Prop sizing yanmar 3gm30f

[Mg451]

Other then using the prop calculator which have and I have the correct prop according to it. Is it safe to say I should run my engine wide open and see if I reach max rpm to decide if I have the right sized prop? The reason I ask is bc ib feel I'm not getting the speed I should and I'm propped at what prop calc calls for.
I have a downeaster 32 with a yanmar 3gm30f, 2.6 gear reduction, max rpm is 3400 ( I've never ran it wide open. I run it at 2400rpm and it just sounds right and happy at that rpm. Yanmar says run it at 2600(?) But it's screaming at that maybe my tachs off i don't know).
I can push the boat at 5knots in flat calm and slight foul tide or chop really slows me down to 4 or even 3 and i can hear cavitation..
My hull speed is 6.8 and I can't get any where near that in dead calm condition 5 or 5.5 if I'm lucky. Maybe she's just a slow boat or maybe I'm running the wrong prop?

[rslifkin]

The first step would be to get a cheap photo-tach and confirm that your tach is accurate (and calibrate it if needed). Then a WOT test is a good second step, as the calculators will get you close, but they aren't perfect. But a WOT test won't tell you anything useful if you don't have an accurate tach.

Looking up the specs for your 3GM30F, Yanmar considers 3400 to be "max continuous", but I think that's a bit optimistic. Looks like rated RPM is 3600 and it's probably governed a little higher than that. So I'd want to see 3600 or a little over at WOT. If you can't reach 3600 at WOT then I'd consider it overpropped. If you reach 3600+ before the throttle is fully advanced then you're underpropped.

What's your current prop? Both the diameter and pitch as well as number of blades and type (fixed, folding, feathering).

As far as cruise RPM, if it's propped correctly I wouldn't worry about cruising as fast as 3000 if needed. It won't hurt the engine, but depending on how well soundproofed your engine installation is or isn't, it may be unpleasantly noisy.

[Mg451]

I've got a 3blade 16rh11. I'm just so damn slow under power and the slightest thing foul sets me way back to 4 or 3knots. I know some one same boat diff motor ( he has a 30hp I have a 27hp) who was told a 17 11 he went with a 17 10 and motors 6knots.

That 3600 rpm sounds ungodly screaming high I'd assume I've had it up at 3k and she was screaming

[rslifkin]

Originally Posted by Mg451

I've got a 3blade 16rh11. I'm just so damn slow under power and the slightest thing foul sets me way back to 4 or 3knots. I know some one same boat diff motor ( he has a 30hp I have a 27hp) who was told a 17 11 he went with a 17 10 and motors 6knots.

That 3600 rpm sounds ungodly screaming high I'd assume I've had it up at 3k and she was screaming

With the minimal sound insulation many sailboats have around the engines, yes, those little diesels revved up high can be pretty obnoxious to listen to. But a bit of sound insulation helps, and knowing that it's "normal" and not about to come apart helps a bit too.

The boat slowing down excessively sounds like it might not have enough blade area on the prop and it's losing grip as load increases. Or the boat might be hobby horsing in waves which will kill speed.

What kind of boat is it? And is your prop one of the sailboat style skinny bladed 3 blades or does it have larger blades?

[Mg451]

It's a downeaster 32 (1700lb displacement and fully loaded for cruising)with a 3 blade michigan prop.

[SailingHarmonie]

Originally Posted by Mg451

I'm just so damn slow under power and the slightest thing foul sets me way back to 4 or 3knots.

The "slightest thing foul" is a big red flag for me here. ANY fouling on a prop has a HUGE impact on its efficiency. Different people have different ideas about what a "little" fouling might be. Any is just bad. Hull fouling is also a significant drag.

But let's assume you have a clean bottom and prop. Before you worry about the prop, you have to know if your engine is big enough. For a 17,000lb boat, with a LWL of 25.8 ft, I calculate the HP needed to run at 6.8 knots is almost 34 AT THE PROP. Your engine at WOT makes 27hp at the crankshaft, so maybe 24hp at the prop. No way you are going at hull speed--no matter what prop you have. But it is not as bad as you might think... Up near hull speed it takes a LOT of power to gain a few tenths of a knot.

Lets assume the 24hp at the prop and work the calculation backwards, and see what speed you should expect...

With 24hp you ought to be close to 6.0 kts at WOT in calm winds, flat water, with a clean bottom, a properly running engine, and a proper prop.

The traditional recommendation is that for a shiny clean bottom and prop, the engine should reach its WOT rated RPM, maybe just a tiny bit more for those times when you have a slimy bottom and have more drag than on test day. Or you have to punch into the wind. The boat speed really does not enter into this, it will be what it will be.

I am going to guess that if you are doing 5 kts in calm water at 2400RPM that's about what you should expect. The data sheets I have seen suggest that for this engine 2800 is the normal "cruising speed" and 3400 is "max continuous output".

There really is no test for the "right prop" other than a wide open throttle test. These engines are small, and high reving. Be sure your tach is correct and your prop and bottom are clean.

It will be noisy. You have three choices, Get used to it, spend lots of time and money doing what you can to soundproof it, or turn off the engine and sail.