PROPS

[Craig Mac]

There have been many threads on props--but don't remember anybody referring to the Slip Factor-----what is the optimal slip % that we can reach with our hull design and shaft angle?

I assume 4 blades have are better than 3 blades--correct?

http://www.rbbi.com/folders/prop/propcalc.htm

[bob lico]

In theory; 5 blade 110 percent,nibral

[bob lico]

You can obtain better then 92% after many hours of trial and error but you are definitely going to chop off wot. Speed to obtain maximum cruise speed at lowest rpm possible.super economical mode will surpass a 26' to 27' center console twin outboard boat .

[Kevin]

Bob,
So how do we do it?
I have a pair of nibrals that should be here this week. I plan on throwing them on and seeing how they compare to my current bronze set up. The nibral have an extra inch of pitch so my comparison won't be apples to apples.

[bob lico]

I may stand corrected but i only know of one source for "blank props" a small casting place in ST. Augustine FL. Very nice people that will give you what ever you need. I sure they will fine tue them to your spec. Just ask " my name is clusterfu-k " and i want to rake a inboard prop. "Yoall kidding me right? " then tell him what you want.( I recommend about 4degrees of rake ),3blade only!use the Biggest diameter you can turn with a inch of hull clearance then cupped the entire prop from hub around blade to hub again with maximum curve in cup this will cause a dramatic increase in prop efficiency, Go light on pitch will you will INCREASE cruise speed at lower rpm. However top speed will drop accordingly. Make sure engines turn up 50 to 100 rpm. beyond your engine rated rpm.at wot. My boat turns up 2950 rpm at a rating of 2800 rpm i cruise at 2150rpm in super economy mode about 30 .

[matt ciarpella]

Bob,
Are you saying you cupped the leading edge of the blades?
Ive aways followed your posts regarding prop cupping for diesels. Couple years ago I had work done to the props and decided
to give it a try, the results were not good. The boat vibrated when attempting to get on plane and wouldn't get up to plane.

Now I don't have the cummins engines, not sure if that had any effect.

standing by

[Kevin]

Yeah, my mind does not grasp how cupping the leading edge of the blade would or could work. I would think that would cause a lot of boiling water. I have tried a lot of things that were debatable for performance on cars and boats. The rake I understand.... I think. But why would our props not come with rake in them from the factory? It is supposed to raise the bow thus reducing wetted surface of hull, correct?

[CaptPatrick]

It's actually the trailing edge of a prop that gets the cup... Cup holds or grips the water before allowing it to spill off and cup also has the effective results as adding diameter. So a 20" wheel with a medium cup is effectively very similar in action as non-cupped 21" wheel.

Slip, relating to a boat, is a commonly misunderstood term. It does not mean the wheel is slipping or loosing traction like you'd think of a slipping belt or transmission. Slip is the difference between actual distance that a prop will move a boat forward or reverse in 1 revolution as opposed to the theoretical distance the prop will progress in absence of all other factors affecting a boats movement.

A prop is a screw that progresses through water much the same as a wood screw moves through wood. So what we are concerned with is a "water screw". A 24" pitch wheel is designed to move 24" in water. However, there are many factors that hold a boat back from motion; friction from wetted surfaces, weight, and bottom surface conditions, & shaft angle to name the 4 biggest factors.

Slip is also generally discussed as a factor of speed.

To find the slip for your boat start by finding the theoretical speed in miles per hr, (not knots). Multiply your pitch and a given RPM then divide that number by the gear ratio times 1056. That gives your theoretical speed for that given RPM.(24x3000 = 2112)/(2.0x1056)=34 mph theoretical speed

To find the slip subtract your actual speed from the theoretical speed and divide it by the theoretical speed. 34 mph minus 30 mph = 4 mph or just under 12% of slip. Slip is never a negative number and slip is not the same as efficiency or inefficiency...

[bob lico]

Capt. Excellent explanation of cup. I just don't have the words!

[bob lico]

Kevin some assho-le somewere figure putting trim tabs negated the need for rake . You have a fixed shaft angle in a inboard so the faster you go beyond 30 kts. The move that fixed angle will drive the boat down in the bow until the bow is ruddering the boat you have to sit back and anticipate that factor then build design accordenly .elliminate the tabs which is the same as holding a oar in the water at 90degrees when you want to stop or slow a kayak and use a rake prop which will steadily work to keep boat on level plane at all speeds. l

[Kevin]

Bob,
I don't see the connection between tabs and rake. Tabs can help level the boat or push the bow down....lift the stern maybe? You say that positive prop rake helps lift the bow right? Have you known anyone that experimented with adding rake to the props on a 28 hyena? I am sure the 28 and 31 are very different rides and I have never been on a 31. I know the Hyena term came from the boat looking like it is dragging its ass. Mine used to have that look but as I increased cruise speed over the years I have found that it runs more level than before. It seems like when I hit about 24-25 knots it is a better running boat. I am willing to experiment a little with the new nibral props when they get here. That is after I run a couple baseline sea trials first to see where I am.
Sorry, I just realized I may have hi-jacked a bit.

[bob lico]

Kevin there is no such thing as hy- jacked on this board say what ever on your mind. There is a 100% comparison between a experimental raked prop and trim tabs! Both are usedto change trim attitude. A good 300+ hp diesel installation in a 31 bertram or 28 bertram needs to bring the bow up as it increses speed ( opposite the effect of trim tabs operation) .the OEM
14 degree shaft angle starts to "push the bow deeper and deeper into the water" as speed increases ,at 34 knots you would start to get into a dangerous situation the boat is now ruddering . The raked prop along with common sense balancing of holding tank,head location,A/C ,batteries,water tank,genny,anchor/chain. Will keep her bow proud at 38 kns. Don't look or listen to other boat owner think how the boat is riding in 4' waves this is the test. Nice to have a travelift with one strap to help you determine balance. A 31' fbc bertram needs about 1100 lbs total weight (one engine with gears )or more in engine compartment along with raked,cupped,21" diameter prop the more you deviate the more you will get your ass kicked in rough water.one more proven theory in Alantic ocean waters with average 5' waves 5 to 6 seconds apart 95% of the boats in the 30' catagory will back off break plane and keep boat bow proud ,given the above info you will maintain 23 knots riding the tops how ever trim tabs will add to you burying the bow by causing a upward motion on the wave you last left.kevin you ever come north i will let you pilot the boat in small craft warnings.

[bob lico]

All pm's the strap from the travelift goes in line with main bulkhekad,bahia- mar in front of engine were engine hatch closest. First make sure port / starboard is dead even with water.the boat was desigh with 413 " Chrysler engine and cast iron gears. In 1961.454 chevy did not come until 1970.

[Carl]

Bob,
Are you saying you cupped the leading edge of the blades?
Ive aways followed your posts regarding prop cupping for diesels. Couple years ago I had work done to the props and decided
to give it a try, the results were not good. The boat vibrated when attempting to get on plane and wouldn't get up to plane.

Now I don't have the cummins engines, not sure if that had any effect.

standing by

Matt...here is a bump so we can get Bobs answer.


You didn't ask for it...but what the hell....here is mine.

Ever drive a manual transmission?
Most boats are setup to run in 1st gear as you need alot of power to get over that initial hump... then your speed/cruise is limited to running in 1st gear.

What Bob is doing is starting out in 2nd gear. You need more power to get over the hump...but once over your running in 2nd gear which is more economical. Now add the cup...its almost like up shifting again. A cup takes effect at higher RPMs...so now your running in 3rd at cruise...plus now you have the advantage of fine tuning the props cruising speed to the sweet spot of the motors.

Problem is...Rake eats up power, cup does as well...that vibrating rumble you heard is your boats motors attempting to get over the hump...but falling short. Basically you were over propped.
Trick is knowing what to cut down on...or reduce the need for the extra power. Lighter boat...better balanced...
It is alot of trial and error...we played with my old boat with huge differences...like picking up 13mph and burning same amount of fuel at cruise.
My father had a less dramatic experience with his 28 Hatteras...went to 4 blade wheels with added rake...he picked up a couple knots at cruise, same burn.

As me and dad had similar setups...I was strong armed into giving the extra rake a shot. "Wanna try it on your new props Carl? Sure, why not dad,"...it was brutal I tell you...just brutal!. Cost me nothing to have props done and I changed them myself in the water...Still was a pain to change them...go out and have boat squat and rumble as it just didn't have the power to get her over the hump....back to the dock, jump back in and change back. Then send wheels back to prop shop...it takes time and usually a good deal of money to get them just right.
If somebody comes into my place to have something like that done...then answer is a simple NO. Very few people want to go thru the expense and time of fine tuning props. They do it once or twice and they get pissed...or they get boat to run faster and just dismiss the fact the motors will not hit WOT...so yes they are cruising faster...but motors are not going to last that way.

The other issue...some prop shops have no idea how to do anything out of the ordinary...but that often does not stop them from giving it a try. It's not hard to ruin a new wheel in the wrong hands.

[bob lico]

A pair of cummins 315 put out 1200 lbs of torque .the object of the game is to make the boat the most economical with highest cruise possible. Rumble? Vibrate? Sluggish ? How about launch out of the water at a 45 degree angle and fall on plane in 5seconds as withess by over 20 people on this board! With 12 people on board and full fuel! Attains 150 rpm over ratings of 2800rpm --------2950 rpm.not a trace of smoke getting up on plane or wot. Also witness by 20 people on this board.cost me nothing to change props and a warehouse full of props to experiment.my prop shop will do anything i want and we put our ideas together after hours. I try my best to explain to my Bertram brothers what to do to help them so they don't have to go thru the cost and labor to do themselves. A 31 Bertram at 26knots should have the water touching the hull directly under the pillar from there forward out of water or just boucing on it .when someone looks at boat and remarks "wet boat" your answer will be "i have no idea what your talking about" the water always breaks just after transom with just a trace of water at hawespipe all due to these props.prop design is clearly keeping bow in the air actual water breaking under pillar .

[Kevin]

Bob,
What would you do in my case?
Current set up is 17x15 bronze with medium cup. Boat runs great at cruise of 25 knots and reaches WOT spec @ 29 knots.
Would 17x14 with heavy cup be better? I don't care about top speed, just the cruise speed and being able to hit WOT spec on the engines.

Carl,
Adding rake eats power? Is adding rake like adding pitch or cup when it comes to the reduction of RPM?

My Simrad is displaying engine load and at cruise I recall it being around 70%. Diesel should run around 80% load right? I don't know how the engine load calculation is derived. Maybe throttle potentiometer position.

Nibral props should arrive today but they are 17x16 cupped. Going to try them as is and go from there.

As always, interesting topic to me.

[Carl]

A pair of cummins 315 put out 1200 lbs of torque .the object of the game is to make the boat the most economical with highest cruise possible. Rumble? Vibrate? Sluggish ? How about launch out of the water at a 45 degree angle and fall on plane in 5seconds as withess by over 20 people on this board! With 12 people on board and full fuel! Attains 150 rpm over ratings of 2800rpm --------2950 rpm.not a trace of smoke getting up on plane or wot. Also witness by 20 people on this board.cost me nothing to change props and a warehouse full of props to experiment.my prop shop will do anything i want and we put our ideas together after hours. I try my best to explain to my Bertram brothers what to do to help them so they don't have to go thru the cost and labor to do themselves. A 31 Bertram at 26knots should have the water touching the hull directly under the pillar from there forward out of water or just boucing on it .when someone looks at boat and remarks "wet boat" your answer will be "i have no idea what your talking about" the water always breaks just after transom with just a trace of water at hawespipe all due to these props.prop design is clearly keeping bow in the air actual water breaking under pillar .

Bob...you have done all your homework, prep work and have a great prop shop to work with. B&S correct?
When it is all said and done, yes some huge dramatic changes over stock can be had...sometimes. Other times changes are subtle and need to be read carefully.

Problem is...not everybody crosses every "T" nor do they dot every "i". Sometimes this is because they lose interest or it just cost too much money and time...sometimes either they do not know better or the prop shop they are working with does not know better.
I have seen alot of pissed off people who choose to play the prop game hoping for better performance and quit along the way...usually when boat was overpropped and they got to cruise faster, but couldn't hit rated WOT...nobody want to pull an inch of pitch out...."and what go slower". Nah...I'll baby it. Or I'd hear No....I won't run it to the pins to check...it looked close...usually meant a few hundred rpms off.

Bob...correct me if I am wrong...your boat shed some weight, correct?..had some weight repositioned". Your shaft angle has been modified from stock? You may have tweaked the motor a tad? played with fuel pump? It all means something.
I just hate getting people started down a road where they expect to get your performance by "just tweaking the wheels". Its more of a process and can take awhile...true you have the knowledge as does your prop shop...but take one of your wheels to a friend of mine and see what comes of it. He is a great friend...can get a wheel back in shape...but rake, tracking...you'll get a blank stare at best. At worst...his guys will do what you ask...move blades aft abit..."no problemo"


Just saying...

[CaptPatrick]

Thank you Carl: The Voice of Reason!

I've witnessed many owners spend hundreds, even thousands, of dollars on prop tweaking trying to get another knot or two out of their boat. And for what? A mear 5% at top end?

The small classic Bertrams are not designed to be drag boats, race boats, or long enough to span 3 crests in a 5' sea. On a 50 mile haul, an extra 2 kt gets you there, what, 10 minutes quicker?

As Vic was fond of sayin': The enemy of good is better...

[Kevin]

"The enemy of good is better" I saw that coming!

Slip with my bronze 17x15 props equals 13% using the calculator. 15 pitch, 1.5 gear reduction, 4000RPM 33MPH
Slip with Nibral is 17x16 props equals 9%. 16 pitch, 1.5 gear, 3800 RPM 35MPH

Since my last post my new to me props arrived. Tested them out and I need 200 RPM on the top end as I figured I would since there is an extra inch of pitch on these new ones. The props will be off the boat again later today and meeting the man with the leather hammer.

If I pull out one inch of pitch I should be the right where I was with the old props. Debating pulling out two inches of pitch and adding more cup.

[CaptPatrick]

Keep in mind that when you change the pitch of a factory prop, much of that pitch change reverts back to the OEM pitch within a few hours of hard running. Especially with nibrals...

[bob lico]

Matt what engines do you have? Carl i only wish you piloted the boat at Greenport quite a few misconceptions would be cleared up.you realize i have nothing to gain i don't sell props or profit on boat performace just trying to help others before they go off on a tangent.sorry captain i diagree on a recent outing with 6 other boat we were dial in at 24 knt while they had to break plane they were 12 miles back before we were 34 miles out .waves were reported at NOAA buoy at 4' and 4.9 seconds apart off our port bow .we had a capt. Whom owned a 32' topaz express on the bridge. " never in my life have i ever experiance a boat like this" he commented his boat would do no better then 13 to 14 in these seas. I removed the 504" v-8 cummins the installed 6cyl. 250 hp yanmars and it felt like up the wave down the wave,sort of fell off the wave crest into the trought i figure 31 bertram was a hoax and i made a mistake.i did not start experimenting with props until i had a total understanding of all the parameters like 24 degree deadrise,strakes,balance .i had just gone thru this with converted 28' cigarette with same hull configuration with 8' beam.
Kevin go with less pitch more cup it will raise cruise speed at same rpm with slightly less wot speed.

[matt ciarpella]

Great explaination Carl. I'd say that's what i was looking for.
As the conversation continued and refering back to you post I can see why what I attemped failed.
This is what I did:
230 hp volvos use a supercharger to get to plane
2:1 gear
Theorectical max rpm is 3900

I had 20x24 prop and could reach 4100 with half tank or better
I thought "OK" Ill take it to 25 with some cup

So essentially I increased the pitch by two instead of one, resulting in what I described earlier
I couldnt believe how bad it was so took them back to original and have been cruising about 19 kts since
I was hoping to achieve a solid 21kt cruise speed with out pushing the engines beyond 3400 rpm

Bob any input would be great

Matt

[matt ciarpella]

That just may not even be possible, still great info to have. The knowledge here is nothing short of impressive

[Carl]

Bob...

I have no doubt about your performance numbers and listen and learn from what you have accomplished.

Where I see an issue is saying its "all" due to the props. Makes people think they walk into any prop shop with a 20knt boat and walk out with a 30 knot boat.


Let say this differently.

If we were to take the wheels off of your boat and put them on any 31 FBC with same Cummings Motors, what performance numbers would we see in your estimation...

BUT please keep in mind "Stock Motors"...no tuning the fuel pump, no cooling the fuel, no Vulcan Drive, no modifying the shaft angle, no moving the engines in any direction, no tweaking the strakes, no extending the shaft log. The excess weight stays put...no moving the holding tank, no moving the water tank, no lighter hatches, no lighter decking etc etc etc. A Stock 31, A Stock Motor...what do you see the performance numbers being?

If you say they should expect to see a 30 knot cruise with 2MPG...I'll back off my statements.



I also have an issue with telling people to just bring them to "your prop guy" and have them increase the rake and add a full cup all around.
If their "prop guy" is not "your prop guy", and as I know "your prop guy" I can say there are not many around like them. I'd bet on seeing more then a few destroyed props in the process and alot of very unhappy people.



That said...work with the right Prop Shop and follow the process to its completion...very good gains in performance can be possible.
Now Bob, your gains are extraordinary....but my guess is alot of that is due to a few % points increase due to this and a few % points due to that that led to the performance gains.
...to me very good gains are picking up 2 maybe 3 knots on the average ride with some fuel savings in the process. The cost is usually a couple hauls along with the cost of prop work. Many lose interest quickly...especially when the wallet needs to be opened to fine tune or the last modification was a dog. AKA- couldn't get over the hump. That is very telling to a good prop shop...but often the end of the process for many.

[bob lico]

Carl you have no argument here! You have a perfect describtion on the long process on my boat.like you said a little here and a little here like like extended F/G shaft log.true you just cannot walk into prop shop.

[Kevin]

Dropped off the props and had a discussion with the man that has been working on my props since 2005ish.
I told him where I was with the new props and where I wanted to be. I told him that I wanted bow lift. He knows me and what I am after. He said we would have to add rake but that he could not do that to these props.
Based on the two sets of props and the given WOT RPM's he suggested progressive pitch. 14 at the leading edge and 15 at the trailing edge.

After looking closely at the new props there is quite a bit more cup in them than my originals and it starts near the hub as opposed to just the outer portion of the blade.
We'll see what happens but I suspect he will nail it on the first try. Should know by Thursday if weather is decent.
I have it down to about 10 minutes now with removing props from the shafts. Practice makes perfect.

[matt ciarpella]

Good luck Kevin
Let us know it works out

[John F.]

Keep us posted.

[bob lico]

Kevin a cup starting at the hub and at least half way around the ear is what you would need in a lessor torque diesel engine.

[wmachovina]

Kevin, how were the Bahamas? By the way we'll be down at Founders Park Thursday through tues if your Round. Seachele II yellow hulled 38,,,, Bill

[Carl]

Kevin a cup starting at the hub and at least half way around the ear is what you would need in a lessor torque diesel engine.

What size cup? and is Kevin removing pitch...if so how much.




Bob, ah...so your cupping the tips to increase rake. Does it work same as cupping the trailing edge for pitch? That is allows slippage at low speeds and comes into effect at cruise...a good way to get over the hump.

[bob lico]

Carl That is correct .this is more then the prop maximun standard # 5 .incidently the props that i currently use are variable pitch 22 to 24" .

[Kevin]

Bill, the trip was good. We covered the whole Berry Loop but did not actually see much of what I intended on seeing. 6 people, one of which was a 2 year old created some challenges that were a bit to overcome. The guys wanted to rough it (camp) but weather did not cooperate. We were always safe and the boat was good to us. The weather made it uncomfortable for much of the voyage. I dodged lots of rain and storms while underway without the aid of radar. Weather limited the amount of nights on the hook/camping we could do.

The one boat that went with us had fuel issues.....the kind where you don't really know your boat capacities and there is no fuel available, so he had to make a run to Andros to get fuel. We ended up solo a lot of the trip.

I will do it again but with less people. Family or guys trip, just never again at the same time.

Props, not sure how my guy values cup. Light, medium or heavy verses a number system. I don't know how to compare them. I am removing pitch. New props are 17x16 cupped. They will be pitched progressively when I pick them up today or tomorrow. 14 on the leading edge and 15 on trailing edge. I think he is leaving cup as is to see how it works. I will take some photos so you can get and idea.

[bob lico]

Craig this is a clear photo of the best prop technology. 110 % props (overlap blades) is what i illustrated in my former post.notice the cup is ground into the beginning of The blade closest to the hub and then brought all the way around to the thin leading edge of prop.ONE more time this only applied to a 31 Bertram with twin engine torque in excess of 1200 lbs where you can take advantage to gain a higher then normal cruise speed and back off for a major increase in fuel economy.this will not work in gas engine nor 4 cylinder diesel in 31 Bertram!

[Kevin]

Picked them up today, installed and sea trialed. Ended up with 17x14PP. I guess the leading 1/3 of the blade is 13P, the middle is 14P and the trailing edge is 15P. That is how it was explained to me. The cup is now only on the outer 1/3 of the trailing edge as opposed to starting at the hub. WOT was 4000 RPM at 29 knots. Cruise was 3300 at 25 knots. In summary, identical performance to my original bronze 17x15 with the same percentage of slip.

What does not make sense is the original sea trial earlier this week as 17x16 with more cup got me 3800 RPM at WOT. I would have expected with so much pitch being taken out, a 17X14PP would have been around 4200 RPM WOT allowing us to add even more cup. Not the case as I am at manufacturer minimum spec RPM.