Jackshaft option

Morning Wood · Post by **Morning Wood** » Apr 7th, '12, 19:34

I wanted to pick the brain trust about a power option for my 25 V Drive boat. Instead of repowering with twins I thought about a power system similar to an Albemarle. Putting a 496 in the current engine bay with a jackshaft connected to a Volvo duoprop outdrive. Any thoughts?

In Memory of Vicroy · Post by **In Memory of Vicroy** » Apr 7th, '12, 19:51

One of the original Faithful here, Stan Smith who is a Hell Diver spearfisherman here in La., had a buddy with a B25 and a big Volvo diesel on a jackshaft to a duoprop and they ran the heck out of it offshore to the oil rigs. Stan had a B31 rigged for diving.

I put a pair of Vulva TAMD30 diesels (110 hp) with duo porp drives in my 1972 B25 at a cost - in 1984 - of over 20K....would not do it again....in fact, would not touch anything Volvo if they gave it to me, they are terrible egines & drives.

But a single in a B25 has been done....in fact, one of the current Faithful is in the process of putting a single DD 6V71 in one as I recall. I'm not a big fan of v-drives but they sho' are better than outdrives.....anytime you ave gears underwater you are asking for trouble. There is a reason the straight inboard is the undisputed standard of the boating world, both commercial and rec. All the IPS crap is a fraud to separate fools from their money faster than a casino.

UV

Michael · Post by **Michael** » Apr 7th, '12, 19:54

If I had a inboard B25, that would be my prefered power option. I am a big single engine fan in smaller boats. Putting the bulk of the weight in the central location seems to be best for these boats. And whilst outdrives are not everybody's favourite, they do provide shallow water operation and great manoverability. I have a couple of boats with sterndrive power, and I do the proper maintenance as suggested by the manufacturer, and have had no problems with the drives. My drives are over 15 years oold and stay in salt water 360 days a year.

Morning Wood · Post by **Morning Wood** » Apr 8th, '12, 11:05

I guess my real concern is the length of the jackshaft, My thoughts were to span the distance from the engine to an I/O by using the existing distance where the twins sat. Roughly 3+ feet or so. I have no clue as to what an unacceptable distance is for jackshafts. Also is the bottom portion of a V Drive hull at the transom the same as an I/O?

CaptPatrick · Post by **CaptPatrick** » Apr 8th, '12, 12:05

With a V drive the engine is mounted backwards and the drive shaft comes off a down angle below the engine so you wouldn't be ale to use that configuration to connect to an outdrive...

You engine needs to be facing forward, a standard transmission configuration and jack shaft from that to the outdrive. This is going to involve a complete re-configuration including heavily reinforcing the transom to handle the weight and torque.

The original I/O setup has the engines back at the transom to keep the shafts short and alignment movement to a minimum. With a mid mounted engine, there's going to be alot of movement between the two fixed points which could lead to a lot of premature wear on the out drive and/or the transmission.

The longer the shaft, the more issues you'll have...

Morning Wood · Post by **Morning Wood** » Apr 8th, '12, 12:14

Thankyou Capt. Patrick. Twins it is! You da Man!

Bruce · Post by **Bruce** » Apr 8th, '12, 17:30

Why use a jackshaft?

Install the outdrive and transom plate assembly on the transom in the center and bolt the engine to the inside plate.

Standard outdrive install.

You'll obviously have some stringer, hull and transom rework and strengthening to do.

Jack shaft would only be needed if you were gonna mount the engine where the fuel tank is.

Morning Wood · Post by **Morning Wood** » Apr 8th, '12, 20:05

I was hoping to use the existing engine bay. Mount the engine centerline facing forward and run a jackshaft to make up the distance the engine would be from the outdrive. That way the engine would be forward a little more than a v Drive due to the transmission length with the current engine mounted backwards. Then I wouldn't need to disturb the original deck and hatch set up in the boat. The fabrication would be hidden from view,transom and engine bedder modifications. My thought was the I/O would be more efficient than 2 inboards. Thrust would be parallel to the bottom and not angled like an inboard. Have 1 large powerplant to service with good room to manuever around the engine. I thought the outdrive weight would compensate for the engine moved a little further forward then existing V drives. Again this was just something i thought I would run by you guys. Putting back twins as originally designed would be the most practical and no modifications. I just thought you could bolt up an outdrive in the existing hull and modify a shaft to deliver the power. But Capt. Patrick addressed that issue. I love the v drive concept and the weight lower in the boat. Just trying to come up with an affordable idea to make the boat more fuel efficient and lower maintenance cost by eliminating one engine.And with the least amount of fiberglass modification to the boat.

Tony Meola · Post by **Tony Meola** » Apr 8th, '12, 23:01

Here is an article on a conversion to a single diesel inboard. Another way to go.

http://boatdiesel.com/Articles/Articles ... cfm&Y=2007

Bruce · Post by **Bruce** » Apr 9th, '12, 07:42

Just an FYI,

Don't let the drive shaft issue stop you.
I've installed and serviced hundreds of boats with shafts. Unless someone assembles one using the parts out of moms 54 buick, drive shafts and universal flex ends are used world wide in more applications than you can imagine transferring over a 1000hp.

Grease the zerts and they are a non issue zero worry factor.

scot · Post by **scot** » Apr 19th, '12, 21:41

one of the current Faithful is in the process of putting a single DD 6V71 in one as I recall

I ressemble that remark. Swapped Stewart & Stevenson the 6V for a 471TA, couldn't figure out where I would hind enough fuel to keep the larger pig fed. Unfortunately I'm still on temporary hold with tooooo many iron in the fire! But the eye is turning back toward the B25 project a little more every day. It's summer and I want that boat!

i think the jack shaft is a great ideal. 25's naturally run bow proud and need the engine pushed forward. All the V-drive owners (engines mid-ship) love the way their boats ride..but hate changing spark plugs, and REALLY hate changing packing!

It has been done, I saw several pictures of a mid-engine jack shaft B25 a few years back, beautiful boat.

Peter · Post by **Peter** » Apr 20th, '12, 12:38

Hey Scott! Good to hear from you again. I was wondering what the status of your project had become. I'm glad to hear it is just on hold, not abandoned.

Shoot us all an update when you get a moment.

Peter

bertram31.com

Jackshaft option

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