# Porpoising and motor height



## justletmein (Feb 11, 2006)

Getting a bit of porpoising on the new boat without using much trim, couple ticks up is about all I can go before it starts bouncing. Called the service manager at the dealer and he said to buy a hydrofoil or install trim tabs. I'm sure both of those would work (and I plan on trim tabs anyway) but seems that they might just be masking a problem that may have a more proper fix. He said when they mount the motor they try to line up the cavitation plate with the bottom of the hull, but after getting home and looking at it the cavitation plate appears to be lower than the hull bottom. Would this be something that would cause the boat to :an6: ?
edit: This is not the flat bottom skiff, if that makes a difference.

Boat is a 2012 Carolina Skiff 218 DLV with a 140 Suzuki by the way, and it runs great as long as I don't try to trim it up. lol *sigh*


----------



## nbell (Aug 18, 2010)

Lots of reasons a boat will porpois . I like the cav plate about an inch above the bottom of the boat . some set ups need a wedge installed ( to tuck the motor further under ) Could be a defect in the hull . Possibly a diffrent prop . I would get the problem solved before I installed a Shallow blaster , Whale tail , What ever brand of cav plate


----------



## Barbarian (Feb 12, 2009)

Any idea what speed you are running when wot trimmed down and is the boat dragging (feeling like it is touching the water front to back)? That boat is heavier than I expected it to be @ 1800lbs dry weight. My guess is 35-38 mph before trying to trim. Also, I know the front is not a flat bottom, but is a flat bottom on the back half of the boat?


----------



## justletmein (Feb 11, 2006)

Barbarian said:


> Any idea what speed you are running when wot trimmed down and is the boat dragging (feeling like it is touching the water front to back)? That boat is heavier than I expected it to be @ 1800lbs dry weight. My guess is 35-38 mph before trying to trim. Also, I know the front is not a flat bottom, but is a flat bottom on the back half of the boat?


40mph by GPS, 45 by speedo at WOT @ 6,100 rpm. That's with two-three brief taps on the trim lever from all the way down. It does feel like it's dragging a little, steering is a bit tough to the right, hard to explain.

Here's a pic of the rear of someone else's boat. The underside in the middle has a little lip, almost like a tiny trim tab sort of thing.


----------



## Fishtexx (Jun 29, 2004)

Ask your dealer to install negative trim wedges. This is cheap and should give you a better hole shot and more usable trim. I agree cavitation plate should be level or slightly above to bottom od the hull.


----------



## Barbarian (Feb 12, 2009)

justletmein said:


> 40mph by GPS, 45 by speedo at WOT @ 6,100 rpm. That's with two-three brief taps on the trim lever from all the way down. It does feel like it's dragging a little, steering is a bit tough to the right, hard to explain.


40mph with no trim is exceptional for that boat with a 115. I think the recommended RPM range for your motor is 5000-6000 so 6100 without any trim is probably too high and you could increase your prop by 1 size. The dealer should to do that at no charge if they sold it you rigged and complete. If you think the cavitation plate is well below the back of your boat, you may also ask them to raise your motor by 1 bolt hole pattern. If your motor is low, this could help some with the bounce and steering torque also. Lastly your weight distribution in the boat could be factor if you have stock piled everything in the rear. If you bought this new completely rigged out, I wouldn't settle until it was running right. Get Carolina Skiff involved if you can't get any help from the dealer.


----------



## richg99 (Aug 21, 2004)

If the dealer adjustments don't work..(try them first, of course, ).....add Smart Tabs. R


----------



## justletmein (Feb 11, 2006)

Barbarian said:


> 40mph with no trim is exceptional for that boat with a 115. I think the recommended RPM range for your motor is 5000-6000 so 6100 without any trim is probably too high and you could increase your prop by 1 size. The dealer should to do that at no charge if they sold it you rigged and complete. If you think the cavitation plate is well below the back of your boat, you may also ask them to raise your motor by 1 bolt hole pattern. If your motor is low, this could help some with the bounce and steering torque also. Lastly your weight distribution in the boat could be factor if you have stock piled everything in the rear. If you bought this new completely rigged out, I wouldn't settle until it was running right. Get Carolina Skiff involved if you can't get any help from the dealer.


Thanks for the info. To clarify, that pic was from google my boat has a 140 on it. The prop is a 20 pitch 3 blade. I'm going to get a broom stick and measure where that cavitation plate actually sits tonight in relation to the bottom of the hull. We did try loading all the gear in the front, spare battery under the front and camping gear, firewood, etc. When running without all that gear for fishing the kids sat up front. My old boat was a 19.5' Sea Fox bay fisher and I couldn't make that thing porpoise if I tried so this is kind of a new problem for me. Before that I had an 18' Kenner and it would porpoise only if I trimmed it up way high.


----------



## pmgoffjr (Jul 30, 2009)

Cavitation plate certainly should not be lower than the hull, but most porpoising is caused by the prop losing it's bite at the current speed and trim angle. 

That isn't a lifting hull, and it's not going to do much with more trim, but...a good designed four blade will cure a lot of your pain.


----------



## Flat's Hunter (Oct 18, 2007)

If its hard to steer one way, you can try to adjust the trim tab directly above the prop.( the little fin hanging down from the cav plate.

I think trim tabs would be a much better value than a compression plate for your problem. I would only add a plate if you are trying to keep water on your prop. Tabs will do a lot more for ya.


Sent from my iPhone using Tapatalk


----------



## Barbarian (Feb 12, 2009)

Flat's Hunter said:


> If its hard to steer one way, you can try to adjust the trim tab directly above the prop.( the little fin hanging down from the cav plate.
> 
> I think trim tabs would be a much better value than a compression plate for your problem. I would only add a plate if you are trying to keep water on your prop. Tabs will do a lot more for ya.
> 
> Sent from my iPhone using Tapatalk


It is amazing that little torque tab can do so much, but you are right. It can help considerably with the steering tightness. If I remember correctly, you angle the tab to the opposite side of the pull - I think. It's been a while since I've adjusted one.


----------



## justletmein (Feb 11, 2006)

I don't know if it matters but this boat has hydraulic steering (came with it). With my two old boats it was the same way though, pulled to the right until I trimmed up to a certain point and then the steering would feel neutral. Problem is with this boat I can't trim, lol, but the steering pull is very slight and I could let loose of the wheel and still track mostly straight.


----------



## MackerelMan (Aug 16, 2005)

*Check motor height first*

Take someone with you. Have them drive the boat at close to WOT. Stand on the back and see where the cavitation plate rides in relation to the water line. If it is submerged you need to raise your motor, where its bolted onto the boat. The cavitation plate at speed should be right at or slightly above the waterline. Do it one hole at a time. This will fix 90% of porpoising problems. I had to have the motor raised on 2 boats, as the dealers set them to low. Where the waterline is at speed in relation to your motor is a function of it's mounting position on the boat as well as how far back you motor sits from the bottom of your boat. Some transoms are angled inward. I can guarantee the cavitaion plate SHOULD NOT be even with the bottom, that's to low.


----------



## justletmein (Feb 11, 2006)

MackerelMan said:


> Take someone with you. Have them drive the boat at close to WOT. Stand on the back and see where the cavitation plate rides in relation to the water line. If it is submerged you need to raise your motor, where its bolted onto the boat. The cavitation plate at speed should be right at or slightly above the waterline. Do it one hole at a time. This will fix 90% of porpoising problems. I had to have the motor raised on 2 boats, as the dealers set them to low. Where the waterline is at speed in relation to your motor is a function of it's mounting position on the boat as well as how far back you motor sits from the bottom of your boat. Some transoms are angled inward. I can guarantee the cavitaion plate SHOULD NOT be even with the bottom, that's to low.


I will do that tomorrow, we'll be heading to Port O'Connor and have a long trip down the ICW and across the bay, plenty of time to take pics and all. I won't be able to update you guys on that until Sunday night though.

My transom is indeed angled inward, and eyeballing that cavitation plate I'm sure that it's actually below the line of the bottom of the boat. I'm going to measure that when I get home today by holding a broom stick to the bottom of the boat and see where it lines up. Will update thread when I know more.


----------



## justletmein (Feb 11, 2006)

Just got this email from the manufacturer so figured I'd post it here. I'll still be running the measurements and such yall asked for over the weekend.



> Good Afternoon,
> 
> The cavitation plate should sit between 3/4" to 1" below the bottom of the boat. Make sure all of the weight in the boat is proportionally even and even trim the motor. If you are still have issues then we recommend having an authorized Carolina Skiff dealership to inspect and possibly installing Smart Tabs.
> 
> Thanks so much for your interest in our products!


----------



## richg99 (Aug 21, 2004)

I've put Smart Tabs on three boats. Two of them were Carolina Skiffs. Smart Tabs have MANY benefits ( as do all tab systems)... well beyond just fixing porpoising.

I ran the original 1500 member Carolina Skiff Owners group way back when we had MSGroups. 
regards, R


----------



## justletmein (Feb 11, 2006)

richg99 said:


> I've put Smart Tabs on three boats. Two of them were Carolina Skiffs. Smart Tabs have MANY benefits ( as do all tab systems)... well beyond just fixing porpoising.
> 
> I ran the original 1500 member Carolina Skiff Owners group way back when we had MSGroups.
> regards, R


I will definitely end up with the Smart Tabs or Trim Tabs if I can find a set cheap enough for me to afford. It looks like my motor height is mounted properly and the prop is within RPM range for the Zuki so I'm ready to just give up and throw the band-aid on and enjoy my boat.


----------



## justletmein (Feb 11, 2006)

OK here's the shot of the cavitation plate. It appears to be about 3/4" below the bottom of the boat. It's a bit difficult to get a straight line with the 2x4 though because the back/bottom of the boat kind of slopes upward so there's a little hump about 1' or so in front of the transom so this line could move a little up or down depending on where exactly I square the board up.


----------



## richg99 (Aug 21, 2004)

IMHO tabs (any tabs) are not band aids. They are put on tons of really high dollar boats automatically. Really big yachts and skinny water skiffs. R


----------



## justletmein (Feb 11, 2006)

richg99 said:


> IMHO tabs (any tabs) are not band aids. They are put on tons of really high dollar boats automatically. Really big yachts and skinny water skiffs. R


Oh I didn't mean to imply that the tabs are bandaids for general use, just that my boat may have a problem that the tabs will hide rather than me fixing the problem first the normal way.


----------



## Barbarian (Feb 12, 2009)

justletmein said:


> Oh I didn't mean to imply that the tabs are bandaids for general use, just that my boat may have a problem that the tabs will hide rather than me fixing the problem first the normal way.


Agree. Only 2 possibilities - 1. something is not right and dealer/manuf. needs to get it right or 2. that boat just needs them and if that is the case, the dealer and/or manuf. should've told you upfront and advised the the tabs before you took the boat. unless you are the first person to run this setup and even with that thought you would think that one of them would've wanted to test run it.


----------



## Redfishr (Jul 26, 2004)

pmgoffjr said:


> but...a good designed four blade will cure a lot of your pain.


I'm thinking a 4 blade with 19 pitch and some cup will solve your problems......Good luck
The dealer should help prop your boat before you start adding a bunch of stuff you may not need.
If the dealer washes his hands of you call Louie.......He knows props by heart.


----------



## [email protected] (Jun 25, 2005)

Some hulls just porpoise and sometimes there is just nothing you can do about it to naturally get it to stop. Trim wedges don't help they are only give you more negative trim but they don't allow you to trim the motor up more, it just seems like you trim it up more since the motor starts out at a more negative angle. Trim tabs will stop the porpoising but will probably hurt your speed a bit. Your boat may just porpoise when you trim up and there may not be a way to fix it without hurting your top end speed. Sometimes people don't really understand what trim wedges do, they think it gives you more trim but in reality the degree of the motor is the same but you have more trim to get to that degree because your motor starts out tucked more under than without wedges. Maybe a whale tail will help.


----------



## justletmein (Feb 11, 2006)

Update: I ran it again this weekend and looked back at around 5,000rpm and cannot see my cavitation plate in the water so seems that it could be too low? Water was a bit choppy and I had a few peeps in the boat with me. I tested out the porpoising to be sure I hadn't been exaggerating and it's pretty close to what I originally though, 3-4 taps on that upper trim is as far as I can go without the boat getting a case of the hops.



Barbarian said:


> Agree. Only 2 possibilities - 1. something is not right and dealer/manuf. needs to get it right or 2. that boat just needs them and if that is the case, the dealer and/or manuf. should've told you upfront and advised the the tabs before you took the boat. unless you are the first person to run this setup and even with that thought you would think that one of them would've wanted to test run it.





Redfishr said:


> I'm thinking a 4 blade with 19 pitch and some cup will solve your problems......Good luck
> The dealer should help prop your boat before you start adding a bunch of stuff you may not need.
> If the dealer washes his hands of you call Louie.......He knows props by heart.


I'm learning an important lesson about buying a boat from an out of town dealer. I'm sure they'd be more than happy to work with me if I drop the boat off, but their closest location is 2.5 hours away. My setup is a fairly standard config from what I understand though, only variable might be the prop so I'll look into that some more.



[email protected] said:


> Some hulls just porpoise and sometimes there is just nothing you can do about it to naturally get it to stop. Trim wedges don't help they are only give you more negative trim but they don't allow you to trim the motor up more, it just seems like you trim it up more since the motor starts out at a more negative angle. Trim tabs will stop the porpoising but will probably hurt your speed a bit. Your boat may just porpoise when you trim up and there may not be a way to fix it without hurting your top end speed. Sometimes people don't really understand what trim wedges do, they think it gives you more trim but in reality the degree of the motor is the same but you have more trim to get to that degree because your motor starts out tucked more under than without wedges. Maybe a whale tail will help.


That's what I was thinking on the wedges, doesn't seem like it would change anything except maybe for hole shot with the motor positioned to launch the rear up from the negative angle.

What do you think would be better, specifically for the porpoising, whale tail or trim/smart tabs?

Anyone know if the tabs or tail will change anything if I were to ever spring for a jack plate?


----------



## MackerelMan (Aug 16, 2005)

*Have the motor raised*

Have it moved up 1 hole on the mounting bracket and take it out again. If you can't see the cav plate the motor is mounted to low on the boat. See where it runs and how your porpoising changed. May have to do it again. This problem is more common than it should be. If you have access to the right equipment you can do it yourself. Both of the last 2 boat I bought had to have the motor raised and it soleved the problem, one needed it raised by 2 holes.


----------



## justletmein (Feb 11, 2006)

MackerelMan said:


> Have it moved up 1 hole on the mounting bracket and take it out again. If you can't see the cav plate the motor is mounted to low on the boat. See where it runs and how your porpoising changed. May have to do it again. This problem is more common than it should be. If you have access to the right equipment you can do it yourself. Both of the last 2 boat I bought had to have the motor raised and it soleved the problem, one needed it raised by 2 holes.


I would love to try it, but my dealer is out of town. What kind of equipment do I need to DIY that? I have a strong 16 year old boy, I could wait until he gets in trouble and sentence him to holding an outboard up for 2 minutes while I fuss with the bolts. hehe


----------



## nbell (Aug 18, 2010)

Put blocks under the skeg and take out the top bolts . The bottom bolts should have a slot so you can loosen then raise the toung jack and reinstall


----------



## KEMPOC (Aug 24, 2010)

[email protected] said:


> Some hulls just porpoise and sometimes there is just nothing you can do about it to naturally get it to stop. Trim wedges don't help they are only give you more negative trim but they don't allow you to trim the motor up more, it just seems like you trim it up more since the motor starts out at a more negative angle. Trim tabs will stop the porpoising but will probably hurt your speed a bit. Your boat may just porpoise when you trim up and there may not be a way to fix it without hurting your top end speed. Sometimes people don't really understand what trim wedges do, they think it gives you more trim but in reality the degree of the motor is the same but you have more trim to get to that degree because your motor starts out tucked more under than without wedges. Maybe a whale tail will help.


James is exactly correct.

Dynamic instability (porpoising) is not caused by motor mounting height or the distance of the cav. plate above or below the water(with all due respect guys). It all about the relationship between center of gravity (CG), thrust vector and load. Raising and lowering the motor changes the arm of the thrust vector thereby changing how much effect it has on the hull's attitude. That gives the impression (incorrectly) that changing the motor height changes the boat's dynamic stability. Actually, when you raise the motor you reduce the arm between the boats lateral center of gravity and the thrust vector (TV) thereby reducng the effect of trimming up. The gain from raising the motor comes from the reduced drag of having less lower unit in the water. Because raising the motor reduces the arm between CG and TV it requires more up trim to get the same hull attitude

Trim up as much as possible before it becomes unstable. That is the best you can do with that center of gravity (CG). Moving the CG forward will allow you to trim up a little more. Trim tabs simply ad some stability at the expense of drag. When you trim up and then run the tabs down to stabilize, you are inducing drag.


----------



## richg99 (Aug 21, 2004)

If moving the engine up doesn't do it....just watch the YouTube testimonials at this site.

http://www.nauticusinc.com/testimonials_about_nauticus.html

In an ideal world, your motor/prop and boat would be married and work perfectly together. Since we live in an imperfect world...(marriages AND boats)...sometimes things have to be fixed by outside influences.

In my three cases, (two Carolina Skiffs and one El Pescador) the Smart-Tabs solved, forever, any porpoising with little or no loss of top end speed. I could also then power down and run on plane at much slower speeds, and the cushioning effect of the gas actuators softened my ride. Carolina Skfifs are not the softest riding boats anyhow, so the cushioning effect was a valuable one.

I STILL have the ST's on my 16 CS that I keep in Wisconsin and wouldn't take them off for any $$$.

Other brands of tabs work every well, too...but just cost more.

Heck, I'm not interested in the last MPH anyhow. I just want to go fishing.

R


----------



## KEMPOC (Aug 24, 2010)

richg99 said:


> If moving the engine up doesn't do it....just watch the YouTube testimonials at this site.
> 
> http://www.nauticusinc.com/testimonials_about_nauticus.html
> 
> ...


Amen brother, amen!


----------



## [email protected] (Jun 25, 2005)

Some hulls just have porpoising built into them, sometimes it happens when the boat builder didn't build the boat to do the speeds you are doing so the boat starts to porpoise. The Shoalwater 23 cat will porpoise with a motor over 200 hp in most cases so to fix it they started putting a wedge on the transom that is like a permanent trim tab to allow the boat to handle more hp and higher speeds without porpoising but that wedge still hurts your top end. Smart tabs are probably the best bet for your problem and may not hurt your top in much at all and actually give you more mph sense you can trim the motor more or your top speed may stay at your current speed but your able to get more hull out of the water. You are probably going as fast as your hull will pretty much go at this point, some hulls just have a limit no matter how much HP you strap to the back.


----------



## richg99 (Aug 21, 2004)

Just to add a bit to James's salient points.....some hulls actually have a "hook" built into them. The "hook" is a downward slant to the hull at the stern. 

( A hook can also be inadvertently caused by improper trailer support)..

Some marine architects purposely design a hook in. It acts as a substitute for trim tabs or for wedges. 

To my knowledge...no Carolina Skiff has ever had a "hook" designed into its hull.

regards, R


----------



## ReelWork (May 21, 2004)

James, you're like the Chinese Buffet of good advice here amigo!


----------



## FAT TIRE (Nov 25, 2004)

If you are dragging the cav plate under water at cruse speed the motor is to low, it needs to be skimming the surface or at least visible in the whitewater when your on plane. Raising the motor may or may not help with the porpoising defiantly wont make it worse. This link has some before and after pics of a motor that was to low.http://www.propgods.com/forum/Default.aspx?g=posts&t=98


----------



## justletmein (Feb 11, 2006)

richg99 said:


> Just to add a bit to James's salient points.....some hulls actually have a "hook" built into them. The "hook" is a downward slant to the hull at the stern.
> 
> ( A hook can also be inadvertently caused by improper trailer support)..
> 
> ...


When I was looking for a spot to mount my transducer I noticed what, in my mind, appeared to be sort of two small maybe 1/2" sloped built in trim tabs in the back. I noticed because I was using a 2x4 to find a straight line from the bottom of the hull and could not. In the very middle there's about 8" that is straight, then on both sides about 12" or so of the sloped area, and then it straightens out to the little V's in the back.



FAT TIRE said:


> If you are dragging the cav plate under water at cruse speed the motor is to low, it needs to be skimming the surface or at least visible in the whitewater when your on plane. Raising the motor may or may not help with the porpoising defiantly wont make it worse. This link has some before and after pics of a motor that was to low.http://www.propgods.com/forum/Default.aspx?g=posts&t=98


Thanks, that link is helpful. I couldn't see the cavitation plate at all when I looked so that seems like it's in order.


----------



## justletmein (Feb 11, 2006)

nbell said:


> Put blocks under the skeg and take out the top bolts . The bottom bolts should have a slot so you can loosen then raise the toung jack and reinstall


Will the Skeg support the weight without any problems? Anything special I need to know? I'm likely to try this, especially if I end up taking the boat out this weekend because the Smart Tabs will not have arrived yet and I want to see exactly what effect it has on things. I may call Suzuki and ask if not having the dealer do it will affect my warranty in any way.



KEMPOC said:


> James is exactly correct.
> 
> Dynamic instability (porpoising) is not caused by motor mounting height or the distance of the cav. plate above or below the water(with all due respect guys). It all about the relationship between center of gravity (CG), thrust vector and load. Raising and lowering the motor changes the arm of the thrust vector thereby changing how much effect it has on the hull's attitude. That gives the impression (incorrectly) that changing the motor height changes the boat's dynamic stability. Actually, when you raise the motor you reduce the arm between the boats lateral center of gravity and the thrust vector (TV) thereby reducng the effect of trimming up. The gain from raising the motor comes from the reduced drag of having less lower unit in the water. Because raising the motor reduces the arm between CG and TV it requires more up trim to get the same hull attitude
> 
> Trim up as much as possible before it becomes unstable. That is the best you can do with that center of gravity (CG). Moving the CG forward will allow you to trim up a little more. Trim tabs simply ad some stability at the expense of drag. When you trim up and then run the tabs down to stabilize, you are inducing drag.


Thanks for the clarification, makes a lot of sense. I've ordered the Smart Tabs so will be installing them. I kind of want to see how the boat will ride with a tick up on the motor before I install them though. I can always lower it back down if I need to.


----------



## ReelWork (May 21, 2004)

The skeg can support the weight but that is flirting with disaster to be quite honest. Any movement and that skeg could snap and then you have a broken skeg to repair. 

If you opt to lower it it onto the skeg, recommend sandwiching the skeg in between a couple 2x6's for added stability and work quick!

Moving the motor will not void the warranty. They don't know where it's mounted to begin with...


----------



## KEMPOC (Aug 24, 2010)

That hull is going to porpoise at the same load (weight and center of gravity), pitch attitude and speed combination. Raising the engine will not change that. Raising the engine will require more up trim to achieve the same pitch attitude at the same load and speed. In fact, if you raise the motor too high, the dynamic stability will be decreased because it will require so much up trim that the motor will begin to blow out. The only reason to raise the motor is to have less of the lower unit in the water causing drag.


----------



## justletmein (Feb 11, 2006)

Thanks everyone for all the great help, lot of great info in this thread.

:an5:



ReelWork said:


> The skeg can support the weight but that is flirting with disaster to be quite honest. Any movement and that skeg could snap and then you have a broken skeg to repair.
> 
> If you opt to lower it it onto the skeg, recommend sandwiching the skeg in between a couple 2x6's for added stability and work quick!
> 
> Moving the motor will not void the warranty. They don't know where it's mounted to begin with...


thx



KEMPOC said:


> That hull is going to porpoise at the same load (weight and center of gravity), pitch attitude and speed combination. Raising the engine will not change that. Raising the engine will require more up trim to achieve the same pitch attitude at the same load and speed. In fact, if you raise the motor too high, the dynamic stability will be decreased because it will require so much up trim that the motor will begin to blow out. The only reason to raise the motor is to have less of the lower unit in the water causing drag.


If I can get away with moving that motor up I wouldn't mind just a bit of extra clearance anyway as I've been dragging it through an awful lot of sand the last couple trips. I may run it out to a local lake and run it WOT without much weight in the boat and look again to see if I can see my cavitation plate or not, if I can't see it from what I understand it may indeed be mounted too low so I may be able to get away with bumping the motor up a little even if it won't help the porpoising. I already ordered the Smart Tabs so I'll be putting those on either way.


----------



## KEMPOC (Aug 24, 2010)

justletmein said:


> Thanks everyone for all the great help, lot of great info in this thread.
> 
> :an5:
> 
> ...


Tabs will certainly help. Good luck and titght lines. And remember _-"In life, if you aint a sponge, you gonna miss some s - - t." _
_ - an old fisherman _


----------

