Power Trim

Derrick · 28 November 2007, 21:38

http://www.ryatraining.org/NR/rdonly...gpowertrim.pdf

I found this advice on the RYA site. May I request a debate/dialogue amongst all the experienced guys and gals on here? The advice is to trim the bow down in a head sea and up in a following sea. Up in a following sea seems to make sense to avoid "burying" in the back of the next wave. Down when approaching the steep face of an oncoming wave sounds a bit tricky. But maybe this works just fine if you know what you are doing. What does the team think and what do you do? I am not talking "perfect storm conditions" here just nice regular rows of waves 4-6ft within the capabilities of the boat and not ugly confused chop where all bets are usually off and a comfortable ride unlikely. Could be there are other issues being considered like prop ventilation and loss of grip on the water. ??????

Pete7 · 28 November 2007, 22:08

Worth having a read of this thread, we get to the right conclusion in the end. However in addition to trim you also need to use the throttle, at the right time and with care or you will go flying off a wave.

http://rib.net/forum/showthread.php?...ght=submarines

Pete

Cookee · 29 November 2007, 11:34

Derrick - In my opinion the broad RYA advice is spot on, and as usual Pete 7 is as well concerning the throttle!

Pablo · 29 November 2007, 12:10

I think a good way to learn the benefits of different trim positions is to drive a boat with a manual trim. You spend alot of time trying to find the best compromise for different situations.

J S · 29 November 2007, 12:24

Quote:

Originally Posted by Cookee

Derrick - In my opinion the broad RYA advice is spot on, and as usual Pete 7 is as well concerning the throttle!

I agree but i suppose what should be said that some boats have different transom angles to some can trim in more than others forcing the bow into the water more.

James

Nos4r2 · 30 November 2007, 09:06

Quote:

Originally Posted by Cookee

Derrick - In my opinion the broad RYA advice is spot on, and as usual Pete 7 is as well concerning the throttle!

Yep.

Personally I find it equates to riding a trailbike fast over fairly rough ground where you'd use your bodyweight to move the C of G as trim, and the throttle in much the same way as on the trailbike. (note-I never did do the fancy 'landing on the front wheel' thing that motoX riders do and it wouldn't work on a boat-I've got 1st hand experience of it

)

It's quite easy to demonstrate on a 4m boat as you're less than 1/2 of most wavelengths.

Nos4r2 · 30 November 2007, 09:16

Quote:

Originally Posted by J S

I agree but i suppose what should be said that some boats have different transom angles to some can trim in more than others forcing the bow into the water more.

James

That's what you use the trim limit pin for.

IMO You shouldn't be able to have the engine trimmed in so far as to have the engine 'leaning backwards' in relation to the keel.

J S · 30 November 2007, 16:26

Quote:

Originally Posted by Nos4r2

That's what you use the trim limit pin for.

IMO You shouldn't be able to have the engine trimmed in so far as to have the engine 'leaning backwards' in relation to the keel.

Yes i agree with that. But if someone gets into a boat that isn't set up correctly and thinks its correct to trim all the way down on a certain type of wave and doesn't have that much experience. It could go wrong.

James

Derrick · 01 December 2007, 19:20

Many thanks to all respondents! I have read right through the thread highlighted by Pete7. I had hoped to attract attention to this question from more of the highly qualified RYA instructors and trainers on the forum.

I can follow the advice to trim up in a following sea this makes good sense and with judicial use of throttles hope not to submarine. What I find more difficult to rationalise is the advice to trim the bow down in a head sea. It may seem that this could set the bow at a disadvantage in two ways,

1......it may tend not to rise quite so readily

2......raising the bow would present the sharp edge of the V section to assist in parting the wall of water ahead.

I appreciate that judicious throttle would be needed too.

Whilst I don't doubt the wisdom of the RYA I would also welcome a commentary on why they give this advice. I am particularly hoping to hear from the RYA folk on here who are probably very familiar with the rationale.

Many thanks in advance.

martini · 01 December 2007, 19:47

Quote:

Originally Posted by Derrick

I can follow the advice to trim up in a following sea this makes good sense and with judicial use of throttles hope not to submarine. What I find more difficult to rationalise is the advice to trim the bow down in a head sea. It may seem that this could set the bow at a disadvantage in two ways,

1......it may tend not to rise quite so readily

Derrick, if you think about the weight distribution on most ribs and other planing powerboats, this is never going to be a problem in a head sea. In reality your biggest problem is keeping the bow from pointing at the stars, hence the need to trim down.
With ribs in particular having a tube full of air round the bow giving maybe a ton or so of buoyancy, it would be very unlikely that the bow would try and go through a wave rather than over it.

Chris · 01 December 2007, 23:18

Derek,

In my limited experience I have found that in a head see you are much better off trimmed in for the opposite of the two reasons you have highlighted below.

1......it may tend not to rise quite so readily
Actually the main problem I have found is that it does rise and far too easily as you go off the top of the wave if you are trimmed out the RIB is more likely to come off the wave very nose high. I have never found my RIB to feel remotely like its going to stuff when going into a head see trimmed in.

2......raising the bow would present the sharp edge of the V section to assist in parting the wall of water ahead.

No quite the opposite in my opinion the sharper bit is where the bow sweeps up to meet the tubes, the more you raise the boat the further back its going to ride and the shallower the VEE becomes. I find the ride much smoother in choppy conditions when trimmed in.

Clearly different RIBS have different attributes however with mine this is certainly what I have found.

Chris

David G · 02 December 2007, 10:25

This also used to be a conundrum to me!

I resolved it by realising that I was not visualising a BIG ENOUGH sea.

If you think of a really big sea compared to your RIB or what you have currently experienced where you really are having to drive up and down the 'fronts and backs' of the waves then I think the recommendations show their worth.

Regards,

Orve

Paul Glatzel · 02 December 2007, 17:16

Derrick

Hi, I think the answers to the question that you raise have really been posted by others but I thought i'd comment given you were keen on a RYA Instructor commenting.

Rational for trim down into a head sea is that this keeps the bow down and thus better uses the V of the bow to cut into the wave and through the top part of it hopefully keeping the bow further down (reducing how much it goes skywards) than it otherwise would be if the nose were trimmed up. As others rightly fully comment this goes hand in hand with use of the throttle to position the boat/bow appropriately to take the approaching wave.

Some outdrive legs on motor crusiers (ours included) have a zero position for leg trim that is not fully trimmed in. This is the optimum position for trim but you can go to negative trim if you so wish. Some larger RIBs can achieve the same effect as they have trim tabs which they can use to push the nose further down.

In my experience each boat is slightly different and you just have to experiment to see how best to set your boat up for the conditions you are facing.

If you are interested in reading more than is covered in the article/RYAPowerboat Handbook then i'd recommned Fast Boats & Rough Seas by Dag Pike (or his newer book Fast Boat Seamanship)

Regards

Paul