05-04-10, 08:44 PM | #1 |
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Porpoising not on purpose
I have a porpoising problem and was wondering if anyone out there has any insight.
The problem: Porpoising before getting to the sweet spot while trimming @ speed. The boat: 93 Hydra-Sports 170 fs. 17’ fish/sk. The motor: Johnson 90 2 stroke runs well as far as I can tell based on my observations, WOT and speed The prop: I have tried 3 and 4 blade, right now; prop is good based on speed/RPM @ WOT and the way it gets up on plane. The load. I have tried it alone and with max capacity. I have tried shifting all the people forward of the wind screen to see if I could get there before the porpoise showed up, no luck. Has room for 2 trolling motor batteries in the rear bilge, I don’t have them in right now. Tank is ½ full and I have been running with the live well empty. Is this a design flaw with these boats? I have only had it for 2 years and it has always done it. |
05-06-10, 07:55 AM | #2 |
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UPDATE
It appears as though the cav plate is 4-6 inches above the bottom of tho boat. Could this be why I'm having a problem? |
05-06-10, 08:53 AM | #3 |
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Could be...I'm no boat expert by any means, and I didn't even stay at a Holiday Inn Express last night, but there's a couple of guys on here that have been playing with boats for years. One of them should be able to help you out, with a little more info.
You'll need an accurate measurement of the distance from prop to pad. In other words, how far the center of the prop is below the bottom of the boat. Here's a link on how to do this properly: http://www.bassboatcentral.com/measuring.htm #2 is the one I'm referring to. That measurement should give some of the boat gurus an idea about what might help you. Good luck.
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05-06-10, 01:16 PM | #4 |
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(just thinkin out loud....)
Photos of trim down and trim 'sweet' spot, on the trailer may also reveal an alignment issue that could cause the fish action.
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05-06-10, 07:54 PM | #5 |
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If I understand what he means by sweet spot it isn't something that you would be able to replicate on the trailer because the only way to know when you are there is by feel and performance when running down the lake.
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05-06-10, 08:05 PM | #6 | |
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05-06-10, 08:41 PM | #7 | |
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P.S. Every boat I have owned has had a trim guage and not one of them had a sweet spot marked on the guage. It's all about seat time.
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05-06-10, 09:53 PM | #8 |
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First, thanx all for your inpuit. I finaly feel as though I'm making progress on this problem.
I do have a gauge and I know where about the gauge was right before it starts to porpoise. I'll take pics tommorrow in that position and all the way down and measure it as mississippiboy sugests......Stay tuned...... Remember, I cannot get it to that sweet spot due to the porpoising. |
05-06-10, 10:35 PM | #9 |
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Doc, it has been my experience that porpoising is a matter of both speed and trim. Mine will porpoise when I throttle back from WOT and don't trim it down. I'm guessing the sweet spot you are referring to is at WOT. I don't know there is one set spot on the trim for it. I think you have to find it every time out because every trip is probably a little different-a different partner that weighs more/less, different gear, etc. can change the weight of the boat so there may be a little variation in where the sweet spot is depending on the conditions.
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05-06-10, 11:11 PM | #10 | |
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05-06-10, 11:25 PM | #11 |
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It sounds to me like it is a motor height problem. Off the top of my head it sounds as if your motor is too low but the best thing to do is get the measurements and go from there.
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05-06-10, 11:33 PM | #12 | |
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Once I have the measurement, I'll post here and then I'm going to try to reach out to Hydra- Sports, even though they have been bought out, to see what the spec calls for on this model. |
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05-07-10, 08:56 AM | #13 |
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The top of the lower unit has a plate. That plate should be at waterline when running. Above it, it will porpoise. To far below it and it will porpoise on trim out.
I had a Gambler with hydrulic lift plate and played with it up and down to figure out why it acted like it did. Capt Mike
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05-07-10, 01:48 PM | #14 | |
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05-07-10, 07:58 PM | #15 |
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Okay, I measured the distance of the center of the wheel to the bottom of the boat and the wheel is 5 3/4" below the bottom. When on plane, I can see the top of the hydrafoils. Also, the pitch on the prop is a 17. I could not replicate the position of the outboard when I was close to the best position. Also, someone earlier mention WOT. I usually dont run WOT, this occours at any speed, but I usually run @ 4,200 rpm and with this prop, thats puts me at about 28mph.
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05-07-10, 10:11 PM | #16 |
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That sounds a bit too low, but I do not know that hull. Most average bassboats that I have messed with are somewhere between 2 1/2" to 3 1/2" below pad. Do you have a water pressure gauge? I realize without a jackplate you don't want to move it around a whole bunch to find out where it should be but I would take it up and watch the water pressure gauge, you don't want that to get low.
I just looked through the Hydrasport Owner's List on BBC and the closest I can find to your boat is a guy running one at 4" below pad and his only bounces from 45-50. He had a fin on and that made it worse. |
05-07-10, 10:35 PM | #17 | |
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05-09-10, 03:16 PM | #18 |
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Update
This motor is a long shaft, 20" I measure the transom @ 22" to the bottom of the boat I'm wondering if I w/b better off with a xtra long shaft 25"? |
05-09-10, 04:46 PM | #19 |
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No your to deep now,propshaft about 3-3 1/2 under the pad with a stock gear case,porpoise is caused by excessive trim that low.
Raising it the motor it will take less trim.Odds are with the motor at a less trim position running level it wont do it now even. And any motor should be run in the power band to insure long life,not exactly wot but in the band 4 to 4200 seems a tad low. When trimmed right the rooster tail behind the boat is no more than motor high,often less . |
05-09-10, 08:02 PM | #20 | |
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05-17-10, 06:15 AM | #21 |
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Ole carolina gets the cookie on the sweet spot.....the sweet spot will vary depending on load etc etc and will be close but not the same every time out. TAKES ALOT of seat time to figure it out.
Second you need to get the prop shaft measurements and start there and as well take a look at the prop. Is the prop in good shape ???? You also need to realize that boat is underpowered. 90 hp on a 17ft boat is bare minimum. Porpising can be caused from a boat "trying to lift" but not having enough hp to sustain the lift.........most generally this is the cause.
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05-17-10, 09:31 AM | #22 |
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Thanks for your input. to answer your questions; the prop sits about 5 1/2 below pad, the cav plate sits about 2" above pad, the prop is near new, no dings or dents. The prop is a Solas 13.25x17p and I also have a 4 blade 15p I use for pulling skiers and it does it as well. This boat max's out @ 32mph with WOT @ about 5,000.
I hear what your saying about the 90, but I have had some corespondance with others with similar hulls and motor combinations and they dont report the problem and they report higher speeds. also, I had a Center console with a 90 on it, it was a heavy boat, double hull with flotation, and I had no porpoising and a top speed of 40mph. How does on test the output performance of an outboard on a bench? Is there a dyno for motors? So, although I agree it is on the lower end of power compared to most bass boats, I also think it should be doing better than this. |
05-17-10, 10:09 AM | #23 |
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Simple solution raise the motor,a lil at a time,it should disappear,raising no higher than needed to go away it wont hurt the hole shot for pulling skiers
With a 2-1 gear ratio,5000 rpm 17 P prop,slip ten percent speed should be right at 37mph. Check with those others getting more speed they arnt 5 1/2 under the pad,I bet. Its easier if on a jack plate if not go up one hole on the motor mounting. |
05-17-10, 12:13 PM | #24 | |
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Quote:
Nonetheless, when it's starts porpising I just trim down a bit and it stops. I also have one of these: http://www.cabelas.com/cabelas/en/te...box.jsp.form23 it helped with the porpising a little bit but made a HUGE difference with my hole shot!! |
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05-17-10, 08:54 PM | #25 |
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Thanx both for your input. I agree and am going to try both. First the Hydrafoil, then the lift.
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