Dolfin

[Jayp]

Ok I have a question for everyone. I just re hung a clean mariner 115 on my old 170 osprey for my son to start to run and me to play with again . My question is a follows. The old motor was a mariner 90 I had put a dolfin on it to help it get out of the hole and plane at a lower rpm . Normal things. So I ran the boat with the 115 and it jumps out of the hole running a laser 2 13.5 x22p was a little worried about being over propped but was able to turn 5k rpm and ran 42 into the wind and 45.6 with the wind and no trim . Boat runs pretty much true to rpm's 3100 = 31-32 mph . Have a fair amount of cavitation in the turns . I am thinking that it will help with that. I also have a ton of torque steer need to adjust the fin . Not sure if the dolfin will help at all or not . I guess what does everyone think. Should I throw it on or not?  Can anyone think of any negatives to putting it on other than drilling the cavitation plate ? Loss of performance or any issues. Thanks in advance .

[RickK]

When I had a 110 Evinrude on my 170 it had a doel-fin on it and if I remember right, it would keep the bow in the water because it raises the stern.  I think it worked fine.
When I changed the motor out to a 115/90 (can't really tell the size as I bought a rebuilt engine and although I paid for a 115, it very well may be a 90 with 115 stickers on it) and it ran about the same without anything on it.  I then bought a Bob's aluminum stabilizer plate hoping to get better speed and lower planing and it was worse than having nothing and it reduced my top speed by 2 or 3 mph - so off it came and I sold it to a member.
Hope my experiences helps you with a decision.

[Georgie]

When I had a 110 Evinrude on my 170 it had a doel-fin on it

I then bought a Bob's aluminum stabilizer plate

The doel-fin and aluminum stabilizer plate are designed TOTALLY opposite.  Only the doel-fin provides the lift you would want via Bernoulli's principle to drop the bow into the water (I have one myself).  Based on design, I'm honestly not sure what the Bob's stabilizer was designed to do...perhaps a bit less porpoising by extending the cavitation plate? , but I can't imagine much more based on basic physical principles.

My personal experience is that there is no reason NOT to install one on a smaller center console. It absolutely improves hole shot and generally reduces porpoising with little/no top end speed loss, so why not?

[dburr]

Think I may have misread.. The steering torque will be taken care of with the trim tab/anode that sticks down vertically from the cav plate, the fin will not help with that (unless it has the vertical tabs and I thought Tiger Tail did that)..

Good luck!

[wingtime]

I have a dolfin or a knock off brand on my 170.  I'm thinking of removing it.  Keep in mind the #1 cause of broken cav plates is a dolfin.  Second since the dolfin generates lift in the middle of the boat.  This causes a teeter totter effect making the boat want to roll side to side which can lead to chine walk.  And the third reason for me is I have to run with the motor trimmed almost all the way down or she wants to porpoise a bit.  I'm going to remove mine and see if I get any cavitation on my hole shot.  If not I'm leaving them off.  The best bet on a 170 would be a set of Smart Tabs or actual trim tabs.  The new electric Bennett tabs look nice.

[Jayp]

Thanks for the input. I think what I am going to do is run it some this weekend to see what it is really going to do with the kids and some gear in it. I don't think I will have much of an issue staying on a plane with the prop set up it has a lot of lift now. I will make some adjustment to find the sweet spot of the trim fin for the cruise mph and go from there. I will keep you posted on the results..

J

[GoneFission]

The Doel-Fin does not work well on boats with the engine at the proper height.  It is OK if the engine is set low, but at the proper height, it will be out of the water when the engine is trimmed out, and will cause a huge porpoise movement at some speeds.   I tried one on my CCP for testing and found it was slower at top speed and no real difference in hole shot or time to speed.  It made the boat very sensitive to engine trim and increased fuel use.  

I have heard more positive results when put on a boat with an I/O setup and lots of weight in the back, but get your outboard set up right and you probably don't need it on an Aquasport IMHO.

[wingtime]

Gonefishing is right.... a properly set up outboard will run with the cav plate on top of the water so a Dolfin will not do anything.  I haven't watched what my 170 does...  but I can tell you my 245 runs with the cav plate out of the water when on a plane.

[Jayp]

I have to agree with you guys. The cav plate is above the water when on a plane at just about any speed. So I don't think it will be of any benefit on this motor . The 90 would struggle a litte when fully loaded. The extra hp helps a lot.  Thanks for the input .