Post Number: 1
|Posted on Monday, December 19, 2011 - 04:13 pm: ||
I have a 43' 1925 Stephens Truck Cabin with a 6 cyl Ford-Lehman. The motor is mated to a velvet drive trans with a reduction gear. I would like to swap out the current reduction gear tranny with a direct drive tranny.
Any help on this topic is appreciated.
here is the boat: http://www.wix.com/tonygranieri/xantippe
Post Number: 584
|Posted on Friday, December 23, 2011 - 07:59 am: ||
The reduction gear as used to get a slower turning propeller.
Slower props are more efficient ( assuming they have been propoerly sized to the boat).
Typical reduction ratios are 1.5:1.
Assuming that your engine turns about 1500 rpm when it is cruising, the prop speed would be about 1000 rpm.
Increasing the prop speed to 1500 rpm by using a direct drive will mean that you'll have to either reduce the prop pitch or the prop diameter (or both). That is to say, you'll have to go through the (endless) fussing of getting a new prop sized and installed.
If you simple replace the reduction drive with a direct drive and leave the existing prop, you'll likely end up with cavitation issues.
Most folks who have done that rarely come out ahead, after quite a bit of cost.
Since the boat is used for charters, it's likely you're not going full throttle most of the time.
All of the above is a complicated way of asking - why are you thinking about this replacement?
Post Number: 2
|Posted on Saturday, December 24, 2011 - 03:05 am: ||
I'd like to replace the reduction gear tranny with a direct drive because it seems like an economical way to gain some extra thrust.
The current tranny is a velvet drive model 10-18-009, ratio is 2.57:1.
I know the boat was re-powered from chrysler gas to the lehman 2715E right before I bought it. I was told that the shaft & prop were un-changed. I think it's an 16-18" prop with about 22 pitch?
Post Number: 64
|Posted on Saturday, December 24, 2011 - 04:20 am: ||
looking at the photo you have pleanty of room around the prop I would advise you to adjust the prop size to get what you want it will cost a lot less, just make sure the engine is reaching its peak or near too peak revs, if you go to direct drive the engine might not be able to carry it and you risk punching holes im pistons,
do some homework and find out what size prop it should be using with that reduction.
Post Number: 44
|Posted on Saturday, December 24, 2011 - 05:55 am: ||
Hi - I guess you are commercial(charters) hence you have a bit of homework. My authorities regulate shaft diameter and material - you need to run engine 120HP? and 2.57:1 gearbox reduction past the code to see if shaft diameter is OK. (1925 engine would not have been Chrysler M8? - If earlier motor was a Chrysler M8 with common 2:1 reduction, 18" prop was usual.) If shaft diameter is OK, run your hull photo and specs past a reputable prop supplier to get their recommendation. Show them your old prop. Also, is the engine able to rev up to just over the advertised power band with the current prop? If you go to direct drive, will you need to remount your engine down to suit the new gearbox? (Are you OK with losing manouvering and are you OK with deliberately losing astern effectiveness?) 2:1 reduction looks about right for a boat like yours. Direct drive and step ups are more for speedboats...
Post Number: 3
|Posted on Saturday, December 24, 2011 - 12:52 pm: ||
I forwarded this same question to my local chandlery and boat yard (Svendson's in Alameda, CA) They believe the best way to go about this with my current prop configuration is to change out the current 2.57:1 to a 1.51:1 tranny. They say doing so would increase the prop speed sufficently and would eliminate the need to either move the engine or extend the shaft to accomodate a direct drive trans.
The whole reason im considering this is beacuse the engine easily revs to peak (& beyond) but my crusing speed is only about 7 knots (@1500-1600 RPM)
I suppose I do have a bit of homework on my hands before I can proceed.