OD leg position when getting on and staying on the plane

[simonfraser]

according to the RYA link below the leg position is used to trim the boat for planing

and the trim tabs for dealing with waves

https://www.rya.org.uk/newsevents/e...d/Pages/top-tips-boat-trim-and-trim-tabs.aspx

from this i see that i have likely been leaving the leg in too deep, minus trim, once on the plane on still water

if the leg is set too high, less minus, the prop may start to loose grip and that would allow for max revs but surely the boat will then fall off the plane too ?

 

[[2068]]

I tend to keep the drives trimmed in all the way when stopped, to minimise fouling on the rams.

On startup, I set to "0" (line of thrust approximately in line with the water line), and keep them there. But, all boats are different. At a cruising speed you can try different positions for drive trim and a little tab to find the optimum speed for a given throttle position. Being too trimmed down in a large following sea isn't a good idea, either.

If you are a little under-powered, then trimming in before taking off does help. If you have plenty of power, then no need.

If going for a "max speed test run" then you can experiment with trimming out, try +5 or so and vary it in both directions to see how the speed varies. As you say, there will come a point where your boat turns into a static generator of sea bubbles, but I think you'll know when that's about to happen.

.

 

[MapisM]

But, all boats are different.

Spot on. There are only three ways to find out the best position: test, test, test.

That said, with respect, I partially disagree with RYA.
The general principle is that the max efficiency of outdrives (and outboards) powered boats at cruising speed should be achieved with no tabs and leg(s) trimmed out just enough to be perfectly horizontal.
That's just a starting point, though: from there, keeping constant throttles, you can play with both trim and tabs a bit and see what happens.
Trouble is, the optimal point (i.e. the one where you reach the higher speed) varies with speed and boat load, so it's not like once you've found the ideal combination you can stick with it forever...

 

[CLB]

Spot on. There are only three ways to find out the best position: test, test, test.

That said, with respect, I partially disagree with RYA.
The general principle is that the max efficiency of outdrives (and outboards) powered boats at cruising speed should be achieved with no tabs and leg(s) trimmed out just enough to be perfectly horizontal.
That's just a starting point, though: from there, keeping constant throttles, you can play with both trim and tabs a bit and see what happens.
Trouble is, the optimal point (i.e. the one where you reach the higher speed) varies with speed and boat load, so it's not like once you've found the ideal combination you can stick with it forever...

That's the key. Adjust trim to the conditions for optimum performance and comfort. However, to sum up trim in one sentence as best as possible, the RYA statement is about as close as you can get to good general advice.

 

[julians]

They say this at the end of that article

"Tip: If you have trim tabs and the ability to trim the drive leg then try to keep it simple. Use power trim for fore and aft trim and trim tabs for side to side trim."

Ime The only way to know for sure for your boat is to experiment and see what works best, but there's no one drive trim position that works for all speeds and sea conditions.

And with that in mind on my boat I don't entirely agree with the quote from the rya above, they are correct to say using trim tabs is the only way to correct for side to side imbalances, the drive is no help here. but I also find that trim tabs can get the bow much lower than using drive trim alone. It makes a big difference when heading directly into waves, ie enables you to really get the pointy bit of the bow slicing the through waves,ot makes an enormous difference to ride Comfort.

 

[Momac]

I tend to keep the drives trimmed in all the way when stopped, to minimise fouling on the rams.

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That's my plan too but sometimes I forget to lower the legs.

 

[Greg2]

according to the RYA link below the leg position is used to trim the boat for planing

and the trim tabs for dealing with waves

https://www.rya.org.uk/newsevents/e...d/Pages/top-tips-boat-trim-and-trim-tabs.aspx

from this i see that i have likely been leaving the leg in too deep, minus trim, once on the plane on still water

if the leg is set too high, less minus, the prop may start to loose grip and that would allow for max revs but surely the boat will then fall off the plane too ?

Still experimenting with our S34 but so far leaving the legs trimmed fully in at -5 seems to be best for speed. It was the same with our Targa 35.

 

[petem]

Still experimenting with our S34 but so far leaving the legs trimmed fully in at -5 seems to be best for speed. It was the same with our Targa 35.

That's interesting as my observation is that might boat goes quickest with legs trimmed fully in (-4 I think). However, logic would suggest that with the slight bow up AOA that my boat has, trimming the legs to +2 or so would create thrust that is parallel to the surface.

 

[MapisM]

Logic indeed suggests that, P.
But if the hull makes less resistance when running flatter on the water, it is possible that the efficiency loss on the outdrives is more than compensated by the hull efficiency gain.
Did you ever try to keep the legs higher and lower the trim tabs a bit to keep the bow down?
The net result might be even better, but even if it would be comparable, I'd rather cruise with legs as close to neutral as possible (to avoid stressing the UJ) and let the flaps keep the bow down.

 

[simonfraser]

'I'd rather cruise with legs as close to neutral as possible (to avoid stressing the UJ) and let the flaps keep the bow down'

+1

 

[mlines]

For me its all done by feel and hearing. Once on the plane you trim the leg upwards. You can feel the boat push forward faster and the revs come up as the engine does less work. There is a point at which this effect stops. It achieves a worthwhile speed increase and a worthwhile fuel efficiency saving

 

[Greg2]

For me its all done by feel and hearing. Once on the plane you trim the leg upwards. You can feel the boat push forward faster and the revs come up as the engine does less work. There is a point at which this effect stops. It achieves a worthwhile speed increase and a worthwhile fuel efficiency saving

I agree with that but wonder if it varies depending upon hull design, engine capacity and size of boat? What you describe doesn’t appear to happen on our S34 and certainly didn’t on our Targa 35. No discernible change to speed or revs until the legs get towards 0 when it starts to sound not so good and speed starts to drop, leading me to conclude that leaving them fully trimmed in is the best option. Still learning with the S34 so may change my view but when we bought the Targa the previous owners advised that trimming the legs out had no discernible effect so they always left them in - over five seasons I experimented but the conclusion remained the same - leave them trimmed in!

 

[jrudge]

I have had 2 outdrive boats. Cranchi Endurance 33 and a Targa 40.

The 33 was very twitchy, and if you trimmed up the drives you had to be careful as the boat was likely to chine ride - which when it happened if going fast was very frightening.

The 40 I used to start on -5 ish and if I was just going say 20 kts then from memory it did not really make much odds where the drives were. The main impact is if you were really going for it then raising the drives would make an appreciable difference to speed, a better ride and the engine note would fall.

On a person front I did not like to run the drives are significantly different trim levels. They had a tie bar so to me keeping them somewhat Level was logical if not required.

Whilst it will depend on the boat and its size I nearly always use tabs in unison to raise / lower the bow. I have every now and then used a bit to level but it is very rare and then the sea state changes or you change course and you correct back - so from a personal perspective tabs to add ( or remove) list tend to be more hassle then they are worth.

Finally we had a Squadron 65 and tabs made no difference to it at all. On the S58 if going rapidly ( which I rarely do) a small bow down trim does seem to help.

So what can be concluded? I suspect that all boats are different and try it and see!

 

[BruceK]

I agree with that but wonder if it varies depending upon hull design, engine capacity and size of boat? What you describe doesn’t appear to happen on our S34 and certainly didn’t on our Targa 35. No discernible change to speed or revs until the legs get towards 0 when it starts to sound not so good and speed starts to drop, leading me to conclude that leaving them fully trimmed in is the best option. Still learning with the S34 so may change my view but when we bought the Targa the previous owners advised that trimming the legs out had no discernible effect so they always left them in - over five seasons I experimented but the conclusion remained the same - leave them trimmed in!

ditto, no appreciable difference on mine in either speed, bow rise or engine rev . Trim tabs conversely make a big difference in both bow rise and lateral trim. I have played aboutseting tabs to hold bow rise down and adjusting legs, still no difference. Now I just leave them fully in unless skirting some very shallow sandbanks at LW around the harbour, gingerly, with the legs up as high as the UJ will tolerate. But that's it.

 

[MapisM]

Now I just leave them fully in unless skirting some very shallow sandbanks at LW around the harbour, gingerly, with the legs up as high as the UJ will tolerate. But that's it.

Fairenuff, but don't forget that constant use of the legs fully tucked in is as bad for the UJs as legs fully out.

 

[MapisM]

So what can be concluded? I suspect that all boats are different and try it and see!

Spot on.
By and large, the more extreme and small the boat, the more sensitive it is to all regulations - and what you are reporting pretty much confirms that.
With my Fountain, fully lowering both tabs and the leg was essential for getting up on the plane.
And even at LOW cruising speed (i.e. 40kts or so!) it was actually possible to steer the boat using the trim tabs alone...

 

[BruceK]

Fairenuff, but don't forget that constant use of the legs fully tucked in is as bad for the UJs as legs fully out.

You say this and I can believe it. It's on a scale of bad that makes little to no difference though. Even though my legs have been fully reconditioned by the dealer, the UJs in it are still 20 plus years old and quite honestly both in perfect condition and I dont believe I am trimming it any different to what any previous owner would have done.

EDIT. I think steering places much more impact on the UJ than any trim setting (bar trailer mode obviously)

 

[Switch]

My old Sealine S42 would always run better with the outdrives trimmed at -4 I tried many different configurations but that's what she most most happy with.

 

[MapisM]

It's on a scale of bad that makes little to no difference though.

Point taken.
I mentioned the "in=out" in terms of stress as a point in principle, more than anything else.
I also agree that steering can be more stressful than lowering/raising, but obviously the time spent by the legs steering is neither here nor there, when compared to cruising time. Besides, obviously nobody would ever steer hard over at WOT.
Another relevant factor is the power/torque: if behind an outdrive I'd have a high torque diesel like the VP D6, I'd handle it with care at all times!

 

[petem]

Besides, obviously nobody would ever steer hard over at WOT.

Hhhhmmm.