Raised My Motor, More Questions

[AguaSki]

I finally got around to raising my motor. The motor was in the lowest position (top hole) and running 5,270 RPM’s at WOT. I know that you should only raise the motor one hole at a time, but it seems that everyone ends up raising more than one hole so I elected to start by raising two holes. I gained an addition 120 RPM to 5,390, but I still have a long way to go to get to a proper RPM. Hole shot felt a stronger. I did not blow out in normal maneuvers around the lake, but while doing quick u-turn’s to pick-up a downed skier the prop would blow out badly. Below is some more information about my boat: <br /><br />• 1978 Glastron SSV 188 (18 foot, fiberglass)<br />• 1978 Johnson 140hp V4<br />• Michigan Wheel Rapture 13.25 X 15<br />• RPM’S at WOT 5,390 (OMC digital tach)<br />• Maximum speed 38 mph (GPS)<br />• 1 season on motor rebuild<br />• 120 PSI on all 4 cylinders<br />• Motor runs great at idle and while cruising<br />• Smart Tabs installed<br />• 28 gallons of gas located near the rear of the boat<br /><br />Where do I go from here to get my RPM’s up? Do I need to consider another prop? I can only go up one more hole to raise my motor. I figure raising will only give another 70 RPM and cause me to really start blowing out. My biggest concern is RPM and not speed, but more speed is always good .

 

[walleyehed]

Re: Raised My Motor, More Questions

would you be game to install a manual plate???? or even some 2" transom shims??

 

[AguaSki]

Re: Raised My Motor, More Questions

I think I am game for just about anything, but the wife and $$ could require me to take it slowly. Eventually I will get there.<br /><br />I have seen jack plates in many catalogs and websites, including Iboats, but I don't recall seeing shims. I assume a shim would allow your motor to be set back, but not have the flexibility to go up and down. Am I correct? Likewise, I assume the shim option is cheaper than a plate. Would a jack plate or shims require me to make changes to my steering system?<br /><br />I think I can do anything, it is just a matter of budgeting and conditioning my wife for more boat things.

 

[AguaSki]

Re: Raised My Motor, More Questions

Why does set back impact performance? I can see how raising a motor eliminates drag, but I don't understand how performance increases with the engine set back.

 

[walleyehed]

Re: Raised My Motor, More Questions

Setback changes the center of gravity and can reduce some drag of wetted surface as it allows the prop to be a bit better at lifting the bow.<br />Ther is a pont, however, when this can adversely affect performance.

 

[JADED]

Re: Raised My Motor, More Questions

Kenny, I would appreciate your opinion. <br />I am about to raise my twin engines I will have to drill new holes but this will only bring the anti-ventilation plates level with the point of hull in front of the centre of each engine. My transom has a thinner section at top that I will have to go through for the top holes.<br />My Question is. Should drill new top holes or maybe use a jack plate. If I drill new holes that will be as high as I can go.<br />Please feel free to give any advice.<br /><br />Tony

 

[bassman_n]

Re: Raised My Motor, More Questions

Don't drill,buy a jackplate

 

[Boilermaker]

Re: Raised My Motor, More Questions

Wow, your getting 100 RPM more then I am.
I have a similar weight & sized boat, running a 1978 Johnson 115 with a Stiletto 13.25 x 17" prop. I would have thought he would be getting a lot more RPM with the 25Hp extra & 15 pitch prop?

 

[AguaSki]

Re: Raised My Motor, More Questions

Boilermaker - I have read some of your posts and I think we are in fairly similar situations.

I have one more hole to go, but I am waiting for the pressure gauge I ordered to arrive before going any higher. Don't want to fry my motor. Currently the prop does not blowout in normal turns, but a sharp u-turn to pick up a downed skier does cause blowout. Since I do a lot of water sports, and sometimes I have little kids in the water, these quick fast u-turns are important to me. Maybe I should be considering a 4 blade prop, but I am concerned my RPM's will drop.

Walleyhed - Can you tell me more about your idea of using shims. Are shims less expensive than a jackplate? Where can shims be purchased? My father still has some machine shop connections so I may be able to fabricate something.