Dol-Fin on 9.9 ??

[Kymasabe]

Took my new-to-me '82 Johnson 9.9 out today on my little 12 foot aluminum boat. With my kid on board, it didn't quite make it to full throttle, barely planed, kinda struggled along under all the weight.
With me alone, it scooted right along.
I was wondering though....would one of those Dol-Fin things on the botton of the motor or similar device by another brand help at all?
Some claim better/faster planing, better stability, better fuel mileage, increase in speed, etc.
All kinds of claims but does anyone actually have one on a 9.9 and does it help?

 

[ratracer]

Re: Dol-Fin on 9.9 ??

Where was your fuel tank? Where did your kid sit? What else did you have in the boat and where was it?

Is the angle adjustment of the motor (the 5 sets of holes at the bottom back of the stern bracket through which you select the best one to slide the angle adjusting rod through) set at the best position for your boat?

If you aren't overloaded and the weight is distributed anywhere near properly you should have no problem getting the boat up on plane, I have no problem getting my 12' planing with myself and my 150 lb dad in the boat. (my 9.9 is an '88 that I've had since new) Make sure that you've got everything set up properly before considering trying using a dol-fin or similar device.

 

[Kymasabe]

Re: Dol-Fin on 9.9 ??

Well, has 5 gallons of fuel right under the outboard, has an old, small trolling motor hanging back there and tilted out of the water and a small deep cycle battery right next to the fuel tank, plus me on the back seat (all 250lbs of me). Add a couple of oars, a small anchor in the bow and a 140lb kid on the front seat. The adjustment pin was set on second to the last hole at the bottom. We were running fresh premium fuel in a new tank with synthetic 2-stroke oil and 1/2oz of the new marine Sta-Bil. 50:1 right? Just want to make sure my mixture was correct. Motor has a new 9&1/2 x 10 prop and the tiller throttle is gear operated, not cable, I checked, all the gears line up nice, nothing jumped out of place.

 

[The-Machinist]

Re: Dol-Fin on 9.9 ??

Thats quite a bit of weight in the rear. I would get some longer cables and move at least the battery forward. If you could move the fuel tank that would help too.

 

[Kymasabe]

Re: Dol-Fin on 9.9 ??

Well, my daughter and I decided we probably don't need the trolling motor very often. And when we do, it'll be in a canal/mangrove situation where we can't get on plane anyway so...most of the time, troller and battery won't be on the boat. When I tried the boat alone, I moved the tank to between the first and second seat to help keep the nose down and I was impressed with how fast a 9.9 HP outboard was on this little boat.

Still doesn't answer the question though is a Doel-Fin would be a worthwhile investment on this motor.

 

[Sea18Horse]

Re: Dol-Fin on 9.9 ??

Personally I don't believe it will help that much. I have always been under the impression that Dol fins are more usefull for outboards on boats that are naturally resitant to planing. Things like prams, driftboats and sailboats. Displacement hulls that tend to want to point their nose to the sky. I could be wrong of course. Wouldn't be the first time and won't be the last . I would think that you would see better results by redistributing the weight or getting rid of some excess balast .

Cheers................Todd

 

[The-Machinist]

Re: Dol-Fin on 9.9 ??

I think on that small of a motor the added drag will take away from any added performance you are hoping to get. I put a doel-fin on my '75 Merc 20 and lost 2.5 mph on top end.

 

[iwombat]

Re: Dol-Fin on 9.9 ??

Couple of things.

1st, you're wasting your money on premium gas. 87 will run just fine.

2nd. Check the location of the anti-cavitation plate with respect to the bottom of the boat. It should be even or 1-2 inches above the bottom of the boat. Any lower and you've got too much drag.

 

[ratracer]

Re: Dol-Fin on 9.9 ??

50:1 ratio is correct. The newer Sta-Bil designed to treat ethanol is fine. Nothing wrong with synthetic oil but if you want to save a couple of bucks the standard Wally-World branded TC-W3 oil works well too. As iwombat mentioned, use regular 87 octane gas. Your prop is fine.

As I suspected, you had too much weight in the back. If you don't have a 10' gas line get one and put the gas tank right in front of the front bench.
Also if possible you should keep the marine battery in between the 2 benches as long as you can lift to move it to the back as needed without having to stand up in the boat. That should be enough to let you get the boat up on plane easily with your 140 lb passenger. Don't waste your money on a dol-fin.

As an FYI the top GPS-measured speed wth my setup is just over 22MPH. I cn easily pop up on plane even with a 160 lb passenger up front.

 

[Timmstng67]

Re: Dol-Fin on 9.9 ??

I have a 12 footer with a 1988 9.9 on it and I put one on to try and help my boat plane. I was on the lake 5 minutes and went back to the ramp and took it off. It was like putting a bucket on a rope and throwing it overboard.
As others have posted get the extra weight out of the back of the boat. My 12 footer will do 22 MPH with two 200lbs men in it set up like it is see pic.

 

[BillP]

Re: Dol-Fin on 9.9 ??

Dolfins help most boats like yours and don't decrease speed when installed correctly. When used to get the boat on a plane faster the fin should be clear of the water once the boat is on a plane. The people with problems have their motors mounted too low (or trimmed wrong) and their fins stay in the water. It's already been mentioned to have the cavitation plate ride above the bottom of the boat. In my case I use a Dolfin to trim up the engine and get more speed.

bp

 

[Kymasabe]

Re: Dol-Fin on 9.9 ??

OK...now I'm confused. It works, it doesn't work, it's good, it's bad. I don't know what to do. I bought some smaller sized Sport 200 hydrofoil thing today thinking it would be OK but now I"m not sure I want to start drilling holes in my motor and find out it's not working out.
Maybe I'll return it and buy one of those triangular fuel tanks for mounting in the bow instead....with 12 feet of fuel line!

 

[samo_ott]

Re: Dol-Fin on 9.9 ??

and move the battery up!

 

[BillP]

Re: Dol-Fin on 9.9 ??

OK...now I'm confused. It works, it doesn't work, it's good, it's bad. I don't know what to do. I bought some smaller sized Sport 200 hydrofoil thing today thinking it would be OK but now I"m not sure I want to start drilling holes in my motor and find out it's not working out.
Maybe I'll return it and buy one of those triangular fuel tanks for mounting in the bow instead....with 12 feet of fuel line!

You need the tank up fwd anyway but Sport sells a clamp that eliminates drilling. If you look at installations that didn't work you will see what I'm talking about. Unfortunately, fin performance take the rap for bad installations. Don't install a fin if your cavitation plate is below or close to the bottom of the boat where the leading edge of the fin will drag...and expect stellar results. It won't be confusing when you look at the whole picture.

bp