Reply With QuoteLooking at what you tried it seems that what you need is more power. What is the current engine rated at and what weight are you pushing? A 79? That has to weigh a bit So I'm thinkin your going to really need to step it up.
i have a 1979 Formula F20, 350 mag, alpha drive, 1.5:1 ratio. the boat runs 52 mph max. propped out many different ways, 23p mirage, 24 chopper, 24 balistic, 25 mirage, 23 high five. tried em all. boat spins up to 5400 rpm with 23 mirage , 4900 with 24 chopper, 4900 with 24 balistic, and 4800 with 25 mirage. all pushing speeds of 50 to 52 mph..... what im looking to do is to get it to hook up top end. i still have quite a bit of trim left at top end, but no increase of speed as trimmed to fullest extent. i think that the top speed seems alil low, but it is what it is. the speeds are gps speeds so they are true. also any help on upgrating alil more power on a low budget would be great. perhaps a carb and intake. loking to get to over the 60 mph mark in the long run. thanx alot
Looking at what you tried it seems that what you need is more power. What is the current engine rated at and what weight are you pushing? A 79? That has to weigh a bit So I'm thinkin your going to really need to step it up.
Something doesn't add up. It appears you have a lot of slip. Averaging around 33-34%. I have a 95 Merc 350 Mag EFI/MP with Bravo I and 1.5 gears and can only spin an 18 and 20" four blade around 4600-4700 rpms in a 20 foot boat. My speed is 52 mph at best as well. I'm surprised you can turn significantly taller props with a similar motor and way higher rpms. I don't think I could swing a 22 inch prop at 5000 with the boat on the trailer.
Your X dimension might be way high.
I'm wondering if you have a 1.86 O/D ratio vs. the 1.5. That would place the slip factor around 17%, which although not great, is more realistic.
Assuming the GPS is the most accurate piece of info, I'd start by verifying the O/D ratio. Right now the numbers just don't make sense. IF the O/D ratio is 1.5, then the tach might be off.
If the tach is correct too, then something else is seriously amiss, and I'd only guess drive height (x-dimension).
Is the engine stock? or seriously pumped up?
Does the trim carry the nose?
If not, I'm gonna go with the very heavy boat theory. If it rides wet, it'll slow it down (and slip more).
I agree a heavy boat will slow it down, and bring the slip numbers up a bit, but he's still turning 4900-5400 rpm. Usually the motors begin to slow down too, not hit 5400 rpm.
I'm simply mystified at how he can turn 5400 with a 23 inch wheel with a 350, that is assuming the motor is not modified.
the boat goes like hell, out of the hole with a 24, i can pull a slolom skier right up with little to no resistance. the boat has plenty of trim, when trimmed for max speed, it stops increasing speed at about half trim. i can trim the same amount again and have the boat stay the same as far as rpm's, lift, and speed, without cavitating: but have lots of trim angle left. it does ride up on the water nice, with the water breaking off the stern, not off the chine. the boat had the original 350 from 79 in it, and a 90 alpha drive on it, spinning a 19p 3 blade for a max rpm of 4500. i got the boat when the motor went bad, and put in a 95 350 magnum, 280hp stock merc motor with about 400 hours. got the motor for the bubble. the interior is all new and redesigned from original setup, but no real added weight. now with the new motor it seems that i can just tach it out of site. im thinking that the slipage is way up around 22 or 23%. the chart i have doesnt even go high enough for the 24 or 25 pitch turning at that rpm and that low of a speed.
again, any thoughts are great. thanx.
You still didn't say how much it weighs... got any pictures??? Whats the length and deadrise?
theres a pic of it a year ago, but thats all i have. sorry, i dont know the weight on it until i get it on the trailer and take it to a friends truck scale. oh yeah, the only thing from stock is the new mallory electronic dist, and the 4" thru hull exhaust straight out the back, no angles.
The boat looks nice. I've always been fond of that style.
I've calculated the slip. You're in the 33-34% range. Assuming the GPS, tach and O/D ratio you gave were all correct there are only a few things that could be wrong.
First perhaps the props were all mislabled. However, the chance of that is slim to none. So that can be ruled out.
Something I just thought of though is perhaps the props are slipping on the hub? That would cause the results you see.
I just don't see how two nearly identical motors (mine and his)could get so wildley different results. My motor turns either an 18 inch or 20 inch 4 blade to 4600-4700 rpm and he can turn 23" and up blades to 4900-5400 rpm.
With that size boat and motor, however, the speed of 52 mph is right on target. The prop size and rpms are not however.
Originally posted by Trash
Something doesn't add up. It appears you have a lot of slip. Averaging around 33-34%. I have a 95 Merc 350 Mag EFI/MP with Bravo I and 1.5 gears and can only spin an 18 and 20" four blade around 4600-4700 rpms in a 20 foot boat. My speed is 52 mph at best as well. I'm surprised you can turn significantly taller props with a similar motor and way higher rpms. I don't think I could swing a 22 inch prop at 5000 with the boat on the trailer.
Your X dimension might be way high.
I'm wondering if you have a 1.86 O/D ratio vs. the 1.5. That would place the slip factor around 17%, which although not great, is more realistic.
Assuming the GPS is the most accurate piece of info, I'd start by verifying the O/D ratio. Right now the numbers just don't make sense. IF the O/D ratio is 1.5, then the tach might be off.
If the tach is correct too, then something else is seriously amiss, and I'd only guess drive height (x-dimension).
Is the engine stock? or seriously pumped up?
As I was reading that post I was thinking the same thing. My 21' ultra (a whole lot lighter hull) with 1.5 Gears goes about 56 at about 4600 with a 21 pitch high five prop. I think he is trying to spin too much prop on that boat....
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