Recessed Transoms?? Motor Backset??? How do you decide??

[Darin Jordan]

Guys,

I've been trying to read up on tunnels, and have a couple of questions that generally revolve around the idea of motor backset...

HOW do you decide if you should run a recessed or flush transom??

What determines this and what are the general effects of one or the other?

What handling/speed characteristic is being affected by increasing or decreasing the backset of the motor??

Is there a specific distance from the transom to the prop, or from the ride-pads to the prop that we are after??

Also, I've noticed on some of the tunnel pics I've seen people have been adding 1/4" or so to the ride-pad area by simply glueing a wooden block extension to the transom of each sponson... I've seen this done DURING construction, as well as after the fact... WHY would one do this over simply making the sponson longer in the first place??

Just trying to get a better understanding of what I'm doing with this tunnel stuff... Any information would be absorbed and greatly appreciated...

--Thanks

 

[Darin Jordan]

One more question... Are there any books out there that would discuss tunnel design that would be "must reads" for someone interested in building/designing their own??

 

[Jerry Dunlap]

One more question... Are there any books out there that would discuss tunnel design that would be "must reads" for someone interested in building/designing their own??

Motor set back(as well as other model tunnel boat design aspects) has sorta evolved over the last 20 years. The set back used on the TS2 was based on the theory of "Let's See How This Works." I've seen a book advertised that delt with tunnel design theory. However, I have always used the theory stated in the second sentence. One nice thing about designing any model boat is no one gets injured or killed if the design doesn't work. The designer of the TS2 is a retired school teacher(30.5 years)/Zamboni driver(9.5 years). After the race on Sunday, I can give you a detailed account of all I know about designing model tunnel boats. Shouldn't delay your trip home more than 5 minutes. :lol:

JD

 

[Darin Jordan]

After the race on Sunday, I can give you a detailed account of all I know about designing model tunnel boats. Shouldn't delay your trip home more than 5 minutes. :lol:
JD

:lol: :lol: :lol:

Hey... wait a minute... I thought the race was on Saturday??? :huh: I'd better go re-read the e-mail Jim sent us!

Thanks Jerry... You can bet I'll be watching and seeing how you guys go about things... I'm definatly not trying to re-invent the wheel here... Just don't have a lot of resources to spend heading down the wrong path...

I've found some stuff on the internet that should help... the rest I'll get from chatting with experts like you and from observation... and, of course... TESTING...

Some of this stuff is already becoming useful in the redesign of my sport hydro that I'm undertaking this winter... especially the aero parts...

 

[Scott Norris]

One more question... Are there any books out there that would discuss tunnel design that would be "must reads" for someone interested in building/designing their own??

Motor set back(as well as other model tunnel boat design aspects) has sorta evolved over the last 20 years. The set back used on the TS2 was based on the theory of "Let's See How This Works." I've seen a book advertised that delt with tunnel design theory. However, I have always used the theory stated in the second sentence. One nice thing about designing any model boat is no one gets injured or killed if the design doesn't work. The designer of the TS2 is a retired school teacher(30.5 years)/Zamboni driver(9.5 years). After the race on Sunday, I can give you a detailed account of all I know about designing model tunnel boats. Shouldn't delay your trip home more than 5 minutes. :lol:

JD

Jerry I've never gotten very excited about teaching school or having tunnel hulls but Dang! I Always Wanted to Drive a Zamboni!!!

 

[Flyguy55]

Hey Dunny

When you reinvent the Bolt a Boat , make the sponsons adjust fore and aft. then you can research and explore set back

I loved my bolt a boat grin

Motor set back(as well as other model tunnel boat design aspects) has sorta evolved over the last 20 years. The set back used on the TS2 was based on the theory of "Let's See How This Works." I've seen a book advertised that delt with tunnel design theory. However, I have always used the theory stated in the second sentence. One nice thing about designing any model boat is no one gets injured or killed if the design doesn't work. The designer of the TS2 is a retired school teacher(30.5 years)/Zamboni driver(9.5 years). After the race on Sunday, I can give you a detailed account of all I know about designing model tunnel boats. Shouldn't delay your trip home more than 5 minutes. :lol:

JD

 

[Mark Poole ModVP]

Recessed transoms are useless on boats and women.

Seriously, I think some designs came from the old school thinking of recessing the engine forward was a good thing. An HTB 360 likes the prop waaaaay back. So does a 290 and it has a flat transom. B)

 

[moparbarn]

darin,

check out screamandfly.com, a full size hot boat forum. easy & free to join. a lot of people that conceived tunnel hulls on there. willing to share, about like here. they can help A LOT with your questions-most everything that applies to our boats applies to the big ones, too. and vice-versa. you can ask the people that made tunnels work when they were a "new" concept. they have a r/c forum, also. plus just some REALLY cool boats. check in racing history & rigging&setup threads/forums. hope it helps, and enjoy.

 

[jetpack]

If you really want to go nuts on theory and design, these boys have it covered...

http://www.aeromarineresearch.com/

 

[moparbarn]

If you really want to go nuts on theory and design, these boys have it covered...
http://www.aeromarineresearch.com/

that book was written by jim russell, on screamandfly as jimboat. if you guys haven't been to that site, you're missing out on A LOT of good info. not doubting anyone on this site, just offering additional sources of info. and REALLY COOL boats, both old and new.

 

[Ed.R]

If you make the motor mount plate flush with the end of sponsons you can't move it forward . If you recess it you can always make a spacer to move motor out and still be able to move it forward if need be. I was told your hinge pin should be even with the end of the sponson as a rule of thumb .

adding my 2 cents

Ed

 

[Grimracer]

Darren,

Its best to keep the hinge pin in front of the back of the sponsons if at all possible. I like to keep my pin about ¼” in front of the back of the sponsons but have had it further forward then that. This is better for heat racing and not necessarily SAW racing..

Also..

YOU WANT a lower unit that has the hinge pin and the stub shaft on different hinge lines. The only lower unit that allows that at this time is the OS.. (shameless OS plug)

I still run some lower units that have the hinge pin and stub shaft tied together but that’s not nearly as good as a separated system and will soon go away on my tunnels. You see, with this type of lower you can adjust the hinge pin to carry the front of the boat or lower the front of the boat in the corners regardless of the angle of the prop shaft.. Think about it and you will see what I mean. It makes for A MUCH better out board mounting system.

ROCK ON

Grimracer

 

[Jimboat]

The location of the motor mount (setback) affects the amount of "bow lift" generated by the drive system (propeller). The "moment arm" distance from propeller to sponson aft location has a determining influence on vertical force generated. Trim (if available) also contributes to solution, of course. So the optimum setback (location of the motor mount) will differ for each tunnel hull design and setup, and will significantly affect performance characteristics.

 

[Tim_Duggan]

YOU WANT a lower unit that has the hinge pin and the stub shaft on different hinge lines. The only lower unit that allows that at this time is the OS.. (shameless OS plug)

Sorry to ruin your shameless plug Mike but thats not quite correct. I run the Thunder Tiger bottom end - it's had that feature since it's inception in the early 90's....... way before the OS was released. Also, the last time I checked, they are still available.

Having the pin angle separate to the propshaft angle allows you to set the trim angle of the boats for the straights and for the turns separately. (think simplified power-trim)

This is not possible on the lower's that have the pin at 90 degrees to the propshaft angle without making it servo operated or extremely complicated.

 

[Jerry Wyss]

Hurray For our Side!,, gettin' kinda narrow in here Tim

 

[Grimracer]

O yea.. O great speed demon from down under.... your right.. I stand corrected.. The TT also has this system.. My apologies to those of you that are still TT fans…

All two of you..........KIDDING MAN..

Grimstinker..

Yea…. What year did that motor come out… 93? 94? We got one to run 43 through the stock muffler but that’s about it.. our K+Bs were dishing out around 47.. Just was not going to cut it for racing around these parts at the time..

 

[Darin Jordan]

Thanks for the responses guys! Grim, I do see what you are referring to and I can see that the REK unit may not be ideal when we really get these going... For now, it seems to be working great... Jerry got some really nice shots... I'll try to post them a bit later...

 

[Tim_Duggan]

O yea.. O great speed demon from down under.... your right.. I stand corrected.. The TT also has this system.. My apologies to those of you that are still TT fans…
All two of you..........KIDDING MAN..

Grimstinker..

Yea…. What year did that motor come out… 93? 94? We got one to run 43 through the stock muffler but that’s about it.. our K+Bs were dishing out around 47.. Just was not going to cut it for racing around these parts at the time..

Mike,

92 if memory serves me correctly (not sure - killed lots of brain cells since then) I started racing in 93 and they were around then.

The stock muffler really holds them back - no question. They are also heavy, and have more driveline friction areas, have a cr@p head button, bad crank seal, low timings in the crank and sleeve.

BUT,

If you spend the time figuring them out they can do OK as a mod engine for a relatively small spend. They are fun for those of us that like to tinker with them.

If I was looking for a "sport" or "Stock" engine I'd reccomend the OS over anything else.

Jerry,

funnily enough no-one told me "they are junk" when I was racing mine. But then again we are all slow here in Australia and aren't competitive, so what would I know.