Prop Sizing and Placement

[GregH]

Morning Folks;

Thought I would start a new thread about the prop I have as I wasn't sure about its size due to being able to go 2-3kn at idle, plus vacuum readings - but the engine needs a good tune up in the spring before I can feel good about those numbers.

The pics below of the prop. it's a 13x7 2 blade in an Alberg 30.

- The gap between blade tip and top of notch is about 1/4"
- nothing is bent or loose for the prop to ride that high in the space. it is angle of the shaft log as built from what I can tell.

As some have mentioned - maybe going with a 12x 7 or the indigo 3 blade (I only cruise about) 10x7

The small gap at the top worries me, but I have no experience with this and so looking at discussion/advice

Sorry for the out of focus....

[Marian Claire]

My understanding is that the space between the prop and the rudder should be 15% of the props diameter. Almost 2" for a 13" prop.
Dan S/V Marian Claire

Edit: If I am doing my math correctly you would need to drop down to a 10" prop to comply with this space suggestion.

[ndutton]

General industry specifications were 1 ~ 1½ X shaft diameter for clearance between the prop tip and the hull. Any closer risks a water jetting effect on the hull.

[joe_db]

I had a similar issue with a 13x7. Getting the Indigo 3 blade was a large improvement in motoring ability and made the boat much quieter. Maybe not so much with the Alberg, but for my boat the prop is under a flatish section that acts as an amplifier.
Seriously, if you are out of the water, get the Indigo. Way cheaper and easier than trying to redo the engine mounts and shaft log!

[Marian Claire]

Neil. Is that spec for a prop in the open or in a aperture? IIRC the info I used was for a prop in a aperture. I should have added that in the post.
Dan S/V Marian Claire

[ndutton]

Other than my own Westsail I never built a boat with a prop in an aperture. However, I'm not sure that matters. Prop tip proximity/water jetting would apply in either case. I'm not advocating for one standard over another, only providing what the standard was at several boat manufacturers. One thing is for sure, ¼" clearance is way too close by any standard so a reduction in Greg's prop diameter will be a significant improvement. Also, I should have mentioned in my earlier post that the dimension was a minimum.

Deadwood is a factor though. Greg's only thrust comes from the outer half of the prop's diameter so a size reduction hits him where it matters most.

edit:
Correction, the Freeport 41 and Gulf 32 both had aperture props and both were built during my involvement at Islander and Capital respectively. Tip clearances applied the same.

Here's another little oddity that comes to mind. If you ever get a chance to see a Gulf 32 out of the water, even a picture, look for the two hernias in the hull underbody a little aft of amidships, port and stbd. They are there to allow interior space for the engine to fit low in the hull.

[joe_db]

I have no direct experience, but a look at the prop page shows Alberg 30s, Alberg 35s, and other similar boats using this prop. You might be able to find another Alberg owner for a review.
If you don't want the 3 blade, you might consider a 12x6.

[Marian Claire]

Neil:Cool. I think you are correct that it doesn't matter. My notes from years ago are a bit hard to follow.
The number I have for the space between keel and prop is 30% of prop diameter. I do not think I have info on the thickness of the keel/deadwood but I know that was a consideration with my choice to go with a three blade instead of a two blade.
Dan S/V Marian Claire

[ndutton]

At Islander we offered both three blade and two blade props for the Freeport 41, the two blade being the recommended prop for the following reason. An option was an indexed prop shaft brake that would align the two blade prop vertically in the shadow of the deadwood. On paper it was pretty cool, in practice it was a disaster.

The problem was it did not incorporate any disengagement safety. It required the skipper to remember to disengage the shaft brake manually before shifting into gear. With a mechanically indexed brake, the result was a number of mangled mechanisms returned to the factory. This could have been easily avoided with a hydraulically disengaged index supplied by engine oil pressure accompanied by a neutral safety switch. It would have eliminated the need for any skipper input.

Perhaps you'll remember my unflattering stories of the in-house Islander chief engineer at the time.

http://www.moyermarineforum.com/forums/showthread.php?p=46926

[Elmo]

First Check Prop Placement (Tip Clearance) and Then Sizing (Pitch)

My 1970 Coronado 34 (with reduction drive) needs a new prop, so I did a bit of research on prop installation.

There seems to be TWO major concerns; tip clearance, and blade clearance. Michigan Wheel's faq addresses both concerns:

What is the minimum allowed clearance between the propeller blade tips and hull bottom? How much space should exist between the propeller and strut or keel?

The clearance between the propeller blade tips and the hull bottom should be at least 15% of the propeller diameter and ideally, 20% or more.
So for example, a 20” diameter propeller would have 3” minimum clearance and better still 4” or more.

A common guideline for recommended propeller to strut or keel clearance is 20% of propeller diameter measured between the propeller blade edge and strut leg or keel.
This is often measured at a point on the propeller blade (leading) edge about 70% of the distance from the shaft centerline to the blade tip

Note that the clearance between the prop blade and the strut/keel/deadwood is more critical for efficiency than the clearance between the blade and the rudder.

Once I knew the recommended tip clearance for MY installation, I found that a 15x10 three blade gave app 15% tip clearance (of 2"-2.25").
It was more efficient, BUT wot was only 2,100 rpm. I couldn't get up to hull speed, so the prop was way over pitched!

I had been using a 13d x 10p, so based on industry info, 2" increase in diameter is roughly equivalent to 4" more pitch, so I need to reduce pitch to 8" and try that. I'm told that 1" pitch reduction is app equal to 200 to 300 engine rpm increase; so when the prop gets re-pitched (max of 2"), I expect to see around 2,700 rpm... still not low enough pitch to CRUISE at 2,800 rpm, but moving in the right direction. I expect to end up with a 15d x 6p cruising prop able to push me through tide rips, head winds, and bow waves, while towing two dingys, and let the engine rpm run as per Moyer Marine recommendations.

You suggested a 13” diameter x 14” pitch propeller for my boat. Would a smaller diameter propeller with more pitch provide the same performance?

A propeller with smaller diameter and larger pitch might provide the same load on the boat’s engine but may not have the same efficiency as the suggested propeller.
Propeller diameter is chosen to be optimal for the boat’s particular combination of horsepower, RPM and speed.

Deviating significantly from the best diameter may result in slower acceleration, reduced cruise speeds and efficiencies, and more difficulty in slow speed maneuvering.

https://www.miwheel.com/resources/faq/inboard/#33700

[GregH]

Thanks for the Reference data Elmo.

I am not 100% sure, but from reading all over the forum, I've never read anyone having their cruising rpm at 2800. Mostly I've read people are in the range of 1800-2100 rpm.

I'm sure someone with info better tucked away than myself can enlighten

[joe_db]

Thanks for the Reference data Elmo.

I am not 100% sure, but from reading all over the forum, I've never read anyone having their cruising rpm at 2800. Mostly I've read people are in the range of 1800-2100 rpm.

I'm sure someone with info better tucked away than myself can enlighten

2800 would only be doable with a reduction gear and/or planning hull. I think I hit about 2400-2600 flat out. 5 inches manifold is probably around 2100-2200 RPM, which is an easy low strain cruise setting for me.

[GregH]

Well with a new season, boat back in the water and working on tuning the engine..

- Idles well ~900 rpm with pressure gauge reading about 15psi
- Power tuned for idle
- In neutral can run the engine 2500 and more
- The dizzy flyweights cleaned and oiled and moving well (though still need to get Moyer springs)
- In fwd , on calm day doing 5 - 5.5kn at 1200 rpm (alberg 30)
- clean bottom fresh water

..and that is the top end. Adding any more throttle while in gear does nothing. From the start of this thread I know the prop is over sized and I think that is what is holding things back at this time. I'm planning to replace it for next season and live with what I have for this season.

[GregH]

A little more of the running story here...

After switching over temporarily to a portable fuel tank until I can get the main de-watered, the engine was idling sweetly.. around 900-1000 rpm.

The idle mixture is at 1-1/4 turns and still rich since when starting cold with choke out, it starts in 2 sec and then I have to put the choke in right away or it runs very rough and eventually dies. How lean do people run?

Had it out on a calm (ie no wind) evening last night and was able to get a top RPM of 1700. At this I was cruising at 6.5 kn in an Alberg 30 with a WL of 22' which is right around what the calculator says should be possible. I did not check the vacuum gauge reading at this point.

Did a nice 30 min cruise around at 1500 rpm to try and get the engine to work some some and maybe help clean up the plugs a little.

Don't think I can do much more this season to finer tune it. Will be getting new smaller prop for next year and EI is on the list.

[joe_db]

That does not sound good at all. The engine appears to be some combination of low on power and/or severely over-propped. You essentially have about a 10 HP engine right now.
I would first off get a manifold vacuum gauge and do a compression test and then think about an Indigo prop or maybe a 12x6 two blade if you race.