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

[jigray3]

I have an 8.5' Apex Roll-up with an inflatable keel. I recently replaced the 5HP Mercury 2-stroke with a 2.5HP Suzuki 4-stroke because it's 15 lb lighter and quieter. The Apex/Merc combo would plane with just me on board, but it was pretty unstable, and we're perfectly happy at non-planning speeds knocking around Chesapeake gunkholes.


I haven't seen this question posed anywhere. WOT range on the Suzuki is 5250-5750 RPM, but we can only achieve 4700 RPM with the Apex/Suzuki combo. Is there any real advantage to changing the prop to be able to achieve WOT on a non-planning inflatable? Current prop is 7.5" diam x 5.5" pitch. If it is advantageous, what size and pitch makes sense? I'm not sure the 1" pitch change equals 150-200 RPM change rule of thumb applies to non-planning inflatables.

[Tin Tin]

I'd be surprised if there is a less than 5" pitch that would fit a Suzuki.

[Franziska]

We have an older 4hp Suzuki and put a lower pitch prop than the original on. Much better.

Btw. Check that the rubber propeller bushing did not detach from the metal hub. It's hard to see, but if it is detached you won't get the full power into the water at full throttle as the prop slips then. This is called a spun prop.

Either fix it with a screw through the prop housing into the hub, or get a new hub in this case.

[Greatest Lakes]

Suzuki designed the outboard with the assumption that it would need to be able to do 2 things. Run wide open without shutting off while being held stationary without damage. Run wide open with a minimal, light load with no damage. It's possible to optimize the prop for your boat with a specific load, but its overall change in performance might lead you back to the original prop after experimentation.

[Tedd]

Originally Posted by jigray3

Is there any real advantage to changing the prop to be able to achieve WOT on a non-planning inflatable?

The advantage to finer pitch is that your creep speed (speed at idle) will be slower. This seems to be something a lot of people don't think about when selecting prop pitch but, in a situation like yours, it might well be the most important consideration. I bought a sailboat last year with much finer prop pitch than my previous boat (of similar size) and I've found the slower creep speed much nicer when maneuvering in close quarters.

Small nit: You can achieve WOT now, you just can't get to full RPM. In terms of fuel efficiency that's actually a good thing because you'll have less throttle drag at all speeds. Think of it as cruising in overdrive. But fuel efficiency probably isn't so important and we're talking about quite a small difference.

[malbert73]

I depitched my Tohatsu 15 on my highfield CL290 which allows planing with more passengers/ weight, and a slow speed planing mode (8-9 knots)
This at the cost of 1-2 knots of top end speed
If you won’t plane, and like to putter around at displacement speed, I’d leave as is since you will get your usual speed at lower RPMs (quieter). But if it’s top end power you’re missing, you’ll get it by depitching

[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.

[rslifkin]

Some 4 stroke outboards are more prone to fuel dilution in the oil when overpropped. Not quite sure why it happens, but the extra load at a given RPM seems to cause it somehow.



In your case where you're not getting anywhere near full max RPM in use it likely won't make a big difference, but if it doesn't reach rated RPM at WOT with the current prop it'll certainly be a bit nicer to the engine to put a lower pitch prop on (and get the engine closer to its designed RPM vs load curve).

[Lee Jerry]

Originally Posted by jigray3

I have an 8.5' Apex Roll-up with an inflatable keel. I recently replaced the 5HP Mercury 2-stroke with a 2.5HP Suzuki 4-stroke because it's 15 lb lighter and quieter. The Apex/Merc combo would plane with just me on board, but it was pretty unstable, and we're perfectly happy at non-planning speeds knocking around Chesapeake gunkholes.


I haven't seen this question posed anywhere. WOT range on the Suzuki is 5250-5750 RPM, but we can only achieve 4700 RPM with the Apex/Suzuki combo. Is there any real advantage to changing the prop to be able to achieve WOT on a non-planning inflatable? Current prop is 7.5" diam x 5.5" pitch. If it is advantageous, what size and pitch makes sense? I'm not sure the 1" pitch change equals 150-200 RPM change rule of thumb applies to non-planning inflatables.

Let me re-phrase your question:
Is there any real advantage to changing the prop to be able to achieve rated RPM at WOT on a non-planing sailboat?

Hopefully you can see that the obvious answer is "yes," and whether the hull is on plane or not, and whether it is an inflatable or something else doesn't really matter. You do this for the health of the engine and to maximize the performance of the prop/engine system across the widest range of conditions.

Note that for any given throttle setting, you will not necessarily be giving up performance (speed) with a lower pitch and may actually improve it since the prop will (probably) be spinning faster.

[malbert73]

The answer is only yes if you want to run at WOT to extract all the HP. But any any RPM lower than WOT which is where engine output matches prop output, your engine by definition is under loaded. So, if you find 1/2 throttle is your sweet spot for dinghying around, why bother

[rslifkin]

Originally Posted by malbert73

The answer is only yes if you want to run at WOT to extract all the HP. But any any RPM lower than WOT which is where engine output matches prop output, your engine by definition is under loaded. So, if you find 1/2 throttle is your sweet spot for dinghying around, why bother

At lower RPM it's running below its max output, but not necessarily below the prop curve the manufacturer expected for continuous operation. The extra power available at those RPMs was likely expected to be used only for short bursts (like during acceleration).

[Lee Jerry]

Originally Posted by malbert73

The answer is only yes if you want to run at WOT to extract all the HP. But any any RPM lower than WOT which is where engine output matches prop output, your engine by definition is under loaded. So, if you find 1/2 throttle is your sweet spot for dinghying around, why bother

Sorry, but I agree with nothing in this entire paragraph.

Rather than repeating the same stuff over and over (and ignoring the part where the OP already went from 5hp to 2.5hp) I'll just ask: why do engine manufacturers give the RPM range at WOT in the first place (if you can just ignore it)?

[Franziska]

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.

We could not get up on a plane with our hard dinghy before. Now one person plus fueltank, a dinghy anchor&chain and some groceries can.

Before we turned throttle about 1/3 any further and the engine sounded chocked and got louder but the boat would not accelerate anymore.

These small engines don't use a very large amount of fuel anyway, so it's not so much worse to consume a bit more while getting over the hump.

Normally we go slow, but sometimes it's handy to plane if distances are a tad longer.

[Tedd]

Originally Posted by rslifkin

The extra power available at those RPMs was likely expected to be used only for short bursts (like during acceleration).

I think you'll find that the operating range for OP's engine is 5250 - 5750 RPM, which means Suzuki fully expected the engine to be cruising in that RPM range.


Edit


I thought it might be helpful to put this into perspective because there's a subtlety here that a lot of people miss. The Suzuki DF2.5 only has a stroke length of 1.5 inches. That means that at 5500 RPM it has the same piston speed as a GM 350 engine (3.48 inch stroke) has at 2370 RPM. It also means that it has the same piston acceleration (and, therefore, for all practical purposes, the same relative load on the connecting rods and crankshaft) as a GM 350 at 3610 RPM. The relationship is similar for valve train components.



It's easy to get into the habit of thinking that 5500 RPM is turning over pretty fast but, for a small engine, it's not, it's just loafing along.

[rslifkin]

Originally Posted by Tedd

I think you'll find that the operating range for OP's engine is 5250 - 5750 RPM, which means Suzuki fully expected the engine to be cruising in that RPM range.


Edit


I thought it might be helpful to put this into perspective because there's a subtlety here that a lot of people miss. The Suzuki DF2.5 only has a stroke length of 1.5 inches. That means that at 5500 RPM it has the same piston speed as a GM 350 engine (3.48 inch stroke) has at 2370 RPM. It also means that it has the same piston acceleration (and, therefore, for all practical purposes, the same relative load on the connecting rods and crankshaft) as a GM 350 at 3610 RPM. The relationship is similar for valve train components.



It's easy to get into the habit of thinking that 5500 RPM is turning over pretty fast but, for a small engine, it's not, it's just loafing along.

That's all correct. But my point was related to running at less than full throttle. Prop load is an exponential curve. So if they expected the engine will reach 5500 at WOT (at which point it's at 100% load for the current RPM), there's a curve expected below that point. If it makes 2.5 hp at 5500, it might still make 2hp at 4000 RPM. But due to prop loading, they wouldn't expect it to actually be loaded to 2hp continuously at 4000 RPM. The expected continuous output at that RPM might only be 1.5hp. If it's overpropped, you'd be asking for closer to the max output at those lower RPM, which has the potential to cause durability problems.