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Thread: Just Thinking

  1. #1

    Default Just Thinking

    sory to talk about props again but just had what i hope is a brain storm. boat had a 302 now has 350 have not changed gearing or prop could this be why i top out at 22 knots, i think i might be cavatating any help from all members would be greatly app.
    20ft Shamrock W/ CHEVY 350

  2. #2

    Default

    Quote Originally Posted by sjofugal View Post
    sory to talk about props again but just had what i hope is a brain storm. boat had a 302 now has 350 have not changed gearing or prop could this be why i top out at 22 knots, i think i might be cavatating any help from all members would be greatly app.

    You'd know it if your boat is having those issues. It will rev higher than you will ever feel comfortable with. So, let me get this right. You had a 302 and your max speed was what? Now, you have a 350 and you max speed is all of 22 knots? What is your boat's rpm when traveling 22 knots? And are you completely sure that you are only going 22 knots? It seems that a decently built 350 that was built for marine applications would go far faster than 22 knots in a 20 foot boat. You already know this, but you have a pretty significant problem with something else as your engine should be screaming at you from relief of too many rpms by being underpropped. Are you completely sure that this 350 is tuned correctly?

  3. #3

    Default

    motor is a pmc w/ only 120 hrs pulling 3000 on the tach and 22 knots on the gps the motor sounds great but at 3 grand the boat dosent want to go any faster, (sorry I'm not a mechanic thats why I am looking for help from the members that have more knowledge then me) sorry to sound rude I am just totally frustrated on what the problem is
    20ft Shamrock W/ CHEVY 350

  4. #4
    Member bhollis's Avatar
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    Are you saying that your new engine won't go any faster than 3000 rpm? Or are you saying that the boat doesn't go any faster beyond 3000 rpm?

    Also, what speed at what rpm were you getting with your old engine?

  5. #5
    Member bhollis's Avatar
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    I don't claim to be an expert on this stuff, but it seems to me that if you didn't change your gearing or prop, you should be getting the same speed at the same rpm as with your old engine. Unless of course your new engine is significantly heavier than the old one, in which case you'll be getting less speed for a given rpm than with your old engine.

    Per my post above, I think we need some more info.

  6. #6

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    Quote Originally Posted by bhollis View Post
    I don't claim to be an expert on this stuff, but it seems to me that if you didn't change your gearing or prop, you should be getting the same speed at the same rpm as with your old engine. Unless of course your new engine is significantly heavier than the old one, in which case you'll be getting less speed for a given rpm than with your old engine.

    Per my post above, I think we need some more info.
    I agree with you. Changing from a 302 to a 350 shouldn't change much. However, depending on how the 350 is configured it could need a smaller prop than the 302 because it was built snotty mean and literally needs the rpms to run the way it was designed to. I built one that way once, but it went into a toyota (yes you read it right) pickup truck.

    Here's what I am thinking: you replaced the 302 with a 350 because the 302 went 22 knots, was kinda worn out, and it was time. Now, that it is done you expected to go faster - and you think you should be. I think your rpms are low - I would want at least 5,500 at full throttle. Who am I kidding, if I built it I would want at least 6500 rpms at full throttle.....But why your rpms are low and your boat is slow is because of too much prop, a mechanical issue in your stern drive (and you would know this because you'd hear it and if you hear it it isn't long before it says goodbye to you), or a tuning/mechanical issue in your new engine. We do need more info...

  7. #7

    Default sorry it took so long

    You guys on this forum have incredible info., sorry I did not post as much info as needed for insight. I have not tryied to push it much over 3000 due to the 4 barrel 650 kicking in wide open, it is a crate PMC 350 RATED @ 325HP. How far up do you guys think I should run this motor up I know I said I want to try and keep the 4 barrels clossed as much as possible but sometimes ther are more important things than economey. If any of you gurus need more info to help me out w/ my problem I will give you all the info I can.
    20ft Shamrock W/ CHEVY 350

  8. #8

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    Hull length have anything to do with this? I don't think the new motor is significantly heavier than the old, or is it? (I doubt that is it anyway) At around 3500 rpm you should be in the 25knot range. Prop??? Get it in some small chop, less friction, less surface area touching the water, mine usually goes faster in a bit of the rough vs smooth.

  9. #9

    Default

    going to seward this weekend and am going to run her up WOT and see what rmps it will pull and see what SOG is then I will have more info to post and hopefully be able to get some more insight
    20ft Shamrock W/ CHEVY 350

  10. #10
    Member Dupont Spinner's Avatar
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    Quote Originally Posted by sjofugal View Post
    going to seward this weekend and am going to run her up WOT and see what rmps it will pull and see what SOG is then I will have more info to post and hopefully be able to get some more insight

    Look for about 4800 =/- 150 wot is a good ball park. 5200 r's is the max most will turn.

  11. #11

    Default whats up

    havent had the boat out since big lake but if I remember correctly runing it to the fire wall i was only getting 3200, just looked in the book from pcm and they say max rpm 5000, contiuous cruise rpm 4000. however ther is a note that stats DO NOT CRUISE AT HIGH LIMITS ABOVE RANGE, UNLESS PROPPED TO TO TURN AT OR NEAR MAX RPMS, AT FULL THROTTLE. WHAT DOES THIS MEAN
    20ft Shamrock W/ CHEVY 350

  12. #12
    Member bhollis's Avatar
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    Sorry, I'm not sure what that note means either.

    But if you're only getting 3200 rpm out of an engine with a redline of 5000 rpm, and a rated cruise rpm of 4000, then something's wrong. Either there's something wrong with the engine preventing it from developing full power, or you're over propped. It seems unlikely that you're over propped, since you're using the same prop and gearing you previously used with a smaller engine. (Unless, of course, you were over propped with the old engine too?) My guess is there's something wrong with the engine.

  13. #13
    Member bhollis's Avatar
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    Just re-read T.R. Bauer's post above, and he makes a good point. It's possible that your new engine is designed to run at a higher rpm than the old one, and may need a smaller prop to get to the higher rpm. In other words, even though the new engine is more powerful than the old one, it's possible that it only develops the increased power at higher rpms. So you may need a "lower gear"--less prop--to get the new engine running at its rated cruise.

    Or, as I said above, there may just be something wrong with the engine (or drive line).

    Here are some questions that I think would really help us help you: What were the redline and cruise rpms for your old engine? What speeds did you get at WOT and at cruise with the old engine?

  14. #14

    Default I had same issue

    went inside and out on my motor turned out the timing was off. might be an easy fix. last year I was getting 4800 wot this year I was at 3200 wot mechinic adjusted the timimg and bam 4800 wot.. easy to check

  15. #15
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    This last winter I had Andy at Noah Marine rebuild my outdrive and asked him about the correct prop pitch for my boat as my boat came with extra props that are pitched differently. He said to run it at WOT with a standard load of gas, gear, people, etc and the rpm's should be between 4300-4400 and to adjust the prop pitch accordingly. I have a 350 with a Volvo 280 Duoprop outdrive. If you don't have a Duoprop the WOT rpm's may be different but I can't imagine by too much. You might ask him - he's always been glad to share any info with me.

  16. #16

    Default just got back

    went to seward and was planning on running the boat wot after we tryed are luck at some flat ones. However on the run in as I pushed it to 3200 my racor light came on so I opted not to push it any further, however did play w/ trim tabs and @ 2700 went from 20.3 to 21.6 knts on the gps just gets a little bit rougher, but was starting to pull 23+ at the 3200. thanks to all that have been giving there input have been taking all into consideration will post again when i get back from dungee crabing in ketchikan in a week or two
    20ft Shamrock W/ CHEVY 350

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