propellers rake and attitude

jayjays1 · Post by **jayjays1** » Jan 8th, '24, 20:54

Ok so I remember while back that overpowered B31's tend to push the nose down at speed this being accentuated by the 15 degree prop angle. I have heard that this is accentuated by the larger prop angle. I understand that reducing this to 10 degrees reduces the amount the bow is pushed down. This has always made sense to me but I also read a post where it argued the exact reverse!

Can someone clear this up for me?

Could someone also address the role that pitch and rake play into the nose attitude of the 31?

JJ

Tony Meola · Post by **Tony Meola** » Jan 8th, '24, 22:27

Actually, I have never experienced the boat losing the bow up angle at speed. Reducing the shaft angle, increases efficiency, and allows you to run a bit faster. I am not sure how much that changes the bow up angle.

If you want to play with your props, you can take the Bob Lilco approach, and I also know someone who did the same thing that Bob did using Acme Props and a local prop shop.

You need to find out what size diameter prop you need. Let's say for this example you need a 21-inch prop. Then you need to take get your hands up on the prop blank that is the next size up from the blank that is used to normally make the diameter you need. You then have that blank transformed into the 21-inch diameter prop you require. What this does is increase the blade area, giving the prop more of a bite when it pushes the boat. Then you need to adjust the pitch to what you need. A 20-inch pitch moves the prop 20 inches through the water.

Most of us some cup to the prop.

The rake is the angle of the blade and impacts how the prop grabs the water and can impact bow angle.

I think this link will tell you what you need to know.

https://bblades.com/props-101/

Carl · Post by **Carl** » Jan 9th, '24, 08:01

Pitch is like the threads on a screw (single screw/twin screw), each time you turn a screw it moves forward the distance of the pitch. A 10mm screw has a pitch of 1.5mm, turn the screw once it moves forward 1.5mm. 12" pitch on a propellor theoretically moves/propels the boat 12" forward. ( Sorry for tossing in metric, but they are straightforward simple. In imperial, we then take that pitch measurement and figure out how many are in an inch...why")

Rake in simplest terms is how far the blades are bent forward or aft. Many inboards have very little rake compared to outboard props. If you want to see the difference, take an inboard and outboard wheel, set them down on a table with the stern/back side down. The outboard prop will be sitting on the blade tips with hub high from the table while the inboard prop may be sitting on the hub or close to it. The outboard prop has more rake, with the blades bent inwards...my understanding is that condenses the wash.

How it relates to Bow attitude...I have thoughts, but not sure enough in them to say much.

Yannis · Post by **Yannis** » Jan 9th, '24, 09:46

Carl wrote: ↑Jan 9th, '24, 08:01 ( Sorry for tossing in metric, but they are straightforward simple. In imperial, we then take that pitch measurement and figure out how many are in an inch...why")

Aaahhh! I never thought my eyes could witness something like this!!

jayjays1 · Post by **jayjays1** » Jan 9th, '24, 17:18

Thanks Guys I do understand what rake and pitch are but I need to understand how each effects the bow attitude of the B31? It was often said on this board years ago, that increasing the pitch and having larger props (and hence a larger blade area) pushes the bow of the 31 down with this attitude making the boat less stable at speed. Is that still the current thinking? I am not sure how rake affects bow angle though? Does anyone know? I know that Bob Lico knows a lot about propping so if he could chime in that could be awesome.

JJ

Yannis · Post by **Yannis** » Jan 9th, '24, 20:09

Much before Bob, may I say that all this discussion about prop angles and various prop sizes and multiple bow attitudes is all a matter of speed. Therefore you may never attain the optimal speed, nor the optimal consumption, but always consider that what made this boat what it is is nothing of all that. That wrecked blue police pic we saw is the optimal weapon!

Snipe · Post by **Snipe** » Jan 10th, '24, 17:43

I changed my shaft angle and moved my engines back and down as far as possible. I was able to get 11.5 degrees. The boat rides great cruise 30 knots. I run a 21x24 four blade Michigan hytorq with a light cup. What I’ve seen before with over powered boats is they push the bow down to far and the boat will bow steer. Also they could bow dive in rough seas (push through the waves instead or riding over them) I have never saw a 31 do this but I also did make changes when I did the repower.

jayjays1 · Post by **jayjays1** » Jan 10th, '24, 17:49

Thanks what engines and gears you running?
JJ

Snipe · Post by **Snipe** » Jan 11th, '24, 16:11

JJ I am running Cummins 370’s and zf 220a.

jayjays1 · Post by **jayjays1** » Jan 14th, '24, 12:35

Ok cool. I am running the 370's but haven't found the model numbers on my ZF's yet. I am running 21x23's. What sort of top end speed do you get and at what RPM at WOT?
JJ

Bruce · Post by **Bruce** » Jan 15th, '24, 05:20

I've rode on a number of 31's over time, most I repowered with a variety of engines and some that I was tasked to survey for a buyer and one where the owner was trying to sell after putting over 400K into it with 440 Yanmar's.
The only one that plowed nose down and was uncontrollable at speed was the later and it was because the owner put a huge tower on it which basically sent the CG way forward.
While CG on a boat is a little more forgiving than an airplane, if pushed far enough it will create a bow down which the bow acts as a rudder making the steering almost impossible.
I've also done a 31 with prop pockets when Cummins first came out with the common rail 370. Really didn't make that much difference with the cost of putting them in but that's what the owner wanted.

jayjays1 · Post by **jayjays1** » Jan 15th, '24, 19:00

Thanks Bruce. My main mission is to attain the highest possible cruise speed as I'll be using the boat to commute back and forth to work each day.

Thanks for the info!

JJ

Snipe · Post by **Snipe** » Jan 15th, '24, 19:52

JJ my top end was 38 knots I cruise at 2400 rpm and I am 29 to 30 knots.

jayjays1 · Post by **jayjays1** » Jan 16th, '24, 18:44

Ok thanks!

bob lico · Post by **bob lico** » Jan 23rd, '24, 19:38

tony ,carl and bruce covered the subject well. for the most part if you walk into a prop shop and ask them to put rake in a inboard propeller they are going to give you a snicker and blow you off so you have to understand the ramification of your request . rake is needed after you have done many steps of performance tuning the boat like pulling the engine ( in this case lets use cummins 6BTA-M3 ) back until the gear mount actually touches the fuel tank bulkhead and the aft end of the gears are 1/2" off the bilge.,two layer FG tubes as shaft log slide up and down shaft with just a slot in the bottom of the hull for shaft log.motified stucts with cutlets bearing sticking out the front, remove rudder port completely glass existing rudder hole thru bottom and make new hole the size of rudder shaft thru the bottom and install tides marine F/G Delrin rudder seal and huge delrin/ ss bearing on rudder shelf.hull is built up to two inches thick for strcts ,rudder support.my 31 does 38 knots with 315 hp cummins . my very special three blade props will try to push the bow dow above 35 knots actually a built in problem with any inboard with shaft angle and trim tabs would probably cause me to be a swan diver off the bridge when i stuff the bow in 5' seas . the propeller diameter ,pitch and cup are all about the particular engine you are using prop load ,the rake (i use 4 degree ) is what i need to keep the water breaking directly under the pillars at cruise speed ( i know you contoured the strakes with straight edge) this is maximum fuel economy.

jayjays1 · Post by **jayjays1** » Jan 24th, '24, 07:18

Thanks Bob you are the guru on this! What do you think the minimal shaft angle can be if you till want to swing a 22 or even 23" prop?
JJ

jayjays1 · Post by **jayjays1** » Jan 24th, '24, 07:19

And without shaft tunnels

Carl · Post by **Carl** » Jan 24th, '24, 09:51

This is how I figured what would work before starting-

Top yellow lone is hull at zero degree

Blue line is the shaft.
Top right square dot is the center of the Output coupling on trans, at 6-1/8" above the hull.
Bottom left verticle line is aft end of strut...
Follow Blue line down to the square dot, that prop hub around the blade tip/ aft end of prop.

From that I had 12-1/8" from center of shaft to bottom of hull. A 20" wheel has 2"'s of prop clearance giving text book clearance. I could go 21" dropping to 1-1/2" tip clearance, not too bad. A 22" wheel would only give an inch tip clearance...too close for me. Although I ran 10 degree angle with my gas motors, changed the trans added reduction, and stuck on 17" wheels from the original 15". Very little clearance, like 1/2-3/4" a little noisy, but ran for a few years and came across 16" wheels which were much smoother. I didn't see any hull burn, but only put 4-500 hours on the 17" wheels.

Anyway, if you wanted to go larger in diameter a little more angle adds extra wheel space quickly. YOu could also move strut and prop aft further. It will interfere with rudder so you cannot go much and you do not want to get too close to rudder period. BUT Carl from Elite Marine does move his rudders to the transom on some of his builds. He used that to lower shaft angle, but you could keep 11 degree or so, going back a few inches would give room for larger wheel.

Question... are you using the boat for trolling or close quarters where you need to run slow at idle? If not much of an issue you could add a good amount of pitch, cup n rake to 21" wheels for lots of go. I'm not sure how much more wheel diameter is advantageous at the speed you're looking to go. I also have a feeling more blade surface may create paddling. I think adding a larger wheel, additional blade, pitch and rake can produce stern lift. That last part is just my gut guess. Whether right or wrong, bigger wheels increase drag. Again outside my area...
Also the 31 was once a race boat, but that was back when boats were slower. I am not sure how well the 31 runs over 40 knots which is where you seem to want to be. Just saying the 31 is the best boat ever built, but not always the best boat for every function. Again...my un-requested .02

jayjays1 · Post by **jayjays1** » Jan 27th, '24, 09:47

Hey Carl this is helpful. I read a post from a long time ago where there were problems if moving the rudders too far back? Or maybe it was that there was uneven rudder positioning. I am totally gutting the boat and will use oversized rudders. I would think that the further aft the struts (and rudders) are then the shallower the shaft angle one could have -perhaps less than 10 degrees? Does anyone know for sure if any problems if rudders are all the way back?

JJ

Carl · Post by **Carl** » Jan 27th, '24, 12:50

I believe the issue was the rudders were too far back from the props, I think you want to keep that relationship between the two very similar. It usually seems to be just enough room to pull the prop off shaft. If you want to find the info I think it was a member from the Caribbean, maybe St. Lucia. Kipp?

Lol… so now that you find you might be able to swing a larger wheel moving gear back, you want to reduce the angle further too. I believe there is some benefit, but I’d think marginal. Member Piezo’s dad had his boat modified with rudders mounted on transom, maybe contact him for performance and see what he thought. I do not recall great excitement

I mentioned moving gear back as the question of can it be done always intrigues me. Yes a bigger wheel can be made to fit, but will a performance gain to offset the effort and cost be seen is another matter. Might be better ways to find gains.

Just thinking out loud.

jayjays1 · Post by **jayjays1** » Jan 27th, '24, 14:50

Thanks Carl, larger wheels are not my priority. I think I'd rather have the props farther back in order to minimise the shaft angle further to maybe even less than 11 degrees and still have room for a 21" or so prop. I don't mind repositioning the rudders as far back as possible as boat is a total redo anyway.

Tony Meola · Post by **Tony Meola** » Jan 27th, '24, 22:31

If I remember correctly the 29 Blackfin had the rudders set up, so they were out behind the stern. I believe there were some issues with the boat handled with the rudders back that far. You do not see any of the newer inboards designed that way.

jayjays1 · Post by **jayjays1** » Jan 28th, '24, 07:13

Yup been reading previous threads about that very issue last night. Seems the problem was improved by placing a trim tab over the rudders.

Carl · Post by **Carl** » Jan 29th, '24, 07:13

Yes, to avoid rudder ventilation a rudder plate extending out from the transom, or a foil incorporated on top of the rudder seems to be part of the design...I'd think trim tabs could also work. Often nothing is as simple as "just move it" as one thing changed leads to another each having its implications.

Another thing...our boats are right at the edge of requiring an intermediate strut. There is a ratio between shaft diameter, shaft rpm, and unsupported/supported length. Moving the strut back further extends shaft length, you may want to look into the books and see if the additional strut becomes more of a need than a recommendation.

jayjays1 · Post by **jayjays1** » Jan 29th, '24, 16:18

Thanks Carl will check it out!

jayjays1 · Post by **jayjays1** » Feb 4th, '24, 18:38

Bob is this the rudder port system that I should get? I will have the oversized rudders from High Tide with 1.25" shafts.

https://www.tidesmarine.com/shop/rudder ... 1250-0827/

Tony Meola · Post by **Tony Meola** » Feb 5th, '24, 22:06

Not sure if this is what Bob used, but most of Tides products are pretty sweet. I have their dripless shaft seal.

jayjays1 · Post by **jayjays1** » Feb 6th, '24, 07:09

I am just concerned about the "new fangled" stuff. I know how the bronze shaft ports work with the packing etc. I don't want to be replacing seals every couple years with something "better".

Tony Meola · Post by **Tony Meola** » Feb 6th, '24, 22:05

What is nice about the Tides is that there is no belows to worry about. They come with an extra set of seals that sits in a carrier on the shaft until you need them. You can install them even offshore. Undo a couple of screws slide the old ones out, cut them off slide the new ones in.

I put two extra sets on my shafts because with my luck I would need them that quickly.

Carl · Post by **Carl** » Feb 7th, '24, 08:03

Just came across this post about transom-mounted rudders on a 31. I remember this post, just do not recall the impressions after the work was done. Some say the results can be a big part of a project.

https://www.bertram31.com/newbb/viewtopic.php?t=2279

jayjays1 · Post by **jayjays1** » Feb 7th, '24, 08:22

Thanks Carl that's the one I remember. General opinion was that it was a bad Idea to have them situated without hull or at least a plate over them.
JJ

bob lico · Post by **bob lico** » Feb 8th, '24, 13:13

yes that is the correct tides rudder tower however you have to put the bottom on a good chop saw with 80 pt carbide tip blade and cut bottom 24 degrees to match Bertram hull that's the secret . just cut angle with tower clamped in saw and just touch one edge with blade and cut 24 degrees. mix epoxy 1/2 high density filler and 1/2 colloidal filler forming a thick paste put tower over rudder shaft and slide down the tower and rotate tower to match with 24 degrees at hull. you first remove old rudder "old fashion leaker rudder support" and seal hole in bottom the new hole will be the size of the rudder shaft. there will be about 1" of exposed rudder shaft and the you go thru rudder support (laminated CDX 1/2" soaked in epoxy ) now you put the new rudder support bearing Delrin, stainless over the shaft and bolt down NOTE the tower is not a bearing support you must use bearing on top of rudders support framing laying on the stringers.

jayjays1 · Post by **jayjays1** » Feb 8th, '24, 19:32

Hey Thanks Bob. How do you find the maintenance on these seals? Do they have to be replaced periodically?

JJ

bob lico · Post by **bob lico** » Feb 9th, '24, 20:06

don`t touch for a hundred years and zero water seepage a perfect seal and delrin,stainless 6" bearing keeps rudder with zero shudder------bullet proof

jayjays1 · Post by **jayjays1** » Feb 12th, '24, 09:16

Hey Bob, just got off the phone with TidesMarine. Guy was saying they recommend the bottom end of the tubes to go right though the hull at the bottom and then be faired off. Also says they don't need top bearings.

What are your thoughts, as I want to do it right and I know that you have much experience in this?

JJ

Carl · Post by **Carl** » Feb 14th, '24, 10:40

On a windy day walking with a flag pole, I place one hand low with the other high, close to the flag.

In my mind, the same logic applies to a rudder. I'd want the low support to be as close to the rudder as it could be with a second as high as possible. The rudder is not static, it rotates to make a turn, I'd want the support to be a Bearing Material over fiberglass for that reason.

johnspiezio · Post by **johnspiezio** » Feb 16th, '24, 12:05

Glad to be back on the board, nice to see the community is still going strong. As Carl mentioned my dad had a boat built with transom mounted struts. THe boat is powered with Yanmar 315s, the last of the mechancical control version that Mastry had in stock. On that boat the engines have been shifted aft and down. This has kept the distance to the rudders where it needs to be without extending the shaft, and it allowed for a smaller shaft angle.
The boat weighs 16,000 lbs, has a half tower, and 310 gallons of fuel plus water etc. with a full tank she makes just over 28 knots at 3600 rpm. I cruise all day at 3200 to 3400 and she is making 25,5 to 26.5 knots in that configuration, full throttle is a tad over 32 knots. As the fuel burns off she makes more speed, 1/2 a tank and I get over 31 knots at 3600 rpm. She is propped to make 4050 rpm at full load with 4 blade.

No problems at all with the running gear or the rudders. I will add some pictures but they may not be clear. I can add close ups once it warms up if you want to see details.

johnspiezio · Post by **johnspiezio** » Feb 16th, '24, 13:39

Not a great pic, but if you zoom in you can see the set up. I have trim tabs, because my dad liked them, but they are mounted outboard of the rudders, not above them.

bob lico · Post by **bob lico** » Feb 16th, '24, 18:29

Carl you are the best, nice explanation on the tides tower apparently the guy on the phone is a yard worker ok IQ test you listen to the idiot and put a stainless clamp on top of the fiberglass tower as the rudder moves back and forth what do you think the SS clamp is doing to the fiberglass tower!!!!!!! ok genius so you put a 3" hole thru the hull for the square tower to fit in a 24 degree dead rise DUH !!!! rule number one " in order to teach the dog tricks you have to know more than the dog" trust me use the Derin/stainless bearing on top of the horizontal rudder support. the rudder is held in roller bearings not the fiberglass / neoprene tower just like the SS collar on your PSS shaft seal

jayjays1 · Post by **jayjays1** » Feb 16th, '24, 19:53

Hey Bob thanks. Will stick to the plan! Stupid question but we presumably need a collar above the Ss bearings right?

bob lico · Post by **bob lico** » Feb 16th, '24, 20:10

yes that's what i am trying to explain the tower is the SEAL the Delrin Stainless bearing is what holding the rudder shaft from moving and the support as well as a roller bearing surface to turn the rudder on

Carl · Post by **Carl** » Feb 18th, '24, 09:56

From Tides Marine website.

If you look at top picture you”ll see their design has 2 Bearings shown in dark green, 1 at the very bottom and another at the top just under the lip seal.

Picture also shows the fiberglass tube is inserted through hull flush to bottom and glassed in place, fiberglass shown as blueish grey. The brown item at very bottom would be the boat mold for new boats.

Looking at picture below-

This is where I think we are all talking about the same thing differently. Starting with Tides stock system

The fiberglass rudder port shows a rudder shelf to supporting the top of their tube.

Tides Marine has a bearing incorporated in the top of the post, 1 in the bottom, 1 in the top. These 2 bearing take the side loads when running and turning.

Tides Marine supplies a 3rd thrust bearing that takes the weight of the rudder, it can be seen in dark grey, it rides on top of the rudder port just under the clamp collar, below the tiller. This bearing is nothing more than a thick washer made of bearing grade plastic.

I’m sure Tides Marine rudder port system works well with the upper internal plastic bearing and plastic washer/ thrust bearing.

But Bob doesn’t seem to like something that is just good n ok. This is where Bob goes above n beyond by adding true heavy duty support system by way of utilizing a Stainless Steel pillow block bearing mounted to the solid shelf above the top of tube, not just a piece of plastic sitting atop the fiberglass tube.

Is going that far over stock too far and unneeded. I’m not sure and not my place to say. I do know turning that tiller underway takes a whole lot of force, especially in seas. I’d also rather see a SS bearing take the abuse over having to change the upper plastic bearing.

jayjays1 · Post by **jayjays1** » Feb 18th, '24, 11:18

Thanks Guys fully understood. Do we need a a rudder collar on top of the pillow bearing set up to stop the rudder wanting to slip downwards?
JJ

bob lico · Post by **bob lico** » Feb 20th, '24, 12:35

OMG can the guy who display this tower installation "walk and chew gum at the same time" i doubt it ,just what i wanted on a sleek 31 bertram hull a bolt, nut and washer sticking out th bottom ,unbelievable the bottom painters turn marine mechanics are taking over

Carl · Post by **Carl** » Feb 20th, '24, 14:50

bob lico wrote: ↑Feb 20th, '24, 12:35 OMG can the guy who display this tower installation "walk and chew gum at the same time" i doubt it ,just what i wanted on a sleek 31 bertram hull a bolt, nut and washer sticking out th bottom ,unbelievable the bottom painters turn marine mechanics are taking over

Bob- Take another look. The depiction is for new boat construction. The hardware is keeping the Rudder Port Aligned to the mold as they layup the boats fiberglass. Hardware gets removed after...

Fixturing a new install similarly is not a bad idea in my opinion.

bob lico · Post by **bob lico** » Feb 20th, '24, 21:53

carl installation is for a ' work boat' with twenty tires wrap around it with a flat bottom actually called a push boat . i see it Carl for lay up purposes obviously the rudder goes were the bolt and nut goes but cylinder tower must be address to the interior bilge of a 31 bertram at 24 degree dead rise ,please listen my is not a proposal it is real life running in the ocean under extreme condition offering ZERO water dynamics with flush installation and zero friction

jayjays1 · Post by **jayjays1** » Feb 21st, '24, 06:43

Hey Bob are the pillow bearings shown by Carl the exact model to get?
JJ

Carl · Post by **Carl** » Feb 21st, '24, 07:34

LOL...OK I see your point Bob, the picture shows a flat-bottom boat.

I think we are on the same side of the fence. I look at the rendering as a basic instructional guideline for installation. The rudder post is plumb and the base is to be cut flush to the outer hull. Flush can be 90 degrees as shown for a flat bottom boat, a 24-degree cut to match our 31, 20 degrees to match a_____ (fill in the blank) just make base flush to hull.

I think this horse is dead. lol

bob lico · Post by **bob lico** » Feb 23rd, '24, 10:38

no that is the wrong bearing . the correct bearing to last a hundred years is 6" in diameter and black Delrin outer circle with a stainless roller bearing inside , has 6 ---3/8' bolts that go thru rudder support with flat washers and nuts below made by Tides Marine the same as the tower.forget the bolt and nut installation kit for the tower lets establish the fact that the rudder tower has a flat bottom and your 31 Bertram has a 24 degree dead rise so some boat bottom painter threw some resin lump in the bottom to fill gap MOST important and Carl could explain better , if the bottom forward edge of the rudder (which would normally hit a object or grounding with larger rudders ) will cause the length of the rudder to act as a lever and rip out the tower and a 3" hole in the bottom of the boat !!!! you would sink the boat for sure . that bearing is essential and would save your life let along the wear and tear on the tower , the tower is the ultimate rudder seal But not a essential rudder support as well as holding the rudder sturdy and make steering bulletproof ,part number 0190 and serial number 280244 for the bearing .

jayjays1 · Post by **jayjays1** » Feb 23rd, '24, 19:08

Cool thanks Bob. Will get the 6" ones!
JJ