Help with motor height adjustment

[MonkeyBird747]

1974 Starcraft 18’ center console
2004 Johnson 90hp 4-steoke
Vance engineering 4” setback manual jackplate
13x21 4-blade turning point prop

My previous motor was a 25” shaft and my transom is for a long shaft. I used the jackplate to get the motor to a more reasonable height. I recently repowered to the Johnson 4-stroke, with a 20” shaft. I thought about pulling the jackplate off, but decided I’d give it a try at its lowest position first. The lowest position is not as low as it would mount on the transom without the Jack plate.

The motor seemed like it was still sitting too low and digging in at cruise. I thought it was either because of the setback, or because the Johnson weighs 130 pounds more than the 75hp Merc 2-stroke it replaced. Or both. It is a heavy beast. Maybe too heavy. The boat was rated for 125hp in 1974, and the heaviest motor in that size from back then is maybe 300-320 pounds. The Johnson 4-stroke weighs in at 430 pounds. The waterline is just below the drains in the splash well, but it seems to be doing well and doesn’t swamp at all when coming off from full throttle. Max speed is basically exactly what it was with the 75hp 2-stroke, about 35mph on the gps. FWIW, I wasn’t trying to get more speed with the 90hp. The fuel economy difference is very noticeable.

I raised the jackplate up about 3” and it seems better. I still have maybe 5”-6” difference between the pad and prop shaft. If I’m doing the measurement correctly. That’s a big if. It pulls to the right out of the hole and at cruise. I can adjust the torque trim tab a little more to the right on the next run.

What I’m running in to is that I can’t trim the motor up to the sweet spot without it starting to porpoise. I can almost get there, and I start to see a speed boost and the right pull goes away. But at about that same point I get the porpoise.

Not sure where to go from here. Attaching a video clip at cruise. You’ll also see my transducer probably needs to be relocated. Lots of spray. You can also get an idea of about how high the motor is above the transom. I’ll need to measure it to be exact. My understanding is that it needs to be raised by roughly the amount of setback. It seems pretty high to me, but water pump still looking strong, and no blow outs.

I’m thinking of just ditching the jackplate to reduce the moment arm of the heavier motor, so the stern doesn’t sit so low in the water.

Thanks!

MB

 

[Chris1956]

The motor looks to be too low. You want the antiventilation plate at the surface of the water or above.

Once you get the A-V plate on the surface, the rest of the up adjustment is by trial and error. The higher the motor, the less effect trim has. You want the motor low enough to allow the proper amount of bow lift.

 

[MonkeyBird747]

The motor looks to be too low. You want the antiventilation plate at the surface of the water or above.

Once you get the A-V plate on the surface, the rest of the up adjustment is by trial and error. The higher the motor, the less effect trim has. You want the motor low enough to allow the proper amount of bow lift.

Thanks for the reply. That should be easy enough to do. I don’t know the name of the plate you can see in the video, but I know the anti ventilation plate is the next one down. I should be able to measure the distance between the two plates and raise the motor by that amount. I’ll have to reattach the motor to the jackplate with a different set of holes, since I’m running out of up travel.

MB

 

[Texasmark]

One has to realize that water leaving the bottom of the boat returns to the surface. Rate of return dependends on viscosity of the water and boat speed. Second point is that boat hulls are seldom level when running in the water. With that said, and the fact that once the transom passes over the water it starts rising and anything you hang on the transom will dig deeper in the water than if it were transom mounted.

Rule of thumb for guys running "fast" with jack plates is 1" height increase per 6" of setback from what you would run if on the transom.

Engine height on the transom depends on a lot of things and the AV plate can be from 1.5" below a straight line extension of the hull...yard stick, to as much as several inches for large engines on 60 MPH or so boats. Hanging one on a jack plate just raises it that much more.

If you are cruising, then for your first reference, even with the hull as Chris said plus your setback adjustment is a good place to start....if your hull/setup will accept it...then you jack the engine up from there an amount determined by setback. Engines sitting too deep in the water are inefficient due to drag and are that much more prone to lower unit damage from striking submerged objects. Actually I'd run the engine on the JP as high as it would go without blowing out due to being too close to the surface at high trim angles. Also, using a "cupped" propeller like you find on most SS props is good at grabbing and holding water in high mounting installations.

On the subject of have the JP or not, in Ocean water or large lakes, I'd opt to not have it...personal opinion due to the depth thing you are talking about and opportunities for engine power head partial submersion caused by following swells.

On Chris' comment about the higher the engine the less effective the trim tab in keeping your steering straight, this really applies to fast boats running 40+ MPH with the trim pushed out to get the bow up to reduce drag and increase speed. Obviously, the tab is at the rear of the engine, and as the tilting angle increases, the rear of the engine will be the first to rise and first to be at-near the surface where effect is minimized.

As stated, it's all trial and error till you get it where YOU want it and the above are my opinions based on my 60 years of playing with boats.

 

[Chris1956]

Mark, I was not referring to the trim tab, although, it is true that the higher the motor the less effect the trim tab has on steering torque. I added a wedge to the skeg to make up for reduced effect of the trim tab, due to the motor height on my speedboat.

I was referring to the trim of the motor. As you raise the height of the motor, trimming it has less effect on boat attitude ( bow lift). Depending on hull type, you will want enough bow lift to decrease the wetted surface of the hull to allow max speed. So the correct motor height allows for the correct bow lift, while being high enough to reduce the drag of the gearcase.

 

[MonkeyBird747]

One has to realize that water leaving the bottom of the boat returns to the surface. Rate of return dependends on viscosity of the water and boat speed. Second point is that boat hulls are seldom level when running in the water. With that said, and the fact that once the transom passes over the water it starts rising and anything you hang on the transom will dig deeper in the water than if it were transom mounted.

Rule of thumb for guys running "fast" with jack plates is 1" height increase per 6" of setback from what you would run if on the transom.

Engine height on the transom depends on a lot of things and the AV plate can be from 1.5" below a straight line extension of the hull...yard stick, to as much as several inches for large engines on 60 MPH or so boats. Hanging one on a jack plate just raises it that much more.

If you are cruising, then for your first reference, even with the hull as Chris said plus your setback adjustment is a good place to start....if your hull/setup will accept it...then you jack the engine up from there an amount determined by setback. Engines sitting too deep in the water are inefficient due to drag and are that much more prone to lower unit damage from striking submerged objects. Actually I'd run the engine on the JP as high as it would go without blowing out due to being too close to the surface at high trim angles. Also, using a "cupped" propeller like you find on most SS props is good at grabbing and holding water in high mounting installations.

On the subject of have the JP or not, in Ocean water or large lakes, I'd opt to not have it...personal opinion due to the depth thing you are talking about and opportunities for engine power head partial submersion caused by following swells.

On Chris' comment about the higher the engine the less effective the trim tab in keeping your steering straight, this really applies to fast boats running 40+ MPH with the trim pushed out to get the bow up to reduce drag and increase speed. Obviously, the tab is at the rear of the engine, and as the tilting angle increases, the rear of the engine will be the first to rise and first to be at-near the surface where effect is minimized.

As stated, it's all trial and error till you get it where YOU want it and the above are my opinions based on my 60 years of playing with boats.

Thanks Texasmark, always appreciate your input. Sound like I’ve got a direction to go from here and will go experiment on the water. I’m only running inland lakes, and so far nothing bigger than about 10,000 acres. One has a lot of submerged timber, so getting the motor up higher would be nice. I’ll look into a stainless prop as well. Been doing trial and error with some aluminum props to get an idea of where I need to be. I had to order a new tach for the Johnson hookups, and have been waiting on prop experimenting until that arrives. With the 21 pitch 4-blade I still have more hole shot than I need, and would rather have a little more cruising speed at less than wot. With the info you and Chris have shared I think it will all come together soon.

MB

 

[MonkeyBird747]

Ok, so after a few setbacks I had a chance to go test the adjustment. First had to reseal the lower unit because I had some milky gear oil in the prop hub. Then, just as about to pull out of the driveway I noticed the angle of the the trailer tongue looked different. Turns out the welds on the sides and bottom of the tongue had failed, and the entire trailer was being supported by a small weld on the top. Had a mobile welder come out and resolve that. Then off to the lake! Motor wouldn’t start. Cannon plug had come loose. Lost one hat (recovered), lost one life vest (recovered).

Pics of the height adjustment attached. Didn’t get a video but will next time out. The motor seemed really high to me, but tested it to see. Seemed fine at first, and AV plate was just skimming the surface. But during bumps and turns it felt like the prop was slipping or blowing out a little. Also, with the motor trimmed all the way down I’m still getting too much porpoise at wot. RPM right at 5500.

Lowered the motor 1 inch with the Jack plate. Porpoise a little better. A little better in the turns. I could trim up a tiny bit and notice a big change in RPM. If I was pulling a tube around and making right turns I feel like it might blow out but I’m not 100% on that. RPM still 5500.

I have plenty of hole shot and I’m thinking of going from the 13x21 aluminum 4-blade to the 13x23 stainless version. Using Turning Point props. I’m open to other prop ideas as well. Do you think a different pitch or type of prop could help with the porpoise at wot?

Pics show the motor height for first test. I lowered it about an inch from the position shown, to its current position. Pic of prop I’m thinking of trying next. Also a pic of the trailer that could have taken out a family of four on the highway. This was really, really scary. It looked like someone had cut and shortened the tongue, but didn’t weld or brace it properly.

 

[Texasmark]

Tell you what...before you spend any money on another prop, drop your engine to the first hole on the JP (I assume you have 4 holes on the engine's clamp bracket and you are all the way up in #4) and loosen the bolts on your JP and drop it all the way down.....bolt everything back up and take it out and run it and come back with the results! Just do it without me telling you all reasons why. I'd like to see the AV plate about "1 inch" above the keel (I know it doesn't have one but if it did, the lowest, center part of the hull which seems to have a pretty good, maybe 20 inch dead rise at the transom.

 

[Chris1956]

Setting the motor up for best speed, may not allow you to pull a tube, without prop ventilation. It is the nature of pulling tubes.

So you may need to lower the motor a bit when pulling tubes.

 

[Texasmark]

Setting the motor up for best speed, may not allow you to pull a tube, without prop ventilation. It is the nature of pulling tubes.

So you may need to lower the motor a bit when pulling tubes.

I understand your comment sir and would agree but with the significant dead rise at the transom, I don't think that is necessary and if he does what I said he will be out of options other than unbolting the JP and lowering it with respect to the transom. Let's see what happens.

 

[MonkeyBird747]

Tell you what...before you spend any money on another prop, drop your engine to the first hole on the JP (I assume you have 4 holes on the engine's clamp bracket and you are all the way up in #4) and loosen the bolts on your JP and drop it all the way down.....bolt everything back up and take it out and run it and come back with the results! Just do it without me telling you all reasons why. I'd like to see the AV plate about "1 inch" above the keel (I know it doesn't have one but if it did, the lowest, center part of the hull which seems to have a pretty good, maybe 20 inch dead rise at the transom.

I can do this, but that is about where I started with this motor if I’m not mistaken. That will put the AV plate submerged I’m pretty sure. I’d love to hear your thoughts on how things may change with this experiment.

I was thinking of removing the jackplate all together. The motor is very heavy, and by adding the jackplate I’ve moved 430 pounds 4.5” (including the transom bracing wood) aft. Do you think the resulting shift in center of gravity could be playing a role in the porpoising? I originally got the plate because I had a XL shaft motor on a long shaft transom. I’ve been running with one person sitting on the center console seat in front of the window, and with 75lbs of ballast in the bow.

im also getting so much spray from (I think) the bottom of the Jack plate that it is virtually impossible to see the tattletale stream at anything but 1/4 throttle. You can see this in the video I posted when viewing the starboard side of the motor. Come to think of it, this could be from the live well inlet screen and my transducer, which is being moved soon. I’ll need a better video angle.

 

[MonkeyBird747]

Well, I stared at the jack plate through several cups of coffee yesterday. Decided something about it was bugging me so I pulled it off and bolted the motor directly to transom as nature intended. Dropped the motor down to where the AV plate is 1”-1.5” above the lowest point of the hull at the transom. Will test and report back.

 

[Texasmark]

Don't leave us hanging and surely before you do something different.

Porpoising is caused by the engine tilted out too far for the boat's speed, determined by lots of things including hull type and weight distribution and HP and prop specs, and all that. For a given rig, when you are up and runing at WOT for example, got her trimmed out to the sweet spot and blasting along, smooth running, bow lifted (but at a constant tilt angle...steady), spray coming off the corners of the transom and all that.....if you do nothing but cut the throttle say 10% the boat will start porpoising. If you want it to stop it you either hammer back down on the throttle, or tuck in your trim till it stops. Simple as that.

 

[MonkeyBird747]

Don't leave us hanging and surely before you do something different.

Porpoising is caused by the engine tilted out too far for the boat's speed, determined by lots of things including hull type and weight distribution and HP and prop specs, and all that. For a given rig, when you are up and runing at WOT for example, got her trimmed out to the sweet spot and blasting along, smooth running, bow lifted (but at a constant tilt angle...steady), spray coming off the corners of the transom and all that.....if you do nothing but cut the throttle say 10% the boat will start porpoising. If you want it to stop it you either hammer back down on the throttle, or tuck in your trim till it stops. Simple as that.

I’ll report back after the next trial. No planned changes until then, other than a new trolling motor .

I did just realize something that might be relevant that is new to me. All my props have cavitation burns in the middle of the blade. Little quarter-size round spots that are apparently (according to one online source) designated as “bubble cavitation” due to their location. I remember having these with my previous motor. I thought I was just hitting too many stumps .

I don’t have any skeg damage, and the bottom of the boat is normal. Both this motor and the previous were raised up via jack plate. It sounds like it could be related to a number of things, including engine height, hull design, or prop mismatch. Does this add anything to the overall picture of getting this motor setup optimally?

 

[Texasmark]

SWAG....indication of excessive air presence caused by prop being too close to the surface. I can't imagine what it was like driving that boat with the prop located where it was...mindboggling. Usually cavitation is on the reverse side of the blade, the low pressure side of the blade, caused by irregular water movement as a result of things like a prop that you ran through a gravel bed and ate up the tips, causing irregular jagged edges to form little vacuums and all. Not a metallurgical engr. so I don't know the ins and outs of cavitation damage mechanisms. Hitting stumps you'd have bent blades, not a cancer eating away at the aluminum alloy.

 

[Chris1956]

Cavitation and ventilation are two different things. Monkeys post was about cavitation, Marks was about ventilation.

Outboard motors have an anti-ventilation plate to combat the latter. This should ride on the surface to ease any ventilation. Different style props have different degrees of ventilation. Some props are resistant to ventilation and you can have the anti-ventilation plate well above the surface of the water for top performance.

Cavitation has nothing to do with air around the prop. It happens when a vacuum is created at the prop blades. I would expect this on high performance boats where their power causes the prop to cavitate.

 

[Texasmark]

I was putting up the dishes from the dishwasher while the coffee was "dripping" and something smacked me in the side of the face........At 80 I'm not as quick as I used to be.

Your initial inputs included a 4 blade Turning Point (I assume Hustler) prop. I have 3 of those sitting on my shelf in 13+ inches diameter and all 3 are "PORTED".
Have you ever run a PORTED prop? There is a dissertation here in iboats in the prop section whereby they talk about props and a lot that has to do with them with porting included.

Here's my version: Ports are for a boat to have a great hole shot and simultaneously have great top end performance (lots of MPH) with the same prop. The "hole shot" ( advancing the throttle rapidly to the firewall and going from a level boat just moving, to boat up on the water and bow back down...15 MPH for a number) puts a big load on the engine limiting RPMS which limits rate of HP development and in turn limits the engine's ability to turn the prop shaft.

Ports allow exhaust gasses to flow across the blades in the hole shot, reducing the density of the water and relieving the load on the engine so that it can develop it's RPM faster and turn the prop shaft faster, quicker.

The port size had to be controlled to determine just how much "boost" the engine gets so that you get the fastest hole shot. I think TP figured that for their props, 3/8" was the right diameter since unlike Mercury's Laser II to name one has rubber inserts where you can dial your size from zero to a larger size...don't have the unplugged port size handy. Trim position is very important also in getting the fastest hole shot and trial and error on engine tilt is needed to find the sweet spot.

So what this means to the to the operator is that the RPM shoots up , I like to run 1000 RPM for the boost amount, above what it would have been without porting...engine seems to be overreving, feels like the prop is slipping....it is with respect to the water.....meanwhile the boat responds rapidly and gets up on the water fast.

At some point the inrush water across the blades of the prop has enough pressure to seal off the ports and the prop "bites" the water.....just like a slipping hub in the prop quit slipping all of a sudden. Back in 1989 I bought a new rig and was running an original Merc. Laser Prop on a Ranger bass boat and lockup was hard and the boat shot forward when it happened, engine RPMs dropped and away I went.....great sport!!!!

Lots of verbiage in this post from both sides and I get lost going back over it but possibly all the jack plate addition and all that "stuff" was caused by your not understanding the little holes in the barrel under the leading edge of your prop's blades and trying to fix something that was doing what it was designed to do.

So, lets get your test you promised, and try some different tilt angles in your hole shot. Now that you know what to expect analyze what's happening and tell us of your experience. (I'm not going to go back over this....if I have a little "misteak" in here please forgive me.)

 

[Texasmark]

One more thing. The picture in my Avatar is a 17 1/2' aluminum bass boat, stepped hull, with a stock 2002 Merc. 90 (2.33:1 gear ratio)....like yours probably is and I was running a 13+ x 24P prop at 5500 RPM. I'm kissing 50 GPS in the picture while driving and snapping the picture with my cell phone. No jack plate, engine on transom, trim just below "blowout".....that means that the trim is pushed out while watching RPMs and the speedometer....sweet spot is the point, backed up slightly from the point where the boat speed starts to fall and the RPMs start to rise.....the prop is so high that it is starting to "Ventilate"...suck in air from the surface of the water.....aka "blow out".

Ref: https://www.go-fast.com/Knowledge-base/Find-your-prop-slip/Prop-slip-calculator

Your current TP prop with it's "ports" at 21" on your probably 2.33 gear ratio engine, on your 18' aluminum boat with high deadrise at the transom should be exactly what you should be running...if you don't have a dirty hull, or have your boat piled high in unnecessary "baggage" weighting it down. I just think you need to understand how it works and enjoy it!

 

[orositevuv]

wow, what a nice discussion ....

 

[MonkeyBird747]

Lot of good info in here. I have to read it all a few times and do bits of side research to make sure I understand the various points being made. Sounds like I’m not far from where I need to be, and looking forward to the next test.

Sticking with the 4-blade TP prop for now since is supposed to promote stern lift. The bubble cavitation burns are interesting, and I might ping TP to see what they think. I’m curious to see if they continue or shift position with the engine sitting lower now. Other than the design of the boat and its riveted hull, the only thing I can think of that might be disturbing the water near the prop is the live well inlet screen. It is inline with the prop arc. Hopefully will have a boat day this week. Weather has not been cooperating.