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Thread: Prop shaft question

  1. #1
    sgdiv7
    Can anyone help me with the max length the prop can be from the strut safely. The prop shaft is 1". Boat porpoises with all the extra HP. I have to use alot of peddal to keep the bow down. I can move engine forward 6" max. and I guess i could make trim plates longer if I need to. What do you guys think. looking for suggestions.
    Bill

  2. #2
    058
    I don't know what kind of hull you run or the length of it but this is what works for me. Both my 18' flats [Hondo & Howard] have the prop within 3/16" from the strut. I like to have the strut support the shaft and prop as much as possible rather than having most of the support on the shaft alone. Both boats take a nice set with no porposing at all with the plate in any position. Perhaps you need to extend and/or re-adjust the cav. plate.

  3. #3
    126driver
    I agree with 058. Check the plates, run it again, then move the prop shaft back one nickel at a time if you're still not happy. Somewhere on this site there used to be a good article on setting the cav plates for a baseline. I think Wilkes wrote it. Haven't looked for a long time to see if it is still here or not. How far away is the prop from the strut right now? We started with the prop almost touching the strut and have it moved back 6 nickels right now. Maybe more to come. IMO play around with everything else before you start moving the motor. Small adjustments can make a big difference. Change one thing at a time and run it. If you still don't have any luck, get the tape measure out and post the length from the transom to the back of the motor plate and the back of the v-drive plate as well as the length of the cav plates so we know what you're dealing with.
    Good luck.
    Scott

  4. #4
    sgdiv7
    Thanks for your help.
    The boat is home made 17' 6" runner bottom. I set the plates level with the bottom of the boat. the changes I made to the boat when I put the new motor in was I put in a powerglide trans, and I went from a 15 gear to a 29 gear. The old motor was stock 454 and the new motor is a 454 w/alu. heads and 871 supercharger, so I figured I added 100 LBS. to the back of the boat. I'll get the other measurements this week and I'll post them.I really don't want to move the motor forward any more because I can already feel the heat from the headers Its hot enough when its 100+ deg outside, lol. I do want the boat to be right

  5. #5
    Sangerboy
    I agree with the previous posts, 3/16" is the ideal distance from the back of the strut to the front hub of the prop. This allows enough room for the prop to move forward 1/8" inch or so under hard acceleration due to driveline flex. If your running 500 or more hp I wouldn't put more than 1/2" of space, to much chance of whipping which could break your shaft with major consequences at high rpm.

  6. #6
    ponponracing
    Just keep in mind that the further you are from the strut, more chances you are going to break the prop shaft. And this is costly.

  7. #7
    Bad Ass Flat
    The only way you are going to stop the porpoising is to either put more power to it to carry the nose ( you can drop gear to achieve that) or put steady pressure on the down peddle at WOT. It needs something tohelp carry the nose or something to keep it from porpoising (down peddle pressue)

  8. #8
    Sangerboy
    You guys are pretty sharp. I had to much bowlift with my old bubbledeck and had to move the prop back quite a bit to get it to set at neutral plate. I didn't like the thougt of shaft breakage at 90+ so I'm having a new strut made to match the new prop position. Besides the safety issue if you've forked over decent dough for a good prop you don't want to lose it.

  9. #9
    superdave013
    Sangerboy, Who is making your strut? Barber Welding makes some nice ones. They made a rudder for my drag boat and it was bitchin.
    If you need to, contact Doug Barber @ 562-928-2570.

  10. #10
    Sangerboy
    superdave, Jim Lange at Hi-Tech Marine in Simi Valley is building my strut for me. He did some bottom work for me, and we moved the v-drive forward and went from a 10 degree strut to a 6 1/2 degree strut. I still had to move the prop back quite a ways to get the boat to set, and it was too far back to just weld an extension to the existing strut so we opted to build a new one for safety. I'll contact Barber Welding for a new rudder, I cut mine down to 10" but it is still chrome plated and I wouldn't mind having it be made of a stouter material. Thanks for the info.

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