Any Benefit to Changing Prop Pitch for Non-planning Inflatable?

[Tedd]

Originally Posted by Tedd

...as a GM 350 at 3610 RPM.

Actually, that's wrong. I forgot to take scale effect into consideration. Component mass goes up with the cube of size whereas load bearing area goes up with the square of size. So, taking the ratio of stroke lengths as a proxy for engine component size, the ratio of material stresses between these two engines in, for example, the connecting rods would be 2.32. That means that the stress on the Suzuki connecting rod at 5500 RPM would be the same as the stress on the GM 350 connecting rod at 1560 RPM. Other component stresses would scale similarly.

All of this is to say that, for those of us who grew up framing engine RPM in the context of cars, a whole new framing is required for much smaller engines such as small outboard motors. Myself included. I'm beginning to realize that I've been running my Suzuki DF9.9 too slowly.

[Tedd]

Originally Posted by rslifkin

Prop load is an exponential curve...

Yeah, that's much clearer, thanks. I know it can be awkward to find the right balance between being accurate and being concise.

[jigray3]

Interesting though it may be, the question may be moot. Despite assurances, I have been unable to find a prop for this engine with a lower pitch, or at least low enough to get me anywhere near the recommended rpm range for WOT.


I understand a 1" change in pitch will result in a 150-200 change in rpm. That should mean reducing pitch from 5.5" to 2" might get me into or near the range. I'm not feeling like this rule of thumb holds up in this case.

[rslifkin]

Originally Posted by jigray3

Interesting though it may be, the question may be moot. Despite assurances, I have been unable to find a prop for this engine with a lower pitch, or at least low enough to get me anywhere near the recommended rpm range for WOT.


I understand a 1" change in pitch will result in a 150-200 change in rpm. That should mean reducing pitch from 5.5" to 2" might get me into or near the range. I'm not feeling like this rule of thumb holds up in this case.

On small outboards, especially if you're significantly overpropped, you may see a much bigger jump in RPM. On the 6hp Tohatsu mounted to our dinghy, a 7" pitch gives about 500 RPM more than an 8" pitch with the same load.

[Lee Jerry]

Originally Posted by jigray3

Interesting though it may be, the question may be moot. Despite assurances, I have been unable to find a prop for this engine with a lower pitch, or at least low enough to get me anywhere near the recommended rpm range for WOT.


I understand a 1" change in pitch will result in a 150-200 change in rpm. That should mean reducing pitch from 5.5" to 2" might get me into or near the range. I'm not feeling like this rule of thumb holds up in this case.

I agree with rslifkin, you won't need to go nearly that far. A reduction of 1" in pitch, or maybe 1.5" if available, should get you pretty close to where you want to be. Heck, you may even try 0.5" reduction. I think most of us are used to higher power engines when thinking about these things, and I'm not sure how well they translate to the smallest end of the spectrum. (For example, the RoT you're using.)

[jigray3]

Originally Posted by Lee Jerry

I agree with rslifkin, you won't need to go nearly that far. A reduction of 1" in pitch, or maybe 1.5" if available, should get you pretty close to where you want to be. Heck, you may even try 0.5" reduction. I think most of us are used to higher power engines when thinking about these things, and I'm not sure how well they translate to the smallest end of the spectrum. (For example, the RoT you're using.)

Thank you, gentlemen. Intuitively, the rule of thumb I was seeing didn't make sense for my application, and you confirmed it. Now, to find the solution. I'll dig harder to see if there's an off the shelf option, or perhaps I can find a shop that can make the adjustment without adjusting my wallet too much.

[George_SD]

Sounds odd.

We have an Avon Rover 310 with inflatable keel and floor + 3.5 4 stroke Mercury/Tohatsu with a carb upgraded to the 4-5hp version and an aluminum prop. It will plane with me and my teen son + fishing gear. What I've also looked at is the outboard hydrofoil wings that you can install. They may drop the top speed slightly, but they appear to help with getting on a plane early.

[jigray3]

What part sounds odd?


I have a 5 HP Merc and it will plane with me (230lbs), the 3.5 gal. fuel tank and some gear, but not with two of us (350lbs). It'll really go on plane at WOT with the 5, but, as I mentioned, it's pretty unstable at that speed due to the design.

The floor is wedged in place, held there by the inflated hotdog that runs under the center of the roll-up segmented interlocking PVC floor, and the inflated side tubes. I suspect if the transom and the floor connected structurally somehow, the rigidity and thus stability would improve. I'll have to consider that, but truthfully in the anchorages and gunkholes we currently favor, we don't find ourselves going very far, and the wake in most cases would be disruptive anyway.

We downsized to the 2.5 Suzuki because we didn't really experience benefit from the 5 and it's additional 15 lbs. If we had wanted a hoist for the outboard, we'd likely have stayed with the 5, but the pushpit is cleaner without one.

[srbaum]

Suzuki only list one prop for the 2.5 four stroke.https://www.boats.net/catalog/suzuki...0001/gear-case

[col50]

Originally Posted by jigray3

At this point, much over 3800 rpm's just makes more noise, drives the transom down, and nose up without any real additional speed. That attitude might be a little dryer in chop, but nothing else.


I'm told I can buy a lower pitch prop for $50, trying to determine if there's value in the move. Slower idle, or "creep speed" as Tedd calls it is something I hadn't considered or even noticed. I'll have to pay attention next time to see if reduced speed at idle could be beneficial.


Franziska, you state the lower pitch prop on your Suzuki is much better. How so?


I do like the reduced noise and lower frequency of a lower RPM cruising speed. Just don't want to put more stress on the motor than necessary.


The internal tank holds .24 gal and gets me about 1hr 15min of range. Fuel economy doesn't seem to be a concern. My 5hp Merc has an external tank, and I can modify the Suzuki to use it if I feel I really need the range and prefer external.

If more throttle lifts the bow and drives the transom down, you might want to tilt the top of the engine farther back. Let us know if that makes any difference.

[GordMay]

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