test Power: 1x135-hp 3.0L MerCruiser w/Alpha I Drive Range
RPM MPH Knots Total GPH MPG NMPG Stat. Mile NM KM KPH LPH KPL dBA
650 3.0 2.6 0.7 4.29 3.73 89 77 143 4.83 2.6 1.82 71
1000 3.1 2.7 0.8 3.88 3.37 80 70 129 4.99 3 1.65 71
1500 7.9 6.9 1.6 4.94 4.29 102 89 164 12.71 6.1 2.1 75
2000 9.8 8.5 2.4 4.15 3.61 86 75 138 15.77 9.1 1.76 78
2500 18.2 15.8 3.6 5.04 4.38 104 91 167 29.29 13.6 2.14 84
3000 24.2 21.0 4.1 5.96 5.19 123 107 198 38.95 15.5 2.53 84
3500 29.5 25.7 6.2 4.80 4.17 99 86 159 47.48 23.5 2.04 84
4000 34.0 29.5 7.5 4.56 3.96 94 82 151 54.72 28.4 1.94 89
4500 40.7 35.3 9.5 4.28 3.72 89 77 143 65.5 36 1.82 94
4800 42.7 37.1 10.5 4.07 3.54 84 73 135 68.72 39.7 1.73 98



Edit: table did not turn out as expected, see post below for proper attempt.

 

[Deleted User YDKXO]

Unusual yes, but not impossible. See my post #22 in this thread.

PS - since you asked about what happens at 40kts, actually on my boat the mpg is still better at 60 than at 5... (1.8 vs. 1.5)

I can understand the difference on the plane in your boat which is designed for high speed; presumably the hull/engine is working more efficiently at 60kts than at 40kts but it's difficult to see why 5kts is less efficient than 30kts in tinkicker's Bayliner unless the engine is grossly inefficient at d speed due to fuelling issues or something

 

[tinkicker0]

Umm, thats v unusual indeed. Normally the faster you go, the more fuel you use in terms of mpg although as you point out, there's usually a decrease in mpg around hump speed. What kind of boat is it and what are the actual figures? Would it use even less fuel at 40kts

As can be seen, 2 rises in fuel economy, one at around 1500 rpm / 5 knots and one at 3000rpm / 30 knots.

Edit:

Need new glasses, 3000rpm / 20 knots

 

[DAKA]

Normally the faster you go, the more fuel you use in terms of mpg

I agree flat out burns a lot of fuel but in my experience a faster cruiser speed is more economic than a slower one.

My boat is sort of on the plane @ 16 knots but does not really settle down until 19 knots.

The bow lowers a little further at 22 knots and boat is at 'cruise level' at 25 knots (max speed 30, but boat stays at same angle).

In my opinion the most economical cruise speed for me is 25 knots.

I explain this by pointing out the angle of the prop shaft.

I agree less fuel is being burned at 19knots but a chunk of power/ propulsion is being directed at the seabed instead of astern of me because the bow is still too high.

 

[DAKA]

As can be seen, 2 rises in fuel economy, one at around 1500 rpm / 5 knots and one at 3000rpm / 30 knots.

Is the Bayliner on outdrives ?
(F44 cant change the angle of his shafts)

 

[tinkicker0]

It is. single 3.0 litre mercruiser (chevrolet) and an alpha one gen 2 drive.

Very economical combo as far as boats go.

 

[Deleted User YDKXO]

As can be seen, 2 rises in fuel economy, one at around 1500 rpm / 5 knots and one at 3000rpm / 30 knots.

Edit:

Need new glasses, 3000rpm / 20 knots

Where did you get the fuel burn figures from, measured or from the manufacturer's data?

 

[tinkicker0]

Where did you get the fuel burn figures from, measured or from the manufacturer's data?

Boattest .com

http://www.boattest.com/oem/boat-test-oem.aspx?id=548&lp_id=354&p=0#TestResult

Mine may not get quite those figures as its the cuddy cabin model, so a teeny bit heavier.

 

[asteven221]

Went out at the weekend and where we ended up was mirror calm so I did some messing around with the throttles. Without doubt the boat starts to lift the bow at anything over 7 knots and 1000rpm. I guess that is conclusive as to my maximum displacement speed. No trim tabs were used at this point. I then eased the boat up in speed, the bow started to rise and didn't apear to ever come back down again even at 20 knots. Maybe I should have increased speed further. The trim tabs brought the bow down but to be honest I am not sure what that proves, other than the trim tabs work!

I am not really sure what to expact as I have not been on anyone elses similar boat despite years of sailing/mobo'ing! With my RIB it's obvious going through the hump speed and then going on to the plane. Likewise it's very obvious when the boat starts to drop off the plane. With a 44 footer - it just doesn't seem to give me the same obvious feedback, so I am not sure where we are with it. SWMBO suggested a spirit level which I thought might be a good idea for reference.

FYI we were carrying half a tank of fuel and there was nothing on the bathing platform e.g. a RIB. Given what I have read it above looks as if I will need to mess around with the trim tabs and check out any speed variations/improvements at different settings.

I take it nobody else has an F44 or 420 that can advise. Interestingly my pal has 42/5 which is the replacement for ours. I notice has large plates (about 25 inches or so) added to the trailing edge of the hull. Could that be an indication that the hull doesn't get on the plane very easily?

 

[KevB]

Of course with an outdrive boat you adjust the legs to change the bow angle and the tabs are just to keep it from leaning over to one side.

Using tabs to adjust bow angle on an outdrive boat only causes drag and makes it less efficient.

 

[Deleted User YDKXO]

Boattest .com

http://www.boattest.com/oem/boat-test-oem.aspx?id=548&lp_id=354&p=0#TestResult

Mine may not get quite those figures as its the cuddy cabin model, so a teeny bit heavier.

Thats interesting. I have to say I've never seen figures like that before. Do you think this is a particular characteristic of your boat or do many petrol engined boats behave like this?

 

[tinkicker0]

Thats interesting. I have to say I've never seen figures like that before. Do you think this is a particular characteristic of your boat or do many petrol engined boats behave like this?

I don't really have the knowledge to give you the definative answer to that question.

Maybe other peeps with gas boats can chip in?

 

[BrendanS]

Thats interesting. I have to say I've never seen figures like that before. Do you think this is a particular characteristic of your boat or do many petrol engined boats behave like this?

My Regal 2150 had a curve that showed steadily rising MPG once on the plane with sharp peak at over 30mph, then rapidly descending, so the most economical cruising speed was about 27-28 knts. You could actually feel it when out on the water, as it was almost instinctive to put it into it's most economical cruising mode. It really wasn't as happy cruising at 24knts with other similar sized boats.

 

[Kawasaki]

Crikey Folks!
I have tried to read through this thread
Too much information as peeps say these days

Jump on yer boat and 'feel' it

Just like yer drives yer car

The boat will 'tell' you when its happy
Be it planing , semi or displacement

Engine noise
Wave 'noise'
Vibration or lack of it

asteven 221
One thing I can garauntee
Another Sealine 44 just like yours will probably handle slightly different.

I have had 8 boats over the last 30 years
Takes a bit to get to know your boat
During the last three years I have helmed about , I dunno, maybe 70 or 80 different vessels.

In June 2008 , (just checked me diary!) I handed over 3 Sealine 34 footers to peeps
All about the same era
They all behaved differently!!!

Props maybe different
Fuel load
Water load
Friggen fridge load!
Types of anchor rode
tv's micro's I dunno!
But believe me there was no'Ultimate setting' for any of them!

Just go out and play with the helm, throttle settings trim tabs etc etc
You will get to 'know' your boat!!

 

[BrendanS]

Oi! I said that!

"You could actually feel it when out on the water, as it was almost instinctive to put it into it's most economical cruising mode"