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Author Topic:   Definition of Porpoise

montaukman posted 02-27-2005 10:37 PM ET (US)
Hi. Silly question: What does it mean when a boat porpoises?
jimh posted 02-28-2005 12:09 AM ET (US)
porpoise (por p&s), n. 1. an aquatic creature. 2. a repeated pitching oscillation of the bow of a boat, usually caused by improper weight distribution or engine trim.
jimh posted 02-28-2005 12:13 AM ET (US)
It is usually possible to induce porpoising in almost any outboard powered boat if you have the engine trimmed improperly.
bsmotril posted 02-28-2005 10:41 AM ET (US)
Porpoising is a vertical oscillation of the bow that does not damp out on its own. Sometimes the boat will run fine, until you hit a wave or wake, then the oscillating will begin and won't stop on its' own. Trimming the motor too high for the speed you are running will usually make it occur too. Also, motor(s) that are set too low on the transom will bring it on as well. Changing trim, or speed will make it stop.
BillS
blkmtrfan posted 02-28-2005 11:00 AM ET (US)
On of the causes of the boat porpoising is the transistion between hydraulic lift and lift from the air. When a hull first planes, it is mostly hydraulic lift that is holding it on top of the water. When the "angle of attack" is such that the bow is up catching air, the air will start to lift the boat out of the water, then so much of the boat is out of the water and there isn't enough air speed to hold it up, it falls back down. You can fix this two ways, 1. adjust the trim so the "angle of attack" is lower and you are not trying to catch so much air at that speed or, 2. speed up so eouch air is available to hold it up at that level, but there will need to be some time adjustment to find the correct balance
BLUEWATERHUNTER posted 02-28-2005 04:39 PM ET (US)
the boat front will go up and down while you are cruising and is not caused by wave action.
jimh posted 03-01-2005 09:35 AM ET (US)
blkmtrfan--that is an interesting explanation. I had not heard that mentioned before. With Boston Whaler hulls the onset of porpoising can occur at rather moderate speeds, say 20-30 MPH.
At those speeds there is much air lift being generated by the hull form?
I think your explanation does explain some of the process in higher speed boats. I am curious if you think if applies at the speed where porpoising is seen in Boston Whalers.
blkmtrfan posted 03-01-2005 12:07 PM ET (US)
jimh regarding air lift at slow speeds generally speaking I would have to say yes, although at lower speeds the start of it may have more to do with improper fore to aft balance and could be purely oscillation of the motor trying to lift the bow and then running out of enough thrust too keep it that high. But when the hull is up high, it does get some lift from air because when you trim it down the oscillations reduce don’t they? The classic whaler hull design with the twin sponsons, this hull shape actually has many similarities to a full tunnel and/or catamaran type of hull, the sponsons push air under the boat to reduce wetted surface and increase efficiency. If you could get the speed high enough, the hull would fly a little with only the keel in the water (but is hard to keep from chine walking because the keel is round). Also think about when the times that a whaler jumps form a large wave and/or boat wake, as you launch off the wave the first thing that happens is the bow goes up higher as the hull traps air, then once the speed reduces (because the prop is now out of the water and you no longer have forward thrust) it falls back to the earth.
I’m am sure there are some actual hull designers that could do the necessary calculation to show at what speed and how much lift comes into play with a whaler (I am just a Monday morning arm chair engineer, LOL). But I truly believe that even at 20-30 mph airlift is part of the porpoise phenomena.
Just as a test, try going both directions on a windy day and see the difference with the same trim setting….
2manyboats posted 03-01-2005 01:46 PM ET (US)
I know I don't know much , but I don't think the air under the hull has every made any of my whalers porpoise . Any difference between an up wind run and a down wind run would be the waves more than the wind.
blkmtrfan posted 03-01-2005 02:34 PM ET (US)
Air alone isn't going to create the condtion, reread my above posts, air with the proper (or rather improper, I guess) "angle of attack". I am sure you have noticed that too much trim a lower speed will cause the condtion whereas the same amount of trim at a higher speed will work fine.
Robob2003 posted 03-01-2005 03:42 PM ET (US)
"could be purely oscillation of the motor trying to lift the bow and then running out of enough thrust too keep it that high"
I think that's the correct explanation.
It's exactly what happened to me when I came out of a very shallow canal with the motor trimmed up. I finally figured out what was wrong and porpoising stopped as soon as I put it back down.
Bob on Tampa Bay
2manyboats posted 03-01-2005 07:50 PM ET (US)
" I am sure you have noticed that too much trim a lower speed will cause the condtion whereas the same amount of trim at a higher speed will work fine."
Do you mean then that the increase in air flow at the higher speed stops the porpoising ?
jimh posted 03-01-2005 11:40 PM ET (US)
Another cause of porpoising can be a defect in a hull. If a hull has any hook or rocker this can contribute to a tendency to porpoise.
blkmtrfan posted 03-02-2005 08:51 AM ET (US)
[quote}Do you mean then that the increase in air flow at the higher speed stops the porpoising ?[/quote]
Yes, but it is all about balance. At a certian angle of attack there will be a speed where the air will help hold the bow of the boat up, too much trim and it amy porpoise again, and if you get going too fast with a lot of trim(not as likely with a whaler) you will see too much lift, you will blow the boat over backwards.
I realize that there are many non-belivers, which is ok (jimh you have not yet said if you think I am full of it or not, LOL), but really go out and try it, especially with the classic bottom, as there is a lot of surface area, the twin sponson design, and relatively ligth in weight. Play with the trim at different speeds and see how the handling changes.
[quote}If a hull has any hook or rocker this can contribute to a tendency to porpoise[/quote]
Agreed, plus cause manu other handling problems. However, because of thier construction, I doubt many whalers have such problems unless the hulls have really been mis treated or damaged and repaired poorly

2manyboats posted 03-02-2005 01:16 PM ET (US)
With my limited boating experiance (45 of my 50 years ) and only owning or having owned 2 13s , 2 Montauks, an 18'6" 19 ft. Outrage , a 22ft. Outrage and a 5.2 Harpoon . I think the
apparent wind that most whalers run in has little or no impact on porpoising.
Just to check I am going to take one of the 13s off the trailer , put it on the ground , then jack the bow up to about a 45 degree angle and prop it up with a 2x4.
The next windy day I will knock the 2x4 out and see if it falls slower that expected.
I will report back my findings

blkmtrfan posted 03-02-2005 03:42 PM ET (US)
Love the experement idea 2many, LOL, make sure you tabulate your results as accurate as possible making sure the distance dropped is always the same...
Unfortunately your method cannot duplicate the "gound effect" phenomena that happens when an object flys close to the water..
One questions for you, with your experence with that many whalers, have you actually ever had a porpiose problem? IMHO a guy with that much seat time could probably make all the proper trim adjustments needed without even thinking about it. Kind of becomes second nature, wouldn't you agree.
I do find it interesting that you don't believe there is any air lift associated with the twin sponson hull.

DillonBW posted 03-02-2005 05:00 PM ET (US)
Going along with 2manyboats idea and with the help of some special effects buddies, I did a test on my 17' Super sport.
We supported the boat at the transom on a 2x6 and saw horses. Then suspended the bow with an industrial spring scale hung from a high frame. We then set a Ritter fan (airplane propeller and motor fixed to a trailer) in front of the boat.
We started the fan and as we increased the speed we would not the weight on the spring scale.
The boat does infact lift to a certain degree... The Ritter fan was almost wide open, equal to a 120mph wind, and then one has to be careful that the whole rig does not fall over and break the outboard leg... Mmmmm!
Will give more definate results after repairs are complete Ha ha.
Si
DillonBW posted 03-02-2005 05:02 PM ET (US)
Sorry forgot to proof read! We would NOTE the weight on the spring scale. Was too worried about getting the motor fixed I guess.
Si
blkmtrfan posted 03-02-2005 05:16 PM ET (US)
DillonBW I am kind of confused, did you run this test, if so what were the results?
If you blew 120mph, or course you would have lift, you can rip the roof off of a house with 120mph ...
2manyboats posted 03-02-2005 08:30 PM ET (US)
I dropped the boat for the first time this afternoon , but I have no results. It did drop fast enough to smash the cat and thats why I lost count at 2 mississippi.
As for proper trim one of the 13s has no tilt and trim and we set it 5 years ago and have never changed it. It is set rather high ( 4th hole from the bottom) and at this setting the boat never porpoises. One hole higher and it does.
All the other boats have tilt and trim and the trim settings are sorta automatic.
While there could be some lift I just don't think there is enough to cause or prevent porpoising in the 20 to 40 kt range that most whalers run in.
blkmtrfan posted 03-03-2005 09:55 AM ET (US)
2manyboats, maybe you could enlighten me on which models of whalers have the porpoise problems. Is it only certian models? or can any whaler be made to porpoise? Is it only at certian speeds? or do conditions dictate the when it occurs?
blkmtrfan posted 03-03-2005 10:18 AM ET (US)
2many I thought about it a little more and I still think that you are missing my point. I don't believe that air lift is the only cause of whaler porpoise. However any planing vessel attains it's ride by a balance of many factors including but not limited too; forward thrust (with all the related propeller dynamics), gravity, hydraulic lift, water friction, aerodynamic lift, and air drag. When any of these are not in balance with each other, funny things (like propoise) can occur.
In the 20-40knt range aerodynamic lift not in balance with the other factors can and will be the cause of certian handling problem.
erik selis posted 03-03-2005 10:45 AM ET (US)
I have experienced porpoising in a 17-ft Alert many times. The main reason for this porpoising was because the center of gravity on the boat was way too far aft. Changing the engine height on the transom as well as adding lead weights inside the anchor locker helped solve this problem partially. The boat still tends to porpoise when the engine is not trimmed correctly.
I must say we often tried to see if the wind had any effect on this porpoising by sailing into a strong head-wind. It honestly had no effect what so ever. The boat would keep on porpoising once it started. Slowing down, speeding up or trimming is the only way to stop it getting out of control.
I think there are many forces at play here. I also have no doubt that the aerodynamics of the hull shape and air lift will play its roll in creating the porpoising effect but only at very high speeds...not at speeds that we are used to with our Whalers.
Here is a link to a report of some very scientific research and data on the porpoising topic.
http://ns.marine.osakafu-u.ac.jp/~katayama/katayama_PP/RCL2.pdf
Erik
blkmtrfan posted 03-03-2005 11:04 AM ET (US)
erik thanks for the link I will read through all of it when I get a chance. One question for you, when you whaler was porpoising was it at of near your top speed or could you ever power through it and get to a higher speed?
On a just for information basis. My 22' V-bottom speed boat (with a top speed in the 80+mph range) can porpoise in the 40-45 mph range, but it has plunty of power to get through this zone and will flatten out in the 50-55 mph range and will not porpoise no mater what the trim level is but the bow is now riding high and the air lift is helping to carry the bow, thus flying the hull. I also notice air lift on my 32' planing cruiser. At about 20 knts if it is over trimmed, it will actually slow down from the air lift and it mst be trimmed to a level position to get the speed back up.
Don't get me wrong weight balance in the hull will also have a great effect on this problem. But like you said with too much weight in the back, the bow starts to porpoise, when it is in the up position you have to admit there is some air lift holding the bow up for a moment, making the porpoise problem worse than just the weight imbalance....
erik selis posted 03-03-2005 11:19 AM ET (US)
The Alert would start to porpoise at fairly slow speeds, right after getting on plane at times. Speeding up would stop the porpoising. I have the impression that when running at higher speeds the conditions to create porpoising in this set-up decreases drastically.
Agreed, I have also felt the influence of a strong wind on my 170 Montauk when trying to get on plane when the aft part of the boat is heavily loaded. I think every boat will if the angle of the wind to the hull is large enough, as is the case when slowly getting on plane.
Erik
2manyboats posted 03-03-2005 02:12 PM ET (US)
Ok I lost another cat today and the boat landed on my foot too. So my experment is over, but I still believe porpoising in whalers and most pleasure boats is caused buy improper engine trim. Even if the the boat is loaded stern heavy , proper trim for those conditions will prevent porpoising.
erik selis posted 03-03-2005 02:18 PM ET (US)
2many,
I agree with you. Especially with our Whalers.
Erik
blkmtrfan posted 03-03-2005 03:43 PM ET (US)
Thank you Erik & 2manyboats for the input.
Without a difference of opinions discussion forums would be pretty boring....
Now I guess we will just have to wait until Bob Kellmer Jr. gets a Michief going over 70mph to test my theories, LOL
erik selis posted 03-03-2005 04:01 PM ET (US)
blkmtrfan,
I also agree with you when you say that airlift has its influence on enhancing porpoising...but IMO only at higher speeds than we are used to on our Whalers.
As I mentioned before, when sailing into a strong headwind the boat will porpoise. When sailing with the same wind behind us the boat will porpoise as well. I'm pretty sure that the wind doesn't have any influence in the later because in this case there would virtually be no wind. Only trimming, slowing down or accelerating will stop the porpoising. (at least on the Alert I was talking about).
Let's hope Bob doesn't flip over at 70 mph. :-)
Erik
blkmtrfan posted 03-03-2005 04:46 PM ET (US)
erik did you perform you upwind and downwind test with the trim at exactly the same level and do you believe that frequency and amplitude of oscillation were identical in both directions? I guess what I am getting at is at the slow speed the effect may be harder to determine.
I never did say it is the only reason for a propoise is the air lift but did say it is part of it.
Assume for one moment that your boat is perfectly blanced and you are running in a vacuum (OK kind of hard to keep the motor running, I know but an actual zero wind condition)with positive trim (you are not plowing) and the ride is sweet. Now all of a sudden add 30mph of wind, your balance is now upset, and it is impossible to maintain the same angle of attack and speed without some sort of adjustment. Depending on how delicate of a balance you have with your rig, if not changes are made some oscillation may begin to occur...
All I ask is that you think about it, Later Scott
bsmotril posted 03-03-2005 05:13 PM ET (US)
2manyboats,
It is very noble of you to sacrifice your cat's in the interest of advancing the science of Whaler's. To show my gratitude, I would be happy to send you two of the three cats that I have. My wife will be out of town, this weekend, so I can ship them off then. Let me know where to send them and please let me know which two you want:
bsmotril posted 03-03-2005 05:15 PM ET (US)
Hit the send button by mistake......
You can have the psychotic tabby, the huge fat lazy Manx, or the Calico which terrorizes the other two.
Sincerely, BillS
2manyboats posted 03-03-2005 08:53 PM ET (US)
The two that are well done

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Author	Topic: Definition of Porpoise
montaukman	posted 02-27-2005 10:37 PM ET (US) Hi. Silly question: What does it mean when a boat porpoises?
jimh	posted 02-28-2005 12:09 AM ET (US) porpoise (por p&s), n. 1. an aquatic creature. 2. a repeated pitching oscillation of the bow of a boat, usually caused by improper weight distribution or engine trim.
jimh	posted 02-28-2005 12:13 AM ET (US) It is usually possible to induce porpoising in almost any outboard powered boat if you have the engine trimmed improperly.
bsmotril	posted 02-28-2005 10:41 AM ET (US) Porpoising is a vertical oscillation of the bow that does not damp out on its own. Sometimes the boat will run fine, until you hit a wave or wake, then the oscillating will begin and won't stop on its' own. Trimming the motor too high for the speed you are running will usually make it occur too. Also, motor(s) that are set too low on the transom will bring it on as well. Changing trim, or speed will make it stop. BillS
blkmtrfan	posted 02-28-2005 11:00 AM ET (US) On of the causes of the boat porpoising is the transistion between hydraulic lift and lift from the air. When a hull first planes, it is mostly hydraulic lift that is holding it on top of the water. When the "angle of attack" is such that the bow is up catching air, the air will start to lift the boat out of the water, then so much of the boat is out of the water and there isn't enough air speed to hold it up, it falls back down. You can fix this two ways, 1. adjust the trim so the "angle of attack" is lower and you are not trying to catch so much air at that speed or, 2. speed up so eouch air is available to hold it up at that level, but there will need to be some time adjustment to find the correct balance
BLUEWATERHUNTER	posted 02-28-2005 04:39 PM ET (US) the boat front will go up and down while you are cruising and is not caused by wave action.
jimh	posted 03-01-2005 09:35 AM ET (US) blkmtrfan--that is an interesting explanation. I had not heard that mentioned before. With Boston Whaler hulls the onset of porpoising can occur at rather moderate speeds, say 20-30 MPH. At those speeds there is much air lift being generated by the hull form? I think your explanation does explain some of the process in higher speed boats. I am curious if you think if applies at the speed where porpoising is seen in Boston Whalers.
blkmtrfan	posted 03-01-2005 12:07 PM ET (US) jimh regarding air lift at slow speeds generally speaking I would have to say yes, although at lower speeds the start of it may have more to do with improper fore to aft balance and could be purely oscillation of the motor trying to lift the bow and then running out of enough thrust too keep it that high. But when the hull is up high, it does get some lift from air because when you trim it down the oscillations reduce don’t they? The classic whaler hull design with the twin sponsons, this hull shape actually has many similarities to a full tunnel and/or catamaran type of hull, the sponsons push air under the boat to reduce wetted surface and increase efficiency. If you could get the speed high enough, the hull would fly a little with only the keel in the water (but is hard to keep from chine walking because the keel is round). Also think about when the times that a whaler jumps form a large wave and/or boat wake, as you launch off the wave the first thing that happens is the bow goes up higher as the hull traps air, then once the speed reduces (because the prop is now out of the water and you no longer have forward thrust) it falls back to the earth. I’m am sure there are some actual hull designers that could do the necessary calculation to show at what speed and how much lift comes into play with a whaler (I am just a Monday morning arm chair engineer, LOL). But I truly believe that even at 20-30 mph airlift is part of the porpoise phenomena. Just as a test, try going both directions on a windy day and see the difference with the same trim setting….
2manyboats	posted 03-01-2005 01:46 PM ET (US) I know I don't know much , but I don't think the air under the hull has every made any of my whalers porpoise . Any difference between an up wind run and a down wind run would be the waves more than the wind.
blkmtrfan	posted 03-01-2005 02:34 PM ET (US) Air alone isn't going to create the condtion, reread my above posts, air with the proper (or rather improper, I guess) "angle of attack". I am sure you have noticed that too much trim a lower speed will cause the condtion whereas the same amount of trim at a higher speed will work fine.
Robob2003	posted 03-01-2005 03:42 PM ET (US) "could be purely oscillation of the motor trying to lift the bow and then running out of enough thrust too keep it that high" I think that's the correct explanation. It's exactly what happened to me when I came out of a very shallow canal with the motor trimmed up. I finally figured out what was wrong and porpoising stopped as soon as I put it back down. Bob on Tampa Bay
2manyboats	posted 03-01-2005 07:50 PM ET (US) " I am sure you have noticed that too much trim a lower speed will cause the condtion whereas the same amount of trim at a higher speed will work fine." Do you mean then that the increase in air flow at the higher speed stops the porpoising ?
jimh	posted 03-01-2005 11:40 PM ET (US) Another cause of porpoising can be a defect in a hull. If a hull has any hook or rocker this can contribute to a tendency to porpoise.
blkmtrfan	posted 03-02-2005 08:51 AM ET (US) [quote}Do you mean then that the increase in air flow at the higher speed stops the porpoising ?[/quote] Yes, but it is all about balance. At a certian angle of attack there will be a speed where the air will help hold the bow of the boat up, too much trim and it amy porpoise again, and if you get going too fast with a lot of trim(not as likely with a whaler) you will see too much lift, you will blow the boat over backwards. I realize that there are many non-belivers, which is ok (jimh you have not yet said if you think I am full of it or not, LOL), but really go out and try it, especially with the classic bottom, as there is a lot of surface area, the twin sponson design, and relatively ligth in weight. Play with the trim at different speeds and see how the handling changes. [quote}If a hull has any hook or rocker this can contribute to a tendency to porpoise[/quote] Agreed, plus cause manu other handling problems. However, because of thier construction, I doubt many whalers have such problems unless the hulls have really been mis treated or damaged and repaired poorly
2manyboats	posted 03-02-2005 01:16 PM ET (US) With my limited boating experiance (45 of my 50 years ) and only owning or having owned 2 13s , 2 Montauks, an 18'6" 19 ft. Outrage , a 22ft. Outrage and a 5.2 Harpoon . I think the apparent wind that most whalers run in has little or no impact on porpoising. Just to check I am going to take one of the 13s off the trailer , put it on the ground , then jack the bow up to about a 45 degree angle and prop it up with a 2x4. The next windy day I will knock the 2x4 out and see if it falls slower that expected. I will report back my findings
blkmtrfan	posted 03-02-2005 03:42 PM ET (US) Love the experement idea 2many, LOL, make sure you tabulate your results as accurate as possible making sure the distance dropped is always the same... Unfortunately your method cannot duplicate the "gound effect" phenomena that happens when an object flys close to the water.. One questions for you, with your experence with that many whalers, have you actually ever had a porpiose problem? IMHO a guy with that much seat time could probably make all the proper trim adjustments needed without even thinking about it. Kind of becomes second nature, wouldn't you agree. I do find it interesting that you don't believe there is any air lift associated with the twin sponson hull.
DillonBW	posted 03-02-2005 05:00 PM ET (US) Going along with 2manyboats idea and with the help of some special effects buddies, I did a test on my 17' Super sport. We supported the boat at the transom on a 2x6 and saw horses. Then suspended the bow with an industrial spring scale hung from a high frame. We then set a Ritter fan (airplane propeller and motor fixed to a trailer) in front of the boat. We started the fan and as we increased the speed we would not the weight on the spring scale. The boat does infact lift to a certain degree... The Ritter fan was almost wide open, equal to a 120mph wind, and then one has to be careful that the whole rig does not fall over and break the outboard leg... Mmmmm! Will give more definate results after repairs are complete Ha ha. Si
DillonBW	posted 03-02-2005 05:02 PM ET (US) Sorry forgot to proof read! We would NOTE the weight on the spring scale. Was too worried about getting the motor fixed I guess. Si
blkmtrfan	posted 03-02-2005 05:16 PM ET (US) DillonBW I am kind of confused, did you run this test, if so what were the results? If you blew 120mph, or course you would have lift, you can rip the roof off of a house with 120mph ...
2manyboats	posted 03-02-2005 08:30 PM ET (US) I dropped the boat for the first time this afternoon , but I have no results. It did drop fast enough to smash the cat and thats why I lost count at 2 mississippi. As for proper trim one of the 13s has no tilt and trim and we set it 5 years ago and have never changed it. It is set rather high ( 4th hole from the bottom) and at this setting the boat never porpoises. One hole higher and it does. All the other boats have tilt and trim and the trim settings are sorta automatic. While there could be some lift I just don't think there is enough to cause or prevent porpoising in the 20 to 40 kt range that most whalers run in.
blkmtrfan	posted 03-03-2005 09:55 AM ET (US) 2manyboats, maybe you could enlighten me on which models of whalers have the porpoise problems. Is it only certian models? or can any whaler be made to porpoise? Is it only at certian speeds? or do conditions dictate the when it occurs?
blkmtrfan	posted 03-03-2005 10:18 AM ET (US) 2many I thought about it a little more and I still think that you are missing my point. I don't believe that air lift is the only cause of whaler porpoise. However any planing vessel attains it's ride by a balance of many factors including but not limited too; forward thrust (with all the related propeller dynamics), gravity, hydraulic lift, water friction, aerodynamic lift, and air drag. When any of these are not in balance with each other, funny things (like propoise) can occur. In the 20-40knt range aerodynamic lift not in balance with the other factors can and will be the cause of certian handling problem.
erik selis	posted 03-03-2005 10:45 AM ET (US) I have experienced porpoising in a 17-ft Alert many times. The main reason for this porpoising was because the center of gravity on the boat was way too far aft. Changing the engine height on the transom as well as adding lead weights inside the anchor locker helped solve this problem partially. The boat still tends to porpoise when the engine is not trimmed correctly. I must say we often tried to see if the wind had any effect on this porpoising by sailing into a strong head-wind. It honestly had no effect what so ever. The boat would keep on porpoising once it started. Slowing down, speeding up or trimming is the only way to stop it getting out of control. I think there are many forces at play here. I also have no doubt that the aerodynamics of the hull shape and air lift will play its roll in creating the porpoising effect but only at very high speeds...not at speeds that we are used to with our Whalers. Here is a link to a report of some very scientific research and data on the porpoising topic. http://ns.marine.osakafu-u.ac.jp/~katayama/katayama_PP/RCL2.pdf Erik
blkmtrfan	posted 03-03-2005 11:04 AM ET (US) erik thanks for the link I will read through all of it when I get a chance. One question for you, when you whaler was porpoising was it at of near your top speed or could you ever power through it and get to a higher speed? On a just for information basis. My 22' V-bottom speed boat (with a top speed in the 80+mph range) can porpoise in the 40-45 mph range, but it has plunty of power to get through this zone and will flatten out in the 50-55 mph range and will not porpoise no mater what the trim level is but the bow is now riding high and the air lift is helping to carry the bow, thus flying the hull. I also notice air lift on my 32' planing cruiser. At about 20 knts if it is over trimmed, it will actually slow down from the air lift and it mst be trimmed to a level position to get the speed back up. Don't get me wrong weight balance in the hull will also have a great effect on this problem. But like you said with too much weight in the back, the bow starts to porpoise, when it is in the up position you have to admit there is some air lift holding the bow up for a moment, making the porpoise problem worse than just the weight imbalance....
erik selis	posted 03-03-2005 11:19 AM ET (US) The Alert would start to porpoise at fairly slow speeds, right after getting on plane at times. Speeding up would stop the porpoising. I have the impression that when running at higher speeds the conditions to create porpoising in this set-up decreases drastically. Agreed, I have also felt the influence of a strong wind on my 170 Montauk when trying to get on plane when the aft part of the boat is heavily loaded. I think every boat will if the angle of the wind to the hull is large enough, as is the case when slowly getting on plane. Erik
2manyboats	posted 03-03-2005 02:12 PM ET (US) Ok I lost another cat today and the boat landed on my foot too. So my experment is over, but I still believe porpoising in whalers and most pleasure boats is caused buy improper engine trim. Even if the the boat is loaded stern heavy , proper trim for those conditions will prevent porpoising.
erik selis	posted 03-03-2005 02:18 PM ET (US) 2many, I agree with you. Especially with our Whalers. Erik
blkmtrfan	posted 03-03-2005 03:43 PM ET (US) Thank you Erik & 2manyboats for the input. Without a difference of opinions discussion forums would be pretty boring.... Now I guess we will just have to wait until Bob Kellmer Jr. gets a Michief going over 70mph to test my theories, LOL
erik selis	posted 03-03-2005 04:01 PM ET (US) blkmtrfan, I also agree with you when you say that airlift has its influence on enhancing porpoising...but IMO only at higher speeds than we are used to on our Whalers. As I mentioned before, when sailing into a strong headwind the boat will porpoise. When sailing with the same wind behind us the boat will porpoise as well. I'm pretty sure that the wind doesn't have any influence in the later because in this case there would virtually be no wind. Only trimming, slowing down or accelerating will stop the porpoising. (at least on the Alert I was talking about). Let's hope Bob doesn't flip over at 70 mph. :-) Erik
blkmtrfan	posted 03-03-2005 04:46 PM ET (US) erik did you perform you upwind and downwind test with the trim at exactly the same level and do you believe that frequency and amplitude of oscillation were identical in both directions? I guess what I am getting at is at the slow speed the effect may be harder to determine. I never did say it is the only reason for a propoise is the air lift but did say it is part of it. Assume for one moment that your boat is perfectly blanced and you are running in a vacuum (OK kind of hard to keep the motor running, I know but an actual zero wind condition)with positive trim (you are not plowing) and the ride is sweet. Now all of a sudden add 30mph of wind, your balance is now upset, and it is impossible to maintain the same angle of attack and speed without some sort of adjustment. Depending on how delicate of a balance you have with your rig, if not changes are made some oscillation may begin to occur... All I ask is that you think about it, Later Scott
bsmotril	posted 03-03-2005 05:13 PM ET (US) 2manyboats, It is very noble of you to sacrifice your cat's in the interest of advancing the science of Whaler's. To show my gratitude, I would be happy to send you two of the three cats that I have. My wife will be out of town, this weekend, so I can ship them off then. Let me know where to send them and please let me know which two you want:
bsmotril	posted 03-03-2005 05:15 PM ET (US) Hit the send button by mistake...... You can have the psychotic tabby, the huge fat lazy Manx, or the Calico which terrorizes the other two. Sincerely, BillS
2manyboats	posted 03-03-2005 08:53 PM ET (US) The two that are well done