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Old 10 September 2004, 10:44   #11
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Quote:
Originally Posted by Scary Des
On an aeroplane you can perform a roll by lowering a wing, as one wing drops the other raises so the plane rolls. I can see the same thing happening with these fins, especially if you were turning in the same direction as the direction of rotation of your prop. The combined effect would cause the boat to roll around the longitudinal centre of rotation.

Des
On an aircraft you only get the roll by shaping the change of the wing, which is of course irrelevant to the doel fin. Once the aircraft is actually rolling the changed flow angles actually produce a force which counteracts the roll (so called roll damping). So I haven't a clue what effect you're trying to describe here - unless you're talking about a damaged fin?

As another Suzi 90 / similar sized RIB owner I've also come to the conclusion that one of these is a good idea - it's the potential lowering of the planing speed which appeals.

Scary Des - but I've still got an open mind and would appreciate any pointers to stories describing their dnagerous effects.

[Apologies if my quoting hasn't worked correctly].

Neil. (Former Aerodynamicist).
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Old 10 September 2004, 10:47   #12
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Quote:
Originally Posted by Scary Des
No I think it all boils down to the fact that they are unpredictable in use and as such are dangerous.

If after extensive testing and sea trials an experienced helmsman finds a benefit sure use them. But most people will buy them load their rib with kit and set of for the weekend only finding a problem when they are tipped into the sea. Des
Ho Scary come of the fence B4 you fall!

Well I fitted these to my Rib some tome ago!
B4 fitting if I was chugging throw a harbor or moorings I would make such a big wake at very low speeds,I know moving people or weight for would would have helped but persuading people to move up is not always that easy!
I know get lift from the Dol-fins & less wake, slower plaining speed!
No detrimental handling what so ever as a result of fitting!
Nick
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Old 10 September 2004, 10:52   #13
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Des,

Is it really that pronounced? We've been having difficulties with stern heavy Delta again with a 4 stroke on it. Slow to plane easily knocked off it etc.

Given the American propensity to sue is the turning effect powerful enough to tip the RIB over and does this occur often.

Advice welcomed as it might save us the cost of buying them our AV plate already has holes in a schulz nozzle prop guard now demised.
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Old 10 September 2004, 11:41   #14
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Quote:
Originally Posted by Neil R
On an aircraft you only get the roll by shaping the change of the wing......
Scary Des - but I've still got an open mind and would appreciate any pointers to stories describing their dnagerous effects.

Neil. (Former Aerodynamicist).
Neil
I accept that I have described it in very basic terms and I understand that in modern aircraft this effect is designed out. Reading it again it might be clearer if said ‘during a turn’ etc. What I am trying to show without being too longwinded is that if one side lifts the other drops inducing roll and it is an effect you don’t usual get in a boat.

I can not find the article now but there was a lot said on one of the boat design websites about fins and stability problems.

In essence what was said was that the hull should produce lift and the propeller propulsion. It was also said that the transom and engine mounts were not designed for this type of loading. There was also talk about the increase in drag,( as an aerodynamic you will understand ) increasing fuel consumption.

I think that anything that introduces instability or the chance of something unexpected happening it bad new on a boat.

I still can't get away from the fact that they are not offered by o/b manufactures even as options and I think this is significant, if you can really get a major improvement without any down side why wouldn’t they offer them after all they are the people spending the most on improving performance and they should know there engines

Des
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Old 10 September 2004, 11:48   #15
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My Redbay has got fins and if you do a search back in the archives you will find postings from Andy Gee the previous owner discribing that you have to basically floor her to get her to get onto the plane now she pops up easly.
The basic problem is all the weight at one end - inboard diesel and console a little further aft then I think she could do with for balance but thats my opinion.
Another boat that had them was a knacker old dell quay that was full of water between the hulls and had been rebuilt a few times, ie not light anymore! By putting fins onto (what I think was) a 40hp she just made the plane.

I think they have there place but a new well set up boat shouldn't need them.

Rgds
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Old 10 September 2004, 11:59   #16
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Quote:
Originally Posted by Nick Hearne
Ho Scary come of the fence B4 you fall!

Well I fitted these to my Rib some tome ago!
B4 fitting if I was chugging throw a harbor or moorings I would make such a big wake at very low speeds,I know moving people or weight for would would have helped but persuading people to move up is not always that easy!
I know get lift from the Dol-fins & less wake, slower plaining speed!
No detrimental handling what so ever as a result of fitting!
Nick
Nick your problem is you have a deep v boat and expect dory performance, these fins will have no effect until you start getting sufficient speed for the boat to start to plan and I would suggest that that is more than the average 6 knots of most harbours. My boat creates an enormous wake at 6 knots so I tend to have to travel at 3 or 4.

You should ask people to move forwards because this loading the boat correctly. The weight figures given for boat are uniform loads, the boat manufacture does not expect you to put all the load at the back. I really think that the problem of over loading the rear of a boat is the real issue to overcome.
Des
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Old 10 September 2004, 12:05   #17
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I use to have a 5mtr GEMINI (great boat by the way) with a 75bhp Mariner 17" aly standard prop and Dol-fins. It was great we had huge amounts of weight on the boat, steering consol in the middle with double bench seat, aft triple seat and 140 ltrs of fuel and an A-frame. The boat was planing like it had no weight on it at all with 6 people on it too.

I also remember these old threads and there is a lot there if you care to look.

IMHO I think that they are pretty good for small engines up to 90-100 bhp. For larger engines I cannot see how they can have an effect in a positive way.
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Old 10 September 2004, 14:35   #18
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Quote:
Originally Posted by Gazza
what is the general opinion with regard to Dol-fins / Stingrays for an outboard.......( Those plastic wing type things)

What are the benifits and the problems.....

Who uses them and why.....

Is it a worthwhile investment......

Any info would be appreciated.....


Regards


Gazza
Many thanks for all the info..........

I did not realise that doel fins are such an emotive topic.....

I now feel fully informed, if not a little confused.....

Would this type of accessory invalidate the engine warranty???????


Regards

Gazza
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Old 10 September 2004, 17:37   #19
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oh dear.....

here we go again..........

des,

have a look on the literature from the stingray manufacturer.

http://www.marine-dynamics.com/StingRay.pdf

i'd be interested on your opinions on the manufacturers claims. i can catagorically say i experienced all they say happens (with the exception of eliminating chine walking, which it still did at WOT with the engine trimmed out for an extra knot or so) having had them on my avon (now sold), but also using the boat for a few months before they were fitted.

i think they must be very confident of what they claim in a country (where they sue you for coffee that's too hot), especially if they are prone to turning boats over!!!!

I also thought most boats dropped the side of the boat that is on the inside of the turn. many ribs whose tubes dont touch the water at rest, still touch the water on the inside of a turn.

i'm still not saying they are the answer for every boat, just that they make stern heavy boats perform noticabley better as said by jelly, nick, manos and myself.

would be interseted on your opinions...

regards

colin
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Old 12 September 2004, 16:25   #20
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Quote:
Originally Posted by ribnwheels
i'm still not saying they are the answer for every boat, just that they make stern heavy boats perform noticabley better as said by jelly, nick, manos and myself.

would be interseted on your opinions...

regards

colin
Exactly what I was trying to say earlier. A boat with more weight is less likely to roll than a light boat (especially if the tubes don't contact the water). If the boat's primary use is to carry weight in the stern area (dive boat etc) then they are a good idea, just remembering when the boat is not full to drive with more caution.

At the end of the day if they WERE that dangerous they would have stopped selling them years ago.

My original comment about the dangers was in reference to a boat similar to mine with a large hp engine on a light boat with the engine raised for performance. On a standard RIB with a hp up to 100hp they will make a difference. All IMHO of course.
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