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Removing folding props.


B00B00

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The main reason for the change is so we can even get close to hull speed! The old setup was pathetic and we would cruise below 5kts and only early 5s flat tack.

The boat was launched with a 12hp drofin diesel and I think its still the same prop but now has a yanmar 2GM20 which should easily be able to get us on hull speed.

A mate had that other prp sitting around so I thought I might as well give it a go.

 

Yes you are right slacko, no rev counter, would be keen to put one on there thou.

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Does anyone know if you can repitch the prop and if so how/where and whats the aproximate cost?

 

I had a Yanmar 2GM20 in my Tracker and went from a fixed 3 blade to a second hand folder. Bri-ski re-pitched and repaired it. From memory it wasn't all that cheap tho.

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You may have a hard time finding it, but Dave Gerr's Propeller Handbook is probably the best resource I've seen. It's published by International Marine with an ISBN # 0-07-138176-7. It sells for US$19.95 retail and can be found for less at Amazon. I've long used the section on propellers found in older editions of Skene's Elements of Yacht Design edited by Frank Kinney. It's out of print and the information in Dave' book elaborates on Kinney such that, at least for propellers, there's no need to own both publications.

 

Beyond that, I agree with the points Wheels made earlier. If you're getting black smoke (at all, but especially at less than full throttle), you're overpropped. And 10-12 mm clearance probably isn't enough.

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I have a handheld RPM digital unit that anyone can borrow if they need. So you aremore than welcome to grab it BB.It is at work, but I can tell KM exactly where to find it in my tool box if you wanted to grab it.

But more importatnly, do the math, and work out what hull sped should be in the real world. And also rmember that pusing at hull speed is the biggest use iof fuel. 75% of Hull speed(if engine RPM and Prop are correct) is your most effiecient of fuel use/speed.

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Everyone says being overpropped is bad for the engine and it probably is but we ran an engine like that for many years no problem.

We were just very careful not to have any black smoke coming out of the exhaust.

What does more harm I think is running diesel at idle or at low revs as this tends to glaze bores.

So in summary slightly overpropped is probably ok as long as you cruise at say 2-2500rpm or 3/4 throttle without any smoke for an engine that maxs out at say 3400 or so.

I am not an engineer but have been running boat diesels for 40 odd years without problems.

I think increasing revs slowly is also better for the engine - dont push the throttle forward too quickly

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You are pretty much correct CG, but it is not so much the "to black smoke or not to black smoke" part, but more for where you are most efficient/have most Hp/torque curve(which is the real work).

Although black smoke is not good either, it does depend on how bad it is. Badly smoking means you are washing oil from the boare and the fuel is running down the bore into the sump. But mainly you are pouring fuel out the exhaust without it ever having done anything and for many of us, it is expensive enough as it is. Or you maybe a cruiser and so fuel consumption is a criticle factor for passage making. For many boats, many of these things may never be of concern. For instance, smaller boats with little engines have a fuel usage resembling KM's Pepe scooter, which just runs on fumes. They are often over powered. Simply because a pretty little 12Hp engine is all she needs, but the even prettier 18Hp was at a great price and will drive the freezer and bigger Alt and so on. Then you have the boats(and this is more for Aucklanders) that use the motor for getting in and out of the marina and that is it. No need to long distance motor, no punching into a good head wind and sea for hrs etc et. So perfect matches are not so criticle. Where as a larger vessel or a true motor/sailer that needs a big enigine, also needs the efficiency and needs to get the engine power coupled to that water as best it can.

All Sailing vessels have a certain amount of compromise when it comes to props, unless you can afford expensive feathering devices. They always tend to be on the lesser efficient in terms of gripping the water, because you also want less drag in the water when under sail. But now I am heading off on a tangent. So back to the correct prop..... having the motor matched to the prop means you should be able to get that engine power to the water better and you should be able to get the max speed of the Hull summed with best fuel saving from that engine and also best bite in the water for manouvering around the marina. Yes I know, you feel like you have more bite with the bigger prop, I hear you say. And yes you probably do and yes that is probably just fine for most people. But for big heavy boats where you need the Hp and the Hp is probably on the small side, then you need to get the most Hp and torque from your engine you can get so as you can accelerate or stop as quickly as possible. For many of todays modern engines, torque is being seen over a far wider bandwidth and is flatter over that band. Unlike the older engines where it shot up and down again and you best engine performance lay at the intersection of Hp and

Torque.

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Craftmaster re-pitch props. I had mine done a few years ago out in Howick. Old age and senility and all that, I can't remember who did it but it worked fine and didn't cost much at all.

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