Prop Sizing Questions

[ak-guy]

I have a Yanmar 3HM. The specs are as follows:
27hp at 3200rpm - continuous rating
30hp at 3400rpm - 1 hour rating
3600 rpm max no load speed

WOT throttle is around 3500-3600 rpm. This is my dash tach display reading. I got a digital laser tach recently and haven't tried it yet but maybe today.

Anyway my question is concerning prop sizing. Is 3500 or so where WOT should be? I have tried prop sizing calculators but I am not sure which rpm number to plug into the space for "maximum rated engine rpm". Would "maximum rated" in this case be 3400? I currently have a 17" three blade prop but I don't remember the pitch.

I usually run the boat at 2500 rpm.

 

[GFC]

AK Guy, let me pose a question to you that might bring things into a clearer perspective: How much time do you think you will spend running at WOT?


You seem to be doing a lot of chasing around to find out where that 100 rpm's went, when in all likelihood you will spend very little time at WOT.

 

[Nomad Willy]

Ak-guy,
That’s a “GM” Yanmar. Very popular in older re-powered Albin 25’s.
They run and run and run.
When running a clean bottom you should get 3400 to 3500rpm WOT.

I had a HM 35F in my Albin and ran it for the first year or two at 2750rpm and 8.5 knots. Later when we went to Alaska I ran at 7.35 knots .. don’t remember the rpm. 11 knots WOT. 3500rpm.

Only problem I had was a seawater leak (very small) on the fwd end of the exhaust manifold. Caused a bit of corrosion at the throttle shaft.

What gear do you have? I had a Kanzaki w close to a 2-1 ratio. Very noisy. I made some mods and put the fix on that. It rattled so much I was embarrassed to come into harbor.

How are things on Icy Straits?

 

[ak-guy]

AK Guy, let me pose a question to you that might bring things into a clearer perspective: How much time do you think you will spend running at WOT?


Pretty much never. But I do want the most efficient prop and rpm WOT is an indicator of that.

 

[ak-guy]

Ak-guy,
That’s a “GM” Yanmar. Very popular in older re-powered Albin 25’s.
They run and run and run.
When running a clean bottom you should get 3400 to 3500rpm WOT.

I had a HM 35F in my Albin and ran it for the first year or two at 2750rpm and 8.5 knots. Later when we went to Alaska I ran at 7.35 knots .. don’t remember the rpm. 11 knots WOT. 3500rpm.

Only problem I had was a seawater leak (very small) on the fwd end of the exhaust manifold. Caused a bit of corrosion at the throttle shaft.

What gear do you have? I had a Kanzaki w close to a 2-1 ratio. Very noisy. I made some mods and put the fix on that. It rattled so much I was embarrassed to come into harbor.

How are things on Icy Straits?

From looking at specs for the GM I see they are a little different. The 3GM is rated at 20hp at 3400 and the 3HM is rated at 27hp at 3200. The no load speed for the 3GM is 3750. I imagine they are very similar engines though.

My reduction gear is 2.83:1 and it is a Kanzaki also. The engine is quite loud perhaps some of that is coming from the gear.

Icy Strait is great but it has been a hot dry summer. Some relief now but we still need rain, can't believe I said that.

 

[Nomad Willy]

Ak-guy,
As I remember the GM was 27hp and the HM was 34hp. The HM being an older and bigger engine.
Guy that’s great you don’t have an issue w the gear noise. My solution was close to a boat buck.
A deeper reduction .. that’s great. You can probably swing a 17 - 18” 3blade. That’s good. I have an 18X13p on the Willard.
The Yanmar engines are a bit loud.

 

[FF]

" But I do want the most efficient prop and rpm WOT is an indicator of that."

Only if you operate at WOT, with great regularity like a sport fish.


With a couple of 30 hp diesels probably 4GPH is your WOT fuel burn


Maybe 2 1/2 would be normal cruise , so a 10% reduction in the fuel bill might take years to see a return..

With the engine operating hard enough (Hot enough oil temp) to burn off combustion blow by,, your engine life should be fine. IF not change the oil more often , way cheaper than a pair of props.

 

[O C Diver]

Likely adding 1" of pitch will keep you from turning rated RPM (3,400). You may gain a little speed at 2,500 RPM or achieve your current speed at a slightly lower RPM with slightly less noise. If I were of a mind to add the pitch, on the next haulout I would take the prop to a propeller shop and ask if it can be repitched. Personally, if the prop doesn't need to be reworked, I wouldn't waste the money repitching it until it needs to be reworked.

Ted

 

[Nomad Willy]

Unless run really hard like 75% load this boat will burn 1/2gph.

FF is right. You would gain efficiency being 1” overpropped. You would go from .5gph to .48gph. WTF. The gain is fly s*It. Especially w this boat.

So many good reasons not to overprop it would be just nuts to do so. I’m quite sure I burned about .8gph w my Albin running a bigger engine at a very inefficient speed (8.5 knots) at a steady 2750rpm. This Allweather is obviously more efficient and would not do well overdriving beyond 1/2 a knot below hull speed. It dosn’t have a hull form for even going hull speed. Neither does my Willard. Best speed for this boat is 3/4 to 1/2 a knot below hull speed. I run the W30 at six knots and that’s w a considerably longer WLL. It would be nuts to overpower. My Willard requires 18hp to cruise 6 knots. This Allweather won’t need half that for the same SLR speed.

With a boat and engine like FF has overpropping has a measurable advantage overpropping and running slow bell. Allweather .... no.

 

[OldDan1943]

I have a Yanmar 3HM.
WOT throttle is around 3500-3600 rpm. Would "maximum rated" in this case be 3400? I currently have a 17" three blade prop but I don't remember the pitch.

I usually run the boat at 2500 rpm.

What is your speed and fuel consumption at 2500 rpm? (Just for my information)

 

[Nomad Willy]

Dan,
I think the “WOT” is expressed as a range because one can only change prop pitch in one inch increments. You can’t have an 18 1/2” pitch prop. If you want to go WOT at exactly your rated rpm you’ll need to cut down the prop edge or tip of the blades. Then if you take off too much you’ll need to cup or repitch again.

So I think Yanmars offering a speed range allows one to get “within tolerance”. Whereas in most situations one would be “off” to some degree. Lots of people don’t like to be “off”. With a WOT range one can be in tolerance. With a single number you’re 50rpm off .. 25rpm off .. whatever. With most engines I’d rather be at the high end of a range. The WOT rpm is not a redline like a never to exceed speed. It’s just the engine speed that your engine makes best (highest) amount of hp.