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Author Topic:   Mounting Twin Engines on Moderate V-Hull Boats
jimh posted 05-27-2006 10:21 AM ET (US)   Profile for jimh   Send Email to jimh  
This thread is for comments or discussion related to my article in the REFERENCE section:

Twin Outboard Engine Installation
on Moderate V-hull Boats

by James W. Hebert

prm1177 posted 05-27-2006 12:21 PM ET (US)     Profile for prm1177  Send Email to prm1177     

Nice article. Have you found any information regarding the correct engine height of twins? The proper placement of the anti-ventilation plates relative to the bottom of the boat is made more difficult by the natural asymmetry of the setup. Typically at planing speeds the inboard side of the AV plate is above the water (as it should be) with the outboard side digging into the flow. I can confirm this on my Conquest. Only when I approach the limits of my trim out, do I lose the tell-tale spray line. Of course, raising the engine would improve this running condition at the expense of propeller traction in rough water.

jimh posted 06-04-2006 10:57 AM ET (US)     Profile for jimh  Send Email to jimh     
I have read some mentions of tests performed on the alignment of twin engines while underway. It has been mentioned that the tie bar has been disconnected from one engine while running at speed, and the engine allowed to take a natural alignment resulting from the pressure of the flow of water past the gear case. Once in this natural alignment, the length of the tie bar is adjusted to match, and the tie bar is reconnected.

It would be interesting to have a first-hand report of this procedure being done. My initial reaction to hearing of it is to proceed with caution. Can we have a report from someone who has performed this procedure?

prm1177 posted 06-04-2006 02:17 PM ET (US)     Profile for prm1177  Send Email to prm1177     
Sounds interesting. You'd need smooth water, two people, and a lot of caution, otherwise you're a Darwin Award candidate.
seahorse posted 06-04-2006 03:20 PM ET (US)     Profile for seahorse  Send Email to seahorse     
I've done the "one loose engine" procedure many times, it is the most accurate way of aligning the twin engines.

The loose engine "follows" the other motor exactly as speed increases due to aligning itself to the water flowing by the gearcase. You can "cut donuts", do gentle turns, and run the motors just like if they were connected.
The loose one tracks the steerable one exactly. It does not jump around on its own. You have to try it to believe it, if you don't believe me.

jimh posted 06-04-2006 04:59 PM ET (US)     Profile for jimh  Send Email to jimh     
That is very interesting! Now we need a volunteer with a twin-engine Boston Whaler V-hull to try this procedure. It would be interesting to see if the engine wanted to align itself with toe-in or toe-out, and how much.
Tom W Clark posted 06-04-2006 07:34 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
Are you saying the disconnected motor is running and under the same throttle load as the connected motor? That does not sound right at all.

Propeller torque will come into play here and tend to steer the loose motor one way or the other depending on the state of trim, won't it?

Would you elaborate on this technique?

jimh posted 06-05-2006 08:04 AM ET (US)     Profile for jimh  Send Email to jimh     
That is an interesting point. Perhaps there would be a more true alignment if the engine that was free from the steering link was not running. To eliminate the propeller influence you could temporarily remove the propeller, too. This would really insure that the gear case alignment was entirely due to the hydrodynamic effects and not from propeller torque.
Plotman posted 06-05-2006 09:10 AM ET (US)     Profile for Plotman  Send Email to Plotman     
I still don't think this is going to work properly, even if the second, "loose" engine is not running and the prop is removed.

Think this through. If the single engine is perfectly straight, the boat is going to turn in a circle when running on just one engine. Therefore the trailing enging is going to be turned in slightly (because the boat will be turning in that direction). This will result in some degree of toe in, but the question is how much.

If you adjust the thrust vector of the running engine to get the boat to go straight (and the dragging engine to align with the long axis of the boat), you will also end up with some toe in, but again, how much.

Either way, it would seem to to be much easier to just measure the front and back of your engines and compare measurements, and adjust to get the proper amount of toe in.

Tom W Clark posted 06-05-2006 11:51 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
I agree with David. This is why both motors MUST be running and providing thrust. The more I think about this the more it makes sense.

Because the direction of the boat is controlled by the connected motor, the loose motor will can do nothing but "weathervane" with the flow of water. Of course this begs the question: Why use a tie bar at all if the loose motor simply follows the connected motor?

I think I will experiment with this technique the next time I got out and see where the motors wind up. As it is now, I have 5/8" toe-in with is tight in the middle of the 1/2" to 3/4" of toe-in that Whaler recommends.

Sal DiMercurio posted 06-05-2006 12:16 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Tom, they have to be connected for when your backing up, & if the connected engine dies, your gonna go flying if the free engione is in gear & running at speed.
I've done the loose engine thing a couple of times & it works much better than you think.
The loose engine takes the path of least resistance.
jimh posted 06-14-2006 08:12 PM ET (US)     Profile for jimh  Send Email to jimh     
Additional material regarding setting the twin engine propeller shaft alignment (toe-in versus toe-out) has been added to the article.

The effect of toe-in and toe-out can apparently be quite significant. Here is a narrative of a boat sea trial in which a relatively minor adjustment to the toe-in/toe-out setting of the twin engines produced a dramatic change in the boat's performance:

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