Propping help with Jackplate!!

100333624

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So I just installed a CMC 5.5" setback manual Jackplate and ran the boat yesterday. At first glance it rode nice but noticed the cab plate being about 1.5" higher than the waterline! I guess I had it set too high at 2" even though water pressure was about 17psi steady, dropped to 15 a couple times but no less.

My boat is a 16.5 Center console which is pretty light for the size I think at 1060 lbs dry. Motor is a '95 Yam 115 V4 (2 stroke) I was doing 49mph with me an another person on board before Jackplate with a 20" laser 2. This first run was with a 19" trophy plus and it seemed the boat was riding higher on the water surface, like the stern was more "out" of the water, but when I dropped the motor 1/2" lower the cav plate was buried and the boat was porpoising really REALLY bad, like I couldn't almost control it at all unless I slowed down a lot, even coming off plane it would start jumping wildly...?!?

I couldn't even take on a wake from a cruiser hitting it directly from the side which isn't good but with the laser I felt the bow was riding wayyy higher and I could take on waves like they weren't even there yes at almost full throttle it was never a problem....

I don't know what's going now I should have raised the motor another 1/4" while I was still in the water but I pulled out as it wasn't a good day, speedo tube burst off and filled my console and wiring with water, it just wasn't good so I called it a day...

Got it home and raised it about 1/4" for next time to try it, it seems that 1/4" height adjustments raise/lower the cav plate more than 1/2" or 5/8"!! So I'm gonna try minor adjustments till it's right.

I was told the trophy would grab and bite better and hold better especially than the laser and also launch better, in fact it was worst in every single aspect??? Also my laser was "worked" from a prop shop whereas the trophy plus was brand new pretty much....

Starting to feel the trophy being more for heavier boats instead? I think the laser runs better on lighter boats but the main thing was I wanted more power out of the hole because hole shot with my laser isn't the greatest but so far much MUCH better than this trophy?

Could is be just the fact that I'm also sitting back 5.5" in the water now? Would that make the boat porpoise? I rode fine when the height was too high but I was scared of maybe it was tooooo high because I've never seen the plate run that high up before, scared of cooling problems so didn't wanna take the risk..

The hull is a V and there isn't any type of hook or anything it's fairly straight and smooth. Just looking for some help and pointers as to what to try with next? Goal is to get a little more speed as the Jackplate is supposed to help with getting higher up so more prop correct? Also forgot to mention when lowered it was pushing me 71kph as opposed to the 80 I would normally get... And I thought trophy was supposed to be faster?
 

flyingscott

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First thing is people don't realize is when you put a jack plate on you actually lower the motor deeper into the water. 2 things cause this, first is the angle of the transom causes it to be lower. The second is because the water actually rises up from under the boat as you are running making the motor run in deeper water. Thats why it looks high but you have good water pressure. When you add a jackplate prop selection becomes a crap shoot, because the higher you raise the motor the better your top speed should be but it will cost you holeshot. The opposite is true if you lower it good holeshot at the cost of top end you are looking for something in between. Thats why hydraulic jack plates are the best. Yamaha also makes excellent performance props with there pro series stainless as far as porpoising your boat.DO NOT waste your money on a nose cone they do nothing at the speeds you are travelling total waste of money.
 

100333624

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Apr 24, 2012
Messages
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First thing is people don't realize is when you put a jack plate on you actually lower the motor deeper into the water. 2 things cause this, first is the angle of the transom causes it to be lower. The second is because the water actually rises up from under the boat as you are running making the motor run in deeper water. Thats why it looks high but you have good water pressure. When you add a jackplate prop selection becomes a crap shoot, because the higher you raise the motor the better your top speed should be but it will cost you holeshot. The opposite is true if you lower it good holeshot at the cost of top end you are looking for something in between. Thats why hydraulic jack plates are the best. Yamaha also makes excellent performance props with there pro series stainless as far as porpoising your boat.DO NOT waste your money on a nose cone they do nothing at the speeds you are travelling total waste of money.

Hi flyingscott,

Thanks for your advice. Actually I already knew those things prior to installing it. I am familiar with what happens but I'm not understanding exactly what causes porpoising?

I was looking into some Yamaha props today online and noticed they also make the "Turbo" line of props, I believe they are a Yamaha owned company... Seems they make some smaller diameter ones too like 13.25 x 21 which I was hoping to run in a Laser but it doesn't come in a 21 for the small hub gear case.... Only even numbers (20/22/24/26)..

I guess I'm looking for some advice as to what to try for the porpoising issue? Regardless of where the motor is on the Jackplate, I know the cav plate should be running at or jus above the waters surface, mine was a good inch and a half higher! Pressure was still sufficient though but I was just paranoid seeing it that high. It did run fine actually and didn't give me any issues.... Maybe I will try it out Monday as I raised it 1/4" higher than the last setting, which was far too low I think, there was a lot of splash back there...

Does a low motor cause porpoising because I was told otherwise, that mounted too high will cause it, but there's no way I'm too high, can't even see the plate at speed and like I said I never had this issue before..

As for the nose cone I never wanted one as there is no need for it, it's not a race boat.
 

flyingscott

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Porpoising usually happens when the prop doesn't have enough bite to hold the bow up. Yes the 5.5" of set back can cause the porpoising try moving some weight to the front of the boat. You changed the balance of the boat when you put the set back on.
 

100333624

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Porpoising usually happens when the prop doesn't have enough bite to hold the bow up. Yes the 5.5" of set back can cause the porpoising try moving some weight to the front of the boat. You changed the balance of the boat when you put the set back on.

See that's what I find strange, this trophy has a lot of cup in it I can literally put my thumb in the grooves, so it should have nothing but bite? But I didn't feel it when testing... It felt like it wasn't staying hooked up..

I can see how putting 350 pounds 5.5" further back can change the balance of the boat, I keep my boat fairly light as I like to run fast, bow has no weight on it except trolling motor.

When I did my first run I had my cousin sitting in the seat in front of the console, maybe that's why it wasn't porpoising? When I lowered the motor he was sitting beside me at the leaning post, so the bow was empty. But also I felt this prop didn't lift the bow at all, night and day with my laser 2 and I'd like others opinions on they're experience with these props with jackplate setup.
 
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flyingscott

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Well good luck with your boat maybe someone has the magic set-up for you. I have set-up a lot of boats with jackplates a lot of trial and error was involved.
 

Sea Rider

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Why do you need a jack plate for ? you are experiencing over or back water transom splashes ?

Happy Boating
 

100333624

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Ok I did another couple runs today and all I can is that it seems anything lower than 1.5" on my Jackplate is just not good, it porpoised very badly very scarey, especially today we had about 12kph winds so a small chop and couldn't even handle it at all, then add the wakes from the 40 footers and it felt like being in the bearing sea...

So I jacked it up to 1-5/8" and it ran nice! Porpoised a little bit coming out of the hole but a quick trim up (out) helped get rid of it. Had a good run of this trophy today and it seems like a good prop, not as well as all the hype going around but still good nevertheless..

My cav plate was running about 1" or less above the water surface and I was getting 15-20psi water pressure steady at full trim out WOT, still couldn't spin it fast enough but it ran 5200-5250ish rpm max at 72kph. My laser was doing 80 at 5400 if I recall correctly, but never tried it with the Jackplate.

Going by this I may increase height just a touch say another 1/8" to see, the prop held really good in tight turns so I think I can go a touch higher but not much, maybe gain a few rpm and some speed.

Will experiment with my laser again at this height before changing the height though as I want to compare it now with the trophy.

Need to know what the "safe" limit is for water pressure? My gauge reads up to 30 psi but if I recall 15 psi being the normal pressure? Anyone..??

Oh and I put the Jackplate because it I wanted to experiment with it to see if I can go a little faster. If results aren't better than without then I will pull it off.
 

100333624

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Yes there was some over transom splashing but splash well covers it, if it was manageable in today's conditions it shouldn't be a problem.. Motor was getting a good splash too but under the cowl not a single drop of water.
 

Sea Rider

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Post if possible a sideway pic of lower leg area showing both upper and lower horizontal plates to have a look. You're losing to much precious boating time guessing which lower tail height is best for that combo.

Happy Boating
 
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100333624

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100333624

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That is perfectly level with the bottom of the boat, cav plate measures exactly 1 - 3/4" above the bottom, this was the last setting I had it at and I haven't tried higher yet. Also the center of prop shaft to bottom of transom (or prop to pad as they say, even though I don't have a "pad" boat) is 5 - 11/16"...

Let me know what you think, and also I wouldn't say its time wasted, its a learning experience and also I am 5 minutes from the lake so its no problem to test :)
 

Sea Rider

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It's always cool to experiment by oneself, but will need a starting point from where to dial the correct OB/transom height.

To start combo troubleshooting would recommend the following.

-Need to check at which lower leg height is water flow passing at plane.
-Remove jack plate and sit OB on transom.
-Go for a wot spin with at least one mate up front to achieve a proper deck weight balance.
-Trim OB for combo to run parallel to water level when on plane.
-Test must be done on flat calm, no wind water cond.

Lower leg heights parameters to have into consideration :

100333624.JPG

-If water flow bangs against (1) will have over or back transom water splashes, will slow combo down too.
-Ideal is for water flow is to pass slight under small water deflector plate (red line), there's a cutting edge (2) along allow for water to enter or exit through (X) intakes. If achieving (1) condition, could shim OB height on transom till this cond it met which dials the best combo performance. Should see a nice wake behind combo, assume that cond will depend on hull shape too.
-If flow passes under (3) yellow line, will experience excessive prop aeration on choppy, windy water cond and at close turns.

If you think that removing jack plate is a too much demanding time and hassle, trial to dial that opt cond as in (2) raising or lowering jack plate accordingly. A jack plate will off-set OB weight out transom, probably will need to compensate deck balance moving more things forward to bow area.

One bummer as there's always one, the only way to check water flow spot on is to :
-Pull head out transom and check water flow at which lower leg height is passing through.
-Have a buddy with second boat have a look or take a clear pic while riding parallel to your combo at speed.
-Have a drone flying out transom taking pics or video LOL!!

It's a one time demanding job, but once you dial it perfectly can even maximize a prop to suit best your boating requirements and boat happily forever after.

Happy Boating
 

100333624

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Apr 24, 2012
Messages
205
It's always cool to experiment by oneself, but will need a starting point from where to dial the correct OB/transom height.

To start combo troubleshooting would recommend the following.

-Need to check at which lower leg height is water flow passing at plane.
-Remove jack plate and sit OB on transom.
-Go for a wot spin with at least one mate up front to achieve a proper deck weight balance.
-Trim OB for combo to run parallel to water level when on plane.
-Test must be done on flat calm, no wind water cond.

Lower leg heights parameters to have into consideration :



-If water flow bangs against (1) will have over or back transom water splashes, will slow combo down too.
-Ideal is for water flow is to pass slight under small water deflector plate (red line), there's a cutting edge (2) along allow for water to enter or exit through (X) intakes. If achieving (1) condition, could shim OB height on transom till this cond it met which dials the best combo performance. Should see a nice wake behind combo, assume that cond will depend on hull shape too.
-If flow passes under (3) yellow line, will experience excessive prop aeration on choppy, windy water cond and at close turns.

If you think that removing jack plate is a too much demanding time and hassle, trial to dial that opt cond as in (2) raising or lowering jack plate accordingly. A jack plate will off-set OB weight out transom, probably will need to compensate deck balance moving more things forward to bow area.

One bummer as there's always one, the only way to check water flow spot on is to :
-Pull head out transom and check water flow at which lower leg height is passing through.
-Have a buddy with second boat have a look or take a clear pic while riding parallel to your combo at speed.
-Have a drone flying out transom taking pics or video LOL!!

It's a one time demanding job, but once you dial it perfectly can even maximize a prop to suit best your boating requirements and boat happily forever after.

Happy Boating


Thanks for your detailed post! But if you read post #8 you'll see that I mentioned anything below 1.5" is NOT GOOD...no amount of weight distribution or trim in/out helped at all. Infact ONLY when I jacked it HIGHER to 1-3/4" (where it is now) did everything feel much much better, less torque on the steering wheel, easy to turn (I have hydraulic steering) trimming up gets rid of all porpoising (only porpoised a little out of the hole) better holeshot, and NO blowouts so far as I was using the Trophy Plus prop, the cav plate like I said was riding maybe half inch or so ABOVE the water line. It is easy for me to see the transom at speed as I was occasionally shifting to the side and looking back while driving, my center console gives me good view of the back and my boat is small so I can see it easily as I checked it about 30 times while driving... I don't think its a problem where it is now as the consensus on outboard height is cav plate at or above waterline at cruise/speed (depending on hull etc..) mine is there already and it drives better than it did before, like I said anything lower was extremely dangerous and uncomfortable even just coming out of the hole, it was obvious it wasn't set high enough...

NOW my question is what is the recommended PSI I should be looking for on the water pressure gauge? I know from research that you can pretty much jack it up till it starts losing pressure/blowout/cavitation with no increase in speed, so I still haven't got there yet so I assume I can go a LITTLE higher? I have 15-20 psi constant pressure at full speed.

I will try the Laser 2 prop next to compare to the Trophy Plus at the same outboard height to see what difference there is, and I will assume it will be a bit faster, bring the bow a bit higher and run more rpm, though I think it will blowout on turns where the trophy didn't... I was really impressed with the turns at this height on the trophy so far. And it was slightly choppy conditions from wind but more so from the wakes of about 50 other boats on the lake mostly cruisers so I got a really good feel for the prop and how it acts at this outboard height. I believe slightly more height is possible but I think the Trophy will blow out but I will experiment with it higher, not lower at this point. Can't max the rpms on the trophy at only 5200-5250ish but I remember the laser hitting about 5400 if I recall correctly before putting the jackplate, so I think in cleaner harder water, a little higher it may hit 5500-5600 which should hopefully gain me a little more speed too.

Will report back after further testing is complete.

Thanks,
Nick
 

Faztbullet

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You can possibly go up at least another 1/2 inch...just watch water pressure if you go higher. When I setup a pad hull I start with the bottom of the hull splitting the water intakes...
 

100333624

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You can possibly go up at least another 1/2 inch...just watch water pressure if you go higher. When I setup a pad hull I start with the bottom of the hull splitting the water intakes...


Thanks for your reply.

By bottom of boat splitting the intake I assume you mean pretty much bottom being middle of my red level in the picture above in post #13?

I think I can go 1/2" higher as you say because the FIRST time I took it out I had it at almost 2" high and the cav plate was a good 1" or 1.25" above the water line at wot, and pressure was 15psi steady, I didn't get to drive around like that only did a WOT speed run and took at look back and freaked out seeing it that high lol, so I got paranoid and lowered it and then the porpoising started happening and now I'm back up to 1.75"

Do you have any experience setting up V hull with jackplates as my hull doesn't have a pad..??
 

Faztbullet

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I have set up hulls for over 35+ year so I might have a little experience. Need a shot of transom from the motor end to see if its a true"V" or modified "V"
 

Sea Rider

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The posted formula will not work as intended if you are just playing and guessing with OB heights without "visually" checking in reality where is water flow passing by at lower leg as a starting point. In real world boating you have to match flat water flow passing at speed under hull at correct lower leg height, the equation is as simple as that.

With the wrong procedure you're only waisting neat and fun boating time, entirely up to you.

Happy Boating
 
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100333624

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Here are some more photos hopefully they can clear it up for you? The hull is pretty much V not sure though how "deep" I would consider it as I've seen a lot deeper. Also I'm not sure if its important to note but the transom angles inwards on both sides, like its flat where the motor is but then on port and starboard is caves inwards, I made a drawing showing what it kind of looks like.
 

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100333624

Petty Officer 1st Class
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The posted formula will not work as intended if you are just playing and guessing with OB heights without "visually" checking in reality where is water flow passing by at lower leg as a starting point. In real world boating you have to match flat water flow passing at speed under hull at correct lower leg height, the equation is as simple as that.

With the wrong procedure you're only waisting neat and fun boating time, entirely up to you.

Happy Boating


Sea Rider I'm not sure your reading my posts correctly? I never said I was guessing.... The water flow is passing right BELOW THE PLATE AS I SAW IT ABOUT 30 TIMES, I think that is visual enough???? Sorry to get so frustrated but maybe my posts are not coming out as clearly as I type them? Take a look at the pic as I drew a blue line where I saw the water passing, maybe this will help.
 

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