Prop question

[Propnut]

I currently have a 26”x 20 pitch left hand three blade prop on my single screw full keel 8 knot trawler. The prop seems to be the correct pitch as I can obtain rated engine rpm and it runs smoothly. I have no idea how old the prop is on this 30 year old boat. Now I’m looking for a spare prop to keep onboard. I’m thinking about just buying another 3 blade, same diameter, same pitch prop.
I’ve considered a 4 blade, but have read that two of the blades being inline with the keel can cause a problem. Then there’s three blade props with different DAR’s. I’ve read that prop technology has changed over the last 20 years . Any suggestions or experience on a more advanced prop than what I currently have.
I know there’s not a magical prop that’s going to drastically change any of my performance figures, but if I’m going to purchase a new prop, I’d like to buy the best one for my application.

 

[Comodave]

I would ask this question at a good prop shop. On a 7 or 8 knot boat minor differences won’t make much difference in performance.

 

[Nomad Willy]

Just get another prop.

It’s smooth and attains rated rpm.

The only possible reason you would need a different prop is the chance that you’ve got too much or too little pitch or diameter. The pitch and dia. (along with the profile blade style (and surface area) and rpm .. produce the blade loading you need.

So engine loading (achieved at WOT and engine manufacturers rated rpm) along w rpm and pitch produces the overall loading you need. It boils down to pitch and dia. because most all common props are about the same shape and overall design.

The only reason to use a 4 blade is if you can’t get a pitch/dia. combination that loads the prop blades and engine properly. For most all rec trawlers the 3 blade is best. But a 4 blade will be better if your boat lacks the propeller swinging room to swing the dia. you need. This often occurs if a boat is repowered w a more powerful engine.

Look up the ideal ratios for propellers. Your 26X20 likely will be good. Too little pitch and you will over rev and you’ll waste power just rotating the propeller blades through the water … surface drag. Think: you’ve got to push water to make thrust.
Too much pitch and your loosing too much thrust by water being flung outwards where it will do little work.

If you don’t have sufficient blade clearance for a larger propeller and thus dia. and can’t change gear ratios you’ll need a 4blade. But if you do … then increase the dia. But only if it dosn’t require you to go well out of the ideal pitch/diameter ratios. Usually rec trawler props will have several inches less pitch than diameter.
There are ideal pitch/dia. ratios that will be most efficient. Try very hard to fall into this ideal range of pitch/dia. ratios.

 

[High Wire]

If you have the right prop now, why change to something that could be wrong? My vote would be like kind if possible.

 

[jungpeter]

Just get another prop.

Yeah, this.

If you have the right prop now, why change to something that could be wrong? My vote would be like kind if possible.

And this. Why try to pick the fly sh^& out of the pepper? And bear in mind, you will be very, very unlikely to ever use your spare prop. While I'm only a sole anecdotal case, in my many decades afloat, I've NEVER had to use a spare propellor. I've met Mother Earth a few times, but my spare propeller(s) have simply occupied space and cost money better spent on other things. Conversely, I've bitched often at having to work around the miserable hunks of bronze that are inevitably in the way (and take up valuable storage) when I perform maintenance. You'd be surprised on the number of propeller services there are around any waterfront, and how resourceful they are at repair and/or replacement on short notice. YMMV for sure, and it's your boat, and your money.

Regards,

Pete

 

[rslifkin]

And bear in mind, you will be very, very unlikely to ever use your spare prop. While I'm only a sole anecdotal case, in my many decades afloat, I've NEVER had to use a spare propellor. I've met Mother Earth a few times, but my spare propeller(s) have simply occupied space and cost money better spent on other things. Conversely, I've bitched often at having to work around the miserable hunks of bronze that are inevitably in the way (and take up valuable storage) when I perform maintenance. You'd be surprised on the number of propeller services there are around any waterfront, and how resourceful they are at repair and/or replacement on short notice. YMMV for sure, and it's your boat, and your money.

This logic is why I took the spare props off the boat and started keeping them in the garage at home. It's not a bad thing to have them, but I'm not equipped to install them in the water without outside assistance anyway. So if worst came to worst and I damage a prop that can't be fixed locally, I'll just have someone ship me the spares. It's unlikely to add much delay beyond arranging a diver or haulout, etc. anyway.

 

[Comodave]

If the props you have now are working well for you then get more of the same. I would carry the spares since you can get a diver or yard to change a prop if you have one. Doesn’t take very long to swap it out.

 

[High Wire]

I would and do carry an identical spare. If you wrap one up, its only a couple hours in any lift to get the props swapped out. Then you can get the bent one fixed at your leisure at the shop of your choice. Without a spare, you get the prop pulled and you are held hostage by someone else. Sometimes it's a few days, sometimes it can be a couple weeks. The prop is one of the top 5 of the many boat parts that can get damaged while cruising. It certainly can be a show stopper for a indefinite period of time.

 

[Bacchus]

I would and do carry an identical spare. If you wrap one up, its only a couple hours in any lift to get the props swapped out. Then you can get the bent one fixed at your leisure at the shop of your choice. Without a spare, you get the prop pulled and you are held hostage by someone else. Sometimes it's a few days, sometimes it can be a couple weeks. The prop is one of the top 5 of the many boat parts that can get damaged while cruising. It certainly can be a show stopper for a indefinite period of time.

My thought exactly.
At worst a short haul and you can be underway.
As insurance it isn't all that expensive in that you can always sell and get $$$ back. I looked and found a used one that needed a little work but in total probably 1/2 $ of new

 

[Jeff F]

Fully agreed. I damaged my prop a couple of months ago while cruising. Was able to find a yard to do a quick lift and swap props, and was back underway less than 48 hours later. I just got the repaired prop back as my new spare.

I'm not convinced that it has to be identical though, for the OP. Reaching full rated rpm indicates that you're not over propped, but doesn't answer whether you're under propped.

My two props are quite different. I have a three blade that's fine, a little under propped. The other is four blade, and I'm over propped with it. Having done extensive cruising with both, the 4 blade will be my primary.

I'm probably on record here arguing against over propping. I now think it's appropriate for my use, and it may be for yours as well.

I'd suggest an opportunistic buy in the used market, and be open to experimentation. Maybe consider something that consumes a bit more HP. Throw it on the next haul and try it out. Prop shops are pretty good at detuning props, so I wouldn't be afraid of going too big on an experimental basis, within reason. There are online calculators that I've found useful for comparing prop A to prop B, with a spreadsheet on the side. Or ask a prop shop.

 

[cafesport]

Check with your prop shop, but there could be advantages to running a 4 blade prop if you have a six cylinder engine, all other things being equal.

 

[Nomad Willy]

Cafesport,
It seems like you’re teasing us.
What is the relationship between 6cyl engines and 4 blade props .. please?

Jeff F wrote;
“but it dosn’t answer whether you’re underpped”

Being underpropped isn’t a problem unless you’re underpropped too much. Being a bit underpropped means you can’t operate overpropped .. and I see that as a good thing. I personally consider being 100rpm underpropped to be about perfectly loaded. 200 would be fine too. You’ll never operate at such a high speed and loading other than to do a system check every 6 months or so. Very few will work a prop over to be spot on to the rated rpm. So you’ll be a bit this way or that. And overpropped/underpropped is not a black and white issue. Being overpropped 100 rpm may seem perfect to many or most but I’m in the 100rpm “under” category. As time goes on most of us will eventually be overpropped because of underwater fowling. That’s one reason I like to be 100 over.