Meaning of terms over-propped and under-propped

[srah1953]

I've read various posts commenting on an engine being over-propped or under-propped. Can I ask what do these terms means- firstly from an end-result perspective: does over-propped mean that a boat will reach hull speed before hitting full revs and under-propped mean that even at full revs you won't reach hull speed? Then, does over- or under-propped mean that the prop is too big/small or that the pitch is wrong?
Many thanks

 

[michael_w]

Over propped means the engine cannot achieve its max rated RPM. Under propped means the engine reaches max RPM too easily ie: the boat doesn't go as fast as one would expect. Both effects are caused by a combination of pitch and prop size.

Generally speaking it is better for sailing boats to be slightly over propped as this gives a bit more "bite" for manouvering and allows the boat to achieve best economical speed at slightly lower revs.

For example, my boat's engine has a max RPM of 3,000. The prop is set to achieve a max of 2700 RPM. I normally fast cruise at 2100.

 

[macd]

This, I hope, is a straightforward answer to a hugely complex subject.
Over-propped is equivalent to overgeared on a car or motorcycle. The load on the prop as it drives through the water will prevent the engine reaching full revs. This can be due to too large a prop, excessive pitch, or a combination of the two (or a dirty bottom, but that's not the prop's fault). Usually a boat diesel's peak power is developed at very close to full revs, so in effect by limiting the revs you're also limiting the horsepower the engine can actually produce. But, just like a car going downhill, downwind the boat might reach peak rpm...but then you'd be sailing, not motoring. Some people like just like a touch of over-prop because it allows more relaxed mid-revs cruising...a bit like overdrive in a car (like michael_w, I see, who posted while I was writing).

Whether or not it will reach hull speed will also depend on the power of the engine. Obviously a 10-tonner with a 7hp single isn't likely to motor at hull speed even with the 'perfect' prop.

Under-propped is obviously the opposite: the engine will reach peak rpm too soon and then not spin any faster, which means the boat won't go any faster, either. In fact you'd throttle back a bit to maintain the same speed. It should be obvious that both waste some of the engine's potential.

 

[Eygthene]

When an engine/boat is overpropped the engine is unable to reach its maximum speed and power output even at full throttle. Conversely when underpropped, the engine will reach full speed but the prop will not be able to demand/deliver the engine's maximum power into the water. Either way the boat will not be driven as fast as it should by the engine fitted.

Overpropping results from a prop which has a combination of too great a pitch and/or too great a diameter and/or too many blades. Underpropping visa versa.

Consult the "Propeller Handbook" by Dave Gerr for more information on the very tricky subject of matching the propeller to the engine and gearbox on your boat.

 

[srah1953]

Thank you all very much. It's nice to have unanimity in replies.
I hit maximum revs (around 3,100 as per Yanmar manual) but only achieve 7 knots in flat water, whereas hull speed should be around 8 knots (with 11m waterline). This would then indicate that I'm under-propped. The engine's big enough (39 hp for around, say, 9.5 tons loaded) and I don't believe hull is very dirty.
I have thought in the past of changing from fixed 3 blade to feathering or folding but never found the arguments sufficiently compelling. I've no guarantee that a new prop will result in a desirable over-propping.
Any further thoughts?
Also, how does being over propped allow “a bit more "bite" for manouvering” in that the number of revs is limited?
Again, thanks.

 

[ianj99]

Thank you all very much. It's nice to have unanimity in replies.
I hit maximum revs (around 3,100 as per Yanmar manual) but only achieve 7 knots in flat water, whereas hull speed should be around 8 knots (with 11m waterline). This would then indicate that I'm under-propped. The engine's big enough (39 hp for around, say, 9.5 tons loaded) and I don't believe hull is very dirty.
I have thought in the past of changing from fixed 3 blade to feathering or folding but never found the arguments sufficiently compelling. I've no guarantee that a new prop will result in a desirable over-propping.
Any further thoughts?
Also, how does being over propped allow “a bit more "bite" for manouvering” in that the number of revs is limited?
Again, thanks.

If you fit a Kiwi feathering prop, you can adjust the pitch yourself by means of a 4mm hex key on each blade root.
The Kiwiprop also has a bigger pitch (not adjustable) when going astern for even more bite.

I have mine set at about 2" more pitch than the original fixed blade prop (17x10") and this gives relaxed 6kts cruisng at 2000rpm, about 7kts at 2400rpm (max rpm with this prop) So overpropped by quite a bit as my Perkins 4108 is rated at about a continuous 40bhp at 3000rpm.

With the old prop, it felt to have no bite - as if the gearbox clutch plates were slipping, and going astern to slow down had little effect no matter how many revs were used. This has all changed with the Kiwi though the downside is more prop walk going astern. However a 10t long keel motorsailer has a mind of its own going astern anyway and definitely benefits from the extra bite provided by the Kiwi.
Ian

 

[chewi]

There is aspreadheet on the web propcalc.xls that you can enter the hull, engine and gearbox parameters.
It will confirm whether your engine can achieve the speed you want with whatever prop you have , and suggest a better size.

http://www.alberg30.org/maintenance/MechanicalPropulsion/Propeller/propcalc.xls

 

[Tranona]

Thank you all very much. It's nice to have unanimity in replies.
I hit maximum revs (around 3,100 as per Yanmar manual) but only achieve 7 knots in flat water, whereas hull speed should be around 8 knots (with 11m waterline). This would then indicate that I'm under-propped. The engine's big enough (39 hp for around, say, 9.5 tons loaded) and I don't believe hull is very dirty.
I have thought in the past of changing from fixed 3 blade to feathering or folding but never found the arguments sufficiently compelling. I've no guarantee that a new prop will result in a desirable over-propping.
Any further thoughts?
Also, how does being over propped allow “a bit more "bite" for manouvering” in that the number of revs is limited?
Again, thanks.

Before investing in a better prop you can try repitching the existing fixed prop by 1" (coarser). Not sure which Yanmar you have, but most will run to 3600 and are usually propped to peak at 3400.

As suggested you can put your boats data into Propcalc on www.castlemarine.co.uk to get a suggested prop size.

 

[srah1953]

The engine is a 3JH5CE with an SD50 saildrive. I don't know whether this answers the comment about potential revs up to 3,600, the SD50 handbook says that the maximum rated output at the crankshaft is 39hp/3,000 rpm. If I race the engine in neutral, the needle just flicks up to around 3,000; definitely not much further.
In looking up the handbook for the above info, I notice that it says, in relation to prop selection, that for a folding type and a 2 blade fixed, the diameter should be 18 in, whereas my 3 blade fixed are marked 17/13. It doesn't refer to 3 blades.
What's involved in re-pitching an existing prop?
Thanks

 

[srah1953]

If you fit a Kiwi feathering prop, you can adjust the pitch yourself by means of a 4mm hex key on each blade root.
The Kiwiprop also has a bigger pitch (not adjustable) when going astern for even more bite.

I have mine set at about 2" more pitch than the original fixed blade prop (17x10") and this gives relaxed 6kts cruisng at 2000rpm, about 7kts at 2400rpm (max rpm with this prop) So overpropped by quite a bit as my Perkins 4108 is rated at about a continuous 40bhp at 3000rpm.


Ian

I haven't seen or possibly missed but there must be a downside to over-propping?

 

[penfold]

I'm not keen on overpropping; it's fine in clement weather, but if you're trying to claw off a lee shore or otherwise punching a headwind and headsea, particularly if well laden with cruising paraphernalia, the engine is labouring below peak power. Add a dirty bottom and/or growth on the prop itself and the stressful lee shore scenario might be made more exciting by the high temperature alarm going off.

 

[planteater]

I haven't seen or possibly missed but there must be a downside to over-propping?

My boat is significantly overpropped since I replaced the engine and gearbox. Under normal motoring it's no problem - I can get close to hull speed even though I can't get to full revs. The big problem comes in marinas - it doesn't do slow. Even at tickover we are going faster than we might like. Lots of pulsing the engine to keep things down to a sensible speed - it just adds to the general marina stress.

 

[Tranona]

The engine is a 3JH5CE with an SD50 saildrive. I don't know whether this answers the comment about potential revs up to 3,600, the SD50 handbook says that the maximum rated output at the crankshaft is 39hp/3,000 rpm. If I race the engine in neutral, the needle just flicks up to around 3,000; definitely not much further.
In looking up the handbook for the above info, I notice that it says, in relation to prop selection, that for a folding type and a 2 blade fixed, the diameter should be 18 in, whereas my 3 blade fixed are marked 17/13. It doesn't refer to 3 blades.
What's involved in re-pitching an existing prop?
Thanks

You are right, it is max 3000 rpm and a 2.3 reduction. Generally a 3 blade a 3 blade prop would be an inch smaller diameter than a 2 blade to give the same thrust at the same shaft speed. If you are getting 3000 easily it could be that the prop is too fine pitch as you should be able to get hull speed with that amount of power. Not sure whether you can repitch a saildrive prop as they are usually aluminium. Quite possible to repitch by an inch on bronze props. Might be worth talking to a specialist such as Steel Developments on the possibilities. Do the sums first with the Propcalc programme so you get an idea what is the right size. If you do go to a folder such as a Flexofold the supplier will work out the correct size for you, dependin on whether you have a 2 blade or 3 blade.