Roundnose scale "porposing" in corners?

Terry Keeley · Jul 3, 2015

Got a buddy working on his Miss Supertest originally built from Newton plans. He's got it running real nice generally, but if he slows and makes a gentle turn it oscillates or porpoises wildly front to back. If he turns hard it seems to hold well and goes through the turn good.

Tyndall fin set "neutral", Andy Brown straight blade rudder on LHS. X455/3, balance point right at the sponson TE with near empty tank.

Any ideas?

SayMikey · Jul 3, 2015

Push the strut up a touch or try a Prather 250/255

Terry Keeley · Jul 3, 2015

SayMikey said:
Push the strut up a touch or try a Prather 250/255

Had the strut up higher, same thing.

Why you think it's doing it?

Duck899 · Jul 4, 2015

Terry Keeley said:
Got a buddy working on his Miss Supertest originally built from Newton plans. He's got it running real nice generally, but if he slows and makes a gentle turn it oscillates or porpoises wildly front to back. If he turns hard it seems to hold well and goes through the turn good.

Tyndall fin set "neutral", Andy Brown straight blade rudder on LHS. X455/3, balance point right at the sponson TE with near empty tank.

Any ideas?

Its hard to say, but on my bardahl I have found the same thing but only at certain speeds. I think it has something to do with the amount of air under the hull at certain times. Mine is set up running an Eric Canto prepped H-18 with 4.2" and the strut very deep (2"), with angle but not sure of the degree's. I run a Mike Hughes turn fin (sport 60 size with the hook). I have also found the turn fin needed to be angled in about 5 degree's even with the hook. This results in full throttle straight aways with a slight chop of the throttle when entering the corner and then quickly back up to full throttle through the rest of the corner. When done properly the boat will rail around the corner. At the last race it was turning times of 1:30-1:35 on a 6 lap one mile course. Its actually faster than my Miss Rock or pay'n pak. Keep the air under and keep the speed up for the best corners. There are times when going 1/2 throttle in a corner and then going hard coming out where it will hop front to back violently but those events are few. I'm still working on the boat ride but its getting better.

Jim Grams · Jul 4, 2015

What angle is your skid fin?

DON MAHER · Jul 4, 2015

Not enough tail weight! Terry you said that the CG is on the heels of the sponsons, so there would be "0" tail weight. Try about 10-12 oz. in the transom. That should be very close to what you need to hold the tail from sorpoisesing in a slow turn.

Don

Terry Keeley · Jul 4, 2015

Duck899 said:
Terry Keeley said:

Got a buddy working on his Miss Supertest originally built from Newton plans. He's got it running real nice generally, but if he slows and makes a gentle turn it oscillates or porpoises wildly front to back. If he turns hard it seems to hold well and goes through the turn good.

Tyndall fin set "neutral", Andy Brown straight blade rudder on LHS. X455/3, balance point right at the sponson TE with near empty tank.

Any ideas?

Click to expand...

Its hard to say, but on my bardahl I have found the same thing but only at certain speeds. I think it has something to do with the amount of air under the hull at certain times. Mine is set up running an Eric Canto prepped H-18 with 4.2" and the strut very deep (2"), with angle but not sure of the degree's. I run a Mike Hughes turn fin (sport 60 size with the hook). I have also found the turn fin needed to be angled in about 5 degree's even with the hook. This results in full throttle straight aways with a slight chop of the throttle when entering the corner and then quickly back up to full throttle through the rest of the corner. When done properly the boat will rail around the corner. At the last race it was turning times of 1:30-1:35 on a 6 lap one mile course. Its actually faster than my Miss Rock or pay'n pak. Keep the air under and keep the speed up for the best corners. There are times when going 1/2 throttle in a corner and then going hard coming out where it will hop front to back violently but those events are few. I'm still working on the boat ride but its getting better.

Yup, does the same when slowed down, "hops front to back violently", strange.

Jim Grams said:
What angle is your skid fin?

Curved Tyndall fin is straight up & down, it's not rocking side to side (ie: fin releasing)

DON MAHER said:
Not enough tail weight! Terry you said that the CG is on the heels of the sponsons, so there would be "0" tail weight. Try about 10-12 oz. in the transom. That should be very close to what you need to hold the tail from sorpoisesing in a slow turn.

Don

We were wondering that too, where should a scale balance?

DON MAHER · Jul 5, 2015

Terry,

I don't think there is a fixed number that would be right for all or most all scale boats. A ballpark number I think would be 1-1/2" to 2" behind the sponsons. I would just add & subtract weight untill you have the handling you feel most comfortable with. You may also try a 1455 or 1457 octura prop, I had good results with these on my scale boats.

Don

lohring · Jul 5, 2015

We had the same issue with my gas scale $ Bill. It was built from a kit from scaled up from Newton plans by ML Boatworks. The first issue was way too much lift under the bow. An air dam helped, but we still needed strut angle to hold the bow down. That meant that you couldn't let off the throttle at speed. It's too bad we learned that while comfortably in the lead. The final solution was the right prop, a COCR AC-1 . We tried a series of ABC and prop shop style props to get the lift right. The boat actually balances slightly ahead of the sponson transom. The bow lift seems to put down force on the prop. The result is the NAMBA two lap gas scale record. Below are some pictures that show the turn fin and bottom. The solution was found by trying everything a talented group of racers could think of.

Lohring Miller

My Gypsy · Jul 5, 2015

Terry,

CG should be behind the sponsons at a distance of 10% of the length from the rear of the sponsons to the transom.

The sponsons could be part of the problem as well. What is the angle of attack on each one and do they have recoveries ahead of the ride surfaces?

Dennis

scott williamson · Jul 6, 2015

lohring said:
We had the same issue with my gas scale $ Bill. It was built from a kit from scaled up from Newton plans by ML Boatworks. The first issue was way too much lift under the bow. An air dam helped, but we still needed strut angle to hold the bow down. That meant that you couldn't let off the throttle at speed. It's too bad we learned that while

any chance you could post a side profile of that turn fin thanks Lohring?

Ken Hildreth · Jul 6, 2015

It would seem to me that a combination may be the fix. But here goes. What Dennis said is exactly right, The balance of that boat needs to be started and hopefully stay right there where he said. The next is also what Dennis said, you can tell Dennis races round nose boats a lot. Then after this you need to go straight to what Rob "The Duck" said. It doesn't have to be that fin, a few others would do also, but I would prefer a hook like "The Duck" . Rob, you need that on the back of your shirt. haha!. The Prop is a key element. But I know terry is a hydro guy and already knows this. Next, In my opinion, You need to have a round strut on the bottom of a round nose. The round nose scales usually easily lift the rear of the boat, If they Tail drag. You need to go back to step one, what Dennis said, and start over. Or try a prop with more lift. If you help lift with a flat strut, the strut then has flat sides and will "bounce in the turns" as it grabs and releases. Thank you Marty Davis for that tip some twenty years ago. Many of you don't know it. But Marty had a Miss Verners scale that was Bad Fast, and it turned like it went down the straights. I don't know if this helped. But I had fun writing it. I run mostly flat bottom struts on every other kind of boat that has a strut. Ken

Terry Keeley · Jul 6, 2015

Thanks guys for all the replys, just got off the phone with him & I think we have plan of attack.

He's done a TON of work on it since running it 25 years ago with a hard shaft:

https://www.intlwaters.com/gallery/thumbnails.php?album=1072&page=1

Since these pics he completely redid the bottom filling in the front air dam as well as reducing the angle of attack on the sponsons:

Here's some more pix of the set up. You can see the big flat strut (my mistake!) he's using:

The plan is to table the boat and measure where the strut is now as it's running pretty good down the straight with an X455/3.

He's then gonna move the CG back to 10% aft of the after plane (2.3" or so), round the strut completely and put it back exactly where is was. I know from working with Mark Stein on his Mutt that strut placement on these full body boats is critical!

If the hull is sitting fairly flat now with the strut on the table we'll prolly keep working with X props. If it's way nose down we may flatten it out and try some 14 series.

In any case I think we're on the right path, thanks gang!

My Gypsy · Jul 6, 2015

I love seeing older boats like this one get pulled out of retirement and put back into action.

Bruce Clark · Jul 7, 2015

Years ago Gary P. told me to take the turn fin off my sport 40 to see if it still had a hop in the corners. I did that and the hop went away. After that all I had to do was get the turn fin and rudder to work together. Hope this helps you with your boat.

Jim Grams · Jul 9, 2015

In picture #6, where the boat is upside down, does it appear the strut is much deeper than the sponsons?

Terry Keeley · Jul 9, 2015

Jim Grams said:
In picture #6, where the boat is upside down, does it appear the strut is much deeper than the sponsons?

Sure does to me too. When it's confirmed I think we'll pull it up and try 14 series props...

Update, just talked to him, it's about 3/16" deeper than the sponsons, 14 series here we come.

rctinman · Jul 10, 2015

Terry, ABC 57x73, and 60x76 are also good round nose props.

Steve Ball

lohring · Jul 10, 2015

I use that fin profile in various sizes on boats from 1/10 scale up to gas scale. Below is the gas scale fin profile. The pictures also show the sponson to afterplane angle as well as the break. This design is one of the most forgiving ones, but still gives enough down force to hold the sponson in a turn. I went through several revisions to make the fin and mount strong enough for the gas scale boat. We ended up with 1/16 aluminum outside and 3/16 aluminum inside the 1/8 inch plywood sponson transom. The pictures show the first bracket that holds the 3/16 heat treated aluminum turn fin. The bracket again proved to be too light and I finally machined a 1/4" thick angle from a solid aluminum block.

Lohring Miller

SayMikey · Jul 10, 2015

Ok it's deeper than the sponsons

How deep is it from transom? How far forward is the ride surface on sponsons with the boat sitting on strut and sponsons.

Roundnose scale "porposing" in corners?

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