hlb
RIP
Here it is. Dont my new fender socks look classy!
Why are shafts so much less efficient?
- Thread starter Nick_H
- Start date
RogerRat
Well-Known Member
Yep, works great and I keep the joystick in the head so if I have problems parking, I don't have to go far to kiss my ass good bye! /forums/images/graemlins/wink.gif
jcmmarine
Well-Known Member
Since when were shafts more inefficient?
Efficiency is the ratio of energy in to energy out. As any marine design engineer will tell you the most efficient propulsion involves large propellors running at low speed to minimise slip, set deep in the water to minimise cavitation. If outdrives were more efficient commercial vessels, that need to make money and where speed is not an issue, would be using them.
Probably the most efficient motor driven pleasure craft is a 60ft 20 ton Narrow boat with a large slow reving prop that can get six Knots from an 18hp engine whilst sipping fuel.
Efficiency is the ratio of energy in to energy out. As any marine design engineer will tell you the most efficient propulsion involves large propellors running at low speed to minimise slip, set deep in the water to minimise cavitation. If outdrives were more efficient commercial vessels, that need to make money and where speed is not an issue, would be using them.
Probably the most efficient motor driven pleasure craft is a 60ft 20 ton Narrow boat with a large slow reving prop that can get six Knots from an 18hp engine whilst sipping fuel.
Channel Ribs
Well-Known Member
[ QUOTE ]
... a 60ft boat that can get six Knots from an 18hp engine...
[/ QUOTE ]
I always KNEW there was a reason I wanted one!
/forums/images/graemlins/smile.gif
*grits teeth* must-not-search-boatsandoutboards
How about a narrow boat with twin forward facing duo-props? Or did I miss the point again?
/forums/images/graemlins/blush.gif
... a 60ft boat that can get six Knots from an 18hp engine...
[/ QUOTE ]
I always KNEW there was a reason I wanted one!
/forums/images/graemlins/smile.gif
*grits teeth* must-not-search-boatsandoutboards
How about a narrow boat with twin forward facing duo-props? Or did I miss the point again?
/forums/images/graemlins/blush.gif
jcmmarine
Well-Known Member
Sorry for taking the question/forum seriously. I will log of for a few scotches then log on again.
MapisM
Well-Known Member
[ QUOTE ]
some method of universal coupling could be used so that the drive was parallel to the hull.
[/ QUOTE ]
What do you mean by universal coupling?
With a planing hull (whose bottom is by definition straight with the keel), the only way of pushing horizontally is either with some kind of leg (i.e. outdrive, sterndrive, ips) or with surface drives.
In a displacement hull, it's instead normal to have (almost) horizontal shafts.
Or am I missing something?
And btw, theorically the most efficient drive should be parallel to the sea surface, not to the hull.
some method of universal coupling could be used so that the drive was parallel to the hull.
[/ QUOTE ]
What do you mean by universal coupling?
With a planing hull (whose bottom is by definition straight with the keel), the only way of pushing horizontally is either with some kind of leg (i.e. outdrive, sterndrive, ips) or with surface drives.
In a displacement hull, it's instead normal to have (almost) horizontal shafts.
Or am I missing something?
And btw, theorically the most efficient drive should be parallel to the sea surface, not to the hull.
Channel Ribs
Well-Known Member
[ QUOTE ]
... taking the question/forum seriously...
[/ QUOTE ]
/forums/images/graemlins/frown.gif I tried to be serious, but the mere sight of half naked thighs sends me bit silly. Anyway, I do have a fascination for narrow boats; also for scotch.
... taking the question/forum seriously...
[/ QUOTE ]
/forums/images/graemlins/frown.gif I tried to be serious, but the mere sight of half naked thighs sends me bit silly. Anyway, I do have a fascination for narrow boats; also for scotch.
oldgit
Well-Known Member
"Since when were shafts more inefficient? "
You have made the fundamental error of assuming anything that is not a gleaming tower of shiny white * glassfibre powered by 2 x 500 hp engines will be recognised on this forum as a "boat".
*You will be allowed a blue hull if you are a Fairline owner,but under no circumstances own a green one and expect to be invited in or spoken to at the boatclub bar .
/forums/images/graemlins/grin.gif
You have made the fundamental error of assuming anything that is not a gleaming tower of shiny white * glassfibre powered by 2 x 500 hp engines will be recognised on this forum as a "boat".
*You will be allowed a blue hull if you are a Fairline owner,but under no circumstances own a green one and expect to be invited in or spoken to at the boatclub bar .
/forums/images/graemlins/grin.gif
omega2
Well-Known Member
Why bother to get involved, we know we have Proper boats.
oldgit
Well-Known Member
/forums/images/graemlins/grin.gif
Nick_H
Well-Known Member
by universal coupling I mean a joint that allows a bend in the shaft. In its simplest form it could be just a universal joint like you'd find on a car propshaft, although there are definitely some issues with doing that because unlike a car the drive from the props is transmitted longitudinally up the shaft
DAKA
Well-Known Member
In theory I agree your universal joint seams a good idea.
The shaft is already pointing down instead of parallel and then by the time you are on the plane the props must be 40' out.
My last boat (Cornich) had outdrives and cruised at 24 knots with zero trim tabs.
Most the time it was too bouncy to mess about with outdrive trim but when it was really calm / Smooth there was time to trim the drives and an extra 2 knots was readily available ( only ever bothered with it if going 100 nm or so, it was very fiddly to get right).
Thats a 10% increase but I expect that is probably equivalent to an extra 15% to 20% power due to extra drag/windage ????
I think if you were to align the props they would need to be face up and you would need even more power to get over the hump which is why I guess it wouldnt be practical.
I have messed about with the tender outboard and have found this is a problem , you just get the maximum speed with the correct prop angle but then you need to lean over the bow to get it on the plane.
If you can design a UV joint that allows the shaft angle to be altered once on the plane I think you would be on to a winner !
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edit
Trim explained better in spannermans post he is talking about 26 knots to 35 knots thats almost your 40% !
http://www.ybw.com/forums/showflat.php/Cat/0/Number/1203570/an/0/page/0/gonew/1#UNREAD
The shaft is already pointing down instead of parallel and then by the time you are on the plane the props must be 40' out.
My last boat (Cornich) had outdrives and cruised at 24 knots with zero trim tabs.
Most the time it was too bouncy to mess about with outdrive trim but when it was really calm / Smooth there was time to trim the drives and an extra 2 knots was readily available ( only ever bothered with it if going 100 nm or so, it was very fiddly to get right).
Thats a 10% increase but I expect that is probably equivalent to an extra 15% to 20% power due to extra drag/windage ????
I think if you were to align the props they would need to be face up and you would need even more power to get over the hump which is why I guess it wouldnt be practical.
I have messed about with the tender outboard and have found this is a problem , you just get the maximum speed with the correct prop angle but then you need to lean over the bow to get it on the plane.
If you can design a UV joint that allows the shaft angle to be altered once on the plane I think you would be on to a winner !
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edit
Trim explained better in spannermans post he is talking about 26 knots to 35 knots thats almost your 40% !
http://www.ybw.com/forums/showflat.php/Cat/0/Number/1203570/an/0/page/0/gonew/1#UNREAD
scubaman
Well-Known Member
Speaking of which. Have a look at that Princess 56
Contents in Finnish (sorry about that), but have a look at the picture. The boat hit a rock, which ripped off the p-bracket. This resulted in a huge hole in the bottom and a sunken boat (sunk only 2 feet deep, thanks to the reef that the boat hit).
I believe IPS can't do a much worse in similar circumstances. /forums/images/graemlins/wink.gif
Contents in Finnish (sorry about that), but have a look at the picture. The boat hit a rock, which ripped off the p-bracket. This resulted in a huge hole in the bottom and a sunken boat (sunk only 2 feet deep, thanks to the reef that the boat hit).
I believe IPS can't do a much worse in similar circumstances. /forums/images/graemlins/wink.gif
ColinHuggins
N/A
[ QUOTE ]
Probably the most efficient motor driven pleasure craft is a 60ft 20 ton Narrow boat with a large slow reving prop that can get six Knots from an 18hp engine whilst sipping fuel.
[/ QUOTE ]
70-footers are even more efficient - and the extra 10 feet saves water too!!
Probably the most efficient motor driven pleasure craft is a 60ft 20 ton Narrow boat with a large slow reving prop that can get six Knots from an 18hp engine whilst sipping fuel.
[/ QUOTE ]
70-footers are even more efficient - and the extra 10 feet saves water too!!
oldgit
Well-Known Member
One would assume that the P brackets would have been secured to the hull bottom somewhere around the lazerette,and that this would have been a water tight compartment ?
hlb
RIP
Dont think mobos have water tight compartments. Bulk heads maybe, but not water tight. Would not matter if they were anyway, water would just come over the top! /forums/images/graemlins/crazy.gif