Prop size problem...................??

[petermills]

I have a 21 ft long Starley Sundowner with a Yanmar 2gm 16 hp engine. My friend has the same boat with a Petter 14 hp engine. He seems to move a lot faster than me without effort so I'm wondering if I have the correct prop fitted?? It is a 14 x 9 and the engine doesn't seem to want to go above about 1800 rpm on the rev counter. My friend does'nt know the size that he has fitted so I wondered if anyone out there might be able to give an opinion. I'm not looking for high speed and my fuel consumption is not at all bad so I could leave things as they are. My main concern is that I might be overloading the engine.

 

[No Regrets]

Overloading would normally result in plenty of black smoke from the engine.

If the boat reaches maximum design speed, and nearly achieves it on a half throttle, it sounds OK.

For a comparison, the Broom 30 should hit 8-9 knots, which ours will flat out, but close the throttle by a large margin, and it runs along sweetly at 7 knots. That seems correct to me...

 

[boatone]

Give Steel Developments a ring. They are prop specialists and should be able to help you identify the correct prop size etc

 

[Latestarter1]

I have a 21 ft long Starley Sundowner with a Yanmar 2gm 16 hp engine. My friend has the same boat with a Petter 14 hp engine. He seems to move a lot faster than me without effort so I'm wondering if I have the correct prop fitted?? It is a 14 x 9 and the engine doesn't seem to want to go above about 1800 rpm on the rev counter. My friend does'nt know the size that he has fitted so I wondered if anyone out there might be able to give an opinion. I'm not looking for high speed and my fuel consumption is not at all bad so I could leave things as they are. My main concern is that I might be overloading the engine.

Lots of apples pears and even oranges here......

Yanmar 2GM is 3,400 rpm rated engine with Kanzaki gearbox having optional ratios of 2.21/2.62/3.2:1 You have near same power as the Lister as Yanmar quote baby metric Hp.

Old Lister AC2W was a 3,000 rpm rated engine generally with Hurth HBW gearbox with 2.72:1 ratio.

Prop comparisons as you can see are totally meaningless.

Regardless of anything else a marine diesel engine has to be able to make rated power with the exeption stated below. Check that throttle is properly adjusted by checking your high idle speed, if I remember correctly your 2 GM will rev to 3,600 out of gear, i.e 3,400 plus 200 rpm governor droop.

If you are making correct high idle then you need to look seriously at your propeller, if all you can make is 1,800 rpm at WOT engine is seriously overloaded plus if you look at the 2GM power curve engine is only making just under 8 Hp and your exhaust temperatures will be uncomfortable.

If your boat is only ever going to be used on the river and you will NEVER use full throttle it is acceptable to over prop slightly, I would not want to run a 2GM under load at any less 2,600 rpm WOT, at least power output will be in double figures. My other concern is your tacho, Yanmar tacho's generally read high, you may not even be making 1,800!

You need to establish:
#1 Correct throttle cable adustment.
#2 Calibrate tacho with cheap hand held laser tacho off Ebay.
#3 Look at data plate on side of your Kanzaki box to establish your reduction.

 

[Tranona]

As suggested you are probably well over propped. It is better to start from scratch as trying to compare with another boat with a different engine gearbox combination will be misleading.

Suggest you get a ball park size by using the Propcalc programme on www.castlemarine.co.uk you will need the basic data on your boat, engine and particularly gearbox ratio as the difference between the optional ratios is significant. Almost certainly you will find you need a smaller diameter or finer pitch (or both) so you can then get quotes from possible suppliers such as Sillette Sonic. The supplier will make a recommendation - take it as if it is not right they have some responsibility - but usually a good supplier will do the calculations correctly.

 

[No Regrets]

On a side note:

spoke to a NB'er today, who had a very long 1929 Narrow boat, with a lovely, and I mean LOVELY gleaming 1948'ish Lister Diesel, which developed 22bhp at around 1200 rpm.

The prop size was 26"!!!

 

[Starly]

I have a 21 ft long Starley Sundowner with a Yanmar 2gm 16 hp engine. My friend has the same boat with a Petter 14 hp engine. He seems to move a lot faster than me without effort so I'm wondering if I have the correct prop fitted?? It is a 14 x 9 and the engine doesn't seem to want to go above about 1800 rpm on the rev counter. My friend does'nt know the size that he has fitted so I wondered if anyone out there might be able to give an opinion. I'm not looking for high speed and my fuel consumption is not at all bad so I could leave things as they are. My main concern is that I might be overloading the engine.

I have the same problem now with a 2GM20 (18 HP) enigine. Boat has no speed and is difficult/impossible to steer with little wind.
How has the problem been solved? I think my prop. is too large.

 

[oldgit]

3400 revolutions per minute
The continuous rating output at the crankshaft is 11.8 kW, at 3400 revolutions per minute (rpm). The maximum output at the crankshaft is 13.4 kW, at 3600 rpm. The engine delivers roughly 16 horsepower, for a weight of about 114 kg
A wild amateur stab in the dark.
If the engine will rev at full throttle under load to something approaching this figure the prop pitch probably will not be too far off.
Boat rudder authourity depends on the the amount of water flowing past it, in the case of motorboat, how much water is being pushed past the rudder blade by the propeller.
If the prop is turning slowly,especially at slow speeds boat might wander all over the shop and need big steering inputs to keep good heading.
The boat in question has probably had the correct area of the rudder to match the engine rev band.
On a boat autopilot you can adjust the amount that the rudder will deflect, at 5knots you might want a large movement of the rudder to change
direction, the same amount of rudder movement at 25 knots will be scary ?
This is also why rudder blades tend to remarkably small on fast boats compared to their displacement cousins.

 

[Tranona]

The advice I gave all those years ago is still valid. You have more than enough power for your boat. The critical figure you need (in addition to the boat data) is the reduction ratio as it is the shaft speed that matters in sizing the propeller. As Latestarter says there are 3 different reduction ratios available on the Yanmar gearbox and generally the larger the reduction ratio the larger the diameter prop. So a 2.2:1 might have a 12" prop and a 3.2:1 as big as 15".

Put your boat and engine data onto here vicprop.com/displacement_size_new.php to get a good estimate of size, but if you need a new propeller then go to a specialist who will do the calculations and supply an appropriate propeller.

 

[Outinthedinghy]

Nice little boats those Starley Sundowners. Not been on one myself but I hear they are good in open water and can deal with the weather.

 

[oldgit]

Several companies who might be able to modify your existing propeller if new prop is out of the question pricewise.
Worth attempting to find out what was fitted as standard, over the years somebody may have tried to make it go faster or simply fitted a s/hand non standard prop.
Next time the boat is out of the water, somewhere on the boss should be stamped the size and pitch of the prop.
If the pitch has been altered usually the original pitch will be overstamped with the new one.