kmoose
11-14-2006, 08:41 PM
I’m still getting used to my new/old boat but after getting a few trips in it under my belt I was ready to start evaluating ways to squeeze all I could from an already outstanding set up. I was getting over the flu and could not stand seeing my boat sit under the shed on such a nice weekend so I decided to make some adjustments to my outboard mounting height and give her a good test drive.
So what drove me to come to the conclusion that I needed to do anything at all to my set up? Like I said, I had made several trips with this boat and loaded her down pretty well. I’ve had it out on both flat days and ridiculously rough and saw that there might be room for some improvements. Here are some key indicators:
• No matter what speed, angle, turn or seas, I couldn’t make the prop “blow out” or cavitate.
• The cavitation plate on the motor is well below the running surface of the water behind the transom regardless of speed.
• Motor needed excessive trim angle to gain desired attitude.
Under many standard outboard mounting applications, the motor is mounted with the cavitation plate parallel, if not just above the horizontal line that is the very bottom center of the hull. This rule generally will get you close to where the outboard needs to be, but changes in modern hull design have changed this and needs to be considered prior to mounting an outboard on a new or re-power installation. Some of the factors that might require a higher mounting height are notched transoms, integrated platform mounts, hull extensions, or brackets. In my case, the boat was retrofitted with a 36” bracket to a hull originally designed for use with an I/O. The 36” setback allowed “clean” un-aerated water to rise up well beyond the bottom line of the hull, thus allowing a much higher mounting height.
My first course of action was to evaluate where my motor is at and how far I am able to move the motor up without modification. I did this by placing a strait edge against the bottom of the hull out to the lower unit. Then I trimmed the motor so as the output shaft was parallel to the strait edge. My cavitation plate was 5/8 of and inch above the parallel bottom of the boat. I opted to move my motor to the highest possible position ( a one and 5/8 inch raise). So what brought me to this decision? Well, the move was a choice of the last 2 adjustment holes and I choose to go the max and work from there.
The adjustment process was rather easy for the fact my buddy had an H frame lift on wheels…….good to have friends with expensive tools. :D The end result seemed as a total win, win. Fuel economy was a gain but little to no speed increase. My daughter and I ran the boat through it paces and still could not cause a cavitation or blow out under reasonable trim. From the pics it is hard to tell but in channel conditions I was able to maintain 3 nmph at 30 knots!!! This all felt good and I was ecstatic with the fuel burn…….but what will happen under a full dive load and real gulf seas?
A couple of weeks later the opportunity arose for a trip in 2-4 decreasing all day. Ahhhh the perfect test of the new prop and motor height. So I loaded up the sled with 3 other guys, 400 lbs. of ice, ten tanks and all the associated gear and headed out 50 nautical.
The ride out seemed to go well and I didn’t need near the usual amount of tab to keep the bow down. Fuel burn didn’t seem to be noticeably better. With a lot of throttle tending we made our destination through some uncomfortably spaced 2-3 footers and had a good day of shooting. NOAA was spot on for their forecast for once and the seas laid out to nearly pond conditions. These were the conditions I felt that I would show the fruits of my labor. Unfortunately, as soon as I started to trim the boat up for max speed and fuel burn I started to see some needle bounce on my tach. :( It was obvious I had lost my ability to trim the bow up under dive load and had actually hurt my cruise speed and fuel burn due to the fact I could not utilize my trim angle to lift my hull out of the water.
So where am I at and where will I go now? I am going to invest into lowering the motor down one hole, (half the distance of the raise) and retest. What will I gain? Who knows, but if I go back to the original set up………which may have been perfect!.......I will never know unless I do it. So this weekend I will move it and hope for the best. I have a good test co-pilot and it beats a day at the mall or pressure washing the pool deck. :D
To be continued……..
So what drove me to come to the conclusion that I needed to do anything at all to my set up? Like I said, I had made several trips with this boat and loaded her down pretty well. I’ve had it out on both flat days and ridiculously rough and saw that there might be room for some improvements. Here are some key indicators:
• No matter what speed, angle, turn or seas, I couldn’t make the prop “blow out” or cavitate.
• The cavitation plate on the motor is well below the running surface of the water behind the transom regardless of speed.
• Motor needed excessive trim angle to gain desired attitude.
Under many standard outboard mounting applications, the motor is mounted with the cavitation plate parallel, if not just above the horizontal line that is the very bottom center of the hull. This rule generally will get you close to where the outboard needs to be, but changes in modern hull design have changed this and needs to be considered prior to mounting an outboard on a new or re-power installation. Some of the factors that might require a higher mounting height are notched transoms, integrated platform mounts, hull extensions, or brackets. In my case, the boat was retrofitted with a 36” bracket to a hull originally designed for use with an I/O. The 36” setback allowed “clean” un-aerated water to rise up well beyond the bottom line of the hull, thus allowing a much higher mounting height.
My first course of action was to evaluate where my motor is at and how far I am able to move the motor up without modification. I did this by placing a strait edge against the bottom of the hull out to the lower unit. Then I trimmed the motor so as the output shaft was parallel to the strait edge. My cavitation plate was 5/8 of and inch above the parallel bottom of the boat. I opted to move my motor to the highest possible position ( a one and 5/8 inch raise). So what brought me to this decision? Well, the move was a choice of the last 2 adjustment holes and I choose to go the max and work from there.
The adjustment process was rather easy for the fact my buddy had an H frame lift on wheels…….good to have friends with expensive tools. :D The end result seemed as a total win, win. Fuel economy was a gain but little to no speed increase. My daughter and I ran the boat through it paces and still could not cause a cavitation or blow out under reasonable trim. From the pics it is hard to tell but in channel conditions I was able to maintain 3 nmph at 30 knots!!! This all felt good and I was ecstatic with the fuel burn…….but what will happen under a full dive load and real gulf seas?
A couple of weeks later the opportunity arose for a trip in 2-4 decreasing all day. Ahhhh the perfect test of the new prop and motor height. So I loaded up the sled with 3 other guys, 400 lbs. of ice, ten tanks and all the associated gear and headed out 50 nautical.
The ride out seemed to go well and I didn’t need near the usual amount of tab to keep the bow down. Fuel burn didn’t seem to be noticeably better. With a lot of throttle tending we made our destination through some uncomfortably spaced 2-3 footers and had a good day of shooting. NOAA was spot on for their forecast for once and the seas laid out to nearly pond conditions. These were the conditions I felt that I would show the fruits of my labor. Unfortunately, as soon as I started to trim the boat up for max speed and fuel burn I started to see some needle bounce on my tach. :( It was obvious I had lost my ability to trim the bow up under dive load and had actually hurt my cruise speed and fuel burn due to the fact I could not utilize my trim angle to lift my hull out of the water.
So where am I at and where will I go now? I am going to invest into lowering the motor down one hole, (half the distance of the raise) and retest. What will I gain? Who knows, but if I go back to the original set up………which may have been perfect!.......I will never know unless I do it. So this weekend I will move it and hope for the best. I have a good test co-pilot and it beats a day at the mall or pressure washing the pool deck. :D
To be continued……..