LU question for fun

G

Graeme2004

Member
Joined
Jan 18, 2005
RO Number
16100
Messages
106
We are looking at buying a single I/O and I was wondering if a person were to drive the boat and set the drive angle so the boat will normally plane at a decent speed with a normal load and then simply leave it there for the rest of the life of the boat, would that LU last longer than one that is lowered and raised as per normal?

I was thinking about this because there must be an ideal gear angle within those units, or maybe I am way off base? and no we are not considering direct drive...
 
Graeme,
It will take longer to get the boat on plane, harder to dock the boat with the drive lifted/set to the optimum cruise angle.

When taking off from a stop you want to tuck the outdrive in to get the best angle for the prop...once on plane you adjust the outdrive up to optimize prop angle and efficiency.

For docking, you also want to tuck the drive in all the way to get a better bite for the prop.

There is an ideal drive angle and its easily found once the boat is on plane...lift the outdrive a bit and you will find that sweetspot...performance vs fuel use.

hope this helps,
Perry
 
Graeme,
As said when stopped the drive needs to be tucked under to get the boat on plain. If you keep the drive down the boat will plow and push water where you will get very poor efficiency, probably get very wet and to get it to speed will require more power. If you trim the boat once on plain it will run faster with lower RPM and probably produce more life out of the motor and drive. That is the same for I/O and even outboards.
 
quote:
Originally posted by Graeme2004

We are looking at buying a single I/O and I was wondering if a person were to drive the boat and set the drive angle so the boat will normally plane at a decent speed with a normal load and then simply leave it there for the rest of the life of the boat, would that LU last longer than one that is lowered and raised as per normal?

I was thinking about this because there must be an ideal gear angle within those units, or maybe I am way off base? and no we are not considering direct drive...
Graeme

Click to expand...


No
 
Maybe it would reduce wear in the trim system, U-Joint and U-Joint bellows and hinge pins, but nothing noticeable. Now, on my IO, I typically set the trim and leave it alone unless there is a need to adjust. I'm not constantly adjusting it everytime a make a turn etc. There was a day when IOs and outboards did not have power trim, you set the trim pin and left it. Most people left the engines outdrives trim more neutral/down that up. As a kid we all had little 12ft boats with 10hp motors on them - a Merc 9.8 was the motor of choice. We wanted them to go as fast as possible so we put the trim pin in a high setting, kept the gas tank in the front of the boat so it would plane better and at WOT (where they basically ran all the time) the boats would run 23-25mph and porpose like heck. We loved it - about every winter we were repairing cracks in the transome of my little fiberglass Gamefisher.
 
I think there's potential in Graeme's suggestion. Why not have a fixed drive angle but use trim tabs to get up on plane? As long as the lower is in the water enough at idle to allow for docking and close-quarter maneuvers....

You'd still need the ability to raise & lower the drive for trailering, no?
 
quote:
Why not have a fixed drive angle but use trim tabs to get up on plane?
Click to expand...


sounds like an inboard now.

adjusting the drive angle is mostly where i/o's get their increased fuel economy. that and the smaller engines the higher efficiency provides.
 
Mine increased efficienty by 200 rpm with the trim properly adjusted. Of course, this was before I had trim tabs.....

After I added TT, I tended to leave the outdrive in the optimal cruise position and use the trim tabs for getting on plane.
 
The outdrive trim is adjustable because there is no optimum trim for all situations. Think of the flaps on an airplane.

Trim the drive in to get on plane the fastest, then out for best performance with the particular load, speed, water conditions, etc. Something else will wear out before the trim pump or anything else associated with adjusting the trim.

If the boat doesn't have trim tabs, consider installing them as well. Prper use of trim tabs in addition to the drive trim will give the best and most efficient performance.
 
I normally set the trim to the position that has proven most efficient and then leave it alone. The outdrive is best positioned a little above the angle where the props are thrusting parallel to the bottom of the hull. I adjust the trim tabs to account for changes in load and water conditions. By fiddling with the outdrive trim around the normal position I can sometimes get another 0.2 mph and then I leave it alone. I do not take hard turns and do not need to trim in to avoid ventilating the props.

Trimming down when maneuvering at idle in to "get more bite on the water" is water nonsensical unless your boat normally runs while trimmed very near the upper limit. You trim in at idle speeds on a single prop to push the bow down and limit bow wander. Duoprops and Bravo 3 drives make idle speed trimming essentially unnecessary. If you have trim tabs the need to trim the outdrive up and down is significantly reduced on my boat as used on inland lakes.
 
quote:
Originally posted by onthecreek

quote:
Why not have a fixed drive angle but use trim tabs to get up on plane?
Click to expand...


sounds like an inboard now.

adjusting the drive angle is mostly where i/o's get their increased fuel economy. that and the smaller engines the higher efficiency provides.

Click to expand...


Exactly, that is the advantage of IOs and OBs vs inboards, if you want to do that do not even bother with an outdrive boat at all, why buy something that needs that much more maintenance than an inboard...the only other advantage of the I/O is being able to trim up in shallow water as an OB can.
 
quote:
Originally posted by LouC
Exactly, that is the advantage of IOs and OBs vs inboards, if you want to do that do not even bother with an outdrive boat at all, why buy something that needs that much more maintenance than an inboard...the only other advantage of the I/O is being able to trim up in shallow water as an OB can.
Click to expand...


That's not entirely true. An I/O steers by "directed thrust"; as the drive is turned from side to side, the trust of the propeller moves the stern of the boat and turns it.

An inboard boat steers by the thrust of the fixed propeller against the rudder. This works fine underway going forward, but there is little steering at low speeds and virtually none in reverse because in reverse, the rudder is actually in front of the propeller and receives no thrust.

Simply put, an I/O (or an outboard) is far more manuverable than an inboard with a single engine.
 
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