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  1. #1
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    im kinda stumped

    This is gonna be a bit of a story to get all the details out but ill try to keep it short.... boat is a 92 Seaswirl Spyder 188 and I am currently running 64.5mph @ 5800 with a 24p scorpion with a G1 Evinrude 225HO mounted with 14" of setback and prop shaft at 5" bellow the pad when jack plate is buried.
    its not bad considering the design of the hull, but when I first rigged this motor It had a 25" mid siting on an old land and sea jack plate with a 4" setback spacer and I was turning the same prop and doing 68. I had to have it jacked all the way up and trimmed all the way up and it would porpoise pretty bad but it would hold 68 and I even saw a blip of 70 once lol.
    I replaced the land and sea plate with a bobs action and switched the 4" for an 8" and I immediately lost 4mph, but the porpoise was gone and it was much more stable. I posted about this once and the consensus was that my mid was now too long so I shortened to a 20".
    the engine blew up immediately after but im finally back on the water and out of break in mode but im still stuck at 64. I tested some 26p props this past weekend and ran slower 60-61mph. put the 24 back on and 64.5mph like nothin. Now im just stumped. the only thing I can think of is that the 25" mid trimmed all the way up was acting as more setback giving me more leverage at WOT. I should be happy its running but im always wondering where my 4mph went.....

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  3. #2
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    I would say the added engine setback was detrimental to your hull. You shifted the weight back and the hull is sitting deeper in the water at the transom when running. Before the transom could "pop" free some and let more of the bottom ride on the keel. Weight was balanced more fwd. Also look at where your prop shaft was with the 25'' eng. And now with the 20''. You may need more prop and lower in the water to increase efficiency. Lowers/bullets also provide lift when submerged and that follows along with the trans needing to lift and pop free some. jm2cts.

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  5. #3
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    I had pondered the fact that I may have went too far back, but if my stability and porpoising improved then did I really? it is possible as I never did test 10" setback with the 20" mid. As for engine height I was everything in the highest bolt and jacked all the way up the full 6", when I went to 20" I mounted the engine on the lowest bolt so in theory I should be 1" lower than I was running with the 25" with the plate buried. I still have yet to test a stern lifting prop to see if there is any gains but I am open to suggestions on what to try.

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  7. #4
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    if i get a spare moment i may swap out the 8" spacer for the 4" and test in the near future and report back, but testing a prop requires much less effort haha.

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  9. #5
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    FWIW, it takes a lot of trial and error to find the sweet spot. I am still looking for mine, but have recently learned that higher with the motor is not always best. I dropped my motor a 1/4" and picked up 3 mph.

    Read here a while back that a guy put a different size thrust washer behind his prop, I think it was thicker, and he gained a few miles per hour.

    I keep a journal of what I did, or changed and what were the results. I also log temperature, humidity, and water conditions. I have made so many changes that I do not remember it all. Nice to be able to look back and confirm what happened. Just my .02

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  11. #6
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    You might be in soft blowout or burning the prop basically high slip.

    Find a nice straight run run and start over with the jack plate.

    Come up until you lose pressure or speed drops.

    Also thats a lot of setback for the spider, unless I’m thinking of a different boat. It’s the outboard wake/ ski boat with strakes and a big flat u transom?

    it will run but if the bow feels higher your probably running into the \ / problem as someone stated.

    Might have developed a hook in the hull.

  12. #7
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    Yeah motv18 I believe you are thinkin of the right boat. One set of full length strakes and a rounded vee that’s a pad but not a pad. I think I’ve got engine height about dialed in, I can go lower but it seems to chine walk if I don’t pick it up at least 1” from bottom. I may just have to put my 4” spacers in for a weekend and see how it runs with only 10” of setback. I do have a 26 promax to test this weekend for ****s and giggles and then I will likely shorten setback if I don’t see any gains.
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  13. #8
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    Quote Originally Posted by Motv18 View Post

    Also thats a lot of setback for the spider, unless I’m thinking of a different boat. It’s the outboard wake/ ski boat with strakes and a big flat u transom?

    it will run but if the bow feels higher your probably running into the \ / problem as someone stated.
    Motv18 im not saying you are wrong about the setback as that's when it all changed, but what do you think it should be? its a nose heavy boat by nature which is kinda how I ended up at 14". not a lot of people running this hull so its all trial and error.

  14. #9
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    It’s a gamble normally set back is to keep you on top of the swell as the water that’s pushed down by the hull recoils back to level. The faster you go the farther from the stern the max rise is. It also changes the max height of the rebound as you go faster.

    The \/ problem is high trim and high bow it can cause prop rocking or strake porpoise. It’s unnerv but not deadly like chine walk.

    As for how much setback eh idk if you can video the stern while running we might be label to spot something.

  15. #10
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    I’ve decided to start from scratch with setback. I am going to remove my 8” spacer and bolt the 6” jackplate to the transom and put some time in with engine height and trim and record. If I don’t gain I will then add my 4” and repeat. If my memory serves me correctly I was my fastest at 10” setback. I did not have any seat time with jackplate directly bolted to transom and although it’s a PIA to do it, it just needs to be done in the sake of trial and error.

    Motv18 I will likely get a bracket to have my go pro shoot the rear as I’m curious as to what’s going on back there too. I’ll report on each step of the way and post vids if I get any.

  16. #11
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    Sounds like a plan

  17. #12
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    Spent a good hour this morning doing passes testing engine height and trim positions with only the 6” jackplate and I am down 1mph from when I had 14” setback. The mid range porpoise at slightly positive trim went away and take off and time to plane are about the same. My fastest passes seem to be at about 3.5-4” bellow the “pad” but requires a lot more trim to get there. Going to test with the 26 promax after a quick bite and rule out any possible benefit from more of a stern lifting prop with each amount of setback. But so far looks like I will be adding the 4” spacers next week.

  18. #13
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    Spinning the 26 promax was kinda interesting.... out of the hole it was absolutely terrible. Spun out like crazy to get on plane but then once I was up it was on. Boat rode very well with significantly less positive trim and I gained 1mph from the scorpion. It also ran very well at higher jackplate positions 1-2” bellow the “pad”. I think I might be on to something here with a more stern lifting prop.....

  19. #14
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    Hopefully you can get back to 68 and stable

  20. #15
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    so while im on this setback testing mission I would like to pose this question.... what are some general symptoms of too much setback besides loss of speed?

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