RPM issue under load

[sailormed]

Lookup the difference in ratios FWD/REV on your transmission and you will probably find the answer.

[Q Xopa]

Originally Posted by sailormed

Lookup the difference in ratios FWD/REV on your transmission and you will probably find the answer.

Sounds like one possibility is the Prop too big, ie pitch and/or diameter.
As salormed said it will be apprent if reverse ratio is greater numerical reduction than fwd.

[Sirius 26]

meaning reverse spins much faster than fwd? makes no sense, how much do we need and use reverse and how fast do you really need to go in reverse knowing steering in reverse with a tiller is nearly impossible over 3000rpm?


how do i find a propeller the right size and pitch to be able to drive my boat to 3600rpm at wot? there's hundreds propellers out there and nearly impossible to chose unless you test many over time, mount on, mount off, repeat 100 times. is there a standard propeller specification by volvo penta for each engine model?

[Q Xopa]

No it sounds like the other way ie The propeller spins slower in reverse for a given Eng RPM. This would have the engine less loaded so could achieve a higher engine RPM. The equivalent of being in 'low gear' in your car (ie when you are just starting to move, not when you are going on the highway. Low gear, is expressed as a larger numerical reduction number.

But if you can identify your transmission and find out what the ratios in forward and reverse this would confirm this one way or the other.

As for a correct sized prop. I would start by talking to a prop vendor, or maybe an engine shop.

They will want some information from you though like-
Current prop diameter,
Current prop pitch,
Engine HP,
Transmission ratio
Boat displacement, etc.

It may have a designation you can see like 12x11R or something similar. Meaning 12 inches diameter x 11 inches pitch Right hand rotation.

Or perhaps it will easier to take it off and take it in.

Ideally you want the engine to be able to attain max normal rated operating RPM. This means that the full engine power can be utilized, when needed. Yours is 3500?

So if your 10 HP engine can only spin to 2500 then its not able to get its full 10HP to the propeller. Normally referred to as 'over propped'. Guessing here but maybe its producing 6 or 7 HP. Like trying to go up a step hill in your car in top gear.

A rough guide to increasing Eng RPM is
+200 Eng RPM for every inch less of pitch, and
+500 Eng RPM for every inch less of diameter.

This cant always be reliable as number and shape of blades etc also effects this.

I hope this is a clearer description.

[a64pilot]

Your overpropped, the prop does spin slower in reverse, but is also much less efficient as its backwards so it can’t absorb as much power, so for both reasons that is why if your overpropped, you’ll turn much higher RPM in reverse.
Autoprop is one prop that flips around for reverse so it’s just as efficient, but even then the prop turns slower.
Also most feathering props are just flat plates so they are inefficient in both directions.

[Sirius 26]

I haven't checked the size or the marks on propeller cos the boat is in water and I'd have to dismantle the prop to see the marks on it near the shaft and i haven't measured its diameter yet either.
So if what you all are saying if the case is my prop being too big, then if i mount a smaller prop to reach 3600rpm at wot in forward motion, then in reverse it will just reach 3600rpm way faster than now but the boat will go much slower in reverse?
Now in reverse when I tested it to see if it reaches 3600rpm it's so dangerous it feels like it's gonna lock the tiller sideways and break it due to water resistance.

The boat originally had a yanmar engine inside and it was replaced with this volvo penta and therefore new shaft back in 2002 and i wonder if this same propeller came from that yanmar engine. Have no idea what hp the yanmar had though, to be suspicious of the prop being too big.

Anyway, have to check the size and pitch of the prop first before assuming anything I guess.

But would the overprop cause the black smoke deposit on the back of the boat as in engine suffering at wot only putting out 2800rpm?
Some people in the marina asked me if I've been hauling ass with this engine cos the boat is black from smoke on the back.

Anyway I think my volvo penta md2010d engine specs for wot is 3600rpm and apparently volvo penta md2010 propeller specs call for a 14x8 regardless of what boat you have as this engine is considered for sailboats and so is the prop.

[Wotname]

Originally Posted by Sirius 26

..............

But would the overprop cause the black smoke deposit on the back of the boat as in engine suffering at wot only putting out 2800rpm?
Some people in the marina asked me if I've been hauling ass with this engine cos the boat is black from smoke on the back.

........

Yes, it would.

At WOT, the governor is trying to reach 3600 and keep chucking in fuel to achieve 3600. The oversized prop is preventing the engine from exceeding 2800 rpm so the excesses fuel is turned into black smoke deposits on the stern.

[Wotname]

Originally Posted by Sirius 26

..........

Anyway I think my volvo penta md2010d engine specs for wot is 3600rpm and apparently volvo penta md2010 propeller specs call for a 14x8 regardless of what boat you have as this engine is considered for sailboats and so is the prop.

The prop selection is dependant on the waterline length, the underwater shape, the gearbox reduction, the horsepower and the RPM of the engine so one size doesn't fit all.

For instance, for a given engine / gearbox, a fin keel sailboat would need a different prop than a full length keel in a similar sized boat etc etc.

[Wotname]

^^ try this prop calculator - I put in some average figures for LOA 26' sailboat and your 10HP engine and came out with a three bladed 11x7. Now these figure won't be exact but you try it with the best figures you have at your fingertips.

https://www.vicprop.com/displacement_size.php

[Sirius 26]

Vicprop calc is a bit annoying cos i don't know half the data needed for it to calculate. Beam at waterline, molded hull draft excluding keel, number of shaft bearings, etc...
I don't even know the gear ratio on my engine cos there's 2 types 2.35:1 and 2.72:1

[Wotname]

Originally Posted by Sirius 26

Vicprop calc is a bit annoying cos i don't know half the data needed for it to calculate. Beam at waterline, molded hull draft excluding keel, number of shaft bearings, etc...
I don't even know the gear ratio on my engine cos there's 2 types 2.35:1 and 2.72:1

You just use approximate figures and you will get a reasonable starting point.

For instance, assuming this is your vessel
https://sailboatdata.com/sailboat/sirius-26

then beam at waterline is ~8' to 8.5', folded hull draft maybe 1.5', shaft bearing will be 1, use 2.5 for gearbox and so on. Juggle the figures to get a range of where your prop should be.

[Sirius 26]

No but this is the correct boat:
https://sailboatdata.com/sailboat/sirius-26-streuer

But how do u get a waterline beam and folded hull draft just ball parking it?
Best gettin in the water and measuring the keel and deducting from draft to get true readings and to figure out waterline beam.

[Wotname]

^^ OK, the max draft is 4.10' and looking at the underwater profile, the moulded draft looks ~ 1/3 of that (and the keel is 2/3 of it) so it ballparks around 1.4'.
For the waterline beam, stand on the dock looking from the bow to stern and visualise the difference between max beam and waterline beam.

Of course, measuring is better but you need to get a rough idea of how far your current prop is from ideal and ball parking will achieve that.

Wishing you good luck.

[kmacdonald]

The critical dimensions are LWL and weight. You can alter the beam at waterline and draft and it won't make much if any difference. Don't forget to use the loaded weight and not the dry weight.

[Sirius 26]

Yes tnx to all, I found all specs for my boat even the beam waterline and it calculated for a 14 x 14 prop which is unheard of (pitch), even volvo penta only manufacture 14x8, 14x9 and 14x11 props for sailboats. Ii think I'm gonna try the stock 14x8 but first I gotta even see what size prop i have on now to cause such low rpm. Apparently my engine wot rpm is rated for 3200 and not 3600rpm. It says engine wot rpm md2010-md2040 is 3200-3600rpm, so I'm assuming the 3600rpm is for the 40hp engine.