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]

Originally Posted by 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

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

Adjusting Outboards To The Transom

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?