Prop size question

[Vancefish]

My 1977 Peterson 34, came with a 14" Martec two blade folding prop. It's in terrible shape, pits, floppy pin, a hole in one blade.

I don't plan to "Race" this boat. Actually I hope to Coastal cruise her from San Francisco to Panama, through the Canal and ???..

So people in my Marina (who seem to know a lot) have convinced me that a 3 blade solid is the way to go.

The pics are the boat's dimensions, measurements etc etc..

For fun I entered wanting to go 8 knots!

I have a 12hp 2 cylinder Universal (Kaboda) diesel reported to have under 100 hours on the rebuilt.

So the Calculator says basically I want a 13" with 7.5 pitch..

Would I be okay with a 14" - 9? (Also pictured). What would be the negatives?

[mlydon]

NO!!!

first number is diameter, second is pitch. If you upsize one, you need to downsize the other, otherwise you will be overpitching (bogging down your motor).
Look at it this way - if you have a 13" column of water, and you move it 7.5" per turn, that will take a certain amount of force from the engine. if you now try to move the same 13" column 9" per turn, that takes more force, correct?

Same goes for diameter - more force to move a wider column the same distance.

Moving a wider column a shorter distance, or a narrower column a longer distance is roughly a wash.

There is a LOT of black magic around properly pitching, which basically amounts to pitch the prop such that you get hull speed at ~80% of max continuous rpm. if you can't REACH max continuous, you are over-propped (black smoke)

add to that that higher diameter lower pitch is more efficient (high aspect ratio) while under power, but a bigger drag when stopped - you can see how it gets very complicated.

I would suggest you look into campell sailor props for fixed pitch - expensive, but cupped, smaller blade area, more efficient for cruising, while still being simple.

they are sold by westbynorthwest.com. Discuss with them the parameters of your boat, engine, etc. and have them suggest a size.

Matt

[DougR]

I ran your boats data thru my prop program and came up with recommendations that are, in general, about an inch more in diameter than what you have. This makes sense, as the recommendations that you have are based on a stated rpm/power reduction of 10%, while mine are based on 100% rpm and power.

My program recommends a 14x 8 X 3.

The 14x9x3 that you show is too much prop to allow your engine to achieve full rpm. It would require about 15 h.p. to achieve 3000 rpm with that prop.....25% more power than you have. You could, of course, have an inch of pitch taken out of the blades, but you might find that the cost of the prop plus the reworking doesn't make financial sense. Better to start with a new prop of the correct size.

DougR

[Pete7]

Originally Posted by Vancefish

So people in my Marina (who seem to know a lot) have convinced me that a 3 blade solid is the way to go.

Bet they do, you are probably way faster than them

You have a nice sleek hull and fin keel, be a shame to put a great big 3 bladed prop on it.

How about having the prop rebuilt or replace with another folder second hand?

Pete

[JimsCAL]

Martec is still in business and reconditions their props. May have an issue with the blade with the hole, but they may have spares. I've done it and they do a great job.

Unless you were unhappy with your speed under power, I would not go with a fixed 3-bladed prop. Maybe a fixed 2-blade if cost is a big issue.

[Vancefish]

Okay, thanks all!

None of the people recommending a three blade are racers. I do appreciate a good performer though. Growing up racing 505s, I really prefer a boat with a lot of close to wind capability. As well as a fast turn.

However as I said. I don't really plan to race her either! Instead I plan to sail it away (Far from California!! [emoji1787]).

How many people will recommend a two blade folding Martec (for a cruising boat) over a solid three blade? Keeping reverse performance in mind as well.

There are always used 2 blades on Ebay.

[DougR]

Rethinking this a little bit, I think I would come up with a different recommendation based on the following:

- the boat is considerably underpowered- It's normally a 5 ton boat, and in cruising trim it may weigh a good bit more. A realistic amount of power for a cruising boat should be 3.5 to 4 h.p. per ton, and that means 20 h.p. However you only have 12 h.p., and that's not surprising for a go fast Doug Peterson design that was only given enough power to get out to the race course and back.

- Two bladed propellers are always more efficient than three bladed props if there is enough room under the boat to fit the proper diameter two blade, and your boat has enough room for a properly sized two blade. If you had an engine with 35 or 40 h.p., that might not be the case, and a three blade would be recommended, but not here...

- If your budget is no consideration, the best prop for you would be a modern design two blade folding prop, such as a Flex-o-Fold or a Volvo. These props give very good thrust and efficiency in forward, with no cavitation or thrust breakdown, and also good reverse power. When sailing they offer low drag and almost no tendency to snag lines or weed. Unfortunately while the Martec that you have gives low drag while sailing, there is very little reverse thrust.

- If budget considerations do come into the picture, the second best option is a properly sized two blade fixed prop. This will give you very good performance under power, with better efficiency and thrust than a three blade fixed prop. It will also give you significantly less drag while sailing than a fixed three blade prop, and it won't break the bank...

DougR

[Pete7]

Couldn't agree more, why sacrifice the great sailing ability?

We currently have a fixed two blade and I keep looking at feathering props but there are other higher priorities.

One day though we will have have either a feathering or folding prop, it would be worth 1/2 - 3/4 a knot under all points of sail. That isn't to be sniffed at even on a short passage. A new laminate genoa would be twice that and may not achieve the same sort of performance improvement.

Pete

[zboss]

I agree with either a feathering or folding. We recently put on a Jprop feathering prop and we have enjoyed an increase in speed when sailing but also better handling. However, it was a very expensive upgrade. I don't think that a 13" or 14" would be that expensive (we have an 18").

Plus, since it has a pitch we can adjust without removing the boat from the water we have been able to tune the prop to the right pitch. When we cross an ocean, we will increase the pitch slightly (do it while we are cleaning the bottom) so we can run the engine at a lower RPM to decrease fuel burn.

[Panope]

I just switched from a 3 blade fixed prop to a 3 blade feathering (Maxprop).

Huge difference. It's like an entirely different boat.

-Faster. Not quite 1 knot faster, but close.

-Boat is now able to come about under the (very small) headsail alone.

-Boat LOST a significant portion of her ability to self steer under sail (I guess the old prop was acting like a drogue).

-Only one tenth of a knot boat speed was lost from my normal cruise RPM power setting. A small price to pay for the much improved sailing performance.

Steve

[MicHughV]

...this is an incredibly complicated subject.....there are many, many variables...engine horsepower....bottom shape of boat, and many, many more..
...my best advice is to get this book...
" The Propellor Handbook" by author " Dave Gerr "
The book is filled with graphs, " how too's"...specifications, etc....
It's a tough read and may requite several reads to thoroughly understand..but after you have read and understood this book, you will be better equipped to answer your own question...

[Cheechako]

Whatever you go to from that Martec, it will be tons better. Even a 2 blade fixed. 3 is better. Go with the biggest diameter that works for your boat. Then adjust your pitch for the engine rpm. I think the rule is you want 15% of prop diameter clearance to the hull. So if your prop is 14" you want a bit over 2" clearance etc.

In 3 blade props there's a big difference in blade shape. The one you show has huge wide blades.... you dont need those and they drag more. The Campbell Sailor or similar have blades that are less like powerboat blades and not as wide. Excellent prop too.
Take a look at the difference, and the bigger prop in this pic isn't even as wide as the powerboat prop in your link.:

[Vancefish]

Just found this. Thin blades, very close to the right pitch. Might be an inch long, but would definitely fit.

[malbert73]

Great deal. Go for it

[Vancefish]

I could see this reducing drag significantly over the original posted prop.

Plus I could easily rotate the prop shaft to aim one blade straight down, further reducing drag.

Also being cheap and simple (my goal).

Obviously I'd need to test my max speed through water (not over land) using this prop. Otherwise I may find current outperforming capability.

However the entire goal is to reduce complications. A solid definitely gives simplicity. While giving nearly equal reverse and forward capability.

I fully agree with everyone who has contributed to this so far. However I need to get sailing as cheap as possible right now. This prop has definitely been one of my hold ups to actually getting on the sail.