Flat Bottom twin rigger setup

[Andy Brown]

I think optimum tub height has has more to do with over all hull design and not just a certain height for a certain size or "the higher the better".

Marty experienced poor handling with a 67 boat that was 5/8" off the water and some are running 21 boats that are much higher than 5/8" off the water.

I raced a 45 SG with a MAC 67 engine at the IMPBA Internats in E Hydro and tied for 1st place in heat racing. The 45 SG is only 5/8" off the water.

I don't need to expound on how many major races the 45 SG/SGX has won running 5/8" off the water.

My flatbottom Mongoose 21 hull was set at 3/4" high. The rocker bottom SGX 21 is closer. As Chris mentioned, we tried the original flat bottom Eagle set at less than 5/8" and it was not a good rough water boat, but the narrow rocker bottom hulls do work well close to the water.

 

[Chris Wood]

Hi andy

Do you think the rocker bottom allows you to be able to run the boat highr off the water or the vis versa?

 

[John Finch]

I think optimum tub height has has more to do with over all hull design and not just a certain height for a certain size or "the higher the better".

Marty experienced poor handling with a 67 boat that was 5/8" off the water and some are running 21 boats that are much higher than 5/8" off the water.

I raced a 45 SG with a MAC 67 engine at the IMPBA Internats in E Hydro and tied for 1st place in heat racing. The 45 SG is only 5/8" off the water.

I don't need to expound on how many major races the 45 SG/SGX has won running 5/8" off the water.

My flatbottom Mongoose 21 hull was set at 3/4" high. The rocker bottom SGX 21 is closer. As Chris mentioned, we tried the original flat bottom Eagle set at less than 5/8" and it was not a good rough water boat, but the narrow rocker bottom hulls do work well close to the water.

Andy,

A bunch of years ago I had three sg 45 riggers all with the curve at the transom bottom. I built up the bottoms on two of the boats so that one was flat and one was a comprimise between flat and recurve. I found the flat bottom to sometimes lift and cause the boat to stuff, but the flat bottom was the fastest of the three boats. I made more changes and liked the one with the 1/8th inch total recurve at the transom for top speed but no stuffing. I tried all kinds of rear sponson depths using the two outsides rears and the center shoe. My favorite was the one with the tub top parallel with the set up table resting on the center shoe with the outside shoes/sponsons 1/8th inch off the table. You were right about the rear feather looking winglets. I installed them on the popstick and saw the results first hand. Something you can appreciate at record trials.

 

[John Finch]

Hi john

i agree. i had a 20 boat one time that was about 5/8 off the water.. it was like a snow plow. my current boat is 7/8 and have wondered if i could go more. What i don't know is how far is to far. does the size of the boat matter? meaning 21 45 80 ect.

I know from past when i ran eagles.. now take in mind this was ALONG time ago. they were real close to the water. we always had issues with the boat. we had to loosen them up. they they got so loose we couldn't keep them on the water. it was a balance act we always had issue's with. i have no idea what the current boats are like vs the old ones. my guess is the tub height is different maybe andy could chime in?

Do you think the closer to the water the more air pressure. or the taller off the water the faster the air is able to escape from underneath? i could see it being closer the more controlled the air is. which might explain why the closer to the water the TIGHTER the boat is. compressing air but not allowing it to escape fast enough? This is a good topic. i wish i was smarter and new more about dynamics and what not.. i am just a schmuck trying the hard way lol

i would love to be able to build a cheap wind tunnel with some sort of smoke to see the effects of some of the things we talk about.

not to get to out of sorts. What are your thoughts on the down force effects of the OUTSIDES OF THE SPONSONS?? everyone talks about tops and bottoms but i think there is ALOT of effects being good or bad on the outside we are missing.

chris

Chris,

I am sure more non trip on the side may add aerodynamic as well as hydrodymanic lift but I have not experimented with that. What I think may be more important on the sides of the sponsons is how the water breaks off them. Especially getting a boat launched. Any water climbing up the sponson is holding the boat to the water and making it hard to break loose. A deep vee has a lot of angle on the bottom of the hull but it will still blow off the water so I feel a sponson with a lot of non trip side would do the same.

 

[Joe Warren]

John what is the highest you have been so far?

 

[Andy Brown]

Chris, The rocker bottom allows you to run closer.

 

[Terry Keeley]

I built my new SAW boat with a completely flat top & bottom with the nose in the center for an aero neutral tub. I bet it still makes lift tho by packing air in ground effect.

 

The faster we get these things going the more they want to fly so making enough lift isn't a problem anymore.

I think some of the things we do are a throwback to what we always did, swept transoms included. They, along with big attack angles on the fronts were needed to help lift the nose, all to counteract a 4* + hard shaft angle in a much heavier boat.

It's all just a balancing act as far as I can tell, you need enough lift on the front to balance the lift in the rear from the prop, rudder and rears/ski. You also need balance side to side to offset torque and propwalk.

The next heat boats I build will have flat tubs, just simplifies things...

 

[DON MAHER]

Terry,

Have you run a flat tub with no rears/ski? If so, what were the results?

Don

 

[Terry Keeley]

Terry,

Have you run a flat tub with no rears/ski? If so, what were the results?

Don

Yup, my SAW boat, was trying to tell if the ski was the making the back blow out. Originally I had a ramped ski which caused lift and when I took it off the back didn't come unhooked as easily, I later made a completely flat ski which worked better for my setup. With the flat ski there was no difference once it got up on the prop, the ski was only there for the launch and slowing down for the corner.

It's all just a balancing act IMHO...

 

[John Finch]

John what is the highest you have been so far?

I am traveling today from Atlanta to home. I will measure and let you know for sure by tomorrow.

 

[John Finch]

Terry,

Have you run a flat tub with no rears/ski? If so, what were the results?

Don

Yup, my SAW boat, was trying to tell if the ski was the making the back blow out. Originally I had a ramped ski which caused lift and when I took it off the back didn't come unhooked as easily, I later made a completely flat ski which worked better for my setup. With the flat ski there was no difference once it got up on the prop, the ski was only there for the launch and slowing down for the corner.

It's all just a balancing act IMHO...

Terry,

I looked at your ski at the spring saw event and was surprised. Are you saying on the set up table the ski has zero angle of attack?

 

[Terry Keeley]

Terry,

I looked at your ski at the spring saw event and was surprised. Are you saying on the set up table the ski has zero angle of attack?

Yup, zero angle of attack, completely flat and parallel to the flat bottom.

You can see it pretty clear in this shot:

ps: I want your avitar! :lol:

 

[Joe Warren]

Great pic, Martins boat was carring the ramp out of the water in huntsville last weekend on his record runs.Just setting on the prop @ speed. Dont know the angle or flat.

 

[John Finch]

Terry,

Have you run a flat tub with no rears/ski? If so, what were the results?

Don

Yup, my SAW boat, was trying to tell if the ski was the making the back blow out. Originally I had a ramped ski which caused lift and when I took it off the back didn't come unhooked as easily, I later made a completely flat ski which worked better for my setup. With the flat ski there was no difference once it got up on the prop, the ski was only there for the launch and slowing down for the corner.

It's all just a balancing act IMHO...

Terry,

I looked at your ski at the spring saw event and was surprised. Are you saying on the set up table the ski has zero angle of attack?

Yup, zero angle of attack, completely flat and parallel to the flat bottom.

You can see it pretty clear in this shot:

ps: I want your avitar! :lol:

Terry,

Your center shoe works well. I have been playing withe the center shoe for some time now. I tried a deep V shoe that works similar to yours. Just for the launch. Then the prop carries the boat down the straights. My twin saw record boat had both the 2 degree sho full length up to the motor and two short shoes on both sides of the center shoe about the same length as yours but still at 2 degrees. The boat sometimes blows out at the transom when letting off the throttle so I might change to zero angle like yours. Thanks for the tip!

 

[John Beardslee]

Hi John,

You've probably already thought of this but here goes anyway. I'm not so sure that the transom blowing out when you let off at high speed is a hull design problem. If you were looking straight on from the rear when it happens the transom probably moves to the right as well. The prop blade that is in the water as the engine slows now creates a ramp and is also on a helix that would cause a simular situation. If you locked up an engine at those speeds it would probably be even more spectacular.

John

 

[John Finch]

John,

That would explain why the pushrods comes back bent when it happens. I also had it happen with my gas sport hydro several times. Come to think about it Steve's saw mono would cut really hard after letting off the throttle probably for the same reason. Those big three inch diameter props are the problem!!!!!

P.S. I don't have that problem with the heat racing boats. Only when slowing down too fast at 100 mph with the SAW boats. I had suspected that after tearing up two servos and some linkages with my gas sport boat during testing last Spring and cured the problem by moving the rudder closer to the transom. The twin boat still had the long rudder extension on it. DUH!!!!

 

[thomaselkins]

again great reading, wish stu would put in his take on it?

 

[John Finch]

Joe,

Just got in from a 12 hour trip from Atlanta to Richmond. All my gas riggers run 1 1/8th to 1 1/4 inches off the table. There is one boat that is 1 7/8ths off the water but that was for SAW use. It is not a rigger and I will be experimenting with it next year for heat racing to see if the boat will heat race that high off the water. Tired and ready for bed. Good night.

John

 

[Joe Warren]

John you are there 1 1/8 to 1 1/4 plus for heat racing.