1979 19' Marktwain 165 hp I/O trimming

[moffitt77]

My Marktwain has 2 up trim buttons. When on plane am I supposed to trim the lower button all the way up for better fuel mileage or should I just leave it all the way down? Will it make any difference if I am pulling a skier to tubber?

thanks

 

[ezmobee]

Re: 1979 19' Marktwain 165 hp I/O trimming

Start out with the trim all the way down until you're up on plane. Then trim up just until she starts to porpoise, then adjust it back down a little bit. I would imagine towing a skier or tuber would affect where this "sweet spot" is in your trim range.

 

[ziggy]

Re: 1979 19' Marktwain 165 hp I/O trimming

yep, trim in/down all the way to start with. then after on plane, trim up to best rpms. best attitude. when trim is too high (out) ya can ventilate (large increase in rpms due to no connection with the water) easy or porpoising occurs. then ya know it's time to trim back in some.

when i pull skiers, i usually leave trim in/down all the way... but that's my boat, yours may be different..

When on plane am I supposed to trim the lower button all the way up for better fuel mileage

that will give you worse fuel mileage. a boat that rides out of the water gets best fuel mileage and goes faster since it is in less contact with the water, which is friction.. pulling skiers isn't good for fuel mileage...

 

[haulnazz15]

Re: 1979 19' Marktwain 165 hp I/O trimming

You have an "up" and and "up/out" button. The up/out button is only used when trailering the boat, as it is basically a bypass switch for the limit switch which keeps you from raising the outdrive too far while the engine is running. The up switch should be used by itself to trim up once you've planed out (or when in shallow water at slow speeds). Like was mentioned, leave trim fully down when starting from a dead stop, then once planed out you can use the up trim to raise the unit until the boat begins to porpoise, then trim down a bit until the boat stops porpoising. That will give you the best fuel economy, easiest steering, and the smallest wake (when towing skiers). If you want a larger wake for tubes or wakeboarding, leave the trim further down which will cause more of the hull to be in the water throwing a bigger wake.

 

[weareallnuts]

Re: 1979 19' Marktwain 165 hp I/O trimming

The purpose of i/o trim is to adjust the angle of the outdrive relative to the transom, affecting the angle of the boat to the surface of the water. I disagree with the "trim all the way in" as it will force the bow down, causing more wetted surface on the hull, greater drag. A runabout is designed to ride on about the after 1/3 to 1/4 of the hull. "Getting on plane" means getting the boat up out of the water and on top of it. Putting the bow down (trim all the way in) causes the boat to ride with more of its hull on the surface, reducing speed and efficiency.

Boats like yours (I had a '72 Cobia Centaur 18') perform better with an i/o when you put a doelfin or stingray on the lower unit. Time to plane is cut in half, at lower speed, which means your up on the water, bow down, moving efficiently sooner than without it. You get up on plane without the nose pointing at the sky. Give it a try, you'll be amazed.

 

[haulnazz15]

Re: 1979 19' Marktwain 165 hp I/O trimming

The purpose of i/o trim is to adjust the angle of the outdrive relative to the transom, affecting the angle of the boat to the surface of the water. I disagree with the "trim all the way in" as it will force the bow down, causing more wetted surface on the hull, greater drag. A runabout is designed to ride on about the after 1/3 to 1/4 of the hull. "Getting on plane" means getting the boat up out of the water and on top of it. Putting the bow down (trim all the way in) causes the boat to ride with more of its hull on the surface, reducing speed and efficiency.

Boats like yours (I had a '72 Cobia Centaur 18') perform better with an i/o when you put a doelfin or stingray on the lower unit. Time to plane is cut in half, at lower speed, which means your up on the water, bow down, moving efficiently sooner than without it. You get up on plane without the nose pointing at the sky. Give it a try, you'll be amazed.

That is an awefully vague generalization. Actually, most people would say that smart tabs are an infinite step up over any "whale tail". In any case, my Mark Twain has no need for either as it planes out about as fast as anyone would need. The purpose of having the trim all the way in on the holeshot is to direct all propultion forward to get the boat moving as quickly as possible (read: reach planing speed faster). As you accelerate, you can begin to trim up in order to get more of the hull out of the water. There are reasons that inboard ski boats have such great hole shots for watersports, and one of those reasons is that the propulsion angle is optimized for it, while not having the ability to "trim up".

 

[moffitt77]

Re: 1979 19' Marktwain 165 hp I/O trimming

Thanks for all of the great information. My boat gets up on plane very quickly even when pulling a tube or skis and actually has been getting good fuel mileage with the trim all the in. This information should make it even better.