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I have a 1990 18 Outrage with Twin Yamaha 70's. The motors are mounted in the lowest position, top mounting hole. I'm looking for advice from others with same boat and motors. How high are your motors? Thanks.
It all comes down to where the anti-ventilation plate is in relation to the top of the water when the boat is running on plane, at a good cruise speed. That will get you the optimal performance out of both the motors and the props.
The saying goes, for optimal performance the anti-cavitation plate should be ~1-1 1/2 inches above the bottom of the keel. With twin engines, this will not be applicable since they are mounted off center.
Observing the anti-cavitation at medium cruise, trimmed out will tell you whether they need to come up. The key is the plates are at the surface and not underwater.
Note that raising the engines will affect your WOT RPM's. For each hole you go up, it will add ~ 150 rpm's at WOT. If, with your current mounting, you are reaching the maximum RPM's for that model, raising the motors may result in you exceeding the max RPM's and require a prop change.
Given the above, I suggest you run the boat with a crew mate and at 30-35 mph, trim out and have the mate look over the transom (be careful) and, if possible take photos to show where the plates are.
It would also be helpful to know the performance of your current setup and what propellers (make, model, diameter and pitch) you have. Note - there is no "standard" propeller for your motors. The diameter and pitch is stamped on either the outside of the hub or on the inside.
If you can report WOT speed in RPM's, speed observed by GPS and note conditions (wind, waves, current) that will help.
[EDIT - Tom posted while I was composing. Go with his recommendation on height. He is a guru at this. ]
Edited by Phil T on 03/14/09 - 10:29 AM
1992 Outrage 17 I
2019 E-TEC 90, Viper 17 2+
2018 Load Rite Elite 18280096VT
Derwd24 wrote:
It all comes down to where the anti-ventilation plate is in relation to the top of the water when the boat is running on plane, at a good cruise speed. That will get you the optimal performance out of both the motors and the props.
Great stuff Dave. You posted this over the summer and because of reading this link, I raised my motor and have managed to gain a few knots.
There are some good pictures of how your cavitation plates should be positioned while on a plane and at cruise speed. The link shows that whether you have one or three motors, that all the cavitation plates should be just above the water while cruising.
As always, great stuff.
Cliff
1966 13' Sport with a 1993 40hp Yamaha 2 Smoker
One important thing to consider with twin engines.
Twin engines are not mounted on the center line of the boat.
What does this mean?
When turning, one of the engines will raise out of the water more because they are not mounted on the centerline of the boat like a single engine. That engine will most likely cavitate even when it is mounted all the way down. Of course this depends on how steep a turn and water conditions.
Just something to keep in mind when setting up twins.
My twin engine 70 hp were mounted all the way down on one of my Outrage 18's. It is possible that they could have been raised up one hole but I didn't see anything abnormal the way they were.
Take a look at this article on the performance of the twin 70hp engines on my prior 1986 Outrage 18'. http://www.whalercentral.com/articles...icle_id=35
I am going to have to disagree with Joe on that point. The height of the outboard is relative to the transom, or more precisely, the flow of water coming out from under the transom. When the boat turns, one motor rises up relative to the surface of the ocean, but not relative to the boat. The motor only reacts to the water flow it encounters which remains esseintially the same (except under extreme maneuvering which is not what we set the motors up for.)
I have no doubt that Joe's Outrage 18 would have perform better and with no added cavitation had his twin OMC 70s been mounted one hole up instead of all the way down. Unfortunately, back in the 1980s it was almost standard practice to mount outboards all the way down propellers design had not evolved as far as it has today. Even so, with the OMC SST propellers made since the mid 1970s, those motors would have tolerated (and benefitted from) a higher mounting height.
Tom's information is correct. As one who bought an 18 Outrage new in 1986, and rigged it with twin Merc 115's, one hole up, using the recommended engine centerline of 29-3/4", that is what someone at the Whaler factory recommended to me.
