Best practices for propeller shaft installation?

[Mark Willoughby]

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?

Why the cardan shaft remote v-drive approach, here's a few reasons:

- allows more flexibility for the placement the engine and transmission to achieve proper vessel trim when underway and keep the shaft angle under 15 degrees
- direct coupled v-drive / engine combos are often challenging to work on

 

[Mark Willoughby]

Here is the data sheet published by Bertram in 1970 that contains some useful information for the discussion. Note that with 2 x 120 HP the vessel appears to attain more than 30 knots top speed.

 

[Bakodiver]

Since I have a fondness for small diesel powered fishing boats, and have direct experience with converting 2- 25 foot deep vee hulls, I thought I would chime in with my 2 cents. We'll call this a tale of two 25 footers.
The first; my buddy bought an Annacapri 25 center console powered with a 350 gasser and straight shaft drive. This is an 8 foot beam boat as opposed to the Bertram's 10, so the Bertram should react better to the increased weight of the diesel. He installed a Cummins 6BT with a 1-1 transmission and retained the 1 1/4" shaft. Clearance of the prop tips and max size available was the reason for going 1-1 reduction. Performance is around 17 knot cruise and 23-4 top end. I personally find the location of the engine box intrusive, as it is quite high and in the middle of what would otherwise be the fishing deck.
The second boat; My Marlborough 26, which is a heavy, solid glass deep vee with 24 degree dead rise from New Zealand, also 8 foot beam. I bought it with a 200 hp Volvo diesel and 290 outdrive. The Volvo was overworked, with large swing loads of divers and tanks leading to it failing at 3500 hours. I'm not in love with sterndrives, but part of this boat's raison d'etra was to land passengers on sandy beaches, so a tilt up prop was desirable.
I ended up replacing the cockpit floor with composite board, adding a second fuel tank in the bow, and installing a Cummins 6BTA 280, velvet drive 1-1 transmission, and a Konrad 520 outdrive with 1.43;1 reduction. I'm still playing with props but I am currently quite under-propped at 18x19 3 blade. Performance is 22 knot cruise and I have had it over 27 knots at 2700 rpm which is 100 over the rated rpm. The engine box is much lower than my buddy's, and of course right in the stern so makes a nice bench and mount for a fighting chair.
I think for usability of the boat my installation is preferable to my buddy's BUT: it is a sterndrive with all the added maintenance that entails. Every boat is a trade-off. My opinion is that the perfect small diesel powered fishing boat would be vee drive leading to a tunnel mounted prop. You would have a low, stern mounted engine box, relatively shallow draft, and the simplicity of shaft drive.

 

[Mark Willoughby]

Thank you for sharing your experience and observations.

Do you recall the fuel burn in the 25' vessel with the 6BT 180?

While the Bertram 25 has a 9'11" beam amidships on the deck, the beam at the waterline appears to be 8' 4"+-.

I'm working on getting a 3D hull scan completed so I can use Rhino CAD for the all design work.

You must carry a lot of weigh in your 26' when you do charter work.

 

[Bakodiver]

We troll a lot of 10 hour sport fishing tournaments here, so the most important fuel burn number is at 9 knots. My buddy's Annacapri burns about 11.5 liters per hour at 9 knots, mine burns about 14, being heavier and bigger.
Yes, I've had up to 9 people on board and 24 dive tanks, as well as weights, food etc. Boat handled it OK but the Volvo engine was overloaded, which is why I went for 2 liters more displacement and 80 more hp.
I hope your engine gives you the performance you require. Personally I think it will. BTW I am from your neighborhood, but relocated to the tropics decades ago. Still have family in the Gulf Islands.

 

[Mark Willoughby]

The BC coast Gulf Islands, IMHO on the list as one of the best places on earth. No doubt the Marshall Islands are on that list too .

The project boat when it is finished, will spend time cruising the Gulf Islands as well as take trips to the north and west coast of Haida Gwaii (Queen Charlotte Islands) for some spectacular fishing.

I worked on Norway Island for summer in the late 1970's,. Do you know it?

All indications point to 185 HP in my boat as having enough power to cruise at 18 knots and top out at 25 knots. Given that the engine is rated for commercial service, I could probably cruise at 22 knots.

While I wait for a chance to use a 3D scanner, I'm conjuring up ways to incrementally increase HP without using an exhaust driven turbocharger or sacrificing reliability.

Here's what I have come up with:

- dry sump engine lubrication
- substitution of water cooled exhaust headers (think marine big block gas engine racing headers fitted an adaptor plate) in place of the log style water cooled manifolds the engine has now.
- add a belt driven supercharger ala Volvo 44 series marine diesels

If I can come up with a modest 15 HP increase, I'll end up with the same power (200 HP) that the boat was originally designed for in 1961

These are probably topics for a different forum category.

 

[gonzo]

A turbocharger will get more air into the engine. It also needs more fuel, which means a larger volume pump and injectors. Then is all needs to be recalibrated. That is a lot of work and expense for 15 Hp extra.

 

[william stokes]

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.

take a look at VICPROP, they will calculate all for you as rgds speed and prop size eh

 

[Mark Willoughby]

Thanks William.

I posted some output from VICPROP in an earlier post in this thread.