Prop diameter

[dognduck]

What do you all think is the smallest diameter prop (lets take proper engine loading out of the equation) it takes to move a 11000-12000lb bertram31 at 27-30 knots and keep the slip numbers down around 10%? obviously alot of mods like cup and blade area would be required. but what's the smallest diameter? 17",19",20"? it sounds like Bob Lico was able to get away with 21"

My gasser 31 currently has 17" diameter props and at 25 mph (not knots) i am at a 12% slip. at 30 mph i am at 19% slip...

[Tony Meola]

Be careful when you say Bob did it with a 21 inch prop. It wasn't quite that simple. First he had to find out what size blank a standard 21 inch prop is made with. Then he went up one size on the blank and had it cut back to 21 inches.

That gave more blade (wider) giving him more of a bite into the water reducing slip, then he added pitch and cup. Depending on the engine I would say 20-inch prop would be the smallest.

Carl is really the best one to answer this question. I am guessing, he understands this stuff better than most.

[Yannis]

If I'm not mistaken, Bob also changed the angle of the shafts which in turn may impact the prop dia.
In other words, a 21 with the stock shaft angle may give different results from the same 21 at Bob's shallow angle.
Plus what Tony said.

[Tony Meola]

That's right Bob changed the shaft angle, but he did not change the distance from the tip of propeller to the hull. Once you start to go bigger than 21 inches you start to get near the point where the wash from the prop will start to wear at the Hull.

The shaft angle changes everything as far as performance, and fuel economy.

I would think if you reduced the shaft angle, you might be able to shrink the diameter of the prop a little more to achieve the results you are looking for.

[Carl]

Problem with reduced slippage and attaining a high top end is you need more torque to get your boat out of the hole.


Here is a link from Capy posed years ago on the old forum. It is all about slippage to prop diameter and very informative.

https://bertram31.com/performance.htm

[dognduck]

Here is why i ask: assuming pitch stays the same, from a leverage stand point, it takes less torque to turn a smaller diameter wheel. on top of that, with a small diameter wheel you can raise the strut making the shaft angle even better. seems to me that the smaller the wheel, the better until you start incurring slippage...

Do i have this right?

[Carl]

My understanding going from 15° to 11-12° is performance gain of a couple percent. About the same performance that will be lost reducing prop diameter an inch or two.

Yes, two wheels of same design and pitch but different diameters, the smaller wheel will need less torque as I expect it will slip more.

Rules of thumb…



My boat came through with 10° shafts, 1:1 gears, 440 gas motors and 15” wheels. Performance was good, but I did better with 1-1/2:1 gears and 16” wheels.

[Tony Meola]

Actually, the way to change the shaft angle is the way Bob did it. You don't change the length of the strut or change the distance to the hull.

Think of the Strut as a fulcrum. You have to customize the strut so that dimensions remain the same, but the strut barrel is pivoted. Think of the strut being set up so it pivots. You lower the engine as far as you can, In Bob's case I believe he has about an inch of clearance between the oil pan and the hull. He also moved the engine back.

Then you line up your shaft and a pivot the barrel to accept the shaft. That is the new shaft angle and the angle the barrel has to be set at. Even a 3-degree change in the shaft angle will enhance performance and efficiency.

[Carl]

I agree that reducing the shaft angle is a benefit, but it is not a huge improvement in and of itself. Look around the information is out there on the expected performance benefit of reducing the angle 1-2 degrees, its a percentage point or two. What is 2% of 20 knots???

http://www.ricepropulsion.com/cartas/TNL49/Tnl49.htm

https://ab-marine.com/knowledge-base-pr ... important/


It is also not as easy as "just" lowering the motors and pivoting the strut to gain 2%.

Motor mounts do not have infinite adjust-ability, the mounts themselves may need to be lowered, meaning cutting the motor bed down.

Shaft is in a different position going through the hull...now your moving, replacing or fabricating a new shaft log.

Move the shaft log and now there is this big hole in the bottom of the boat that needs to be addressed. How good are you at glass work? Hopefully pretty good as its the bottom of the boat. Bang up the shaft by hitting debris and that log area is going to see some stress.


I tackled lowering the angle as I was converting to diesel, the motor beds needed work, the shaft log had to be replaced as I was going larger on shaft diameter, I needed new struts as mine could not be bored open to accept a larger shaft. I was into the job anyway, sure I'll take the 1 or 2% gain for the work.

If your going into it like Bob going though everything and racking up the percent points here and there, yes it makes sense. Twenty upgrades at a point or two is a huge difference.
But similar to changing your spark plugs and going to a high energy ignition and expecting to becoming a speed boat, your not changing shaft angle and becoming a speed boat and passing by the fuel dock.

I obviously do not know what your intention is, but if after a little extra performance, economy, there are easier ways.

[Tony Meola]

I obviously do not know what your intention is, but if after a little extra performance, economy, there are easier ways.

Carl

Outside of a straight repower nothing is easy and sometimes even a straight repower gets crazy. We set a budget then about a month later that is out the window. As I tell everyone while you are there you might as well.......

But you are right the easiest is to just upgrade the power.

[bob lico]

the FBI says i have my own version of the English language and i purposely shut off spell check so i will try to help and Tony has it just about down but let me clarify one aspect of changing shaft angle. the strut ALSO is rotated to match shaft angle and with a 4 degree of rake the actual effective diameter is reduced slightly. keep this in mind .this is why i tell you never allow a marine mechanic in the boat because the hold shaft arrangement is counter to what he has learn and apply from day one. the propeller fitting IS NOT SHIMED it is directly bolted to output flange of the transmission (after both are trued exactly 90 degrees on a air chuck mounted on a lathe .i also up the bolt diameter from 7/16" to 1/2" bolts there is zero play in propeller fitting

[Snipe]

I lowered my motors moved them back as far as I could dropped them into the bilge as far as I could glassed in new shaft logs,modified the struts,fabricated all new motor mounts. I run a 21x24 four blade Michigan hytorq propeller with a light cup and can cruise 30 knots. It was a lot of work but well worth it. I am running Cummins 6bta 370’s