prop selection for heavy 37 foot sail

[doog]

Ive seen boats with similar design and engines with much smaller and steeper props. I have an 18 by 18 inch three blade and perkins 4.108 power with 2.8 to one hurth gear. 37 foot boat at 23K lbs.
Im very pleased to get under two gallons per hour fuel consumption, at around 1800 to 2000 rpm. But on doing the math I find the rule that says at 1000 RPM (prop) one inch of pitch equals one knot of speed and a slip of 50% is to be expected. So half of 18 is nine and I dont get 9Kts of hull speed. And my prop turns about 500 to 700 RPM.
So I have a 16/16 four blade available and with this the rule says 8 kts with a 50% slip is to be expected at 1000 rpm And maybe this prop would actually be working better with less slip and load the engine less than the steeper and bigger prop I have installed.
The original was simply purchased because it fit the shaft and it was a very good deal. I had no guidelines. Just guessed.
And since I see smaller props with less pitch even than 16..similar powerplants and boats, Im considering the downsize since Im out of the water and can do it.
Thanks so much for the voice of experience on this cool forum!
If anyone can share their prop speed and powerplants and hull speeds it would be very informative in this choice!
If anyone has a reference chart that would be awesome.

[doog]

https://www.rbbi.com/folders/prop/propcalc.htm

This calculator tells me an 18/18 prop is just right so maybe the 16 pitch is too steep.

Glad I ran this calculator.

[TrentePieds]

You are doing just fine :-)

I take it your boat is something like a Tayana 37 with a 36 foot LWL. Your theoretical "top speed" (Hull Speed) is 8 knots on FLAT water. You have a 10 ton boat, with a 50-horse engine. Generally, we spec required power at 4 hp per ton for operation in all weathers and waters.

Check the power curve for your engine at your operational RPM to see what HP you can expect at that RPM - it won't be 50 :-)! Your operational RPM is only half of the engines Max, so you are well away. You might want to check that at WOT ("Wide Open Throttle") you get something like 3,300 or 3,500 RPM on the clock. If you do, Bob's yer uncle. If you can't get above 2,800 RPM or so at WOT you are probably over-propped, but we can come back to that once you've actually checked your RPM. Being "over-propped" is something you can live with till your next scheduled haul-out :-)

I doubt that you have any real idea of what your ACTUAL speed over the ground (SOG) is. You instrumentation will only mislead you. You need to do a speed trial over a measured distance to get to know it. When you've collected the data from multiple runs of the "measure mile" (or any other accurately known distance) and you done the TDS calculation you'll know your SOG with as much precision as modern science can contrive.

Your STW ("Speed Through the Water is less important. If you really want to know it, make yourself a chip log :-)!

Propeller calculations are always approximations only. Another "close enuff" factor is introduced by whether you have a clean bottom or not, and that -obviously - changes over time.

So don't place to much reliance of what you see, or hear about, what other people do. Since you ran the calculator and the answer you got is "close enuff for guvmint work" just let it rest there and enjoy your sailing :-).

Bonne chance!

TrentePieds

[doog]

Yep, Thanks, Maybe over propped but its working for me. Although I would like to know what similar boats are using, Valiant, Hans or Westsail.

Im out now and just wanted to look into it.

PS. according to a chat on Sailnet I am WAY overpropped! I like the great fuel economy but I think with the right pitch and diameter I will do just as well. Yeah, I dont want to go by other's applications but I do want to know what the other guys are doing and how its working for them.

Still very interested in input. 2.8/1 gear.

[doog]

"4108 in the 20,000 # displacement Gulfstar 37 is deliberately overpropped (3 blade 15 diameter x 12 pitch)" Previous thread info.

So from this discussion, I'd say my 18-18 is way overpropped! Ive got a 16-16 available and still on the high side but probably better all around.

[NYSail]

A prop specialist can take all the numbers and tell you exactly what you need. On my 43 foot / 23,000lb boat with a 53 hp engine I burn less than 1 gallon /hr at 2,400rpm. Fixed three blade prop

[Searles]

Heavy boats need high reduction g/boxes large diameter props ,low pitch ,plus low disc area ratio ,there is no ideal as drag under sail must be also considered ,most 4108 are happy at about 3 to 3.5 k rpm .⛵️⚓️

[GordMay]

You might be interested in this free download:
“The Propeller Handbook” ~ by Dave Gerr
The Complete Reference for Choosing, Installing, and Understanding Boat Propellers
Free ➥ http://kashti.ir/files/ENBOOKS/Propeller%20Handbook.pdf

[doog]

Originally Posted by NYSail

A prop specialist can take all the numbers and tell you exactly what you need. On my 43 foot / 23,000lb boat with a 53 hp engine I burn less than 1 gallon /hr at 2,400rpm. Fixed three blade prop

Wow NY, could you share what your gear ratio is and the engine you have. And also your gear ratio. Amazing claim but you leave out the important details.
And you must have a very well designed and maintained vessel.

And of course your waterline length is also in your favor.

I hope that you know these details and can share if you get this reply.

Thanks so much..And yes, three blade is best. Id rather not go to the four blade.

[doog]

Originally Posted by Searles

Heavy boats need high reduction g/boxes large diameter props ,low pitch ,plus low disc area ratio ,there is no ideal as drag under sail must be also considered ,most 4108 are happy at about 3 to 3.5 k rpm .⛵️⚓️

Indeed the slower the prop turns and the larger the diameter, the more efficient. And Im finding many installations are limited to diameter.. 12 to 16 inches. Then with a pitch of ten or twelve they must have a higher shaft RPM.
The book (below) gives all the engineering math which suggests ideal reduction gear and estimates speeds with pitch and displacement, but this is a story problem for the math guys. I own what I own and just want help choosing a prop, and Im doing well with my 18-18 but find this is steeper than 90% of the others I hear about.

And FYI the 4108 is very happy to cruise at far less than 3000 RPM. myself and many others run it at around 2000 rpm and we know it can carbon up and take time to run it higher or clean with a treatment, periodically, but its an old diesel and happy to just chug for hours as they do in gensets and refrigeration applications literally for YEARS...

Thanks for the input.

[FPNC]

As mentioned before. The question is what is the maximum RPM you can achieve with the current propeller, while underway, on a calm sea, with a fully laden vessel? If you can turn 3000, your fine. If not you are over propped.

Your running a relatively deep gear. which is why you are turning a square propeller. Nothing wrong with that.

You could turn up to 20"-21" diameter with that deep ratio. The larger diameter would give better efficiency.

[doog]

Thanks FPNC!
and yes, I love the deep gear ratio and aspire to efficiency.

I dont have room for 20" but perhaps 19..

Ive never accelerated till the engine would no longer increase speed which would be up around 3000 rpm because its just too much thrashing around the prop and transom. The boat accelerates nicely around 2500 and I dont wish to operate the engine faster than this.

Now I find myself asking if props increase by two inch increments, or if one can find the dimeters in odd dimensions..

[doog]

Okay I found a really good prop calculator program and feel that with 18-18 Im currently over propped and that's fine. I like low rpm, efficient operation and am also over powered so no need for a steeper pitch.
Above commenter was right< Im just fine due to the deep reduction and no need to shop for props. And a 16 or 17 inch pitch would be just fine...

[sailcrazy]

We gave up trying to "guesstimate" the best prop, and installed an Autoprop. It changes blade angle based on boat speed thru the water, torq, RPM, etc, and continually adjusts to balance load vs torq. In addition, gives better speed & less fuel burn. We've had ours since 2001, have rebuilt it 2x, and happy. Also....less propwalk in Rvs.

[Nicholson58]

We are on the other end of this. Heavy boat, 40 tons, 58 feet. 115 hp 6-cylinder tractor engine. 2:1 reduction, 24 inch 3-blade with a Hundested variable pitch transmission. We run a prop speed of 350 to 450.

I have had to re-prop many smaller engines and outboards to optimize performance to use. Changing from 9.25 down to 8 pitch SS made a world of difference on the dinghy. I highly recommend using several prop calculators and the assistance of the prop techies. The things are too expensive to get it wrong.