why don't boats have transmissions?

[overload]

Re: why don't boats have transmissions?

I'll try make this easy (maybe) in two steps...

1) Props don't coast, so if you changed gears, the prop would slow down so much in the split second between shifting gears that you'd lose enough speed that you'd have to start back at the gear you just shifted out of.

2) You can only spin a prop (or any object) so fast through water. If you keep spinning a prop faster and faster through water, it will eventually cause cavitation and prop blow out. If you throttled up your engine to the max and decided to "up-shift" to a higher gear to gain more speed like on a car, you would have to first overcome step 1, which is difficult, and if you did, you wouldn't have very much speed to gain before your prop cavitates and blows out.

Hope this helps.

 

[45Auto]

Re: why don't boats have transmissions?

Not quite that easy.

1) The prop and boat will slow down a little, not nearly enough to affect any half-way competent shifting mechanism.

2) Boats that use transmissions use the prop sized for the top gear. The lower gear is used to allow the boat to plane quicker. You can drive the boat around in top gear without worrying about cavitating and blowing out the prop, no different than a normal boat. It'll just accelerate slower.

These people make transmissions for Volvo outdrives used with older diesels. They have some interesting info on their site.

http://www.stepdrive.co.uk/

 

[scoutabout]

Re: why don't boats have transmissions?

In the airplane biz this transmission issue is addressed by either variable pitch propellers that you control manually or constant speed props which use engine oil under pressure running through the crank into the nose cone actuators to change prop pitch automatically. In the early days or on some simpler aircraft, cetrifugal weights shift the pitch as engine speed increases. I believe there are also some electrically-actuated props out there too. Interesting to hear of wca_tim's experiences with similar applications on his destroyer.

Anyway, except for Bondo's prop (which I'd love to try behind the Scout) certainly most of this is way too costly for the average weekender runabout.

 

[Silvertip]

Re: why don't boats have transmissions?

I didn't read the entire article but the tranny was more likely a two-speed PowerGlide which is used in drag racing. That aside, there were a couple of other interesting transmissions (actually torque converters) that were used in the 50's and 60's. Two that come to mind were the late fifties Chevy TurboGlide that didn't have any forward gears at all, other than those required to make the car go forward and reverse. It instead used a three element torque converter rather than the single that is common today. The three different diameter turbines provided a seamless (CVT-like) operation. The other was a "switch-pitch" torque converter used on some '64 to '67 GM cars. At idle the torque converter blades sat at a flat angle so you could literally sit at a stop sign without your foot on the brake or at least with very minimal braking. The second you touched the gas pedal, the blades switched to a sharper angle and off you went. In passing, at about 2/3 throttle the blades switched shallow again to provide added performance without an actual downshift. The downside was that the blades were always at a shallow angle at wide open throttle -- but I fixed that on my '65 and '67 Cutlasses with a switch so I could manipulate the feature manually. It would be interesting to see what either of those systems would achieve using a higher pitched prop. It would obviously not be workable for production using the entire transmission but for experimentation purposes it would be an interesting exercise using "gearless" technology.

 

[oops!]

Re: why don't boats have transmissions?

btw oops... "superpro" I'm flattered, but to set the record straight, I'm a backyard hack... especially compared so some of the folks on here...

lol.....so am i my friend....so am i !

but watch out or ill post the info you gave me on the 383 (i still have it and read it from time to time ) and blow the back yard theroy ourra the water

 

[oops!]

Re: why don't boats have transmissions?

Ayuh,....


But,....
I've got the next Best thing,....
My L&S Torque-shift prop starts out at 11" of pitch,+ shifts up to 26" of pitch.....

im nor framilliar with this set up.....is it a manually controled set up outside the craft, or manual in side the boat or auto?

that's funny... and quite frankly sitting here thinking about it torque shift props really probably are the best way to go from a cost / performance benefit standpoint...

In the airplane biz this transmission issue is addressed by either variable pitch propellers that you control manually or constant speed props which use engine oil under pressure running through the crank into the nose cone actuators to change prop pitch automatically. In the early days or on some simpler aircraft, cetrifugal weights shift the pitch as engine speed increases. I believe there are also some electrically-actuated props out there too.
.

this technology works very well in fluid dynamics

It would be interesting to see what either of those systems would achieve using a higher pitched prop. It would obviously not be workable for production using the entire transmission but for experimentation purposes it would be an interesting exercise using "gearless" technology.

heh heh heh......boys in the backyard "putzin" with ideas....is the mother of production

 

[wca_tim]

Re: why don't boats have transmissions?

Interesting to hear of wca_tim's experiences with similar applications on his destroyer.

Anyway, except for Bondo's prop (which I'd love to try behind the Scout) certainly most of this is way too costly for the average weekender runabout.

I don't know... if it could be managed well enough to significantly reduce the size (and drag) of the lower gearcase and provide good control of torque transmission to the water, would actually make the gearcase a lot simpler and stronger, if effective prop could be designed. Shaft only has to turn one direction and at one gear ratio... prop could be optimized to good guess at where needs to be for max speed at wot... make it a surface piercing prop, and being able to drop the blade angle of attack could make a big difference in being able to geting it to plane out quickly...

Regarding cost... have you priced a new bravo xr and imco shorty lower lately?

(yea, I know that's not the average weekend toy...)

 

[NSBCraig]

Re: why don't boats have transmissions?

Neat thread just had to chime in about somethings.

Zf trannies work great and are run with Yannmar' s and Arneson surface drives in quite a few offshore race boats v hull and cats.

Velocity built a 41' with that package and it runs just as strong as my brothers 41' without burning nearly the fuel.

I went on a poker run with a cat ( MTI I think ) that ran strong with the same package.

In fact 2 speed trannies have been common in europe offshore racing for years. It just isn't big here because of class rules don't allow them.

Whoever commented about coasting to switch gears is way off, I'm pretty sure noone is thinking about throwing a stick in it. ( seen it though ) It's not a factor.

Pitch changing props are neat, anyone remember pitch changing torque converters? Buick used them way back when on two speeds to get them big cars to move.