25D Repower - Prop Sizing

[Dick Barthel]

Dream Weaver is having a Betamarine 13.5 h.p. installed in January.

From our archives I've discovered that Campbell and Michigan Wheel are the manufacturers of choice for a propeller. I also learned that the last 25Ds came with larger Westerbeke 10-2 rather than the 1 GM 6.5 (3400 rpm)/7.5(3600 rpm) which DW had. DW had a 12x9 two blade from Michigan.

John V and others seemed to think that the "right" propeller if you go with Campbell is the three blade 12X7.

My question is would the additional h.p. of the Beta require even less pitch? I guess the good news is that changing the pitch an inch or two is not that dear - $40 according to John (a few years back). With engines ranging from 6.5 h.p to 13.5 h.p, I suppose there could be quite a difference in the suitable pitch.

I have sent prop sizing info sheets to both North by East Enterprises (Campbell) and Michigan Wheel and I'll also hear from my mechanic as well. I will pass all of this info on to the board.

Thanks to all for your thoughts and ideas. Happy Holidays.

Dick

I hope to get some pictures of the installation but I'm on a jury the first part of January.

[rtbates]

That's quite a nice increase in power. Keep us abreast of the details, please.

[John Vigor]

John V and others seemed to think that the "right" propeller if you go with Campbell is the three blade 12X7.

No, no. I have always recommended a three-blade 12 x 9, and I think you should try that first with the new engine, also.

John V.

[Dick Barthel]

Of course you are correct John. You told us you had reduced from a 12x11 to 12x9 with excellent results. It was a Jeff G (guest) who got a recommendation from Campbell to use a 12 x 7. Sorry I misquoted you but at least you're not running for President.

I just heard from Campbell this morning and they are recommending 11x6! They invite me to call to discuss it which I will do after the holidays. I can only guess it’s because of all the extra power and their blade design. I'll listen to what they have to say and put it on the board.

You have lots of sailing experience in general and with the 25D in particular so I will be certainly telling them what you think. Thanks John.

Calder warns prop sizing is as much art as science. The concepts are easier to understand when you have a real life decision to make. It is still not completely clear to me how more hp and prop size/pitch tie together. But I'm going to try and understand it before I'm finished.

Dick

“I use not only the brain I have but all I can borrow."
--Woodrow Wilson--

[Kurt]

I sail a CD27 which has the Beta 13.5hp engine. I experimented with a used 12x8 3 blade and a used 12x11 2 blade. I settled on a new Michigan Wheel 12x9. When I talked to the Campbell folks....there calculations seemed like witchcraft to me...I wouldn't trust them. Campbell told their 12x6 3 blade is equal to Michigan's 12x9 3 blade....I don't believe that's possible.

[John Vigor]

Kurt, I agree with you. I think the best prop for this set-up is the plain vanilla Michigan RH 3-blade 12 x 9. Campbell's withcraft might spring from the fact they they make cupped props, which don't offer much benefit for the speeds at which sailboats move. But because they deliver more thrust, they need to be of smaller diameter than plain vanilla props. Because of their smaller profile, cupped props do offer slightly less resistance under sail, it's true, but the difference is minimal. You'd lose just as much time from a couple of bad windshifts or a little inattention at the helm when the skipper accidentally spills his rum.

Stick with a non-fancy Michigan and have yourself a merry Christmas and a trouble-free New Year.

(Truth in advertising declaration: I have no shares in Michigan and they aren't paying me for this unsolicited endorsement, though if they had any sense they would.)

John Vigor
CD27 "Sangoma"
Bellingham, WA

[Dick Barthel]

John/Kurt:

[Warning: This post can induce sleep...please don't operate heavy machinery, etc. while reading it.]

I haven’t heard from Michigan Wheel yet but with John’s recommendation and Kurt’s actual success with a Beta in a 27 that seems to be the way to go. But since DW doesn’t need a prop until May I might as well fully explore the facts and circumstances. Others on our board have raved about how happy they are with their Campbell Sailors. Anyhow, there’s not much else to do sailing-wise in Connecticut this time of year.

I’ve come across something that might explain a two inch difference in recommended pitch between the 1GM and the Beta and that’s the reduction gear. The reduction gear ratio of the Kanzaki KM2A which came with the 1 GM is 2.62 to 1 while the TMC 40 that comes with the Beta BZ482 is 2:1. Both engines have similar maximum rpms but obviously the BZ482 is spinning the shaft considerably faster at the same hp.

I’m using Calder’s Cruising Handbook and his propeller calculations which he borrowed from David Gerr’s Propeller Handbook. Briefly the math:

Determining Pitch

1. You start with your desired hull speed (just less than the maximum theoretical hull speed, say 95%...the idea is you burn a lot of fuel getting the last 5% when motoring because the stern squats in the bow wave).

CD 25D: 1 GM & Beta both 5.5 kts = 557.15 ft per minute. (1 kt = 101.3 ft per minute x 5.5)

CD27: 7.5 kts = 101.3 x 5.7 = 577.41 ft per minute

2. Maximum shaft rotation:

1GM = 3600/2.62 = 1374 rpm
Beta = 3600/2= 1800 rpm

3. Provide cushion for adverse conditions:

1 GM = 1374 x 95% = 1305 rpm
Beta = 1800 x 95% = 1710 rpm

4. Determine pitch without slip:

1 GM = 557.15 ft divided by 1305 rpm = .4269 ft = 5.12 inches
25D Beta= 557.15 ft divided by 1710 rpm = .3258 ft = 3.9 inches
27 Beta= 577.41 divided by 1710 rpm = .3376 ft = 4.052 inches

5. Determine “real world” pitch by factoring slip (assuming 55-60 percent effectiveness for the Cape Dory hull shape and heavy displacement or 57.5%):

1 GM = 5.12 inches divided by 57.5% = 8.9 inches of pitch rounded to 9
25 D Beta = 3.9 inches divided by 57.5% = 6.78 inches of pitch rounded to 7
27 Beta = 4.052 inches divided by 57.5% = 7.04 inches of pitch rounded to 7

You can see that mathematically the big difference is the gear reduction ratio.

John teaches us that changing the pitch a few inches is very doable and pretty cheap so I suppose going with a 12 x 7, 12 x 8 or 12 x 9 wouldn’t preclude finding your boat’s maximum performance.

I’ll post again after I hear from Michigan Wheel, Campbell and our mechanic at the boat yard. It does seem that a 12 inch diameter and three blades is a no brainer.

Merry Christmas,

Dick

[Dick Kobayashi]

This is an interesting discussion. I have a 25D with the 1 GM and original two blade prop. I have been having trouble with the engine for the last two years. Starts fine but rpms now max out at 2700 under load. This enough to get the boat to almost hull speed in calm conditions. I have been advised to have the head pulled with a view to examining the valves, etc. I am contemplating doing this and at the same time making the switch in prop that John Vigor recommends. Together, based on a close reading of the archieved posts on this mb, this would represent a real increase in effective power at a modest expense compared to engine replacement.

Any comments or suggestions on this strategy are most welcome.

[Dick Kobayashi]

This is an interesting discussion. I have a 25D with the 1 GM and original two blade prop. I have been having trouble with the engine for the last two years. Starts fine but rpms now max out at 2700 under load. This enough to get the boat to almost hull speed in calm conditions. I have been advised to have the head pulled with a view to examining the valves, etc. I am contemplating doing this and at the same time making the switch in prop that John Vigor recommends. Together, based on a close reading of the archieved posts on this mb, this would represent a real increase in effective power at a modest expense compared to engine replacement.

Any comments or suggestions on this strategy are most welcome.

[bobdugan]

This is an interesting discussion. I have a 25D with the 1 GM and original two blade prop. I have been having trouble with the engine for the last two years. Starts fine but rpms now max out at 2700 under load. This enough to get the boat to almost hull speed in calm conditions. I have been advised to have the head pulled with a view to examining the valves, etc. I am contemplating doing this and at the same time making the switch in prop that John Vigor recommends. Together, based on a close reading of the archieved posts on this mb, this would represent a real increase in effective power at a modest expense compared to engine replacement.

Any comments or suggestions on this strategy are most welcome.

Dick I had a problem like this last year. It turned out that the primary fuel filter was clogged.

Bob

[Dick Barthel]

Update:

I just heard from Michigan Propeller and they recommend starting out with the original 12 x 9 two-blade to make a performance assessment. I'm not sure I am comfortable staying with a two-blade when the horse power is being doubled. I should be talking with the mechanic doing the install in next few weeks.

Dick

[Clay Stalker]

Hi Dick:

I don't know anyone sailing our kinds of boats that doesn't have a 3-blader going....successfully.....course, that depends on a few things...none of us can back up very well...and we can be slow...but we do go ahead pretty well...

CS

[Al Levesque]

This discussion has has been very informative since I had already ordered a new Michigan prop to go with our new 3YM30 in our CD33. In the calculation of pitch I did not see mention of prop diameter, nor of number of blades, nor of style of blade. I assume these all relate to the slippage. One rule of thumb had been that an inch of diameter equals an inch of pitch. I have not seen rules related to number of blades nor to style of blade.

I simply ordered on the recommendation of our diesel mechanic and will wait to see how lucky I am. I stayed with the three bladed sailor style that came with the boat. The increase in pitch was guesswork since the engine maxed out with the old prop. Thanks for the extra guidance.

[Dick Barthel]

Al,

Al,

Again, referring to Calder's Cruising Guide, chapter 5, after you've decided on engine size, he calculates pitch, then diameter, and then blade area. He gives a formula for determining diameter but because it’s fairly complicated he also provides a graph which is meant to be used if you are using a standard three-bladed propeller. A most important measure is the size of the aperture. I’ve consistently seen 10-15% of the diameter of the propeller as minimum clearance between the tips of the blades and the boat.

First you start with your maximum shaft speed which you determined in arriving at the proper pitch (see my earlier post).

Next you determine “maximum shaft horsepower.” He started with the maximum engine horsepower and multiplied by 95% to take into account power loss for the gear box. In his case he then reduced that hp a few more for a high output alternator. He also mentioned a refrigeration compressor as another reason to reduce it some more. You would look at the horsepower draw for any auxiliary purposes. In his case he started with a 50HP yanmar and ended up with a shaft horsepower of 44. In his case he had an also already computed a maximum shaft rpm of 1374. You take those two numbers to the graph and it indicates a 17.7 inch diameter which he rounded up to 18.

For me the graph did indicate a 12 inch diameter blade.

Al, if you let me know your maximum shaft speed and your estimate of your maximum shaft horsepower, I would be happy to look up the indicated diameter for your situation on the graph (assuming you have a three bladed propeller).

He ends with an analysis of blade area which determines “blade loading.” In my case I’m going to need to study this part some more because I thought for sure I would be using a standard three-bladed propeller but now Michigan Propeller is suggesting that I at least start with my old two- bladed 12x9 which would be much less blade area. “The higher the engine output and the lower the blade area, the higher is the blade loading.” Too much blade loading can result in cavitation which can damage things.

You could just let the yard mechanic take care of all this but what fun would that be.

Dick

Felix qui potuit rerum cognoscere causas …Happy is he who gets to know the reason for things. (Motto of Churchill College, Cambridge)

[Al Levesque]

The engine data is all in the boat and I won't be getting to it until early Spring so I won't be able to take advantage of your offer. Since the prop is already in process I will get to test it at launch time. If it doesn't work as expected I will have to adjust from there.