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Freddo if that is your RPM at WOT (5000) you are killing those engines, they will not last .3 of the hours they should. Your numbers will improve vastly if you reprop and no longer lug the poor things, also your fuel economy will jump also.
cheers fnq
PS dont want to sound dramatic but if it were mine I wouldn't even risk running it until it is fixed.
yes right on FNq u re- read and then thought he may only get 5,000 WOT surely does need some 'tuning" of props/trim something, I know a guy with a 5.2 with 90 yammies and it goes way faster than his speeds ,by the way, he just repowered also with a couple of 4 strokes and is SORT OF happy he did, to keep the weight down he went smaller in HP but is now kinda regretting the power difference, but I guess it will be OK after he gets used to it, the fuel side of things is great, but then he went from tired old carby 90HP to new 60HP efi so there should be a difference.
This speed "thing" (as usual) is always overrated. BUT whatever the speed at 5000rpm, now that is a real worry and should be the FIRST thing to get right!
For what is only a relatively short boat (less than 17 feet actually) one should be very happy being able to sit around that 22-26 knot mark at 4200-4600 type rev range and being able to stay there in conditions that would see a lot of even larger vessels pulling back below 20.
Sure they might have the legs on good days but big deal, not important.
If one weighs up the size/range/fuel capacity/deck space/capability etc if you can do an honest 22-24 knots with good econ and maintain that speed in abaove average conditions then what more would you want?
Don't want to hijack the thread but you may have to slow down and spell it out for me as I’m still v new to all this boating.
Why is 33knots, 5000rpm, WOT so bad and what would be the cause?
I don’t think anything is wrong with the motors as they have just been serviced and have only done 16hrs anyway so they should be in perfect condition. One of the props however is quite badly pitted but I haven’t thought much of it as it’s only the surface that’s pitted/flaking off, most likely due to sitting in the marina for the last 18months.
Also, the underside of the right pod was covered in barnacles where they obviously missed while water blasting it. That’s since been fixed. The boat also has been anti-fouled which will slow it down a touch but not that much.
What can I do to “fix” it if indeed there is a problem and what rpm at WOT should I be aiming at? With tax time coming up spending $1000(?) on new props is out of the question, at least until next financial year. Given this, would it still be ok to run it at 20 odd knots if the solution is indeed re-proping?
Cheers
ff
ff, 33 knots at 5000rpm, WOT is not good as the motors must be able to reach the top end of the WOT rev range and that would be a lot more than 5000rpm.
It's not the motors but the props, too big!
For a WOT rev range of 4500-5500 on a boat like this I would want 5600rpm WOT!
I wouldn't suprised that they are way too big. The guy I bought it off didn't do things by half but by his own admission, didn't know much about boats (much like me I guess but I don't the same amount of cash!). So what size props should I be using?
FF it's not the speed that's in consideration it is the max RPM's you engines can make.
The more rpm's the engines max out at the easier it is doing 'work' at any and all rpms below that, pistons are made of aluminum which expand and contract differentially to the cylinder wall which is steel.
Working (lugging) the engine forces the engine run consistently high combustion temperatures at the piston crown which over expands relative to the cylinder this is a form of seizure, the rings (piston coolers) are trapped usually by carbon (insulator) due to the abuse it has already seen and they snag an exhaust port and snap or bind between piston and cyl wall and scrape the side.
This will happen to your engine/s -no doubt about it this is not a fable, instead of engines that should run until corrosion kills the entire engine it may see 3 rebuilds before corrosion sends it to the wreckers.
I have the exactly same engine as you on a mono hull and my target for heavy -all gear 3 blokes and full fuel is 5750rpm WOT, I can then be assured that at all times the engine is doing 'work' easily.
When the boat is light it will wot higher but it is the fool behind the throttle's choice what is the light weight rpm ceiling.
To get some idea of combustion temperatures and how variable they are/can be, have you ever driven a diesel vehicle and towing with an exhaust gas temperature guage fitted?
Well gents the time has come,
I am leaving Macaky now to head down and pick up the 5.2m KC. I should be back home tomorrow night but the weather is looking really really bad for the rest of the week so i dont think i will be getting out anytime soon. Ironic hey, once you get a boat the weather is too bad to use it.
Are there any Ausfish members living in Mackay area that are keen for a fish as I am always looking for people to join me. I will put up some pics when i get her home tomorrow.
I repowered mine about 12 months ago with the 60 high thrust yamahas and am very happy with it. I have done just over 210 hrs in the last 11 or 12 months and its proved to be very fuel efficient, and they have not missed a beat!! My boat also has the small tanks ( 112l of which 107l is usable), and I get plenty of range out of it. I fish from Mlba alot, and run out for an hour or so, troll at 8 kts for 9 -11 hrs, then run home at 23 kts, and average around 120l for the day!!! A good mate of mine was the original builder/designer of the kevlacats, and he thinks its the perfect match. The big problem is the weight. I have been in one with hondas on it, and it goes ok, but the extra weight is a problem. The pods haven't got the required flotation, and as cat owners will tell you, cats dont handle weight down the back very well. I think you will be very happy with the yamahas if you decide to repower with them.
Regards
Trent
I own a 1989 5.2KC, (personally i think the best trailer boat in the land) hehe, i started off with 60hp johnson 2 strokes, they were the originals, i recently re-did the whole boat, motors, trailer, canopy, and i have put 90hp e-tecs on it, i have just done 100hrs on them and i couldnt be happier. the boat handles 100% better, planes so easy and quite good fuel economy, around 2.7km per ltr each motor, motors are extremely quite and so reliable, starts first time EVERY time, i say its like going out fishing in a new car. i get around 70kmh top speed, however i have the long shaft, i suspect if you got extra long shaft you would get even higher speed, but does fine for me! if you have anymore questions to ask feel free,
[planes so easy and quite good fuel economy, around 2.7km per ltr each motor,
cheers,
Heath[/quote]
Heath,
Does that mean that both motors combined you go 1.35km per litre?
I ask because I'm looking at changing from 90 Yamahas which give me a 1km/litre economy (both motors combined).
I'm interested in the "real life" figures you are getting and not the dealers figures I've been quoted, which have been that I should achieve 2km/litre(both motors)
Hi Darren, I am based in Adelaide, my rev range on a flat day ofcoarse is 4500rpm to 58-60km/h depending on trim, that is with a full rap around bimini top canopy with front clears, so that drags a fair bit of wind, i actually havent tested with out the clears on, i enjoy the warmth to much, haha. i was looking at putting 4 strokes but i wanted a power increase (from 60's) and i if i put 4's on it i would have to put larger pods on and then it just gets pricy, my e'tecs wiegh 140kg and i certainly wouldnt go any heavier, i have had to put spring loaded scuppers on because with a couple of large guys standing at the back the water does come in a bit otherwise.
Thats correct WA, i cross the st.vincent gulf here in adelaide a bit, i go from port adelaide to a place called Ardrossan which is 65km one way (130km return) and on a return trip i do around 50 to 55 ltrs per motor and if my maths serves correct i think thats around 2.7km per ltr or 1.35 km/ltr combined. that is with me and another guy my size (100kg) fairly large, hehe and crap you generally take, bait beers food not that its that heavy. i hope that helps
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