davedpc
Well-Known Member
If they are so efficient then why not ?
Why dont outboards have duoprops?
- Thread starter davedpc
- Start date
Sneds
Well-Known Member
Bloody good question!
jfm
Well-Known Member
Some biggish yamahas do
But I agree, would be great if there were more of them. I dont know what weight penalty they add though
But I agree, would be great if there were more of them. I dont know what weight penalty they add though
G
Guest
Guest
Not to mention fuel...............
So I wont! /forums/images/graemlins/grin.gif
So I wont! /forums/images/graemlins/grin.gif
brianrunyard
Well-Known Member
[ QUOTE ]
Not to mention fuel...............
So I wont! /forums/images/graemlins/grin.gif
[/ QUOTE ]
I thought Duoprops were more efficient.
Not to mention fuel...............
So I wont! /forums/images/graemlins/grin.gif
[/ QUOTE ]
I thought Duoprops were more efficient.
[2068]
...
I remember a test (not very scientific) that showed the optimum number of props, and how it varied with boat speed and weight.
The results were something like:
40 kts+, light boat - Single prop best
20-40kts and a mid weight boat - Two props best
Below 20kts, heavy boat - Three or four props best !!!
dv.
The results were something like:
40 kts+, light boat - Single prop best
20-40kts and a mid weight boat - Two props best
Below 20kts, heavy boat - Three or four props best !!!
dv.
jointventureII
Well-Known Member
I dunno!! Prop effect is very noticable i find, our 23fter goes to stbd far more easily than to port with a 115hp o/b. I suspect below about 60-80hp it would be pointless but definately over that it would seem to make sense. Maybe another prop + gearing would add more to the price than people would be prepared to pay for the benefits it brings
MapisM
Well-Known Member
[ QUOTE ]
Prop effect is very noticable i find, our 23fter goes to stbd far more easily than to port with a 115hp o/b.
[/ QUOTE ]I guess you mean at cruising speed, don't you?
In fact, unless you have a left prop - which is unlikely on a single engine boat - the prop walk effect goes the other way round. You should notice that when maneuvering at low speed: doesn't the boat turn more easily and tighter to port than to stbd?
The effect which you mention is instead due to the lack of servo-steering, and can easily be neutralized with the small compensating fin under your leg (turning it clockwise, when looking at it from the bottom).
There's nothing you can do to compensate the real prop-walk effect at low speed, on the other hand.
Prop effect is very noticable i find, our 23fter goes to stbd far more easily than to port with a 115hp o/b.
[/ QUOTE ]I guess you mean at cruising speed, don't you?
In fact, unless you have a left prop - which is unlikely on a single engine boat - the prop walk effect goes the other way round. You should notice that when maneuvering at low speed: doesn't the boat turn more easily and tighter to port than to stbd?
The effect which you mention is instead due to the lack of servo-steering, and can easily be neutralized with the small compensating fin under your leg (turning it clockwise, when looking at it from the bottom).
There's nothing you can do to compensate the real prop-walk effect at low speed, on the other hand.
jfm
Well-Known Member
Mapis, i think whitefin has it the correct way round. With a single outboard r/h prop and 115hp you should mount the engine about 20mm to starboard of the centreline. If whitefin's outboard is on the boat centreline (as many badly set up boats are) it will tend to turn the boat to starboard at high speed (I'm not talking about low speed prop walk)
MapisM
Well-Known Member
I suspect that's not the case, for a couple of reasons:
1) he doesn't mention that the boat is leaning to port at cruising speed, which would be another noticeable effect of the wrong installation which you mention;
2) he says that his boat "goes to stbd far more easily". In my understanding, what he meant is that it takes more effort to turn the wheel to port (or that the boat tends to turn to stbd when leaving the wheel, in other words). Which is the typical effect of a badly oriented (or totally missing) compensating fin.
WhiteFin, what's your view? Mind, what jfm suggests is also possible, of course.
1) he doesn't mention that the boat is leaning to port at cruising speed, which would be another noticeable effect of the wrong installation which you mention;
2) he says that his boat "goes to stbd far more easily". In my understanding, what he meant is that it takes more effort to turn the wheel to port (or that the boat tends to turn to stbd when leaving the wheel, in other words). Which is the typical effect of a badly oriented (or totally missing) compensating fin.
WhiteFin, what's your view? Mind, what jfm suggests is also possible, of course.
jointventureII
Well-Known Member
The engine wasnt the originally installed one, so that could be it. I had no idea that you could compensate by moving the OB off centre, but it definately turn to stbd easier, to the point that if you let go of the wheel at a decent (20knot+) speed it'll turn itself.
As for tight turns, its the same, easier to stbd, but she doesnt lean to port, possibly as the helm is on stbd side. The anode fin is set up perfectly perpendicular to boat transom
As for tight turns, its the same, easier to stbd, but she doesnt lean to port, possibly as the helm is on stbd side. The anode fin is set up perfectly perpendicular to boat transom
MapisM
Well-Known Member
[ QUOTE ]
The anode fin is set up perfectly perpendicular to boat transom
[/ QUOTE ]See? That's definitely not correct.
Just turn the fin clockwise 20 degrees or so (looking at it from below).
After that, try again to let go the wheel at cruising speed. Should the boat still turn herself to stbd, increase the angle (or obviously decrease it, would the boat turn to port).
Finding the right position to get a neutral steering (same effort when turning port/stbd) should be a matter of 2 or 3 trials max, but I'd bet you'll get already very close with a 20° angle.
What jfm suggested remains a possibility, but that's not as easy to check/adjust properly as the fin angle, hence I'd check that first.
PS: all the above assuming that the prop is R/H, of course (turns clockwise in forward, looking at it from behind).
PPS: when I said that the prop walk effect goes the other way round, allowing tighter turns to port, I meant at very low, manuevering speed. That's even more noticeable in reverse, where you should be able to turn much tighter to port than to stbd.
The anode fin is set up perfectly perpendicular to boat transom
[/ QUOTE ]See? That's definitely not correct.
Just turn the fin clockwise 20 degrees or so (looking at it from below).
After that, try again to let go the wheel at cruising speed. Should the boat still turn herself to stbd, increase the angle (or obviously decrease it, would the boat turn to port).
Finding the right position to get a neutral steering (same effort when turning port/stbd) should be a matter of 2 or 3 trials max, but I'd bet you'll get already very close with a 20° angle.
What jfm suggested remains a possibility, but that's not as easy to check/adjust properly as the fin angle, hence I'd check that first.
PS: all the above assuming that the prop is R/H, of course (turns clockwise in forward, looking at it from behind).
PPS: when I said that the prop walk effect goes the other way round, allowing tighter turns to port, I meant at very low, manuevering speed. That's even more noticeable in reverse, where you should be able to turn much tighter to port than to stbd.
