What causes a boat to porpoise

Big Billy · 01-07-2006, 04:48 PM

I am having problems with my boat acting like a dolphin. I have tried different things and decided to try a plastic hydro-foil to help with this problem. This helped get me out of the hole faster and helped somewhat with bouncing up and down, but I still have the problem. Would trim tabs solve this problem? The boat is a 21' NR seahawk with affshore bracket, motor is a yamaha 225 4 stroke. Thanks

BM-FISHER · 01-07-2006, 04:58 PM

Yes trim tabs would help with the problem also you should be able to control it some what by triming the motor but this could lower your over all performce a little. trim tabs are the way to go

Bait O' Eggs · 01-07-2006, 05:32 PM

Not the first one of those boats I have heard the same problem with.

I believe they can bend the small lip at the transom where the bottom extends back an inch or two past the transom down a tad to help remedy the problem. Which creates full time more drag, but helps keep the nose of the boat down.

Redistributing the weight can help also if you have that option.

Bluto · 01-07-2006, 06:49 PM

I'd put trim tabs on. Will solve your problem and help with making adjustments to account for weight distribution. I could be wrong, but aren't trim tabs standard equipment on the new seahawks?

Tacklebuster · 01-07-2006, 09:56 PM

Take the boat back and have them re position the tab on the back of the boat. Do a water test to make sure it's done. Trim Tabs are nice but they shouldn't be the reason your boat doesn't porpoise. The only time I even use my tabs is to correct the list when more weight is on one side. Before my boat had tabs, they had to mess with that tab a few times to get it right.

Starfish · 01-07-2006, 10:04 PM

What kind of prop are you running? Sometimes a prop with more bow lift is needed to reduce porpoising. With a Yamaha I presume you got a stainless prop... if not, that's a good place to start. If you're already running stainless, make sure it's a high-lift prop like a Merc Tempest, Yamaha, etc. that can carry a heavy boat instead of a Laser or some kind of light boat prop.

As mentioned in another post, "peening" the hull may be needed to add a slight hook at the trailing edge. Just a tiny amount may be needed. You can also try raising or lowering your motor on the transom.

Mr. Carp · 01-07-2006, 10:37 PM

A lot of times when the motor is trimmed up too high it will do that also.

jimh · 01-07-2006, 10:43 PM

Porposing usually occurs because the bow is too high on plane. Try moving some of the weight forward in the boat. You can fix it with trim tabs or just move weight forward.

Since it is an outboard prop, you can also just trim the boat less or not run as fast.

Gundog · 01-08-2006, 05:15 AM

I had this same problem with my 21 seahawk with 150 hp. They bent the lip at the back of the boat and changed the prop to SS from aluminum. The boat was like night and day after that. I also added a fin on the motor and that helped to let me plain at lower speeds. I hope this helps you.
GD

Big Billy · 01-08-2006, 10:51 AM

I have tried trimming the motor and it does porpoise less at different angles, thanks for the advise

Tacklebuster · 01-08-2006, 12:12 PM

Your boat should run fine trimming the motor up under the right conditions. If you can not trim your motor up without your boat bouncing, take it back so they can bend the tab.

EZLoader · 01-08-2006, 12:14 PM

You have the motor trimmed up to much. Pull out and reset your motor's trim pin so that the motor is perpendicular to the water while at rest. If you still porpoise then adjusting your load more forward should take care of it. The trim pin set may need to be adjusted each trip if your load placements vary.

backlash442 · 01-08-2006, 09:14 PM

My Seahawk which has an I/O porpoises too. Only have to hit a boat wake to get it going and boy will it go. Trimming the motor out will cause it to stop pretty quick.

Some say to move the weight forward to stop porpoising. My question is what weight and where you going to move the weight to? It's not like there is a lot of room. With just me and my son sitting in the forward seats the boat can porpoise. There is nothing in the rear that can be moved forward.

Okie · 01-08-2006, 09:22 PM

Porpoising isn't a big thing to worry about. Water conditions can cause purposing. On the big river when another boat has made some waves or there are small swells you can start purposing. slowing down a small amount will usually stop it. It isn't something to worry about. There are certianly more important problems that purposing.
Good Luck:

Flatfish · 01-08-2006, 10:09 PM

Hey Boater! You sure are quiet on this thread.

Mark and the dog.

boater · 01-09-2006, 05:24 PM

Quote:

Hey Boater! You sure are quiet on this thread.

Mark and the dog.

Big Billy · 01-10-2006, 07:52 PM

I am taking my boat to North River anything else I should have them do to it?

toddsnorthriver · 01-11-2006, 01:06 PM

Wow, I didn't know that this was a common problem with the SeaHawk. I have a 21 ft with the off shore bracket and a 225 Yamaha. It has had the porpoise problem from day one. It partialy goes away trimming the motor. I can get rid of the problem completely if I use the trim tabs but this takes away lots of speed and creates so much splash that I can't pull a tube behind with out drownding the person on it. It also is a problem fuel wise. I love running out 30 to 35 miles off the oregon coast to fish for halibut. With the tabs down I use a considerable larger amount of fuel. What are they talking about bending on the back portion of the boat. I have the stock aluminum prop that came with the motor. Thanks, Todd

As yet un-named · 01-11-2006, 01:17 PM

Starting to notice a bit of a pattern here.... Hmm... I have a 17' Fish-Rite Performer with an offshore bracket and it too will porpoise when the motor is trimmed to it's most efficient spot. I put a doel-fin on and pretty much took care of it. 'Course I have a 120hp Force and wasn't as scared about drilling holes in the cav. plate as I would be if I had a "nice" motor like most of y'all have. One other benefit was that I can hold plane at a much lower speed now and tend to burn less fuel on a typical day.

Anyway, I'm starting to wonder if the offshore bracket isn't the common denominator here. It would make sense to me since you're moving the center of mass of the motor farther from the back edge of the hull? I personally would be more than a little leary of bending the back edge of my hull, but maybe that is the correct way to fix it.

Gundog · 01-11-2006, 01:36 PM

I would like to clear something up about my boat. The problem I had did not go away by trimming the motor down it did it all the time. My boat will porpoise now if I trim the motor way up. If you are not getting good performance from your motor you may have a problem but if you are just trimming the motor up to high I believe that would be normal. My boat was made much better with the prop change and bending the overhanging lip down a little. Before it was fixed it also ran very bow high all the time. I will post a picture of the back of the boat that shows the lip it is the very bottom of the hull. North River was very good about finding a solution to my problem and fixing it. I don�t want to see my post a s flame post or cause undo problems for anyone.
GD

toddsnorthriver · 01-11-2006, 02:53 PM

Hey Mike,
Did they have to take the boat back down to Roseburg to fix the lip or did they do it at the Wilsonville shop. Also did your motor come with a aluminum prop? Never mind, I just re-read your ealier post about your prop. I have a 225 with the same problem. I thought it was because of the heavier engine. I think I would have a hard time drilling holes in my motor for a fin though. Thanks, Todd

backlash442 · 01-11-2006, 07:51 PM

Well my Seahawk is an I/O so that throws out the offshore bracket theory. So I wonder what causes it?

jimh · 01-11-2006, 08:17 PM

My thought is NR thinks you will have plenty of weight up front and will never hit the problem. They could always start out with a boat that would never porpoise, but then it might be bow low if you have a load.

Tacklebuster · 01-11-2006, 10:36 PM

Take it back to Wilsonville, they can do it right there. Most of our boats porpoised the first day we picked ours up as well. Be sure and do a water test with them after they finish. Your trim tabs shouldn't be the tool to fix it. I rarely use my trim tabs at all, when I do, it's mostly to correct the list my boat has when to much weight is on one side.

Starfish · 01-12-2006, 05:59 AM

Basically the porpoising is caused by the bow lifting to the point where the prop can no longer carry the load, then the prop loses its grip and the bow drops again. Having a slight upward bend on the trailing edge of the boat can make the problem worse. Slightly bending the trailing edge downward-- sometimes by as little as a quarter inch-- can fix this (of course performed by the boat manufacturer, I wouldn't try it myself). As stated earlier, getting the right prop can be a huge factor, especially with a big motor like a 225hp. That motor can carry a lot of weight and it takes a high rake/heavy lift, stainless prop to pick up the bow and hold it up. You might want to try a Merc Tempest (3 blade) or Rev 2 (4 blade), just make sure you keep tweaking till you get the right pitch. A Vensura might also be a good choice. Either of those props will probably require 2" less pitch than what you're running in a stock aluminum prop.

backlash442 · 01-12-2006, 06:23 AM

NR obviously knows about the problem so I wonder if they've taken the steps to correct it in the 06 line. What would be interesting to know is if other models have the same issue. For that matter what other manufacturers. Before I bought my Seahawk I test rode a Weldcraft and it porpoised too but having had no previous experience with porpoising I didn't know what it was. Never even heard of the term until I read about it on this website. My last boat, which was glass didn't porpoise.

boater · 01-12-2006, 07:38 AM

why dont you guys with the northrivers that porpoise mount one of those fake bulls on the bow and charge people to ride it

Bait O' Eggs · 01-12-2006, 07:43 AM

The 8 second North River bull ride would be cool, unless you fell off and went thru the prop

Probably worse than getting stomped on by a bull

boater · 01-12-2006, 08:10 AM

i realy think that the porpoise problem is from the hull being over straked, the lifting strakes are to aggresive and the boat is always trying to lift it self out of the water

backlash442 · 01-12-2006, 08:54 AM

My kids love the bull ride.

So does Weldcraft have lifting strakes? I can't remember. I know the Jetcraft I test rode didn't porpoise and I purposefully went through lots of boat wakes to see how it handled since it was right after the Weldcraft ride. Maybe it has something to do with the style of boat (forward windshield)? Length? We know the 19' and 21' Seahawk will do it but how about a 23'? What about sleds? It would be interesting, at least to me, to hear what others have to say. Maybe should ask this question on the main board.

jimh · 01-12-2006, 10:16 AM

I spoke to one NR winshield sled owner who's boat porpoised according to him. His fix was to add a 60 lb anchor to the front. He said it cleared it right up!

bluebackbil · 01-12-2006, 10:28 AM

I have a 20' thunderjet with an offshore transom and i've never had a problem with porpoising. of course, i am only running a merc 90 4 stroke so it isn't too heavy. a guy i know runs a fire patrol boat (custom weld) with an offshore transom and a 150 4 stroke johnson and he said it doesn't have a problem with porpoising.

reelfine · 01-12-2006, 10:33 AM

I have an '05 North River Scout (18') with a 115 4-stroke on the back.

It porpoised *badly* when I picked it up, basically un-usable. I tried weighting the bow, but it took ~450 lbs. of added weight before the porpoising subsided (5 bags of 50lb. lead shot, + me). I took it back to the dealership and they tried a few different things:

* bending the lip down a little (didn't help and cracked the welds. They had to re-weld and re-paint).

* weld on what they call "porpoise-pads" to the hull. These are little triangular pieces of metal that helped to lift the stern and drive down the bow.

The end result is that porpoising no longer occurs, but it was a pain getting there.

boater · 01-12-2006, 10:42 AM

:lurk:

Coastalfisherman · 01-12-2006, 01:03 PM

My 20' fishrite wanted to porpoise at about half throttle. Once I got above that it leveled out. I bent the tab on the transom donw about 1/4". No more problems with the boat.

Big Billy · 01-12-2006, 07:29 PM

So Gundog, you had your overhang bent down, was it bent down flush with the bottom or further down where it wis making resistance pushing your nose down, I am taking my boat to roseburg the end of this month and want to make sure I know what I need to do to get this thing swimming right, thanks everyone for the imput

Starfish · 01-12-2006, 08:22 PM

If you want to do a "before and after", take a ruler under the bottom stern of your boat and lay the edge against the bottom of the boat. Note whether the straightedge touches flush all the way to the rear or whether it has to be rolled along to get it to touch (a "rocker") or whether there is a gap just in front of the trailing edge (a "hook"). From the sounds of your performance it's probably either straight or a rocker. The dealer may induce a slight hook by banging downward on the trailing edge. A little bit goes a long way. Too much will cause the bow to plow.

Also take note of what pitch, manufacturer make and model prop you're running, and how many RPMs your motor turns at wide open throttle and what GPS speed you're getting. (If your max RPMs are lower than the recommended range, that could be the cause right there.)

Finally, take note of which hole your motor is mounted in (what height it is sitting on the transom). You can also measure the height of the cav plate compared to the bottom of the hull but for comparison to see what gets changed you can just check which hole you're mounted at.

Good luck!

BARCHASER · 01-13-2006, 10:37 AM

I have an 18' Seahawk with a transom mount Yam. I have had no purpoise problems. I use it in Puget Sound a lot where you always have wave action that would start the purpoise if the boat is prone to one.

My old 19' Bayliner purpoised much worse. But I put one of those wings on the motor and that pretty much stopped it.

Length may be a factor on the Seahawk hull.

Gundog · 01-13-2006, 11:24 AM

BB,
They are the experts take it to them and they will decide how much if any it needs to be adjusted. The amount mine was bent was maybe 1/4 to 1/8 of an inch. The picture I sent you was after the bend and you can't even tell from the picture you must look real close to see it and if it wasn't pointed out you wouldn't even know.
GD

Big Billy · 01-13-2006, 03:59 PM

Thanks everyone

Regular Joe · 01-13-2006, 08:26 PM

An opinion

The offshore brackect is design for a maximum outboard motor weight and if you go beyond that, you are asking for complications. The other thing is that maybe theses offshore brackects generate too much lift at a certain speed and the bow has no choice but to drop and this process is repeated continuously. Another thing to check is your hull with a straight edge to see if the hull has a depression in it. A small buckle will start this porpoising affect. A buckled hull could be fixed depending on the boat, but costly. For an aluminum boat you would pound it out and add continous hull stringers 6061 t6 for extra strength. A bottom hull depression can also be caused by a trailer with rollers that only support the hull at certain areas unlike a craddle type trailer. The heavy four stroke outboard motors extending from the off shore brackect can put severe stress on the hull. Going on a bumpy road with a trailer that dosen't fit right and having too much overhang weight on a offshore brackect will damage your boat. It is important that your hull has a very strong structure and that your trailer is not too short and fits the boat. Some boat have a stonger hull structure than others and this could be the reason why boat "X"is more money than boat "Y".

Big Billy · 01-15-2006, 07:15 PM

Thanks for the info

5 Salt · 01-15-2006, 09:44 PM

Quote:

i realy think that the porpoise problem is from the hull being over straked, the lifting strakes are to aggresive and the boat is always trying to lift it self out of the water

Not sure about that one.

Motion Marines were/are likely the most agreessively staked of any jetsled on the market. Look at pics of Motions on plane - below. By design, only the last 2-2.5ft of hull is in the water due to large lifting stakes that add huge lift, as well help them to stick hard in corners.

That said, I have owned two Motions and neither had porpoising probelms.

My 25ft walkaround will porpoise with the right combo of trim, power, speed and seas. Change one of the variable and porp stops. Porpoise is a hull design/power/trim issue, not a strakes problem.

Lifting Strakes:

In the water

Out of the Water:

Last 2-3ft in the water - big lift from stakes

Stick in turns from Stakes:

boater · 01-16-2006, 08:14 AM

they call him flipper, flipper, faste...., whoops wrong thread, soory

backlash442 · 01-16-2006, 09:18 AM

5 Salt Fever you have a G.W. now right? How big of seas do you have to have in order to porpoise? I would imagine any boat could porpoise but when do you know your porpoising compared to just having an up and down motion over swells/waves?

I'm not convinced that strakes are the issue. At least not entirely.

Bluto · 01-16-2006, 09:20 AM

Quote:

i realy think that the porpoise problem is from the hull being over straked, the lifting strakes are to aggresive and the boat is always trying to lift it self out of the water

Not sure about that one.

Motion Marines were/are likely the most agreessively staked of any jetsled on the market. Look at pics of Motions on plane - below. By design, only the last 2-2.5ft of hull is in the water due to large lifting stakes that add huge lift, as well help them to stick hard in corners.

That said, I have owned two Motions and neither had porpoising probelms.

I'm pretty sure the last pic is actually my 22ft MM during the water testing. From the pic you can see the new drop chine/strake design which gives their boats even more lift than they had before. It also makes for an increadibly stable ride (no porp, no listing, better tracking), so no need for trim tabs.