Adjusting the trim on my outboard for a heavy displacement hull boat.

[Mohawkriverman]

I have a MacGregor 25, which weighs about 2,700 lbs, sleeps 3 comfortably, and is being converted into a motor sailor instead of a sailboat with a little tiller throttle 8 HP Evinrude/Johnson Sailmaster kicker which came stock with the boat. The new (to me) motor is an Evinrude 20 HP, new heavier motor mount so the boat is safe, and it has remote steering throttle, ignition & transmission controls. Before anyone points out the boat is rated at 10 HP - that is due to the stock motor mount & the boat has never handled this well since the motor upgrade, she is a pleasure to cruise under sail or power. The concern I have is I noticed a trim control this winter when I took the boat out for the winter. It has a large rod holding the trim in place & would be difficult to adjust with the boat in the water, but I could do it now while the boat is on the hard. What is the best trim angle for such a boat? Perfectly parallel to the transom? Or is a slight angle away from or toward the transom more efficient?

Thanks,
Mokawkriverman

 

[Scott Danforth]

not sure it would make much difference on such a heavy boat with such a small motor.

if the bow tends to sit low, angle the motor up a notch. if the aft tends to sit low, angle the motor down a notch

 

[ondarvr]

Set it so the motor sits as vertical as possible, but like Scott said, it may not make much of a difference on that size of boat.

 

[dingbat]

Prop shaft should be parallel to the surface of the water.

 

[wrvond]

the leg of the outdrive should be as perpendicular to the water as you can get it for maximum efficiency.
Quite frankly, if you are happy with how it is performing now, there's no real need to even mess with it.

 

[Mohawkriverman]

Thanks for the input guys!

 

[Sea Rider]

If wanting to get the max out of that motor with minumum lower leg water drag. Raise/lower leg for water flow to pass skimming right under small water deflector plate at speed. Motor must be trimmed at 90? for anticav plate and prop shaft to ride parellel to water level. Install an induction tach and check max wot revs achieved, check if inside middle to max wot rpm range which assume should be 5-6 K wot revs ?

If under 5.5 K rpm, can go for a prop maximization for a less prop pitch to pull wot revs higher. Won't plane that combo but will achieve a better overall fast displacement speed than before.

Happy Boating

 

[Mohawkriverman]

Yes - I have a separate thread for the prop/wot/rpm issue and I installed a tach for that purpose (see link)- I do have the depth of the main lower unit shaft right where you mentioned. Thanks for the tip on the prop shaft angle!

http://forums.iboats.com/forum/engi...-1991-20-hp-2-stroke-just-mounted-on-sailboat

 

[ondarvr]

Depth of motor isn't going to matter much at these speeds, to a point, the deeper it is the better though. When it gets rough you need to keep the prop in the water, which means mounting it slightly lower if possible. Having a longshaft, or extra long shaft helps keep the powerhead out of the water and the prop in it.

On a planing hull you would want the motor higher so there's less drag when running at WOT.

 

[Sea Rider]

Deeper is the better as long water flow at speed is not banging above small upper water deflector plate, that portion being a non edged cutting area as found under small plate, will generate water drag and slow combo down a bit. Correct OB/transom height along a prop maximization is the way to go for any combo, enlightens the water performance of any under powered OB.

Once you dialed both working parameters won't matter if boating at choppy windy water cond, prop will do its homework right.

Happy Boating

 

[ondarvr]

At these speeds drag of the lower unit and midsection aren't of much concern, the big concern is clean water to the prop, and it normally means going deeper. This allows better thrust in forward and reverse, and yes, keeping the prop in the water is a big deal, and sometimes difficult to do, that's why longer shafts are preferred.

 

[jimmbo]

Only rated for 10 hp. Hmm, a friend of mine is looking for one of these second hand. Most of them around here have 40s, 50s, and 60s hanging on them.

 

[Sea Rider]

Long shaft OB's for sailboat uses along a prop maximization is the way to go to get the max HP out of that under powered 10. If a 3.5 HP OB can push a heavy J-24 sailboat with crew at displacement speed on flat calm no wind water cond why can't a 10 HP push same that sailboat.

Happy Boating