2 Blade props

[bsa]

Hi, being very new to wooden boat restoring i hope you guys can give me some advice on setting up our first project boat
we currently have( what i have been told) is a seacraft it is 15ft ply boat.

We are installing a 350 chev into it. Liking to keep most of the original parts on the boat it has a 7/8" 2 blade prop and 7/8"skeg, should i be changing the prop to a three or four blade and going to 1" as every one is telling me the 2 blade prop will be useless.
one other concern i have if i keep with the 7/8" gear the shaft will be 1" at the dog clutch then machined down to 7/8" at the log it that going to be a problem. I don't know to much on how these things can make a boat perform.

Any ideas, thoughts would be greatly apprieciated. i'm sorry on asking what is probley silly Q's.

[Doc]

Normally 7/8" shaft 6 Cyclinder.
1" shaft 8 Cyclinder.
1 1/8" shaft Big Block 8 Cyclinder.
My advice ring Peter Adcock who built seacraft boats , he is a life member of this club .

[bauchy]

hey mate,
stick with a one inche shaft, you can buy the parts to suit anywhere as just about every boat now is one inche shaft.
you wont need a 1 1/8th unless you are running a huge horse power chev. i personally wouldnt run a smaller shaft then one inche for the reason most boat shops are stocked with all one inche skeg bushs and log glands.

if you are chasing any log or skeg parts let me know as i bought a few new ones for me race boat which turned out to be the wrong angle etc so may be able to sell ya a new one cheap

brian

[ED174]

I`ve been experimenting with different props lately as I reckon a lot of this stuff is trial & error. Found out a few years back when mucking around with a scale model radio controlled nitro boat that the 2 blade blitzed all over the 3 blade prop. My 15 ft Apache came with a couple of three bladers. These I found to restrict the engines revs to a max of about 4000 rpm & that was it. Bought myself a 2 blade thinking its got to spin more freely & wouldnt u know it, the thing took off. Bit of cavitation off the start but top end is about 10 - 15 kms faster.
Then I decided to have one of the 3 blades worked, they suggested a bit more pitch & a balance. Got it back, threw it on, took it for a spin & wow could`nt believe it , about the same speed as the 2 blade.without any slip off the start & it now revs past 4. I guess if the 2 blade was a slightly larger diameter then it would probably beat the 3 blade.
So for skiing, definately a 3 blade & for top end speed that would be a 2 blade. Am I right or am I right! Somebody through me a frickin bone.
Ed.

[bauchy]

yeh agreed,
my bond wood wont rev past 5000rpm so im going to start playing with a few two blades. spent $5500 on the engine which should rev to 6000 easily so im feeling abit robbbed with out that 1000rpm

brian

[Bob Vic]

My experience is that prop selection is a black science. You can have all the theory in your head, but what works best is actually fitting the prop to your boat and trying it. I've tried a heap of props on my boats (and will keep trying ones that I think might still work better) and the differences are significant. Ultimately you need to get a prop that allows the engine to make optimum revs to achieve max power AND at the same time a prop the delivers good handling. ON Turbo Fire for instance I tried two 12x15 three blade props - both were a bit big and held engine rpm down, but one delivered quite nice handling, the other torqued to boat to port. The props were identical diameter & pitch but different blade shape.

Once you get a prop that works okay for you, get it run on a prop scan machine to ensure that all the blades are working in harmony.... it's worth the extra few bucks.

Bob