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Old 29 June 2003, 06:21   #1
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Diesel Trim

Hi folks

Now we have a few hours on out new RIB I have noticed that trimming the outdrive has very little difference to the speed and ride of the RIB.

On our previous Ribcraft 4.8/50HP outboard the trim made a hugh difference to the speed and ride of the RIB.

I find that most of the time the difference between fully trimed in and out to the point where you start cavitating only makes 2Kts difference in speed, and does not make much difference to the ride between a smooth sea and moderate sea (not had any rough yet). The best being almost trimmed fully in.

Is this normal ?, does the trim behave the same on bigger diesel ribs as smaller outboard ones?.

Also I changed form 22" cupped props to 24" strights would this make any difference to the trim behaviour and range.

Thanks Gary
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Old 30 June 2003, 04:19   #2
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Diesel Trim

I know quite a few people running large ribs with the same engine package as yourself. They too found that the boats didn't trim as they thought they should and speed difference was minimal.
However when they fitted Doel-fins to the cavitation plates on the engines the difference was amazing. The boats have started to respond to trimming the leg and speeds have increased as well as fuel consumption dropping.
I know of one 11m rib with twin diesels using the Doel-fins and the owner says it has changed the entire handling of the boat.
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Old 30 June 2003, 05:12   #3
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Re: Diesel Trim

Quote:
Originally posted by tsunami
... when they fitted Doel-fins to the cavitation plates on the engines the difference was amazing. The boats have started to respond to trimming the leg and speeds have increased as well as fuel consumption dropping.
That sounds seriously impressive. But if it's the lift that is generated by the fins that helps speed and fuel consumption, surely the same effect would be achieved by using trim tabs instead of Doel Fins.

Or am I missing the point?
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Old 30 June 2003, 05:30   #4
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I'm a fins fan but I've not tried them on an out drive.
However Mike, fins affect the trim out as well as in, therefore they can help lift the nose too.

JW.
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Old 30 June 2003, 05:58   #5
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I also find the trim does a lot less on my Humber than it did on my previous Ribcraft with out-boards. I'm presuming, (apart from the fact the hulls are different), one reason may be the extra forward weight of the diesel engine?
Great if Doel-Fins can help here but my fear is that if they do help..... by pushing the boat up out of the water... is this good for the leg?
Maybe I'm not giving my Alpha One its due, but constant upward pressure, more than it was designed for, may at the very least give the warranty people an excuse not to pay out in the event of a claim. The leg would certainly not be my favoured lifting point by crane.... but maybe the forces are less than I imagine.... but how else would it work? My impression is that it doesnít seem so bad on an out-board for some reason!
Maybe proper trim tabs are the answer!

Any thoughts?

Mike C
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Old 30 June 2003, 07:00   #6
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Hi folks

Thanks for ther feedback.

Tend to agree with Mike, would like to know where you stand with warrenty etc before fitting fins.

Trim tabs are a option but as you say they only work in one direction. Would the extra drag of using trim tabs not slow you down or is the extra drag outweighed by the better trim of the boat.

Maybe the cupped props was better that i starte with, as I belive they help lift the bow. I always thought of diesel RIBS as stern heavy but now Mike points out while the emgine has more weight its much further forward then a outboard. it make me think again.

Plan to have a chat to Ribcraft to see what they think.

Regards Gary
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Old 30 June 2003, 07:38   #7
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Mental thoughts........ and perhaps contradicting part of my last post. What is exactly happening with a doel-fin?
Correct me if I'm wrong, but when we trim out we are pushing the force of the prop in an upward angle away from the boat. That has the effect of raising the nose, (or lowering the stern slightly). Now the oncoming rush of water will hit the hull further down the length of the boat, thus making the whole boat rise further out of the water on the plane. Less boat in the water, less drag, faster speed etc. As the speed increases and if we get the trim right and the boat is balanced, we should get level again and be an inch or two higher and happy.
So, how is the doel-fin helping? Is it causing more downward force to make the bow rise at the start of the sequence, or does it help to put the boat back onto a level plane by upward force when maximum speed is reached? Or both? Or neither? Got me thinking now!

Any Hydrodynamics experts out there?

Mike C
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Old 30 June 2003, 11:51   #8
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Quote:
Originally posted by jwalker
...Mike, fins affect the trim out as well as in, therefore they can help lift the nose too.
Of course. Thanks, JW. I'm tempted to give it a try but think I'll wait untill I'm well out of warranty.
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Old 30 June 2003, 11:59   #9
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I don't think you need worry about your warranty. The fins are made from plastic which is not too strong (I have broken one). I would hope your out-drive is a good deal stronger!

JW.

PS. go to Land&sea site and look for the outdrive extension which they make. It only fits the alpha one but it's interesting in principle anyway.

You Mercruiser 1.7 folk might be interested, and the torqueshift will fit the Alpha too.
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Old 30 June 2003, 12:04   #10
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Just a thought. Put the fins on upside down. Now that would lift the bow!
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Old 30 June 2003, 12:47   #11
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Quote:
Originally posted by jwalker
PS. go to Land&sea site and look for the outdrive extension which they make. It only fits the alpha one but it's interesting in principle anyway.

It's available for both Alpha and Bravo, and they offer a money back guarantee if it doesn't increase your top speed by 10%.

My only concern would be whether or not it was man enough to handle the extremes that you rib boys put your boats through!!
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Old 01 July 2003, 04:26   #12
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Doel-fins

Doelfins simply work by giving the boat a larger planing area aft of the transom of the boat. This in effect gives a greater trimming action due there location.
I would doubt very much if warranty would be an issue as the fins are widely accepted on the market place. If such large forces were being exerted on the leg the fins would break as they are made of plastic!
Trim tabs would do a similar job but can be very expensive and require a separate set of controls as well as screwing or bolting into the transom which isn't very desirable.
Doelfins and trim tabs simply allow a boat to perform closer to its optimum as they help the stern of the boat to run higher in the water and not dig in! This means the hull is more efficient therefore reducing fuel consumption etc..
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Old 01 July 2003, 07:03   #13
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Joe, when I trim out the leg of Magellan Alpha (which, unless the water is really flat, gives a very hard ride) the nose duly starts to rise and I have less wetted hull area.

However, before I get very far the prop starts to slip badly and I trim in until it again grips. Would Doel fins enable me to get the boat higher out of the water without slippage?
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Old 01 July 2003, 07:14   #14
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SNAP

I get the same Mike

In the trim range (upto the soft stop on the control which lifts the leg) I can only trim out about half way before I start to cavitate bad. I run mostly fully in or upto about 1/4 of the trim range out.

Spoke to Ribcraft and they said this was to be expected on a bigger diesel RIB as the trim had far less effect on the ride of the RIB than on smaller lighter outboard RIBS. However they still think I might need to play round with the props a bit.

Regards Gary
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Old 01 July 2003, 08:15   #15
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Can anyone point me to the "land and sea" site or any other which may discuss doelfins on diesel outboard legs?
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Old 01 July 2003, 08:50   #16
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Trim Cavitation

Sounds very similar to what was happening on the 11m! The boat always ran very well but on trying to trim the nose up that bit more on calmer days the props would start cavitating without having achieved any noticable difference in trim!
Now with the doel-fins fitted the boat also comes on the plane easier. What has been noticed about trimming with the fins fitted is that it takes much less trim on the leg to achieve more noticible bow trim!
I think you would find that you wouldn't have to trim the leg out as far to get the desired running angle on the boat so therefore you wouldn't be getting the cavitation.
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Old 01 July 2003, 08:53   #17
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So, you would recommend it's worth a go fitting fins on my heavy ol' 9m beast then, Joe?
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Old 01 July 2003, 09:21   #18
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Go Fit Them

They are so cheap to buy and take about 10 minutes to fit. If someone has a boat which they consider to run perfectly then it probably isn't worth fitting them.
However if there are obvious things like lack of trim or a boat running stern heavy then the answer has to be definitely fit them. You will more than likely end up with a boat which handles much better and is cheaper to run.

Just found one of there adverts in a magazine stating;

- Doel-fins improve overall performance even on well designed hulls.
- Save up to 30% on Fuel Consumption
- Faster onto the plane
- Greater stability
- Lower planing speeds
- Higher top end speeds

I say give them a go
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Old 01 July 2003, 09:26   #19
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Thanks, Joe. I'll give it a whirl. Anyone got any idea where to get the things in the UK?
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Old 02 July 2003, 15:00   #20
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Mike,

Try here: http://cgi.ebay.co.uk/ws/eBayISAPI.d...category=15263
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