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| Help - Porpoising J-Craft http://ww.w.hpbc.ca/viewtopic.php?f=19&t=3374 |
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| Author: | Ctice [ May 18th, 2016, 10:11 am ] |
| Post subject: | Help - Porpoising J-Craft |
Hi everyone, Im new here - need help with a J-Craft Last year I bought a "barn find" near mint 1985 J-Craft Regatta 17 with a matching Johnson 140 looper in northern Quebec. Boat runs ok, Idle is rough - stalls in gear (particularly bad with the motor trimmed under and exhaust relief holes underwater - yeah is sits low in the back). I have done the following too it: Plugs, Gear Oil, Compression check Bypassed the VRO and onboard fuel tank - for now; running 50:1 premix from a portable tank, ran Ring-Free thought it. Water pump is working but will get done this year. I need help with the set-up May main issue is that it porpoises like crazy in the midrange - 26-40 MPH unless trimmed all the way in and my objective is a non-porpoising, 55 MPH, skiable ride. I have two OMC Raker props 19 and 21 The 19 does 48MPH at 5500 RPM The 21 does 51MPH at 4900 RPM The motor is essentially sitting on the Transom in the lowest mounting holes. Cavitation Plate is about 1" above the bottom of the boat. No blowout on turns, has the original J Craft fin and turn's really well. Transom looks solid, no stress cracks in the gel, hardwood bracing is not rotten and looks tight - Transom doesn't flex on the trailer when I push on the bottom end of the motor tilted. I have checked for a hook or a bow on the bottom when it is on the tailer and it all looks straight. Here is what I am planning on trying next: I bought a whale tails doel fin thing that I was going to try but I wanted to get some opinions on the set-up before I drill the cavitation plate; seems like a bit of a band aid solution but I need to stop the bouncing. Carb rebuilds. I think it should spin the 21 up to 5500 Any suggestions would be helpful |
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| Author: | OneCycle [ May 18th, 2016, 11:09 am ] |
| Post subject: | Re: Help - Porpoising J-Craft |
i think you should try lifting the motor so the water intakes are level with the back of the boat. you would gain speed, RPM and the hull is more likely to ride on the surface with less hydrodynamic input from the lowerunit. |
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| Author: | wildcatt [ May 18th, 2016, 1:10 pm ] |
| Post subject: | Re: Help - Porpoising J-Craft |
OneCycle's idea is definitely worth trying. No holes drilled and just a little time messing around. You could also try moving some weight around to see it that helps. I tried the props, weight (200 lbs of sand in the bow), cav plate thingys, and nothing seemed to work until I got the smart tabs. These made the boat work like it was suppose to. Low speed porposing was gone, as soon as I put them on. http://nauticusinc.com/smart_tabsSX_aut ... m_tabs.htm |
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| Author: | Culprit [ May 18th, 2016, 1:27 pm ] |
| Post subject: | Re: Help - Porpoising J-Craft |
Are you using the bow tank or remote tanks in the back? I had a 17 ft. J with a 140, it was a 1978. It never porpoised at all that I recall. I had the motor bolted 1 set of holes up on the transom. 21P OMC stainless I believe and it ran a solid 50mph. I recall it being close to 6k rpm, maybe it was going faster but this was 20 years ago so pre GPS. It had a bow tank and the battery was in the back. What I am getting at is you could be dealing with a slight weight bias issue if you are running remote tanks in the back right now, or a bottom issue from sitting on a less than supportive trailer? I would check the bottom closely if shifting weight does not help. |
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| Author: | hec2buck [ May 18th, 2016, 2:25 pm ] |
| Post subject: | Re: Help - Porpoising J-Craft |
You need to check bottom 4-6ft up from transom with a carpenters' level. If there are any air gaps between level and hull, that could cause problems. Usually a bow in the bottom or a rocker bottom will cause the bouncing. Any air gaps that exceed .015-.020" needs to be addressed. |
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| Author: | Ctice [ May 19th, 2016, 6:21 am ] |
| Post subject: | Re: Help - Porpoising J-Craft |
Thanks everyone, the under deck fuel cell is about half full but I am using a portable which is in the back. The fuel is contaminated so I need to figure out how to dispose of it. The previous owner also had a sand bag under the deck. I took it out after I tried it and it didn't make much of a difference. I looked at the bottom, it is a shallow v and it looks to have a pad for the last foot before the transom. I will put a level to it I on the weekend to see if it is bowed. |
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| Author: | Culprit [ May 19th, 2016, 9:32 am ] |
| Post subject: | Re: Help - Porpoising J-Craft |
if it had a sand bag in it I suspect the problem may have been there for a long time. You have to check the whole bottom as hec2buck noted. For the bad fuel it should still make fire starter! Perhaps you could take it to the local auto recyclers? |
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| Author: | voo-doo_child [ May 19th, 2016, 10:35 am ] |
| Post subject: | Re: Help - Porpoising J-Craft |
post some pics of the boat as well |
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| Author: | David [ May 19th, 2016, 9:13 pm ] |
| Post subject: | Re: Help - Porpoising J-Craft |
You need to find Sean Goodman, the guru of J crafts. |
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