Will Trim tabs fix Porpoising?
03-18-2005, 10:13 AM
#12
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Re: Will Trim tabs fix Porpoising?
TCLANO
You posted the PDF's at the same time I posted my last reply. I will check those out (once I upgrade my membership that is)
Thanks
Tom
You posted the PDF's at the same time I posted my last reply. I will check those out (once I upgrade my membership that is)
Thanks
Tom
03-18-2005, 11:22 AM
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Re: Will Trim tabs fix Porpoising?
Tabs certainly will help porpoising.
Regardless of why the boat is bouncing (wrong prop, bad wieght distribution, junk hull, whatever), tabs effectively extend the length of the hull thereby negating the bouncing.
While tabs do not cure the root cause of the instability, they most certainly will help reduce the effects of the instability.
thsorens, throw some tabs on that rig. You'll be glad you did.
Regardless of why the boat is bouncing (wrong prop, bad wieght distribution, junk hull, whatever), tabs effectively extend the length of the hull thereby negating the bouncing.
While tabs do not cure the root cause of the instability, they most certainly will help reduce the effects of the instability.
thsorens, throw some tabs on that rig. You'll be glad you did.
03-18-2005, 11:31 AM
#14
Re: Will Trim tabs fix Porpoising?
Thsorens,
Short answer: tabs will help pretty much every boat and give you control over a variety of loading conditions. On a 23', I'd stay with a fairly small tab, certainly not even as big as a K-280. A high-performance Bennet might be just the ticket.
Without knowing what speeds you are running, I can assume that you are probably in the 50 mph range when you start to porpoise. You can get it to stop by deflecting the tabs some. If you find that you are deflecting them a significant amount, causing you to lose speed, it's time to shift weight forward. My own personal boat has 50 lb that I bolted in the nose. I can get it trimmed out with the tabs basically up at WOT (around 80 mph), but as I slow back down into the 50-60 mph range, I lose stability and have to deflect the tabs. I have K-380's on a 26' boat which is overkill, but that's what it came with... I have a forward fuel tank which I only transfer to the main tank when it starts to get low.
To summarize, trim tabs will let you run with LCG further aft than otherwise, allowing you to attain higher top speeds while making the boat controllable in the midrange. They also offer you the ability to trim the nose down while running upwind, and minimize the motiions in the vertical plane, while leaving LCG far enough aft to turn and run in a following sea without stuffing the nose.
These properties are what get a lot of folks in trouble while trying to make turns. There are other things at play trying to turn a stepped boat with very high x-dimension setup, but in straight V-bottom, when you come off the throttle to enter a turn, and fail to trim tabs and drive down, you can get into viscious porposiing oscillations, which have proven problematic in the turn. This, as you probably have seen, doesn't happen on lower performance boats with LCG's further forward.
Short answer: tabs will help pretty much every boat and give you control over a variety of loading conditions. On a 23', I'd stay with a fairly small tab, certainly not even as big as a K-280. A high-performance Bennet might be just the ticket.
Without knowing what speeds you are running, I can assume that you are probably in the 50 mph range when you start to porpoise. You can get it to stop by deflecting the tabs some. If you find that you are deflecting them a significant amount, causing you to lose speed, it's time to shift weight forward. My own personal boat has 50 lb that I bolted in the nose. I can get it trimmed out with the tabs basically up at WOT (around 80 mph), but as I slow back down into the 50-60 mph range, I lose stability and have to deflect the tabs. I have K-380's on a 26' boat which is overkill, but that's what it came with... I have a forward fuel tank which I only transfer to the main tank when it starts to get low.
To summarize, trim tabs will let you run with LCG further aft than otherwise, allowing you to attain higher top speeds while making the boat controllable in the midrange. They also offer you the ability to trim the nose down while running upwind, and minimize the motiions in the vertical plane, while leaving LCG far enough aft to turn and run in a following sea without stuffing the nose.
These properties are what get a lot of folks in trouble while trying to make turns. There are other things at play trying to turn a stepped boat with very high x-dimension setup, but in straight V-bottom, when you come off the throttle to enter a turn, and fail to trim tabs and drive down, you can get into viscious porposiing oscillations, which have proven problematic in the turn. This, as you probably have seen, doesn't happen on lower performance boats with LCG's further forward.
Last edited by tcelano; 03-18-2005 at 11:35 AM.
03-18-2005, 11:52 AM
#15
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Re: Will Trim tabs fix Porpoising?
tcelano,
Whoaaaa! I love knowledge and experience! Based on your testing, do you have an opinion on the top speed performance potential of the traditional Mercury type 380 trim tab at 3/4" to 1" above the bottom installation vs. the Arneson Rocker Plate concept with the plate even with the bottom but with the ability to add hook or rocker?
I may have missed it in you posted article, but I not clear on what impact the tab had on lift at top (non-porpoising) speed.
After a lot of bottom work involving: straightening, pads and strake length, I have my 24 degree vee stable at 100+ mph. What I am looking for is more speed as it relates to optimizing trim tab devices and there set up.
Sprague
Whoaaaa! I love knowledge and experience! Based on your testing, do you have an opinion on the top speed performance potential of the traditional Mercury type 380 trim tab at 3/4" to 1" above the bottom installation vs. the Arneson Rocker Plate concept with the plate even with the bottom but with the ability to add hook or rocker?
I may have missed it in you posted article, but I not clear on what impact the tab had on lift at top (non-porpoising) speed.
After a lot of bottom work involving: straightening, pads and strake length, I have my 24 degree vee stable at 100+ mph. What I am looking for is more speed as it relates to optimizing trim tab devices and there set up.
Sprague
03-18-2005, 12:20 PM
#16
Re: Will Trim tabs fix Porpoising?
The concept of adding rocker is an interesting one, but my opinion is that's what your drive unit is for. To achieve bow lift, trim up or put on a larger diameter prop, or one with more blade rake angle. Anything extra you drag in the water at 100+ mph costs you big time in resistance. To get more out of your rig, I'd start looking at raising the drive height, or getting rid of unnecessary weight.
You should be set up to run at top speed with the tabs clear of the water, or ever so sligtly deflected. That's where the 1" offset comes into play, by letting the tab gain some angle of attack prior to having it cost you in wetted surface area.
The posted article doesn't exactly address the implications on porposing on resistance, but what is apparent in the computations is that the total resistance is high at very low trim angles due to excessive wetted surface areas. As you trim up, you make the hull a more efficient lifting surface (the boat still weighs the same and therefore doesn't have to generate any more lift). Without porpoising in the pitcture, you could theoretically keep trimming up well past the point where porposing inception occurs, until the hydrodynamic lift vector (acting perperdicular to the running surface), and increase aerodynamic drag due to presenting more frontal area start to catch up with you. For most high-performance applications I've simulated, you could run faster if porposing didn't occur, but you end up having to set up for stability, and end up running right near where you start to lose it.
Similar to what I mentioned in the previous post about the trim tab lift unloading the hull and causing it to be more stable, aerodynamic lift does the same. Some setups benefit from getting the trim angle up to the point where the boat is getting a lot of aerodynamic lift, and this really helps you reduce the wetted surface area and achieve stability at the same time.
You should be set up to run at top speed with the tabs clear of the water, or ever so sligtly deflected. That's where the 1" offset comes into play, by letting the tab gain some angle of attack prior to having it cost you in wetted surface area.
The posted article doesn't exactly address the implications on porposing on resistance, but what is apparent in the computations is that the total resistance is high at very low trim angles due to excessive wetted surface areas. As you trim up, you make the hull a more efficient lifting surface (the boat still weighs the same and therefore doesn't have to generate any more lift). Without porpoising in the pitcture, you could theoretically keep trimming up well past the point where porposing inception occurs, until the hydrodynamic lift vector (acting perperdicular to the running surface), and increase aerodynamic drag due to presenting more frontal area start to catch up with you. For most high-performance applications I've simulated, you could run faster if porposing didn't occur, but you end up having to set up for stability, and end up running right near where you start to lose it.
Similar to what I mentioned in the previous post about the trim tab lift unloading the hull and causing it to be more stable, aerodynamic lift does the same. Some setups benefit from getting the trim angle up to the point where the boat is getting a lot of aerodynamic lift, and this really helps you reduce the wetted surface area and achieve stability at the same time.
03-18-2005, 12:29 PM
#17
Re: Will Trim tabs fix Porpoising?
Oh,
I should have mentioned that Arneson uses that because of the ASD's inability to give bow lift. ASD's work best on catamarans and other types of hulls which don't need the bow lift induced from having a deeper set , bevel gear drive.
I should have mentioned that Arneson uses that because of the ASD's inability to give bow lift. ASD's work best on catamarans and other types of hulls which don't need the bow lift induced from having a deeper set , bevel gear drive.
03-18-2005, 02:01 PM
#18
Platinum Member
Platinum Member
Re: Will Trim tabs fix Porpoising?
tcelano,
I have dialed in each of the systems/components you have mentioned. Last year after a lot of re-rigging, I managed to scrub off 6 mph! If was a bad year! The two major changes were switchable mufflers (the flap is the only restriction) and a bottom flush mounted tab system. I plan to test shortly; set a base line and remove the mufflers. If the speed is still missing, the Mercury 380 trim tabs are coming back! Does your experience say that the tab at +1" above the bottom is optimum?
Sprague
I have dialed in each of the systems/components you have mentioned. Last year after a lot of re-rigging, I managed to scrub off 6 mph! If was a bad year! The two major changes were switchable mufflers (the flap is the only restriction) and a bottom flush mounted tab system. I plan to test shortly; set a base line and remove the mufflers. If the speed is still missing, the Mercury 380 trim tabs are coming back! Does your experience say that the tab at +1" above the bottom is optimum?
Sprague
03-18-2005, 03:22 PM
#19
Registered
Re: Will Trim tabs fix Porpoising?
THOSORENS, imagine that. I could tell from your picture it looked like the mighty MO. I have a slip at Sandpiper Marinia. Looking at another one at Dodge park this week. Boating is just around the corner 
JIM
JIM
