a prop's rake

[Sindel]

First some history...
I've got a 12 x 14 prop that makes my boat preform very well.
At wot my engine runs at 3400rpm (3200 is supposed to be top...)
This gets the boat up to around 32mph (gps)
However the prop is brass and doesn't fair well with even the small sticks in the river. It's been fixed many times...
I bought a new 13 x 13 nibral prop some time ago, but it made the boat ride nose high w/ wot of around 2800 rpm. (before gps)
When I first had this prop worked on, I told the guy I needed to turn the prop about 400 rpm more.
He convinced me to change the pitch to a 13 x 11 and to add a cup.
Upon trying the new prop the boat's wot became 3200 rpm (which was goal)
The boat's slow speed maneuverability was way better, but...
The boat still rides nose high from 9 to 32 mph. (flat bottom)
At 32 mph there is so much of the nose out of the water a strong wind can turn the boat suddenly without warning.

This year I busted up the brass 12 x 14 just after the july 4th weekend...
I quickly pulled the boat home, put on my nibral 13 x 11 prop, dropped her back in the water and 3 minutes from the marina a wot the shaft broke and toasted this prop (actually there was minimal damage from hitting the rudder, but still damaged...)
I thought I had checked everything out good enough, but some things you miss?
Anyways, when I took my props in to be repaired, I saw that they had a nibral 13 x 13 cupped prop for sale. I bought it, installed a new coupling, new 1" aqualloy shaft, and this new 13 x 13 prop...

The outcome...
Boat runs at 2800 rpm wot (32 mph w/ gps)
Nose still dangerously high.

I think it has something to do with the "rake" of the blades...?

How do I measure the rake on the brass prop?
Can the rake of my nibral prop's be modified by the repair shop to match?

[Guest62110524]

i think adding trim tabs will save you a great deal of bother, is this a go?

[Sindel]

I'd rather not add trim tabs to my 1951 Chris Craft Riviera.
I try to keep her as original as I can...

[Frosty]

In 51 years there has probably been some alterations made to the boat. If some one has moved fuel tanks rear or any alterations affecting trim.

I think rake will have minimal effect on your shaft system and you will need to make to some trim adjustments by moving weight around.

For instance is this the original power plant or some lighter or more likely a heavier equivalent.

Do'nt you have a small fin under the boat around the middle?

[Ad Hoc]

the history is good, but needs hard data now.
Engine Power (as of tdoay not when installed)?
Is the RPM you've given, at the prop or engine, (or no g/box?)
Not familiar with the boat, is it single or dual prop?
Boat displacement?
At rest, how does the boat sit in the water: level, slightly aft very aft, fwd very fwd etc?
What appendages, if any, are on the hull bottom surface?
Does the large trim occur when lightly loaded and when heavily loaded?

Just as a few bits needed really, before being able to answer some of your queries...otherwise it's just guessing

[Frosty]

Ad Hoc, you've never heard of a Chris Craft? I don't believe you.

Have you heard of Rolls Royce?

Never seen the aquatic shows of Florida 50 years ago, the swim suited girls in a pyramid waving?

Thats a Chris Craft not now but were then.

Bridlignton UK, 1960's trips from the end of the pier,--- Miss Velocity ?

[Ad Hoc]

Frosty

Ah, sorry, perhaps didn't make myself clear. Yes I am familiar with the boat, in the sense that I know what one looks like from pictures in magazines etc, very sleek stylish boats of the yester-year. But in the technical sense, to answer the Q, i know nothing about them.

Ahh..we had the paddle steamer the Ryde Queen at the end of 'our' pier!

[hwsiii]

Sindel. any good prop shop will be able to measure the amount of rake in your prop. Adding cup to a prop can either pick up the bow more or lift the stern. If cup is added to the trailing edge it will have a tendency to lift the stern, but if it is added to the blade tips all it does is increase the bow lift.
In my opinion you need a zero rake prop with cup on the trailing edge and it needs to be a 4 blade for extra lift at the stern. If you will take the boat out and run it and give me the RPM and GPS speed numbers from 1,000 RPM up to Wide Open Throttle in 250 RPM increments I will be glad to run some analysis and try to see what is happening, also if there is a way to gauge the amount of degrees of logitudinal increase in trim as you increase speed after attaining plane that would help as well.
While you are at it have the prop shop identify the rake and cup on your 12 x 14 prop as well, so we an do some comparisons.


H

[Frosty]

Sindel. any good prop shop will be able to measure the amount of rake in your prop. Adding cup to a prop can either pick up the bow more or lift the stern. If cup is added to the trailing edge it will have a tendency to lift the stern, but if it is added to the blade tips all it does is increase the bow lift.
In my opinion you need a zero rake prop with cup on the trailing edge and it needs to be a 4 blade for extra lift at the stern. If you will take the boat out and run it and give me the RPM and GPS speed numbers from 1,000 RPM up to Wide Open Throttle in 250 RPM increments I will be glad to run some analysis and try to see what is happening, also if there is a way to gauge the amount of degrees of logitudinal increase in trim as you increase speed after attaining plane that would help as well.
While you are at it have the prop shop identify the rake and cup on your 12 x 14 prop as well, so we an do some comparisons.
H

This is a shaft driven boat!!! he might do better by looking on the Mastercraft forum.

[Sindel]

Perhaps there's a place to have a copy made/forged... (in the usa)

The boat runs very well with the brass 12 x 14 prop.
Which has had a cup added to the trailing edge...
Actually could use a little more pitch, perhaps a 12 x 15
WOT is about 200 rpm high...
But, I don't want to mess with the best prop I have...
(prop originally can off a similar 19' Chris Craft 10 years ago...)

The repair shop said they couldn't change the rake of my nibral prop?
What is up with this?

As for ordering something with the rake, I'm still lost?

Hard data on the boat is hard to determine, but here it goes...
Engine: Chris Craft model KL - original specs say 105 hp - bored .030 over
Drive: 1 to 1 with single prop
Boat "Loaded" Weight: 2600 lbs (questionable scale used...)
Gas tank location original...
Battery moved 2 to 3 feet forward of original.
At rest she sits level (stays level till around 7 mph)
The bottom has only the basics...
- Shaft, prop, rudder, siphon, water pickup, drain plug hole near front, strip of 1/8" x 3/4" brass along the keel.
The bad trim happens whether the boat is loaded or not. (around 20*)
I'll have to take a notepad with me tonight to record rpm = mph

[Boatpride]

Hi Sindel,

It sounds like your propeller rake is set to far aft, this has the effect of lifting the bow out of the water. In order to lower the bow, adjust the angle of propeller rake to be more negative.

This could be achieved by adjusting the angle of outboard trim, so the propeller is closer to the transom. This lowers the bow. You may lose boat speed as more wetted area is in contact with the water. But you wont flip over at high speeds either!

Resources I used for this included:

Propeller Terminology (http://www.propline.com/Propeller-General-Information/Propeller_Terminology.htm)

Adjusting Outboards To The Transom (http://boatpride.com/questions-visitors-are-asking-and-answers/how-to-adjust-an-outboard-engine-to-the-transom.html)

Good luck!

[Frosty]

Some one else does'nt know what a Chris craft is.

It shaft drive!!!!! Thats a shaft under the boat.

Geeez sorry but the information is there.

[Ad Hoc]

Frosty

Now you see why i asked some basics Qs first..since all i know about them is just pretty shots from glossy magazines!! :)

Don't even know what length they are....i know not big though!

[Sindel]

The boat is 18' long...

[Ad Hoc]

how did the rpm readings go?

[Frosty]

It may be night time for you but in Us its still early morning.

He did say tonight he would take notes.

But your in the Med at the moment are'nt you.

[Ad Hoc]

No, I'm in Japan....and after a glass of wine with my meal, i do get a bit confused about time differences..hic! ;)

[Sindel]

Here's more data...

With the nibral 13 x 13 prop:
rpm - mph (gps)
500 - 4.7
750 - 6.1
1000 - 7.0
1250 - 7.6
1500 - 9.9
1750 - 12.3
2000 - 16.2
2250 - 21.2
2500 - 26.1
2750 - 29.0
2900 - 31.1 (wot)

One thing to note about is that the tachometer is mechanical and bounce 100 to 200 rpm sometimes...

Also found some old data from the brass 12 x 14 prop:
rpm - mph
375 - 3.3
500 - 3.9
800 - 5.5
1000 - 6.2
1500 - 7.8
1750 - 8.2
2000 - 12.6
2250 - 14.4
2500 - 17.3
3000 - 26.3
3200 - 28.6

Another thing I noticed today...
The boat needs to slow down to around 12 or 13 mph for descent steering. (nibral prop)
The brass prop would let me steer at more like 22 to 25mph

[Ad Hoc]

Looking at some old charts i have of prop data etc, assuming you have around 10% slip, the prop ratios are in the order 12x14 to 12x16, so you're in the right ball park, again assuming certain conditions, such as the rpm being correct, the engine power being correct etc...which it all probably isn't, but the range of 13x13 does seem about right. If it wasn't quite right, you would just change it slightly to suit your exact conditions (since it is bit of a guess anyway), as you have from the 12 x14 to the 13x13, just minor trial and error.

So this comes down to the boat.

the power to weight ratio 'predicates' a speed around 28~29knots, so that's about ok.

So, it must be down to the weight distribution on the boat, since just changing a prop, would not have such an affect, there must be some other effect going on, or something caught up underneath when running.

Can you run the boat with the 13x13 and move as much weight aft as possible run it, even add some small weights aft to get the LCG far aft and then repeat with as much weight fwd as possible?

See what happens. This is most odd, but interesting.

[Frosty]

[QUOTE=Ad Hoc;

So, it must be down to the weight distribution on the boat, since just changing a prop, would not have such an affect, there must be some other effect going on, or something caught up underneath when running.
/QUOTE]

Which is what i said in the first place- post number 2 from just plane experience.

However, what is the gearbox ratio, how you work out slip without knowing that?

[Ad Hoc]

Frosty
"...Which is what i said in the first place- post number 2 from just plane experience..."
Didn't say it wasn't, just need more facts to eliminate what it is not.

"..how you work out slip without knowing that?.."

If i may para-phrase...just plane experience.

Without seeing the lines and the fact he has not said a g/box is used (which i asked for, so hence non), and lines of boats like these and the position of the shaft angle and prop from endless previous vessels (and old predicted data) indicates around 10%. (These are from old series data of boats and props at various speeds rnageing from 5 to 80knots).To err on the side of caution 10% value i used, and it seemed to stack up with his figures. That's it..no magic. If you wish to disagree, please say so and why.

[Sindel]

Gearbox ratio again: 1 to 1

These are the responses that made me give up on trying to get a nibral prop that make my boat preform like the brass one several years ago...

Even the prop repair shop people looked at my funny when I ask them to change my nibral prop's rake forward; instead they convinced me to make my 13 x 13 into a 13 x 11 and add a cup. The 13's only gives me about 1.25" of clearance under the boat.

I can visually see that the blades on the brass prop are raked forward...
Is there a way to measure this?

I've found places to order props with different rakes, but which do I order?
I don't really understand their terminology or what the number mean...

How do I spec out a prop?

[Ad Hoc]

Sindel

It is not an exact science as such...just lots and lots of old charts showing which is 'best' and then interpolating between them. But other factors come into play as well, hence the iterative nature, as every boat is slightly different. The more boats you do the more of a "feel" one developes for a particular hull or arrangement that is then given as 'experience'. Personally, i wouldn't worry about the rake of the props or cupping..other factors are at work here.

[Frosty]

Well you tell me how you work out slip without knowing gearbox ratio, and ille tell you how I knew his problem was weight distribution. Which by the way has not yet been proven before either of us claims success.

"About 10% slip" will just about cover most low speed planing light weight pleasure boats.

[Ad Hoc]

".."About 10% slip" will just about cover most low speed planing light weight pleasure boats..."
Eaxctly...told you there is no real magic to it. Just confirmation from my old data, that's all.

[Frosty]

Ok lets look at this again . You say your boat is dangerously high at the front when planing. Why is it dangerous? Have you driven others?

[Sindel]

Are you guys more interested in each others discussions?
need a ruler?

Anyways, when I put the 12 x 14 on my boat it lays almost level once you get to around 22 mph.

This prop's blades are raked forward to remove water from directly under the boat bringing the nose down...

It has never been necessary (in the past 10 years) to move weights around in this boat to make it ride correctly.

Only when I install either of the two 13" props is there a problem with trim.

Many commercial working boats (ie. tug boats) use a prop with forward rake. The applications are there, I just need to know how to use them...

My brother-in-law has a 17' sportsman with a very similar hull. He runs a 12 x 12 prop. I tried this prop years ago and it did not cause the trim problem just excessive rpms.

When I say the boat rides dangerously nose high, the boat can quickly be turned port or starboard 90 degrees by a strong gust of wind.
The 13" props lift so much of the boat's keel out of the water that the boat rides on only the flat part of the bottom. (aft of trailer tires)

http://woodie.ppworks.com/images/trailer.jpg

[pistnbroke]

I think mr boatpride is just trying to advertise his alternative site Boatpride.com ..hey frosty its like your motors ... crap

[Ad Hoc]

Well, lets look at this technically then.

What is rake?
When the face profile of a prop is not normal to the axis, this is called rake. It can be raked fwd or aft and the only reason why this is ever usually done is to improve tip clearance, and sometimes aids flow into the prop to provide a more uniform flow, nothing else.

Hence the questions.

[Frosty]

Sindel ,is that yours or your brothers? the engine is far back is it a V drive gearbox?

Do you have a small fin under the boat. can you take some more pics, one of underneath and one of the engine installation and one of the prop that gives you trouble.

Ide like to see a pic of the boat planing in its "dangerous" situation. but!!!

If your brothers prop works well but gives you too much RPM have that one made with more pitch.

Im running out of ideas!!!!

[Frosty]

Well, lets look at this technically then.

What is rake?
When the face profile of a prop is not normal to the axis, this is called rake. It can be raked fwd or aft and the only reason why this is ever usually done is to improve tip clearance, and sometimes aids flow into the prop to provide a more uniform flow, nothing else.

Hence the questions.

Fwd or aft??? Ive never come across negative rake!!!

Rake is meant to keep the prop operating in cavitating conditions.

Its not normally used on a shaft boats, I cant think why it should be raked or what effect if any it would have.

Rake is of course extremely effective on surface propellers where just the submerged blades rake will make a big difference as it has non to oppose it.

Something is wrong here Sindal. I am trying to balance you obvious experience of 10 years owning the boat and its seamingly suddenly mis behaving itself for no real reason.

I am thinking increasingly worse reasons for this such as bad storage and the bottom of the boat being distorted some how, hooked.

Are the bilges flooded and water rushes to the back???

[Sindel]

Rake?

...to improve tip clearance, and sometimes aids flow into the prop to provide a more uniform flow, nothing else.

Exactly what I'm getting at...

As for pictures:

at rest
http://woodie.ppworks.com/albums/cruising/DSCF04601.sized.jpg

moving 5 to 7 mph
http://woodie.ppworks.com/albums/cruising/Pentwater.sized.jpg

wot
http://woodie.ppworks.com/Fast.jpg

a peek around picture of the bows bottom
http://woodie.ppworks.com/albums/cruising/DSCF0462.sized.jpg

and aft
http://woodie.ppworks.com/albums/cruising/cam186.sized.jpg

an oldie showing the bottoms flatness
http://woodie.ppworks.com/BottomsUp.jpg

another
http://woodie.ppworks.com/BottomOver.jpg

That wot picture above is the only shot I have at high speed, it's hard to take pictures when your driving :D

So, anyone...
How would one go about measuring a prop's rake?
Can it be done by hand or do I need some computer to do it?

[Sindel]

This is not something that has happened recently, it has happened everytime I've used either of the two 13" props.

Whenever I have messed up my 12 x 14 prop, I've used the 13" as a spare to use during the repair of the 12" prop. Because of the 13" prop's poor performance with trimming out my boat, I always pulled the boat and reinstalled the 12 x 14 as soon as I get it back from the repair shop.

I have 3 props now (2 are in for repairs)

I'd like to be able to not have to play musical props...

[Ad Hoc]

"..Fwd or aft??? Ive never come across negative rake!!!.."

I assume then you've never heard of the Classic paper "Propeller Design Model Experiments" by Emerson and Sinclair?

I use them frequently, well, when a prop is required....these experiments were using a prop with a 10 degree aft rake.

I know that rake of SC/CP props can theoretically ahve an effect on the trim of a boat owing to vertical forces exerted, but, the amount and for what size of prop, power delivered, boat size, displacement etc, I've no feel for since, i don't use SPs/CPs so can't confirm this myself to their real effect.

But from these pic's, the prop is a normal 3bladed submerged prop.

When you chnage the prop's over...does the shaft move in anyway or move any weights accidentally???...all seems very odd.

[Guest625101138]

H........

Another thing I noticed today...
The boat needs to slow down to around 12 or 13 mph for descent steering. (nibral prop)
The brass prop would let me steer at more like 22 to 25mph

Sindel
Does the boat want to steer to port noticeably more with the bigger diameter prop?

Also when you have done the 90 degree lose in a good breeze has it always been in one direction or has it gone either to port and starboard?

Rick W

[Sindel]

The prop pictured is the 12 x 14 prop

The explanation I got about rake from an old geezer who used to race these old wood boats is:
Forward rake draws more water from under the boat's center line helping to keep the boat drawn tight to the water then also the force of water going out the back of the boat is spread out more and since there's no water above the water the resulting force helps to push the nose back down...

Clearance do change a little...
13" prop on the shaft shows 3/4" before the strut - 1 1/4" clearance from bottom
12" prop on the shaft shows 1" before the strut - 1 3/4" clearance from bottom
The 13" props do have a beefier hub than the 12", but everything else about the same...

I don't have the 12 x 14 here yet to look at, but I'll try to keep myself from changing it back right away to get some good photos and maybe a plaster cast or something...

And here's my dilemma, when I get the prop back from the repair shop I like to put the prop back on asap (boating on the brain syndrome) and I lose my ability to physically handle the prop, then all I'm left with is the 13" prop wondering how to make it preform better... (like the 12 x 14)

[Sindel]

The boat pulls noticeably more to port with the 13" prop and there's actually not enough slip during acceleration (you can notice the engine working really hard)
The newest 13 x 13 will become a 12 x 15 when it needs repaired, but I'd like to add the proper rake also

Now that I have 3 props, I should be able to have a prop on the boat, a spare, and one in the repair shop being modified...
I always keep a spare shaft, coupling, lots of brass keys, stainless cotter pins... Pretty much the whole drive-train...

The quick unexpected turns have happened in both directions (always 28+ mph)
We ride mostly on the river with spots only 40' wide in spots, plus there are bridge columns, trees, rocks, other boaters, people skiing, tubing, etc.

[Sindel]

Also, a note on efficiency...
Every night we go for a ride to the damn, let the dogs out, and back home.
The trip takes 1 to 2 gallons with the 12" prop.
With the 13" prop it takes 2 to 4 gallons of fuel.
(averages dependent on time 1 1/2 to 2 1/2 hour rides)

[Frosty]

Ok nice pics. A lovely boat and what I expected. these boats don't trim you get what you got and you got what you get.

This boat is designed for 4 people you are bit of a fly weight yourself so if you were to seat 4 people in there your trim would be perfect.

I would be very happy with that trim. It would handle a bit of chop or another boats wake.

As far as blowing about in the wind these kind of boats usually have a fin under neath amidships, I mentioned it before, if it is missing it will blow about. If it never had one then!!!!

Lovely boat --I like that a lot.

I would be more concerned about the rudder, that big lump where the shaft is welded in is too big and right in the way of prop thrust. A higher aspect ratio (longer one ) might help the blowing about, certainly increase control.

[Sindel]

Does no one read?

That's the original rudder used during production. (kept as original as posible)

The boat preforms very well with the 12 x 14 prop (trim and steering at all speeds)

Even does well with another old brass 12 x 12 prop borrowed from my brother-in-law's boat years ago (runs high rpms, pegs the tach, breaks the needle, money out the wazoo...)

I don't use the 12 x 12, nor can I modify it because, my brother-in-law still uses it...

I can get them in brass for the same cost as the nibral ones, but one usually wants the best materials for the drive train to help with any hits... Besides each additional prop would be a gamble on how it will preform.

[Frosty]

Well then get your 12x12 prop back off your brother in law. Find a prop you like and fit it. I doubt if thats the original rudder but if you say so.

Fit the 12x14 if it performs well ???

I cant help you any more my magic wand is at the menders.

[Guest625101138]

The boat pulls noticeably more to port with the 13" prop and there's actually not enough slip during acceleration (you can notice the engine working really hard)
....

Pulling to port is to be expected with the shaft inclination you have. The bigger prop has quite low slip. You can do the comparison based on the numbers you provided:
14" pitch 3200rpm 28.6mph gives what slip?
Compared that with:
13" pitch 2900rpm and 31.1mph?

The lower slip with the larger diameter (17% larger area) means the down-going blades on the starboard side are working very hard in thrust and the up-going blades on the port side are giving little thrust or even acting as a brake. It means the prop is providing a lot of vertical lift force and also a significant steering moment, forcing to port.

Ski boats with this sort of set up that we used in Australia many years ago had a deep brass skeg (I have seen up to 16" deep) located slightly ahead of midships to maintain good control.

Reducing the prop diameter increases the slip so both sides of the blades provide thrust. Also the smaller radius results in slightly less steering arm from the unballanced forces so it is a double benefit.

As far as fiddling with rake and the like to get things to work better it is probably a matter of trial and error. The fundamental problem is the angle of inclination of the shaft. It limits the efficiency you can get by increasing the diameter of the prop. As you have seen the smaller diameter is actually better. A prop with smaller blade area should give better efficiency. The prop is spinning quite fast and the boat is reasonably easily driven.

Going even smaller diameter and increasing pitch may work better for controllability as the loading on each side of the prop becomes better ballanced as the slip goes up.

The prop efficiency is quite good anyhow - up around 75% if it was on a horizontal shaft. Would take a lot of effort to work out what is gets with the inclined shaft.

Rick W

[baeckmo]

The lower slip with the larger diameter (17% larger area) means the down-going blades on the starboard side are working very hard in thrust and the up-going blades on the port side are giving little thrust or even acting as a brake. It means the prop is providing a lot of vertical lift force and also a significant steering moment, forcing to port.

Rick W


Naah, Rick, that's not correct! Let's look at the inflow velocities (for the sake of simplicity using Va without asymmetrical corrections for induced speed), and check the situation with a shaft inclination of 15 degrees. The vector sums will show that it is the upgoing blade that has the max angle of attack and thus does most of the job, in particular with the 13"x13" prop!

For the 0.7 radius case on the 12x14 prop we get the following:

Downsweep: peripheral velocity (U) =35.75 m/s, advance vel. (Va) =12.79 m/s, relative vel. (W) =40.97 m/s, flow angle (fi) =17.57 deg.

Upsweep: U=35.75, Va=12.79, W=34.71, fi=20.85.

The blade angle at 0.7 radius is 27.95 deg., which leaves us with an angle of attack "upsweeping": 10.4 deg., and "downsweeping": 7.1 degrees.

Doing the same for the 13" prop we get the following:

Downsweep: U=33.89 m/s, Va=13.90, W=39.82, fi=19.72.

Upsweep: W=33.13, fi=23.90.

Blade angle at 0.7 radius is 24.45 degrees for this prop, resulting in an angle of attack of 0.55 degrees for the downbound blade and 4.73 when rising on the port side. This propeller is producing its thrust asymmetrically; the starboard side actually running idle. The small prop is working with high angles of attack (probably close to stalling) all the way around and with less asymmetry in pressure loading.

This explains why the 13x13 propeller is "digging down" the boat aft, (not creating lift, Mr Willoughby!!!), which in turn results in the nose-up attitude. The asymmetry of the propeller backwash is also creating a side-paddling effect to stbd. If this velocity check were done with correct inflow velocities (takes some iterating time) the tendency would be even stronger.

A flat bottom like this needs a fin to avoid skidding around as has been said earlier in this thread. In order to find the optimum prop and trim, use an inclinometer to avoid guessing on trim angles!

[Guest625101138]

Well done baeckmo. You agree with my diagnosis of the shaft inclination and the fix I proposed.

(I will leave others to check the vectors and get the right answer)

Rick W

[Ad Hoc]

baeckmo

I'm surprised you haven't been hit with the usual childish negative points that I get for pointing out the usual errors in Ricks arguments!.

However, nicely put.

I don't normally go into this much detail when i design a prop, i just check the basics, run a few worse case scenarios and then hand over the data to the prop manufacturer, as it then becomes their warranty, not mine!

Not having done the vector forces analysis, well except when i was a student some 25years ago but not since, is the vertical force, on a 100-ish HP motor really that high and really that different between a 12x14 and a 13x13 prop to influence a some-1.1 tonne run-about? In other words, would the increase/difference really be in the order of magnitude sufficient to create a large trim, assume a large trim exists, even at a shaft angle of some 5~10degrees (looks like)?

I can image on a very light mono running at some 80~90knots with a huge amount of power, then perhaps yes, but on a small run about? Since such a slight difference in prop size, relatively speaking, would have been notice when the vessel was designed, again I'm assuming "normal design practice" here.

Bearing in mind I'm speaking from a normal fast boat commercial field, as opposed to the pleasure-run-about market, design with props.

[Sindel]

The part number on that rudder matches what they used in production.
Everything chris-craft related has been saved at the mariners museum in wv (specs, blueprints, original hull cards, etc)

The boat's trim and steering are perfect with the 12 x 14 brass prop... (single right-hand prop)

The historic nature and value of owning one of these classic boats is to maintain its originality... (she's a 1951)
I've been to 30+ boat show and very rarely have seen one with an extended keel or forward fin/rudder.
Have seen one with a forward rudder for steering though...
I personally know 5 other owners with the same model boat and have ridden in 3
One had been modified with a modern v8 and required the addition of trim tabs, but it rides rough and drinks gas like crazy.
My boat with its 12 x 14 only drinks 1 to 2 gallons per hour...
Another that I rode in had a KLC model engine and got up and went around 42 mph with good trim and handling...
Mine runs a KL model engine with 105 vs the KLC's 135 hp and they weigh about the same (different pistons and carb setup...)

So, as stated before, there will be no modification, additions, etc. to the bottom of this boat.

...with the 12 x 14 prop, the boat handles and trims out well even with an additional 700 lbs of passengers in the back. (do it a lot...)

I'm not trying to better what I already have, just duplicate it! (in nibral instead of brass)

Everyone I know with one of these old wooden boats (that care about going fast and handling) uses an old brass prop.
One guy I ran across had purchased over 50 props from e bay before settling on one. (I don't have this kind of loot)

Are there prop manufacturers that could duplicate mine? (in the us)

Again I am not trying to optimize the prop or boat, rather I'd like to get a nibral prop just like my brass 12 x 14 prop...

[baeckmo]

If this was my boat, I would NOT go for a stronger propeller material. It is far better to fix a dented propeller blade now and then, than to have the "expensive" pieces (shaft, bearing bracket, bottom.......) destroyed. Btw, are you certain that the old one is BRASS? I would have expected bronze!

And, Willoughby, we agree on the source of the behaviour, but your analysis is completely wrong; in fact you are upside down! (Result from living down under.......?)

[Guest625101138]

....
And, Willoughby, we agree on the source of the behaviour, but your analysis is completely wrong; in fact you are upside down! (Result from living down under.......?)
baeckmo

I have not shown you my analysis. Your simplistic calculations give the incorrect direction of the forces.

If you want to have a more realistic stab you need to consider the velocity profile over the prop disc. You also need to remember the the lift force on a foil is perpendicular to the direction of flow onto the foil.

If you do the analysis correctly you will determine that the steering moment forces the boat to port and the moment is considerably higher with the 13" prop compared with the 12" prop.

Rick W

[baeckmo]

baeckmo

........ the steering moment forces the boat to port and the moment is considerably higher with the 13" prop compared with the 12" prop.

Rick W

On that we agree, the "paddling" works two ways; first the wash from bottom half of disc is directed to port, with no straightening from a rudder surface, giving a stern reaction to stbd, ie turning the boat to port. Second, the wash from the upper section is straightened by the rudder area, equally pushing the heck to stbd. Running straight fwd, both side-forces are counteracted by a slight stbd rudder.

Now, Sindels problem was that his boat was running nose-up (=stern trimming down) with the 13" propeller, not vise versa! In dealing with rotodynamic turbomachinery, like propellers, it is essential to understand the in- and outflow combination of velocities; the vector sum of W+U equals Va. In this case it is not necessary to go further to give an understanding of the phenomenon. In my world of trouble-shooting, knowing wether a force is directed downwards or up is not trivial........!

An additional factor is that the 12" prop is running with heavy cavitation, while the 13" has a cyclic cavitation pattern. If non-cavitating, the 12" prop would use some 160+ hp instead of the ~100 it is using today.

So Sindel, I think that Michigan Propeller Co has a prop series called "DynaJet" that is designed for this kind of operation. Its blade profile is cambered to work better in a cavitating environment. The optimum pitch for you is probably slightly less than 14", say 13,5 but with a cup.

[Guest625101138]

beackmo
The attached might help you understand how p-factor forces are determined for a propeller. While still simplistic it may get you to the next level of understanding.

Your notion of prop wash causing the steering moment is hogwash.

Rick W

[baeckmo]

But holy smoke, that is what I have tried to make you understand! What is not shown in your thumbnail is the important factor that the angle of attack is varying around the perimeter as well, and with that, the lift or better the flow deflection. The "thumbnail propeller" is rotating the same way as Sindels prop. The velocities and angles I gave earlier are the result of exactly the same considerations, with shaft angle 15 degrees.

This is a well known phenomenon in industrial pump installations and also a great problem in most jet propulsion arrangements; reducing efficiency and causing high radial forces on impellers. Any textbook on pumps will cover the issue.

So, go back to basics and try to understand that, then we can discuss Sindels problem (or is it your ego that is most important here?)!

[Guest625101138]

beackmo
It is good to have agreement on the cause of the problem. So you now accept that the downgoing side (starboard side) is creating the high thrust, as explained in the above attachment, and increasing vertical force and the upgoing side has lower load and less downforce. Net result being steering moment to port (or left as noted in the diagram) and overall uplift. (I believe this is opposite to what you originally stated)

If Sindel does not want the skeg then he needs to stick with lower diameter props that operate at higher slip. The negative pressure on the front of the the blade on the starboard side is limited by partial cavitation. The larger diameter prop has less of the blade in cavitation hence it is able to produce higher differential forces across the prop.

Rick W

[baeckmo]

beackmo
...you now accept that the downgoing side (starboard side) is creating the high thrust, as explained in the above attachment, and increasing vertical force and the upgoing side has lower load and less downforce.

Rick W

Nope, the blade force here is roughly a product of velocity x angle of attack. If the aoa > zero (as for the downgoing 13" blade; 0,55 degree), there is no lift no matter the relative inflow velocity, just drag. The change in momentum is small. In fact, if induced inflow velocity is taken into consideration, the angle of attack at the 90 degree position is negative. A flat face profile has often a zero-lift aoa of ~ -1 to -2 degrees, so the downmoving blade on the 13"x13" prop is only producing drag.

On the port side however, with blade moving up at an angle of attack of 4,7 deg (close to max L/D for these profiles), the product of W*(angle of attack) will be higher than on the stbd side in spite of the lower inflow velocity. The result is a flow leaving the blade with a higher peripheral composant (swirl), on that side. Most of this propeller's momentum change is produced in the sector 120 degr>60 degr, the rest is more or less "idling" due to low angle of attack.

[Yellowjacket]

Interesting analysis and results.

One point not made is that the higher trim angle (created by the prop down wash) actually makes things worse (and contributes to an even higher trim angle) in that the shaft angle will go up from 15 degrees to maybe one or two degrees higher, so the angles will all go up on the thrust side and down more on the "non'thrust side. It isn't "running away", but a couple of degrees more is clearly making a difference.

So, if Sindel wants a nibral prop, what specs should he be using to get closer to where he wants to be? That is, assuming that he can go to a prop supplier, what rake and/or cup and pitch should he be trying to spec that will get him what he wants?

[baeckmo]

The resulting forces, vertical and longitudinal, are totally depending on the lift and drag coefficients in the respective position of the blade. These coefficients change dramatically as the local cavitation number is reduced; I suspect that Mr W is not including this effect in his analysis, so let's have a look at the 90 degree and 270 degree positions with the 13" propeller.

I use profile data from a crescent shape profile (no S710) that has been used for both industrial pumps and waterjets. As we are only interested in the relative values for comparison, we may lump the water density and the infinitesimal area dA into a common factor K. With a NPSH of 10.8 m we get a local cavitation number on stbd blade of 0.134, compared to 0.193 on the rising blade, all referring to the 0.7*radius.

The rising blade, with angle of attack 4.7 deg will then have: Cl=0.21, Cd=0.03 and a total force coefficient Cf=0.212. the descending blade will have Cl=-0.05, Cd=0.03 and Cf=-0.058.

With these figures, the force on the descending blade is -0.058*39.72^2*K. That equals -91.99*K, acting rearwards and directed down 3.8 degrees; the vertical force just a fraction.

The force on the rising blade on the port side has the value 0.212*33.13^2*K = 232.7*K, acting forwards and down with the angle 17.03 degrees. The vertical composant is here 232,7*K*sin(17.03) =68.2*K, directed downwards. This equals 31% of the horizontal thrust from this area strip!

With profiles, less suitable for a cavitating environment, the degradation of profile performance is even worse than in this example, resulting in erratic propeller behaviour, often occurring suddenly and without warning.

Without cavitation, there may well be a lifting force from this propeller, depending on the balance of lift and drag, even if the main thrust is still generated by the port side blade.

Yellowjacket: The streamlines into the prop are more or less directed by, and following the hull bottom, so the influence of trim is not significant. As for the original question about rake: it cannot change things in this case. It is the blade profile that is important, it should be designed with cavitation in mind!

[Guest625101138]

Nope, the blade force here is roughly a product of velocity x angle of attack. If the aoa > zero (as for the downgoing 13" blade; 0,55 degree), there is no lift no matter the relative inflow velocity, just drag. The change in momentum is small. In fact, if induced inflow velocity is taken into consideration, the angle of attack at the 90 degree position is negative. A flat face profile has often a zero-lift aoa of ~ -1 to -2 degrees, so the downmoving blade on the 13"x13" prop is only producing drag.

........

baeckmo and others following
It may help you if you read through this whole link. It explains p-factor and some of the misunderstandings around it.

http://www.qmfc.org/school/asym.htm

You have reached the right conclusion regarding cavitation but do not understand how the blade forces are generated.

Pushing propellers are self-stabilising. There are strong forces that are trying to align the prop with the flow. When the prop is angled downward the forces are pushing it upwards to get it aligned. It is possible to operate a pushing prop without a shaft strut. It locks itself in the flow as if supported by a rigid support. Holding a prop at an angle to flow induces large forces in the shaft and strut.

Rick W

[Yellowjacket]

Beckmo..

Thanks for the clairfication on relative prop flows...

How far away does the prop have to be from the hull for the flow field not to be considered to be flowing along the lines of the hull? For instance, cleary up where the tip is in close proximity to the hull, the flow field will be parallel to the hull surface. As you move down toward the bottom of the prop the flow field will slowly straighten out. There are also probably induction effects where the flow is being pulled toward the prop at work here too. Lots of things going on down there...

[Guest625101138]

...
How far away does the prop have to be from the hull for the flow field not to be considered to be flowing along the lines of the hull? For instance, cleary up where the tip is in close proximity to the hull, the flow field will be parallel to the hull surface. As you move down toward the bottom of the prop the flow field will slowly straighten out. There are also probably induction effects where the flow is being pulled toward the prop at work here too. Lots of things going on down there...

YellowJacket
The flow field is a complex picture. It will be slower relative to the prop near the hull. This means that the bottom of the prop will be producing more thrust than the top.

Depending on the blade shape and how much of the blades are in cavitation through their rotation, the thrust and moment produced by the prop could be resolved by a single force acting lower than the bottom of the prop, further to starboard than the blade tip and angle upwards by as much as 40 degrees.

The smaller prop has more blade in cavitation and this limits the variation in force on the prop disc due to the complex flow field. There is also less variation in velocity over the prop disc from proximity to the hull because clearance is greater. The smaller diameter also results in smaller moment arms.

A larger diameter prop should work OK if the blade area was reduced. This will increase the required operating pressure and more blade will cavitate.

With this boat and set up the prop cavitation is helping with boat stability.

Rick W

[Ad Hoc]

From RickW
"...If you do the analysis correctly.."

Hmmmmm, as always, you seem so 100% convinced of this, but when asked to show your numbers, in the same way you ask everyone else ad nauseam your reply is:
"I have not shown you my analysis..."

Why is this, why don't you show, so we can all see...

It is very easy to follow beackmo's very well explained reasoning and analysis, as he does this more times in one day in his real job than i suspect you ever do in reality. So, where is your analysis then, other than just words copied from websites?

And since you love to "show and tell"...once you have posted your calculations and analysis, for the record, just so everyone is clear, how many high-speed monohulls with single prop propulsions have you designed?

[Ad Hoc]

yellowjecket

Sorry you're getting the usual flip-flopping from Rick...on one post the flow is simple and easy to understand, in a patronising manner and very dismissive, but suddenly now it is all very complex. Well, that is the MO of a non-naval architect. Anything to twist away from answering the real question....

In most normal cases anything that is 20% diameter from tip to hull is considered acceptable. Some Class societies allow down to 17%, but anything below 15% is considered unacceptable, period. However i do know of a patrol boat that has 12% clearance, it performs ok..but is not exceptional! It has the associated vibration problems etc...

[Sindel]

Interesting discussion here bring up cavitation...

In the 30's and 40's it was commonplace to do.
Miss England II made by the british had 2 "secret modified" rolls royce aircraft engines that turned its props like 12,000 rpm...

So decreasing the area of the blades (say 13" to 12.5") will create more cavitation and perhaps a better trim alone or is the negative rake creating the additional outward force of water bouncing off the bottom of the boat a necessary component?

And say I was to spend the money to do a prop modification, how close would it be to make the newest 13 x 13 into a 12.5 x 13 in comparison to a 12 x 14 prop?

Ya, I know it's a Bronze prop, but lots of people refer to them as brass because they are so soft...
I thought I said that I live on a river full of debris:
http://woodie.ppworks.com/Shaft-RIP.jpg
...and with that; shaft material and general drive train strength are another issue I should address, but that's another discussion...

The shaft angle is supposed to be 12* according to the drawings I have.

The props:
12 x 14
http://woodie.ppworks.com/12x14.jpg

and the 13 x 11
http://woodie.ppworks.com/13x13.jpg

[Guest625101138]

Interesting discussion here bring up cavitation...


So decreasing the area of the blades (say 13" to 12.5") will create more cavitation and perhaps a better trim alone or is the negative rake creating the additional outward force of water bouncing off the bottom of the boat a necessary component?

And say I was to spend the money to do a prop modification, how close would it be to make the newest 13 x 13 into a 12.5 x 13 in comparison to a 12 x 14 prop?

......

Sindel
Under cavitating conditions the force on the front side of the blade is reduced. The back face has to do a higher proportion of the work.

The factors that contribute to cavitation are the velocity of the blade through the water, the area of the blades and the shape of the blade section. A flatter section will have less tendency to cavitate than a thicker section at the low angles of attack that the blades normally operate at.

You will get increased cavitation by reducing blade area through reducing diameter. Whether this achieves the same result as you get with the 12 x 14 prop depends on the blade section and the chord length of the blades.

The larger diameter prop seems to have the blades skewed more as well. This can affect cavitation.

You could spend a fortune on analysis to try to determine what is going on with some precision in the complex flow field around your prop to get better understanding. However the most economic way is to modify and test. You already have a prop that satisfies your requirements. The easiest way is to try to copy it as close as you can. Taking the 13" down to 12.5" will be a good start. Also compare blade profiles and blade plan forms. A single section at the 70% radial position will be useful for comparison.

Rick W

[Guest625101138]

If you are able to measure the blade sections I am able to provide values for the pressure coefficients that will allow comparisons of the blade velocity that promote cavitation.

Also a propeller of modern design is likely to be better profiled to prevent cavitation, which is not what you want.

Rick W

[baeckmo]

Ad Hoc, thanks for your kind remarks! In your #45 thread, you intuitively note that the magnitude of the vertical force fom the prop may not be enough to create the behaviour. I think that's a correct observation. Due to the asymmetry with this propeller, we have a high speed, low pressure jet blowing almost parallell to the bottom. This, we know, has the effect of reducing the surface pressure over a significant portion of the bottom, resulting in a noticeable trim change!

As for this Willoughby character, I may have overestimated his qualifications. Now that he has revealed that he is not even familiar with basic hydrofoil theory, I believe you are right; his nonsense is best left unattended, hoping that people don't loose orientation in his endless fog!

[Ad Hoc]

baeckmo
I'm always very interested in your more detailed hydrodynamic assessment/analysis than I "usually" do when i design (don't have the time); i just check the basics and the "what if" scenarios, and focus on the remaining design of the vessel. Your expert diagnostic skills are very clear and much appreciated. It is easy to spot the professionals on this site.

As you've already notice Rick is not one, doesn't have the education, diagnostic skills, comprehension nor experience of real design save for his computer playstation and 5m peddle boat. But sadly those that come on here who need help get lost in his fog. Very sad, as you noted, more ego than help..

Pity really, his practical knowledge on small 5m peddle power boats and what wire needs to go where electrically, could be of good use to many, but he can't help himself. As you noted, the ego doesn't allow for the "oh hang on, I've never designed a real boat before perhaps i should listen and learn from professionals who do this for a living"...oh well. I know it stops many from wishing to contribute where their professional advice would be very useful, since I've had many emails from others stating as such...very sad!

He still hasn't said how many real full-on high speed prop boats he has designed....i'm not holding my breath!

PS..i have my stopwatch on now...seeing how long it takes for the very predictable and childish neg. points!...ergo, not a professional!

[apex1]

Ya, I know it's a Bronze prop, but lots of people refer to them as brass because they are so soft...


Just as a aside Bronce is´nt soft!!! And it has nearly the strength of steel! If yours are "soft" they are probably "Marine Bronce" which is just brass!
Richard


And I like to say, that I fully concur on Baeckmo´s and Ad Hoc´s statements about Mr. Willoughby and his (limited) knowledge! It is a shame to see his endless fights against professional knowledge. He knows quite much, but does not know his limits, neither his softwares limits.

Richard

[Frosty]

Sindel ,--the damage on those propellers shown are not from driving over river debris but from reversing back over something like a rock.

The damage is on the trailing edge.

No propeller of marine brass or stainless steel will protect you from careless driving.

By the way that is so easy to repair.

[baeckmo]

One thing Sindel, before we dig into the details on propeller: You are running direct drive 1:1, and have a right-hand screw, how come? Is the engine really rotating this way, or is there a V-gear or something along the line? Or have I missed this info somewhere?

And: are 3200 rpm the correct speed for max power for this engine?

[Frosty]

I asked this question too. if you look at the picture the engine it is far aft meaning there must be a V drive.

The shaft looks like 15 degrees meaning the prop is trying to push the boat in the air meaning trim will be sensative to weight distribution.

[Ad Hoc]

Exactly....hence post #19

[Sindel]

Hold on...

I didn't realize we had smart___ rocket-scientist here?

http://woodie.ppworks.com/12x14-b.jpg

Damaged caused me to lose about 1.5mph and probably contributed to shaft braking later with 13 x 11 prop...

Cause of damage above...
Set screw points sheared off... (possibly caused by bad 12 x 14 prop shaking?)
http://woodie.ppworks.com/SetScrews.jpg

Upon arriving at the damn to let the dogs out, the shaft popped out and the prop hitting the rudder caused the trailing edge damage...

And yes, it's a very easy fix... Just cost about $100 (I wish santa or one of his 'friends' would give me the money...)

The 13 x 11 prop is damaged too...
http://woodie.ppworks.com/Damage13.jpg

That's another $70...

http://woodie.ppworks.com/BrokenShaft.jpg

And look... The shaft is broken, another $200 (aqualloy)
That's close enough to call $300, the repair shop is 80 miles away...

When can I expect my new friend to send me some money?

Okay now that I got that out of the way...

The drive (1:1)
http://woodie.ppworks.com/Drive.jpg

The engine turns the same direction as the prop, it was just designed that way...
http://woodie.ppworks.com/albums/cruising/DSCF0460.sized.jpg

And yes that IS a lot of motor...
Contributes to 1700 lbs of the boat's weight...
And yes the "book" says 3200 peak rpm...

Now before weight distribution is mentioned, might I suggest examining the "Miss America X"
It held the water speed record for nearly 5 years going 124.86 mph
The old boats like mine were designed based on winning race boat designs, before computers and all the math...

http://woodie.ppworks.com/MissAmericaX.jpg

http://woodie.ppworks.com/MissAmericaX-2.jpg

How is that for weight distribution?
Can they move those engines any further back?

I also might add...
Years ago, I went to a lake with a nice marina and was able to test drive several of the props they had...
Tried 12 x 16 first... (nose high trim...)
Next tried 13 x 12... (nose high trim...)
Even tried 12 x 14 new out of the michigan wheel box (nose high trim...)

The rake is what I have always thought was the key, but everyone says that it would make the prop not work as well...

I don't really care if the prop is working at its best...
The one I have now (12 x 14 with its unoptimized rake) allows me to go out for 2 to 3 hours pulling someone on a tube or kneeboard and use less than 10 gallons of fuel...
It works go enough for me...

I just want a spare like the one I've already got...

[baeckmo]

Nice piece of machinery; certainly deserves some care! Now, one problem with installations from this era is that the engine is mounted stiff to the boat and propshaft is flanged stiff to the transmission. On one of your photos we can see a shaft broken within the coupling hub. The fracture is a typical fatigue breakage, caused by the combination set screw dents plus key recess. A few more blows to your prop, and the shaft bracket may be at risk as well!

Thus I would strongly recommend that you install a flexible shaft coupling, rubber "donut type" (Centaflex, Bullflex......). It is a deviation from the original, but a reasonable compromize to save your beauty. It may require sliding the engine slightly forward on its girders, but it will save you a lot.

First, they come with a clamping hub for the flange, so there are no dents or key recesses to initiate a shaft fracture. Second, the rubber "donut" will take much of the energy when your prop hits the unavoidable log, saving both propeller, shaft and gearcase. And third; the costs you have had for propeller and shaft repairs are a fart in the wind, compared to the costs for repairing a mashed gear, wich is taking all the punishment at the engine side of the propline.

So, for propellers: There are several varieties on the market; a 12"x14" from one series is not equal to a 12x14 from another series. Take Michigan for example; if the 12x14 you tested was from their "M" series, it has a blade section shape that is totally unsuitable for the cavitating performance you need (but it is perfect in ordinary, lowspeed, submerged applications).

This difference in shape may boil down to a few mm:s deviation in blade camber; it is not possible to distinguish one from the other without measuring the section. It is therefore not possible to say other than that the 12x14 you now have, is probably of a high-speed type, most certainly with a cambered pressure side, as opposed to most of the others you have tested.

I suggest that you contact a propsupplier that is familiar with props for skiing; these boats often use direct drive. Modern engines often reqiure 13 or 14 inch diameters, but there are 12:ers around. For similar operation we have good experience from "Mikado propellers", type E, which is a 3-bladed prop with area ratio 0.73.

On top of this, I would like to see the actual running trim angle of your boat (no rocket science,just a cheap inclinometer, mounted longitudinally, base parallell to keel aft), to get a better idea of what your "nose-up" actually means!

[Sindel]

Thanks for the compliment baeckmo...
Mines the little 18 footer you know?

Bent the "strut" 2 years ago and had to replace it...

In the picture of the engine from above; can you see it? (drivesaver coupling...)

Got 4 of those gearboxes behind the garage? Couple engines too...

So back to my original question, so to speak...
How can a prop be measured to determine its components?
And what do I need to measure? (point to point from where to where...)
Where are the various angles originate?
etc.

I'll see about getting a inclinometer, but I usually ride 1 hour before sunset to 1 hour after dark... (kind of hard to see)

[baeckmo]

Hah, so I can refer to you about spares then.......? The Drivesaver was noticed, but unfortunately it has far from the torsional flexibility you get with the rubber couplings I mentioned; there we are talking about several degrees of torsional flexing. The main virtue of your drivesaver is to allow a veeeery slight difference in shaft inclination (fraction of degree), plus a valuable resistance to Galvanic currents through the shaft.

I understand you have access to a lathe? If yes, then I will prepare a message to you on the measuring topic. I guess your driving habits are not exactly reducing your repair budget......, but would they really stop you from reading an instrument?

[mark775]

Damn. The dam where the dogs are dropped is spelled "dam". Hate to be a dissenter, you guys are having so much fun by yourselves ( "need a ruler?" LOL). Getting mighty technical here and you guys are way over my head with your forces generated, analyses, etc. but didn't the gentleman say that the boat doesn't work with a 13" diameter prop and does work with a 12" diameter prop? I would think about (not to start any trouble - maybe I missed something!) maybe using a 12" diameter prop (rake...What rake?).
Now, that is one ugly rudder but true to original, I'm sure (and good for shallow water). That is an uglier shaft coupling. This beautiful boat deserves (as Frosty somewhere pointed out) the repeatable, sustainable accuracy in alignment afforded by a matched tapered shaft/coupling. A split-coupling is in a distant second place. What you have there is an Obamanation (one of my new favorite words! don't need to start that here - just a joke). Take that coupling and throw it and its safety-wire away. Set screws have no application here. Through bolt and use a nylock, if you use a split-coupling. There is no need for keys on this set-up. Don't use them. However, the coupling and prop must be lapped to the shaft. Do I see never-seize(?) on the tapers you have - don't use that either. The cotter-pin is superfluous, as well. Stick with the bronze prop. I, personally, feel that any kind of drive-saver, etc. is just something to go wrong and not too clean. Simple is elegant. Orange plastic thing down there is anachronistic and ugly.

[Sindel]

baeckmo
Need parts? Just let me know what your looking for... (Chris Craft and Greymarine stuff)
I take that ugly drivesaver off for boat shows and never really noticed any damage, even after hits... I check the alignment of the engine to the shaft each time (2 to 3 time per month) Might the drivesaver be oversized for my conditions?
Also there's no room for it to fail... Isn't this another problem?
Yes, I do have a neighbor with a lathe... And Dad's got a mill (2 miles away)

mark775
Sorry about the dam...
Way to technical for me too, but interesting none the less...
Well, whatever is different between the prop I have/use and the one's I've tried (ie. a different 12 x 14 tried at Clear Lake, my 13" props, etc.), I'd really like to know...
Yes an ugly rudder, but true to origin...
That uglier coupling always get forgotten about... I know I should do something about that, but the prop shop called and I'll soon forget again... (winter project? hopefully)
Those safety wires...
Well, I put a tapered drive coupling on my father-in-law's boat and he got knocked at the next boat show he attended for it... We had to take that new coupling off, drill and tap 3/8" deep dummies for looks like they're there set screws and safety wire...
Don't use never-seize? Thanks, didn't know I shouldn't...
Don't need the cotter-pin? Really... Could someone else confirm?
The drivesaver... I have to take that stupid thing off for boat shows, but it gives me a chance to check alignment... Should I just not use it at all?