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Old 24-12-2008, 13:55   #1
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Best combination of Prop to Gear Ratio & Engine

When we bought the Adams35 almost 3 years ago, we used the old engine to bring the vessel to Botany Bay from Sydney Harbour where it had resided. We were impressed with the power of the old motor and how fast it pushed the vessel through the water with some good growth underneath.

Now we have a newer engine, the same prop and a clean hull and it does not seem to do the same knots in the same conditions ???

The old engine was a 1982 vintage Yamaha (not Yanmar) ME120EH, big 2 cylinder long stroke type beast with a hydraulic gear box and a ratio of 1:2.45. It has a continuous rating of 23HP at 2600 revs or 26HP at 2700 revs(1hr rating).

The new engine is a mid 80's vintage Nanni 30HP 3600 revs (must be a peak power rating) 4 cylinder with a Hurth HB100 gearbox and a ratio of 1:2.75

The prop is a 3 bladed fixed Austral A75 17GR3\M or 17GR3VM (hard to read). 1.25" shaft.

The only alteration was that the prop is about 3/4" closer to the hull above than before as the angle of the shaft was too steep (20 degrees), now it is about 14 degrees.

Any ideas as to what is happening ?
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Old 24-12-2008, 14:18   #2
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When we bought the Adams35 almost 3 years ago, we used the old engine to bring the vessel to Botany Bay from Sydney Harbour where it had resided. We were impressed with the power of the old motor and how fast it pushed the vessel through the water with some good growth underneath.

Now we have a newer engine, the same prop and a clean hull and it does not seem to do the same knots in the same conditions ???

The old engine was a 1982 vintage Yamaha (not Yanmar) ME120EH, big 2 cylinder long stroke type beast with a hydraulic gear box and a ratio of 1:2.45. It has a continuous rating of 23HP at 2600 revs or 26HP at 2700 revs(1hr rating).

The new engine is a mid 80's vintage Nanni 30HP 3600 revs (must be a peak power rating) 4 cylinder with a Hurth HB100 gearbox and a ratio of 1:2.75

The prop is a 3 bladed fixed Austral A75 17GR3\M or 17GR3VM (hard to read). 1.25" shaft.

The only alteration was that the prop is about 3/4" closer to the hull above than before as the angle of the shaft was too steep (20 degrees), now it is about 14 degrees.

Any ideas as to what is happening ?
The key is that you should be able to reach your peak RPM from the engine (under-way) with a clean hull, flat seas and little to no headwind. You should not be able to over-rev the engine while under-way.

With the figures that you laid out, you may have been getting peak performance from the prop with the old engine. I have a feeling that you may be able to over-rev the new engine and are loosing your peak performance. If this is the case, you may be able to have a greater pitch put into your old prop but more than likely will need a larger prop.

The prop being closer to the hull could be an issue. You may not be able to get a good "Bight" on the water passing past the hull. This could cause prop cavitation (kinda like a slipping clutch in a car). It shouldn't be too difficult to place a spacer between the crankshaft flange and the prop-shaft flange (if there is room behind the prop).

Do you have before & after pics of the prop/hull?
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Old 24-12-2008, 16:24   #3
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Buyers Remorse...

I had the same feeling about the old Ford that I junked.
It pushed the boat through the water beautifully. Too bad it was hard to start and sprayed oil though the boat.
Now my new John Deere does not seem to fit the boat quite as well. The prop needs the pitch reduced a few inches, and when I apply full power it just seems to dig a hole in the water.
Now all it does is get me where I want to go with no fuss...
I'd suggest playing with what you have for a while, provided the engine is not clearly over reving, or overloaded. Work out exactly what you want and then get the prop repitched if necessary.
Try using a GPS and a log to get some hard data.
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Old 24-12-2008, 17:04   #4
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The engine is revving up to max revs with no real indication of being under strain. it does feel that the prop is not grabbing the water, if that makes sense. Placing a spacer in the shaft system is out of the question for the short-mid term, that whole system has just been done with new engine beds, shaft flange, shaft saver, etc. There is 12mm clearance between the stern tube and the flange. I will post some photos on to our Images file.

I will use a GPS next week when we get back to SYdney and get some data, could be good to compare to other Adams35 owners, if we can find some.
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Old 24-12-2008, 17:18   #5
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The photos do not show the distance clearly between prop and hull as the blade is not in the top position, but it will give an idea.
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Old 24-12-2008, 17:49   #6
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The engine is revving up to max revs with no real indication of being under strain. it does feel that the prop is not grabbing the water, if that makes sense. Placing a spacer in the shaft system is out of the question for the short-mid term, that whole system has just been done with new engine beds, shaft flange, shaft saver, etc. There is 12mm clearance between the stern tube and the flange. I will post some photos on to our Images file.

I will use a GPS next week when we get back to SYdney and get some data, could be good to compare to other Adams35 owners, if we can find some.
It is best to set it up so that the engine can turn no faster than 3600RPM while motoring in flat conditions. 3650 is too much and 3500 is too little.

Some may argue that you should have a certain % in reserve (ability to achieve higher RPM) for extreme conditions. I simply disagree. IMO, a sailboat should be set-up to achieve hull speed at cruising RPM. On your engine that would be about 2500RPM. This will save you as much as 50% on your fuel consumption.

If you are under propped, you may have to push the engine to 3000RPM to go the same distance as a larger prop will travel @ 2500RPM. Your engine will be producing wasted HP at the higher RPM and a shorter life. The larger prop will push the boat farther @ lower RPMs and less HP. This will dramatically increase efficiency and increase engine life.

I'm not sure what good the GPS will do. It will give you your speed over the bottom but that's not really helpful. All you need is your tach. It tells the whole story. If you want to know how fast you are going, use your knot meter, not your GPS. Your knot meter will give you your speed through the water. That's helpful info but makes little difference to what is most important.

When I 1st bought my Passport 45 the 88HP Ford Lehman burned 1.5G per hour. The engine would practically free wheel. Max RPM on that engine was 2500. I could easily push it to 3000 and beyond. I put a 2" larger prop on it and it still reached 2600 and burned 1.0 Gal per hour. I had the prop pitched in NZ so the engine could achieve 2500 max. For the remaining 25K miles, we achieved .75 gal per hour while motoring in calm weather at 1600RPM @ 7kts.

I still had plenty of power to motor into a small channel going into Wellington NZ with 50kts of wind on the nose.
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Old 24-12-2008, 21:09   #7
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Originally Posted by ribbony View Post
The old engine was a 1982 vintage Yamaha (not Yanmar) ME120EH, big 2 cylinder long stroke type beast with a hydraulic gear box and a ratio of 1:2.45. It has a continuous rating of 23HP at 2600 revs or 26HP at 2700 revs(1hr rating).

The new engine is a mid 80's vintage Nanni 30HP 3600 revs (must be a peak power rating) 4 cylinder with a Hurth HB100 gearbox and a ratio of 1:2.75
This is puzzling. Your Nanni at max RPM is turning the prop at 1309 RPM (3600/2.75). Your old Yamaha was turning the same prop at 2700/2.45 = 1102 RPM, but you think you were going faster. I don't think reduced hull clearance would explain it. Perhaps at higher RPM your prop is cavitating? Do you know the diameter and pitch of your prop? What happens when you run your Nanni at 2900 rpm (this should be the same shaft rpm as your Yamaha at 2600)?
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Old 24-12-2008, 21:10   #8
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GPS calculations...

If you only have a GPS for speed I suggest doing one run at set RPM, then doing a reciprocal course at the same RPM. It should then be possible to calculate your through the water speed.

I believe that you need to know the speed relative to the water so that you know what proportion of hull speed you are doing, and also to determine the effect of any changes that you might make. The GPS is going to be way more accurate than any log.

Your LWL should be about 28.24' so using the usual hull factor of 1.4 your hull speed would be about 7.5kts. My waterline length of about 32'11" gives me a theoretical hull speed of 8 kts which is the best that I have been able to manage. This is at 2100RPM (my max is 2500) so I live in hope that repitching will give more speed however I suspect that it will only save a little fuel and maybe make the boat a little more controllable when docking.

My experience would lead me to believe that if you are getting 7 kts without too much drama (engine overreving, black smoke etc.) than you might be OK. I'm waiting till I come out of the water next before I start repitching.

My prop is not that far from the hull either leading me to believe that the interference effect between hull and prop is possibly overrated. I would expect to hear (or feel) if there is a pressure build up as the propeller blade passes the keel. If any members have experience in this their comments would be appreciated.
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Old 24-12-2008, 21:38   #9
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With a 17" diameter prop, the pitch should be about 12". You should be able to get over 7 knots at max rpm. Another alternative would be a 16x15 prop. I don't think you can expect much over 7.5 knots with a 28' waterline.
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Old 24-12-2008, 23:58   #10
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We do not have a log as yet, hence the GPS. Will try the combining the of up and down legs on a run to get a water speed reading.

Out of interest repitching a prop, is that something that can be done on an old prop or is a new prop with the altered pitch the solution ?
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Old 25-12-2008, 05:00   #11
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Repitching...

Propellers can be repitched to a limited degree, provided they have not been previously done.
I have a vague memory of the limit being 2".
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Old 25-12-2008, 05:29   #12
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I was also told by a prop shop that 2" pitch change is about the limit.
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Old 03-01-2009, 18:57   #13
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We do not have a log as yet, hence the GPS. Will try the combining the of up and down legs on a run to get a water speed reading.

Out of interest repitching a prop, is that something that can be done on an old prop or is a new prop with the altered pitch the solution ?
My understanding is also 2" max on re-pitch.

The prop should have the original size and pitch stamped on it. If it has been re-pitched, the new pitch should also be stamped on it but don't count on it.

Take off the prop and take it to a prop-shop along with the LWL, tonnage, beam make and model of engine and gear box ratio. They will be able to calculate what is the best size and pitch for your vessel. I have a feeling that you need a bigger prop. There are places to buy used props but you must be careful. Bring a magnifying glass and check the prop carefully for tiny cracks. Only buy a used prop if the seller agrees to refund your $ if the prop shop turns it down.
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Old 03-01-2009, 21:07   #14
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I have to strongly disagree with one of the previous posts. If your engine cannot reach its maximum governed speed, then you are over propped...period.

If you want a close analogy, this is like running your car up a steep grade in fifth gear. All you are doing is lugging the engine which will dramatically reduce the engines life. This is irrespective of which speed you are choosing to run the engine. Running an engine at a slower RPM will extend its life some, but running it at a slower speed while over propped, will decrease its life.

In fact Cummins Marine will not warrant an engine which cannot reach its maximum governed speed.
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Old 04-01-2009, 00:36   #15
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On our run up to Sydney for the NYE fireworks we used teh GPS in a variety of conditions to record the vessel speed, the maximum wa 7.3knots with a tail wind with the engine running at 3600 rpm.

But, the average speed produced at 3600 was 6.4 to 6.8 knots.

The engine seems to run OK at max rpms, but seems happier at 3200-3400 rpms, little vibration, not too noisy and the boat speed is still good.

Diving around the boat we cleaned off most of the slime that was on the hull, but it was thin and there were no barnicles or weeds on the hull. I checked the prop and it was clean.

I still have to do a propper still water test and the weather looks windy for most of the week.

The engine reaches its max rpms without difficulty and we would gather that an increase in pitch may help with boat speed. Not expecting miracles though !
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