Hydro Turn fin questions

[Raydee]

Ok I know there are some hydro EXPERTS here so here is my question. I would like to know what effect I will see on my hydro with different turn fin positions.

What effect will I see if:

1) The fin is moved foward

2) The fin is moved backward

3) The fin is moved in on the sponson

4) The fin is moved out on the sponson

5) The fin is angled back

6) The fin is angled foward.

Any help would be great.

 

[PakMan]

Generally, regarding scale/sport boats:

1) More Responsive (in straights & turns), and potentially less stable/forgiving

2)Less responsive, and generally tracks better (straights)

3)Inboard fins have less leverage and can cause inefficient water flow coming off of them

4)Outboard fins work well on scale/sport boats, my suggeston is at or near the riding point of contact (hopefully an anhederal STBD Sponson)

5)Softens (effectively moves the fin back) response -vs- more vertical

6) Not advised unless bracket and fin are VERY rigid and hull parameters are inflexible

Rudder, Prop, Skid fin location, CG and stance as well as personal preference all play factors in getting the optimal set-up for you and your driving style/requirements.

Hope this helps.

David Jensen

 

[Raydee]

David, thanks for the post.

The problem I am having with my hydro is that the hull is very sluggish in the turns. Down the straights the boat runs fine, nice and straight and level but when I hit the turns the engine sounds like its laboring and the hull slows. It almost seems like the nose of the hydro is being pulled down to much and causing it to dig and slow down. I am going to try raising the strut some tomorrow and see how it does. I also have a larger turn fin that I want to try out and see if that helps or hurts the hull.

 

[PakMan]

David, thanks for the post.

The problem I am having with my hydro is that the hull is very sluggish in the turns. Down the straights the boat runs fine, nice and straight and level but when I hit the turns the engine sounds like its laboring and the hull slows. It almost seems like the nose of the hydro is being pulled down to much and causing it to dig and slow down. I am going to try raising the strut some tomorrow and see how it does. I also have a larger turn fin that I want to try out and see if that helps or hurts the hull.

Well, many things can impact this. First, what type of hull Sp40, or 1/8 scale, what weight, when/who built the hull, and what motor/pipe/prop combo? Is the fin angled, curled(hook) or both?? I'll try to help.

David

 

[tracerbob]

Remember...do one thing at a time.

 

[Mike Hughes]

Sounds like the motor can't pull the prop in the corners. Have you try a smaller prop? Or maybe one with less blade area IE backcut?

Mike

 

[Mark Anderson]

Have some one help you watch the boat. Does the transom drop when you turn it? does the front suck down? Pay attention to what the boat is actually doing. Most time I find that the transom is dropping loading the prop more. It could also be too much curve or tuck in the turn fin pulling the front down too aggressivly.

 

[Duck899]

Sounds like you may need to drop the strut 1/8" at a time untill the boat loses that urge to slow down in the corners. What may be happening is the prop is cavitatating from being to shallow in the corners, resulting in a huge loss in speed. This occurs as the hull drops in the corners and the prop is not deep enough to push the hull with all the water pressure now acting on the hull in the corners.

Its something I have seen on riggers and sport boats. It also depends on the various designs of the hulls.

Just something to try......then once its running fast through the corners, you can play with turn-fin angles and effects.....

 

[Mike McKnight]

You may have a balance point issue as well. If you have set up the boat with regards to weight your balance point down the long axis of the boat should be within 1" either way of the trailing edge of your turn fin. If you have to much weight forward of the turn fin, the minute that fin bites in the turn it will suck the nose down and cause the transom to lose bite at the prop sometimes even making the boat spin out. Be careful in your distribution of the weight on both the BP (long axis down the length) and the CG (laterally accross the boat). If you drop the strut to try and get more push without checking your BP and CG all you are going to do is compound the problem by loading up the motor trying to get the ride pads to lift the boat in the turns.

 

[Mark Anderson]

If the boat is running good in a straight line it is not a strut height issue. If the motor is laboring in the turn it is being loaded by something not unloaded as it would be with cavitation.

The best turning scale hydro that I have ever driven had the c.g. 1 inch ahead of the sponson transom and 3" ahead of the turn fin leading edge.

I try to put the leading edge a little behind the c.g.

If the boat is dropping the transom getting into the turn pull the bottom of the rudder back so the pivot angle will cause the rudder to lift the transom alittle. If the front end is being pulled down take some fin angle out. If it is slapping the front end down try pushing the rudder in. When adjusting the rudder it has to change the pivot angle not just the blade.

The strut and c.g. adjust the boat ride, the turn fin and rudder need to be tuned so the boat ride remains constant in the corner. Watch and listen to what is really happening as the boat enters the corner.

Most boats that I see bog the motor in the corner due to the transom dropping loading the prop.

 

[Raydee]

Mark, great info! I was told that my CG should be on the back third of my fin and that may very well be my problem in the turns. When I have some better weather I ma going to try moving my fin back some and see if the hull turns better.

 

[Russ Williamson]

Most boats that I see bog the motor in the corner due to the transom dropping loading the prop.

Great information Mark!

What if the engine is un-loading in the turns?

Rw