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Old 07 April 2008, 09:46   #1
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Doel Fins or Lifters??

Has anybody any experience comparing Doel Fins and large trim tabs like Lifters (from Maxi-Marine)? I've now got my little 2.9m Ribtec tender and 15hp two-stroke outboard and am very impressed with its performance, but it does seem to be all or nothing. To get on the plane you've got to open the throttle almost wide open, then it accelerates like mad and goes very well. Two up it will plane, three up it really struggles, as the bow rise is massive and its pushing a huge wave of water.
I have the rib on an inland waterway where there's a speed limit of 10kts but going flat out draws attention to yourself and is likely to get you fined. I'd like to be able to plane below this speed if at all possible on a reduced throttle, and think that either hydrofoils or the lifters are the way forward, especially as they should help improve load carrying ability.

I'd be happy with either solution, think i'd prefer the fins as you don't have to mess around removing them for the wheels to go down, but if the lifters would perform better then I'd fit those.
I'd really like to try both out, but don't fancy drilling holes in the cavitation plate and the transom, only to have to fix one set of them later!!

Any thoughts please??
thanks, NIck.
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Old 07 April 2008, 14:30   #2
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a few years ago i had two smaller ribs one with 25 hpfitted tabs on the cavitation plates made a huge diference in poping the rib up onto the plane i would recomend this is the way to go
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Old 07 April 2008, 17:07   #3
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Try a tiller extension to bring the CG forward when getting on the plane. My mates similar sized boat would plane 2 up with a 6hp. Just.
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Old 07 April 2008, 17:38   #4
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Iam about to give some smart tabs a try . I have searched lots for results for them and not really seen any negatives , look out here they come
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Old 08 April 2008, 07:17   #5
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Peter, I take it you mean fins rather than tabs? Tabs mount on the transom, fins on the outboard... Thanks for the recommendation anyway.

Ian, you seem to have decided that smart tabs are the way to go. Are you fitting them on your main rib or a smaller one? There seems to be a consensus that on larger boats the trim tabs are a better bet than hydrofoils.
Hydrofoils do seem popular on smaller outboards though.

As holes in glassfibre are much easier to fix than holes in the cavitation plate of an outboard though I think I might try the lifters first and see how I get on.

Just a pity that you can't get a decent clamp-on hydrofoil so that you don't have to drill holes in the outboard. Or can you???

Nick
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Old 08 April 2008, 09:40   #6
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I have fitted them to my last two boats & they work very well & are a cheap alternative to trim tabs!
I have them on my alpha 1 drive with a 4.3Lt 220 hp & B4 I fitted them going under plaining speed the bow would be up in the sky, also with say 10 people on board all sitting around the stern it was one hell of a job to get it on the plain! (I know I could ask them to move forward)
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Old 08 April 2008, 15:56   #7
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Peter, I take it you mean fins rather than tabs? Tabs mount on the transom, fins on the outboard... Thanks for the recommendation anyway.

Ian, you seem to have decided that smart tabs are the way to go. Are you fitting them on your main rib or a smaller one? There seems to be a consensus that on larger boats the trim tabs are a better bet than hydrofoils.
Hydrofoils do seem popular on smaller outboards though.

As holes in glassfibre are much easier to fix than holes in the cavitation plate of an outboard though I think I might try the lifters first and see how I get on.

Just a pity that you can't get a decent clamp-on hydrofoil so that you don't have to drill holes in the outboard. Or can you???

Nick
yes I have fitted them to my main boat but have yet to try them out , its not a rib , but that makes no difference for this topic .
Take a look at smart tabs website , I know its thier own advertising but it does give a good comparison between a doel fin and a trim tab .
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Old 08 April 2008, 16:31   #8
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Thanks ian, I'll have a look. The more I look at it the more I think that the lifters are the way to go. It would be nice to do some comparitive testing, but getting any meaningful results must be dificult otherwise you'd have thought the manufacturers would already have done it for their advertising literature.
Nick
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Old 09 April 2008, 06:01   #9
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I've had Doel Fins on a 4.7 Avon for years. They work well and as I've posted before, allow a very convenient platform to use when re-boarding the boat, from the water. We spend a lot of time in the water here which may not be such a pressing issue where there are ice cubes floating around the boat!
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Old 10 April 2008, 05:41   #10
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Personally, I've removed doel fins from two boats I bought second hand. The drag took 2 & 4 knots off the top speed, and I didn't like the handling. Also drilling to fit them will kill the paint / corrosion warranty if it's new. Mind you if the speed limit is 10 Knots it may be the way to go as Vmax will be irrelevant.

Have you tried playing with the engine trim? You may be a hole or two too far out. Alternatively if you regularly have that number aboard and need to plane slower, try a slightly lower pitch prop.

As Matt says, a tiller extension might be the best bet - thing with boats as small & light as that is you & your crew are a probably a good 2/3 of the weight of the setup, so your positioning will be crucial. I've regularly used an "OMC Express 2.8" with an 8Hp. if you just sit there at the stern it takes forever to plane and behaves exactly as you describe. Move forward to get over the hump and move back once planing it goes like stink! If you have a crew, use them as mobile ballast!
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Old 10 April 2008, 06:24   #11
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Personally, I've removed doel fins from two boats I bought second hand. The drag took 2 & 4 knots off the top speed, and I didn't like the handling. Also drilling to fit them will kill the paint / corrosion warranty if it's new. Mind you if the speed limit is 10 Knots it may be the way to go as Vmax will be irrelevant.
In my continued defense of Doel Fins...shows you how much I have to do....two to four knots off the top end is irrelevant in six to ten foot seas, at least in a boat less than 5 meters OAL. I can't speak to handling other than to say that I handle the boat....the boat doesn't handle me.

In return for sacrificing speed(?)....the aforementioned platform, at the engine side to re-board the boat, is a big plus in 80 degree water with use as a dive platform and swim platform and....I do a lot of trolling and realized a 100% improvement at maintaining a steady 7-8 knots after the fin install. Before it was bog down or gather too much speed, no matter the engine trim.

We have some of the most saline water on the planet outside of Great Salt Lake and the Red Sea. Ten years after my fin installation an inspection revealed zero corrosion adjacent to the drilled holes in the cavitation plate. Then again I've got good zinc and don't leave my boat in the water between trips.

I don't find any justification for me to suggest anything negative about alternatives to Doel Fins. That's not the point, as far as I'm concerned. Their use on my boat, in my ocean, with my float plans prove them to be spot on. The corrosion bleep is a little shrill. I'd rather drill my cavitation plate and lose sleep over the paint warranty than drill my hull or transom for lifters, any day.

As always, evaluate the mission and match the equipment that most efficiently performs in that envelope
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Old 10 April 2008, 11:54   #12
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9D280, engine is trimmed right in, and I move forward to get the boat planing and aft once its up and going as you suggest. The performance when planing is much better than I was expecting, its just that its all or nothing; flat out or slightly less, or back to pushing water out of the way. I'd like to achieve lower speed planing with less bow rise, which is why I'm looking at hydrofoils or tabs.
Thanks for the recommendations anyway.

Tomas, consistent planing at 7-8kts is just what I'd like to achieve, so maybe I'll try doelfins first. Seems like they work for some people in some circumstances and not others. Depends what you're trying to achieve or correct I guess. The reason I was planning to try the tabs first is that I'm pretty happy repairing holes or damage to GRP and gelcoat, more than mending holes in aluminium.

thanks, Nick.
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Old 10 April 2008, 13:12   #13
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Have to come in on the side of fins - only a small 4m avon , but huge advantage in planing at lower speed ( now about 9 knots) & i can live with a 2knt reduction in top speed - how often do you run at max speed compared to everything else ( cruise, slow speed trol etc)!

Like wise on no corrosion showing - I though also keep the boat out of the water when not in use & it comes out the end of most days - only left in for a week or two over the summer.

I did loads of reading before I fitted them & with a narrow transom woudl not really have the room for any kind of tabs.

PLus they are very cheap fix & if they dont work just unbolt them & do something else .
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Old 11 April 2008, 05:07   #14
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Quote:
Originally Posted by Tomas View Post
.

As always, evaluate the mission and match the equipment that most efficiently performs in that envelope
errrrr and I quote myself:

Quote:
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Mind you if the speed limit is 10 Knots it may be the way to go as Vmax will be irrelevant.
As I've said before , one man's ideal is another's worst nightmare.

The reason I took them off the old SR was becasue it nose dived quite spectacularly in a rather unnerving manner there was so much lift. If Nick needs to plane at low speed, then they will not cause half the drag they will flat out. As I said, if the speed limit is 10 knots...... I have a lot of open and speed limit free (well, most of it! ) water to play with. Time to plane & slow planing speed is irrelevant to me compared to the fuel consumption increase brought on by the drag. In Nick's case the fuel consumption will probably drop, as the hull drag not planing will be far geater than dragging the fins through the water at relatively low speed.
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Old 11 April 2008, 06:05   #15
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Honda Like Them

Was looking at engines for another reason a couple of days ago and saw OEM lifting foils in Honda's catalogue.

In a very fetching colour to match the engine.

Our Yam 100 on 5.5 metre Delta has benefitted from Tigershark VG fins for some years now. The effect they have is clear, lifting what is a heavy engine for the size of boat.
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Old 11 April 2008, 09:42   #16
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[QUOTE=nickd;244527] To get on the plane you've got to open the throttle almost wide open, then it accelerates like mad and goes very well. Two up it will plane, three up it really struggles, as the bow rise is massive and its pushing a huge wave of water.

Any thoughts please??

Before placing doel fins or trim tabs I would:

1-Trim the engine so the anticavitation plate is 90 with respect of the rib bottom, usually second hole from transom.

2-Move forward and tie the gas tank to the prow if possible, or al least under the front seat. ( prow balast)

3-Infalte tubes to correct working pressure, all must have the same pressure, minimum 3.0 PSI on water.

4-With 2 passengers, sit towards the prow extending your arm to a confortable driving position, your buddy must face you in the same position.

5-With 3 passengers the same as above, + the third buddy must sit in the prow seat to obtain a better balanced boat.

6-Full throtle is always required to break the water inertia faster, specially in a short boat fully loaded. I always use full throtle to plane fast a well balanced boat, the bow rises shortly and comes down quickly, then reduce to fast cruise speed. It's all a matter on how well you balance your boat, you'll be surprised on the results.

After trial and error, if this data doesn't better your needs, then go for the doel fins or trim tabs.

Note: Had a pair of doel fins on my last 15 HP engine, now with a 18 HP and a well passenger/gear balanced boat no need to put them back again.

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Old 11 April 2008, 11:52   #17
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Loco, thanks for the tips, I'd already seen your photos on another thread regarding the fuel tank in the bow, which is where it has lived so far. I ought to get a strap and fasten it to the bow eye like you do to stop it bouncing around, but haven't as yet.

I tried moving the engine trim to the second hole last weekend but preferred it in its no1 position, though I think I need to do it and experiment more with ballast trim at the same time.

Are tube pressures that critical in a rib?

Looking forward to Sunday and playing a bit more, the wind chill must have been about two degrees last weekend with the Northerly wind blowing down off the fells. Not really good conditions for much experimenting!!!

Nick.
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Old 11 April 2008, 17:09   #18
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[QUOTE=nickd;245014] Loco, thanks for the tips, I'd already seen your photos on another thread regarding the fuel tank in the bow, which is where it has lived so far. I ought to get a strap and fasten it to the bow eye like you do to stop it bouncing around, but haven't as yet.

*Strap tight the gas tank on bow.

I tried moving the engine trim to the second hole last weekend but preferred it in its no1 position, though I think I need to do it and experiment more with ballast trim at the same time.

*Experiment with better balancing the rib in 2nd hole or the hole you can obtain the 90 angle before experimenting trimming to 1st hole and balance.

Are tube pressures that critical in a rib?

*If they touch water, yes, you could experiment chine-walking problems. Is like your car tires, correctly inflated performs better than badly done.

Looking forward to Sunday and playing a bit more, the wind chill must have been about two degrees last weekend with the Northerly wind blowing down off the fells. Not really good conditions for much experimenting!!!


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Old 12 April 2008, 04:20   #19
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Take a look at smart tabs website , I know its thier own advertising but it does give a good comparison between a doel fin and a trim tab .
Out of interest, what are they like when you go backwards?
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Old 12 April 2008, 07:15   #20
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Going backwards i have never noticed any differance with the fins fitted compared to when they weren't installed. I think that as usually you only go very slow backwards they have no noticable effect. I cant remember all the maths , but am confident that due to the low speed any lift/down force created is so small (in the region of a few Newtons) its negligable. I did wonder if they would create ' lift' in reverse with engined trimmed a long way out ( as when in very shallow water) but no problem at all . Just my personal experiance though . am 100% fan ! of them - I thnk especially in small boats that are relatively stern heavy.
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