picking a prop light moderate to heavy load?

OddballNo3 · 05 February 2025, 14:57

I have a 12' Avon that I just bought a new tohatsu 30d for. I didn't realize it doesn't come with a prop. on the Tohatsu website they list propellers but list light moderate and heavy load, what exactly does that correlate to? the boat is a rigid, and we're looking to carry around 6-700lbs with us included. I'd like it to have a higher top end speed. how do I figure out what prop i need when I have none?

Pikey Dave · 05 February 2025, 15:04

I’d say 6-700lb in a 12’ boat falls well within the heavy category.

OddballNo3 · 05 February 2025, 15:18

so what's considered light? two Americans hit the 400 mark pretty easy, there has to be like a percentage chart or something. I'd like to go fast but we also want to load our lil boat with camp gear (within reason lol) also, comparing aftermarket props to tohatsu oe and they all seem to be a bigger diameter than what tohatsu's are why is that? and won't that bog down my rpms?

spartacus · 05 February 2025, 23:06

Page 95 of the manual.

Your rev range at WOT will be 5500-6500. See if you have a local dealer that would loan you a couple to test. Genuine Tohatsu are vey good quality.

Pikey Dave · 06 February 2025, 10:15

“Go fast AND 2 Americans + camping gear”
If you hang around boats long enough, you’ll soon learn that everything is a compromise. You need to manage your expectations from your outfit. Overpropping the engine (I.e. fitting a prop from the Light end of the scale, but loading the boat to the heavy end) will result in the engine struggling to get the boat planing & will stress the engine, possibly leading to premature failure.
You could always buy a couple of props & swap them over to suit your needs on the day, carrying a spare prop is always a good idea anyway.

Fenlander · 06 February 2025, 10:37

I know our SIB itself will be lighter than your hard hull but it's the same length at 12ft.

We've not used a 30hp, maximum was a 20hp and we've used multiple varying pitch props on 10-20hp motors to know how they behave.

Our usual load is almost 500lbs with us two, dog, larger fuel tank and day out kit etc. Sometimes adding our daughter put us nearer 600lbs.

With the 20hp the dealer wanted us to take it with a 12" pitch, I knew that would be too much but his only offering was to swap to an 11" which I reluctantly accepted. It was clearly over-pitched so I bought a 10" and that was perfect.

You will never know exactly how your outfit will perform until you try a prop and see the revs you are reaching, on a cheap mini tach if you don't have one as part of an instrument set. As Spartacus says ideal if the dealer or someone else will lend you a prop to establish a baseline. If not I'd tend to go towards a lower pitch as should that prove to be too low you just have to watch top speed but otherwise the outfit will feel responsive. If you go too high on pitch the outfit will feel bogged down and unsatisfactory at all speeds as PD says.

Aftermarket props are often great value at half OE price but be aware sometimes they do not run true to pitch.

OddballNo3 · 07 February 2025, 19:16

So I think I'll start with an 11 and see what we get in the spring. I don't have one of those mini tachs after seeing they're $15 I think ill get one, since they have hour meters, I think ill mount one to the boat and one for the toolbox. I still want to believe that there is a "formula" like weight / hp * "x" i mean it is physics, there should be something that gets you close, without needing an aquatic drag calculating fluid computing super computer lol

Pikey Dave · 08 February 2025, 09:39

Quote:

Originally Posted by OddballNo3

So I think I'll start with an 11 and see what we get in the spring. I don't have one of those mini tachs after seeing they're $15 I think ill get one, since they have hour meters, I think ill mount one to the boat and one for the toolbox. I still want to believe that there is a "formula" like weight / hp * "x" i mean it is physics, there should be something that gets you close, without needing an aquatic drag calculating fluid computing super computer lol

It is physics, but once you delve into it, there are so many variables, you get into Chaos Theory, that's where the super computer comes in

. As previously mentioned, you have to establish a base line/datum/origin, call it what you will, and work from there. You're going down the right road with the mini tach, without knowing your RPMs you're just guessing.

jeffstevens763@g · 08 February 2025, 22:09

My 4.35m sib me fuel,camping gear I’d say 400/500 pounds with a 25 hp Suzuki I run 10 1/4 x 10,11,12 inch pitch all perform with the 10 being more responsive in rough weather a sib has more drag that your solid hull and you have 5 hp more power hope that helps

Ps the 11 inch prop is supplied with the engine new