Fergus
Well-Known Member
I believe my boat is over-propped (won't rev above 2k) so would I be correct in assuming that if motorsailing I should be able to rev higher?
over-propped question
- Thread starter Fergus
- Start date
Bilgediver
Well-Known Member
YES!
G
Guest
Guest
Depends. How much higher do you expect?
If you are limited to 2k due to it black smoking then yes motor sailing will help. This is the case of an undersized engine for the water line length.
If it is just stuck because the speed of the hull is limited by its water line then no real sustained increase of speed will be achieved with the sails up. So you would have the same problem trying to move water significantly faster past the hull than the boat is going. In this case you will see little change. This is the case of an oversized engine with too much pitch.
If you are limited to 2k due to it black smoking then yes motor sailing will help. This is the case of an undersized engine for the water line length.
If it is just stuck because the speed of the hull is limited by its water line then no real sustained increase of speed will be achieved with the sails up. So you would have the same problem trying to move water significantly faster past the hull than the boat is going. In this case you will see little change. This is the case of an oversized engine with too much pitch.
dickh
Well-Known Member
I always understood that the engine should be able to almost reach it's maximum rated speed when fully loaded. I've just re-engined with a Beta 25 and at the season start I can only get about 3300/3350 RPM, when it should be 3600 RPM, so in theory I should reduce the pitch by about ½". But I will probably leave it as it is for various reasons.
JackFrobisher
Well-Known Member
What are your various reasons?
Feel free to tell me to sod off if I'm being nosey...
Feel free to tell me to sod off if I'm being nosey...
Pye_End
Well-Known Member
When you think of the speeds the prop runs at, I can't see that feeding it with water traveling at say 5kn will make a huge difference on engine RPM. Whether it is noticable - I would be surprised (not that I am unwilling to be surprised!).
oldsaltoz
Well-Known Member
Check your trim, make sure heavy items are stowed midships to keep the ends light, remove all junk items and any other items not required on a particular trip, every pound helps.
Motoring under sail will get yo to maximum revs on most yachts.
Avagoodweekend......
Motoring under sail will get yo to maximum revs on most yachts.
Avagoodweekend......
Robin
Well-Known Member
If you cannot get max revs in gear then it is over propped regardless and you will not get the full quoted hp for the engine. In light conditions and not full throttle you might get more speed for the revs used, but meet a headwind or sea and you are not getting the power you need. What it does under sail is irrelevant really unless you motor sail everywhere. We have a Brunton which self pitches so under sail pitches up (goes faster or you can reduce revs for same speed) or against a head wind/sea pitches down (so you need more revs for same speed), but if you have a fixed prop it needs to be able to produce the hp the engine is rated for and it cannot do that if it doesn't reach the design revs.
roly_voya
Well-Known Member
Running the engine overloaded will damage it so you do need to re-prop and go down enough to lave the enging with 10-20% reseve for adverse conditions.
jimbaerselman
Well-Known Member
First, check your governor is not limiting revs. Check you can reach max revs in neutral.
if you can, you should be able to reach max continous revs when travelling at your hull speed (I'm assuming 7kts or so) and about 10 to 15% less when the boat is static.
If you can't reach those revs (and the governor isn't interfering), either your gearbox ratio is too low, or you're over-propped (dia or pitch) and you're likely to see black smoke coming from the exhaust at full throttle (and less) and you're not reaching full engine power.
If you can fiddle with pitch (feathering prop types) reduce it. If you can't, take advice on the correct pitch/dia for your HP, boat speed, gearbox ratio and see where the prop is wrong.
Have fun!
if you can, you should be able to reach max continous revs when travelling at your hull speed (I'm assuming 7kts or so) and about 10 to 15% less when the boat is static.
If you can't reach those revs (and the governor isn't interfering), either your gearbox ratio is too low, or you're over-propped (dia or pitch) and you're likely to see black smoke coming from the exhaust at full throttle (and less) and you're not reaching full engine power.
If you can fiddle with pitch (feathering prop types) reduce it. If you can't, take advice on the correct pitch/dia for your HP, boat speed, gearbox ratio and see where the prop is wrong.
Have fun!
Gumpy
Well-Known Member
Since on most engines the max Torque is not when they are flat out and since torque is what actually turns the prop against the water, then it is perhaps better to run at revs that produce the most "power". I have a ford and max revs is 2500rpm where it produces 120hp, however max torque is at 1600rpm (280ft/lbs) the boat goes no faster after 1600rpm just uses more fuel. It will pull 1900 in gear with no smoke black or otherwise.
J
J
jleaworthy
Well-Known Member
Over-propping is pointless and even harmful. The previous owner of my boat was very proud of his large coarse prop! After getting the pitch reduced by two inches(!) the engine runs much more freely, quietly and, surprise surprise, economically - the original reason given for over-propping.
Gumpy
Well-Known Member
I didnt say to over prop what I did say was that the most power transfered to the water comes not at full revs but below that.
I changed mine from 23x21 to 23x16 as it was with the old blade on, overpropped. Calculations done and prop supplied by Crowther Marine, so I think with their experience they might have got it about right!
J
I changed mine from 23x21 to 23x16 as it was with the old blade on, overpropped. Calculations done and prop supplied by Crowther Marine, so I think with their experience they might have got it about right!
J
DaveS
Well-Known Member
Since power delivered is proportional to torque X rotational speed, this would only be true if the engine torque curve was very "peaky" and fell away at a faster rate at high revs than the rate of speed rise. This is possible for, say, a fast revving 2 stroke motor bike engine, but unlikely for a marine diesel which tends to have a "flattish" torque curve, falling away relatively slowly at higher speed.
Like an earlier poster, I'm currently experimenting with reduced pitch to get higher revs (Kiwi) so that all of the engine power can be used if required and, hopefully, also improve further the already good fuel economy. While doing a bit of research before starting this exercise I unearthed the interesting fact that it is apparently "traditional" in some places to deliberately overprop to reduce engine speed, but there seemed to be no underlying logical reason for this.
Like an earlier poster, I'm currently experimenting with reduced pitch to get higher revs (Kiwi) so that all of the engine power can be used if required and, hopefully, also improve further the already good fuel economy. While doing a bit of research before starting this exercise I unearthed the interesting fact that it is apparently "traditional" in some places to deliberately overprop to reduce engine speed, but there seemed to be no underlying logical reason for this.