34' Tollycraft prop question.

bshillam · Oct 21, 2013

Currently I run 17x16 which Tollycraft installed installed my convertible. Max RPM is about 3600 at WOT. I thought this was low on a 5.7 350 CID 270HP Crusader, did some research and contacted the manufacturer. Found out the max RPM for this engine at WOT should be 4400.
Thoughts about pitch on the prop? I was thinking 17x14 would be in order? Is it best to take a more conservative approach and just do 17x15?

bshillam · Oct 21, 2013

Email receive from Crusader,
The spec back then was 235hp @ 4400 RPM’s Wide Open Throttle, for a standard 350 CID model CH270 from 1981 with a Quadrojet 4bbl carburetor. Cruising RPMs should be around 3300 RPMs. (75% of WOT)

C lectric · Oct 21, 2013

You can do a very rough estimate of pitch changes by using %.
Revs 3600/4400= 0.82
pitch 17" x 0.82= 13.94
The 14" rather than 15" would be the better bet.

However, before doing anything you should double check the tachs. They can be and likely are out and maybe by quite a bit unless good quality digitals. Even then double checking is a good idea.
Get a digital phototach, laser pointer type. They can be had from e-bay and others sources for between $50-$100. and take a run double checking what the phototach says vs what the dash tach says. Keep notes. Do it at several points, particularily at what you think you want to run at for cruise. Then run up to your current WOT and double check that 3600.

This info will be of use to you and the prop shop to better zero in on what you need.
You may not want to go for 14 in one shot but less than 15 but do the other checks first.

Al · Oct 21, 2013

greetings Bshellam

This site works well. I confirmed my current wheel with this site.

Vicprop - Propeller Calculator

From other information a increase of one inch diameter= 400 RPM drop and one inch of diameter = 2 inches of pitch. For what it is worth.

A.M.Johnson-Ketchikan
You're just jealous because the voices only talk to me

robbie · Oct 22, 2013

This site works well. I confirmed my current wheel with this site.

Vicprop - Propeller Calculator

Hey guys, I don't want to hog this thread,
I did a calculation with my boats specs using the link, It came up with a 4 blade prop size of 24.8 x 20.4.
I checked the survey report for my boat, as I have just bought it, it's fitted with a 4 blade 22 x 24.
So my question is: how close should I be in size and pitch before considering her over or under propped?.
I entered, for the required speed, 8 knots, the "speed & power" in the calculator gives a max speed of 12 knots.. the last owner stated she will make 11 knots at a push.
Any thoughts guys?.

Brooksie · Oct 22, 2013

robbie said:
This site works well. I confirmed my current wheel with this site.

Vicprop - Propeller Calculator

Hey guys, I don't want to hog this thread,
I did a calculation with my boats specs using the link, It came up with a 4 blade prop size of 24.8 x 20.4.
I checked the survey report for my boat, as I have just bought it, it's fitted with a 4 blade 22 x 24.
So my question is: how close should I be in size and pitch before considering her over or under propped?.
I entered, for the required speed, 8 knots, the "speed & power" in the calculator gives a max speed of 12 knots.. the last owner stated she will make 11 knots at a push.
Any thoughts guys?.

Maybe your boat can't swing a 24-25" prop due to clearance issues so diameter was reduced for optimum and pitch increased accordingly. Usually the larger the diameter the more efficient but you must maintain proper clearance. Does your boat have a Novi or a Northumberland hull.
As mentioned above, FIRST verify that your tach reads correctly then get your engine's specs, & your gear ratio.

robbie · Oct 22, 2013

Hi Brooksie,
She's a Northumberland, I have all the engine and GB ratio specs, so there's only the tacho calibration left to check.
I have no idea of the prop clearance, but you're right that could be the limiting factor, I do know there's a 3 blade prop kept as a spare, it was removed a few years ago, I hope they did the calculations for that..who knows?.

Baker · Oct 22, 2013

Prop calculators.....

Prop calculators are just a reference. Or they are a starting point if there is no info/history on the prop.

If you have history with the prop/engine/tranny set up, THAT is what will be of most value to your prop shop....the calculators are basically worthless unless that is all you have.

Al · Oct 23, 2013

Hummm, If one inch of diameter = 2 inch of pitch then a 22 inch wheel over a 20 inch wheel in diameter = 4 inches of pitch. Your pitch is 24 inch vs the 20 inch of the site results, Looks like you are equal pretty much. I'd have agreed with Brooksie on the clearance question. As I understand the equation it is somewhere around 3 inches optimum.
Having said that, the clearance is for cavitation allowance (I think). In high shaft RPM turning application the tips of the blades can be effected by some element that I can not readily identify or forgotten!. On wood boats, I recall a shipwright commenting on the need for clearance so as not suck the corking from the planking over the top of the wheel. All of this is good, though RPMs on our trawler applications would tend to be somewhat light compared to that of a large diameter commercial wheel lacking clearance. Just some thoughts and comments. Good thread.

Al Johnson-Ketchikan

robbie · Oct 23, 2013

Al said:
Hummm, If one inch of diameter = 2 inch of pitch then a 22 inch wheel over a 20 inch wheel in diameter = 4 inches of pitch. Your pitch is 24 inch vs the 20 inch of the site results, Looks like you are equal pretty much. I'd have agreed with Brooksie on the clearance question. As I understand the equation it is somewhere around 3 inches optimum.

Thanks Al, It makes sense the way you explain it..I think..of course I had basically arrived at the same solution just a few minutes ago.

cheers
Robbie

Baker · Oct 23, 2013

I seem to remember 10% being the general rule of thumb for clearance.....ie a 20 inch prop will need 2 inches of clearance.

34' Tollycraft prop question.

bshillam

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bshillam

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C lectric

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Al

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robbie

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Brooksie

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robbie

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Baker

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Al

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robbie

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Baker

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