Getting Screwed!

[rslifkin]

Originally Posted by merrydolphin

Your prop is pretty deep but I suppose that extreme pitching might cause some venting. Your CCA designed boat has a lot of overhang and short waterline length. When the seas are space so as to coincide with your pitch moment, you get a lot of hobby horsing. This slows you forward progress a good bit.
I agree that if you had the space (which you don’t ). A bigger reduction of say, 3:1 and a bigger prop would make a big difference. This is a common issue with auxiliary sailboats. The designer/builders do not have proper reduction and propping for real life cruising. Eg. My Cal ll-46 had a near 3:1 reduction with a 26” prop! She could motor like a trawler! If you had to go seriously to windward you could stow sails and put the hammer down. She could make 5.5 kts even on the Baja bash into 6-10 footers and 20-30kts wind. All of the other cruising boats that I had, as with yours, you have to get well off the wind and seas ( probably 30 degrees or more and get that mainsail drawing well in order to motor sail into blustery conditions. No other way to get around it. It’s the way aux sailboats are designed. Bad way to do it. We need good reduction and larger props to punch into it. Come on designers. I could make 200 mile days under sail with that prop.

When diameter limited, you also have the option to add more blade are to give the prop more bite for better low speed thrust. Things like going to a 4 blade prop with big blades will help.

In general, more blade area (and a pitch reduction to allow reaching proper WOT RPM) may reduce top speed in calm conditions slightly (unless you've got enough power to reach hull speed with power to spare). But it will also improve low speed thrust for maneuvering, fighting a head sea, etc. That extra blade area can come from more diameter (and possibly a change in reduction ratio to keep diameter vs prop RPM in a good balance) or more blade area if you're diameter limited.

[rhubstuff]

Just for fun, print a paper copy of the lines plan that you posted, then put the point of a pencil at the midpoint of the boat's waterline. Place a straightedge on the waterline and rotate it using the pencil point as a fulcrum to simulate the position of the propeller at various angles of pitch. You will see that as the bow is immersed, the prop comes closer and closer to the surface, and it doesn't take much pitch to do this. I think you're getting ventilation. Instead of powering directly into waves, consider motorsailing at about 25 degrees off the wind, with just the mainsail, sheeted hard in the reefed if necessary. This will provide a bit of extra drive, steady the boat down, lessen the pitch angle, slow the rate at which you meet oncoming waves, and help keep the prop in the water. And because you are not slowing down so much when a wave hits your bow, you might make better SOG and SMG, and be more comfortable too.

[MJH]

Originally Posted by Renegde_Sailor

I have a propeller question...

Right now I am sailing my Spencer 42 to Alaska, she has a 12-8 pitch 19" 3 blade prop, I have noticed every time we wind up pushing through some waves running against us that the prop cavitates and the boat slows way down.

As well with my 40hp engine, it can easily spin the prop, I never have to run full throttle to get max speed because if I push it too far, it just cavitates.

I cannot go any bigger due to the aperture limiting the size, but I could go to a 4 bladed pitching prop like a MaxProp.

I am curious if anyone more knowledgeable than myself might have some insight on doing so, right now at 2200 rpm where the engine runs real nice, she will cruise at 6 knots, if I push her a bit harder she will clip along at 7.2 above that the prop just cavitates.

Or maybe it is not even worth messing with, but it would be nice if I could get a bit more speed under motor out of her without having to rev her so hard, it feels to me like she is under-propped, and I am wondering if going to a 4 blade would put her in better order when motoring against waves and current?

Assuming the prop is in good condition, it doesn't sound to me that you have the proper prop/pitch for your boat.

Let's get to the basics. With your boat in neutral at Wide Open Throttle (WOP) you should be able to get max RPM the engine was designed for; no more, no less. When in forward gear at WOP you should be able to get to that RPM without any significant vibration or prop cavitation; either of those conditions means you need some change or adjustment.

For starters I recommend you go to the Michigan Wheel Prop web site, input your boat data and see what they recommend for prop size and pitch for your boat.

Finding the sweet spot for your engine-trans-prop combination is not always an easy task...it wasn't for me.

Good luck.

[Rothblum]

Originally Posted by rhubstuff

I think you're getting ventilation.

The problem is almost certainly ventilation, as several responses have pointed out. Rhubstuff's suggestion is very good. Reduce pitching and very likely the low pressure region on the prop blades will not have access to the atmosphere. A path to the atmosphere is one of the necessary conditions for ventilation. The prop doesn't actually have to broach the surface, though. If the separated wake of the boat (low momentum water with respect to the undisturbed water "flowing past" the boat) is in proximity to the low pressure on the prop, air will be sucked into the prop, resulting in a dramatic change in the lift on the prop blades. Outboard motor props deal with this problem with "cavitation plates" or fences -- a misnomer. Another condition that must be met to cause ventilation is that there must be an area of separated flow on the prop blades as well. In this regard, the suggestions that the prop is not properly matched to the RPM are probably valid. Likewise, more blade area would help. The easiest fix is to try to reduce pitching as Rhubstuff suggested. Motorsail slightly off the wind and seas.

[Icarus]

Originally Posted by Renegde_Sailor

I just pulled out my old survey, it says 16" 3 blade 12 pitch, but 12 seems pretty steep to me, the prop had numbers stamped on the side which I believe were 12-8

12-8 means it is a 12" diameter by 8" pitch prop.
Seems rather small.
18"x12" or there about is more likely
.

[fstbttms]

Originally Posted by Icarus

12-8 means it is a 12" diameter by 8" pitch prop.
Seems rather small.
18"x12" or there about is more likely
.

The OP told you in his initial post that he has a 19” diameter prop.

[MVDarlin]

My last reprop on my 52' twin diesel motor yacht included a switch from 4 blade to 5 blade props. Smoother running, less cavitation and 1/2 knot better speed at same rpms. I recommend you explore with a trusted prop expert.

[Renegde_Sailor]

Originally Posted by drdoyle

Your prop is marked as 12-8. This means it is 12 inches in diameter and 8 inches in pitch. It appears you are under propped. Most sailing vessels use small props so as not to create sailing drag. It could be cavitation, but probably is air injestion from surface conditions. But another possibility about the noise would be a bad cutlass bearing, they get very "rumbly" as they wear out. If you are diameter limited, either 4 blades and or wider blade area will help. I would maximize diameter - check for maximum clearance. If you have some back space available then you can add a shaft spacer between the Transmission and the shaft coupler to push the prop back a inch or so. The complicated option is to add a "kort nozzle " to the vessel (square tipped prop rotating in a fixed nozzle). Or there are som exotic new propellers that have the "kort" type ring cast onto the propeller (ring rotating with the propeller). Or the newest propeller design with each blade being a complete loop back to the hub - very intriguing design.

It is a 16" prop, 8" pitch from what we can tell.

I cannot move the prop at all as it is centered in the aperture.

[Renegde_Sailor]

Originally Posted by merrydolphin

Your prop is pretty deep but I suppose that extreme pitching might cause some venting. Your CCA designed boat has a lot of overhang and short waterline length. When the seas are space so as to coincide with your pitch moment, you get a lot of hobby horsing. This slows you forward progress a good bit.
I agree that if you had the space (which you don’t ). A bigger reduction of say, 3:1 and a bigger prop would make a big difference. This is a common issue with auxiliary sailboats. The designer/builders do not have proper reduction and propping for real life cruising. Eg. My Cal ll-46 had a near 3:1 reduction with a 26” prop! She could motor like a trawler! If you had to go seriously to windward you could stow sails and put the hammer down. She could make 5.5 kts even on the Baja bash into 6-10 footers and 20-30kts wind. All of the other cruising boats that I had, as with yours, you have to get well off the wind and seas ( probably 30 degrees or more and get that mainsail drawing well in order to motor sail into blustery conditions. No other way to get around it. It’s the way aux sailboats are designed. Bad way to do it. We need good reduction and larger props to punch into it. Come on designers. I could make 200 mile days under sail with that prop.

Yeah, pretty sure it is a 16" prop, making me think a lot about it, I think the pitch is ok because right now running 2400 rpm I am pushing ~6.5-6.8 Knots, it's been doing pretty solid like that but we have been in mostly flat water.

I think to get the performance in heavier seas when motoring, gonna need to go to a 4 blade. Or could always build a new rudder and carve out a bigger aperture, but that is kinda nuts as far as projects go...

[Renegde_Sailor]

Originally Posted by Cheechako

nature of the beast I think. Boat hits waves, slows , prop still turns the same, boat wallows, prop cavitates until boat starts moving again. A problem with heavy cruising boats. The heavier and deeper the boat the worse it is. A lightweight racer or cat will move along on top of waves, a heavy blunt boat like my old Hans Christian was wallows terribly is a stiff chop.
Angling through the waves, like tacking ....but when you are motoring, helps a lot in a stiff chop. Motor sailing helps immensely.

That is what I would expect in a lot of boats, the Spencer 42 is pretty fine on her entry, and when sailing to windward points pretty high and can charge on with good speed.

That is where I noticed the huge disparity when motoring, and it was in an area where I didn't have the sea room to sail, well not without tacking 5000 times to get where I was going!



I think the only thing I can really do about it at this point is go to a 4 blade prop which would help somewhat, or just dealing with it which is probably what I am gonna be stuck doing for now.

[mvweebles]

Originally Posted by Renegde_Sailor

It is a 16" prop, 8" pitch from what we can tell.

I cannot move the prop at all as it is centered in the aperture.

16x8 is an odd pitch for an inboard boat with reduction gear. 8-inch pitch is something you'd find on an outboard. Usually, the numbers are closer to the same, a so-called 'square' prop. There are variations, but 8-inch pitch is pretty low. I doubt that's correct. Picture of the prop - even at some distance - might help.

Question: can you turn the shaft easily by hand from inside the boat?

[Renegde_Sailor]

Originally Posted by mvweebles

16x8 is an odd pitch for an inboard boat with reduction gear. 8-inch pitch is something you'd find on an outboard. Usually, the numbers are closer to the same, a so-called 'square' prop. There are variations, but 8-inch pitch is pretty low. I doubt that's correct. Picture of the prop - even at some distance - might help.

Question: can you turn the shaft easily by hand from inside the boat?

I haven't tried turning it by hand. Not sure.

[Renegde_Sailor]

This is a pic of the prop from when I got it back from the shop after servicing, these are the only numbers on it.

It is 16 inches in diameter.





This is it installed on the boat.

[mvweebles]

Appears to be more than 9-inch pitch. But looking at your hull I don't think it's a material issue. Prop seems low in water and fairly close to fore/aft center of boat so doubt its ventilation. I don't have any great ideas, but I doubt it's the prop. May be something mechanical - cutless, etc.

Good luck -

[rslifkin]

Nothing about that install seems particularly wrong to me. I'd be inclined to find a good prop shop and get their thoughts on the matter. A good prop guy can seem to work magic with finding a prop that fixes various complaints.