Best practices for propeller shaft installation?

Greetings:

I'm planning to remove the twin stern drives in my 25' deep V planing hull vessel and replace with a single shaft. The plan is to use a remote V-drive with cardan shaft.

I'm looking for guidance on:

- the recommended distance between the transmission output coupler and the top of the shaft log
- optimum length of shaft log
- the distance from the outboard side of the shaft log to the propeller strut
- finally, the distance from the strut to the propeller, if any

I understand that improper installation will lead to unwanted results such as excessive vibration, shaft whip, etc.

Are there rules of thumb to apply here? I will likely use a 1.5" diameter propeller shaft.
The v-drive transmission location hasn't been finalized but it looks like a nominal 6.5 foot long propeller shaft would be needed.

Thanks in advance.

 

It would seem that the Glen-L book will be a good place to start.....

 

What's the engine power and all up weight of the boat fully loaded? Those numbers are crucial for the determination of propeller and shaft dia's, so needed before anything else can be said. In addition: state present power, weight and speed.

 

Power will be 185 hp @ 3000 RPM with 1.5:1 ratio transmission. Fully loaded weight with full tanks for fuel, water, plus stores and 2 persons onboard is estimated at 7000 lbs. With these assumptions, I'm hoping to achieve a top speed of 25 knots and a cruising speed of 18 knots. Dry weight of vessel is 4400 lbs according to manufacturers literature. That includes 1200 lbs for two inline six gassers and sterndrives. New engine all dressed should come in at 1200 lbs plus another 150 lbs for the remote v-drive. New dry weight is assumed to be 4600 lbs leaving a hopefully generous 2400 lbs for fuel, stores and crew.

 

I have searched the internet for a line drawing but have not yet been successful. Note that the subject vessel is the Bahia Mar model with more superstructure than is shown above thus shifting the C/W forward.

 

What was the power and speed before? Is that a diesel you want to install? If you get maximum power at 3000RPM it does not seem like a reduction is needed. Your expectations for speed may not be realistic. I am curious about why the cardan drive. As a rule of thumb, the shaft angle should not be more than 15 degrees. The more reduction on the gears, the larger the propeller diameter needs to be. An inboard installation will add close to 2 feet of draft. That means modifying the trailer for extra height. Also, if you normally launch at a ramp, the depth may not allow after the increase in draft. Since you have two sterndrives and want a single engine, why don't you simply remove on and install the other in the center?

 

All About Marine Transmission V-Drives https://www.sbmar.com/articles/all-about-marine-transmission-v-drives/

I've not read the full thing but know of a couple of their systems. Acquaintance had some konrad outdrive issues so decided to go shaft, but could only add a few feet to the water line length due to harbor stall limitations. They ended up with a remote gear setup and a few extra feet for a smidgen more rudder and prop room. They mentioned sb marine doing what they called a reasonable long distance consult. To my knowledge they are happy with the setup.

 

With a single shaft drive in a 25* deep vee hull, you will almost certainly need to utilize a tunnel or a surface piercing prop, or twin engines to keep the draft within reason.
I think you will be underpowered with only 185 hp, as deep vee hulls demand a lot of horsepower to comfortably plane.
I usually draw out on the garage floor in full size a side view of the components as a quick feasibility study.

 

About the only advice we can give straight away is about the distance from strut to prop, it shouldn't under any circumstances exceed 1.5 shaft diameters, usually one diameter or less is used.
The rest is a function of shaft stiffness, for wich there are formulas and even online calculators. If the shaft is to long it needs intermediary bearings to keep it straight. Some types of shaft logs act as a bearing, some don't, you need to take that into account when determining unssuported lenght. The "rule of thumb" is a maximum of 40 diameters between supports.

 

Existing power is 2 x 165 HP inline 6 with Mercruiser Alpha drives. Bertram brochure claims a top speed of 39 mph under ideal conditions. Perhaps 35 mph or 32 knots is more realistic. I don't know what the test weight of the vessel for the published performance figures was but Bertram published a "cruising weight" of 5500 lbs. for the Bahia Mar model. I am trying to find that literature and I will post it when I track it down.

I may have to scale back my fully loaded weight from 7000 lbs. to 6000 lbs. to achieve my expected performance.

Yes, I want to install a diesel and it will be the International 7.3 IDI normally aspirated engine. I have a full marinization kit for the engine from Peninsular Marine from the late '80 s when they sold them as a fully dressed marine engine. "The most powerful version of the naturally aspirated 7.3L IDI in the Ford F-Series was rated at 185 horsepower and 358 lb-ft of torque @ 1400 RPM." I will keep in mind that 1:1 reduction is probably appropriate with a little tweaking to maximize propeller efficiency. I like this engine because it is a dead simple mechanically controlled diesel, it is a designed to be a medium duty truck engine at the outset and has a proven track record of reliability in service. In this configuration we are only asking to produce 25.3 HP per liter of displacement.

Here's a screen cap of the predictions of a propeller size calculator I found at Free to use Propeller Sizing Calculators https://www.vicprop.com/free-propeller-sizing-calculators. I used a 150 value for "C" to get the results shown below. Is there a better online calculator out there now that boatdiesel.com is no longer online?



I'm aware that the direction I'm headed has some trade offs. There is no perfect boat and there are always compromises.

Yes, it will increase the draft and be less friendly at the boat launch. I have access to a good launch with a steep ramp and I will have to modify the trailer to accommodate the new configuration. With the new setup. I may actually be comfortable with mooring the boat and not be afraid that galvanic corrosion will eat up my stern drives. I will lose some overall hydrodynamic efficiency with shaft drive setup. I will gain some increased fuel economy dropping from 2 gas engines to a single diesel. It won't be as friendly in reverse or docking it but I will manage.

"Since you have two sterndrives and want a single engine, why don't you simply remove on and install the other in the center?"

I am beyond fed up with stern drives. That is a whole separate rant.

 

"Acquaintance had some Konrad outdrive issues so decided to go shaft"

As I have said, I am beyond fed up with stern drives.

 

A single shaft setup on this vessel without a tunnel or surface piercing prop has been successfully completed before so we are not breaking new ground here.

Widgeon - A Very Special 25 ft. Bertram https://www.sbmar.com/repower-projects/widgeon-special-25-ft-bertram/

They used a 330 HP engine to accommodate 135 gallons of fuel and a 150 gallon bait well.

 

Thanks, this is useful information.