Intake Side Fins

« **on:** November 25, 2008, 04:12:18 AM »

3AM and I'm reading old posts... I'm addicted

. Ran across this comment from Bottom Feeder:

"I know you So Cal types would never think about removing the side fins on you intake

But that is a good place to start. They have an effect on the roll center and dont do much for loading. The worst part is the placement. If you run a forward skag and the skags on the pump you have one fighting the other. Think about it the way it is. The forward skag is your pivot the nozzel is your force and direction. With the fins on the intake the pivot and fulcrum are way to close together."

This guy knows his shit. I cut mine off (B4 reading this thread) but he is dead on. Those things don't make any sense. I can't see any upside to em, but lots of downside. They are in the perfect place to dork up handling or (in my case) block flow to the pump in a hard turn. I think they musta looked good on paper back in the 60's when Berk made their castings. But IMO they are a mistake.

How many of you out there have removed them? Did you do it with a saw or just use the bottom of the lake?

Anybody got ideas about what good they do?

« **Reply #1 on:** December 06, 2008, 07:38:09 AM »

Anybody have any results or testing? The only thing I think might be a factor would be the safety. I can see the pivot point or fulcrum being the turn skag, but the intake skags provide some directional stabilty. without those, it seems to me that the boat would be very prone to spin outs when the forward skag would grab water and there is no "ruder" so to speak in the rear. Let alone if the power shut down from an engine failure. I've heard alot over on the other site with the drag guys that the intake skags do very little for pump loading. In fact, I haven't seen one that has them removed. However, they aren't turning, so the skags might come to play much more in a circle boat. Interesting topic, I'd like to hear if anybody has tried it and what happened...

« **Reply #2 on:** December 06, 2008, 08:01:08 AM »

I'm undecided on this topic at the moment.

I have damage to the fins on my intake currently and was planning on replacing the complete intake housing. I do know that with one fin mostly missing and the other with a chunk out of it my boat cavatates bad when turning left (I have a hard time turning right being an old circle track racer

). My boat does not have a forward turning fin currently so I don't know how that would affect things at this point.

Maybe when I'm ready to replace the intake housing I'll cut both fins off completely and go out for a test then try the same run with a new intake. Gonna be a little while before I get to it though.

I also would like to see some opinions.

So, what do you think?

« **Reply #3 on:** December 06, 2008, 08:42:29 AM »

Each boat & setup is different of course, but each has it's own turn axis (an imaginary line vertical out of the boat where it pivots in a turn). Most boats turn axis will be forward of the pump inlet. Maybe a few boats have the axis farther back - somewhere near the inlet. Mine is well forward - probably 2-3 feet. (I'm rigged more like a competition ski boat.)

For a boat with turn axis at or near the inlet area, those fins never see any side/cross flow, so in my opinion they are un-necessary for this setup and can be cut off (or left alone) without ever noticing a difference.

But for most boats (including mine) where the turn axis is forward, then as you turn, these fins start to get a portion of flow across them instead of straight at them. In a hard turn, this cross flow will go turbulent and this will get sucked into the pump. In other words you will cavitate your pump. That is exactly what happened with my boat. Because my turn axis is so far forward I would get it even in a mild turn and the boat speed would fall off dramatically. (My setup is just a more extreme case of turn axis.)

So I took a deep breath last Spring and put a fresh blade in the sawzall. One of the best moves I've made. Now I can turn as hard as I want (& I mean real hard) and no cavitation whatsoever.

For a flattie or other hull with nothing on the bottom to keep it from sliding sideways, the pump inlet fins are too small and in the wrong place. Turning fins are the correct thing as is discussed in some other posts.

And this same phenom comes into play with certain loaders and certain rock grates. The slight cross flow in a turn (which is natural and unavoidable) will hit the side of the loader/grate and go turbulent. Some loaders take this into account and others don't.

« **Reply #4 on:** December 06, 2008, 11:17:29 AM »

I've been racking my brain over this for some time and I can sit here and argue both side all day . My brain is hard at work .

-beerjet-

« **Reply #5 on:** December 06, 2008, 11:18:28 AM »

In the whitewater world, we just wear them off

GT

« **Reply #6 on:** December 06, 2008, 02:08:02 PM »

Anyone cut them off and had it be BAD?

« **Reply #7 on:** December 06, 2008, 02:14:22 PM »

Quote from: crewchief22 on December 06, 2008, 08:01:08 AM

I'm undecided on this topic at the moment.

I have damage to the fins on my intake currently and was planning on replacing the complete intake housing. I do know that with one fin mostly missing and the other with a chunk out of it my boat cavatates bad when turning left (I have a hard time turning right being an old circle track racer ). My boat does not have a forward turning fin currently so I don't know how that would affect things at this point.

Maybe when I'm ready to replace the intake housing I'll cut both fins off completely and go out for a test then try the same run with a new intake. Gonna be a little while before I get to it though.

I also would like to see some opinions.

So, what do you think?

Quote from: Stevecat on December 06, 2008, 02:08:02 PM

Anyone cut them off and had it be BAD?

Used to have an old beat up Tahiti with a 350 Olds and a Berkeley, the fins had both been badly broken and bent, so we just whacked em with a Sawzall and went boating, the next week I ordered a rudder, the boat would get into this really wierd powerslide and once in a while would just spin, kinda fun really...the rudder made it work very well, although the litttle Olds only ran the boat a little over 50 MPH (using the "Ass-o-nometer" as a speedo)..

GT

« **Reply #8 on:** December 06, 2008, 03:10:03 PM »

Someone should put a pressure gauge to see it the fins create anymore intake pressure than without

-beerjet-

« **Reply #9 on:** December 07, 2008, 08:55:17 AM »

Seems like there are some mixed reviews. I have no practical experience to back this, but it seems to me that the pivoting axis would be well forward of the intake. Just imagine driving your boat into a turn and feel where the boat is "grabbing" and pivoting around. I think it's somewhere near the front seats, which would make sense. The pivot point should be where the turn skags are mounted, right?? Otherwise, the skags would be fighting the natural tendency of the hull in a turn. The boat would still want to turn where it does, and the skags would have to slide sideways or see a lateral movement. Kind of like turning a tight turn with a tandem trailer. The forward tire becomes the axis and the rear tire "slides" throught the turn.

There are 2 aspects being talked about. One is pump loading and the other is handling. I can see the benefit in tight turns where the skag could interfere with pump loading, and it sounds like some have had good luck removing them. Would be cool to see the results with and without the skags with an intake pressure gauge with back to back runs.

I still think the boat needs to have something in the rear to keep it from grabbing the skag and sliding out completely throught the turn into a spiin. Maybe just a nozzle rudder?? Somebody said it got very squirley when they removed them and that was with a small block. Very good thread, I've always wondered what would happen if they were removed, just never got the balls to crawl under the boat with a sawzaw

« **Reply #10 on:** December 07, 2008, 09:40:33 AM »

Quote from: River Dad on December 07, 2008, 08:55:17 AM

Seems like there are some mixed reviews. I have no practical experience to back this, but it seems to me that the pivoting axis would be well forward of the intake. Just imagine driving your boat into a turn and feel where the boat is "grabbing" and pivoting around. I think it's somewhere near the front seats, which would make sense. The pivot point should be where the turn skags are mounted, right?? Otherwise, the skags would be fighting the natural tendency of the hull in a turn. The boat would still want to turn where it does, and the skags would have to slide sideways or see a lateral movement. Kind of like turning a tight turn with a tandem trailer. The forward tire becomes the axis and the rear tire "slides" throught the turn.

There are 2 aspects being talked about. One is pump loading and the other is handling. I can see the benefit in tight turns where the skag could interfere with pump loading, and it sounds like some have had good luck removing them. Would be cool to see the results with and without the skags with an intake pressure gauge with back to back runs.

I still think the boat needs to have something in the rear to keep it from grabbing the skag and sliding out completely throught the turn into a spiin. Maybe just a nozzle rudder?? Somebody said it got very squirley when they removed them and that was with a small block. Very good thread, I've always wondered what would happen if they were removed, just never got the balls to crawl under the boat with a sawzaw

Hey Riverdad, I feel like you are close, however it was aleways my understanding that the turning skags were meant to relocate the pivot point to keep the boat held in a turn at a more desireable speed (I could be wrong but I think the natural pivot point you speak of woul be while moving extremely slow) the pivot point moves at different speeds and different centers of gravity, I've never been able to totally understand it though.

GT

« **Reply #11 on:** December 07, 2008, 06:57:32 PM »

Quote from: GT Jets on December 07, 2008, 09:40:33 AM

the pivot point moves at different speeds and different centers of gravity,

I agree with this. The pivot axis location depends on how much of the hull is wetted or down in the water. At slow speeds there is a lot of hull in the water and the pivot point is as far forward as it will get. Think of your hull as a giant, blunt weird shaped fin. As you increase speed and the hull starts to plane then less wetted area and most of it further back, so now the pivot point shifts rearward. And as you get ultra fast and start flying the hull like Beerjet always does, then there's very little hull in the water and the pivot point is way back at or near the intake area.

All of what I just said is b4 adding turning fins/skags up in front of the pump inlet. When these are present, then the pivot point will "cling" to their location, because they become the fulcrum for the turn in stead of the hull itself.

« **Reply #12 on:** December 07, 2008, 07:13:49 PM »

Quote from: Stevecat on December 07, 2008, 06:57:32 PM

Think of your hull as a giant, blunt weird shaped fin.

I always do

I think you just came up with the name of my next boat..

LMAO

GT

« **Reply #13 on:** December 07, 2008, 10:35:39 PM »

Ive been thinking about this (never a good thing) and looking at the thread about rudders. On my boat the rudder made no difference, and I always thought it was because of the fins on the intake. In my way of thinking (

) the hull should want to go straight (with no outside influences) and the intake fins will do the same. But the rudder (or thrust from the pump) needs to slide the back of the hull around to turn it (right/wrong ?) . So the fins and rudder are fighting each other, one trying to go straight and one trying to turn $:-\$ .

I also remember there has been a boat at the circle races that used a fixed nozzle and a big V-drive style rudder. Does anyone know if that boat had fins on the intake ?

« **Reply #14 on:** December 07, 2008, 10:45:52 PM »

Quote from: propless on December 07, 2008, 10:35:39 PM

Ive been thinking about this (never a good thing) and looking at the thread about rudders. On my boat the rudder made no difference, and I always thought it was because of the fins on the intake. In my way of thinking ( ) the hull should want to go straight (with no outside influences) and the intake fins will do the same. But the rudder (or thrust from the pump) needs to slide the back of the hull around to turn it (right/wrong ?) . So the fins and rudder are fighting each other, one trying to go straight and one trying to turn $:-\$ .

I also remember there has been a boat at the circle races that used a fixed nozzle and a big V-drive style rudder. Does anyone know if that boat had fins on the intake ?

In that case, would that mean that the rudder would need to be larger than the surface area of the intake fins? Would only make (common) sense right? Also, I've seen some rudders with some surface area before the kingpin, and some with none, why? Somebody knew something...right?
I have seen rideplates with the fins turned down, this always seemed to be a bad idea to me.
I always wanted to try hooking up cavitation plates to the steering linkage and use them like ailerons on an airplane, probably end up killing myself, but could be cool, I should try it on my R/C boat first

GT

« **Reply #15 on:** December 07, 2008, 11:18:52 PM »

What I would realy like to know is do the intake fins realy do anything for the pump (loading) in a straight line. I do remember an old thread somewhere that the old circle boats would cut the outside fin off to help load the pump in the turns.

It just dosnt make sence to have two sets of fins (one beeing the rudder) that close together trying to do different things

. I cant imagine a boat other than a jet adding a fin in that location and expecting the boat to turn. But maybe I'm just looking at it wrong (wouldnt be the first time).

« **Reply #16 on:** December 07, 2008, 11:33:27 PM »

Quote from: propless on December 07, 2008, 11:18:52 PM

What I would realy like to know is do the intake fins realy do anything for the pump (loading) in a straight line. I do remember an old thread somewhere that the old circle boats would cut the outside fin off to help load the pump in the turns.

It just dosnt make sence to have two sets of fins (one beeing the rudder) that close together trying to do different things . I cant imagine a boat other than a jet adding a fin in that location and expecting the boat to turn. But maybe I'm just looking at it wrong (wouldnt be the first time).

On the other side, I've seen them extended too, maybe I'm retarded and thick, but wouldn't they keep the intake from ventilating in a turn?

I don't know, my brain is mush right now and tomorrow is Monday...I'm glad I'm not an engineer.....

GT

« **Reply #17 on:** December 08, 2008, 07:48:20 AM »

Just for fun here are some pics to contemplate!

First 2 are what I think is happening and why I cut mine off.

Last pic shows a comp skiboat and how it is rigged. Not saying that's what you all want to do but shows how they intentionally are putting the pivot axis right under the rope pylon.

« **Reply #18 on:** December 08, 2008, 12:58:37 PM »

First the disclaimer, I am not a very smart man.
It seams to me that you need these forces working against each other to help stablize the handling charactoristics of a jet drive. Dont the intake fins help to keep you from spinning out, a too loose situation. They just add a little more bit for the ass end.
So, how about leave the intake fins and use them as your pivot point. Add a tiller to the turn fin and when you turn the pump it turns the fin. Maybe a larger fin, and like GT said a fin with some surface area ahead of the kingpin to assist in turning the fin. Use the waters energy to turn the front of the blade.

« **Reply #19 on:** December 08, 2008, 09:55:26 PM »

I realistically dont see the water "Rolling" over or under the fins . I always thought the idea behind the fins was to have a water directed straight into the pump , keeping the pump loaded . Yeah , it will unload in a turn but realistically , you should slow down to a safe speed to make a tight turn and at that point , cavitating a pump shouldnt even be an issue .

-beerjet-

« **Reply #20 on:** December 14, 2008, 11:50:53 AM »

Never thought of the speed issue.. I guess it would change with speed. But i agree with Omar, I was considering a fairly hard turn which would mean slowing down to a speed where at least more of the boat is wetted. if you're turning hard enough to unload the pump, that's not going to be a wide open throttle movement (At least I hope not). Once again, good thread. I always like to hear how others are looking at the same thing I am

« **Reply #21 on:** January 02, 2009, 09:20:23 PM »

I have had several tournament boats, they all had at least 2 skegs, and they all turned like they were on rails, flat...in my Kona, the turns at slower speeds feel kinda mushy and the boat kinda "banks" into turns. With a rudder on the diverter, I wonder if skegs would have the same handling properties in a jet? I wonder if adding skegs would help with maintaining a straightline underway, without having to constantly steer a straightline...

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