Strut location on TWINS ..... Need input ?

[Scott Schneider]

For you guys whom have run, Do run or know a thing about twins ?

I am doing my final assembly of a GAS twin and feel as designed / manufactured is not optimum as it came to me.

This hull was never finished and never been wet, so changes are getting done now rather than later on.

Questions that come to mind are:

As built, the strut C/L's are 4 1/2" which is engine C/L as well ... Should I move them closer together aiming for @ 1/4" between blade tips ? ( Which would be @ 1 1/4" closer )

What happens when the two struts are moved too have a right side offset ? ... In other words what ever there C/L spacing is, just get moved over to the right say 1/2" or so

Will an offset help counter prop walk ?

 

[Piston1]

Roadrunner Twin Xtreme prop centers 3-1/4" space. Running 1667s. CMB 91 HRs. B)

 

[dr gary turner]

We do not seem to have problems with prop walk with our twins. The wider spacing will keep the props hooked up in rough water AND there is NO smooth water in twin racing!!!!!!! Running more space between the props will often require less pitch to the prop. When they are closure they cavitate with each other and allows one to run bigger props with more pitch.

We run H 50's and a wide spacing between the props, more than the road runner. What you havei s not too bad. BTW I have been running twins for over 25 years. GTT

 

[Joe Warren]

For you guys whom have run, Do run or know a thing about twins ?
I am doing my final assembly of a GAS twin and feel as designed / manufactured is not optimum as it came to me.

This hull was never finished and never been wet, so changes are getting done now rather than later on.

Questions that come to mind are:

As built, the strut C/L's are 4 1/2" which is engine C/L as well ... Should I move them closer together aiming for @ 1/4" between blade tips ? ( Which would be @ 1 1/4" closer )

What happens when the two struts are moved too have a right side offset ? ... In other words what ever there C/L spacing is, just get moved over to the right say 1/2" or so

Will an offset help counter prop walk ?

Looks Good!! Run it as is....... I`am sure it will turn some Big Wheels. ( Bigger than 67mm ). Nice Work!

 

[Tom Foley]

Yeah , I say don't change anything and get it wet !

 

[Mark Bullard]

For you guys whom have run, Do run or know a thing about twins ?
I am doing my final assembly of a GAS twin and feel as designed / manufactured is not optimum as it came to me.

This hull was never finished and never been wet, so changes are getting done now rather than later on.

Questions that come to mind are:

As built, the strut C/L's are 4 1/2" which is engine C/L as well ... Should I move them closer together aiming for @ 1/4" between blade tips ? ( Which would be @ 1 1/4" closer )

What happens when the two struts are moved too have a right side offset ? ... In other words what ever there C/L spacing is, just get moved over to the right say 1/2" or so

Will an offset help counter prop walk ?

Doc was right. Him and I have had this discussion many times. What I have learned is no less than 3 3/4" C/L. I don't think that 4 1/2" will be a problem looking at the way you have your boat built. I built a boat sort of like that in 89 with K 90's and it was bad fast. Got a bad start in the first heat of F hydro at the 89 Internats. Overtook the field so fast in the back straightaway that I drove the boat into a bad position at bouy four. Blew the boat off the water and threw parts all over the water for 60'. Boat was broke in half at the front of the radio box, sponsons blew into pieces, no doubt having a bad day. But glued the boat back together and all of it's pieces back together and ran the boat the next couple of heats.

 

[rhomodel]

Scott I would run them as they are. Moving them closer would prevent you mounting/turning big props such as 31xx's. With twice the hp and not double the weight you need to think differently about props. Forget about H-50, 1667, think 2170, 2175, 3016, 3114. My twin I run 3418 & 3317, but that is an inline.....

 

[AWaddle322]

We do not seem to have problems with prop walk with our twins. The wider spacing will keep the props hooked up in rough water AND there is NO smooth water in twin racing!!!!!!! Running more space between the props will often require less pitch to the prop. When they are closure they cavitate with each other and allows one to run bigger props with more pitch.We run H 50's and a wide spacing between the props, more than the road runner. What you havei s not too bad. BTW I have been running twins for over 25 years. GTT

Doc,

actually they are not cavitating together. The closer two props are the more stern lift they willl create. two props close together act as if they are producing one thrust cone instead of two. You are able to run more pitch with them close together because it is surfacing and supporting the stern lift that the propellers are producing. It is a trade of though because the propellers will slip more due to not having as much propeller surface in the water. Look at the full size race boats with stagered set ups to understand more. Stagered set up boats are bad fast in straights and give up some in the turns because the propellers will not support the sternlift they produce while going straight

Scott, if you try as it is and it does not lift the stern of the boat enough try 3 blade props. the more blades on a propeller the more it will surface the prop.

Allen

 

[Scott Schneider]

Thanks for the input thus far, it's all good stuff !!

No one has addressed the question of right side offsetting on both drives to counter prop walk ?

 

[Jeff Lutz]

For you guys whom have run, Do run or know a thing about twins ?
I am doing my final assembly of a GAS twin and feel as designed / manufactured is not optimum as it came to me.

This hull was never finished and never been wet, so changes are getting done now rather than later on.

Questions that come to mind are:

As built, the strut C/L's are 4 1/2" which is engine C/L as well ... Should I move them closer together aiming for @ 1/4" between blade tips ? ( Which would be @ 1 1/4" closer )

What happens when the two struts are moved too have a right side offset ? ... In other words what ever there C/L spacing is, just get moved over to the right say 1/2" or so

Will an offset help counter prop walk ?

Doc was right. Him and I have had this discussion many times. What I have learned is no less than 3 3/4" C/L. I don't think that 4 1/2" will be a problem looking at the way you have your boat built. I built a boat sort of like that in 89 with K 90's and it was bad fast. Got a bad start in the first heat of F hydro at the 89 Internats. Overtook the field so fast in the back straightaway that I drove the boat into a bad position at bouy four. Blew the boat off the water and threw parts all over the water for 60'. Boat was broke in half at the front of the radio box, sponsons blew into pieces, no doubt having a bad day. But glued the boat back together and all of it's pieces back together and ran the boat the next couple of heats.

1989? With K90's I think you may have meant 1999?

 

[Tom Foley]

Thanks for the input thus far, it's all good stuff !!
No one has addressed the question of right side offsetting on both drives to counter prop walk ?

I have thought about doing it ....I just try to hold it wide open and find a hole to go through !!!!!!

 

[Andy Greene]

Thanks for the input thus far, it's all good stuff !!
No one has addressed the question of right side offsetting on both drives to counter prop walk ?

I have thought about doing it ....I just try to hold it wide open and find a hole to go through !!!!!!

Tom I'm always so blown away watching them run , I never really paid attention to the driver-

do you have to put left in it at WOT ??

 

[Tom Foley]

Thanks for the input thus far, it's all good stuff !!
No one has addressed the question of right side offsetting on both drives to counter prop walk ?

I have thought about doing it ....I just try to hold it wide open and find a hole to go through !!!!!!

Tom I'm always so blown away watching them run , I never really paid attention to the driver-

do you have to put left in it at WOT ??

Yeah sometimes depending on how good you came out of the previous corner etc .

 

[Buckshot]

Thanks for the input thus far, it's all good stuff !!
No one has addressed the question of right side offsetting on both drives to counter prop walk ?

I don't think that is needed. The biggest twin I've ran was a twin 90 with 1470's cupped up and the boat tracked straight in all sectors of the course. It is all in the props and set up. Riggers should be set up with the drive train centered and true.

Looks like you have a Waterloo rigger from the photos. Have fun.

-Banned-