Propeller Replacement - Should I?

[salt water sailor]

My question has two parts. My 78 Hunter 30 is grossly under powered with a Yanmar YSB 12 hp which drives a two bladed 13" 10 pitch left-handed prop. I have been out in a good blow with 8' seas and struggle to make headway- almost to the point of loosing steerage. Will going to a 3 blade prop help improve propulsion in these type of conditions?
Secondly, I cannot find the gear reduction ratio anywhere on my engine or in any documentation I have read; including the owners manual. I'm told that I will need this in order to establish the size and pitch of a replacement prop. Any feedback would be appreciated.

 

[centerline]

some thoughts before spending any money on re-propping,

how is the prop you are using now, for normal motoring?...

does the engine run at the correct rpm during WOT (wide open throttle) operation?
a diesel engine has a speed governor on it so sometimes its hard to tell if you could use a bit more pitch, but the problem is if you get too much pitch in the prop, you will cause the engine to lug, and thats really really bad...

is the throttle lever on the fuel pump actually opening up all the way to allow maximum fuel flow? too many times its out of adjustment and the motor isnt even getting full throttle.

if it seems to lug, then re-propping may be an option, but if you are already running at the correct rpm, and the prop is still pushing, it may be that you are just under powered... there isnt much you can do....

when pushing against a rough sea with wind and current against you, about all that 12hp will be able to do with a 30ft boat is to maintain some steerage.... making headway will be slow to non existent...

re-propping could either cause you to lug, or it will not drive hard enough, which in either case will cause you to be worse off than you are now...

 

[centerline]

I cannot find the gear reduction ratio anywhere on my engine or in any documentation


I'm told that I will need this in order to establish the size and pitch of a replacement prop.

is there a tag on the gearbox? the numbers on the tag are actually a code, and with it you should be able to order anything within the confines of the paint on it, and in turn, also find out what kind of gearing it has...

and just because you know the gearing, that will get you close to the right size of prop you need, but it wont give you a definitive part number that you need to buy.... there are too many variables when choosing a prop... you gotta try a couple different sizes within the proper range to see what works best in actual practice.

another big factor is how clean the bottom of the hull is.... or isnt!

 

[Jeff10528]

Why not try to buy a prop on the condition that it works ok. Shop around marinas and service shops especially focused on sail. Some have a used prop collection they are eager to sell. I swapped a 2 blade for a 3 blade on an '81 H30 for a similar reason. Just make sure you can get close to your cruising and full speed RPMs, you may need to drop an inch of pitch to do that.

Good luck!

 

[Terry Cox]

Re-proping

Find a local prop shop. Gather all the information that they will need; engine model/horsepower rating, boat size/displacement, prop spec, etc. They can help you determine whether your boat is over/under propped. They can also recommend steps you can take to accomplish your performance objectives.

 

[Benny17441]

The prop is not the answer. The engine is just an auxiliary and not intended to be the sole source of propulsion much less in a blow. A change of prop will not do significant improvement. Reef and trim the mainsail and let out a bit of jib and take a favorable course. If you do not want to loose much gained ground just heave-to.

 

[salt water sailor]

Thanks to all for the very good feed back. For normal motoring, I can achieve approx 5 knots depending on current/tide. I am pretty sure the throttle is opening all the way. Up here in the north atlantic, the hull only gets wet from June to Sept. so the bottom is very clean. I didn't see a tag on the gearbox but will check again.

 

[kito]

My ysm12 shop manual on my 79 H30 says it has 2:1 gear reduction....3000 rpm at the crank and 1500 at the prop. I would think the ysb12 would be similar but not sure. I believe the 13 x 10 2 blade was what came from the factory. I bought a Campbell Sailer 12 x 5 used on Ebay but have yet to use it. Campbell recommends a 12 x 7 but figure I can re-pitch it.

 

[centerline]

Thanks to all for the very good feed back. For normal motoring, I can achieve approx 5 knots depending on current/tide.

the speed of your boat should not depend on current or tide, but only on the cleanliness, condition and shape of the bottom, and the amount of force driving it.

the boat floats in its medium, which is water, NOT land, so hull speed is always measured in relation to the water...

and any current or tide will be taking your boat along at its pace, so in a sense, you are stationary in the water, although you may be moving briskly along in relation the the land.

running in to the direction the current is coming from, you should be able to get about the same hull speed in relation to the water, as you could if you were to turn down current and run with it... this is measured with an impeller, NOT a GPS..
the biggest factor that will make any change in this is the wind, which is another force acting on the hull... it can work for you or against you.

as you live your life in relation to the land, your boat lives its life in relation to the water... so even though you want to make headway over ground, but the boat is only capable of doing as fast as she can in her medium which is the water, so as Benny said, "Reef and trim the mainsail and let out a bit of jib and take a favorable course.... If you do not want to loose much gained ground just heave-to".

so you have options to change course and take the long way around, but the boat has no options and only has so much to give...

and 5 kts with that horse power on a 30ftr doesnt seem too unreasonable....

 

[Ian McGain]

Replaced two blade with 3 blade on my H30'

I have replaced my two-bladed prop with a Campbell three-blade on my '79 Hunter 30' powered by a YSM12. The performance improvement was smaller than I expected. The rpm tops out at about 2200 before starting to push black smoke. One unexpected hitch was the shaft was coupled by two square headed set screws. These broke and scored a grove in the shaft until one sailing day coming back into my marina the shaft just spun with no power to the prop. I had to have the marina pull my boat and replace the shaft with a stainless steel one and reset the engine and rebalance. Must say the boat never ran better after the repair.

 

[Steve Dion]

Ian: I do not know what the rated RPM of that engine is but you may want to talk to Norm about repitching that prop so you can run it in the recommended RPM range. The fellow in our marina with the same boat/engine had measurable increases in performance.

 

[kito]

I am trying to make out the numbers..........is yours a 12 x 6?