O/B on jack plate, how high is too high???

rphillips

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May 18, 2012
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19' fiberglass bass boat with 150 merc. What will be first thing I will notice when I get the eng. too high. I've been raising it a little at a time & nothing seems to change. Nose goes quite high on take off then plains off nicely, I'm thinking the higher the eng. the less the nose will lift. Right now cavitation plate is appx. 2" above bottom of boat with no adverse effects, no blow out, no porpoiseing, no big rooster tail, good water pressure. What will be the first thing I should notice when it starts to get too high?? Thanks
 

ondarvr

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The prop will start to ventilate or the water pressure will drop.
 

roscoe

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What will be the first thing I should notice when it starts to get too high?? Thanks


That depends on how observant you are, or how distracted you are at the time.

Could be a slight drop in speed, or an increase in rpm.
Or an overheat alarm :eek:
 

Chris1956

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Mar 25, 2004
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You are correct that trim will have less effect on the bow lift, as you raise the motor up on the jackplate. You should use a GPS and accurate tachometer to find best speed and minimum slip. If your boat does not "carry" the bow easily, you will likely need to have the engine further down to allow more trim influence.

The distance of setback also influences the amount of raising of the motor, she will tolerate.

Trial and error, I am afraid....

Also, drive-ability plays a part. If just you are going to exclusively drive the boat, she can be set up for higher performance, then if more inexperienced drivers will run her.
 

Sea Rider

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A simple method : on calm water cond with boat running parallel to water level and trimmed to 90?, with well deck weight distribution, if OB sits way low will experience water back splashes or even over transom, if too high will have prop aeration on choppy, windy sea cond and at tight close turns even on calm water, only good for straight water courses...

Other form, when ridding same pull head out transom and check at which lower leg is water flow passing at plane. Report if passing above or slight under upper splash plate or towards AV plate and if AV is exposed when at plane.

Why are you using a jack plate, what if sitting OB straight on transom and testing that way..

Happy Boating
 

rphillips

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May 18, 2012
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The boat was set up with the jack plate when I bought it. I think I would want to run with the motor as high as possible, without being too high, thus I'd like to know what is the first sign of too high, then back down just a touch. thanks
 

Chris1956

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What performance goal are you looking for? Jacking up the motor to the max may not give the highest speed, but it will certainly make the boat more difficult for the average Joe to drive.
 

rphillips

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May 18, 2012
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Don't care much about top speed, it rarely gets to WOT anyway. Right now the boat is as stable as any boat I've ever been in, at WOT, (appx. 50mph). My only complaint is how high the bow rises on take-off, & just logical thinking would make me think, the lower the prop in the water, the more bow lift, the higher the prop, the less bow lift.
 

ondarvr

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Yes, the higher motor will generally mean less bow rise, but other things can affect it too, just changing props or moving stuff around in the boat.

Take it up and see what happens, just make sure it's getting water.
 

Sea Rider

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If don't want to lose precious boating time wild guessing wihich is the Tip Top Transom height for that OB post a pic shot sideways showing both lower leg plates to have a look..

Happy Boating
 

Faztbullet

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What model boat? 4"5"6"8"10" jack plate? What year motor? Standard or Xr lower unit? Stainless or alum prop?
 

rphillips

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May 18, 2012
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Did yall notice, im pretty green. 1997 BMC mod. 1860. 1984 merc XR2 150. Lower unit uses the large hub prop. 4 blade stainless prop with no brand name visable, marked 24P. I didnt realize this would be sooo technical, thought I would hear the prop will lose grip in turns, /ou will start losing water pressure, handeling will start to get squirrelly. Guess ill just keep going up a half in. at a time till I feel, see, or hear something neg. Going on, then back it back down a little. Thanks for your help. Rick P
 

Sea Rider

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Visually checking what's going on at back transom to see at which lower leg height water flow passes at plane is faster and better than by ear method, once you know this height can raise/lower OB accordingly till Tip Top OB/Transom height is dialed and combo performs spot on. As you have checked, this is not a car....

Happy Boating
 

comstox

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Can anybody confirm what this guy is saying as a rule of thumb? At the 30 second mark he says, "That 'plane' (Anti cavitation/ventilation plate,) wants to be even with the keel of your boat." Is that a good rule-of-thumb?
 

Sea Rider

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Will totally disagree on that statement, boater doesn't know that each OB manufacturer states their own AV plate/lower hull parameters heights on dry installations which doesn't mean a thing, once combo floats and runs at plane at speed it's a different story...

Boater should go for a wot spin with OB sitting "fully down" and check what's going on at back of OB, if with water splashes backwards or over transom, need to raise a bit OB till splashes completely disappears, if with prop aeration, that's other horrible story..

Happy Boating
 
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rphillips

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May 18, 2012
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I laid on the back deck, watching, while a buddy drove. with the boat WOT, plained all the way up, the water that was rolling up from the bottom edge of the hull, was hitting against the bottom of the ventilation plate & spraying out both sides. No water on top of ventilation p[late, except maybe some little splashes. Boat is still very stable, good water pressure, & prop not blowing out, even in turns. I think I'll leave it like it is & be happier than I've been in the past 17 yrs., ever since I've owned the boat. Thanks for all the help.
 

jbcurt00

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YES, it is a good rule of thumb, but typical for an OB mounted directly on the transom. Depending on how much set back the jack plate provides, the vent plate is ABOVE the keel, not even w it.

Not 100% on every combo, but always a good place to start, w or w out the jack plate. Gotta start somewhere.

To the.original poster: Glad you got it sorted out and you're happy w results.
 
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