Prop Size and Aperture

[terry_thatcher]

I am contemplating a repower with either a Beta 35 or a Beta 43. I have no interest in the top end rpms of the Beta 38. I have been reading Dave Gerr's book on propellers. He says the MINIMUM diameter propeller for a vessel of Morgan 382 proportions is 18" We all have 16" wheels. Keefer Douglas tried 17 and 18' wheels and rejected them because of high cavitation. The Brewer "owner plans" show an aperture of 21" in height and a prop drawn to be 18." Does someone have available an actual measurement of the aperture? Thanks. I will try looking in archives, as well.

 

[jimcleary]

Terry

I am hauling out this week. I'll measure the aperture and let you know.

Jim

 

[terry_thatcher]

thanks

 

[Warren Holybee]

Does the book talk about position fore/aft in the aperture? I added a driversaver, which moved the propellor aft, and it seems to have reduced cavitation. My prop was previously forward enough that a "standard" anode wouldn't fit, and I needed a special thin one. Now a normal anode fits.

I expect the thickness of the keel/skeg/rudder also plays a large part in cavitation, as (particularly in front of the prop) it blocks water flow and thus affect the pressure drop. If the prop is too far forward, the part of the prop that is blocked by the skeg would cavitate, as not enough water would be able to flow to the prop. Moving the prop as far aft as possible would mitigate this. Only my theory though, but I would/will consult an expert about placement fore and aft when changing props.

Also, Isn't the skeg filled with "stuff"? If you wanted an additional inch of clearance, I would think that grinding off and re-glassing half an inch on the top/bottom of the aperture wouldn't be a big deal.

 

[struell]

Our boat is out of the water- (1978 382 #41). The aperture is 22" high and 11 1/2" fore and aft.
I'm not sure how to define the position of the prop but the bronze prop hub is 3 1/4" in the fore and aft dimension and the aft face of that is 8" from the forward side of the aperture.
I think Warren is correct that the aperture could be increased if it was cit larger and then reglassed.

 

[terry_thatcher]

It appears to me from old photos that the prop comes out at an angle so the prop blades are closer to the bottom of the aperture than to the top. My boat is in the water, so next time you are at the boat, could you confirm or deny that? Thanks very much.

 

[struell]

The boat is in my backyard. so I just looked again. The prop shaft and the bottom of the aperture are parallel and 11 inches apart. The top slopes upward relative to the shaft as you go aft. The overall aperture is 22" high at the propeller blades and about 23" at the rudder end. The forward end has a large fillet top and bottom so it is hard to get really clean measurements, but yes the propeller would look a little closer to the bottom than the top depending on the angle of the camera.

 

[terry_thatcher]

Thank you for doing this.

 

[john english]

I concur with Warren. I have a 4-blade,16" VariProp with a long hub that makes it necessary to move the prop forward which results in cavitation. If I run above 1800 rpm on the 3QM30 its quite unpleasant.

 

[david_buckley]

I just went through this exercise with my repower. I wanted a 18" wheel based on the common knowledge that bigger diameter is more efficient. I was advised against it by PYI who sold me my max prop. I went with that recomendation especially given the experience of other owners in the past where 18" did not work for them. When I measured the aperature there was not the required tip clearance recomended by Gerr's book. The 16" that I installed just starts to cavitate at WOT so it was the right call for our Yanmar 4jh45.

 

[Warren Holybee]

What is the tip clearance Gerr recomends?

 

[jimcleary]

Dana was just hauled out yesterday. I photoed the prop and aperture. Here are the photos and a sketch of the measurements I took. Again, my engine is the Yanmar 3QM30H and the prop is a 16 x 11 three blade Michigan wheel. I use a 1" thick Drive Saver at the shaft coupling, The prop hub is 3 1/4' long. Years ago when I put a new shaft in I had it shortened so that, by removing the drive saver, the shaft can be slid forward and there is just enough room to remove the prop without having to drop the rudder or move the engine forward. There is 3+ inches of room from the tip of the blades to the top and bottom of the aperture. We do not experience any cavitation at any speed. The three blade fixed prop pushes Dana at 6.2 knots in calm seas and a clean bottom. When we are sailing we always allow the wheel to freewheel. Been doing that for 34 years now and this past winter the transmission was given a clean bill of health. And yes that is a 384 rudder on a 382.

Jim

 

[royaltern]

Jim, your bottom looks clean and smoth after a summer sailing.

 

[jimcleary]

Hi Bert
The yard power washes with a powerful machine so it makes the paint look good. There are a couple of barnacles on the prop which I spray with cold galvanizing spray. I'm happy with the way it comes out. This year Long Island Sound has seen all boats coming out of the water with a fairly thick layer of scuz that we have never seen before. It come right off with the washing but it certainly slowed the speed through the water towards the end of the season. And we usually have the bottom scrubbed twice during the season.

Haven't heard from you in a while. How are you doing?

Jim

 

[david_buckley]

What is the tip clearance Gerr recomends?

15-20% Gerr mentions that it is speed related where tugs will use less than 15% and accept vibrations and high speed boats need at least 20% He then goes on to discuss the benifits of fairing the trailing edge of the apperature above and below the shaft to allow water to the prop. I was not to eager to reshape the skeg.

 

[jimcleary]

Dave
The Brewer aperture has a shaped area forward of the prop to feed the water into the prop. I agree with you about not changing the shape of the skeg any further.

Jim

 

[Mitchell S Allen]

This is the Rudder/Prop opening on Sonata from 3 years ago when I hauled her for paint. She is a 383, and seems much different from the other photos here.
As you can see there is a supporting fairing at the prop shaft. The path to the prop is faired in withcovers. I have to thin down anode collars to make them fit. Are the later boats all like this? Just 383/4s?
Mitchell

 

[BJoslin]

Mitchell,
Here is a pic of mine. Can’t remember hull number off the top of my head but she is a 1978 382 model. Looks as though your shaft log just extends out farther than mine and others I’ve seen on here. Just going from memory I believe my shaft log only sticks out maybe 1/4 inch and cutlass bearing a 1/4 inch beyond that.
I’m no expert so can’t speak on whether there is a benefit to having the shaft log stick out farther or not. Maybe something to do with less unsupported shaft at the prop end? Not sure. But if you wanted to fit a full size zinc you could probably get away with trimming some off.

Barry

 

[jimcleary]

In the photos Dana's aperture you will see there is only 3/8" of the cutlass bearing sticking out of the skeg. The shaft log is flush with the skeg. The 3/8" of the bearing that is left sticking out is for the purpose of getting a wrench or a channel lock on it to remove it. I think the shaft log is well glassed into the skeg and doesn't need to extent out from it .

Jim

 

[Mitchell S Allen]

Thanks guys. I will look into removing a littlev of the shaft log next time she is out of the water for bottom paint (soon). That would make fitting standard Zincs a breeze. It won't take much in this case. I need to mill the thickness down on the zincs about 3/16" or a little more to get them to fit.

 

[terry_thatcher]

I have 382 number 163, built in 1979. I think I do not have the scalloped areas shown on your skeg, altho it has been 2.5 years since she was out of the water and I am embarrassed to admit I don't remember. I assume that improves prop efficiency.

 

[Mitchell S Allen]

I agree Terry. I figure it must help water to the prop.
They may have implemented the scalloped areas later on. I'm just guessing.
Sonata is a 383, from 1982. Hull 30, (but for some reason all headliner panels, etc. are marked 029. As is her sail #)
Mitchell

 

[Warren Holybee]

I just looked at some old pictures. 382 #256, and I have the taper. I doubt this area changed.

 

[struell]

Our 1978 382 hull #41 has the same scallops that the others have shown.

 

[john english]

Mitchell,
The collar zinc is so close to the cutlass bearing that I would be concerned about the amount of water getting to the bearing that provides cooling and lubrication.

 

[Mitchell S Allen]

Thant's a good point John. Although it has apparently been this way through the life of the boat. I think as I said, I will explore removing and making more space there. It will make it possible to use standard anode collars as well as more water flow.
Where have you had bottom paint done in the past?
Bay Marine did Sonata almost 3 years ago. I was very satisfied with their work. Sonata is due again.

 

[Warren Holybee]

I don't think there will be any issue with water flow. It should only drip a few drops per minute, and there is plenty of space to support that.

 

[terry_thatcher]

I have a bullet zinc at the back or the prop, to protect it, and a donut zinc on the shaft forward of the prop to protect the shaft. I think Mitchell's set up does both with one zinc. And I agree, my donut is about the same distance from the cutless bearing and it does not impede water flow.

 

[Mitchell S Allen]

Thanks Guys, I do get the mandatory drip, drip, drip. And it hasn't appeared to create any problems. When the boat was hauled to paint, I did check the shaft play at the cutlass bearing and was fine anyway.
The biggest nuisance is having to kill every collar anode.

 

[john english]

Mitchell,
Don't mean to hijack this thread but any mention of a haul out in the SF bay area is hazardous to your bank account.
My last 3 haul outs have been a KKMI. Had quotes from Bay Marine but the prices appeared to be the same. Any thoughts?

 

[Warren Holybee]

Spaulding was reasonable when i last hauled out in the bay area. 2015 i think. But it was the cheapest place and does good work.

 

[Mitchell S Allen]

Ok, so hijacking this further...
I've only (fortunately) hauled out once so far. That was almost 3 years ago shortly after we purchased Sonata. She was overdue. Bare gel coat is again showing now. So, I need to once more paint the bottom. I'm debating with myself on whether to use ablative or hard paint this time. Trying to learn more about what it takes to do hard. She has always had ablative.

I must say I was very pleased with Bay Marine's work and speed to splash. They qouted me about a week and she was back in the water in under 4 days. I asked if they would put a 3rd layer of paint along the waterline to about a foot down, they did and didn't charge any extra. They were substantially less than KKMI at the time. These were the only 2 prices I explored.

I've considered doing the painting myself if I can find a reasonable yard. I recently checked Napa Marina to do the work myself. But by the time all the fees and space rent were added up, it would be more than having another yard do the whole job. The other impression my wife and I left with, was that boats go there to die. Both the storage/work yard and the marina. So many boats there that will never go back in the water.

I have visions of heading to Mexico for bottom paint this spring.
I expect it may be a little less $$$ but don't really know. But a reason (excuse) to go! So, anyone have suggestions?
Mitchell

 

[Warren Holybee]

Mexico won't be less, unless you travel far off the beaten path. The popular spots will charge Southern California Prices, which are even higher than SF Bay. In La Cruz, the price of an oil filter off the shelf in the Chandlery was $50. Amazon shipped one to me for $5. Absolute robbery for everything, parts/labor/food. If you might get really far south, Chiapas has a very nice marina and yard that does good work stupid cheap. While I was there a friend had his boat stripped to bare gel coat, then 2 coats of epoxy barrier coat, then 3 coats of Ablative, for under $2000. 6 people working on it and done in 1 week. But..... Chiapas is far. Unless you are planning to cruise to Central America, you would never otherwise get there.

The few people I know that tried hard paint, switched back to ablative. Ablative works well.

 

[Mitchell S Allen]

On Mexico, I know the berths in the Sea of Cortez, mainland and Baja are surprisingly high. So I guess I'm not surprised that work and other prices are high too. Or would be any different. Further South might be an option. We'll see. I need to spend more time offshore before I feel comfortable.
Ablative does work well. Though I have a friend with hard epoxy paint that swears it's still going strong after 7 years. Not sure I believe that.

 

[_Jason_]

With regards to the original thread topic, anyone have success with a 17" prop?
We have the higher reduction ZF replacement transmission with a rebuilt 4-108, but still original 16x11 prop. This results in much higher rpms and difficulty to hit hull speed. Looking to size up prop or adjust pitch.

 

[terry_thatcher]

What ZF transmission and reduction ratio in forward do you have? Hull speed on a 30,5' wl displacement hull is only 7.4 knots. Recommend you find a good prop shop.

Member Coy MacDonald installed a 17" diameter MaxProp 4 blade with a Beta 50. He says he is happy with his setup, but I don't know what pitch he has. Nor have I seen anyone query him about cavitation. More pitch will help speed for sure and the Perkins has plenty of power to accept more pitch, even more than a Morgan probably needs. I think Gerr says if you can't increase diameter, boost the pitch. But be aware that pitch is backwards in reverse and in reverse diameter is quite important (unless you have a MaxProp or other feathering prop.) . So, you might have to rev up quite a bit to stop the boat.

If it arrives from England, I am installing a Beta 35. It will have a ZF12M with 2.136/1 reduction. MaxProp suggested a 16 x 12 prop. We will see. One can adjust the pitch. My Perkins has a 1.88/1 ZF and I turn a 16x11 fixed three blade now. I get 6 knots or close to it at 2000 rpm. I hasve never tried to get to 7.4 knots under power.

 

[_Jason_]

Thanks for the reply Terry! We have the ZF 12 with 2.63/1 ratio, and a 16x11 prop. Based on prior posts, it seems the original Hurth 2.5/1 and 16x11 prop was already slightly under propped/pitched to promote higher rpms on the powerful Perkins. However, our ZF just takes it further. We can get 5 kts at 3,000 rpm in calm water. During survey the boat reached near 7 kts at max rpm of 4100 rpm, if the photo tach was reading properly. It's my understanding the desired max intermittent rpm of the 4-108 is 3,600 and max continuous rpm is 3,000. I talked to a prop shop in S. Fla and they recommended a 17" prop, and I would rather increase size before pitch, but wonder if anyone else had cavitation issues based on aperture. We are just looking to get a little more cruising speed at a reasonable rpm, particularly when adding wind/waves/current.

 

[terry_thatcher]

Understood. Wish you luck. Any chance you can just swap out the tranny? zf15 is a great unit, at least in my experience. and 1.88/1 works well.

 

[Warren Holybee]

I would suggest looking into Terry's idea. Increasing diameter will add power (think towing or bashing into waves) but not much speed. Increased pitch will increase speed, but too much pitch risks cavitation.

 

[_Jason_]

Understood. Wish you luck. Any chance you can just swap out the tranny? zf15 is a great unit, at least in my experience. and 1.88/1 works well.

That is definitely an option. Was thinking a re-pitch might be more economical, or get a new prop and keep a spare.

 

[_Jason_]

I would suggest looking into Terry's idea. Increasing diameter will add power (think towing or bashing into waves) but not much speed. Increased pitch will increase speed, but too much pitch risks cavitation.

Good point about size vs pitch. I guess increase in size might help with thrust and slippage, but perhaps not as much at higher ends where those influences might taper. Thanks for all the feedback!

 

[_Jason_]

A summary of information garnered today for future forum reference. Had a helpful conversation with folks at Michigan Wheel today - they were very willing to offer guidance. They recommended 16x13 or a re-pitch to on the 16x11 to get as close to 13 as the shop would be willing. They advised to stick with size 16", as that was what came with the boat and there was likely intent there, and their algorithm agreed. Plus recommended gap between blade tip and hull at 15% of prop diameter, an increase to 17" would push that. Their philosophy was to run out of HP (~50hp) right around peak engine RPM (~4,000). Thus you are getting max load at max RPM. The speed variable would then come as a byproduct. You may get hull speed before max rpm, etc. I've heard elsewhere to shoot for hull speed at 85% of peak RPM in order to be in a good RPM range for normal operations. In any case, looks like we will attempt to re-pitch up to around 13 on forthcoming haul out and see where we land. I figure it will be another set of data points to compare with our 11" pitch. Then if we opt for a new prop (campbell or otherwise) we will have some real world data to use and a good spare. Hope this is helpful to someone down the road.

 

[terry_thatcher]

Glad the Michigan Wheel folks were helpful. Let us all know how it works out. Search for posts from Mark Pearson. He put in a Yanmar 45 with I think a 2.45 reduction. Put on a 16x14.