Driveline for Dumas SK Daddle (.40 size, 36")

[dannyUAL767]

Hello everyone. I'm finishing up my old style flatbottom hull and I've been reading up on bufferoo's experiences with his smaller SK Daddle Too. I'll agree that these Dumas wood boats have parts that don't fit very well, parts that are warped and instructions that just flat suck :mad: ! I've used all of my 41 years worth of experience on God's green earth to precisely assemble this boat, straight and warp free. I've measured and remeasured, used bubble levels, straightedges and a flat building board to end up with a very true hull.

I'm thinking about using a surface drive system, exiting the transom, with a prop that hangs way off the transom like the modern, "state of the art" racing monos use. BTW, I modified the boat's stringers to be the standard 5" spacing used in most fiberglass boats these days. I've got several engines in my collection to use: OPS .65, .67 and K&B .67, .82. And, I could always try out my Z230 gas engine in my Aeromarine Avenger hull :D .

In another thread, someone said that the rudder and prop should be under the boat for proper handling. It just seems like "old school" to install the running gear there :confused: . With the flatbottom design of the boat, I know that handling is not going to be very good anyway. It's certainly not going to carve turns through the water. It'll probably skid more than anything. I think that my surface drive system will give great straightline speed on smooth water. What do y'all think?

[bufferoo]

I'm thinking the same thing. I have also thought about adding a small turn fin right where the keel meets the transom just to give it a little bite in the turns. Maybe hold 1 - 1 1/2" into the water to help the turns.

But isn't the squirrelyness (is that a word :confused: ) part of the fun of these flat hulled boats? What would a crackerbox be like with a nice deep vee?

Bufferoo

[Hydro Junkie]

Gentlemen, you are forgetting one little word.........PROPWALK!!!!!! Yes, the boat was designed for a subsurface drive and yes, prop and rudder under the hull may be old school and yes, the boat will skid out of control without a skeg under the hull. With this style of boat, you almost have to take corners like a hydroplane. Having a surface drive will mean probably needing to dial in a little left rudder trim to hold the boat straight. This means drag, meaning lower top speed. If you want to have something different, why not look at a flex shaft and subsurface drive? That might be the best of both worlds, new and old :)

[dannyUAL767]

Hydro Junkie, I get propwalk with my gas powered Aeromarine Avenger and it's not fun. It pulls pretty good at full throttle :mad: . I'm sure this SK would be worse with the flat bottom.

Now, when you say subsurface, are you talking about the traditional flex cable system that exits the bottom of the hull ending with the prop parallel to the bottom of the hull? I have a Dumas DV20 with this setup. I'm using a Marine Specialties VOA outdrive with the prop about 2 1/2" behind the transom. The prop's top blade would just touch the bottom of the hull if it were right under the boat instead of 2 1/2" behind it. This boat runs and handles well and has no propwalk. However, it's not very fast. I'm running a tired 26 yr old Thunder Tiger .25 airplane engine with an Octura Kool clamp that I bought when I was a sophmore in high school :D . The wood boat is very heavy since I covered it in 3/4 oz cloth and polyester resin. It's tough as a tank and just as heavy :( .

If this is what you are talking about, I'm all ears! I don't want propwalk! Yes, I know the boat is going to skid in the turns and won't carve like my DV20 or even like my Avenger. I could run a strut mounted off the transom similar to the way my DV20 is. I'd also mount the rudder off the back as well. I'm planning on using the new, high tech, machined aluminum hardware that you see on many of the high dollar, fast gas boats these days. The same stuff that I've got on my Avenger. I guess I could start with a prop depth with the top blade even with the bottom of the hull and go from there.

bufferoo, that's a neat idea that I'll keep in the back of my mind about the turn fin. I was thinking about adding a skeg under the middle of the boat but I thought that I'd read somewhere that that can make these boats grab and spin out :confused: .

[Hydro Junkie]

I wasn't thinking about 2+ inches behind the hull, but yes. The reason your other boat isn't very fast has more to do with the TT .25 power plant than the extra weight of the glass. The airplane motor has considerably less power than the marine versions, as well as sacrificing almost 10K RPM. You might consider pulling the .25 and drop in a marine .21, or if you're really feeling crazy, a .45. If you keep the nitro down to around 20%, a .45 would be a good upgrade. Getting back to the new boat, if you run the flex drive and put a short wood glue block between the stuffing tube and the boat bottom, you probably wouldn't need a skeg. The flex shaft would act like a centerboard on a sailboat(to a point) helping the boat to corner more like a deep vee. The wood block would be right behind where the shaft exits the bottom, bracing the shaft against impact as well. Hope these ideas help, but don't feel obligated to use them if you don't wish too :D

[bufferoo]

Here's what I was thinking...and by all means let me know where I've gone wrong.

To counter the propwalk I had thought about not having the shaft exit the transom at a 90 degree angle but rather shifted maybe 1/8" off to the right (or is that left? It's early here). This would shift the thrust pattern a bit to the side and "hopefully" ballance out the propwalk situation.

Now I have no objections to a subsurface drive, but I REALLY don't want the strut under the hull. I have stuff like that and I just don't like it. Not to mention that as far as I can see it can really mess with the driveline angles with the shorter cables.

Am I wrong on any of this?

Bufferoo

[Hydro Junkie]

If you want a transom mounted strut, go for it. You have to remember that in R/C boating, suggestions come in all forms and from all directions, but the last word comes from the owner. If I remember right, you would have to angle the shaft to the left, as the prop would push to the left. A word of caution though, playing with shaft side angles is an inconsistant science. What might work well at one speed may screw up handling at another.

[dannyUAL767]

Hydro, I've been considering an engine upgrade to my DV20 since I first got it on the water about 2 yrs ago. I've been thinking about the O.S. .21RG-M that I've seen in the TowerTalk for as low as about $115 a while back. Of course I'd still need a header and pipe for it but with 1.7bhp@28,000rpm, I'm sure the boat would at least double it's speed!

Like bufferoo, I want to use a transom mounted strut. A machine shop (where I used to live, near Ft Lauderdale) makes some incredible looking stuff and their mono gear was just wicked looking. The thought of having an old school, 1960's design boat with 2004 hardware hanging off the back is just too cool to pass up :D . However, I'd rather have it run right than look super cool and run poorly. I guess I'll lean towards running a subsurface prop with the transom mounted strut.

Now confirm for me please; propwalk is a problem for surface drives only? Do riggers experience propwalk? Like I posted earlier, my Avenger experiences it at full throttle. It really pulls :( . What can I do to lessen the propwalk on that boat? I thought that I read a while back that a super sharp prop can help minimize propwalk. How do all the racing monos deal with propwalk with their surface drives :confused: ?

[Hydro Junkie]

Unfortunately, propwalk is a problem with ANY surface drive. The reason it doesn't effect subsurface drives is the prop is in a balanced state while under water. What I mean by balanced state is the prop is pushing constantly with ALL BLADES. This means that the drag is always even on both sides of a two blade prop. With a surface drive, the prop IS NOT in a balanced state. This is because only half of the prop is in the water at any one time, while at speed. This makes the prop, regardless of how sharp, act like a paddle wheel, pushing the transom to the left with a standard direction prop and engine. To take advantage of the propwalk, most races are run clockwise, using the propwalk to help push the transom out, improving cornering. While running down the straights, the drivers usually trim the boats with some left rudder, hold some left rudder or offset the prop angle to the left(not recommended) to hold the boat in a straight line. Hope this answers some of your questions :)

[dannyUAL767]

Hydro, that's a great easy to understand explanation of propwalk and why it affects surface drives! I'm glad to know that nothing is "wrong" with my Avenger and that the racers experience the same thing with their boats. It's always annoying to have to add in some rudder when heading straight under full throttle.

Now, knowing what causes propwalk and why it affects surface drives, why would it affect my flatbottom, 1960's boat more than a new design racing mono. Aren't some of these monos a very flat bottom design as well? I'm sure they're not as flat as my SK Daddle's bottom!

[mjmsprt40]

I used to have a SK Daddle .40. Now, I know you're trying to avoid "old school" stuff, but this boat is really old school. Mine had a straight shaft, with a short stub shaft connecting it to the engine to allow for any slight mis-alignment. It had a skeg that served to hold the shaft log where it exited the hull, and I have little doubt it helped with directional stability too. A turn fin on these boats is a requirement, because that flat bottom gives no purchase whatsoever for turning otherwise. I ran mine with the K&B .40 Sport Marine engine, and that seemed to be a happy combination for this boat. The propeller and rudder were both underneith the boat, and as shallow as this boat runs at speed I have doubts that a surface drive would do more than raise a fuss and cause unhappiness at the poor performance.

[Hydro Junkie]

Adding to what mjmsprt40 said, all the surface drive monos have the same problem. It's more pronounsed with a totally flat bottom boat. I would think about what he said he had to do to get his SK to work and incorporate some of it into your boat

[dannyUAL767]

I'm pretty sure that I'm going to run a subsurface drive with a transom mounted strut. My flex cable will exit the bottom of the boat and go parallel to the bottom and then extend behind the transom. I'll be able to adjust the height of the prop with this setup. This is how I've got it on my little DV20 and it rides pretty good, just slow with that tired airplane .25! Since I'm going to be powering this SK with either of my 2 OPSs (.65, .67) or either of my K&Bs (.67, .82), I don't think that it'll be too slow :D .

mjmsprt40, when you say that a turnfin is a requirement, are you talking about the skeg under the center of the hull (right about under the engine) or are you talking about something hanging off the transom on either side like I see on some deep vees?

[mjmsprt40]

The turn fin is the short skeg underneith the engine. It doesn't need to be much, mine was a short piece of metal, triangular in shape with the trailing edge being sraight up. I think it was about 1" deep, but it has been awhile since I had that boat. There were no fins, other than the rudder, on the stern.

[dannyUAL767]

Thats what I thought. I have a real 1973 Sanger flatbottom dragboat with a big block Chevy and it has a skeg right under the center of the boat. This boat doesn't turn very well either. It takes wide sweeping turns with a lot of radius. It sure does go good in a straight line, though ;) !

I believe that Marine Specialties sells two different size skegs for rc boats.

mjmsprt40, did you ever run yours without the skeg? It seems like you might have. Tell me a little more about how it ran without and then with the skeg. Thanks!

[mjmsprt40]

I never ran it without the skeg. I got the boat from a fellow club member, and it had the skeg on when I got it. However, any question about the need for a skeg can be cleared up with a little thought on the matter. You have a flat, or nearly flat, bottom trying to go into a turn. The rudder does its job well enough, but with no skeg that boat is going to sideslip something fierce. Think about a skipping stone and you get the idea. Now, add in another possibility. With the boat sideslipping through the turn, there's every possibility the outer chine could dig in and flip the boat.

(Note: I got much good advice concerning turn fin/skegs from my fellow club members, which is where I learned of the need for these things. I have never seen a flat-bottom boat, and few hydros, designed to run at speed without one. I am presently building a .40 size Crackerbox, and of course it has a skeg mounted just aft of the engine mounting. There is no way it could be run successfully without it, unless you're doing straight-line runs only.)

[dannyUAL767]

I feel like a total moron :o . I just took another look at my SK instructions and here is what the very last section says: The SK-Daddle and SK-Daddle Too will require a fin on the bottom under the flywhell to keep the boat from sliding in a turn. For SK, 1 1/4" wide by 1 1/2" deep. For SK Too, 1" wide by 1 3/8" deep.

I guess I'll order one from Marine Specialties!

[bufferoo]

getting rid of all that silly external hardware; prop, strut, shaft, skeg, rudder etc and going with a jet drive unit? I'm seriously thinking about using one now. I kind of like the way the boat may slide as it turns. Just saw a jetboat race on TV and it got me thinkin'.

Thoughts?

Bufferoo

[mjmsprt40]

I've been thinking of doing the jet-drive on a Prather DV-40 that I have. It's been the absolute pits as a propeller boat, it barrell rolls something fierce. The jet drive just might fix that. Then again, maybe not but it might still look cool with the rooster tail that jet drives produce.

As far as trying to do this with a SK-Daddle, hmmm.... They do it with full-size flat bottom boats, it ---might--- work ok in this one. The problem is that it's expensive if it doesn't work out as well as you'd hoped.

[Hydro Junkie]

Why not try it and let us all know how it works. At best, the boat will handle like a dream. At worst, you pull out the jet drive and put it in something else

[dannyUAL767]

I second what Hydro Junkie said!

I'm a prop type of guy and thats why I bought a v-drive boat back in 1986 vs a jetboat. When it comes to rc boats, the idea of all this "complex" gear underneath the boat intrigues me! But then again, I'm a real mechanically minded type of person.

BTW, those skegs from Marine Specialties are only $9.50 each plus shipping. I've got some other things that I'll get from them at the same time.