# Cant stop flats boat from porpoising



## Tails up (Aug 18, 2017)

This boat is going to drive me nuts! Had a mechanic kindly nuke a motor for me so I now have a new motor but same dang problem.
16' high tide flats boat
Yamaha 90 sho with a prop tech 18p four blade.
20 gal fuel tank up front. Group 31 tm batt and a motorguide wireless.
Console has a group 24 under it
Smart tabs
So by myself I get about 38-40 mph but when you get the trim about right and the spray at the console it will porpoise with any movement. I cant ever get a decent ride. Motor is mounted in the top hole but the cav plate is right at the top of the cut out. I'm getting 58-6000 rpm. I've tried a 17p alum 3 blade, 17p/3 blade, 19p/3blade and now the 18p/4 blade which will get that rig out of the hole fast enough to snap your neck. I'm just not sure what the heck to do with it. I'm being told it should be running a lot faster.


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## CodyW (Jan 26, 2016)

Some boats don't need the motor trimmed more than where it sits. My silver king does the same and I just run it all the way down all the time. Speed sounds bout right. My silver king is 16 ft with a 90 etec and I get 40mph.


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## Rookiemistake (Jan 30, 2013)

Pics


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## Tails up (Aug 18, 2017)

I'm on shift tonight I'll post some tomorrow if it's not raining again. The problem with trimming down is by the time I trim down enough I'm only running in the low 30s


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## devrep (Feb 22, 2009)

some boats do this. I had a mitzi 17, one of the many problems I had with it was chronic porpoising. I tried everything, moving weight around, dif trim angles, moved the motor up hole by hole and back down hole by hole, dif props, wedges, hydrofoils, checked hull for hook. finally put trim tabs on to control it but of course you lose some speed. My Silver King is the same way, won't tolerate much trim up at all.


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## Tails up (Aug 18, 2017)

Even with the smart tabs you cant tell a difference whether they are deployed or not and I've tried different pressure shocks. I was wondering about raising the motor. I tried with the 19p but didnt do a whole lot


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## Guest (Jan 12, 2019)

Tails up said:


> I'm on shift tonight I'll post some tomorrow if it's not raining again. The problem with trimming down is by the time I trim down enough I'm only running in the low 30s


Yes, post up some pics! If you can get a pic of it running from over the transom to see where the water is at on the cav plate that would help too. With the pocket, I am willinh to bet your motor can come up some!


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## RunningOnEmpty (Jun 20, 2015)

My big flats boat doesn't run right until I push it past 40 mph. At 56 mph I'm trimmed out like crazy and dropping a few psi on the water pressure gauge. I never use my tabs because they kill my top speed. I've learned how to balance my load so it runs just right at high speed.


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## Smackdaddy53 (Dec 31, 2012)

Some boats need adjustable trim tabs to run as fast as possible. Without Lencos my hull would only hit 27-28 but once I trim her out and tab down some I hit 33-34 and throw a roostertail that a bass boat would envy. I’m not a fan of Smart Tabs, if you are going to drill holes in your transom put some Lencos on it so you have full control. Moving crap around all the time gets old, just get in and go.


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## Tails up (Aug 18, 2017)

I so agree. I wish I would have gone with lencos too. How about the stepback? The recessed area where the drain plug is. Does that have anything to do with motor height? Where should the cav plate be in relation to it?


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## Vertigo (Jun 3, 2012)

Things that will stop porpoising:
Raise the motor
Trim the motor in
Redistribute weight
Add trim tabs
Get a stern lifting prop


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## Guest (Jan 13, 2019)

Tails up said:


> I so agree. I wish I would have gone with lencos too. How about the stepback? The recessed area where the drain plug is. Does that have anything to do with motor height? Where should the cav plate be in relation to it?


If it’s a pocket then yes! 
Level or even higher in many cases!


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## el9surf (Dec 23, 2008)

Bigger trim tabs that can be controlled should put an end to the porpoising. The smart tabs probably have too much give to them under pressure on your boat. That and maybe look for a stern lifting prop. Powertech can get that dialed in for you.


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## Tails up (Aug 18, 2017)

Raining pretty good here so I took some pics of the motor height from under the cover and one of the prop to see what you guys think.


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## Guest (Jan 13, 2019)

Ok, try this...
Raise that motor up until the center line of gearcase bullet is level with the bottom of the hull. You may have to re prop after but she won’t porpose any more amd I’d be willing to be hole shot improvement also even though it’s good now.


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## lemaymiami (Feb 9, 2007)

- and toss those "smart tabs" - then install a decent set of either Lencos (electric) or Bennetts (old school hydraulic)... Make sure to contact each trim tab company first with the hull measurements, etc. They'll tell you exactly what model you need... 

Hope this helps


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## Jakehollender (Mar 22, 2014)

https://bobsmachine.com/product/true-tracker-stabilizer-plate-90-hp/

My boat has the bobs stabilizer plate, and it definitely helps with porpoising, but it took off a couple mph on top end.


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## No Bait / Lures Only (Apr 10, 2011)

Raise it up, n stern lifting prop. I had similar issues with my 18x60 flat boat n added 10x10 trim tabs n adjusted down until porpoise stopped. I think I made about 10 trips to the launch and 2 propeller changes n 2 trim tab sizes n bunch angle changes before it all worked.


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## Tails up (Aug 18, 2017)

I'm kinda hoping to adjust for the 18p four blade or the 19p three blade as I already have them. The power tech one is supposed to be a good all around prop from what they said but not sure if it's a stern lifting one.


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## Smackdaddy53 (Dec 31, 2012)

Jakehollender said:


> https://bobsmachine.com/product/true-tracker-stabilizer-plate-90-hp/
> 
> My boat has the bobs stabilizer plate, and it definitely helps with porpoising, but it took off a couple mph on top end.


That’s not going to hold true with every boat, mine needs the compression plate to get top speed.


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## No Bait / Lures Only (Apr 10, 2011)

Proping a boat a boat is like RnD or voudo trial n error. Most give up n disgusted n don't follow thru. Call prop gods n power tec listen to their suggestions.


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## State fish rob (Jan 25, 2017)

Some hull designs will do it at certain speeds , no matter what. Good luck. The smart tabs i have aren’ t so smart. Adjusted them out of the way


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## Smackdaddy53 (Dec 31, 2012)

No Bait / Lures Only said:


> Proping a boat a boat is like RnD or voudo trial n error. Most give up n disgusted n don't follow thru. Call prop gods n power tec listen to their suggestions.


A good prop shop will eliminate a lot of headache but a good prop is no good if your motor is not set up correctly on the transom.


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## No Bait / Lures Only (Apr 10, 2011)

Smackdaddy53 said:


> A good prop shop will eliminate a lot of headache but a good prop is no good if your motor is not set up correctly on the transom.


My bad


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## Tails up (Aug 18, 2017)

How does this look? 3rd hole down. Of course I cant run it till tomorrow.


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## Smackdaddy53 (Dec 31, 2012)

No Bait / Lures Only said:


> My bad


You didn’t do anything wrong, my comment was a general statement. Your comment was fine.


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## Smackdaddy53 (Dec 31, 2012)

Tails up said:


> How does this look? 3rd hole down. Of course I cant run it till tomorrow.
> View attachment 56392


Try what was mentioned earlier, get the prop shaft even with the bottom of the boat.


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## Guest (Jan 13, 2019)

Tails up said:


> I'm kinda hoping to adjust for the 18p four blade or the 19p three blade as I already have them. The power tech one is supposed to be a good all around prop from what they said but not sure if it's a stern lifting one.


Gotta get that motor up!



Smackdaddy53 said:


> A good prop shop will eliminate a lot of headache but a good prop is no good if your motor is not set up correctly on the transom.


Yes sir!



Tails up said:


> How does this look? 3rd hole down. Of course I cant run it till tomorrow.
> View attachment 56392


That lools better! But I bet she’ll go higher! When you go to jump it up have the motor tucked down and then start trimming up. She might try to cavitate a little on holeshot now as she begins to level out, as long as it grabs pretty quick this is normal with that hull style.


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## No Bait / Lures Only (Apr 10, 2011)

How wide is the hull? Kinda looks my v6 mercury on it. Think it's overpowered?
The new 90 sho is 100# heavier than the original 2 stroke 90 engine. Maybe that is the issue causing the problem? Just a thought?


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## Tails up (Aug 18, 2017)

16' 2" long, 78" wide. Max rating 90hp. Came with a yamaha 90 2s new.


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## FlyBy (Jul 12, 2013)

Can you move the battery to the front?


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## Tails up (Aug 18, 2017)

The group 31 is the smaller is under the console with all the wiring


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## Guest (Jan 13, 2019)

No Bait / Lures Only said:


> How wide is the hull? Kinda looks my v6 mercury on it. Think it's overpowered?


Is there such a thing?


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## hipshot (Sep 29, 2018)

I may be off track here; bear with me. Is your outboard trimmed in all the way in those photographs. It may just be the angle, but it looks like it's too far out.


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## Tails up (Aug 18, 2017)

It's just trimmed level, Where I could squat down and take a pic with the cav plate and keel flat in the pictures


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## Tails up (Aug 18, 2017)

Alright here's the results, 3rd hole down, ran 44-45mph solo with no porposing! So added my brother n law still no proposing if he rode on the bench but if he moved to the seat in front of the console it would try to start a little. Went back moved motor all the way up. Boat ran same speed but again no porpoising unless he moved up but it was very easy to control. Boat ran great! Tuff to steer but I will adjust the tab. Funny thing is it runs no diffrent with the smart tabs in play or pulled all the way up now. Corners fast and tight with no blow out. Very loud now though, sounds like a race car. I am so grateful to all yall for your help taming this beast! Two final thoughts/concerns, is there any additional stress on the transom I should worry about with it mounted all the way up? And should I be concerned with water pressure, the cav plate is not completely dry so the prop is all the way in the water and it appears to be peeing just fine. Again thanks so much for all the help.


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## Guest (Jan 15, 2019)

There may be a little added stress do to the added height but nothing to worry about IMO. A jackplate would add even more! If you ran it on plane at speed... then I’d say no water pressure issues as she didn’t overheat on ya during wet test! I am glad it worked out for you as I tend to get doubted on this hull style when I tell people to raise that motor lol!


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## Stauter (Jan 20, 2012)

Noticed 2 things about your smart tabs. Your port tab is mounted upside down and it can effect its performance.
Also noticed you are running 20 pound cylinders. With your engine you should be running 60#. Smart tabs work best when you have a good amount of force and you then use your engine trim to make the fine adjustments. Smart tab is good about swapping the cylinders.


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## Tails up (Aug 18, 2017)

That's funny about the strut! Never even noticed it. I started with 80#, swapped for 60#, someone gave me the 40#. I wanted to use the 60# ones but the head of them dont unscrew so I can use the retractor kit I have on the boat. So I'm kinda stuck with the 40s. But i may give them a call and see what i can do.


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## Tails up (Aug 18, 2017)

Boat rains, you were absolutely spot on about engine height! Your knowledge should now be concrete and not doubted. I have been to the boat builder, place I bought my motor and other odd and end people. No one ever mentioned the pocket hull design!


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## State fish rob (Jan 25, 2017)

Tails up said:


> Boat rains, you were absolutely spot on about engine height! Your knowledge should now be concrete and not doubted. I have been to the boat builder, place I bought my motor and other odd and end people. No one ever mentioned the pocket hull design!


Nice to have knowledgeable folks to count on ! Microskiff has 
been a great resource for me as well , thanks again to ever body


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## Steve_Mevers (Feb 8, 2013)

el9surf said:


> Bigger trim tabs that can be controlled should put an end to the porpoising. The smart tabs probably have too much give to them under pressure on your boat. That and maybe look for a stern lifting prop. Powertech can get that dialed in for you.


Powertech prop helped my performance. In my opinion a lot of people worry to much about top end speed. I want a prop and setup that performs the best with hole shot, porpoising and speed combined.


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## Godzuki86 (Nov 30, 2013)

Well I’m late to the party. I had the same issue with my 16’ action craft. The motors need to be higher with the pocket tunnels.

It’s also a good idea to run the boat with a second person and see where the anti-cav plate is on plane.

Glad you got it fixed.

I like this motor. I was hoping Yamaha would have gotten 90hp out of the 70 four stroke platform though. Benny’s blancos new Pro is now my dream skiff with this motor. Maybe one day...


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## PureCoastal (Aug 11, 2018)

CodyW said:


> Some boats don't need the motor trimmed more than where it sits. My silver king does the same and I just run it all the way down all the time. Speed sounds bout right. My silver king is 16 ft with a 90 etec and I get 40mph.


Hey Cody - I have a 16 SK, just put 90 SHO, and was wondering what prop you are using? Thanks v much


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## Tails up (Aug 18, 2017)

So wanted to share an update. Took the boat out to winyah bay today and she ran 45mph didn't porpoise one time but here is the kicker, yesterday I purchase a new lowrance 7ti and while installing it I put the tabs in the stow position with the retraction bracket. I forgot to put them down! So all day I rode the flats and creeks and crossed the big bay with no tabs! Came home and took em off! Now to figuring out that new totalscan transducer that looks like a giser behind the boat. Thanks again for everyone's help! BTW self bragging time, 34 reds today!


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## Guest (Jan 17, 2019)

Raise that ducer too!!! You can also build a spray rail/top that will mount with it to knock the spray down if raising causes issues.


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## makin moves (Mar 20, 2010)

34 reds sounds like a arm workout! Good job.


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## Tails up (Aug 18, 2017)

No doubt. Most were in slot under docks on vudu shrimp. Great day especially at 48 degrees.


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## DuckNut (Apr 3, 2009)

Boatbrains said:


> Raise that ducer too!!! You can also build a spray rail/top that will mount with it to knock the spray down if raising causes issues.


He can even raise it a bit above the bottom of the hull if he does not use it for high speed. When puttzing around, poling or trolling it will be in the water.


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## DuckNut (Apr 3, 2009)

Tails up said:


> Boat rains, you were absolutely spot on about engine height! Your knowledge should now be concrete and not doubted. I have been to the boat builder, place I bought my motor and other odd and end people. No one ever mentioned the pocket hull design!


Now your gonna give him a bigger head than he already has.


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## Guest (Jan 18, 2019)

DuckNut said:


> Now your gonna give him a bigger head than he already has.


Ha, only a 7 3/4 hat size!


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## RunningOnEmpty (Jun 20, 2015)

Do you have a water pressure gauge? I can trim and jack up my motor until my water pressure goes to nothing. I don't have low water pickups on my motor.


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## DuckNut (Apr 3, 2009)

Boatbrains said:


> Ha, only a 7 3/4 hat size!


Up from 7 1/8"?


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## Tails up (Aug 18, 2017)

No water pressure gauge. And I raised the transducer last night. Gonna rain a few days again so have to check it out next week.


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## dranrab (Oct 9, 2016)

Great thread. I learned something!


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## No Bait / Lures Only (Apr 10, 2011)

RunningOnEmpty said:


> My big flats boat doesn't run right until I push it past 40 mph. At 56 mph I'm trimmed out like crazy and dropping a few psi on the water pressure gauge. I never use my tabs because they kill my top speed. I've learned how to balance my load so it runs just right at high speed.


What size your engine?


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## Mako 181 (May 1, 2020)

lemaymiami said:


> - and toss those "smart tabs" - then install a decent set of either Lencos (electric) or Bennetts (old school hydraulic)... Make sure to contact each trim tab company first with the hull measurements, etc. They'll tell you exactly what model you need...
> 
> Hope this helps


I had Bennett hydraulic and converted to Lenco electric. 
Love the electric. 

If they ever quit I will head for the dock and fix them tomorrow. Got to have adjustable tabs at all times.

The boat will porpoise without the adjustable tabs.


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## Steve_Mevers (Feb 8, 2013)

I had porposing issues also, the Powertech LNR 4 really helped. I also use a little trim on my tabs. I agree, ditch the tabs you have and put on a set of Lenco.


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## MattGent (Nov 12, 2009)

Show a close up of the tabs from the side. 

I friend’s Egret came with the tabs mounted below flush and it would porpoise like crazy. Had to re-mount tabs per instructions. The leading edge of them should be above flush to the running surface


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## Lee Singleton (Jul 3, 2018)

I got a 70 on my 16 Dragonfly. I get 37-39 running smooth, but if I pull the tabs up to go faster it porpoises. I think light boats (less than 600lbs) aren’t meant to run any faster than 40 mph.


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