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Comments
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RY, if you were to increase pitch to a degree that you struggled a bit to reach maximum WOT RPM, you would have another problem surface.. your drives can't handle it.. the resistance the higher pitched prop would encounter likely wouldn't be transferred to the input shaft of the engine, because the engine can keep up- the resistance would be manifest at the coupler and lower- which would get hot as hades in july, especially as time and distance increased... heat would destroy those things in short order, at the top end..
but.. and this is the trick: you would see 30mph at a lower RPM than you would see using a lower pitch prop, and still have more throttle left IF needed, to 'dodge the big boys'.. :-) ... the heat generated at the coupler and lower would likely be nearing the same at, say, 3200RPM with a 24p prop set as it would be at 3500RPM with the 21p you're running now... maybe a shower would help with that, i dunno.. maybe using 'hard gears' would eliminate the concern <- but now we're back to dropping tons of $$$.
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the diesel can achieve plane, and hold it easier with more load.. the gasser can out run it in a sprint.. if you get heavier, the gassers are taxed to the point they can't produce enough torque to achieve or sustain speeds above plane, so you trade for a diesel which can sustain torque, easily get it on plane, but not much faster afterward..
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Definitely a great thread! I may have to do some number comparison with the two 390s I know that have diesels this upcoming year. They are basically the same boat but have the diesel with volvo outdrive.
I also know someone that has my exact boat (fmba from NJ) but has B-3 drives instead of the B-2 that I have. I'd love to know any difference in performance numbers, hole-shot, etc. Both of us have vessel view, so we could compare fuel burn (maybe next summer when we meet up, we can cruise a bit side by side, did it last year but I didn't have the VV installed yet)
Dream 'Inn III -- 2008 400 Express
the fact the bucket trucks were outfitted with gassers kinda drives the point home we are discussing here, that there is confusion all around and between multiple applications where and when to use a gasser, or a diesel...
when I was in Africa, we had three generator sites to provide power to the compound- each working in parallel of the others so losing one to a wayward mortar round wouldn't kill operations.. two of these were freaking HUGE diesels- twin 16 cylinder jobs that were the size of a car each.. they each used a literal barrel of oil each, which was changed every 100hours, or every four days.. those things never burped, and provided steady and clean(ish) power... the one site was gas.. it was a terrible idea, but born in concept and used in the case we couldn't find oil to burn... it was perpetually 'down', it seemed.. it would rev up and down to meet load requirements, and the power was constantly spiking from it.. it's call name on the radio was 'jaguar' due to the fact it was in the shop more than 'on the road'.. :-)
at trawling speeds, the resistance of the hull is huge, and the load on the engines greater.. at planing speeds, the resistance is greatly reduced on the hull, and the load is shifted to the screw trying to allow it to spin faster, as the engine translates torque into work..
is there a formula to determine load/resistance drop commensurate with percentages of hull in the water? as in, (and knowing displacement) at full waterline trawling having, say, 28' dragging the water @ 'x' entry angle, and then another showing a planing speed w/ 20' of waterline dragging @ 'y' entry of angle?
my thinking, for what it's worth, is to match the required torque, where it's curved in engine output, and then where a shift into work translation would best be decided.. and using this to determine whether using a big block, a small block, or a diesel would be most advantageous..
a boat requiring 400# of combined torque @ around 1500RPM prop speed to push/lift a hull on plane would benefit from maximum torque at the engine speed needed to produce 1500RPM shaft speed (dependent on gear ratio of drive).. if the engine was capable of maintaining a range of torque at that engine speed, and had plenty of range to either side (for altering weights of the boat due to passengers, gear, ect), it would be safe to apply a small block engine, as the engine provides enough torque at that range to push on plane... after planing, the torque required diminishes and it can be translated to HP and make you go fast..
a boat requiring 700# of combined torque @ around 1500RPM output speed to push/lift a hull on plane would benefit from a engine speed commensurate to the shaft speed at 1500rpm- a small block ain't gonna give you that very easily, but a big block can.. the big block can spit that torque out and watch it diminish slower than a small blocks, and translate HP using less of the torque (%'s) to do so..
a boat requiring #1000 of combined torque @ around 1500rpm prop speed to push on plane will tax even the big block, though once the big block gasser fights it up it will go hella faster than a diesel as there is more RPM range to translate TQ into work... it will hang and extend the 'hole shot' time, drinking fuel at the rapid rate, and hold a zero vacuum whilst pushing through the weight, where as the diesel will just casually flex it's muscles and do the same thing...
there has to be a weight/resistance ratio that could be used here to determine which engine platform would best suite a persons needs..
a small block provides decent top end HP but is limited in low end torque..
a big block produces ample torque and good HP on the top end, but may be too much for smaller vessels and not enough for bigger rigs 40'+.
a diesel provides a LOT of low end and mid range torque and will plane the heaviest boats with little (comparative) effort in terms of fuel usage and load, but has a limited top end due to RPM limits of engine.
where is the cut off for any of these? a formula would be a good thing to have when figuring this stuff out.. if you knew that, then you could flip open the boat's brochure and see what engine would best suit your needs.
your drive reduces the spin, but makes it harder to stop the spin, using gears to amplify torque.. your input shaft turns 1.8 times to the prop(s)... the overall effective gearing can be altered at the prop or through the gear box.. if you're stock 21p props were to be swapped to 25p (hypothetically and for point of discussion) it would be closer to nearing 1:1, effectively for overall ratio...
that is trickery, though, and easily confused with what I'm suggesting: I wanna know how much torque it takes to spin a known prop size at a certain engine speed (taking into account gear reduction) and on a specific hull to achieve planing speeds.... and then match the torque curve of the engine/drive combo to the required torque level needed by the boat..... you may find that a small block(s) is having a hard time as that RPM is beyond its peak torque output, where a big block(s) could be on either side depending on weight, and where a diesel produces abundance of TQ, but not enough RPM above the TQ level to gain speed..... <- which is what I'm thinking you're battling.. you could over prop your rig to a degree and likely not overtax the engines, but your idle handling will disappear as you move too quickly at idle speeds to be able to maneuver....
all of this discussion is hung on the age old issues of cost effective production of engines and couplers that can be used on a multitude of applications, so as nothing has to be custom produced from the ground up for a specific application.. If you were to build a 20' boat perfectly married in power-to-purpose, and I'm talking perfectly, you'd likely have as much invested in that thing as you would a flippin' 50' egg harbor convertible due to the one-off custom parts and pieces, down to the camshaft profile, drive ratio's, absolute weight/loads.. ect... you know.. like one of those off shore racing rigs that nail 180mph and have digital and computer controlled engine trims based on sensors detecting weight of fuel, computer controlled trim tabs that measure vibration and resistance and adjust accordingly (or tell the co-pilot to do so) and other things like that...