1964 Speeditwin 28hp / WOT RPM - Prop sizing

[oldboat1]

More drag but more thrust with a larger diameter. Speed props have a smaller diameter.

You need an old time shop that has load of used props --can usually buy/try and return.

 

[racerone]

Don't buy a prop for your motor through this website.-----The listing for 40 HP 1960 to 1976 shows totally wrong prop !!!----40 uses same prop as the 28 HP.

 

[kdiddle]

Yeah it looks like a Michigan Match 9x10 from Tacoma is in my near future. I wanna err on the side of slower rather than faster on the pitch, as its already goes plenty fast enough. Moreso just concerned about over-revving my new engine...

Thanks for all the input everyone! This site is simply amazing...

 

[racerone]

The prop that shows up ( picture ) on this website is a through hub exhaust.----Not a pin drive either.-----There is no way you should be putting a 9" diameter prop on this motor.-----Hurry find a boat shop for some old time factual help.

 

[kdiddle]

Racerone - not sure that I have a choice...every site I look at for pin drive props of this size only come in 9" diameter size...

 

[F_R]

kdiddle, those props will NOT fit your motor!!!

 

[F_R]

Supposedly, these are options for your motor, although they seem to be showing a generic picture.

Propco Marine Propellers | Propcopropellers

 

[racerone]

kdiddle------Please be more thorough in your research.-----The 28 in that 15-28 is a totally different motor than the 28 HP that you have.----Hurry and visit a boat shop for some help.----The internet searches are failing you !

 

[kdiddle]

@F_R and @racerone - thanks for the reality check. I was apparently looking at the wrong category from when I purchased the replacement prop...this entire thread has been misdirected - I pulled the existing prop off and looked at the part # stamped on it and it turns out its a 10 1/2 x 12 (not sure if I can post this, but here is the link to the prop I'm currently running - https://www.tacomapropeller.com/product/michigan-match-10-1-2-x-12-3-blade-aluminum-propeller/).

Since that seems to be the case, I guess I'm back to believing that my WOT rpm is indeed 4200. But it appears the only other down pitched prop I can find is still 10 3/8 x 11 1/2 and I can't imagine that the 1/2" decrease in pitch is going to do much to get my rpm much higher...

Off to play with an online prop calculator to see if I can confirm the #'s (speed/rpm/pitch)...

 

[kdiddle]

@racerone since you said earlier it should be running 30mph with an 11in pitch...I'm guessing since I'm running 12in pitch and its only turning 4200 rpm (supposedly) and 22mph (via GPS), then can I assume that the boat is just too heavily loaded for the prop I have (with no real alternatives to get a shorter pitch that I can find online)...

 

[F_R]

Reviewing this whole matter, I'm going to give you an opinion. My opinion is that your 10-1/2 x12 is ok for your boat. Full throttle operating range for your motor is 4000-5000 RPM. So you are within the ballpark, albeit at the lower end of the ballpark. You also say you are not interested so much about top speed and find that it is almost too fast now, and you don't even normally run it at wide open throttle. With all that in mind why increase revs and fuel consumption? Overstressing the motor is not an issue if you aren't even asking it for maximum effort.

Just my opinion. Others may disagree.

 

[kdiddle]

Thanks for your input/advice @F_R . That's the same conclusion I've come to, that there isn't much to be gained by changing my prop. I def have no intent on winning any races in this shoebox of a boat (I rarely even fish where I know there will be much boat traffic at all)...I just wanted to ensure I was running the appropriate prop for my situation. And since it seems I am, I suppose I'll leave well enough alone...