Bruntons Autoprop performance

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MGambling

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I have recently added a rev counter to my hunter which I purchased last year and find that the max revs i can get is about 2300. Engine is Yanmar 2GM20 which peaks at 3300. The hull and prop are clean and the rev counter accurate which leads me to think yacht is overpropped!!. Can other owners achieve near max revs on full throttle?
 
It is difficult to overprop a self pitching prop which adjusts to suit the loads, the real adjustment being in the diameter. If you don't reach max design revs then you are not going to get the full HP of the engine, 2300 is way too low if max is 3300 IMO. Are you really sure the new rev counter is correct?

Have you spoken to Bruntons because they are very helpful? We have a Yanmar 44hp and a 3 bladed Brunton Autoprop and we get max revs in gear no probs.
 
The self pitching of the Autoprop makes the prop efficient at low revs and you tend to reach maximum hull speed at lower revs.
If you know the waterline length of the Hunter you can calculate approximate maximum speed in knots from 1.3 times the square root of the waterline length in feet.
As Robin says it is difficult to overprop with an Autoprop so perhaps a rev counter check is next.
 
i understand what you say bout max hull speed being achieved at lower revs but the rev counter is accurate. it indicates 3400 when not in gear. Bruntons are not that helpful since I didnt purchase the unit. I think perhaps the prop diameter is too large
 
Hi,
My Parker 275 has a 1GM and a two bladed Autoprop. When I first fitted the prop, the boat got to 4.5knots and then started to produce black smoke and over heat. I sent the prop back and Bruntons and they reset the angle of attack, so the prop does not pitch up so qucikly (so Bruntons know how to effectively change the pitch, although not in the convetional sense). I now get 6.3 knots (about hull speed) at full throttle, don't know the revs, but no black smoke. I got 5.8knots out of the original 2 bladed fixed prop. I'd send the prop back to Bruntons and ask them to reset it, my guess is that you should be seeing at least 6.5knots at 2,800 to 3,000rpm. Hope this helps.
Cheers
Ian
 
The assumption here is that the prop is the reason you don't get max revs. If you have dirty fuel, a fuel restriction, or an air leak, or your fuel pump is not set properly etc etc you will reach max revs at no load but not under load. If your engine was being restricted that much by a prop (which is very difficult with a Brunton anyway) you would see loads of black smoke. Try changing filters, flushing through your fuel lines and see if that makes a difference. Also check the blindingly bleedin' obvious things like the throttle on the engine is actually reaching the stop and the cable has not slipped.
 
I didn't buy our Autoprop either because it came with the boat. In our case the previous owners gave figures in the brokerage info that I was concerned about, that the max boat speed under power was low and at less than the max revs for the engine, as was the quoted cruising speed. I was convinced that either the broker data was wrong or there was a serious question about the prop hence I called Bruntons before we signed for the the sale, the boat was ashore at the time. In our case Bruntons double checked all the spec points, including which of 3 different gearbox ratios was used, and said the prop was correct.

On each Brunton is a stamped serial number, on the root of each blade, in case they need to supply a replacement. Given this number they can check the boat/engine/gearbox ratio data to see if it is correctly sized. In our case once we were eventually able to do a seatrial we discovered that a) the log was reading significantly low b) that the engine did pull max revs in gear under load. Later we discovered the previous owners had wrongly assumed the recommended cruise revs of 2,600 quoted by Yanmar in the manual was the MAXIMUM (actually 3,600) and had never opened the throttle beyond that 2,600 point - daft but heyho fact, and it was this wrong data that was quoted by the broker details.

When the Autoprop comes under load, like a headwind/headsea, it effectively drops down in pitch and boatspeed drops off but more revs will put it back, just like changing down a gear in a car. That way under heavy load you should still be able to reach max HP, which occurs at or near max engine revs. Conversely with the Autoprop. it pitches up (like 5th gear or overdrive) if there is any outside help like a bit of push from wind in the sails and the same boatspeed can be achieved at lower engine revs.

You don't say which size of Hunter you have, but hull speed will be 1.34 x sq root of the waterline length in feet, so for example a 25ft waterline would give a hull speed of around 6.7kts. In many cases you might even get a tad more, say 7kts. I would guess that you have enough engine HP to get to hull speed so for sure something isn't right.
 
There is a simple check.

Run the boat at full power in reciprocal directions and note the speed on the log each time. At the same time as you are under full power look at the exhaust water. If it is black or grey and you are not reaching max displacement speed averaged over the two runs then you are overpropped.

If you are reaching max displacement speed and have a grey exhaust don't worry too much but a black exhaust means you are seriously overpropped and the engine is not running efficientlywhich causes damage in the long run. An autoprop is supposed to automatically adjust . If overpropped send it back to brunton's for repair under warranty etc. You will find them most professional and helpful.

All IMHO of course.:-)
 
I have always felt my 2GM20 did not deliver enough power. In fact they are only delivering about 14HP at the revs you are quoting. Recently we lifted ours to replace the engine mounts and I found the throttle lever was dirty and stiff and not opening fully. I also fitted a new primary fuel pump since the old one seemed ineffective when operated by hand. It did seem to rev more freely on reinstallation. I also fitted a Kiwi prop at the same time so the improvement in sea trials was not directly comparable. When did you last replace the engine fuel filter? You can also pump dry the diesel tank using your oil change pump to remove any water and muck, and replenish with fresh fuel. Hve you looked at the state of the air filter?
 
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Robin may have the answer.......the log is under-reading. The odd barnacle or bit of slime wrecks the calibration. If you have GPS you can recalibrate easily otherwise you may have to find a measured mile or do some accurate chartwork.

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I rather suspect though that something else is wrong because of the significant difference between revs in and out of gear. It may therefore be the engine as boatmike says, just not getting enough fuel through (gungy filters?) when it needs it under load or simply a badly adjusted throttle cable. Personally I think the least likely suspect is the prop but I'd still talk nicely to Bruntons for their confirmation. If it genuinely isn't getting enough revs when in gear then it isn't reaching it's potential HP rating and simply may not have enough HP at the revs actually achieved to reach hull speed - assuming the log calibration is correct or it isn't fouled up. Even if the log was reading slow, it should get close to max revs even though it won't go much faster than hull speed, but just digs a bigger hole and squats.
 
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