Getting Screwed!

[nrx500]

I repowered my boat 2 years ago with a Beta 20 HP and installed a Max Prop. I learned that the new prop had to allow the engine to spin at it's maximum RPMS, for the Beta 20 that was 3600 RPMs. They installed the largest diameter 3 blade prop the aperture would accept and adjusted the pitch until the max RPMs hit 3600.

My question for you is will your engine turn its maximum RPMs (no more and no less) at full throttle with your current prop? If you run lower, you are over propped and need less pitch. If running higher, you are under propped and need more pitch.

A Max Prop will let you adjust pitch by changing screw inserts in the hub of the prop. Which is what I did to get my engine to turn 3600 RPMs at maximum throttle.

In your case, check your prop for fouling and damage. Is it the biggest diameter prop that will rotate in your aperture? Run the engine to max RPMs per your owner manual. If the prop is in good shape and you hit the max RPMs at full throttle, everything is as it should be. If you get cavitation before full throttle, something is wrong. It may be you're under propped or that not enough water is flowing to the prop blades in the aperture. You may want to ask other people with your model boat if they have the same cavitation issue.

[Lee Jerry]

Originally Posted by Renegde_Sailor

Fixed, I believe 19" and the number stamped on the side is 12-8 so I am assuming a 8" pitch? Prop is clean, was serviced and balanced last summer while the boat was out of the water.

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

Originally Posted by Renegde_Sailor

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

Based on this data:

Originally Posted by Renegde_Sailor

...right now at 2200 rpm where the engine runs real nice, she will cruise at 6 knots.

Originally Posted by Renegde_Sailor

...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 can say with pretty high confidence that the pitch is NOT 8"- the slip would be nearly zero. 12" pitch is a possibility. (A little higher wouldn't surprise me, like 14".)

As mentioned above, boats slow down in a seaway. All boats. The waves add drag. Exactly how much depends on height and period (steepness). So it's possible your expectations are just too high (in terms of speed in those conditions, but I don't think you quantified the size of the waves). So again mentioned above, you could try taking them on an angle (effectively lengthening the wave and reducing steepness). It's my experience that when heading into "big" waves, speed is often voluntarily reduced from what the engine is capable of (either by throttling back or changing heading) for the comfort of crew and the vessel (reduced slamming, for example).

I'm not sure what you're hearing. It could be cavitation. Or maybe just the waves coming off the transom. I can't say "from here."

If you did want to change the prop, you really should figure out first what size it is for certain. (The prop shop should be able to measure this, if they didn't already.) If you do, you have a starting point for the new one, otherwise you're essentially starting blind (which a prop shop can also do, but why not start with the best info available?). You'll also want to know, which has been asked several times, what the RPM is at WOT (in calm water).

You mentioned hitting 7.2 kt (which I already deleted from the quotes and I'm not going back for it ) but it's not clear if this was max speed or if you didn't want to push the engine harder. Or at what RPM it was at. Hull speed is about 8 kt for your hull, and I'd expect you to be able to get a little closer to that. But maybe the engine is a little small on the 1960s boat? You should be able to run a diesel at WOT without worry, but it does often sound scary. (It's actually recommended to do it occasionally for the engine.)


Edit: RPM should be the actual RPM, not just the gauge reading. So if you haven't "calibrated" the gauge recently, put an optical tach on the engine (and then you'll know the calibration going forward).

[Renegde_Sailor]

Originally Posted by nrx500

I repowered my boat 2 years ago with a Beta 20 HP and installed a Max Prop. I learned that the new prop had to allow the engine to spin at it's maximum RPMS, for the Beta 20 that was 3600 RPMs. They installed the largest diameter 3 blade prop the aperture would accept and adjusted the pitch until the max RPMs hit 3600.

My question for you is will your engine turn its maximum RPMs (no more and no less) at full throttle with your current prop? If you run lower, you are over propped and need less pitch. If running higher, you are under propped and need more pitch.

A Max Prop will let you adjust pitch by changing screw inserts in the hub of the prop. Which is what I did to get my engine to turn 3600 RPMs at maximum throttle.

In your case, check your prop for fouling and damage. Is it the biggest diameter prop that will rotate in your aperture? Run the engine to max RPMs per your owner manual. If the prop is in good shape and you hit the max RPMs at full throttle, everything is as it should be. If you get cavitation before full throttle, something is wrong. It may be you're under propped or that not enough water is flowing to the prop blades in the aperture. You may want to ask other people with your model boat if they have the same cavitation issue.

I have seriously been considering going the Max Prop route, everyone who I have run into who has them seems to love them!


Right now it is just a mater of $$$ as I am fixing to start buying sails and rigging, and re-rigging her a bit to make her easier to single hand.

[Renegde_Sailor]

Originally Posted by Lee Jerry

Based on this data:
I can say with pretty high confidence that the pitch is NOT 8"- the slip would be nearly zero. 12" pitch is a possibility. (A little higher wouldn't surprise me, like 14".)

As mentioned above, boats slow down in a seaway. All boats. The waves add drag. Exactly how much depends on height and period (steepness). So it's possible your expectations are just too high (in terms of speed in those conditions, but I don't think you quantified the size of the waves). So again mentioned above, you could try taking them on an angle (effectively lengthening the wave and reducing steepness). It's my experience that when heading into "big" waves, speed is often voluntarily reduced from what the engine is capable of (either by throttling back or changing heading) for the comfort of crew and the vessel (reduced slamming, for example).

I'm not sure what you're hearing. It could be cavitation. Or maybe just the waves coming off the transom. I can't say "from here."

If you did want to change the prop, you really should figure out first what size it is for certain. (The prop shop should be able to measure this, if they didn't already.) If you do, you have a starting point for the new one, otherwise you're essentially starting blind (which a prop shop can also do, but why not start with the best info available?). You'll also want to know, which has been asked several times, what the RPM is at WOT (in calm water).

You mentioned hitting 7.2 kt (which I already deleted from the quotes and I'm not going back for it ) but it's not clear if this was max speed or if you didn't want to push the engine harder. Or at what RPM it was at. Hull speed is about 8 kt for your hull, and I'd expect you to be able to get a little closer to that. But maybe the engine is a little small on the 1960s boat? You should be able to run a diesel at WOT without worry, but it does often sound scary. (It's actually recommended to do it occasionally for the engine.)


Edit: RPM should be the actual RPM, not just the gauge reading. So if you haven't "calibrated" the gauge recently, put an optical tach on the engine (and then you'll know the calibration going forward).

Part of the deal is the last boat I was driving before this one was a 30' Boston Whaler with counter rotating 250's on it, it was a lot of fun but when you got into chop you had to cut the throttles to keep from yeeting it through the air!



This sailboat had a terrible bottom on it when I bought it, a home brew copper coat job that was peeling and creating a ton of drag, I only got it back in the water with the fresh bottom on it back in August, so I have still been adjusting my expectations of her, I went from being able to chug along under motor at 5 knots to now being able to pull 6.5-7 pretty regular.

And under sail she has picked up some amazing manners with the bottom that is more fair.



It is probably just not knowing what to expect, except that under sail the loss of speed was not as apparent.

The chop I was running through were 5'ers on a 5-7 second period. Real close and steep.

[skipperpete]

A bit of propeller trivia from the Thai /Malaysia border, here the propellers are described as “rabbits” or “elephants” so if you’re ordering a prop on the phone, diameter/ pitch and shaft size don’t fully describe what might arrive.
There’s not too much difference between those 2 props other than blade area

[Renegde_Sailor]

Originally Posted by skipperpete

A bit of propeller trivia from the Thai /Malaysia border, here the propellers are described as “rabbits” or “elephants” so if you’re ordering a prop on the phone, diameter/ pitch and shaft size don’t fully describe what might arrive.
There’s not too much difference between those 2 props other than blade area

As with anything in sailing, once you scratch the surface it is amazing how much there is to it to learn!

[Renegde_Sailor]

Diving back into this thread, I am back in WA now and have the boat hauled, had intended to sail south last fall but the shaft alley started leaking as the bronze has gone pink.

This now leaves me with a bit of a conundrum...



The engine is a 36.8 KW or 50HP standard, 41.2 KW/55HP max.

I ran the calculation at https://retmarine.com/propeler-shaft-size-calculator/

Which returned a safety factor of 6.384, to which it says anywhere from 5-8 is appropriate that is assuming my shaft is a 70KSI shaft.

So I am assuming that my shaft size is ok however, if I want to upgrade my propellor I am not sure.

Lets start with the calculations using the VicProp calculator: https://vicprop.com/displacement_size_new.php









The current prop is a 16" Diameter and is stamped 12.8 on the hub, so I am assuming it is a 12.8 pitch.

So currently when I am running about 2400 RPM at max speed, that makes a lot of sense.

What I am considering is going to a 4 blade PIY Max Prop and setting it at about a 10 pitch as the calculator suggests.

What I am up against here are a couple of thoughts:

1. I am adding a 400A alternator to my engine to charge the 1320AH house bank and 200AH windlass battery.
2. I may consider going with a larger engine when it comes time to replace this one although currently it has less than 4000 hours on it and runs like a sewing machine.
3. If I were to go to a larger engine in the future a 1" shaft would no longer be within that safety factor and I would be looking to move to probably a 1 1/4" shaft.
4. Going to a larger engine of 80hp with a 2.5:1 gearbox, it would recommend a 17.6" 4 blade prop at 11.8" which would require a bit of clearancing on the rudder and installing the new shaft tube about an inch lower which can be achieved by throwing about 1.9 degrees more angle on the engine as right now it sits fairly level.

It is a tough call because I am doing the balancing act between not wanting to spend money unnecessarily and future proofing my boat.

I worry that with the 4 blade prop that the 1" shaft will be more likely to snap, although I do plan on running a line cutter and the prop is very well protected where it is.



I did measure from the tips of the blade to the top and bottom of the aperture...



As you can see it is ~1 7/8" on the top, 1 3/8" on the bottom.

Being a 16" prop now if I wanted to go to an 18 I could simply cut out the bottom of the aperture in the rudder and add another 2" of clearance there, then move the shaft down an inch which would center the prop back in the opening.

That is my conundrum, do I bother with this work, or do I stick with the 1" shaft, knowing it will limit the boat to 55hp, or do I do the bit of extra work and wind up able to run the larger engine at a future date and have the stronger shaft?

Considering right now with a clean bottom at about 2500 rpm, she will do 7.2 knots but wind her any faster than that and she cavitates like crazy.

Additionally my bigger concern is when running against waves and current, she is prone to cavitating when you throw more power at her.

The Ideal would be a bigger slower turning prop, but it will take quite a bit of money, time and effort to get there.

I would love to hear your thoughts if you have been through this or have some insight.

[rslifkin]

With a 6+ safety factor I wouldn't be concerned about the shaft strength. Plenty of powerboats have lower safety factors without issue.

[Renegde_Sailor]

I have been putting a lot of thought into it over the last 24 hours, I am thinking the right move is going to be to see what is available shaft and prop wise on some boats that are fixing to get crushed over here, if I can find a 1 1/4" shaft and prop on them that I can steal off one of them and cut down to work on mine I will, otherwise I will stick with the 1" shaft and call it good.

[Statistical]

Going to a larger engine of 80hp with a 2.5:1 gearbox, it would recommend a 17.6" 4 blade prop at 11.8" which would require a bit of clearancing on the rudder and installing the new shaft tube about an inch lower which can be achieved by throwing about 1.9 degrees more angle on the engine as right now it sits fairly level.

That seems like a lot of work and complexity. Just increase the pitch. Yes higher diameter is more efficient than more pitch but we all operate within design constraints. If we didn't every sailboat would be using a 32" 2 blade folding prop running at 600 rpm coupled to a 6:1 transmission.

[Renegde_Sailor]

Originally Posted by Statistical

That seems like a lot of work and complexity. Just increase the pitch. Yes higher diameter is more efficient than more pitch but we all operate within design constraints. If we didn't every sailboat would be using a 32" 2 blade folding prop running at 600 rpm coupled to a 6:1 transmission.

I mean if money was no object I would be refitting to a nice diesel electric operation where the engine's primary function became charging batteries with the motor optimized to be peak torque at my ideal cruising RPM...

But that takes me back to a saying my grandpa used to throw at me about wishing in one hand... 🤣

[Lepke]

Seems like a lot of expense for 1 knot. A 4 blade prop will increase the drag sailing. Your keel adds a lot of drag for the hull speed equation that seems to work for power boats.
If going faster is important, why not buy a longer boat. My hull speed is 12.2 and before looking for a new boat knew I wanted to cruise at an easy 10 knots. That set the length before I started looking.

[Renegde_Sailor]

Originally Posted by Lepke

Seems like a lot of expense for 1 knot. A 4 blade prop will increase the drag sailing. Your keel adds a lot of drag for the hull speed equation that seems to work for power boats.
If going faster is important, why not buy a longer boat. My hull speed is 12.2 and before looking for a new boat knew I wanted to cruise at an easy 10 knots. That set the length before I started looking.

Well there are a couple things there, one way or another I have to do the shaft alley because the bronze has turned pink and is leaking, so it is going to get bored out and replaced with a piece of G-10 that will get epoxied back in removing the metal from the equation.

The top speed of the boat is good, mainly what concerns me is more torque when we get into heavier seas as right now it slows me way down, and wit that when I go to a 4 blade I will be going with a PIY MaxProp so the drag will not be an issue.


I would love to get a bigger boat, but right now it is not financially in the cards, I want to get the work to this one finished where I am happy with it and enjoy it for a while after having put so much time and money into rebuilding her.

Well that and I just love how this boat sails, she is the finest sailing boat I have ever been on and I am quite looking forward to getting the roller reefing headsail and all that I am doing to her done up which will make her even more of a pleasure to operate.

[skipperpete]

Yanmar differs a little with your propeller and shaft size but due to your aperture, there are few options. The Yanmar installation manual suggests a diameter of 440 mm- 460mm and a pitch of 250mm for a 3 blade prop to achieve 8 knots.
About the shaft size, if the material is SUS304 they suggest a 28.58 mm shaft and for SUS630, a 22.67 mm diameter shaft (30 and 25mm)so if you have the high strength 17 alloy I think you’re ok.
I’ll post a screen shot of the Yanmar prop sizing later.
On the maxprop, excellent choice but again , the aperture size can be a problem as the blades rotate, I had to cut a bit out of my boat to install a maxprop classic because of this.

[Jim Cate]

Do remember that Maxprops (and all other conventional feathering props) are somewhat less efficient than fixed props, due to their flat (no twist) blade shape. This means a bit less thrust for a given shaft horsepower. If you are actually HP limited now, it could be worse with the new prop.

Jim