How to select prop

[spamn]

I just purchased my first boat and the prop that came on it is in pretty bad shape. Having it machined is out of the question.

The engine is a Mercury 402 40hp and I am unsure of the year. The previous owner told me that it was a 1986. I am unsure of where to locate the serial number plate to verify.

I took the old prop off and found a number stamped into it. Unfortunately, I am at work and cannot remember it. I will post it when I get home. The last part was "10p", so I'm assuming that is the pitch. I measured the diameter by measuring from one blad to the center and then doubling it and came up with ROUGHLY 11. I would like to stick with an aluminum prop and have no idea whether to go with 3 or 4 blades. Any information on blade selection?

Lastly, I know that you can change diameter and pitch minimally to gain/lose certain characteristics of the engines preformance and fuel consumption. Any elaboration on this would be highly appreciated. Also, if anyone can tell me of a great place online to order the hub and prop for a low price, that would be great as well. I spent all of my money on the boat and do not want to spend $200-$300 on a prop. I saw some at my local Gander Mountain sporting good store for a great price but did not know my diameter or pitch at the time nor anything about props.

Thanks
Sam

 

[steelespike]

Re: How to select prop

Iboats has a prop store and lots of prop deals.My info indicates 402 is 72-78. max rpm is 5,500.Prop selection is made to get the motor to operate close to the max rpm at wot (wide open throttle) Wer usuall;y like to have your present prop size and the max rpm and speed but lacking that start with your present pitch.Don't worry about diameter it is a function of the design and a secondary consideration.Before your final selection tell us the details of your boat. length,width,weight.brand/model
etc.typical load.
If you look up a 10" pitch 3 blade for your motor
It should get you close to your requirements.

 

[spamn]

Re: How to select prop

I appreciate the information. The boat is a 14.5 foot Tidecraft bass boat. I am unsure of the width. I found some props on the iboat store and according to them the 402 40hp was 1977-1984. I counted 13 splines on the shaft so the information sounds correct on the iboat store. They list 11-1/4, 12, and 11-7/8 as diameters that they sell with 10 pitch. The one I am considering (mostly due to price) is 4 blade. Can you elaborate on the difference in 3 and 4 blade props please?

Thanks again
Sam

 

[usskydvr]

Re: How to select prop

3 Blade - a little more speed 4 Blade - a little more power (provided you have an engine that has the power to turn it) My self, I wouldn't put a 4 blade on a 40 hp motor. If I needed more power I'd just drop my 3 blade down to a lower pitch. Also interesting in that 4 blade props usually cost more than 3 blades.