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Author Topic:   Porpoising: Why Trim Tabs Help

jimh posted 07-18-2007 09:19 AM ET (US)
Porpoising, or the repeated oscillation of the bow of the boat up and down while the boat is on plane, is often seen in boats which seem to be heavy in the stern. There have been reports that porpoising can be eliminated by adding trim tabs.
The root cause of a lack of stability in a boat's fore and aft pitching when on plane is probably related to how much of the hull is in contact with the water and the angle the hull makes with the water. On hulls which rise onto plane but maintain a long section of the hull in contact with the water, there is much greater fore and aft pitching stability, and the porpoising behavior is not seen. Such hulls also tend to run with only a small angle of rise. On the other hand some hulls tend to rise onto plane at a greater up angle, and leave only a small amount of the hull in contact with the water. The angle the hull makes with the water tends to try to force the bow down. This force is overcome with lift from the engine's trim. If only a small portion of the hull is in contact with the water, the hull can lack stability on plane and go into pitching oscillations (porpoising).
A trim tab mounted on a transom can only exert a downward force on the bow. Because the hull itself (from its angle of plane) is already exerting downward force on the bow, it does not seem like adding trim tabs would be beneficial. The usual problem in hulls which porpoise is there seems to be insufficient lift to carry the bow at a particular angle. Bow lift comes from the propeller thrust and engine trim, not from trim tabs. Tabs can only help the bow go lower, not higher.
So how do trim tabs affect porpoising? It appears that the main benefit is that a trim tab extends the running surface of the hull, and by in effect making the hull a foot longer, it increases the amount of hull surface in contact with the water. This gives the hull more stability and resistance to fore and aft pitching.
Tabs are also capable of providing stern lift, so they can lift the stern of the boat when on plane. This results in a lower angle of attack for the planing attitude, and this puts more hull in contact with the water, too, in turn increasing fore and aft pitching stability.
I suspect that in boats where trim tabs have made a significant improvement in the fore and aft pitching stability when on plane, if the angle of the tabs was observed we would find that in most cases they have been set to just extend the hull's planing surface aft, or perhaps with just a very slight downward angle to provide some stern lift. Reports which confirm or contradict this assumption are welcome.
Buckda posted 07-18-2007 10:28 AM ET (US)
Jim-
That is exactly my opinion and experience on a stern-heavy 18' Outrage.
The benefits of tabs extend beyond the correction of porpoising though. They make a huge difference to correct list when your courseline has you in a crosswind; they can also greatly improve the ride (and lower planing speed) into a headsea.

In short, they do make boating a bit more comfortable at a modest cost per year of use.
Dave
Royboy posted 07-18-2007 01:19 PM ET (US)
I would agree with your treatment of how trim tabs help to stabilize the hull (keep it from porpoising). In my case, using Smart Tabs, the hull is extended and the tabs are deployed down seven degrees at rest,and move according to angle of attack of the hull itself. Actual trim is then done with the engine as it should be, or as one might expect.
I think it may also change the moment, by moving the center of gravity farther aft under planing conditions. I think this is another way of saying what you've already written.
Roy
cc13 posted 07-18-2007 06:07 PM ET (US)
Jimh,
Forgive me if I am not following the thought thread exactly. You seem to say trim tabs would not assist in alleviating porposing due to the trim effect, but do assist due to lengthening the hull.
Then you say that trim tabs, when down, raise the stern and lower the bow. I believe that is true. It seems to me that it is the lowering of the bow, changing the angle of attack, that alleviates porpoising.
If it is solely the lengthening of the running surface that eliminates porpoising, then is there is minimum length hull that will not porpoise? Assume a 14 ft hull porpoises and you add 1 foot of trim tabs and the porpoising stops, does that mean 15 feet is the magic number? No, because other lengths can porpoise, or not, as well.
I think it is the angle of attack that matters, which can be influenced by the prop, motor trim, or trims tabs.
jimh posted 07-18-2007 07:29 PM ET (US)
It is not the length of the hull that matters, but how much of the hull is still in contact with the water when the boat in on plane that determines how stable the fore and aft pitching oscillation will be.
If you just added fixed trim tabs which extended the line of the hull bottom aft and extra foot I think their effect would be to reduce the tendency to get into these pitching oscillations. It is because they put more hull in contact with the water. Or, you can look at it like this--they provide more hull to overcome the heavier weight aft. Even if these extensions were fixed and never adjusted to provide the sort of trim angle changes that real trim tabs provide, I am certain they would help reduce the tendency to porpoise.
Also, when a boat is on plane not much of the hull is in contact with the water. A boat might be 22-feet long, but when on plane only the last three feet of the hull is in contact with the water. So if a trim tab is added which adds about one foot more of running surface, its effect is not to add one foot to a 22-foot surface but rather to add one foot to a three foot surface, or a 33-percent increase in the length of the surface in contact with the water. This is why they can have such influence on the porpoising tendency.
cc13 posted 07-18-2007 08:30 PM ET (US)
OK. I'll accept that. If you imagine a five foot long tab, weightless so that that tab weight did not bring the bow up, I accept that bringing that tab into contact with the water surface will reduce porpoising. More length is in contact with the water, changing the fulcrum point, making it is harder to rock/oscillate.
Also, if you have a shorter tab and trim it down, thereby raising the stern and lowering the bow, you have the same effect, more length (actual hull bottom) in contact with the water and thus harder to rock/oscillate.
I'm not smart enough to figure which effect is greater. They both get the same result.
Although, rudder trim tabs on airplanes have a definite effect without increasing overall length.
fishgutz posted 07-23-2007 08:42 PM ET (US)
I think you guys are close but a little off about the lengthening of your hull. The trim tabs do not lengthen the hull as you say. They in effect move your lower unit or prop forward and move the "fulcrum" of the porpoising closer to the middle of the boat (or closer to the middle of the running surface), leaving some surface "in effect" aft of the motor or aft of the fulcrum preventing it from porpoising. To demonstrate this in an extreme example, think of those fishing boats with an outboard roughly 1/4 back from the bow in a well and all that running surface behind the motor. You couldn't get a boat like that to porpoise at all ever. Also, when your boat is porpoising, its running surface is constantly changing from short to long and vise versa.
Jimh's last paragraph in his opening post touched on this. And cc13's 5 ft tabs illustrate this ,too.
Now, many new boats with standard trim tabs are integrated into the bottom of the hull and do not extend aft so the effect isn't lengthening at all but just forcing the bow down.
Does this make sense to y'all? I would have never thought of it but I just saw one of those fishing boats last week down in Ft. Myers FL and I looked at some new scout and grady boats that had "recessed tabs" as the salesman called them that don't extend past the transom.
jimh posted 07-23-2007 08:52 PM ET (US)
If a boat has recessed trim tabs and those tabs are raised so they are not in the stream of water coming off the hull, then I suggest you should call their effect "hull shortening."
fishgutz posted 07-23-2007 09:37 PM ET (US)
You know "salesmen". Recessed may not be the correct term. They were recessed the thickness of the metal plate and they did not protrude past the back of the boat. They in fact did nothing to shorten or lengthen the hull.
Interesting: the salesman said if a boat porpoises or has trouble getting on plane fast it is a design problem. I know I could make a Grady porpoise and without tabs I'm sure a Grady would take longer to plane. He didn't impress me.
He did have a nice tan and a neatly pressed white Columbia fishing shirt. He also had no idea what a High-thrust Yamaha or a Bigfoot Merc were for.
elaelap posted 07-24-2007 03:52 PM ET (US)
Here we go again...
I think that properly trimming a well-designed hull should totally alleviate the need for tabs; i.e., I think that tabs represent failure somewhere along the line, either with initial design or with weight distribution, especially weight aft.
I've never had a porpoising problem with my Whalers, even though each of them (Katama 16, classic Outrage 18, and now a banana Revenge 19) has had a relatively heavy four stroke main motor and a kicker. Nor has any of those boats looked like she was waving her bow in the air at rest. My simple solution: moving batteries forward out of the splashwell and up under the console, and adjusting trim, if necessary, with the addition of a surprisingly small amount of weight (70-100 lbs of lead 2- and 2.5 lb salmon weights)up forward (in the anchor locker on the 16 and 18). I don't want to start a controversy here--just try it for yourselves, you folks who are having porpoising difficulties. I initiated this idea as a topic a couple of months back and it led to a pretty interesting thread of commentary, but I haven't heard any feedback from those who said they were going to give it a try.
By the way, it seems to me that most of the comments I've read here concerning porpoising problems come from owners of post-Dougherty Whalers, which doesn't surprise me one little bit. I'd feel like I was blaspheming altering the external fluid dynamics of a Dougherty hull. I was taught early as a sailor that every boat has its own design quirks, its own 'personality', and that a competent sailor must adjust himself to these idiosyncracies. Modest alterations in trim and balance (and sail set on a sailboat, of course) are part of this adjustment, but most of it involves learning and then exploiting the inherent design characteristics of ones craft. Obviously this is easiest on a well designed boat rather than a 'crank' craft; and when I was fortunate enough to discover the brilliantly designed Dougherty-designed classic Whalers, it was relatively simple to become comfortable with their handling and seakeeping characteristics. Yeah, shift a little weight around to trim up your craft, be careful that you're not trimming out the motor too much or not dealing with the chop and swells in ways optimal for your boat's design, or maybe your boat is just too damn powerful or you're asking for too much speed from a workboat hull, but if she's really a sweety you shouldn't need tabs.
Tony
elaelap posted 07-24-2007 05:34 PM ET (US)
Here's a link to that recent thread on trimming by adding weight to offset a heavy motor. I think some interesting and thoughtful comments were made by folks with various opinions about this important matter:
http://continuouswave.com/ubb/Forum4/HTML/005172.html
Tony
elaelap posted 07-24-2007 06:32 PM ET (US)
You know, I didn't want to derail this thread or bore you guys and gals with this stuff, but I've learned so much from so many of you over the years and all I really feel that I've been able to contribute of practical worth are the following:
--Try redistributing (and adding if necessary) weight to overcome porpoising and/or to offset the weight of a heavy motor.
--Make sure your Whaler (or any other small boat) has midships/springline cleats and use them FIRST when docking and LAST when leaving a dock, usually...especially when boating alone.
--Tack back and forth into chop and steep swells rather than pounding straight into the stuff.
--Motors spec'd "mid-range" work very well indeed on some of the smaller classic Whaler skiffs, depending upon ones use and the conditions in which one operates ones craft.
I know that most of these little hints are second nature to many of you, but others might benefit from at least giving a couple of them a try.
Tight lines,
Tony
Riverwhaler posted 07-24-2007 08:52 PM ET (US)
Hope Perry sees this..
Took all day but I installed Lenco 12 by 12 edge mounted tabs today. Took a run on the river tonight and the boat behaves more what I think it should. Remember it is a Honda 150 on a Nantucket. I have not been able to trim out much at all before porpoising takes over. I think the motor is a little heavy on this boat. I first tried a four blade prop that helped some then I moved up 1 hole that made a slight change. Last week took it up another hole and it got worse. So ordered Lencos and voila can trim out half way on gauge and half way down on tabs with no porpoising. Boat steers easier and just feels better. Motor is down to one hole up from bottom. Perry should try this.
Kencvit posted 07-25-2007 09:58 AM ET (US)
Riverwhaler,do you have pictures of your installation?
I have a Yamaha 150 on a Nantucket and hope to add tabs.
Thanks, Ken
n
jimh posted 07-25-2007 02:21 PM ET (US)
There is an article in the REFERENCE section which discusses the installation of trim tabs on the transom of Boston Whaler boats, and the article includes many pictures of how various trim tabs have been installed on various models of Boston Whaler boat. If others have installed trim tabs and have available similar pictures, I could append those pictures to the article, and, in this way, the value of the information could be increased.
If anyone has pictures of the installation of their trim tabs and would like to contribute to the collection of information about trim tab installation I have organized in the REFERENCE section, please send me an email with the digital images attached and an explanation of what is being shown.
Cf.: http://continuouswave.com/whaler/reference/trimTabs.html

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Author	Topic: Porpoising: Why Trim Tabs Help
jimh	posted 07-18-2007 09:19 AM ET (US) Porpoising, or the repeated oscillation of the bow of the boat up and down while the boat is on plane, is often seen in boats which seem to be heavy in the stern. There have been reports that porpoising can be eliminated by adding trim tabs. The root cause of a lack of stability in a boat's fore and aft pitching when on plane is probably related to how much of the hull is in contact with the water and the angle the hull makes with the water. On hulls which rise onto plane but maintain a long section of the hull in contact with the water, there is much greater fore and aft pitching stability, and the porpoising behavior is not seen. Such hulls also tend to run with only a small angle of rise. On the other hand some hulls tend to rise onto plane at a greater up angle, and leave only a small amount of the hull in contact with the water. The angle the hull makes with the water tends to try to force the bow down. This force is overcome with lift from the engine's trim. If only a small portion of the hull is in contact with the water, the hull can lack stability on plane and go into pitching oscillations (porpoising). A trim tab mounted on a transom can only exert a downward force on the bow. Because the hull itself (from its angle of plane) is already exerting downward force on the bow, it does not seem like adding trim tabs would be beneficial. The usual problem in hulls which porpoise is there seems to be insufficient lift to carry the bow at a particular angle. Bow lift comes from the propeller thrust and engine trim, not from trim tabs. Tabs can only help the bow go lower, not higher. So how do trim tabs affect porpoising? It appears that the main benefit is that a trim tab extends the running surface of the hull, and by in effect making the hull a foot longer, it increases the amount of hull surface in contact with the water. This gives the hull more stability and resistance to fore and aft pitching. Tabs are also capable of providing stern lift, so they can lift the stern of the boat when on plane. This results in a lower angle of attack for the planing attitude, and this puts more hull in contact with the water, too, in turn increasing fore and aft pitching stability. I suspect that in boats where trim tabs have made a significant improvement in the fore and aft pitching stability when on plane, if the angle of the tabs was observed we would find that in most cases they have been set to just extend the hull's planing surface aft, or perhaps with just a very slight downward angle to provide some stern lift. Reports which confirm or contradict this assumption are welcome.
Buckda	posted 07-18-2007 10:28 AM ET (US) Jim- That is exactly my opinion and experience on a stern-heavy 18' Outrage. The benefits of tabs extend beyond the correction of porpoising though. They make a huge difference to correct list when your courseline has you in a crosswind; they can also greatly improve the ride (and lower planing speed) into a headsea. In short, they do make boating a bit more comfortable at a modest cost per year of use. Dave
Royboy	posted 07-18-2007 01:19 PM ET (US) I would agree with your treatment of how trim tabs help to stabilize the hull (keep it from porpoising). In my case, using Smart Tabs, the hull is extended and the tabs are deployed down seven degrees at rest,and move according to angle of attack of the hull itself. Actual trim is then done with the engine as it should be, or as one might expect. I think it may also change the moment, by moving the center of gravity farther aft under planing conditions. I think this is another way of saying what you've already written. Roy
cc13	posted 07-18-2007 06:07 PM ET (US) Jimh, Forgive me if I am not following the thought thread exactly. You seem to say trim tabs would not assist in alleviating porposing due to the trim effect, but do assist due to lengthening the hull. Then you say that trim tabs, when down, raise the stern and lower the bow. I believe that is true. It seems to me that it is the lowering of the bow, changing the angle of attack, that alleviates porpoising. If it is solely the lengthening of the running surface that eliminates porpoising, then is there is minimum length hull that will not porpoise? Assume a 14 ft hull porpoises and you add 1 foot of trim tabs and the porpoising stops, does that mean 15 feet is the magic number? No, because other lengths can porpoise, or not, as well. I think it is the angle of attack that matters, which can be influenced by the prop, motor trim, or trims tabs.
jimh	posted 07-18-2007 07:29 PM ET (US) It is not the length of the hull that matters, but how much of the hull is still in contact with the water when the boat in on plane that determines how stable the fore and aft pitching oscillation will be. If you just added fixed trim tabs which extended the line of the hull bottom aft and extra foot I think their effect would be to reduce the tendency to get into these pitching oscillations. It is because they put more hull in contact with the water. Or, you can look at it like this--they provide more hull to overcome the heavier weight aft. Even if these extensions were fixed and never adjusted to provide the sort of trim angle changes that real trim tabs provide, I am certain they would help reduce the tendency to porpoise. Also, when a boat is on plane not much of the hull is in contact with the water. A boat might be 22-feet long, but when on plane only the last three feet of the hull is in contact with the water. So if a trim tab is added which adds about one foot more of running surface, its effect is not to add one foot to a 22-foot surface but rather to add one foot to a three foot surface, or a 33-percent increase in the length of the surface in contact with the water. This is why they can have such influence on the porpoising tendency.
cc13	posted 07-18-2007 08:30 PM ET (US) OK. I'll accept that. If you imagine a five foot long tab, weightless so that that tab weight did not bring the bow up, I accept that bringing that tab into contact with the water surface will reduce porpoising. More length is in contact with the water, changing the fulcrum point, making it is harder to rock/oscillate. Also, if you have a shorter tab and trim it down, thereby raising the stern and lowering the bow, you have the same effect, more length (actual hull bottom) in contact with the water and thus harder to rock/oscillate. I'm not smart enough to figure which effect is greater. They both get the same result. Although, rudder trim tabs on airplanes have a definite effect without increasing overall length.
fishgutz	posted 07-23-2007 08:42 PM ET (US) I think you guys are close but a little off about the lengthening of your hull. The trim tabs do not lengthen the hull as you say. They in effect move your lower unit or prop forward and move the "fulcrum" of the porpoising closer to the middle of the boat (or closer to the middle of the running surface), leaving some surface "in effect" aft of the motor or aft of the fulcrum preventing it from porpoising. To demonstrate this in an extreme example, think of those fishing boats with an outboard roughly 1/4 back from the bow in a well and all that running surface behind the motor. You couldn't get a boat like that to porpoise at all ever. Also, when your boat is porpoising, its running surface is constantly changing from short to long and vise versa. Jimh's last paragraph in his opening post touched on this. And cc13's 5 ft tabs illustrate this ,too. Now, many new boats with standard trim tabs are integrated into the bottom of the hull and do not extend aft so the effect isn't lengthening at all but just forcing the bow down. Does this make sense to y'all? I would have never thought of it but I just saw one of those fishing boats last week down in Ft. Myers FL and I looked at some new scout and grady boats that had "recessed tabs" as the salesman called them that don't extend past the transom.
jimh	posted 07-23-2007 08:52 PM ET (US) If a boat has recessed trim tabs and those tabs are raised so they are not in the stream of water coming off the hull, then I suggest you should call their effect "hull shortening."
fishgutz	posted 07-23-2007 09:37 PM ET (US) You know "salesmen". Recessed may not be the correct term. They were recessed the thickness of the metal plate and they did not protrude past the back of the boat. They in fact did nothing to shorten or lengthen the hull. Interesting: the salesman said if a boat porpoises or has trouble getting on plane fast it is a design problem. I know I could make a Grady porpoise and without tabs I'm sure a Grady would take longer to plane. He didn't impress me. He did have a nice tan and a neatly pressed white Columbia fishing shirt. He also had no idea what a High-thrust Yamaha or a Bigfoot Merc were for.
elaelap	posted 07-24-2007 03:52 PM ET (US) Here we go again... I think that properly trimming a well-designed hull should totally alleviate the need for tabs; i.e., I think that tabs represent failure somewhere along the line, either with initial design or with weight distribution, especially weight aft. I've never had a porpoising problem with my Whalers, even though each of them (Katama 16, classic Outrage 18, and now a banana Revenge 19) has had a relatively heavy four stroke main motor and a kicker. Nor has any of those boats looked like she was waving her bow in the air at rest. My simple solution: moving batteries forward out of the splashwell and up under the console, and adjusting trim, if necessary, with the addition of a surprisingly small amount of weight (70-100 lbs of lead 2- and 2.5 lb salmon weights)up forward (in the anchor locker on the 16 and 18). I don't want to start a controversy here--just try it for yourselves, you folks who are having porpoising difficulties. I initiated this idea as a topic a couple of months back and it led to a pretty interesting thread of commentary, but I haven't heard any feedback from those who said they were going to give it a try. By the way, it seems to me that most of the comments I've read here concerning porpoising problems come from owners of post-Dougherty Whalers, which doesn't surprise me one little bit. I'd feel like I was blaspheming altering the external fluid dynamics of a Dougherty hull. I was taught early as a sailor that every boat has its own design quirks, its own 'personality', and that a competent sailor must adjust himself to these idiosyncracies. Modest alterations in trim and balance (and sail set on a sailboat, of course) are part of this adjustment, but most of it involves learning and then exploiting the inherent design characteristics of ones craft. Obviously this is easiest on a well designed boat rather than a 'crank' craft; and when I was fortunate enough to discover the brilliantly designed Dougherty-designed classic Whalers, it was relatively simple to become comfortable with their handling and seakeeping characteristics. Yeah, shift a little weight around to trim up your craft, be careful that you're not trimming out the motor too much or not dealing with the chop and swells in ways optimal for your boat's design, or maybe your boat is just too damn powerful or you're asking for too much speed from a workboat hull, but if she's really a sweety you shouldn't need tabs. Tony
elaelap	posted 07-24-2007 05:34 PM ET (US) Here's a link to that recent thread on trimming by adding weight to offset a heavy motor. I think some interesting and thoughtful comments were made by folks with various opinions about this important matter: http://continuouswave.com/ubb/Forum4/HTML/005172.html Tony
elaelap	posted 07-24-2007 06:32 PM ET (US) You know, I didn't want to derail this thread or bore you guys and gals with this stuff, but I've learned so much from so many of you over the years and all I really feel that I've been able to contribute of practical worth are the following: --Try redistributing (and adding if necessary) weight to overcome porpoising and/or to offset the weight of a heavy motor. --Make sure your Whaler (or any other small boat) has midships/springline cleats and use them FIRST when docking and LAST when leaving a dock, usually...especially when boating alone. --Tack back and forth into chop and steep swells rather than pounding straight into the stuff. --Motors spec'd "mid-range" work very well indeed on some of the smaller classic Whaler skiffs, depending upon ones use and the conditions in which one operates ones craft. I know that most of these little hints are second nature to many of you, but others might benefit from at least giving a couple of them a try. Tight lines, Tony
Riverwhaler	posted 07-24-2007 08:52 PM ET (US) Hope Perry sees this.. Took all day but I installed Lenco 12 by 12 edge mounted tabs today. Took a run on the river tonight and the boat behaves more what I think it should. Remember it is a Honda 150 on a Nantucket. I have not been able to trim out much at all before porpoising takes over. I think the motor is a little heavy on this boat. I first tried a four blade prop that helped some then I moved up 1 hole that made a slight change. Last week took it up another hole and it got worse. So ordered Lencos and voila can trim out half way on gauge and half way down on tabs with no porpoising. Boat steers easier and just feels better. Motor is down to one hole up from bottom. Perry should try this.
Kencvit	posted 07-25-2007 09:58 AM ET (US) Riverwhaler,do you have pictures of your installation? I have a Yamaha 150 on a Nantucket and hope to add tabs. Thanks, Ken n
jimh	posted 07-25-2007 02:21 PM ET (US) There is an article in the REFERENCE section which discusses the installation of trim tabs on the transom of Boston Whaler boats, and the article includes many pictures of how various trim tabs have been installed on various models of Boston Whaler boat. If others have installed trim tabs and have available similar pictures, I could append those pictures to the article, and, in this way, the value of the information could be increased. If anyone has pictures of the installation of their trim tabs and would like to contribute to the collection of information about trim tab installation I have organized in the REFERENCE section, please send me an email with the digital images attached and an explanation of what is being shown. Cf.: http://continuouswave.com/whaler/reference/trimTabs.html