Neutral Trim

[brucehawsker]

On the principle you never stop learning, when do people use neutral trim?

In the Solent I am either trimmed right down to cut through the chop or ride almost above it in effectively what feels like a horizontal attitude.

Or I am in relatively calm conditions trimmed out to just before cavitation to obtain top speed.

Or I am facing a monster sea (to me!)and it's following me and I trim out to perhaps three quarters of the way between the former and the latter to try to avoid stuffing.

But my normal use is full in or full out.

I read in Yamaha blurb that a boat runs best (= most efficiently?) in neutral trim, is half way between full in and full out.

Have I missed any tricks here? How do people use trim? Do you use that 50% position, and if so when and why?

[Bushrider]

I usually trim manually to around 50% and trim up or down depending on conditions .
My G2 250 has also auto trim function which sets the best possible trim for high efficiency .
In my case it sets the trim to slightly below 50%

[jwalker]

Bruce, there's not going to be a conclusive answer to your question because different hulls respond very differently.
My Humber responds sensitively to trim and full in I only use for rising onto the plane. If I trim up, way before ventilation occurs, the boat slows noticeably and feels poor in the water. When the drive is trimmed correctly the boat feels good.

Now, I once drove the Jackeens' boat in the Solent, I believe it's the same hull as yours, it was notable that engine trim made little difference to the way the boat travelled. There obviously was a difference but after my Humber it felt very insensitive to trim adjustment. I'm not sure whether this is a good or a bad thing.
My understanding is that your hull is based on a Phantom hull and that was designed primarily for fast sport travel rather than deep sea work. I guess there would have been very different design parameters from a traditional rib hull.

Having your engine lowish may be to your advantage because you'll have more leverage on the hull which will improve trim response.

Trim it to where it feels best for the conditions you find yourself in.

IMHO, of course.

[Fender]

Once your on the plane, trim out till you feel the boat go light and you'll also notice the engine revs rise slightly, the boat correctly trimmed has less drag and consequently unloads the engine - that's pretty much the boats sweet spot, adjust in small increments from there to fine tune. I was on a long run the other day, tweaking the trim and boats tabs to get her running at the best efficiency, its amazing how much extra speed you get from just playing with the boats altitude and leaving the throttle alone.

[Poly]

Bruce - is your transom vertical? Neutral presumably isn’t the same on all hulls as the transom rake varies?

If it was quite as easy as your Yamaha quote suggests there would be little demand for power trim - simply set the angle up and leave it there like people do on small boats/engines without PTT.

[Dan Gurney]

Fwiw, in my opinion it's not an exact science. I trim
it by feel mostly. And also fuel burn vs speed. Knots vs lph: looking for at or below 1litre per nautical mile. Once up on the plane, as I trim out the figure gets better and better then it stops and gets worse again. When there is a bit of a chop I'll trim it in gradually to try and keep the boat 'flying straight' as it were: trying to avoid the feeling that the bow is scooting skyward of every wave.
The difference can be quite subtle but you will get to notice it. My boat is a Redbay 6.1 and the engine is mounted quite low in the water (Redbay tend to do this for seakeeping as opposed to top speed) so when the weather is suitable it really needs a good dose of trim! The Humber Attaque I had previously was much lighter and had a less pronounced V. It would react much more noticeably to trim changes,

[brucehawsker]

Thanks guys. You are right. There seems little difference between fully trimmed in and neutral trimmed (ie cav plate horizontal). Just slightly more attack into the chop when fully in.

Going out beyond neutral there is a huge effect and I use quite well trimmed out to get to the high speed sweet spot when the sea permits it.

Yes it's a Phantom hull.

Putting the Suzuki 150 on it makes it feel very different. I think when the prop is raised to an inch or more above the bottom of the transom it will be different again..... Probably going to an L engine.

[charliee]

Hi Bruce, glad to read you’re probably going to an L engine. I positive it’s the right thing to do and you won’t regret it.

[jwalker]

Originally Posted by brucehawsker

....
Yes it's a Phantom hull.

Putting the Suzuki 150 on it makes it feel very different. I think when the prop is raised to an inch or more above the bottom of the transom it will be different again..... Probably going to an L engine.

Contrary to popular opinion, I don't think you notice much difference if you keep the same prop.
You're using a sports boat hull and driving it with a 16" dia prop. That's work boat diameter and it's more than likely a general purpose characteristic too. Typically for your application you'd be looking for about 14" possibly slightly less.

You've also given us other clues in that you're trimming out to almost ventilation level to get the bow high for maximum speed. That indicates you would be better using a prop with some bow lifting tendency, you could then use a trim level to give best propulsion, also, if you're habitually needing to keep the prop high in the water a prop which can tolerate a little surface piercing without loosing grip may be an advantage.

It's difficult to recognise prop characteristics by just looking at the blades but slightly swept back blades with rounded ears which look kinda spoon-shaped is an indication that it may be worth trying.

A little info from the past; I ran a Suzuki motor for some time using a non-suzuki prop. The shaft diameter, the splines and the taper were all ok BUT, the position of the taper on the shaft was 6mm forward of the normal position, this meant the hub of the prop contacted the rear of the gearcase before the prop taper seated on the shaft. It was easily remedied by making a 6mm thick spacer which fitted onto the shaft before the prop went on. This may not now be the case but it's worth checking before you lay out money for a prop. Are you in touch with Nasher? He did a similar modification and also has, or had, a lathe so could be helpful to you if necessary. Finally, I may say that the standard Suzuki prop supplied with my engine looked the part but gave a dreadful performance and a harsh ride.
Anyway, tread wearily, you're getting lots of opinions, filter advice carefully, only you know what you are feeling driving your boat. I suspect you may not achieve the same level of performance using a 4 stroke motor as you enjoyed with your 2 stroke engine. Just the way of it - shoulda but 200 horses instead of 150.

[Last Tango]

Originally Posted by charliee

Hi Bruce, glad to read you’re probably going to an L engine. I positive it’s the right thing to do and you won’t regret it.

+1
Once you've got the right engine on there you'll probably find there's no need to mess about with props for anything other than "fine tunning" to suit a particular application.

[boristhebold]

uuum, never really thought about it but here goes,

1. trimmed fully down when crusing along poole quay or holes bay (less than 6 knots) to keep wash etc down. And approaching anchored boats etc.

2. never trimmed fully up at high speed as it feels a tad light.

3. in a following sea probably trimmed the most up to avoid waves breaking over bow, low to medium speed.

4. Normal cruising in normal conditions speed between 20 and 33 knots probably trimmed up about 50% to 60%

it kind of depends on conditions alot but never out to extent anything feels twitchy or too light.