Unknown 80's 42" Mono quest for high speeds

[John Finch]

I've always been a fan of aerodynamics. Anything that may have any affect on attempting faster travel of anything... That being said... John Finch did an article on pushing a mono hull to higher speeds. You may or may not know about that article. In any case it's well worth reading. And, if you've read it before it's worth reading again! I have to admit some of what he found surprised me more than once... It's in Advanced R/C Boat Modeling. Published in '92 and republished in '94. Sorry, I am NOT a fan of mono's. But, I found this well worth reading! Chapters one and two.
Thanks, John!

Ken

Ken,

I am glad to be of help. I have learned so much since that book was published and still learning. last weekend me and my buddy Steve got to put the concepts of weight, cg, rudder placement and prop design to work, and after 5 hours of constant testing we gained 11 mph on his mono saw setup. Things change every 10 mph gain in speed and aerodynamics is a big part of it when only one inch of the boat is touching the water.

 

[K F Wheeler]

Hi, John.
Definitely sounds like you've made good progress though... I have a lot of patience. I better have since I run nitro. But, I don't know that I would have enough hair on my head or the patience to do what you two have done!! A high speed camera would certainly help. Even several action shots may help. That may be what you'll ultimately need to gain much more speed at all. Even two sets of eyes can only see so much.
It would be nice to read more of what you've learned since then!!
Good luck!!

 

[John Finch]

We have been using cameras and you are right! Liberty told me about the insta 360 camera and I have been using that attached to my ball cap. Still learning how to use it. One time everything was upside down. Jim Skelton has been filming us as well on his phone. Everything happens so quickly that the only way to figure out what happened when the boats blow off the water is go to the film. With one boat the transom was lifting out of the water then the boat went crazy. With another boat it just got so light the boat took off like an airplane just as smooth. Another time the boat was rocking left to right so slightly but the camera told the story. Rounding the bottom of the rudder took care of the rocking because the rudder was causing too much lift. Amazing the tools we have today that we did not have years ago..

 

[LibertyFrancis]

We have been using cameras and you are right! Liberty told me about the insta 360 camera and I have been using that attached to my ball cap. Still learning how to use it. One time everything was upside down. Jim Skelton has been filming us as well on his phone. Everything happens so quickly that the only way to figure out what happened when the boats blow off the water is go to the film. With one boat the transom was lifting out of the water then the boat went crazy. With another boat it just got so light the boat took off like an airplane just as smooth. Another time the boat was rocking left to right so slightly but the camera told the story. Rounding the bottom of the rudder took care of the rocking because the rudder was causing too much lift. Amazing the tools we have today that we did not have years ago..

Still lots for me to learn here...

Could you tell me more about the rounding the bottom of the rudder? My assumption was that rounding the bottom leading edge would create additional lift, but it sounds like you are saying the opposite?

 

[John Finch]

Liberty,
The area you have marked in red is a good leading edge but what I am referring to is the very bottom of the rudder from leading edge to trailing edge. When a mono gets up on the last inch or so the bottom of the rudder acts as a lifting surface. I know the bottom is small but the boat and rudder are at a positive angle of attack when at top speed so the rudder bottom lifts the boat on the side that it is mounted on. In some cases I slope the bottom and round it off so when the boat is at full speed the rudder has no lift from the bottom of the rudder. you can also just kick the rudder back so the bottom of the rudder does not lift.
keep in mind that the rudder itself causes lift despite the bottom configuration. The deeper the rudder the more the lift. Also, the more distance from the center of the boat the more it lifts that side of the boat. So, the rudder comes to the forefront in trimming the boat for saw runs.

 

[LibertyFrancis]

Liberty,
The area you have marked in red is a good leading edge but what I am referring to is the very bottom of the rudder from leading edge to trailing edge. When a mono gets up on the last inch or so the bottom of the rudder acts as a lifting surface. I know the bottom is small but the boat and rudder are at a positive angle of attack when at top speed so the rudder bottom lifts the boat on the side that it is mounted on. In some cases I slope the bottom and round it off so when the boat is at full speed the rudder has no lift from the bottom of the rudder. you can also just kick the rudder back so the bottom of the rudder does not lift.
keep in mind that the rudder itself causes lift despite the bottom configuration. The deeper the rudder the more the lift. Also, the more distance from the center of the boat the more it lifts that side of the boat. So, the rudder comes to the forefront in trimming the boat for saw runs.

Understood, thanks for explaining that. Almost like a small ride pad on the bottom of the hull.

 

[K F Wheeler]

YUP! See there! John you just taught me something else. Had never thought about rounding the bottom of the rudder front to back. Only the tip and across from side to side. I think I may have thought about from side to side only as a possible lifting feature. But, not so much as front to back as a lifting feature. That little tiny as it may be, fact, means you're really thinking hard on the subject. Typical John Finch... My defense is I only run hydroplanes! Lol

Ken

 

[Timbo Whalen]

I was told once by a full size boat engineer
with Volvo Penta that once the critical speed of a common mono hull has been reached, it requires like double the horsepower to gain only a small percentage more speed...

Don't know how true this is, but seems plausible due to the hydrodynamic forces involved in the surface area of the wetted surface.. which is why you see stepped hulls. Less drag.

Of course full size doesn't always equate to our small boats, as evidenced by our boats blowing over as the speed goes up.

If full size commercial boats blew over with the frequency of our little boats, there wouldn't be any boat builders left on the planet..

Your engineer buddy is partially correct. 'Double the horsepower' isn't quite true. The factor is exponential. The most efficient mono hulls are of the ride pad design-ie- Allison, Hydrostream, etc. Prop slip % also is relevant. Generally speaking, with the above hulls in mind, for every mph over 85-90 mph an additional 10-15 hp is required- to a point. However, as speed increases so do parasitics, which also increase slip % and reduce efficiency.
So that's where things become exponential.
Yes, there would come a point where doubling of horsepower would be required, but that would be well beyond a given hulls efficiency potential.

And yes, full size performance boats do blow over with the same regularity as rc boats. Some physical laws don't care about the effects of scaling.

 

[John Finch]

Ken,
You say you only run hydroplanes. I respect that. My favorite hulls are hydros as well. I love to watch a hydro floating down the saw with the sponsons riding on just a film of air beneath them with little to no water coming off the trailing edges of the sponsons. The slippage rate is so low with hydros that the sky is the limit. With the mono hulls the slippage rate is the key to saw speed. You have to get slippage down to get record saw speeds. All the setup strategies come down to that one thing: Reduced Slippage! After playing with the mono saw stuff Steve and I plan to pull the equipment from the boats and possibly tackle some sport hydro records. Sport and scale hydros are my favorite racing hulls. That will be really exciting. Got to keep it interesting.

 

[LibertyFrancis]

I think it will be time to retire this boat.
Earlier today I went to remove the strakes and although the inside of the hull makes it appear they are additions on top of the V-shape not all of them are.
I burnt through the hull in several places. I don't think I'll be patching it up although I could.

In some ways I was looking for an excuse to upgrade to a modern mono hull. A friend has been pushing me to get a Seaducer and will likely go that route

 

[Timbo Whalen]

I think it will be time to retire this boat.
Earlier today I went to remove the strakes and although the inside of the hull makes it appear they are additions on top of the V-shape not all of them are.
I burnt through the hull in several places. I don't think I'll be patching it up although I could.

In some ways I was looking for an excuse to upgrade to a modern mono hull. A friend has been pushing me to get a Seaducer and will likely go that route

Can't go wrong with a Seaducer....really important to follow Jerry's rigging instructions implicitly.

 

[john v]

Oh yeah, seaducer is a fast boat but fussy about setup, once you have it dialed in you will love this hull

 

[Mark Poole ModVP]

Curious to know what brand hull that was you were testing. Never seen a mono with three strakes per side and they are wide strakes too.

 

[LibertyFrancis]

Curious to know what brand hull that was you were testing. Never seen a mono with three strakes per side and they are wide strakes too.

I have asked around and no one seems to know.

 

[David Bryant]

Liberty,
the very bottom of the rudder from leading edge to trailing edge. When a mono gets up on the last inch or so the bottom of the rudder acts as a lifting surface.

interesting, didn’t realize such a small area could create that much lift

 

[John Finch]

David,

With heat racing boats the rudder does not effect the ride of the boat as much as when doing saw runs. As the boats get faster and lighter on the water it is easily seen how much the bottom of the rudder causes lift. The boat will lean to the left or chine walk at 90 mph when it rides great at 60 mph.

 

[Steve Bobbitt]

I really have enjoyed this thread. It has great support too.

 

[LibertyFrancis]

Small update. I have the 39" SD4 Seaducer and enjoying this boat so far. It takes turns at speed so much better than the old 42" unknown brand mono did.

My first outing (using the same motor, esc, LiPos, and prop) it was running too wet and hit 80 mph. Another attempt with the LiPos shifted as far back as possible the boat came up out of the water fully and almost blew over at 86 mph. I believe next time out I know where the batteries need to be and should be ready to rock n' roll.