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Thread: Proper Trim of sterndrive and trim tabs, fuel

  1. #1
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    Default Proper Trim of sterndrive and trim tabs, fuel

    I was just researching how to improve fuel economy on my boat this season and came across an article about proper trim of the sterndrive and trim tabs. As for the trim tabs it talked about the less you use them (in the down position that is) the less drag there will be, therefor increasing your fuel economy. Makes sense. Talking about trim tabs, do you guys mess with the trim tabs to help get up on a plane?
    The other thing it stated was that most people put their sterndrive in the full down position, which helps the boat get up on a plane but end up keeping it in that position (which I do this as it seems to put the boat at the right angle in the water). The article talks about after you are on a plane to adjust the sterndrive to raise the bow as much as you are comfortable to take the boat (bow) out of the water… he stated this will also help fuel economy. I have just been making sure that my bow isn’t plowing into the water. Do you guys try to do this adjustment to the sterndrive?
    Trying to help improve fuel efficiency this summer.

  2. #2
    Member NewMoon's Avatar
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    My understanding is that for efficiency you want the boat relatively level, and the propshaft level with the water surface. This involves having the drive trimmed up some, and how much depends on how level your boat runs.

    I was taught something like this:

    Use tabs down to help you get up on plane, and if necessary drive down some as well. When you're up on plane, set throttle to the RPM you want for cruising, and adjust drive trim to roughly what you think is a level propshaft.

    Fiddle with trim tabs until the boat runs as level as practical without slowing it down. You don't want to be lifting the stern so much that you're forcing the bow down into the water, but if you're dragging butt, you're making the motor try to push the boat uphill more, and that can be much more inefficient than having tabs down. Using trim tabs less or none is great if your boat planes fairly level without them, but that's often not true for heavier deep-V boats.

    Then adjust the drive trim until you reach the highest speed at the throttle setting you have chosen. On my boat that's at +1 or +2 on the Volvo Penta trim gauge. In flat water, more like 1. Choppier, more like 2 or even 2.5.

    It's an iterative process if you want best results. If you change the throttle setting, or the chop increases, you may want to re-adjust.
    Richard Cook
    New Moon (Bounty 257)
    "Cruising in a Big Way"

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    Member Maast's Avatar
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    When I bought my Yam 75 for my small 17' boat I got to talking to them about trimming, they said to only use the trim tabs to correct for leaning.

    When getting up on plate they said to trim it up until you find the highest RPM for the throttle setting you're at, that'll be your most fuel efficient trim for the throttle setting. On mine I ended up with my nose up in the air a good bit.

    To find your best MPG throttle setting either install a fuel meter or calculate a rough approximation based on the distance you travel and how much your gas guage says you've used (assuming you can trust it - which you probably can't).

    Following their advice I gained 200 RPM at my best cruising throttle setting. You might also want to look into permatrims, which is a aluminum hydrofoil you bolt on to your cavitation plate - some people swear by them, others say they're worthless, YMMV
    2696 Sea Raider Pilothouse
    "Dominion"

  4. #4

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    I use the tach and GPS speed to dial in the trim. Basicly, when the RPM's change and speed changes I use the tabs to adjust back to the 'sweet spot'. Certainly need to 'bow down' the tabs when starting out, the outdrive I set at the same setting from the first plane of the trip and never move it, except to get back where I need it if it moved.

    In a following sea, set the tabs up as high as possible (bow up position) this helps lesson the surface area of the stern getting 'pushed' by the following sea.

    I set my throttle, adjust it if it drops RPMS and then constantly work the tabs. My boat rides to one side so I work one slightly off balance from the other, no matter the speed or conditions.

    Ideally running on the 'tail' is great, that's totally planing with not much touching the water but the very tail end of the boat.

    A bit of chop or rough bottom paint helps break the surface adhesion between the water and hull, thus less friction and more speed. (Actually read an article "recommending" a little bit of buffing up of a smooth surfaced bottom, wasn't totally sold on that idea, but I understand the concept.)

  5. #5

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    Outdrive trimmed all the way in and tabs all the way down. Get up on step and bring tabs all the way up. Then adjust as necessary to level the boat (side to side). Then raise the outdrive until you hear that it's starting to cavitate (if that's the right word) a little. Then lower it back down until you find the sweet spot. I have a fuel flow meter so that's easy to do. You could instead watch your speed and rpms. Agree with what Meyer said. In fact, the hull as I understand it should never be waxed (the part that's in the water anyway).

  6. #6
    Member Cap'n Ron's Avatar
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    Default I understand that some boats....

    may need the trim tabs down to get up on step faster, but not all. My boat (26' Hewes, twin 135 Hondas) gets on step in a max of about 1 1/2 seconds, so I leave my tabs all the way UP until I am on step (Plane) and then just use them to level out...wind, current, wave angle can all make the boat lean, and the tabs level the ride.

    About the following seas, tabs up = bow up like you say would put the stern down, with MORE exposed to the push of the seas, wouldn't it Also, makes you more at risk if the following seas break, so wouldn't you want the bow down, stern up (so tabs DOWN)to reduce the surface area the seas push and also give you more stern freeboard above the seas???

  7. #7

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    I've aways heard and read that in following seas the trim tabs should be in the up position. There's even factory markings next to my trib tab buttons that say that. I believe that if you have a following sea and have the tabs down (bow down) that you may be more likely to plow into the upcoming wave. Maybe pitch pole. If I'm in a following sea that's not rough, I may lower my tabs a bit to make it a little more smooth.

  8. #8

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    I've also found that the best tactic is to raise the outboard until it starts to cavitate, and then take it down slightly till you find "the spot". That spot changes with boat speed - I seem to be able to raise the motor more as speed increases. Trim tabs are only used to level side to side, or to bow down in rough weather. I'm sure all that is totally boat specific. I've been told that using hydrofoils on outdrives/outboards puts a lot of strain on them, and can wear things out faster. They also increase drag. If you have a cavitation problem, need to get on step faster, or need more stern lift, try a different prop. Droping a pitch and switching to stainless helped me out in all of these areas.

  9. #9
    Member Cap'n Ron's Avatar
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    Default Diver that makes sense...

    Quote Originally Posted by skydiver View Post
    I've aways heard and read that in following seas the trim tabs should be in the up position. There's even factory markings next to my trib tab buttons that say that. I believe that if you have a following sea and have the tabs down (bow down) that you may be more likely to plow into the upcoming wave. Maybe pitch pole. If I'm in a following sea that's not rough, I may lower my tabs a bit to make it a little more smooth.
    Makes sense about not wanting the bow down if you are going faster than the following seas and might plunge the bow into the next wave. I've heard on here a lot about tabbing down to put the bow into rough seas, I guess it lets the sharp bow plow thru without getting beat up flapping up and down?

    Only big following seas I've been in was my old boat and it had no tabs, and rode it fine, but that was like having tabs up! Lots of 3 foot seas into the bow, I'll try tabbing down this year and see how that works...

    Sorry to veer a little off the topic, I guess when the seas demand it you position the boat for the best ride and maybe sacrifice the fuel economy a bit!

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