Bow Up/Down??

[CaptainCaveman]

Hi,

May be a ridiculous question but what does bow up/down switches do, I know they are related to the trib tabs but does bow up = trim tabs up and bow down = trim tabs down? also I have the bow up/down switches but not sure when the tabs are fully up or down as there is no indication beep etc, when do you usually know they are fully up or down?

thanks
CC

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[hlb]

Hmm. Trim tab buttons are wired a bit back to front, depending on your point of view. Right bow down button moves the left trim tab down and vicky versa. Best to watch from the pontoon whilst some one presses the button, and time how long it takes. Normally start with bows up, get to revs you want. Then put the trims down for say 3 seconds. See what happens to speed, then try another 3 seconds and another. You will find the best setting for your boat. Course this might change acording to weight or sea conditions.
In rough conditions. Trim the bow down heading into sea, to put more boat length in the water. With following sea it's important to have the bows up, or might cartwheel!!

<hr width=100% size=1> <font color=blue>No one can force me to come here.<font color=red> I'm a volunteer!!.<font color=blue>

Haydn

 

[Solitaire]

Always an interesting debate this one. The effects of trim tabs are very much based on how your boat is loaded. The easiest way to check how your switches work is to get someone to operate the switches in each direction and look over the stern and see the results. So if you push to bow down switches, you will see the effect and vice versa with bow up. Then swop places with whoever is operating the switches and you operate the swictches and get the observer to tell you of the position of the tabs as you push them, this way you will get a "feel" of how much the tabs move as you push. Now when you are moving on the water of course you can't see this so you have to gauge it be "feel" as to how the boat reacts to the change. If you have the two black switches side by side on the dash you will know the position for when the tabs are down or up. On my boat with, the tabs are forced down by pushing the top part of the swictch and up by pushing the bottom part. As a rule, I normally have the tabs down when getting on the plane (they act like an aircrfat wing and will give you more lift on the stern) as the boat gets up I'll adjust each side to level the boat out. When you first do this, the boat will have an interesting attitude, a heavy list to one side or another! Now adjust each tab by briging the tab up and watch/feel how the boat attitude changes. Because you started with the tabs down you know that you will be bringing them up and will observe how the boat levels out by the amount of adjustment to each side. Once the boat is running level I will then adjust the leg trim. So to answer your question is by touch and feel for what is right for your boat, based on number of people/load, sea state wind direction etc. Hope that helps/forums/images/icons/smile.gif

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[jimh]

that's an interesting technique - is this a common practise among forumites for sterndrives? I normally start off with my tabs fully raised, leg in, and then trim the leg out once on the plane to give the bow the right 'attitude'. I then level the boat (port/stbd) by lowering a single tab (which is a constant exercise with kids keep moving about) ??

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[whisper]

I do as per Solitaire. At displacement speeds tabs down and leg at -2 or -3 until on the plane. Once on the plane I tend to run in flat seas with leg at -1 and tabs about level. In head seas tabs down and leg at -2, in a decent sea from astern leg +1 to +2 and tabs up. Seems to work for us but every boat is different.

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[MedDreamer]

Yes that's the way I do it to. My understanding is that I should trim the leg to get the right "attitude" for the boat overall and only use the tabs for port/starboard levelling (to correct boat loading). In fact I have a warning sticker on the dash telling me that the tabs should ONLY be used for this - the implication being that to use them any other way could be dangerous.

Clarification from the old salts on here of correct and safe usage would be very welcome. Am I missing a trick in the way I use them or are others taking risks?What is the correct planing angle for a 30 foot cruiser - In flat conditions should it feel level or should the bow be perceptiby pointing up?

Also what effect does the use of the tabs have on fuel consumption, presumably if they are down they significantly increase the "drag" through the water?

<hr width=100% size=1>Martyn

 

[hlb]

Correct is whenever the boat is going fastest at given revs. Less of course your trimming for sea conditions.

<hr width=100% size=1> <font color=blue>No one can force me to come here.<font color=red> I'm a volunteer!!.<font color=blue>

Haydn

 

[Observer]

Trim tabs

1. Outdrive/outboard leg trim is to help the boat get on the plane and to achieve optimum attitude in cruise. So, trim in/down (legs in) to get planing and then trim out/up to achieve best boat speed for any given throttle setting. Most/all planing hulls will naturally have some bow up attitude when planing but if this is excessive, it's inefficient (more of the aft section is being pushed through the water rather than riding on it) and possibly dangerous (poor visibility etc). There is no "correct" planing angle. Depends on the hull, sea conditions, speed etc. The absolute indicator of best trim is best boat speed for the given throtlle setting, and subject to sea condition.

2. As mentioned, one should also trim in/down (bow down) in a following sea and trim out/up (bow up) in a head sea. This is because, in a following sea, with a big wave coming up behind the boat, you want some bow down trim to reduce tendency for wave to push the stern down (and therefore bow up and over). (This assumes the waves are going faster than the boat). In a following sea, it's best to be either slower or faster than the wave speed but NOT the same speed. Unless you have a very powerful boat, a bit slower is better because you then have power in reserve to straighten the boat if the stern gets pushed round.

Conversely, in a head sea, you ride up an approaching wave and go down into the trough, so you want some bow up to reduce tendency to stuff the bow at the bottom of the trough. Of course, in a heavy sea, you should also use the throttle to power up the oncoming wave and then ease back at the top of the wave so you don't fly off it.

3. If you have outdrive trim and trim tabs, leg trim should be used primarily for attitude correction and trim tabs for lateral correction. In a cross wind a planing boat will tend to lean into the wind and this can be corrected by adjusting trim tab (usually trim down = bow up on the same side as the heel - but this depends on how the switches are wired). Can also be used to correct heel/list caused by unequal loading.

4. If you have shaft drive, trim tabs will be used for attitude correction and lateral correction. You should always start with trim tabs in the fully up position (bow down). Then, at desired throttle setting, adjust both tabs simultaneously in small increments until maximum boat speed is achieved. Then (if necessary) apply a little more down tab on one (or a little down on one and a little up on the other) to correct wind/load induced heel. Always trim up (retract trim tabs) fully when leaving boat so that rams are not exposed to fouling which can make them jam.

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[MedDreamer]

Re: Trim tabs

Thanks for such a full and detailed answer.

I'm a bit confused about how to use in head/following seas though. I thought that in a following sea I should have the bows up so that it reduces dip in the trough of the wave and in head sea I should have the bows down to cut the wave? I think this is also what Haydn is saying in his post?

Which is correct?

<hr width=100% size=1>Martyn

 

[hlb]

Re: Trim tabs

As I said. Bows up in following seas. It's very important. If in any doubt always trim the bows up. It's safest. Trim the nose down if heading into the sea. Stick the sharp bit into the wave and lengthen the hull in water.

<hr width=100% size=1> <font color=blue>No one can force me to come here.<font color=red> I'm a volunteer!!.<font color=blue>

Haydn

 

[gjgm]

well, i m also a bit confused by observer comments. Id agree with hlb.Into a headsea, you need bow down somewhat and power into the wave to drive the bow up the wave face, easing off before the crest. In a following sea, a raised bow will keep the bow at a better angle riding down the wave face you ve just passed. But maybe that applies more to smaller powerful sportsboats? Only once got it wrong into Chichester wind over tide, and I wont make that mistake ever again.

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[hlb]

Observer has got his knickers in a twist. The important thing is to have your bows high when riding down a wave, so as not to stuff the bow into the wave in front, this could cause you to broach. Which means that the front of the boat stops, leaving the back end to either come round sharply or in worst case, cart wheel. If you broach, you find yourself stopped, beam on to sea. Then the next wave rolls you over. Here endeth the leason.!!

<hr width=100% size=1> <font color=blue>No one can force me to come here.<font color=red> I'm a volunteer!!.<font color=blue>

Haydn

 

[Observer]

Re: Trim tabs

Yes, Haydn's right and I'm not. Too much flying from the seat of my desk. Sorry for bad advice.



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