Dinghy Propeller Pitch - Who's experimented?

[Cheechako]

Originally Posted by Cowpoos

I am looking to get a second prop for my dinghy for higher loads. I haven't been able to find a decent online tool to help with the decision, so I was curious what other people's experiences are.

I have a 2.9m (9.5 ft) Aluminium RIB - Approximately 380 - 400kgs (830 - 880 pounds) all up with a heavy load - 15hp Suzuki Two Stroke outboard. I currently have a 9 1/4 x 10 3-blade prop, which easily gets on the plane with two adults with a total boat weight of 270kgs (595 Pounds).

I am thinking of going to a 4 blade prop, but I can't decide which pitch. So I am after actual experience with a similar set-up in the real world.

With a 10ft RIB and a Yamaha 2 stroke 15, I found no perceivable difference in two widely varied prop pitches. Both were Yamaha aluminum props. Only thing I can figure is with 15 hp on a 10 ft light boat, pitch didn't matter much, it has plenty of power anyway.

The other side is there are likely limited prop choices for a small OB. (?) Pick one with a flatter pitch and give it a try.

[pcmm]

My Highfield 290UL with a yamaha 9.9, I switched to a 4 blade with slightly less pitch that the OEM prop. the 4 blade has more "push' but it costs me slightly in top speed. I wanted the extra push in order to be more effective when towing (like hip towing) the main boat or helping others

[MicHughV]

I've read Gerr's book inside out....and twice over for that matter..an excellent read for those that are technically inclined..

As previously posted, if the engine doesn't reach it's rated rpm, it also won't achieve it's rated horsepower. A difference of 500 rpm on the top end can be the difference between getting 10 hp or 15 hp.

To that end, my old 15 hp could do most everything I wanted with just a tiny turn on throttle as it had power to spare and WOT was never needed.

My 8 hp, on the other hand, needs WOT to get moving...

[nfbr]

OC tender 350 with 20HP Suzuki EFI 4Str

Had everything on it with three different motors from 13" to 8"

The Suzuki has 10" stock. Currently running 8" for family of 5.

I have a 4 blade I tested, but it won't plane. I haven't played with it yet to figure out why. Seems to ventilate.

Lower pitch is like lower gear, better in rough weather, waves, heavy loads, but less top speed and less fuel economy.

[rslifkin]

Originally Posted by nfbr

I have a 4 blade I tested, but it won't plane. I haven't played with it yet to figure out why. Seems to ventilate.

For engines that have manually adjustable trim where you have to set it before departure, different props may want a different trim setting. And if you've got any wiggle room for mounting height of the outboard, the optimal height will be different depending on the prop as well. And if the props have different blade shape, sometimes one will be a better match than another.

Prop selection (especially on something like a dinghy where load varies widely) is only partially a science, it's also a bit of an art. So it can take some experimenting to find what works best.

[Nicholson58]

We have a 10 foot aluminum double floor RIB with mercury 2-stroke 15 Hp. The lower end has a large whale tail. Similar loading as the OP described. The standard prop is too steep and fails to plane when loaded. I switched to a Solas SS three blade with 1 inch shallower pitch. The Solas blades are also larger surface area. SS blades are thinner, more efficient and durable.

Keep a spare prop for when you hit something and spin the hub.

[OSBNZ]

I have 15hp Yamaha on 3.2m West Marina quite heavy RIB. I dropped down 2" of pitch and it is a great improvement all round. So much easier to pop onto plane whatever the load. Can't believe I didn't do it years ago!


I used these guys for advice and supply. Very helpful, and great service. Highly recommended!

https://www.marinezone.co.nz

[Jetx]

It’s definitely worth experimenting! We have an AB Lammina 10AL dink (10.5 ft, double floor aluminum) with Tohatsu 18hp 2-stroke with hydrofoil fins.

With the stock Tohatsu 9.25” x 9” prop, we could easily plane with 3 adults, and 4 adults with a little weight shifting. So plenty of power overall. It was almost scary fast wide-open with only one or two aboard. But it absolutely sucked at below-planing speeds. The outboard would bog down as soon as it fell off plane, and just generally struggled at mid-rpm.

After banging up the original prop, I replaced it with a solas 3-blade 9.25 x 8”. Only one step down in pitch, but this made a huge difference in usability. We get on plane a little easier now, including with 4 adults, although that wasn’t really an objective. What’s really changed is at mid-rpm. I can now stay on plane at much lower speeds than before, and can also run at moderate speed while keeping the bow up without jumping on plane, which wasn’t possible with the stock prop. Both of these things, but especially the latter, make a huge difference in comfort and dryness when dinghying upwind in heavy chop.

Before the prop change, I was honestly fed up with the dinghy because there were too many times we either got soaked or got pounded, or both. I thought the hull design was just flawed… too heavy and too much drag from the deep v bow section. But going down just 1” in prop pitch has completely changed the dink. Before, it was either dead-slow and wet, or haul-ass just skimming the water. Now I have some in-between modes that I never had before, and we’re comfortable much more of the time. I’m sure I’ve given up a few MPH at the top end when we’re 2-up in dinghy in smooth water, but that’s a trade off I’ll happily make any time.