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View Full Version : Donzi 16 sacrilege - dolphin tail??



Northern Sweet
01-17-2011, 05:56 PM
I think I know the comments I am going to get before I ask, but I have to. Has anyone ever tried a hydrofoil on a 16 or 18. I know there is a lot of negative press about "gimmicks" and don't work, but hear me out. I have a 16 with 350 retrofit. And I am not sure if it has to do with the extra weight or just part of the 16 hull design, but porpoising is pretty bad at about 30 MPH. I have Bennet TT's so I can adjust it away or drive through it, and it goes away at about 35MPH and stays away.

I previously had a 18 Baja and it already had one on. My friends made fun of me so I took it off. Well it was night and day. With the dolphin tail, it came up on plane much quicker, little bow rise, and did not effect top end one bit. And it actually felt more stable at top end (on a good day 54). So here is the question. Has anybody tried this and would they recommend one, and if so what brand? Or is there anybody out there that would like to talk me out of this because it's a bad idea. I have heard negatives that it hurs outdrive and may become unstable at higher speeds (p.s. I am still exploring that area so I don't want any surprises!!

Thanks for any insight into this.

MOP
01-17-2011, 06:06 PM
My 16 had a Dolefin on it the season before I bought it, it was on the boat for a few weeks. My buddy thought it would help with skiing, it did it popped up quite a bit quicker. Now the BAD, when he trimmed for speed the azz end sucked down. The trim had to be left in the neutral position cutting top end quite a bit, He pulled it off and gave it away. I filled the holes in the cav plate when I re-painted drive after I bought it, IMO don't waste your time go for tabs that is the one thing 16's needs. The addition of tabs makes the boats so much better in many respects, some say they don't need them me I like full control of my boats all have ended up with tabs.

Phil

BobinCovington
01-17-2011, 06:06 PM
Whatever works... I almost bought a fin then went to the Nauticus Trim Tabs and love them for my boat. No big deal to install and they seem to give the boat a much better feel in the choppy stuff and no porpoising whatsoever. They are adjustable (not with controls like your bennetts) but set and forget. They have a shock actuator and adjust up a little when you are at planing speeds.

But you already have trim tabs so you probably don't want to replace them with cheaper versions.

Maybe somebody else can jump in with the right answer.

fogducker III
01-17-2011, 06:24 PM
Whatever works... I almost bought a fin then went to the Nauticus Trim Tabs and love them for my boat. No big deal to install and they seem to give the boat a much better feel in the choppy stuff and no porpoising whatsoever. They are adjustable (not with controls like your bennetts) but set and forget. They have a shock actuator and adjust up a little when you are at planing speeds.

But you already have trim tabs so you probably don't want to replace them with cheaper versions.

Maybe somebody else can jump in with the right answer.


MOP did jump in with the right answer......:wink:

BobinCovington
01-17-2011, 06:33 PM
yes he did, but when I was forming my words of wisdom to reply, I had not seen his post yet.

Rob M
01-17-2011, 07:26 PM
I had one on my 1994 16 classic - can't recall the brand of fin. It made a huge difference for getting on plane quickly. There was no impact to top end speed gauging by the top mph that others have posted over the years for their 16s.

Any porpoising at I was able to control with the trim switch - but I guess this won't help you out with your Eaton drive. I never felt the need for trim tabs as the boat was always stable, other than chine walk at very high speed and high trim.

And...who really cares what anyone has to say about whether you have one on your boat if it sorts out a performance or handling issue.:)

Rob M
01-17-2011, 07:45 PM
Ignore the Eaton drive comment, I mistakenly thought this was Bob's original thread.

Northern Sweet
01-17-2011, 09:17 PM
Thanks for the feedback. I think I will spend another season playing with the trim tabs first, before I drill holes in the outdrive for the fin. Has anyone heard that they actually can cause damage or stress to your outdrive?

younger
01-17-2011, 09:49 PM
The 16 is a seasaw at lower speeds, and the tabs work. The best thing I have found is to drive fast, really fast!

The Hedgehog
01-17-2011, 10:19 PM
We had one on the 16. It worked great but in time we accomplished the same effect by reworking the prop and picked up a few mph. Not a bad short term fix but there are better ways to go.

Sweet little 16
01-18-2011, 11:21 AM
but in time we accomplished the same effect by reworking the prop and picked up a few mph. ..... are better ways to go.


this is true for every 16 well maybe except an Eaton 16( not sure if there are any props that will help ) The best money spent will be on a prop. the porpoise cycle can be limited to a small speed range or almost eliminated with the correct prop. The cause is the prop to begin with so why try to cure the symptom and not the cause???( porpoise is caused by the presence of bow lift then the absence of bow lift , some props cycle is very large and can take away all mid range cruise without some other force like tabs. I guess this happens at certain speeds were the prop cannot maintain the lift the bow falls and and as the prop builds lift the cycle repeats) I have seen props that will do it from 25-35 mph and others that only do it at 25-27 mph

Tabs are a good investment for rough water handling and load balancing but get used less and less once the boat is propped correctly. so stop and prop before you drill

Carl C
01-18-2011, 12:20 PM
I'd rather die than being seen with one of those on my boat....:hangum:

DonziJon
01-18-2011, 01:09 PM
I'd rather die than being seen with one of those on my boat....:hangum:

Carl, if you had a really nice Bimini, no one would notice the flippers on the tail. :nilly: DJ

Marlin275
01-18-2011, 01:35 PM
My boat had the phins on when I bought it.
Made it real easy to climb the drive getting out of the water.
Keeps your feet off the prop.
I took them off and got 2-3 mph faster.

BobinCovington
01-18-2011, 02:16 PM
So maybe you can solve some with prop selection. In our case, the old Eaton had the shaft replaced with a stronger inboard shaft and better prop and after a little trial and error, it took the porpoising totally out of it (according to my dad). The original Eaton prop hubs were twisting apart with our motor. It doesn't look like it should work, but it does.

60993



When I added the trim tabs (Nauticus) they helped it come up on plane faster and also nearly eliminated the chine walk effect

Northern Sweet
01-18-2011, 08:52 PM
Thanks for all the advice. I never would have thought the prop was part of the problem. I know that I do want to change the prop. The boat was refit with a 350 (had a v6) and although the gearing was changed, I am getting the impression the prop was not changed. The boat runs out of RPM before running out of horsepower. Actually it can quite easily over rev. Does anyone know if I prop for higher top end will that help with the porpoising, or make it worse???

As "younger" pointed out driving fast makes everything better, but there is some times when you have your mother-in-law with you and 30 mph without porpoising is a good idea.

The Hedgehog
01-18-2011, 09:15 PM
this is true for every 16 well maybe except an Eaton 16( not sure if there are any props that will help ) The best money spent will be on a prop. the porpoise cycle can be limited to a small speed range or almost eliminated with the correct prop. The cause is the prop to begin with so why try to cure the symptom and not the cause???( porpoise is caused by the presence of bow lift then the absence of bow lift , some props cycle is very large and can take away all mid range cruise without some other force like tabs. I guess this happens at certain speeds were the prop cannot maintain the lift the bow falls and and as the prop builds lift the cycle repeats) I have seen props that will do it from 25-35 mph and others that only do it at 25-27 mph
Tabs are a good investment for rough water handling and load balancing but get used less and less once the boat is propped correctly. so stop and prop before you drill

Great answer.

Stop and prop.