getting it up and keeping it up

[Dean]

I thought that might get your attention!

I've got a 25' footer that I can get onto plane at about 12 Knots in the right conditions. I use the trim tabs to help.
Now the question, when it is planing do I return the tabs so the bow rises or do I leave them down?
I ask because I was going about 20 Knots the other day and it started to drift off course I corrected this for it to all I can describe as "dig in" and lean the way I was trying to turn.
Apart from shutting the power off, we all nearly sh*t ourselves.
I tried again and found I could alter the ride level according to where I stood at the helm seat, but again when it came to turning it frightened the life out of us.
Am I doing something wrong, is this normal, or am I just a big jessy?

Advise please.

Dean

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[Dave_Snelson]

Dean - you don't say what boat you have or what powers it, or what configuration. The tabs are their to trim your bow downward and therefore make planing easier. Also, they allow you to apply trim to correct side windage etc.

Falling to one side or another when turning sounds like you are riding with the bow too high, and therefore unstable.

Do you have more detail?

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[Colin_S]

A friend of mine has a new boat and it does exactly as you describe, including the need for clean underwear. After reading the manual and consulting the manufacturer it was resolved by trimming the tabs fully up and trimming the sterndrive up too (not fully). This lifted the bows out of the water and stopped the stern trying to overtake.
I've heard the expression 'chine walking' before and wonder if this is a 'technical' term for this experience.
Friend's boat is a Hardy 24' fisher with 200HP inboard BTW

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[Dean]

The boat is a 25' sports cruiser similar to a Bayliner. It has a 4.3l volvo petrol engine.
The tabs are Bennets and the switches are rockers that say Bow Down when pushed forward.

does that help?

Dean

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[adarcy]

Re: but what do I know?

I regret to say I think Dave_Snelson has got it the wrong way round.

You said
<<get onto plane at about 12 Knots ......use the trim tabs to help>>
that is a low speed to get onto the plane so I presume you have the tabs hard down. As you go faster, the bow SHOULD rise, if you look at all fast planing boats the entry point to the water is well back. In fact it then goes faster for the same power as it has a smaller wetted surface (= less drag).

Keeping the tabs down prevents all this and is IMHO completely wrong.

Colin_S is right, take the tabs off completly when up on the plane and then trim the leg out - not too fully as he says otherwise it then chine walks. As you trim out the boat should speed up but too far and it seems to wobble from one side to another (onto one chine, get pushed back up goes over centre and onto the other). It feels unstable but the driver notices before anyone else panics and then trim a bit in and you have a perfect setting for that speed/power.

Sorry dave, isn't that what you really do?

Anthony

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[Dave_Snelson]

Re: but what do I know?

Yes, I think chine walking was where I was coming from, but two bottles of wine on a sunny Sunday afternoon distorted my abiliity to think...

Ah, well...

Thanks for filling in my gaps Anthony

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[Dean]

Re: but what do I know?

I'll try that next time then.

But from now on I will pack extra undercrackers just in case.

Dean

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[Colin_S]

Re: but what do I know?

Sounds like problem's sorted and I now understand what chine walking is too.

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[Chris_B4]

I think adarcy has answered your question but I would say that you shouldn't bring the tabs all the way up because that will actually pull the stern down, ideally they should be left half way such that the water flows off the hull and then continues straight along the tab.


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[hlb]

I'm not a leg man but.

The reasons for trimming a boat are.

1/ Safety
2/ Speed/Economy
3/ Sideways heal.

In calm conditions you would trim for speed/ economy, so lower the tabs till best speed is achieved. (This may well be with no trim at all)

Before I go on. If you have any doubt always keep the bows high, this is the safest.

Now in a head sea the boat needs as much area in the water as posible and the bows need to cut through the waves, so trim bow down.

In a following sea there is a danger that the bows can stuff themselves into the wave in front causing the boat to cartwheel in extream circumstances. To stop this you need the bows UP.

I think you have been trying to keep the bows trimmed down to much.

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Haydn

 

[adarcy]

Re: No more than yes.....

Chris
I'm not at all sure running with the tabs fully up could suck the stern down by any appreciable amount as they would break away from the water flow but many do not go up that much anyway.

I had advised what I thought was a newbie to do things in an orderly manner ie (for his benefit) leg/s full in tabs down to get onto the plane then tabs fully up and only then trim the leg out.

Therefore if you leave the tabs half-down, as you suggest, they will come back into play. Then when the bow is too high does he take up the tabs or put the leg down/in a bit - which to juggle with at the same time as steering and throttle. I respectfully suggest my plan and then, finally, maybe a bit of tab one side if needed for side to side trim. Also I have found that until people know a boat well (and even then not sometimes) they are unsure where the tabs are so I think there is something to say for fully up and then go from there.

All IMHO

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[hlb]

Re: No more than yes.....

The tab dont suck the stern down. The boat should (If designed right) have a natural tendancy to ride with the bows up with no trim which is safest and should be near max performance. Course on a small boat extra weight, people etc will make alot of difference.

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Haydn

 

[ArthurWood]

As others have indicated, the reason the boat became unstable was because you had the tabs too far down for the sea conditions and speed. I would think that you might try starting off with your tabs fully up and the trim (outdrive) tucked fully in and see if it gets on plane at about 18mph. If not, try some trim tab, but once on plane, put the trim tabs up and gradually trim out the drive till you get to max speed for that throttle setting. The faster you go, the more you raise the drive. At max speed, the objective is to have only the prop in the water!!!!If you raise it too high on a flat sea, the boat will begin to porpoise which indicates it's trimmed out too far. If you have an uneven, side-to-side load, use a light touch of either trim tab to level the boat. Practice in different sea conditions, preferably without nervous guests/forums/images/icons/smile.gif

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