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Old 29 June 2003, 07:21   #1
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Country: UK - England
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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, 05:19   #2
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Country: Ireland
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Boat name: Tsunami/Saol Eile
Make: Excalibur Ribs
Length: 9m +
Engine: 350hp Volvo Diesel
Join Date: Apr 2001
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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, 06:12   #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, 06:30   #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, 06:58   #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 doesn’t 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, 08:00   #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, 08:38   #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, 12:51   #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, 12:59   #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, 13:04   #10
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Just a thought. Put the fins on upside down. Now that would lift the bow!
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