Trim Tabs, Plane speed and fuel usage ?

[lenny2113]

I start acceleration with trim tabs all the way down. Once the boat is on plane and at about 3000-3200 RPM I rase the tabs all the way up to reduce drag on the water. <br />It is twin 350 Mercs inside of 26' Formula. I know I should not be looking at fuel efficiency with that kind of setup. Anyway, at 3800-4200RPM range I see fuel gages drop by the minute so I try to stay at lower range 3000-3200 RPM. I run the boat with 17p aluminium props. <br />Any advice on how trim tabs can help reduce fuel consumption? <br />Thanks.

 

[rabidfish]

Re: Trim Tabs, Plane speed and fuel usage ?

If you want hard evidence, try a fuel flow meter. You install it in-line. It will give you fuel usage in real time. <br /><br />Actually, it's kinda cool, watching it change as you make changes to the trim, tabs, engine, etc...<br />You will be able to know for certain how to trim for fuel economy.

 

[Northern Eclipse]

Re: Trim Tabs, Plane speed and fuel usage ?

Lenny what rpms are the motors turning when the boat is at WOT, I would think with that set up you should be able to spin 21p, it is possible the 17's are slipping alot, thus you need to maintain higher rpms to maintain a decent cruise speed which in turn sucks gas, I'm assumming this boat is sterndrive.

 

[rodbolt]

Re: Trim Tabs, Plane speed and fuel usage ?

hello<br /> tabs hurt fuel consumption. no other way. but tabs can correct for a list to port or stbd and excessive boiw rise. once actuated as the tab drops it uses water pressure(drag) to lift the transom. if this is a stern drive try useing the drive trim to lift the bow. the more hull that is out of the water the faster and more fuel efficient you are.

 

[LubeDude]

Re: Trim Tabs, Plane speed and fuel usage ?

Originally posted by rodbolt:<br /> tabs hurt fuel consumption. no other way.

Well, I Think you are one bright guy, and really hate to call you on this one, but I beg to differ. Naughtyjohn may have to help me out here, I can see how you might think that the drag of the tabs would cause more fuel consumption, but I believe that if the stern is lifted a bit, and the drive or drives are a bit higher in the water, the drag may be actually less, and the better efficiency of the hull may make for better fuel economy. I may be all wet here, (Been wet before), But that is how I see it.

 

[waterone1@aol.com]

Re: Trim Tabs, Plane speed and fuel usage ?

In my experience (with twin engine V-8 cabin cruisers) I can always watch the speed increase, without touching the throttles when I bring the tabs back up. I can also notice a real drag and speed loss if I let the tabs down while on plane. The only time I see a benefit, speed wise, is if I am trying to keep the boat on plane when running just below the normal planing speed...the tabs will keep me on plane. Other than that, they are usefull to : 1) get me up on plane faster from low speed or dead stop 2)keep the bow down when running just above plane speed 3) balance out the boat from listing if in a cross wind 4) balance out the boat if I am running with more weight on one side. As a side note, if trying to "get moving fast" I always start out with both tabs all the way down and if in a sterdrive boat with both drives all the way down. As I reach planing speed I raise both tabs all the way up and after a few seconds, tap the drive trims up a few notches....that is when the boat seams to really take off.

 

[rodbolt]

Re: Trim Tabs, Plane speed and fuel usage ?

yep<br /> thats how tabs work. any protrusions from marine growth to tabs eat fuel and horsepower. however most inboards and heavy boats have to have them to balance the boat and to lift the stern. a friend of mine has a 33 wellcraft St Tropez with a pair of v-drives and 7.4 mercs that without lowering the tabs it takes a football field to plane and you get an excellent view of the moon. that bieng said tabs eat fuel. no other way. thats the whole point for drive trim. to get the bow out of the water. less hull in the water more speed for the fuel consumed. with tabs the opposite occurs. but tabs eat fuel its a law of physics. drag eats power and power eats fuel. and in the case of inboards if you lift the stern high enough to get the struts,rudders and "drives"out of the water you tabbed down to far or ran aground playing with the switches

 

[Doug Durako]

Re: Trim Tabs, Plane speed and fuel usage ?

Agree on tabs----use them to launch, but then get them up and out of the way.<br /><br />Nothern Eclipse is on to something---17 pitch seems way too low for these power units and this size boat. Have you always used the 17s?<br /><br />With the right props, I would think you should be able to cruise comfortably at under 3K rpm with tabs completely up and out of the way.

 

[lenny2113]

Re: Trim Tabs, Plane speed and fuel usage ?

At WOT both RPM gauges read 4500. That is with 14 1/4 x 17p props. I got this boat with these props and have never tried different once. I am looking to buy 2 new Hustler (2 piece setup) aluminium props. Now, I am a bit concern that I should no be getting 17p again. However, if I switch to higher pitch I will decrease WOT RPM's. <br />What are your thoughts ? <br />Thank you all.<br /><br />And guys, happy boating season to all of you!!! I can't wait until we get some real warm days nere on the East Coast.

 

[Northern Eclipse]

Re: Trim Tabs, Plane speed and fuel usage ?

Lenny is your Formula a Cruiser or Sport boat, and how old is it and what type of drive do you have?

 

[rabidfish]

Re: Trim Tabs, Plane speed and fuel usage ?

I agree with rodbolt on the tabs/fuel economy issue... But I always allow room for exceptions to the rules.<br /><br />I rigged a 115EFI Mercury on a C-Dory (including a fuel flow scan.) <br /><br />(I no longer have the notes), but it did get better fuel economy with tabs down a bit.<br />(I have always considered it to be a lesson of "experience" )<br /><br />Now... I am not saying it always works this way...<br />I realize it's just one boat, and with a different engine, it might react differently. but it did do it in this particular case.

 

[rabidfish]

Re: Trim Tabs, Plane speed and fuel usage ?

I just remebered why I posted again...<br /><br />Get your engines running in the recommended range (4200-4600 @ WOT) Use stainless props to gain efficiency, and look into a fuel flow scan...<br /><br />It will literally tell you how fast the fuel is moving through your fuel line, while the boat is underway. If you tab up and the fuel slows down, then you know for certain how it affects economy.

 

[lenny2113]

Re: Trim Tabs, Plane speed and fuel usage ?

Northern Eclipse <br />It is a Cruiser with twin 5.7 Mercs and Alpha 1 drives. It is a 1976 Formula 26SC. Both engines are pre 1986, 260hp. Both engines are rebuilt 1year old and 2 year old. <br /><br />Thanks.

 

[rodbolt]

Re: Trim Tabs, Plane speed and fuel usage ?

rabidfish<br /> on a light small outboard rig if you play with the tabs and the trim you can actually lift more hull out of the wter to cmpensate for the extra drag. works well with old whalers with 110-140 HPmotors. k

 

[lenny2113]

Re: Trim Tabs, Plane speed and fuel usage ?

 

[lenny2113]

Re: Trim Tabs, Plane speed and fuel usage ?

oops, sorry, linked to a full size original image, not the editted.

 

[Boatist]

Re: Trim Tabs, Plane speed and fuel usage ?

Lenny<br />To start with sounds like you have the right props. If you went to a 19 pitch props your WOT RPMS would drop to 4100, this I belive would give your better fuel mileage and about 2 more MPH speed. But 4100 if you run WOT alot may not be the best for your motors. <br /><br />Fuel mileage for almost all boats is best right after your boat reaches a clean plane with the trim tabs all the way up and your motor trim out until 2/3 of the hull is out of the water. Trim motor foward or trim tabs down and fuel mileage will go down. In general trim tabs down or motor trimed foward = bow down = more wetted surface = more drag = less speed and less fuel mileage.<br /><br />There is one time when trim tabs can increase fuel mileage. If the water is too rough for you to run fast enough to reach a clean plane with out trim tabs then your boat would be plowing. In this condition adding enough trim to bring the bow down and get on plane it will make it safer and increase your speed at the same throttle and you will get better mileage. Still your best mileage will be with the tabs all the way up increase the throttle slowly until the boat climbs over the bow wave and reaches a clean plane. At this point add just a little more throttle then adjust your engines trim slowly for best speed and RPMs without moving the throttles. This will be your best crusing speed for fuel mileage. When you advance the throttle more if your have 4 barrel carbs then you open the back two barrels and add more fuel for more power and speed but less fuel mileage.