Moving the running gear further aft?

[Dimmer]

Currently setting the 315hp Cummins in our Bahia mar. I have a set of stainless struts I have modified to fit our 31. The barrel angle is 11°… where I’ve currently got the starboard engine and dummy shaft mocked up, the propeller will be sitting around 7”-8” further aft, leaving me scratching my head for the rudders. But I do have a set of blackfin transom mount rudders that I’m getting ideas with.. anyone have any input before I commit?

[neil]

We have a Bahia Mar with 270 Cummins, the trailing edge of our rudders stick out past the transom a couple inches and with no trim tabs over top of them to hold the water down, the boat couldn’t steer at all once planed off, even with oversized rudders. We put big trim tabs on the boat and it’s a night and day difference, now with something on top of the rudders holding the water down the boat steers like it’s on rails at any speed. Not sure how the black fin rudders would work out for you, but they are a similar concept with the top fin, just not sure it’s enough to make it bite.

[Tony Meola]

I am confused as to why the props are falling that far back. If you changed the shaft angle and had to do so you had the clearance from prop tip to the hull then you did not set up your struts correctly.


If you follow Bob Lilco's posts on this and hopefully I can explain this as well as Bob.

Think of the strut as a fulcrum. Using the old strut, envision the shaft coming down and trying to fit in the barrel. To do that you change the angle of the barrel but everything else on the Strut remains the same. That should leave you with plenty of hull clearance to still spin up to a 21 inch prop. That means you would have had the new strut made to match to old strut but with the barrel angle only changed.

I do remember seeing a 31 with the Stern mounted rudders. Unfortunately, I have no idea how it handled.

Hopefully Bob will see this and correct me where I went off base.

I find it interesting that Neil was able to fix his issue with Trim tabs, working as a type of tunnel for his rudders causing more water to stay in contact with them so the boat handled better.

[Craig Mac]

I think Elite did some 31's with the transom-mounted rudders----never saw any comments on how they ran.



https://www.thehulltruth.com/boats-sale ... ation.html

[Yannis]

There is a 28 in my yard with same external rudders.
If anyone is interested I can take pics.

[Dimmer]

So, I’ve dropped the starboard motor pretty low to the hull, I have about 5/16” clearance between the hull and the lowest point on the ZF220A gear.. I don’t want to bring the motor any further forward though. It’s definitely eating my lunch. Tempted to just cut barrel off and reweld it to settle for higher degree. I’ll try to post some pics.

[bob lico]

ok i will bite why would you make the propellers further aft ?????????

[scenarioL113]

I would imagine it is because the engines are sitting at a height that is prob too high combined with the 11 degrees struts.

Sounds like the engine beds need to be lowered and news logs installed but I am only guessing...

[bob lico]

you say you have lowered the engines until you have 5/16" clearance from the aft most part of the ZF gears. ok good and the gear Barry mounts are TOUCHING the fuel tank bulkhead. that is as low and far back as the cummins can go, you can go one step further by putting one strap on the travel lift and pick up boat 4"out of water with the strap directly under the main bulkhead or in your bahia mar about 4" forward of engine boxto check balance but i can tell you your there unless you have 50' of chain and 30 pound anchor in V -berth. next problem you must cut the shaft to keep props in original place and knowing you read all my information on this you have a shaft log made of FG that can slide up and down thru a oblong hold in the hull were the old shaft log was . now you scribe around the strut to recess the strut flange to match the 11 degree angle then you go about making bulletproof 2" thick strut backing and new bronze bolts .NEVER moved the propellers backward!! you will be using 21" props with minimum prop to hull clearance and 1" between end of the strut barrel end and the back edge of the prop hub for maximum abuse going forward and reverse.without snapping a 1 1/2" aquanet 22 shaft. i hope i answer your questions.

[Dimmer]

https://hosting.photobucket.com/images/ ... fit=bounds

[Dimmer]

https://hosting.photobucket.com/images/ ... fit=bounds

[Dimmer]

https://hosting.photobucket.com/images/ ... fit=bounds

[Dimmer]

When I get some more time, I’m gonna look at everything again. I just can’t seem to get the shaft even ball park close to where it needs to be to maintain the same propellor location. Motor is sitting almost as low as it can go without transmission hitting the hull. Only thing I can think of to do is moving the motor further forward.

https://hosting.photobucket.com/images/ ... fit=bounds

[Yannis]

Did you have different motors before?
Were they shorter?
If these Cummins are longer, then you might just have to shorten the shafts.

[Carl]

This shows 11 degree down angle with the stern end of the strut 90" from the gears output shaft. (stock Strut location). This also shows the center of the Gear Coupling 6" above the hull.
My gear are 8 degree down, I had to angle my motors about 1-1/2degrees to achieve this. Your gears should be 10 degree down...you should be at zero to maybe 1 degree angle. Remember if boats on land to account for angle boats sitting.

With this setup the center of the shaft where props sit is 12-1/8" allowing a 21" wheel.

The center of the stern edge of your strut should be sitting aprox 11-3/8 from the hull.




As to your question about moving the struts and props back then relocating the rudders...I think your going to start creating problems and work. BUT yes, Elite Marine has been doing just what you said, transom mount rudders. JPiezo from this site, his dad had his Bertram done that way. BUT its a learning curve and not sure you want to start re-engineering.

In short, I'd keep the stock strut location but I think maybe your motor is not at the right angle. In the stock Strut configuration, the stut base is angled/shimmed from the hull. It doesn't sit flush as the stock strut is 15degrees. Unless its the 10" stuts which is too short to turn a wheel ove 16" diameter. As yours are SS, they may not be either but something a bit different.

In any case..."I would try to keep strut real close to original location". You may have to angle motor a degree or two or raise motor a hair to achieve fitting a 21" wheel with strut in stock location.

At 90" shaft length...a very small adjustment on the motor is huge back by the prop. Your in the ballpark...just have to tweak.

[bob lico]

wow hold on here let`t go back to square one that is NOT how the ZF gears fit on the engine. those gears are rotated clockwise giving you a false indications of 5/16" clearance from rear of gears to bilge. my cummins are at least 6" further back toward transom. you did make the correct slot in the bilge floor for F/G shaft log. normally we make the rear (threaded end of the shaft ) and cut the front end of shaft because only machine work required is a keyway slot and spooned as opposed to threading at prop end of shaft.one more time the struct get rotated into the hull to make the 11 degrees correct.the only transom mounted rudders are for sailboats with a tiller doing 10knts. think about it!!!!

[Carl]

.the only transom mounted rudders are for sailboats with a tiller doing 10knts. think about it!!!!

Blackfin


Jersey Speed Skiff

[Craig Mac]

blackfin, donzi, cary all had transom-mounted rudders

[bob lico]

and drop the design in a few years sort of a stop gap measure!!!! very much aware of Blackfin i built one and that was the first change i made with 3126 cats @385 hp. we removed cat 3208 @ 375 hp and installed the 3126 @ 385 hp,then put rudders under the boat even with transom (maximum undisturbed water flow) change shaft angle and custom propellers. the boat went from 23 knots cruise to 29 knot cruise and great steering especially docking.i hate to say this on Bertram forum but that extra room in cabin was nice as well as cockpit and bilge space.32" Blackfin

[Carl]

First, I am not for changing the Strut and rudder location just to make it fit. I am saying good performing boats have used transom-mounted rudders, but those were designed that way OR redesigned that way as Elite marine has done on their 31 Bertram Refits.

Change where the prop wash hits the rudder is a big difference in how a boat steers.

[bob lico]

as usual Carl caught the theory immediately .you cannot have rudders swinging in the breeze in a 20 knot plus boat.

[Dimmer]

So I can rotate and clock those ZF’s at an angle, bob?

[bob lico]

looking at the gears from the rear end of the ZF gears the two 1 1/8" plugs at the bottom of the gear (port is the gear filter and starboard side is the transmission oil drain plug) should be level or exactly horizontal . now you take a dimension from bilge to rearmost edge of gear and it will be a lot more than 5/16" as you indicated because somehow gears are rotated clockwise let`t start with this condition first!!!!!!

[bob lico]

thank you Craig nice photo of outboard rudders that makes it rather easy to explain even to the most casual boat builder. the props turn causing forward propulsion and the water is "pushed" into the leading edge of the rudder for steering other then Carl can anybody viewing this photo see something obviously wrong with a outboard rudder installation and why Blackfin dropped that idea. does anybody know of a high performance powerboat that uses this antiquated idea.

[Dimmer]

Not really high performance, but our 32 Cary has transom mounted rudders. She turned just fine, that’s why I was curious if transom mounted rudders would work on the 31 Bertram. Haha.. With 250hp cummins in that 32 Cary, the cruise speed was 27 knots and wot was 34 knots

[Carl]

does anybody know of a high performance powerboat that uses this antiquated idea.

Jersey Speed Skiff

Drag Boats

Boats with Surface Drives



My point is a boat can be designed/engineered to work well with outboats rudders. But just moving a rudder from under the hull in the contained prop wash and slapping it on the transom is asking for a whole new set of problems.

[bob lico]

cmon Carl you know i meant high performance inboard fishing boats . BTW surface drives have the rudder on the drives ,drove a Apache years ago.

[Carl]

cmon Carl you know i meant high performance inboard fishing boats . BTW surface drives have the rudder on the drives ,drove a Apache years ago.

Yes I know exactly what you meant.
I still think if designed right they work...not sure of the benefit though. Aside from lowering the running angle and ez access to rudders "when on the hard". The other thing is when fishing you have rudders protruding from the stern...tabs are bad enough for those that have...but now rudders are hanging out there.

Yes many or most surface drives articulate, I was talking about the straight shaft and prop setups like on some Sunseekers with that rudder hanging way back. Maybe its surface piercing and not drives.


Then there is :

https://www.youtube.com/watch?v=Vm7g1o9Yt80

[Raybo Marine NY]

I would throw a rod out to try to catch some dinner