Prop Question

[Jackman]

How close can you get to the hull of your boat when sizing up props? I have 20x18's and still have 2.5 inches to the hull from the blade when its closest to my hull. I figure maybe 22" props would be my max?

[sammidog]

From Dave Gerr's book, "Propeller handbook". "Generally there should be a tip clearance of at least 15% of the overall propeller diameter between the blade tips and the hull. The ideal tip clearance is 20%or more..."

Jackman, see if you can find Gerr's book in the library. The math is a little rough but if you take your time you can get through it. I did and barely passed my second class of calculus! Great information. I can send you my copy if you like, but I want it back!

[Jackman]

Sammi,

I probably will take you up on that offer. Where are from? I know that I should probably look for the biggest prop my boat can handle and that will tell me what HP to look for, I'm assuming the 330B's are the optimum engines but I'd like to prove it, being a scientist and all.....

[sammidog]

I live in Erie, Pa. The book won't cost much to ship. 8X11 and 152 pages. Excellent reference.

[ADaily3224]

Jackman,

Your hull and mine are very much the same design, the later model 36 and your 32 both have tunnels for the props allowing for a much closer tip clearance. My propellers are 26x25 with a 3.125 inch or 12% tip clearance. It's OK to be a little under with a tunneled hull.

Tony D

[Boatsb]

From the old timers in the boatyards where I worked they looked at approx 2 x shaft diameter as optimal. 1.5 x shaft diameter as functional and any less as dangerous.

Doing the basic math I have 1 1/2 shafts refitted to 1 3/4 and about 2 1/2 -3" clearance I am ok with 23 x 24 3 blades

Remember the length of the shaft also makes a difference. Adding an inch or so to the length if you are still in tolerance with the distance behind the strut gains another 1/2" or so. You do not want to be too close to the hull or it becomes an issue as the water will not flow over and around the prop correctly.

Add a blade if you need because the diameter and the pitch and the # of blades all work together.

[Jackman]

I have 1.5" shafts A22. From what I've been hearing that is in the middle of the safety range for sizing, but please correct me if I am wrong. I figure that with the torque of the diesels I should be able to swing the biggest props my boat can swing, but with safety in mind. That's what I am trying to do...fit the shaft to the prop to gear to the engine...in no particular order, before I make more moves on this conversion.

[sammidog]

Again from Gerr's book: multiply 321000 x shp x sf/ St x rpm. Take the cube root of that number and you have your shaft dia.
shp= shaft hp
sf=safety factor (gerr recommends 3 for yachts and light commercial craft)
St= Yield strength in torsional shear, in psi. For aquamet 22 it is 70,000
RPM= revolutions per minute of the shaft.
You should be fine with 1.5 inch shafts if you know you have A22.

[jim rosenthal]

You can go up to 22" wheels, but not any bigger than that. That would still give you 1.5". That's 7 percent. A bit tight, but I think permissible. 21" with a larger disc area ratio and/or more pitch would be better, probably. The shaft size is fine with A22 shafts.

[Lucky Dog]

Gerr's Propeller Handbook is $13.57 from Amazon. A bargain for the amount of info. in it.

[34Hatt]

With 330B in your 32' I think you will land up needing something like a 21"x21" 4 blade this is also what I landed up with. The old rule also was to try and keep it square. I don't think there is a need to go any bigger dia. Dia gives you lift and pitch gives you the speed. The 21" dia gives my 34' plenty of lift which is heavy in the back end.
1.5" shafts are fine just do a good job Installing the props and make sure they are scanned. Tom Slane run 330b's with 1.75 to 1 reduction on a 1.5" shaft swinging a 22"x26" wheel.

[Jackman]

Is there a need for me to go 1.75 to 1? I spoke with a lot of people and looked at Tom's Got Bait repower and they used the 1.5 to 1, I believe. I'm just checking...Once they're in I don't want to have to take them out for a long time.

[sammidog]

Get Gerr's book. It will tell you what reduction to use. I think 1.5 would be fine. Although heavy for a 32 footer it really doesn't weigh that much.

[34Hatt]

Well 1.75 should be better higher ratio usually is. I stayed with 1.5 since 21" was the biggest wheel I could fit. Also I knew the 1.5 work well in a 34C and I knew 21"x21" worked had a 34C in Cape may to compare with. So it came down to being a known package verse unknown and It is more stress on 1.5" shaft you safety factor goes down more!

[Jackman]

The other thing was the HP factor. I could have gone with 370's but I think there comes a point at which you have to decide is too much too much? I think I'll be happy with the 330's, even though the 370's aer only 1k more each.

[REBrueckner]

Do not extend the shaft beyond the aft end of the strut such that the front of the propeller hub is more than 1/2 the shaft diameter or you'll run the risk of vibration....

[REBrueckner]

As the prop tip clearance from the hull bottom gets smaller, water turbulence increases causing noise, possible cavitation corrosion at the prop tips, and some loss of efficiency. Instead of increasing prop diameter to such an extent, you can try larger pitch or cupped props.

[Banshee36]

My series One 36c, 1.5:1 TD trans, 330b Cummins, 1.5 Shafts, 22x22.5 3 Bladed Austral props. 1370 Hours no problems. with a 1:5:1 gear in your boat you will be fine, but remember. When you put her in gear she goes.
JW

[the kuz]

My 1977 36C is set up just like Banshee's, except 22x23 3 blade Michigan Nibrals and 1.5 ZF trans. Next time out for spring cleaning, I will most likely take out .5 to 1.0 " of pitch since I now turn up right at 2800 WOT (600 hrs on engines) vice about 2850 I got during the sea trial when first installed. Before I do that though, I'm going to clean up everything below the waterline really well, and put her on a diet. She is loaded up much heavier than when we first got her back. Be sure you leave some room for 1000 lbs of stuff when you figure things. If I get a good weight this spring, I will post it. I'm guessing somewhere around 22K. If you PM me, I will mail you a copy of my sea trial data sheet. Regards, Bob K

[Maynard Rupp]

Let us not forget that the 32' Hat has tunnels so none of the discussion about extending shafts or the normal hull clearence applies. I am told that it is better to fill most of that tunnel with propeller. Our 36C has roughly 21/2" clearence around the prop tip to the tunnel. An earlier thread here said it is OK to decrease that number to 1". While the early 36C has trouble swinging 23" props, our later 36C,(same design as the 32'), uses 26" props. I would go larger dia. long before I would increase pitch. That statement only applies to boats with tunnels. They are a different animal. :cool: