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Prop Size and Aperture

T

terry_thatcher

Terence 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.
 
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.
 
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.
 
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.
 
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.
 

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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.
 
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.
 
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
 

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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
 
Warren Holybee said:
What is the tip clearance Gerr recomends?
Click to expand...
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.
 

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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
 
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
 

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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
 

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

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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.
 
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.
 
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.
 
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.
 
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.
 
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?
 
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.
 
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
 
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.
 
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.
 
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.
 
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.
 
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.

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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.
 
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.
 
terry_thatcher said:
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.
Click to expand...
That is definitely an option. Was thinking a re-pitch might be more economical, or get a new prop and keep a spare.
 
Warren Holybee said:
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.
Click to expand...
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!
 
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.
 
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.
 
For those that might be interested and to document our findings. Following the above discussion, the shop repitched our 16" wheel from 11" to 12.5", as they weren't confident to go further based on age and condition. We definitely saw an improvement in speed at the lower RPMs as anticipated, however, it also introduced some cavitation around 2600-2800 rpms. I hypothesize this is due to slippage, as the propeller wants to progress further in the water, but the boat isn't allowing, as I notice an improvement with winds abaft. In any case, it's been performing well otherwise, with better performance at the lower RPMs but still room for improvement overall. Thus, after a long interesting discussion with Kelsey at West by North (Campbell), we opted to replace with a 17x11 Campbell prop on our upcoming haulout. This should give us more thrust with the size increase and performance improvements with the propeller type, as I understand a 16x12.5 Michigan wheel is approx. equivalent to a 16x10 or 17x9 Campbell. As mentioned earlier in this thread, the Perkins has the power to handle a bit of load, and with our trans gear ratio, there is quite a bit torque available at the shaft. After all the theory discussion, looking forward to see the outcome in practice.
 
I installed a Beta 35 with a new Max-Prop in 2023. We used a 16x12 prop, which works very well. Gets me cruising speed of 6.1 to,6.2 knots at 2100 rpm and top rpm of 2750, just below engine's max of 2800. A Perkins could probably handle more pitch-bigger engine. I just have always lived with some cavitation sound (a kind of roar).
 
I installed a 17" max prop last year. I forget what pitch I am at, but I did have to reduce it from what I started with due to cavitation. I still get a lot of cavitation, but I'm not sure it's any worse from the 16", as I am going over 7 kts now. I can just make 8 kts wide open. At 6 kts it's pretty comparable to what the 16" was at 6 kts.
I have a very different engine from the rest of you, a 50 HP Volvo md22. My top RPM is 3500 with 2.3:1 gearbox.
 
I suggest that we will write over here the engine type and shaft rpm to compare apple to apples.
I repower from Perkins to Beta 38. New gearbox ratio is 2.45.
After one year of using 16X11 Michigan I switched to 16X13 Federal which is I think the same as Michigan.
I can easy reach max rpm of the engine with about 7.5 knots of speed and terrible cavitation.
Cavitation starts abut 2500 rpm (1020 shaft) . I'm cruising at 2000 rpm (816 on shaft) at 5 knots.
New prop 16X13 performs better than 16X11 with similar cavitation rpm.
Gearbox ratio was chosen by Beta , I think should be higher.
Jason I'm interested how 17" prop will perform.
Terry, what transmission ratio do you have ? We have the same engine, yours has lower max rpm.
 
I was not impressed withe the standard transmission used by Beta. I got a ZF12M with a reduction 2.136. Installed a simple transmission cooler as well, which uses the flow from the Beta raw water pump. Speaking of which, what a stupid little pump. My old Perkins had a Sherwood pump. The impellers lasted probably well over 1,000 hours . The Beta ate up the impeller (and deposited two blades in the heat exchanger) in 500 or so. I know to be more watchful now.
 
I found that impellers used by Beta can not stand one season.
I was replacing them as fast as I was replacing anodes.
I bought from Defender Jabsco prop made in UK and will try it.
Jabsco has 10 blades and blades are softer than blades on Beta or Johnson impellers.
Beta, Johnson impellers have 6 blades.
 
Not sure if it works on a Beta, I removed the raw water impeller cover and the impeller itself on my Perkins and started the engine. This blasted the old rubber impeller pieces back from the exhaust heat exchanger and out the open pump. Of course you also have to remove the cooling heat exchanger end cap to get the rest of it.
This was when I first bought the boat, who knows how long all that had been blocking cooling flow tubes.
 
yurek said:
I found that impellers used by Beta can not stand one season.
I was replacing them as fast as I was replacing anodes.
I bought from Defender Jabsco prop made in UK and will try it.
Jabsco has 10 blades and blades are softer than blades on Beta or Johnson impellers.
Beta, Johnson impellers have 6 blades.
Click to expand...
Not sure I understand. Did you install a Jabsco impeller in your Johnson pump? BTW, Beta no longer uses Johnson pumps on the 35 hp. They found some company to make a similar pump that uses a Johnson impeller.

My anodes lasted longer in Mexican warm water than northern cold water. Makes no sense to me, but I was glad to replace the anode less frequently. In my home fresh waters (Columbia River) I use magnesium engine anode. In salt I use zinc. BoatZincs.com say aluminum engine anodes deteriorate even faster. In haven't tried aluminum.
 
I did not try Jabsco impeller yet, but I will do it. Beta, Johnson and Jabsco impellers have the same dimensions, I measured them with decent caliper.
The risk is that 10 blade impeller will not push water properly thru not matched diameters of inlet and and outlet holes in installed pump housing.
Jabsco has marking Made in UK, Beta and Johnson are not stating where their impellers are made. I thin this all is in the quality of the rubber.
On previous boat I had old Yanmar, and almost forgot what impeller is for.
 
I have 800 hrs on engine and I replaced 3 impellers. They all had cracks where the blades are attached to the body of the impeller.
 
There is an outfit, Globe Impellers, that makes impellers from silicon. They claim run dry survival of several minutes. Has anyone tried them? I just installed one on my engine( VP D2-40 ), will report if I encounter problems.
 
I am interested in your results. I have heard mixed reviews, ranging from very good, to no better than stock. Of course, I don't expect you to test it by running it dry for an extended period.

I have done well with impellers. I change it annually, or sometimes bi-annually if I forget. Year round sailing in California, so we don't have that maintenance season to get stuff done. During that time I almost always forget and run it dry for a few minutes before I realize what I have done, and never had any problems or cracked blades. It's probably due to the relative locations of the strainer and pump, and that the pump is never truly dry, just not pumping any water, when the through hull is closed.
 
yurek said:
I suggest that we will write over here the engine type and shaft rpm to compare apple to apples.
I repower from Perkins to Beta 38. New gearbox ratio is 2.45.
After one year of using 16X11 Michigan I switched to 16X13 Federal which is I think the same as Michigan.
I can easy reach max rpm of the engine with about 7.5 knots of speed and terrible cavitation.
Cavitation starts abut 2500 rpm (1020 shaft) . I'm cruising at 2000 rpm (816 on shaft) at 5 knots.
New prop 16X13 performs better than 16X11 with similar cavitation rpm.
Gearbox ratio was chosen by Beta , I think should be higher.
Jason I'm interested how 17" prop will perform.
Terry, what transmission ratio do you have ? We have the same engine, yours has lower max rpm.
Click to expand...
I just received the Campbell and will report back once installed and tested. We are scheduling a haul out to swap while we do the bottom. Debating further scope creep - cutlass, stuffing box packing, rudder...will be searching on here for info about dropping the rudder and servicing, was hoping to avoid, but may be time.
 
Dropping the Rudder isn't hard as long as the boat is blocked high enough. I think 14" under the keel is enough, which is more than you would normally do, but isn't unreasonably high. I can drop the rudder by myself in about 2 hours total. I don't think you can change the prop without dropping the rudder, so that might be an unavoidable task anyway.
The hardest part about the cutlass is getting the shaft coupling off the shaft. The Gudgeons probably need cleaned up, and a new bushing fabricated. You want to avoid direct contact between the stainless shaft and the gudgeons, or the bronze gudgeons will slowly dissolve away. An anode on the gudgeon is a good idea too.
 
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